ABSORPTION OF NITROGEN OXIDES
INTO DILUTED AND CONCENTRATED
NITRIC ACID
J. B. Lefers
Delft University Press
I
I
ABSORPTION OF NITROGEN OXIDES
INTO DILUTED AND CONCENTRATED
NITRIC ACID
T3
m o
o
o
U1 U l
BIBLIOTHEEK TU Delft
P 1608 4302
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456614
í
ABSORPTION OF NITROGEN OXIDES
INTO DILUTED AND CONCENTRATED
NITRIC ACID
PROEFSCHRIFT ter verkrijging van
de graad van doctor in de
technische wetenschappen
aan deTechnische Hogeschool Delft,
op gezag van de rector magnificus
Prof. dr. ir. F. J.Kievits,
v o o r e e n commissie aangewezen
door het college van dekanen
te verdedigen op
woensdag 12 maart 1980
te 16.00 u u r d o o r
Jan Bernard Lef ers
scheikundig ingenieur
geboren te Enschede
£0 02250
Delft University Press/1980
% D0ELEHSIR.101 £ ~
Dit proefschrift is goedgekeurd door de promotor
PROF. DRS. R J. VAN DEN BERG
I
!
Aan mijn ouders
i
i
/
i
I
VOORWOORD
Dit
proefschrift
personen.
i s t o t s t a n d gekomen met
Langs deze weg
de medewerking van een g r o o t a a n t a l
w i l i k hen voor hun e n t h o u s i a s t e i n z e t h a r t e l i j k
en
o p r e c h t danken. In het b i j z o n d e r gaat m i j n dank u i t naar
de a f s t u d e e r d e r s : Okke de Boks, Arend Bos en Jan Laverman, d i e aan d i t onderzoek hebben meegewerkt;
m i j n c o l l e g a Cock van den B l e e k voor de p r e t t i g e samenwerking b i n n e n de
onderzoekgroep
en voor z i j n
s t i m u l e r e n d e b i j d r a g e i n de
discussies;
a l l e medewerkers van v e r s c h i l l e n d e s e r v i c e g r o e p e n voor de v e r v a a r d i g i n g ,
stelling
NO^-
her-
en v e r b e t e r i n g van de a p p a r a t u u r en de u i t v o e r i n g van de t a l r i j k e
l y s e s , zonder w i e r werkzaamheden d i t p r o e f s c h r i f t
zeker n i e t
ana-
t o t s t a n d gekomen
was;
Wim
J o n g e l e e n en Koos Kamps voor het tekenen en v e r k l e i n e n van de
figuren;
S a u l Lemkowitz voor het c o r r i g e r e n van de e n g e l s e t e k s t ;
Marian Wijnen
manuscript.
voor het z o r g v u l d i g u i t g e v o e r d e typewerk
en de l a y - o u t van het
C O N T E N T S
SUMMARY
1
SAMENVATTING
3
1.
INTRODUCTION
5
1.1 G e n e r a l remarks
5
1.2 Aim o f t h i s work
9
1.3 O u t l i n e o f t h e t h e s i s
9
References
10
2. THE ABSORPTION APPARATUS
11
2.1 I n t r o d u c t i o n
11
2.2 S e l e c t i o n o f l a b o r a t o r y a b s o r b e r
11
2.3 D e s c r i p t i o n o f the a b s o r p t i o n a p p a r a t u s
14
2.4 Mass t r a n s f e r i n a l a m i n a r f a l l i n g l i q u i d
2.4.1
film
16
Theory
16
2.4.2 E x p e r i m e n t a l
18
2.4.3
Results
2.5 Gas phase mass t r a n s f e r i n l a m i n a r p l u g flow gas streams
2.5.1
Introduction
18
19
19
2.5.2 Theory
20
2.5.3
22
Experimental
2.5.4 R e s u l t s
2.6 C o n c l u s i o n s
References
23
27
28
3. SPECTR0PH0T0METRIC DETERMINATION OF NITROGEN OXIDES AND NITRIC ACID
VAPOUR
30
3.1 I n t r o d u c t i o n
30
3.2 E x p e r i m e n t a l
30
3.3 R e s u l t s
33
3.4 C o n c l u s i o n s
39
References
41
4. THE ABSORPTION OF NO„/N„0_ INTO DILUTED AND
2 2 4
NITRIC ACID
CONCENTRATED
42
4.1 I n t r o d u c t i o n
42
4.2 Review o f l i t e r a t u r e
42
4.2.1 A b s o r p t i o n o f NO^/I^O^ i n t o aqueous s o l u t i o n s
4.2.2 NO^/N^O^ a b s o r p t i o n i n t o c o n c e n t r a t e d n i t r i c
42
acid solutions
4.3 E x p e r i m e n t a l
48
53
4.4 R e s u l t s
55
4.4.1 The a b s o r p t i o n o f NgO^ i n t o d i l u t e d n i t r i c
4.4.2 The a b s o r p t i o n o f ^ ^4
i
n
t
o
2
acid solutions
concentrated n i t r i c
55
acid
solutions
59
4.5 C o n c l u s i o n s
65
References
65
5. THE OXIDATION AND ABSORPTION OF NO BY NITRIC ACID
67
5.1 I n t r o d u c t i o n
67
5.2 Proposed mechanism
67
5.3 E x p e r i m e n t a l
70
5.4 M a t h e m a t i c a l model and r e s u l t s
70
5.5 D i s c u s s i o n
86
5.6 C o n c l u s i o n s
87
References
6. AN ABSORPTION MODEL FOR THE DESIGN OF A DILUTED NITRIC ACID
ABSORBER AND METHODS TO DECREASE THE NO -CONTENT IN TAIL GASES
x
6.1 I n t r o d u c t i o n
89
89
6.2 A b s o r p t i o n model f o r t h e p r o d u c t i o n o f d i l u t e d n i t r i c
6.3 Methods t o d e c r e a s e
t h e NO^-content i n t a i l
acid
89
gases o f n i t r i c
acid plants
94
6.3.1 Wet p r o c e s s e s
95
6.3.1.1 Extended a b s o r p t i o n
6.3.1.2 H °2
s
c
r
u
b
b
2
6.3.1.3 N i t r i c
i
n
S
process
acid scrubbing
6.3.2 Dry p r o c e s s e s
95
95
97
100
6.3.2.1 A d s o r p t i o n
100
6.3.2.2 N o n - s e l e c t i v e r e d u c t i o n p r o c e s s e s
100
A
6.3.2.3 S e l e c t i v e r e d u c t i o n p r o c e s s e s
References
101
102
APPENDIX I. THE ADDITIVITY OF RESISTANCES FOR MASS TRANSFER IN A
WETTED WALL COLUMN
105
1. I n t r o d u c t i o n and g e n e r a l t h e o r y
105
2. R e s u l t s
107
3. C o n c l u s i o n s
113
References
NOMENCLATURE
113
114
SUMMARY
The
subject of t h i s thesis
into n i t r i c
absorbers
i s d e a l i n g with the a b s o r p t i o n o f n i t r o g e n oxides
a c i d s o l u t i o n s to o b t a i n data f o r the design o f i n d u s t r i a l
f o r t h e p r o d u c t i o n o f d i l u t e d and c o n c e n t r a t e d n i t r i c a c i d . From t h e
l i t e r a t u r e c o n c e r n i n g t h e a b s o r p t i o n o f n i t r o g e n o x i d e s i n t o aqueous s o l u t i o n s
it
i s known t h a t NO, NO^, N^O^
a b s o r p t i o n . Moreover, n i t r i c
and N^O^
a l l p l a y an important
r o l e during the
a c i d and n i t r o u s a c i d can be formed i n t h e gas
phase and i n t h e l i q u i d phase. T h i s complex a b s o r p t i o n mechanism was
investigated i n a specially
d e s i g n e d wetted
w a l l column w i t h a known
facial
a r e a between t h e gas phase and t h e l i q u i d phase. A l a m i n a r
liquid
film
and a l a m i n a r p l u g flow o f t h e gas phase w i t h o u t
g r a d i e n t p e r p e n d i c u l a r t o the g a s - l i q u i d
wetted
falling
a velocity
i n t e r f a c e c o u l d be r e a l i z e d
w a l l column. The mass t r a n s f e r i n t h e l a m i n a r f a l l i n g
i n v e s t i g a t e d by a b s o r b i n g pure carbon
o
inter-
liquid
i n the
f i l m was
d i o x i d e i n t o water at a temperature
20 C and at a p r e s s u r e o f 1 b a r . I t was found
of
t h a t w i t h i n t h e measured
c o n d i t i o n s t h e l i q u i d phase mass t r a n s f e r c o u l d be d e s c r i b e d by t h e p e n e t r a t i o n
theory.
The gas phase mass t r a n s f e r was i n v e s t i g a t e d by a b s o r b i n g ammonia
a n i t r o g e n gas stream
into 2 N s u l f u r i c
from
a c i d s o l u t i o n s . The e x p e r i m e n t a l
r e s u l t s showed a good agreement w i t h t h e t h e o r e t i c a l l y p r e d i c t e d v a l u e s d e r i v e d
from t h e s o l u t i o n o f t h e G r a e t z
The
nitric
problem.
a b s o r p t i o n o f NOg/N^O^ gas m i x t u r e s
from
a c i d was c a r r i e d out at a temperature
o f about 1 b a r . The e x p e r i m e n t a l
a n i t r o g e n gas stream
o
o
into
o f 20 C and 30 C and a t a p r e s s u r e
r e s u l t s c o u l d be i n t e r p r e t e d w i t h t h e f o l l o w i n g
model:
a) NOg and N^O^,
which a r e i n c o n t i n u o u s
from t h e gas phase t o the g a s - l i q u i d
b) N^O^
i s t h e o n l y s p e c i e s which d i f f u s e s i n t o t h e l i q u i d
c) In t h e experiments
N^O^
e q u i l i b r i u m w i t h each o t h e r ,
diffuse
interface.
with d i l u t e d n i t r i c
acid
i s accompanied by a r a p i d pseudo f i r s t
phase.
(25% and 40%) t h e d i f f u s i o n o f
order r e a c t i o n i n the l i q u i d
phase between N 0^ and water. I t was found t h a t t h e a b s o r p t i o n r a t e o f N^O
2
into diluted n i t r i c
In t h e experiments
NO
a c i d decreases with i n c r e a s i n g a c i d s t r e n g t h .
with concentrated n i t r i c
acid
(63%-80%) t h e r e a c t i o n o f
w i t h water can be n e g l e c t e d and N O
dissolves physically
a 4
^4
l i q u i d phase.
i n the
1
The
s o l u b i l i t y o f N^O^ i n c o n c e n t r a t e d n i t r i c
a c i d s o l u t i o n s was c a l c u l a t e d
from t h e t o t a l vapour p r e s s u r e d a t a o f t h e system NgO^-HgO-HNOg. I t was found
t h a t w i t h i n t h e c o n d i t i o n s s t u d i e d Henry's law i s v a l i d .
s o l u b i l i t y o f NgO^ i n c o n c e n t r a t e d n i t r i c
c r e a s i n g a c i d s t r e n g t h and d e c r e a s i n g
The
Furthermore, t h e
acid increases strongly with i n -
temperature.
o x i d a t i o n o f NO i n a n i t r o g e n gas stream
by 40%-80% n i t r i c
acid
o
s o l u t i o n s was i n v e s t i g a t e d
i n t h e wetted
and 30°C. In t h e experiments
experimental
w a l l column a t a temperature
w i t h 63% and 80% n i t r i c
o f 20 C
a c i d s o l u t i o n s the
r e s u l t s were i n t e r p r e t e d w i t h t h e f o l l o w i n g model:
a) The o x i d a t i o n r e a c t i o n t a k e s p l a c e i n t h e gas phase between NO and n i t r i c
a c i d vapour and can be c o n s i d e r e d t o be i n f i n i t e l y
vapour a r e t r a n s f e r r e d by m o l e c u l a r d i f f u s i o n
b u l k and t h e g a s - l i q u i d
f a s t . NO and n i t r i c
from, r e s p e c t i v e l y , t h e gas
i n t e r f a c e t o t h e r e a c t i o n zone o r p l a n e .
found t h a t Danckwerts' s o l u t i o n
acid
f o r instantaneous
I t was
irreversible reactions i n
the l i q u i d phase can a l s o be a p p l i e d t o gas phase r e a c t i o n s .
b) The NO^ and N^O^ produced, which a r e i n continuous, e q u i l i b r i u m w i t h each
o t h e r , d i f f u s e from
t h e r e a c t i o n p l a n e o r zone t o t h e gas b u l k and t o t h e
gas-liquid
i n t e r f a c e . At t h e g a s - l i q u i d
i n t e r f a c e o n l y Ng0
physically
into the concentrated n i t r i c
acid.
Experiments
partially
w i t h 57% n i t r i c
i n t h e l i q u i d phase. Under t h e s e c i r c u m s t a n c e s
t h e gas phase r e a c t i o n
a c i d vapour p r e s s u r e . In t h e experiments
the f i n a l
proceeds
f a s t , a f a c t which may be caused by
a c i d i t was found t h a t t h e o x i d a t i o n t a k e s p l a c e c o m p l e t e l y
Under t h e s e c i r c u m s t a n c e s
dissolves
a c i d showed t h a t t h e o x i d a t i o n o f NO a l s o
i s t o o slow t o be c o n s i d e r e d t o be i n f i n i t e l y
the r a t h e r low n i t r i c
4
o x i d a t i o n product
w i t h 40% n i t r i c
i n t h e l i q u i d phase.
i s mainly n i t r o u s a c i d .
Based on g e n e r a l c h e m i c a l r e a c t i o n e n g i n e e r i n g c o n s i d e r a t i o n s a mathematical
model was developed
t o d e s c r i b e t h e a b s o r p t i o n mechanism which o c c u r s i n the
absorber
f o r the production of d i l u t e d n i t r i c
decrease
t h e amounts o f n i t r o g e n o x i d e s c o n t e n t
p l a n t s were b r i e f l y
2
discussed.
acid. F i n a l l y
in tail
v a r i o u s methods t o
gases o f n i t r i c
acid
S A M E N V A T T I N G
Dit
p r o e f s c h r i f t h e e f t a l s onderwerp de a b s o r p t i e van s t i k s t o f o x i d e n
t e r z u u r , hetgeen
van b e l a n g i s b i j het ontwerp van industriële a b s o r b e r s
de p r o d u k t i e van verdund
derzoek
i n salpevoor
en g e c o n c e n t r e e r d s a l p e t e r z u u r . U i t een l i t e r a t u u r o n -
i s g e b l e k e n d a t de a b s o r p t i e van s t i k s t o f o x i d e n
i n waterige oplossingen
z e e r g e c o m p l i c e e r d i s , w a a r b i j NO, N^O^, NO2 and N^O^ een b e l a n g r i j k e r o l spelen.
Bovendien
kunnen s a l p e t e r z u u r en s a l p e t e r i g z u u r i n de v l o e i s t o f f a s e en i n
de g a s f a s e worden gevormd. Om een d e r g e l i j k g e c o m p l i c e e r d a b s o r p t i e p r o c e s t e
bestuderen
i s een n a t t e wand kolom o n t w i k k e l d waarin het c o n t a c t o p p e r v l a k t u s -
sen g a s - en v l o e i s t o f f a s e goed bekend i s . In deze kolom kon een l a m i n a i r e s t r o ming van de v a l l e n d e v l o e i s t o f f i l m
zonder
en een l a m i n a i r e p r o p s t r o m i n g van de g a s f a s e
snelheidsgradiënt l o o d r e c h t op het g a s - v l o e i s t o f c o n t a c t o p p e r v l a k worden
v e r k r e g e n . Het s t o f t r a n s p o r t
i n de l a m i n a i r v a l l e n d e v l o e i s t o f f i l m werd
ondero
z o c h t door z u i v e r CO^ t e a b s o r b e r e n
i n water b i j een temperatuur
een druk van 1 b a r . U i t de experimenten
transport
Het
kon worden g e c o n c l u d e e r d dat het s t o f -
i n de v l o e i s t o f f a s e b e s c h r e v e n kon worden door de p e n e t r a t i e t h e o r i e .
stoftransport
i n de g a s f a s e werd onderzocht
s t i k s t o f gasstroom
overeen
van 20 C en
i n 2 N zwavelzuur.
door NH^ t e a b s o r b e r e n van een
De e x p e r i m e n t e l e r e s u l t a t e n b l e k e n goed
t e komen met de t h e o r e t i s c h v o o r s p e l d e waarden u i t het Graetz-model.
De a b s o r p t i e van NOg/NgO^ gasmengsels van een s t i k s t o f gasstroom
o
o
t e r z u u r werd u i t g e v o e r d b i j een temperatuur
ongeveer
en 30 C en een druk van
1 b a r . De e x p e r i m e n t e l e r e s u l t a t e n konden worden beschreven met het
volgende
model.
a) N 0
en NgO^,
2
van 20
i n salpe-
w
e
^
k
e
v o o r t d u r e n d i n evenwicht
zijn,
d i f f u n d e r e n van de gas-
f a s e naar het f a s e g r e n s v l a k .
b) N 0
2
4
i s de a c t i e v e component d i e i n de v l o e i s t o f f a s e
c) In de experimenten
met verdund
diffundeert.
s a l p e t e r z u u r (25% en 40%) gaat de d i f f u s i e
van N^O^ gepaard met een s n e l l e pseudo I e o r d e r e a c t i e i n de v l o e i s t o f f a s e
t u s s e n N^O^ en water.
van NgO^ i n verdund
In
de experimenten
U i t de experimenten
b l e e k dat de a b s o r p t i e s n e l h e i d
zuur afnam b i j toenemende z u u r s t e r k t e .
met g e c o n c e n t r e e r d s a l p e t e r z u u r (63%-80%) b l e e k dat de
r e a c t i e t u s s e n water en ^ 0 ^ v e r w a a r l o o s d kon worden. In dat g e v a l
NO
lost
s l e c h t s f y s i s c h op i n de v l o e i s t o f f a s e .
3
De o p l o s b a a r h e i d van N^O^ i n g e c o n c e n t r e e r d
s a l p e t e r z u u r werd berekend u i t
l i t e r a t u u r g e g e v e n s b e t r e f f e n d e de t o t a l e dampdruk van h e t systeem N^O^-HgOHNOg. Binnen de beschouwde c o n d i t i e s b l i j k t
Verder b l i j k t
de o p l o s b a a r h e i d van N 0
2
4
d a t de wet van Henry g e l d i g i s .
i n geconcentreerd
toe t e nemen met toenemende z u u r s t e r k t e en dalende
salpeterzuur sterk
temperatuur.
De o x y d a t i e van NO i n een s t i k s t o f gasstroom door 40%-80% s a l p e t e r z u u r werd
onderzocht
i n de n a t t e wand kolom b i j een temperatuur van 20°C en 30°C. De ex-
perimentele
r e s u l t a t e n met 63% en 80% s a l p e t e r z u u r konden worden beschreven
met
het volgende model.
a) De o x y d a t i e r e a c t i e v i n d t p l a a t s i n de g a s f a s e
t u s s e n NO en s a l p e t e r z u u r -
damp en kan a l s o n e i n d i g s n e l worden beschouwd. Salpeterzuurdamp en NO d i f funderen
r e s p e c t i e v e l i j k van h e t g a s - v l o e i s t o f c o n t a c t o p p e r v l a k
b u l k n a a r h e t r e a c t i e v l a k o f de reactiezöne. E x p e r i m e n t e e l
en de gas-
b l e e k dat
Danckwerts' o p l o s s i n g e n voor i n s t a n t a n e i r r e v e r s i b e l e r e a c t i e s i n de v l o e i s t o f f a s e tevens kunnen worden t o e g e p a s t
op i n s t a n t a n e g a s f a s e
b) Het gevormde NOg en N^O^, welke v o o r t d u r e n d
i n evenwicht z i j n ,
reacties.
diffunderen
van h e t r e a c t i e v l a k o f de reactiezöne n a a r de b u l k van de g a s f a s e en n a a r
het
f a s e g r e n s v l a k . Op h e t f a s e g r e n s v l a k l o s t
alleen
N
2
°
4
f y s i s c h op i n de
vloeistoffase.
Experimenten met 57% s a l p e t e r z u u r toonden aan d a t de o x y d a t i e van NO ook gedeeltelijk
niet
v e r l o o p t i n de v l o e i s t o f f a s e .
In d i t g e v a l kan de g a s f a s e
a l s o n e i n d i g s n e l worden beschouwd, hetgeen wordt v e r o o r z a a k t
salpeterzuurdampdruk.
reactie
door de l a g e
B i j 40% s a l p e t e r z u u r v i n d t de o x y d a t i e van NO v o l l e d i g
i n de v l o e i s t o f f a s e p l a a t s , w a a r b i j
s a l p e t e r i g zuur h e t u i t e i n d e l i j k
gevormde
produkt i s .
Toepassing
van de algemene b e g i n s e l e n van de chemische reactorkunde
op de
a b s o r p t i e van s t i k s t o f o x i d e n i n s a l p e t e r z u u r r e s u l t e e r d e i n een w i s k u n d i g
mo-
d e l voor h e t ontwerpen van industriële a b s o r b e r s b i j de p r o d u k t i e van verdund
s a l p e t e r z u u r . T e n s l o t t e werden de v e r s c h i l l e n d e m o g e l i j k h e d e n
te i n afgassen
4
van s a l p e t e r z u u r p l a n t s t e v e r l a g e n met e l k a a r
om h e t N O ~ g e h a l x
vergeleken.
1. INTRODUCTION
1.1 GENERAL REMARKS
Nitric
a c i d i s one o f t h e most important
production of f e r t i l i z e r s ,
t i o n s are s t a i n l e s s s t e e l p i c k l i n g
the n i t r i c
nitric
acid with
and metal e t c h i n g . About t h r e e - f o u r t h s o f
a c i d produced i s used i n t h e f e r t i l i z e r
p r o d u c t i o n o f ammonium n i t r a t e ,
The
i n o r g a n i c a c i d s and i t i s used i n t h e
d y e s t u f f s , r e s i n s and e x p l o s i v e s . F u r t h e r a p p l i c a -
i n d u s t r y , mainly
ammonium phosphates and compound
a c i d needed i n t h e f e r t i l i z e r
f o r the
fertilizers.
industry i s usually diluted
nitric
a c o n c e n t r a t i o n o f 50-70%. F o r most o t h e r a p p l i c a t i o n s , such as
n i t r a t i o n r e a c t i o n s , 90-100% n i t r i c
acid
i s used.
S i n c e t h e development o f t h e Haber-Bosch ammonia s y n t h e s i s i n 1913 n e a r l y
all
nitric
a c i d p l a n t s a r e based on t h e o x i d a t i o n o f ammonia and t h e subsequent
absorption of n i t r o g e n oxides
Diluted
An
nitric
acid
production
example o f a flow sheet
the D.S.M. n i t r i c
f o r the production of d i l u t e d n i t r i c
a c i d process
Ammonia mixed w i t h
passed over
i n t o water.
i s g i v e n i n F i g . 1 (mono p r e s s u r e
a i r enters a converter
a platinum
gauze c a t a l y s t
a c i d based on
system).
(B) i n which t h e gas m i x t u r e i s
a t a temperature o f 850-920°C. The
ammonia i s o x i d i z e d t o NO a c c o r d i n g t o
4NH
The
+
3
50
-»• 4N0
2
+
c o n v e r t e r can be o p e r a t e d
(7-10
6H 0
(1)
2
at atmospheric,
medium (3-5 b a r ) o r h i g h
pressure
b a r ) . The hot gases l e a v i n g t h e c o n v e r t e r a r e c o o l e d i n a waste heat
b o i l e r t o generate
cooler-condenser
2N0
+
0
2
steam. The temperature i s f u r t h e r reduced
t o 20°-40°C i n a
(D), and at t h e same time t h e NO formed i s o x i d i z e d t o N 0
X
2N0
2
2
(2)
5
7
12
-H 0
2
NH 3
0-
B
T
11
10
8
60°/.
Fig.
1
The
A:
D.S.M. n i t r i c acid
air
compressor,
condenser,
process
B: converter,
E: absorption
(mono-pressure
C: tail
column,
gas
system).
heater,
F: bleaching
HNO3
D:
column,
cooler-
G:
expansion
turbine.
1: air,
gas
2: NH , 3: 10% NH
containing
10: bleached
in air,
200-2000 ppm
60% n i t r i c
4: NO,
5: N0 ,
8: unbleached
acid,
11: air,
12:
absorber
The
The n i t r i c
60% n i t r i c acid,
9:
NO^,
some weak a c i d i s
H0
2
•*
2HN0
3
+
NO
(3)
a c i d f o r m a t i o n i s accompanied by NO
i s subsequently
absorbed
e v o l u t i o n which i s r e - o x i d i z e d
i n t o the l i q u i d
a b s o r b e r the r e - o x i d a t i o n r a t e o f NO
may
tail
i s v e r y slow,
( 2 ) . The
N0
2
phase. In t h e top o f the
and
as a f i r s t
approach
be assumed t o be t h e r a t e d e t e r m i n i n g s t e p i n the a b s o r p t i o n p r o c e s s .
gas,
c o n t a i n i n g about
200-2000 ppm
Energy
i s subsequently
stream
i s vented t o t h e atmosphere.
The
60% n i t r i c
a b s o r b e r and
6
acid
phase at ambient
i n the gas phase by m o l e c u l a r oxygen a c c o r d i n g t o r e a c t i o n
produced
e n t e r s the
and at the o p e r a t i n g p r e s s u r e i n the c o n v e r t e r :
+
2
tail
10 volume % NOg,
(E) where i t r e a c t s w i t h water i n the l i q u i d
temperature
3N0
gas m i x t u r e , c o n t a i n i n g about
7:
water.
The water formed condenses i n the c o o l e r - c o n d e n s o r and
produced.
6: weak acid,
2
N0 ,
x
r e c o v e r e d by expansion
l e a v e s the a b s o r b e r and
acid
this
The
i s heated.
i n a t u r b i n e a f t e r which the
gas
a c i d c o n t a i n i n g some d i s s o l v e d n i t r o g e n o x i d e s l e a v e s the
i s s t r i p p e d w i t h a i r i n a b l e a c h i n g column
(F) .
A v a r i a n t o f the mono-pressure p r o c e s s
i s the d u a l p r e s s u r e p r o c e s s , at
which the c o n v e r s i o n t a k e s p l a c e at a lower p r e s s u r e than the a b s o r p t i o n . In a
d u a l p r e s s u r e system a n i t r o u s gas compressor i s needed. The main advantage
o f f e r e d by a low p r e s s u r e i n the c o n v e r t e r i s the d e c r e a s e
platinum c a t a l y s t
l o s s e s . On
o f the ammonia
the o t h e r hand the i n v e s t m e n t s
and
r e q u i r e d are
higher.
S i m i l a r processes
such as the D.S.M. n i t r i c
a c i d p r o c e s s can be found i n
the l i t e r a t u r e [ 1 ] .
Concentrated
nitric
acid
production
There i s a s u b s t a n t i a l need f o r s t r o n g e r a c i d , p a r t i c u l a r l y
f o r acid with
a
c o n c e n t r a t i o n i n the range o f 90-100%. Such a c i d i s , f o r example, used i n
n i t r a t i o n r e a c t i o n s . However, c o n c e n t r a t e d n i t r i c
p r e p a r e d by
form
distillation
o f d i l u t e d a c i d o f 60%,
a constant b o i l i n g mixture
( a z e o t r o p e ) between an a c i d
E x t r a c t i v e d i s t i l l a t i o n with s u l f u r i c
c o m p o s i t i o n w i t h magnesium n i t r a t e and
mixture
are r a t h e r e x p e n s i v e
R e c e n t l y some new
nitric
Fig.
a c i d o f 80%,
a c i d process
which can be d i r e c t l y
NO
produced
i n a waste heat
(B). In the c o o l e r - c o n d e n s e r
(1) i s condensed and
10-12
gas m i x t u r e
bar. The
gas m i x t u r e
2N 0
2
NOg.
+
2
column.
+
0
2
-
4HN0
3
(C)
a c i d at 0°C-10°C and at a p r e s s u r e o f
80%-85% n i t r i c
i s converted to n i t r i c
2H 0
produced
i s produced. The weak a c i d
a b s o r b e r c o n t a i n s some n i t r i c
T h i s can be d e c r e a s e d
c o n t a i n s about 15-30% d i s s o l v e d
4
at a
column (D) where the excess o f water i s r e -
l e a v i n g the p h y s i c a l
about 2000 ppm
i n which the NgO^
( 2 ) . Moreover, the water
l e a v e s the bottom o f the d i s t i l l a t i o n
i n 80%-85% n i t r i c
s c r u b b i n g w i t h water. The
absorber
further cooled i n a
i s o x i d i z e d t o NO
from the c o o l e r condenser i s f e d t o a p h y s i c a l a b s o r b e r
i s dissolved
a c i d vapour and
boiler,
t h e NO
some weak a c i d
e n t e r s the weak a c i d d i s t i l l a t i o n
2
n i t r i c acid.
by o x i d a t i o n o f ammonia i n a c o n v e r t e r
o f 15°-70°C a c c o r d i n g t o r e a c t i o n
The
resultant
t o produce c o n c e n t r a t e d
d i s t i l l e d t o produce 100%
temperature
where N0
of the
flow sheet o f the Du Pont de Nemours c o n c e n t r a t e d
[ 2 ] . The
a c i d o f 68%
s t r e n g t h o f 68-69%.
subsequent d i s t i l l a t i o n
by r e a c t i o n
moved. N i t r i c
water
methods.
(A) i s , a f t e r r e c o v e r y o f energy
cooler-condenser
directly
a c i d and
a c i d o r m o d i f i c a t i o n o f the a z e o t r o p i c
p r o c e s s e s have been developed
2 gives a s i m p l i f i e d
nitric
a c i d can not be
because n i t r i c
t o 200
ppm
a c i d l e a v i n g the bottom o f
N
2
°4•
by
the
T h i s s o l u t i o n e n t e r s r e a c t o r (E)
a c i d a t 40°-100°C w i t h a i r .
(4)
7
NH
3
2~A
B
6
10
<90"/.HN0
S
c
3
14
c:
12
<68 °/o HNO3
Fig.
2
The Du Pont de Nemours concentrated
A: converter,
B: cooler-condenser,
d i s t i l l a t i o n column,
strong
acid
n i t r i c acid
E: reactor,
distillation
C: physical
F: strong
dissolved
diluted
N^O^,
acid
5: <_ 68% n i t r i c
7: 80% bleached
scrubber,
acid
9: 85% n i t r i c
Nitric
air, 14: > 90% n i t r i c
acid,
acid,
nitric
11: NOg in air, 12: 85% n i t r i c acid
13:
absorber,
D: weak
bleaching
column, G:
acid,
acid
solution
absorber,
4: weak acid
6: 80% n i t r i c acid
8: tail
to bleacher,
10: N0^ in air,
to d i s t i l l a t i o n
effluent
2
4
from
15: water.
t h e r e a c t o r (85% n i t r i c
bottom p r o d u c t
[ 3 , 4 ] . The
a c i d ) c o n t a i n i n g about 5-10%
i s s t r i p p e d w i t h a i r i n a b l e a c h i n g column ( F ) , A f t e r
the c o n c e n t r a t e d n i t r i c
acid
i s d i s t i l l e d t o produce 90-100% n i t r i c
(80-85% n i t r i c
the l i t e r a t u r e o t h e r p r o c e s s e s
a r e r e p o r t e d t o produce c o n c e n t r a t e d n i t r i c a c i d
such as t h e SOLNOX-process o f Ugine Kuhlmann
the c o n c e n t r a t e d n i t r i c
stripping
a c i d . The
a c i d ) i s r e c y c l e d t o t h e p h y s i c a l a b s o r b e r . In
[ 5 , 6 ] , t h e SABAR-process o f Davy
Powergas GmbH [ 7 , 8 ] , the HYCON-process o f Chemico C o n s t r u c t i o n Corp.
3
column,
a c i d o f about 68% i s f e d t o t h e r e a c t o r t o s u p p l y the water r e q u i r e d f o r
dissolved N 0
Ltd.
to
with
gas to water or
the r e a c t i o n . Note t h a t d u r i n g t h e a c i d f o r m a t i o n no NO i s formed
liquid
acid
column.
1: 10% NH^ in air, 2: NO, 3: NO2 to physical
d i s t i l l a t i o n column,
process.
a c i d p r o c e s s e s o f Sumitomo Chemical
[1,12] and F r i e d r i c h Uhde GmbH [13].
[9,10],
E n g i n e e r i n g Co.
Regulations
Tail
of NO^
emission
gases o f n i t r i c
a c i d p l a n t s c o n t a i n n i t r o g e n o x i d e s which are
reactants i n the formation o f photochemical
p o l l u t a n t s themselves.
kg NO
In the U n i t e d S t a t e s the p r e s e n t e m i s s i o n l e v e l
( c a l c u l a t e d as N0„) per ton a c i d f o r new
x
essential
smog, i n a d d i t i o n t o b e i n g
p l a n t s . This i s equivalent to
ppm.
F o r e x i s t i n g p l a n t s a l e v e l o f 400
ppm
will
be r e q u i r e d . In
Europe the l i m i t v a r i e s from c o u n t r y t o c o u n t r y . P r e s e n t l y f o r new
v a l u e o f 400
In
AIM
ppm
may
be assumed, depending on t h e l o c a l
plants a
situation.
OF THIS WORK
the a b s o r b e r
f o r the p r o d u c t i o n o f d i l u t e d as w e l l as c o n c e n t r a t e d
nitric
a c i d s e v e r a l r e a c t i o n s and e q u i l i b r i a o c c u r . Many i n v e s t i g a t i o n s can be
i n the l i t e r a t u r e c o n c e r n i n g the a b s o r p t i o n o f N0
2
s o l u t i o n s , but the a b s o r p t i o n mechanism i s s t i l l
not w e l l understood.
h i a t u s i s caused m a i n l y by the f a c t
important
t h a t NO,
N
2
°3'
i n t o water and
N 0
2
3 1 1 1 1
N
2°4
r o l e i n the a b s o r p t i o n p r o c e s s . Moreover, n i t r i c
a c i d a r e produced
a
1.3
P
l a
acid
absorbers
be
a
y
n
solutions
i n v e s t i g a t e d to o b t a i n data f i r s t l y ,
f o r the production of n i t r i c
a c i d and
the d e s i g n o f s c r u b b e r s f o r the removal o f n i t r o g e n o x i d e s from
nitric
This
In t h i s work the a b s o r p t i o n mechanism o f n i t r o g e n o x i d e s
acid solutions w i l l
design of i n d u s t r i a l
of
1
i n the gas phase as w e l l as i n the l i q u i d phase. I n v e s t i g a -
a r e p a r t i c u l a r l y poor.
into n i t r i c
1
found
aqueous
a c i d and n i t r o u s
t i o n s concerning the absorption of n i t r o g e n oxides i n t o n i t r i c
for
1.5
*j
about 200
1.2
is
for
secondly
tail
gases
a c i d and n i t r a t i o n p l a n t s .
OUTLINE OF THE
THESIS
A l a b o r a t o r y absorber with a w e l l d e f i n e d i n t e r f a c i a l
phase and the l i q u i d phase i s developed
mechanism (Chapter 2 ) . The
phase i s g i v e n i n Chapter
t o i n v e s t i g a t e the complex a b s o r p t i o n
3. A b s o r p t i o n measurements o f NOg/NgO^ gas
a c i d are c a r r i e d out
r e s u l t s a r e d e s c r i b e d i n Chapter
a b s o r p t i o n mechanism o f NO
by n i t r i c
4.
gas
mixtures
i n the l a b o r a t o r y
In Chapter
5 an o x i d a t i o n -
a c i d s o l u t i o n s i s proposed.
Experimental
r e s u l t s a r e compared w i t h t h e t h e o r e t i c a l l y p r e d i c t e d v a l u e s . Chapter
a mathematical
gas
q u a n t i t a t i v e a n a l y s i s o f n i t r o g e n o x i d e s i n the
i n t o d i l u t e d and c o n c e n t r a t e d n i t r i c
a b s o r b e r . The
a r e a between t h e
6 gives
model f o r the d e s i g n o f a d i l u t e d n i t r i c a c i d a b s o r b e r based
g e n e r a l c h e m i c a l r e a c t i o n e n g i n e e r i n g c o n s i d e r a t i o n s . At the end o f
this
on
c h a p t e r v a r i o u s methods t o d e c r e a s e
acid plants are b r i e f l y
the N0
discussed. F i n a l l y
x
content
intail
gases o f n i t r i c
i n Appendix 1 t h e a d d i t i v i t y o f
i n d i v i d u a l phase r e s i s t a n c e s f o r mass t r a n s f e r
i n the l a b o r a t o r y absorber i s
discussed.
REFERENCES
1. H o n t i , G.D., The N i t r o g e n I n d u s t r y , Akademia K i a D6, Budapest, 1976.
2. Du Pont de Nemours, B r i t .
3. Franck,
H.H. and Schirmer,
1419645, 1975, December 31.
W., Z. Elektrochem.
,1950, 54, 254.
4. Shneerson, A.L., M l n o v i c h , M.A. F i l i p p o v a , Zh.M. and P l a t o n o v , P.A., J.
Appl.
Chem. USSR (Engl.
Transl. ) , 1965, 38, 1627.
5. Ugine Kuhlmann, Ger. O f f e n . 2128382, 1971, December 23.
6. Anon., Nitrogen
No. 106, , 1977, M a r c h / A p r i l , 35.
7. Anon., Hydrocarbon
8. H e l l m e r ,
Process.
, 1975, November, 164.
L., Chem. Eng., 1975, December, 98.
9. Newman, D.J. and K l e i n , L.A., Chem. Eng. Progr.,
1972, 68, 62.
10. Chemico C o n s t r u c t i o n Corp., U.S. P a t e n t
3542510, 1970, November 24.
11. Komiyama,
Y., Hydrocarbon
D., O h r u i , T. and S a k a k i b a r a ,
Process.,
A p r i l , 145.
12.
Sumitomo Chemical
Co. L t d . , Ger. Offen. 2125677, 1971, December 2.
13. F r i e d r i c h Uhde GmbH, Ger. O f f e n . 2148329, 1973, A p r i l 5.
10
1972,
2. THE ABSORPTION APPARATUS
2.1
INTRODUCTION
A b s o r p t i o n phenomena a r e commonly s t u d i e d i n a b s o r b e r s w i t h a w e l l d e f i n e d
interfacial
of
a r e a between gas
i n d u s t r i a l equipment
and
liquid
i n o r d e r t o o b t a i n d a t a f o r the
(packed beds and p l a t e columns).
c o n c e r n i n g the a b s o r p t i o n o f n i t r o g e n o x i d e s i n t o n i t r i c
From l i t e r a t u r e
gas and
[1,
i s necessary.
a l a b o r a t o r y absorber with a w e l l d e f i n e d i n t e r f a c i a l
liquid
occur
i n v e s t i g a t e such a complex a b s o r p t i o n mechanism on
l a b o r a t o r y s c a l e a s p e c i a l d e s i g n o f the a b s o r p t i o n apparatus
t h i s Chapter
data
acid solutions i t i s
known t h a t gas phase r e a c t i o n s as w e l l as l i q u i d phase r e a c t i o n s may
2,3,34,35,36,37], To
design
i s developed
i n which gas a b s o r p t i o n w i t h
In
a r e a between
simultaneously
o c c u r r i n g gas phase r e a c t i o n s can be s t u d i e d . L i q u i d phase and gas phase mass
t r a n s f e r behaviour
i n t h i s a b s o r p t i o n apparatus
a b s o r b i n g pure CO^
i n t o water and NH^
a c i d s o l u t i o n s and
i n t o water. The
from
i s i n v e s t i g a t e d s e p a r a t e l y by
a n i t r o g e n gas
stream
r e s u l t s are compared w i t h the
into
sulfuric
theoretically
predicted values.
2.2
SELECTION OF LABORATORY ABSORBER
S e v e r a l types o f l a b o r a t o r y a b s o r b e r s can be found
i n the l i t e r a t u r e
[4]. Table
1 g i v e s a survey o f the normal o p e r a t i n g c o n d i t i o n s o f v a r i o u s a b s o r b e r s .
To i n v e s t i g a t e gas
r e a c t i o n s the b e h a v i o u r
a b s o r p t i o n w i t h s i m u l t a n e o u s l y o c c u r r i n g f a s t gas
o f the a b s o r b e r has
t o be
l i q u i d phase and the gas phase. In a c o n t i n u o u s
may
"ideal"
phase
i n r e l a t i o n to the
f l o w i n g system f a v o u r e d
cases
be:
a) complete m i x i n g
i n the l i q u i d phase and
i n the gas phase, so t h a t
b u l k c o n c e n t r a t i o n s i n each phase a r e u n i f o r m
(and e q u a l t o the
the
outlet
concentration).
b) p l u g f l o w o f t h e gas phase and the l i q u i d phase w i t h o u t
g r a d i e n t p e r p e n d i c u l a r t o the g a s - l i q u i d
a velocity
interface.
11
LAMINAR
JET
WETTED WALL
COLUMN
LIQUID
WETTED
SPHERE
LIQUID
GAS
À
1
STRING OF
WETTED
SPHERES
ROTATING
DRUM
LIQUID
g°
LIQUID
area
cm
cm
laminar
half
parabolic
velocity
listribution
'flow
conditions
of the
gas phase
turbulent
laminar
absorption
penetration
model
model
> 100
a l s o as
batch
reactor
-0.25
diameter ;
11,4
lenrjht
12.4
laminar
u n i f orrr,
velocity
liquid
BAND
10
flow
conditions
of the
liquid
phase
gas
^ i
4M-
time of exposure
of f r e s h
liquid
elements t o gas
in seconds
interfacial
,
2
5
FILM
gas
It
STIRRED
CELL
GAS
gas
I
MOVING
BAND
laminar
uniform
velocity
turbulent
laminar
turbulent
penetration
model
penetration
model
turbulent
laminar
turbulent
penetration
or
film
turbulent
penetration
model
penetratio
model
film
model
model
moderate
general
reference
Table
[4,5,6]
[4,7,8,9,33]
[4,16,17,1a]
[4,10,lî]
[4,12,lJ]
1 A comparison
of various
Ad a ) . A c o n t i n u o u s l y
laboratory
stirred
c e l l w i t h independent c o n t r o l o f t h e a g i t a t i o n
r a t e s i n t h e gas phase and i n t h e l i q u i d
complete m i x i n g . To i n s u r e
absorbers
a smooth
phase may be s u i t a b l e f o r r e a l i z i n g
i n t e r f a c e excessively high s t i r r e r
have t o be a v o i d e d e s p e c i a l l y i n t h e l i q u i d phase. T h i s
for
t h e maintenance o f u n i f o r m i t y
Godfrey
o f the bulk c o n c e n t r a t i o n
[16] s t u d i e d m i x i n g phenomena o f t h e l i q u i d
w i t h and w i t h o u t b a f f l e s by u s i n g
t r a c e r techniques.
liquid
for
the l i q u i d
12
i n the l i q u i d
phase i n t h e s t i r r e d
phase.
phase.
cell
The m i x i n g time was found
i n the l i q u i d
i f t h e m i x i n g time i s l e s s than 3% o f t h e average r e s i d e n c e
the
speeds
critical
phase i n a s t i r r e d
t o be 2-5 seconds. G o d f r e y [16] assumed t h a t u n i f o r m i t y
realized
f a c t o r may be
c e l l . T h i s means a r a t h e r
b u l k was
time o f
long residence
time
M i x i n g i n the gas phase i s much l e s s c r i t i c a l
r a t e s which can be a p p l i e d without
Very
little
i n f o r m a t i o n can be
i n the gas phase i n s t i r r e d c e l l s .
the gas phase was
t o the h i g h e r
agitation
d i s t u r b i n g the smooth g a s - l i q u i d
found
interface.
i n the l i t e r a t u r e c o n c e r n i n g m i x i n g
Sada et a l [17]
time o f about 10 seconds and w i t h a s t i r r e r
cell
due
speed
times
found t h a t at a r e s i d e n c e
o f 100
c o m p l e t e l y mixed. The most important
rev/min
the b u l k o f
advantage o f the
i s t h a t the d e s c r i p t i o n o f a b s o r p t i o n phenomena can be based
stirred
on the
two-
f i l m t h e o r y , which means t h a t even f o r complex mechanisms r a t h e r s i m p l e mathem a t i c a l e x p r e s s i o n s a r e o b t a i n e d . On
mental r e s u l t s
i s r a t h e r low.
r e a c t i o n o f NO
with n i t r i c
the o t h e r hand the a c c u r a c y o f the e x p e r i -
F u r t h e r i t s h o u l d be n o t e d t h a t the gas
a c i d vapour i n the p r e s e n c e
phase
of l i q u i d n i t r i c
acid
produces water, which condenses on the w a l l s o f the gas compartment. T h i s i s an
unrealistic
situation
for industrial
absorbers
such as packed beds and
plate
columns.
Ad b ) • P l u g flow o f the gas phase and
wetted
gas
w a l l column
stream
[7,19,20,21]
the l i q u i d phase can be r e a l i z e d
i n which an i d e a l
falling
liquid
are f l o w i n g l a m i n a r and c o c u r r e n t l y i n a v e r t i c a l
v e l o c i t y p r o f i l e o f the gas
stream
i n the wetted
f i l m and
tube. A
tube can be c r e a t e d by
f i l m e q u a l t o the gas v e l o c i t y .
flow model i s p r e s e n t e d
the v e l o c i t y d i s t r i b u t i o n
ideal
laminar f a l l i n g
liquid
film
i s h a l f p a r a b o l i c , p l u g f l o w o f the
f i l m can be assumed i f the p e n e t r a t i o n depth
compared t o the f i l m t h i c k n e s s . In t h i s way
a
flat
c h o o s i n g the s u r f a c e v e l o c i t y o f t h e l i q u i d
i n F i g . 1. A l t h o u g h
in a
o f the a b s o r b i n g gas
The
i n an
liquid
i s small
the f o l l o w i n g advantages can
be
obtained:
1. Known hydrodynamic b e h a v i o u r
2. F l a t
o f the l a m i n a r f a l l i n g
v e l o c i t y p r o f i l e o f the l a m i n a r f l o w i n g gas
liquid
stream
film.
without
v e l o c i t y d i s t r i b u t i o n p e r p e n d i c u l a r t o the g a s - l i q u i d
interface.
3. E q u a l c o n t a c t time v a l u e s i n t h e a b s o r p t i o n apparatus
o f the gas
and the l i q u i d
a
phase
phase.
4. The mass t r a n s f e r i n the gas phase and
a molecular d i f f u s i o n process
l i q u i d phase can be d e s c r i b e d by
i n r a d i a l d i r e c t i o n . Because o f the
equal
v e l o c i t i e s o f the gas phase and the l i q u i d phase, t h e r e i s z e r o drag at
the i n t e r f a c e and no
t r a n s f e r can be
i n f l u e n c e o f the moving i n t e r f a c e on the mass
expected.
A b s o r p t i o n phenomena i n wetted
w a l l columns a r e commonly d e s c r i b e d w i t h
p e n e t r a t i o n t h e o r y , and w i t h complex a b s o r p t i o n mechanisms t h i s may
difficult
mathematical
e x p r e s s i o n s . On
lead
the o t h e r hand, the r e s u l t s w i t h
w a l l columns are a c c u r a t e ( T a b l e 1 ) . The water produced
by t h e gas
the
to
wetted
phase
13
r e a c t i o n o f NO and n i t r i c
this situation
The
a c i d vapour condenses on t h e l i q u i d
i s realistic
f o r the absorption process
in industrial
mean advantages o f a wetted w a l l column compared t o a s t i r r e d
studying the absorption o f nitrogen oxides
accuracy
in
i n t e r f a c e and
and t h e f a c t
industrial
into n i t r i c
t h a t a b e t t e r correspondence w i t h
absorbers
absorbers.
cell for
a c i d are the higher
the absorption
process
may be r e a l i z e d .
On t h e c o n t r a r y t h e m a t h e m a t i c a l d e s c r i p t i o n o f t h e a b s o r p t i o n p r o c e s s i n
wetted w a l l columns i s more d i f f i c u l t
than i n s t i r r e d c e l l s .
c h o i c e between b o t h l a b o r a t o r y a b s o r b e r s
use
Although the
i s rather arbitrary
i t was d e c i d e d t o
a wetted w a l l column f o r s t u d y i n g t h e a b s o r p t i o n mechanism o f n i t r o g e n
oxides
into n i t r i c
acid solutions.
liquid
film
gas
\
e
(
y
R
fc
-r
r
Fig.
I
Flow model and coordinate
system.
2.3 DESCRIPTION OF THE ABSORPTION APPARATUS
The
wetted w a l l column i s s c h e m a t i c a l l y p r e s e n t e d
upper and lower s t a i n l e s s s t e e l c a l m i n g
The
upper c a l m i n g
gas
eddies
section contains
and t o c r e a t e a f l a t
the upper c a l m i n g
(glass) with
section f i t s
i n F i g . 2. I t c o n s i s t s o f an
s e c t i o n each w i t h
a l e n g t h o f 25.0 cm.
a porous s t a i n l e s s s t e e l
filter
t o suppress
v e l o c i t y p r o f i l e o f t h e gas. The lower end o f
i n t o the top of a c y l i n d r i c a l
absorption
an i n n e r diameter o f 3.45 cm. The l e n g t h o f t h e g l a s s
cylindrical
a b s o r p t i o n column i s 13.0 cm, 33.0 cm and 50.0 cm. The l i q u i d
f i l m was
duced t o t h e wetted w a l l column t h r o u g h an a d j u s t a b l e a n n u l a r
slit,
normally
o f t h e same w i d t h (^0.4 mm)
as t h e t h i c k n e s s o f t h e l i q u i d
f i l m c o v e r i n g t h e i n n e r s u r f a c e o f t h e tube flowed
14
column
intro-
which i s
f i l m . The
down and was i n c l i n e d t o an
a n n u l a r p o o l w i t h a s m a l l s u r f a c e a r e a . In t h i s way t h e gas was s e p a r a t e d
the l i q u i d . The l i q u i d
level
from
i n t h e r e s e r v o i r was kept on a c o n s t a n t h e i g h t by
means o f a l e v e l c o n t r o l l e r . P r e s s u r e taps were l o c a t e d 11 cm above t h e wetted
w a l l s e c t i o n and 11 cm under t h e wetted
w a l l s e c t i o n . Temperatures o f t h e i n -
and o u t g o i n g gas and l i q u i d were measured by means o f
thermocouples.
2.4 MASS TRANSFER IN A LAMINAR FALLING LIQUID FILM
The
p h y s i c a l a b s o r p t i o n r a t e o f a pure gas i n t o an i d e a l
falling
liquid
film
may under c e r t a i n c o n d i t i o n s be d e s c r i b e d by t h e t h e o r y o f p e n e t r a t i o n . In
o r d e r t o check t h e hydrodynamic b e h a v i o u r
d e s c r i b e d wetted
of the l i q u i d
film
i n the p r e v i o u s l y
w a l l column,experiments were c a r r i e d out by a b s o r b i n g COg
into
water. The measured a b s o r p t i o n r a t e s were compared w i t h t h e v a l u e s p r e d i c t e d by
the p e n e t r a t i o n t h e o r y .
2.4.1 Theory
The
p h y s i c a l a b s o r p t i o n o f a pure gas i n t o a l a m i n a r f a l l i n g
liquid
f i l m may
under c e r t a i n c o n d i t i o n s be c o n s i d e r e d as a n o n - s t a t i o n a r y d i f f u s i o n p r o c e s s i n
a s e m i - i n f i n i t e medium. The a b s o r p t i o n p r o c e s s
following
equation:
dt
The
i s then d e s c r i b e d by t h e
initial
dx
and boundary c o n d i t i o n s a r e
t = 0
X > 0
t > 0
X = 0
t > 0
X =
C
l
=
°l,o
C
0 0
C
£
£,i
= °SL,o
The
s o l u t i o n of t h i s equation i s :
The
a b s o r p t i o n r a t e can be found be d i f f e r e n t a t i o n o f e q u a t i o n ( 5 ) :
3C
i ' " it*
16
(2)
(3)
(4)
The
amount o f gas absorbed p e r u n i t o f s u r f a c e a r e a d u r i n g c o n t a c t
time T i s
g i v e n by:
m(T) = 2 ( C . - C
£
For
an i d e a l
vertical
laminar
l o
)
V* — TT
1
falling
(7)
liquid
f i l m without
ripples
f l o w i n g down a
tube we have:
_ _h_
(8)
v
s
,
= 2 (g/3V)
s
[ - J
(9)
3V<j> \ 1/3
B
6
(
f=l-igTJ
A laminar
equation
falling
liquid
f i l m has a h a l f p a r a b o l i c v e l o c i t y
(7) can o n l y be a p p l i e d i f t h e gas p e n e t r a t e s
whose v e l o c i t y
does n o t d i f f e r
that penetration
0
)
d i s t r i b u t i o n and
into a l i q u i d
layer
t o o much from t h e s u r f a c e v e l o c i t y . T h i s
depth o f t h e a b s o r b i n g
1
implies
gas i s s m a l l compared t o t h e f i l m
thickness. According
infinitely
t o N i j s i n g [5] t h e l i q u i d f i l m can be c o n s i d e r e d t o be
2
2
deep i f D ^ T / S f
< 0.04. In o u r experiments D ^ T / S f
i s s m a l l e r than
0.065, and t h e d e v i a t i o n from e q u a t i o n
(7) i s about 0.15%. The f o r m a t i o n
falling
a d e v i a t i o n from t h e steady
film
relationship
contact
film
through a s l i t
f o r the v e l o c i t y given
time i s n e g l i g i b l e
thickness
order
introduces
i n equation
i f the f i l m height
(9). I t s e f f e c t
on t h e t o t a l
[ 5 , 7 ] . The c o r r e c t i o n needed f o r t h e end e f f e c t
i s g e n e r a l l y an
o f magnitude more.
found t h a t t h e s e
r i p p l e s g i v e no important
R i p p l e s cause some m i x i n g
penetration theory
and C l e g g
and g i v e enhanced mass t r a n s f e r r a t e s compared t o t h e
("Teepol") t o t h e a b s o r b i n g
a d d i t i o n a l r e s i s t a n c e t o mass t r a n s f e r
A falling
film. Portalski
increase of the i n t e r f a c e .
[ 2 4 ] . These r i p p l e s a r e e l i m i n a t e d by a d d i n g
o f s u r f a c e a c t i v e agents
an
state
i s more than about 20 times t h e
In p r a c t i c e some r i p p l e s may appear on a f a l l i n g
[23]
of a
f i l m has a stagnant
s m a l l amounts
l i q u i d without
introducing
[5,7,8].
surface with
o f a few cm above t h e
In t h i s
neglected
h has t o be c o r r e c t e d by s u b s t r a c t i n g t h e h e i g h t
and t h e t o t a l h e i g h t
stagnant
a height
l e v e l o f the r e c e i v i n g l i q u i d .
surface the absorption
can be
o f t h i s end e f f e c t [ 5 ] .
17
2.4.2
Experimental
O r d i n a r y d i s t i l l e d water c o n t a i n e d i n an overhead
t o the a b s o r b e r
shown i n F i g . 2.
o b t a i n e d which was
necessary
a b s o r b e r was
34.9
15.9,
vacuum, and 0.05%
a constant
f o r a smooth f i l m . The
and 51.9
cm.
The water was
by weight o f " T e e p o l " was
e l i m i n a t e r i p p l e s o f the l i q u i d
water vapour at the e x p e r i m e n t a l
w a l l column. The
In t h i s way
r e s e r v o i r was
f i l m . CO^
2
flow
column l e n g t h o f
completely
gravity
was
the
degassed by
high
added t o t h e water i n o r d e r to
from a c y l i n d e r was
temperatures
CX> a b s o r p t i o n r a t e was
liquid
f e d by
saturated with
and then s u p p l i e d t o the
wetted
measured from the d e c r e a s e o f the
C0
2
volume at c o n s t a n t p r e s s u r e w i t h a soap f i l m i n a c a l i b r a t e d tube a c c o r d i n g t o
N i j s i n g [5].
2.4.3
Results
In o r d e r to check the hydrodynamic b e h a v i o u r
o f the l i q u i d
f i l m the e x p e r i m e n t a l
a b s o r p t i o n r a t e s were compared w i t h the r a t e s p r e d i c t e d by the p e n e t r a t i o n
t h e o r y . In our experiments
the c o l l e c t i n g l i q u i d
the h e i g h t o f the stagnant
r e s e r v o i r was
found t o be 1.6
d a t a found by Lynn et a l [8-10] and N i j s i n g
Regression
l i n e through
f i l m formed above
cm which agrees w i t h
the
[ 5 ] . In t h i s p a r t o f the f i l m
a b s o r p t i o n r a t e can be n e g l e c t e d . A p l o t o f m ( T )
give a s t r a i g h t
liquid
the
v e r s u s ^ ( h - AlO/v^ s h o u l d
the o r i g i n w i t h a s l o p e o f 2C^
iy^jj/^"
a n a l y s i s shows t h a t the d e v i a t i o n o f the e x p e r i m e n t a l
3
)•
points i s
always l e s s than 2%
and the upper bound and the lower bound o f the 95%
-5
2
c o n f i d e n c e i n t e r v a l f o r the i n t e r c e p t was r e s p e c t i v e l y 0.155 x 10
kg/m
and
-5
2
-0.102 x 10
kg/m . The upper bound o f the 95% c o n f i d e n c e i n t e r v a l o f the
"™™
—5
2
4
s l o p e 2C„
WD./l
was found t o be, r e s p e c t i v e l y , 7.97 x 10
kg/m .sec
and
2
4
-5
2 4
7.68 x 10~
kg/m .sec which agrees w i t h the v a l u e o f 8.04 x 10
kg/m
.sec
5
found by N i j s i n g
Hence i t was
[5].
concluded
t h a t the a b s o r p t i o n r a t e i n the l i q u i d phase i s w e l l
p r e d i c t e d by the p e n e t r a t i o n t h e o r y and
liquid
18
t h a t the hydrodynamic b e h a v i o u r
f i l m agrees w e l l w i t h the t h e o r e t i c a l model.
of
the
12
Fig.
3 Absorption
rate
of CO^ into
water as a function
of the contact
time
(P = 1.013 bar, t = 20°C).
Symbol
h (cm)
A
15.9
0
34.9
V
51.9
2.5 GAS PHASE MASS TRANSFER IN LAMINAR PLUG FLOW GAS STREAMS
2.5.1
The
Introduction
effect
laminar
o f gas and l i q u i d
flow
r a t e s on t h e gas phase mass t r a n s f e r i n
gas streams was t h e o r e t i c a l l y and e x p e r i m e n t a l l y
columns and r e c t u a n g u l a r
g a s - l i q u i d flow
I f however t h e gas v e l o c i t y i s u n i f o r m
s t u d i e d i n wetted w a l l
[7,20,22,25,26].
and i s equal
t o the surface v e l o c i t y
o f t h e l i q u i d , no i n f l u e n c e o f t h e moving i n t e r f a c e on t h e gas phase mass
t r a n s f e r can be e x p e c t e d . S t r i c t l y
s p e a k i n g t h i s i s t r u e when t h e r e
i s zero
19
drag at t h e i n t e r f a c e . H i k i t a and I s h i m i
situation
i n wetted w a l l
numbers. Dekker
[22] s u p e r f i c i a l l y
columns, but no i n f o r m a t i o n
[7] i n v e s t i g a t e d t h e a b s o r p t i o n
studied
i s given
this
at h i g h
Graetz-numbers from 135 up t o 245 i n a w e t t e d w a l l column which was
in
a s p e c i a l way t o c r e a t e a f l a t
his
measured a b s o r p t i o n
rates with the t h e o r e t i c a l l y p r e d i c t e d values
t r a n s f e r which was not e x p e r i m e n t a l l y
In t h i s p a r t
constructed
v e l o c i t y p r o f i l e o f t h e gas. Comparison o f
however, based upon t h e assumption o f a s m a l l
described
Graetz-
o f ammonia i n t o water at
confirmed.
t h e s e gaps o f i n f o r m a t i o n
i. <
are i n v e s t i g a t e d i n the p r e v i o u s l y
wetted w a l l column ( F i g . 2) by a b s o r b i n g
stream i n t o water and s u l f u r i c
was,
l i q u i d phase r e s i s t a n c e f o r mass
ammonia from a n i t r o g e n gas
a c i d s o l u t i o n s . The measured mass t r a n s f e r r a t e s
are compared w i t h t h e t h e o r e t i c a l l y p r e d i c t e d
rates.
2.5.2 Theory
Gas
and l i q u i d a r e assumed t o flow c o - c u r r e n t l y and i n a l a m i n a r way i n a
v e r t i c a l wetted w a l l column. The gas v e l o c i t y i s assumed t o be u n i f o r m
out
t h e column and e q u a l t o t h e s u r f a c e
model and t h e c o - o r d i n a t e
relevant physical properties
gas
remain c o n s t a n t
. 3C
2
6
8
=
D (
g
3h
^
3r
l
2
+
1
r
w i t h t h e boundary and i n i t i a l
h=0
and t h a t mass t r a n s f e r from t h e
h>0
E
3r
)
'
should
be p a i d
C
=C
g
g>o
3C /3r = 0
g,o
i n which J
-
-c
C
.
6 , 1
g,i
and J
o
that
=
2
E
,
the i n t e r f a c e c o n c e n t r a t i o n
n=l
r
ITjTÜT
n 1
are Bessel
n
(14)
g. i
C
o
(13)
=C
g
to the fact
(12)
g
C
. along the f i l m . According
g. i
the s o l u t i o n of these equations i s :
C (h,r)
value
(11)
conditions:
r = R - <$„
f
have a c o n s t a n t
c
in radial
3C
0 < r < R - 6,
f
r = 0
h > 0
Attention
flow
a l l the
t h e d i f f u s i o n e q u a t i o n can be w r i t t e n as:
3C
v
20
f i l m . The
i n F i g . 1. Assuming t h a t
phase t o t h e l i q u i d phase t a k e s p l a c e by m o l e c u l a r d i f f u s i o n o n l y
direction,
[32]
v e l o c i t y o f the l i q u i d
system a r e g i v e n
through-
should
t o Carslaw and J a e g e r
. -D
2
h a
.
V \ i r r ' - p p - 7
f
functions of the f i r s t
v^R-ö,)
'
k i n d and, r e s p e c t i v e l y , o f
the z e r o and f i r s t
W
The
=
first
order, while a
are the roots of the equation
0
( 1 6 )
e i g h t r o o t s a r e g i v e n i n T a b l e 2.
a
1
=
2.4048
=
5.5201
a
3
=
8.6537
a
4
= 11.7915
g
= 14.9309
a
a
a . = 18.0711
o
a
= 21.2116
?
a
= 24.3525
o
Table
2
First
eight
eigenvalues
The b u l k average
value of C (h,r)
g
H-S
C (h) =
From e q u a t i o n s
g
(
h
)
_
C
-S
C
(15) and (17) t h e v a l u e o f
g i
§ ^
S,o
r.C ( h , r ) d r
- C
g,i
(17)
J
2
f
C
f
»
(R-6 )
on a g i v e n h e i g h t h i s :
I
1
»
= 4Z
n=l
—
a
n
a
n
v
c
g
(
n
) i
s
obtained.
\
exp ( - - 5 — J
V
Gz
(18)
'
i n which Gz i s t h e G r a e t z number d e f i n e d as
G
The
z
(
= ^DTE
g
average mass t r a n s f e r c o e f f i c i e n t k^ between t h e i n l e t
1
9
)
and o u t l e t o f t h e
mass t r a n s f e r s e c t i o n may be d e f i n e d i n terms o f a l o g a r i t h m i c mean d r i v i n g
f o r c e as f o l l o w s :
-ir(R-Ô.)
i
_
_
(C
-C
. ) - ( C (h)-C
v (C
-C ( h ) ) = k 2Tf(R-6" ) h x — ' °
s g.o g
ft
i
r—
*"
-.(h) - C
g
8
,
1
g
.)
(20)
- c
o
g,i
21
A f t e r s u b s t i t u t i o n o f e q u a t i o n (18) i n e q u a t i o n (20) the average
number can be w r i t t e n
G z o o
= - In Z
IT
Sh
=1
Sherwood
as:
4
a
n
2
,
a TT .
( - — Gz
exp
(2D
r
I f the G r a e t z number i s l a r g e r than about
t o be i n f i n i t e l y
150
the gas phase may
deep, and t h i s s i t u a t i o n c o r r e s p o n d s w i t h the p e n e t r a t i o n
t h e o r y . Under t h i s c o n d i t i o n t h e l o g a r i t h m i c average
represented
Sh
g
Sherwood number may
be
as:
Gz
/
—
In I
=
be c o n s i d e r e d
e)"*
J
The
d e v i a t i o n o f e q u a t i o n (21) w i t h e q u a t i o n (22) at G r a e t z numbers l a r g e r
150
i s s m a l l e r than
2.5.3
The
6%.
Experimental
absorbing l i q u i d
c o n t a i n i n g 0.05%
same way
( d i s t i l l e d water,
by weight
IN s u l f u r i c
" T e e p o l " was
as d e s c r i b e d f o r the CO^
a c i d and 2N
sulfuric
meted
a b s o r p t i o n experiments.
w i t h f l o w c o n t r o l l e r s . The
were determined
capillary
The
experimental
from
cylinders
gas f l o w r a t e s o f ammonia and n i t r o g e n
from the p r e s s u r e drop a c r o s s a c a l i b r a t e d s t a i n l e s s
steel
t u b i n g immersed i n a t h e r m o s t a t i c a l l y c o n t r o l l e d water b a t h
m a i n t a i n e d at 20°C. A f t e r m i x i n g the ammonia and n i t r o g e n gas
m i x t u r e was
streams
the
l e d i n c o - c u r r e n t flow through the wetted w a l l column. The
r a t e o f the gas m i x t u r e was
was
acid)
f e d t o the wetted w a l l column i n the
equipment i s shown i n F i g . 4. Ammonia and n i t r o g e n were s u p p l i e d
and
chosen
i n such a way
e q u a l t o the s u r f a c e v e l o c i t y o f the l i q u i d
t h a t the average gas
film.
c o n c e n t r a t i o n o f ammonia i n the i n g o i n g gas stream was
flow
In our experiments
varied
determined
+
4
by adding 0.1N
sulfuric
a n a l y s e d i n the same way.
s u l f u r i c a c i d was
d e v i a t i o n was
22
c o n t e n t o f the i n - and o u t g o i n g l i q u i d
In the experiments
was
reaction
was
w i t h water a known amount o f 0.1
sample t o p r e v e n t d e s o r p t i o n o f ammonia.
the ammonia a b s o r p t i o n c o u l d be e s t a b l i s h e d , and
found t o be l e s s than
by
a c i d t o a gas sample and then a n a l y s i n g on
added t o the l i q u i d
A mass b a l a n c e around
the
from 2% t o 10%
c o n t e n t by means o f a c o l o r i m e t r i c method based on the B e r t h e l o t
i n an A u t o - A n a l y z e r . The NH^
gas
velocity
volume. The c o n c e n t r a t i o n o f ammonia i n the i n - and o u t g o i n g gas streams
NH
than
1.5%.
the
N
rfn.T.
- g ,
Fig'. 4
""Vo-c
Expérimental set-up for ammonia absorption
experiments
in a wetted
wall
column.
2.5.4
The
Results
a b s o r p t i o n o f ammonia i n t o s u l f u r i c a c i d s o l u t i o n s i s accompanied w i t h
fast chemical
r e a c t i o n i n the l i q u i d phase. The
ammonia i s e q u a l
does not
to zero
absorbing
i n t e r f a c e concentration of
i f an i n c r e a s e o f the a c i d i t y o f t h e a b s o r b i n g
f u r t h e r i n c r e a s e the a b s o r p t i o n r a t e . E x p e r i m e n t s w i t h water,
s u l f u r i c a c i d and
2N
s u l f u r i c a c i d showed no
concluded
t h a t d u r i n g the a b s o r p t i o n o f ammonia i n t o 2N
(18)
and
liquid,
and
i s p o s s i b l e . The
the r e s u l t s were compared w i t h
c o n t a c t time v a l u e s between gas
seconds and
i n the i n g o i n g gas
stream from 2.3%
up t o 10.5%
a
time
the
l i q u i d were v a r i e d from 0.25
t o 1.2
and
a b s o r p t i o n r a t e s i n 2N
t h e o r e t i c a l l y d e r i v e d e x p r e s s i o n s . The
up
the
sulfuric
s u l f u r i c a c i d s o l u t i o n s were i n v e s t i g a t e d as a f u n c t i o n o f the c o n t a c t
v a l u e s between gas
IN
3).
a c i d s o l u t i o n s the i n t e r f a c e c o n c e n t r a t i o n o f ammonia i s e q u a l t o z e r o ,
f u r t h e r s i m p l i f i c a t i o n of equation
liquid
i n f l u e n c e o f the a c i d i t y o f
l i q u i d on the a b s o r p t i o n r a t e (see T a b l e
Hence i t was
a
and
the c o n c e n t r a t i o n o f ammonia
by volume.
23
C
total
g,o
% vol
water
bar
s
m/sec
C (h)
g
C
S,o
2 72
1 184
0 543
0.516
2 71
1 191
0 539
0.479
2NH S0„
2 4
2 76
1 201
0 537
0.505
water
4 85
1 095
0 399
0.424
4 84
1 096
0 400
0.430
4 93
1 111
0 401
0.415
1NH S0
2
4
o
1NH S0
2
4
2NH-S0.
2 4
Table
V
pressure
3
Influence
of the acidity
ammonia into
wall
of the liquid
water and sulfuric
column with a height
acid
phase on the absorption
solutions
rate of
at 20°C in a wetted
of 51.9 cm and with an inner
diameter of
3.45 cm
Due t o t h e a b s o r p t i o n o f ammonia from t h e gas phase i n t o t h e l i q u i d
the gas v e l o c i t y d e c r e a s e s .
Therefore
at Graetz
e x c e s s i v e l y h i g h ammonia c o n c e n t r a t i o n s
the maximum ammonia c o n c e n t r a t i o n
by
phase
numbers s m a l l e r t h a n 100
i n t h e gas phase s h o u l d be a v o i d e d ,
i n these
and
experiments was t h e r e f o r e about 5%
volume.
In t h e experiments w i t h 2N s u l f u r i c a c i d t h e stagnant
liquid
f i l m above t h e
r e c e i v i n g l i q u i d was o b s e r v e d t o be about 2.0 cm. F o r a h i g h l y s o l u b l e gas such
as ammonia i n s u l f u r i c
the stagnant
absorption
liquid
acid solutions, i t i s doubtful
f i l m may be n e g l e c t e d
dioxide
experiments.
From l i t e r a t u r e d a t a
stagnant
i f the absorption rate i n
as was done i n t h e carbon
[27,28] i t i s known t h a t t h e e v a p o r a t i o n
water s u r f a c e i n t o d r i e d a i r f l o w i n g a c r o s s
this surface
rate of a
i s strongly
reduced by t h e a d d i t i o n o f s m a l l amounts o f s u r f a c e a c t i v e agents. Long
s t r a i g h t - c h a i n a l c o h o l s , f o r example, may reduce t h e e v a p o r a t i o n
12%
o f t h e r a t e a t a c l e a n water s u r f a c e . D.W.
Thompson [29] found t h a t
amounts o f s u r f a c e a c t i v e agents d e c r e a s e t h e a b s o r p t i o n
water i n an u n s t i r r e d c o n t a i n e r . E x p e r i m e n t s w i t h
decanol
24
r a t e t o only
small
r a t e o f ammonia i n t o
1 - o c t a d e c a n o l and 1-hexa-
showed a r e d u c t i o n o f r e s p e c t i v e l y 41% and 51% i n t h e a b s o r p t i o n
rate.
c
P
g.o
% by volume
bar
0 .782
3.77
1 295
0 0134
0 .772
2 31
1 276
0 0136
0 .780
1 193
0 0167
0 739
5 35
1 189
0 0167
0 .731
2 45
1 208
0 0161
0 744
1 113
0 0194
0 700
1 143
0 0196
0 694
1 113
0 0234
0 683
4 72
1 107
0 0237
0 688
5 34
1 117
0 0225
0 690
5. 20
1 092
0 0262
0 653
5. 49
1 092
0 0256
0 658
4. 05
1 215
0 0391
0 593
5. 30
1 117
0 0567
0 541
4. 53
1 120
0 0571
0 527
2. 74
1 200
0 0621
0 503
2. 78
1 201
0 0626
0 507
4. 98
1. 111
0 0891
0 419
4. 88
1. 111
0 0908
0 411
Absorption
experiments
of NH„ into
acid
acid
solution
coefficient
at 20 C
liquid film
f o r the a b s o r p t i o n o f ammonia
(Ah = 1.5 cm), and t h e G r a e t z numbers were c o r r e c t e d
For the c a l c u l a t i o n
diffusion
2N sulfuric
from the above-mentioned we assumed 75% o f t h e stagnant
i n the wetted w a l l column t o be i n a c t i v e
sulfuric
g.o
0 0131
10 5
effect.
C
1 296
5 83
Reasoning
Gz
(h)
g
3 67
10 7
4
C
red
10 4
Table
TT
into
f o r t h i s en
o f t h e Sherwood and G r a e t z numbers t h e gas phase
o f ammonia
i n n i t r o g e n was taken from t h e d a t a g i v e n by
Mason and Monchick [ 3 0 ] .
(D
= 2.3 x 10
3
-5
2
o
m / s e c at 25 C and 1.01325 b a r ) .
2
25
Corrections
f o r temperature and p r e s s u r e
r e l a t i o n o f Wilke-Lee
The
TT/Gz
red
The
d e v i a t i o n s were c a r r i e d o u t u s i n g t h e
[31]. The e x p e r i m e n t a l r e s u l t s a r e g i v e n
r e l a t i v e concentration
i n T a b l e 4.
o f ammonia i n t h e gas phase as a f u n c t i o n o f
i s p l o t t e d i n F i g . 5.
l o g a r i t h m i c mean v a l u e
o f t h e Sherwood number as a f u n c t i o n o f t h e
G r a e t z number (Gz .) i s p l o t t e d i n F i g . 6. These f i g u r e s show t h a t t h e
red
d e v i a t i o n o f t h e measured p o i n t s
from t h e t h e o r e t i c a l l y p r e d i c t e d v a l u e s i s
s m a l l . At G r a e t z numbers o f more than 150 t h e d e v i a t i o n from t h e p e n e t r a t i o n
theory
Fig.
5
i s s m a l l e r than 6%.
NH
absorption
3
experiments
20°C; comparison
theoretical
Symbol
26
between
lines).
h (cm)
0
14.9
V
34.9
A
51.9
in a wetted
experimental
wall
results
column into
and theory
2N F.^S0^ at
(
Fig.
6
Mean Sherwood number as a function
(21),
assymptotic
Symbol
of the Graetz
number (
equation
solution).
h (cm)
0
14.9
A
34.9
V
51.9
2.6 CONCLUSIONS
A wetted
w a l l column was developed
i n which gas a b s o r p t i o n w i t h
simultaneously
o c c u r r i n g gas phase r e a c t i o n s such as t h e a b s o r p t i o n o f n i t r o g e n o x i d e s i n
nitric
a c i d may be i n v e s t i g a t e d .
liquid
film
In t h i s wetted
w a l l column a l a m i n a r
and a l a m i n a r p l u g flow o f t h e gas phase w i t h o u t
falling
a velocity
g r a d i e n t p e r p e n d i c u l a r t o t h e g a s - l i q u i d i n t e r f a c e c o u l d be r e a l i z e d . The
hydrodynamic b e h a v i o u r
o f t h e l i q u i d f i l m was checked
by a b s o r b i n g COg i n t o
water. I t was found t h a t t h e a b s o r p t i o n r a t e was w e l l p r e d i c t e d by t h e
p e n e t r a t i o n t h e o r y . Gas phase mass t r a n s f e r was i n v e s t i g a t e d by a b s o r b i n g
27
ammonia from a n i t r o g e n gas stream i n t o s u l f u r i c
a c i d s o l u t i o n . The e x p e r i m e n t a l
r e s u l t s show a good agreement w i t h t h e G r a e t z model.
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L. , J. Chem. Phys. , 1962, 36, 2746.
31. R e i d , R.C., P r a u s n i t z , J.M. and Sherwood, T.K., The P r o p e r t i e s o f Gases and
L i q u i d s , M c G r a w - H i l l , 1977.
32. C a r s l a w , H.S.
and J a e g e r , J.C. C o n d u c t i o n o f Heat
i n Solids,
Oxford
U n i v e r s i t y P r e s s , 1959.
33. L e f e r s , J.B.,
Van den B l e e k , C M . , Bos, A.S. and Van den Berg, P.J.,
paper
p r e s e n t e d at t h e 6 t h I n t e r n a t i o n a l Congress o f C h e m i c a l E n g i n e e r i n g ,
Chemical Equipment Design and Automation,
Prague,
August 1978.
34. K a i s e r , E.W. and Wu, C.H., J. Phys.
Chem., 1977, 81, 1701.
35. K a i s e r , E.W. and Wu, C.H., J. Phys.
Chem., 1977, 81,
36. S t r e i t ,
Phys.,
187.
G.E. and W e l l s , J.S., F e h s e n f e l d , F.C., Howard, C.J., J. Chem.
1979, 70, 3439.
37. McKinnon, I.R., M a t h i e s o n , J.G. and W i l s o n , I.R., J. Phys.
Chem., 1979, 83,
779.
29
3. SPECTROPHOTOMETRY DETERMINATION OF NITROGEN OXIDES AND NITRIC ACID VAPOUR
3.1 INTRODUCTION
F o r p o l l u t i o n c o n t r o l purposes much a t t e n t i o n has been p a i d t o the
of n i t r o g e n oxides
and s e v e r a l a n a l y s i s methods have been d e v e l o p e d
d i s a d v a n t a g e o f most methods i s t h a t they
range which o c c u r s
spectroscopy,
however, can a l s o be used f o r t h e d e t e r m i n a t i o n
for
at higher
concentrations.
i n t h e manufacture o f n i t r i c
Infrared absorption
acid. Infrared
of nitrogen
c o e f f i c i e n t s o f NOg and NO
p o l l u t i o n c o n t r o l have been measured as a f u n c t i o n o f t h e o p t i c a l
l e n g t h and t h e temperature
[ 6 ] . Guttman
i n t e n s i t i e s o f pure N0„ and N O .
pressures
[7] i n v e s t i g a t e d i n t e g r a t e d
i n gas m i x t u r e s c o n t a i n i n g n i t r o g e n
and/or n i t r o u s a c i d vapour can be formed
concentrations
nitric
absorption Fontanella
1
number o f 1915 cm , o f N 0
a c i d and p o l l u t i o n c o n t r o l purposes.
a t 1606 cm * and n i t r i c
2
concentrations
the N0
2
absorption
of N0
2
NgO^
Using
a c i d vapour at 1326 cm
1
in
T h i s method i s n o t a p p l i c a b l e at
due t o t h e s t r o n g o v e r l a p o f n i t r i c
a c i d vapour
band.
In t h i s Chapter a method i s d e v e l o p e d f o r t h e d e t e r m i n a t i o n
NgO^ and n i t r i c
2 >
Such i n f o r m a t i o n may be o f import-
[10] s t u d i e d t h e c o n c e n t r a t i o n o f NO a t a wave
the s t r a t o s p h e r e u s i n g t h e sun as s o u r c e .
and
i n f o r m a t i o n , however, can be
t h e q u a n t i t a t i v e a n a l y s i s o f NO, N 0
a c i d vapour i n such gas m i x t u r e s .
ance f o r t h e manufacture o f n i t r i c
oxides.
a c i d vapour
[8,9], e s p e c i a l l y at higher
o f n i t r o g e n o x i d e s . Very l i t t l e
found i n t h e l i t e r a t u r e c o n c e r n i n g
higher
absorption
up t o 2 MPa. The r e s u l t s o f Guttman i n d i c a t e t h a t Beer's law i s v a l i d .
Due t o t h e r e a c t i o n o f n i t r o g e n o x i d e s w i t h water vapour n i t r i c
infrared
path
a t temperatures o f 50°C up t o 100°C and a t
O f t e n water vapour i s a l s o p r e s e n t
and
[ 1 - 5 ] . The
a r e not a p p l i c a b l e i n the higher
concentration
oxides
determination
a c i d vapour i n gas m i x t u r e s a t c o n c e n t r a t i o n s
the manufacture o f n i t r i c
o f NO, NOg,
which o c c u r i n
acid.
3.2 EXPERIMENTAL
All
30
s p e c t r a l measurements were c a r r i e d o u t on a P e r k i n - E l m e r
Model 117 i n f r a r e d
spectrophotometer.
The
w i t h an i n n e r diameter
i n f r a r e d a b s o r p t i o n gas c e l l was
o f 3.5
cm
and a p a t h l e n g t h o f 10.0
windows were cemented on the gas c e l l which was
o
kept
c o n s t a n t temperature
The
o f 25.0
C by a t h e r m o s t a t .
the sample p r e p a r a t i o n o f n i t r i c
mixtures
i s g i v e n i n F i g . 1.
experimental
to
apparatus,
o x i d e gas m i x t u r e s
3 and
cm.
Silver
chloride
i n a l l experiments
experimental
at a
apparatus
for
and n i t r o g e n d i o x i d e gas
In the f i r s t s t e p o f sample p r e p a r a t i o n the
i n c l u d i n g t h e gas c e l l , was
remove the oxygen and then evacuated.
always checked
constructed of glass
f l u s h e d with dry n i t r o g e n
In e v e r y experiment
the e v a c u a t i o n
was
by means o f a mercury vacuum gauge. A f t e r e v a c u a t i o n v a l v e s 1,
5 were c l o s e d and n i t r i c
made t o p u r i f y the n i t r i c
o x i d e was
oxide.
l e d i n t o the system. No
(Matheson Gas
Products,
attempts
2
were
p u r i t y : 99,2%.) The
s m a l l amounts o f n i t r o u s o x i d e and n i t r o g e n d i o x i d e which a r e p r e s e n t i n
commercial n i t r i c
o x i d e were s m a l l enough t o be n e g l e c t e d . A f t e r f i l l i n g
system w i t h n i t r i c
o x i d e the p a r t i a l
pressure of n i t r i c
o x i d e was
measured w i t h
a d i f f e r e n t i a l manometer f i l l e d w i t h bromo-naphthalene, i n which the
of
n i t r o g e n o x i d e s i s v e r y low. The
sufficiently
The
low t h a t no o p t i c a l
gas c e l l was
The
as d e s c r i b e d f o r NO
temperature
c o n t a i n e r and
apparatus
2
p r e s s u r e s o f NO^
4
was
o x i d a t i o n o f NO
o f N0
2
and NgO^
i n T a b l e 1.
kept
o x i d e i n the
p r e s s u r i s e d w i t h dry n i t r o g e n t i l l
2
which i s i n e q u i l i b r i u m w i t h NgO^
found t o be v e r y i n a c c u r a t e . Due
i n the
t o the s t r o n g
[11] a l l temperatures
such as i n t h e gas-sample c e l l , the
sample
Small
cause l a r g e e r r o r s i n the c a l c u l a t i o n o f the
and NgO^
w i t h the e q u i l i b r i u m c o n s t a n t . T h e r e f o r e
NO^
o x i d e s u p p l i e d as d e s c r i b e d above
sample c e l l at an a b s o l u t e p r e s s u r e o f 0.1067
i s complete w i t h i n a few minutes and the p a r t i a l
MPa.
pressures
were then c a l c u l a t e d by means o f the e q u i l i b r i u m c o n s t a n t
In t h i s way
o n l y the temperature
a c c u r a t e l y c o n s t a n t . The
p r e s s u r i s i n g the gas
an
obtained.
p r e p a r e d by o x i d i z i n g n i t r i c
w i t h dry oxygen i n the gas
The
i t s vapour.
i n t h e d i f f e r e n t i a l manometer s h o u l d be kept v e r y c o n s t a n t .
d e v i a t i o n s i n t h e temperature
partial
was
from
dependence o f t h e above mentioned e q u i l i b r i u m
i n the e x p e r i m e n t a l
and N 0
was
p r e p a r a t i o n o f samples o f N0
same way
found
and a f t e r removing the n i t r i c
sample c e l l was
a b s o l u t e p r e s s u r e o f 0.1067 MPa
solubility
vapour p r e s s u r e o f bromo-naphthalene i s
i n t e r f e r e n c e was
c l o s e d by v a l v e 4,
sample c o n t a i n e r the gas
the
o f the gas
sample c e l l
given
should
be
use o f oxygen i n p l a c e o f n i t r o g e n f o r
sample c e l l had no
i n f l u e n c e on the i n f r a r e d a b s o r p t i o n
measurements.
Nitric
a c i d vapours were p r e p a r e d by b u b b l i n g d r i e d n i t r o g e n gas
concentrated n i t r i c
acid solutions
a c o n s t a n t temperature
o f 20°C. To prevent c o n d e n s a t i o n
vapour and t h e water vapour the gas
through
(Merck a n a l y t i c a l grade) which were kept
stream was
o f the n i t r i c
then heated
t o 25.0°C
at
acid
and
31
continuously
not
l e d t h r o u g h the gas
i n f l u e n c e the i n f r a r e d
tion
i n the n i t r o g e n gas
nitric
was
The
c o n c e n t r a t i o n of n i t r i c
adding a known amount o f 0.1
In t h i s method the n i t r a t e
to
nitrite
content
with
by
t o a gas
concentrations
with
azo
stream
and
reductor
conditions
N-l-naphthylethylene-
dye
[12]. In t h e
s m a l l compared t o the
of the n i t r a t e
the
a c o l o r i m e t r i c method.
a copper-cadmium
coupled
were always v e r y
( l e s s than 0.1%
concentra-
sample
i o n then r e a c t s w i t h s u l f a n i l a m i d e under a c i d i c
diamine d i h y d r o c h l o r i d e t o form a r e d d i s h - p u r p l e
concentration
acid
a c i d vapour i n t h e gas
N alkali
i s reduced to n i t r i t e
form a d i a z o compound. T h i s compound was
the n i t r i t e
nitric
changed by v a r y i n g t h e c o n c e n t r a t i o n o f
then a n a l y s i n g on the n i t r a t e and n i t r i t e
column. The
p r e s e n c e o f some water vapour d i d
a b s o r p t i o n measurements. The
stream was
a c i d s o l u t i o n . The
determined by
cell.
samples
nitrate
content).
O,
Np
to vacuum
pump
UJ
NO-
to
Fig.
1
Experimental
A:
sample
set-up
container;
f i l l e d with
for
sample
preparation.
B: molecular
bromo^naphthalene;
manometer; F:
32
thermostat
vacuum gauge;
D:
1, 2,
sieves;
C: differential
infrared
sample
3, 4,
valves.
5
gas
manometer
cell;
E:
mercury
3.3
RESULTS
I n f r a r e d a b s o r p t i o n c o e f f i c i e n t s can be determined
F o r our purposes
A = log —
at c o n s t a n t
temperature
by u s i n g Lambert-Beer's
law.
the absorbance (A) can be w r i t t e n as:
= a.b.P
(1)
o
A
= Absorbance
1,1
= the r e s u l t a n t
o
and
incident
intensities
a
= absorptivity
MPa
.cm
b
= path l e n g t h
cm
P
= partial
MPa
bP
= o p t i c a l path length
Of each component
were p r e p a r e d
found. The
pressure
(NO,
N0 ,
and n i t r i c
4
at wave numbers at which no
a c i d vapour) c a l i b r a t i o n
o p t i c a l p a t h l e n g t h s and
f o r NO
determined
low o p t i c a l path l e n g t h s
law
w i t h the b a s e - l i n e
at a wave number o f 1908
i n F i g . 2 and F i g . 3. L e a s t square
i n d i c a t e t h a t Beer's
i s only v a l i d
curves
absorbance o f o t h e r components were
absorbance o f each component was
method. C a l i b r a t i o n c u r v e s
presented
Ng0
2
MPa.cm
(< 0.015
fits
1
cm
for high
MPa.cm) a r e
t o the e x p e r i m e n t a l
points
at low o p t i c a l path l e n g t h s (<
0.015
MPa.cm). At h i g h e r o p t i c a l p a t h l e n g t h s the i n f r a r e d absorbance f a l l s o f f as
t h e square
r o o t o f the o p t i c a l p a t h l e n g t h . Campani et a l [6] found a much
lower absorbance f o r NO
T h i s d e v i a t i o n may
at 1908
be due
cm
* compared w i t h the r e s u l t s p r e s e n t e d
here.
t o t h e h i g h e r r e s o l u t i o n at our i n f r a r e d a b s o r p t i o n
measurements.
C a l i b r a t i o n curves of N0
cm
a r e g i v e n i n F i g s . 4,
at 2908 cm"
2
and
N
2
0
at 2960 c m
4
-1
cm
^ ^2^4
cm
P
^2^4
e a
k
band. A d i r e c t
is d i f f i c u l t
m a
y
p e a
k
a
r
be due
e
3120
5 and 6. R e g r e s s i o n a n a l y s i s shows t h a t Beer's
i s v a l i d and t h a t the d e v i a t i o n s from t h e o r i g i n o f the 2908 cm
t h e 3120
and at
s m a
H-
The
* NOg
law
peak and
d e v i a t i o n from the o r i g i n o f the 2960
t o the s m a l l o v e r l a p o f the N0g
band and t h e N g 0
comparison o f these r e s u l t s w i t h the measurements o f Guttman
o
s i n c e h i s r e s u l t s were measured at temperatures
100°C. N e g l e c t i n g the i n f l u e n c e o f the temperature
o f 50 C up
4
[7]
to
on t h e absorbance at 50°C
the r e s u l t s p r e s e n t e d here agree w e l l w i t h those o f Guttman [ 7 ] .
It
s h o u l d be n o t e d t h a t a t h i g h c o n c e n t r a t i o n s o f n i t r i c
( o p t i c a l p a t h l e n g t h > 0.003 MPa.cm) i n gas m i x t u r e s
^
s m a l l o v e r l a p o f the 2960 cm
N
a
a c i d vapour
c o n t a i n i n g N0„
and N O
^
£
2 ° 4 ° s o r p t i o n band w i t h a weak n i t r i c
a
4
acid
33
band was o b s e r v e d .
nitric
Therefore the determination of
i
n
t
h
concentrations
i n F i g . 7. From l e a s t
Calibration
curve
(A = 1.3918 P .b
NQ
34
e
s
e
n
c
e
ot h i g h
a t known NOg
a c i d vapour o f t h e 895 cm * peak i s
square
i s v a l i d and t h a t t h e d e v i a t i o n from
2
r
(see T a b l e 1 ) .
c a l i b r a t i o n curve of n i t r i c
presented
Fig.
P
a c i d vapour c o n c e n t r a t i o n s s h o u l d be c a r r i e d out w i t h t h e 3160 cm ^ " 2 ^ 4
a b s o r p t i o n peak o r c a l c u l a t e d from t h e e q u i l i b r i u m c o n s t a n t
The
e
fits
t h a t Beer's law
the o r i g i n i s small.
of NO at 1908 cm
- 0.065.)
i t can be c o n c l u d e d
1
(25.0° C, 0.1067 MPa).
Fig.
3
Calibration
curve
of NO at 1908 am~
for low concentration
(25.0°C,
0.1067 MPa).
(A = 7.0186 P .b
NQ
In
NOg,
o r d e r t o study the a c c u r a c y o f t h i s method gas m i x t u r e s
NgO^
and n i t r i c
p r e p a r e d by p a r t i a l
partial
The
containing
a c i d vapour were a n a l y s e d . These gas m i x t u r e s
o x i d a t i o n o f a known amount o f NO,
w i t h a i r i n the p r e s e n c e
MPa.
+ 0.00210.)
o x i d a t i o n the gas
o f water vapour i n the gas
sample c e l l was
NO
+ 0
+ N0
2
2N0
3N0 (N 0 ) + H 0
2
NO
2
4
+ N0
2
2
+ H0
2
•+ N 0
2
sample c e l l . A f t e r
the
o c c u r i n the gas sample c e l l :
(2)
2
t N 0
3
(3)
$ N 0
4
(4)
2
2
were
dosed as d e s c r i b e d above,
p r e s s u r i s e d w i t h n i t r o g e n t o 0.1067
f o l l o w i n g r e a c t i o n s and e q u i l i b r i a may
2N0
NO,
2
X 2HN0 (g) + NO
(5)
X 2HN0 (g)
(6)
3
2
o
The
e q u i l i b r i a c o n s t a n t s a t 25 C are g i v e n i n T a b l e 1. The
NO,
NOg,
N 0
2
4
and n i t r i c
a c i d vapour i n the gas
concentrations of
sample c e l l were
determined
a f t e r the o x i d a t i o n w i t h i n f r a r e d a b s o r p t i o n u s i n g the c a l i b r a t i o n c u r v e s .
s m a l l amounts N 0
2
3
which a r e p r e s e n t
i n t h e s e gas m i x t u r e s
were c a l c u l a t e d
u s i n g the e q u i l i b r i u m c o n s t a n t . A r e p r e s e n t a t i v e r e c o r d o f t h e
a b s o r p t i o n spectrum
o f such a gas m i x t u r e
The
infrared
i s g i v e n i n F i g . 8.
35
36
0-20
0-15
01 0
<
0 0 5
0 01
Opical
Fig.
5
Calibration
curve
(A = 5.794 7
0 02
path
0
length
of N 0 at 2960 cm
2 4
b- 0.01245.)
MPo.
1
03
0 0 4
cm
(25.0°C, 0.1067 MPa).
2 4
0 - 15
-
0 - 10 -
jQ
<
0 05
-
0 01
0 02
0
Optical path length
Fig.
6
Calibration
curve
(A - 3.902 P
of N 0. at 3120 cm
2^4
b - 0.00194.)
9
1
03
0
04
MP .cm
a
(25.0°C, 0.1067 MPa).
N
2°4
37
•
Fig.
7
Calibration
curve
Optical
of n i t r i c acid
(A - 71.897 P
vapour
Equilibrium
P
2
constant
.P
HNO
K
^ l
3
P
N 0
P N
2N0
X
2
N
\
2°4
c m
1
at 895 crn
(25.0°C, 0.1067 MPa).
V a l u e a t 25°C
0.00130 MPa"
3
2
a
Reference
NO
3N0 +H 0 X 2HN0 +N0
2
M P
.b - 0.0153.)
UMn
Equilibria
path length
1
[18]
.P
2
»2°
2°4
2
_1
0.654
MPa
[11,13]
0.0517
MPa"
1
[13]
0.140
MPa"
1
[14-17]
p
N0
2
P n
NO + N 0
2
X
N
\
2°3
P
2°3
.P
NO
N0
P
2
2
HN0
N0 +N0+H 0 X
2
Table
38
2
1
2HN0
Equilibrium
2
\
constants
P
2
.P
.P
NO
N0
H 0
2
2
wavenumber (cm* )
1
3500
3000
2500
~i
2000
1S00
1600
1400
1200
1000
r
800
600
N 0
N 0..
2
2
NoO,
u
Fig.
8
Small
Infrared
absorption
spectrum
of nitrogen
oxides
and nitric
acid
vapour.
amounts o f water vapour d i d not produce s e r i o u s o p t i c a l i n t e r f e r e n c e . The
initial
amount o f NO
s u p p l i e d b e f o r e t h e r e a c t i o n was
nitrogen oxides
and n i t r i c
sample c e l l
e q u i l i b r i u m had
and
a c i d vapour a f t e r r e a c t i o n had
occurred
i n the
been a t t a i n e d . From T a b l e 2 i t can be
t h a t the d e v i a t i o n i n the mass b a l a n c e
band o f n i t r o u s a c i d vapour at 850
occurrence
compared t o the amount o f
cm
i s s m a l l . In the s p e c t r a an
^ [19] was
sometimes found.
absorption
The
o f the a b s o r p t i o n band o f n i t r o u s a c i d can be e x p l a i n e d
equilibrium
(6) and
the N0
was
i n the gas
only detected
by
i t s p r e s e n c e i s a f u n c t i o n o f the r a t i o o f the
c o n c e n t r a t i o n and
2
c o n c e n t r a t i o n i n t h e gas
sample. N i t r i c
sample at r a t h e r low NO
gas
concluded
NO
a c i d vapour
concentrations
(see
Table
2).
3.4
CONCLUSIONS
A method has been developed
f o r the d e t e r m i n a t i o n
a c i d vapour i n gas m i x t u r e s
by means o f i n f r a r e d
important
f o r the manufacture o f n i t r i c
a c i d and
o f NO,
spectroscopy
2908 cm
-1
cm
; for NO
: 2980 cm
2
o r 3160
1
cm
and
0
^4
a
n
d
n
for nitric
1908
which may
cm
t
i
2
r
c
i
be
for pollution control.
f o l l o w i n g i n f r a r e d a b s o r p t i o n peaks can be used: f o r NO:
1
NOg,
The
, f o r N0„:
a c i d vapour:
895
4
. At r a t h e r h i g h c o n c e n t r a t i o n s o f n i t r i c
1
N
containing nitrogen oxides
the 2980 cm
o v e r l a p w i t h a weak n i t r i c
a c i d band. T h e r e f o r e t h e d e t e r m i n a t i o n
2
°4
a c i d vapour i n gas
the presence o f high c o n c e n t r a t i o n s of n i t r i c
mixtures
a b s o r p t i o n band shows a s m a l l
of N 0
2
4
in
a c i d vapour s h o u l d be c a r r i e d
out
39
O
No.
P
N0'
kPa
s
u
p
p
l
i
e
After p a r t i a l
d
P
%
oxidation
p
P
HNC 3
kPa
(89E cm
2°3
kPa
P
NO
kPa
(2908
N0
kPa
kPa
1
cm" )
(3160
-1
cm )
(1908
-1
cm )
1
12.67
4 80
1.51
5.27
2
11.85
5 95
2.48
0.771
0. 091
3
12.21
6 12
2.60
0.817
0. 091
4
13.37
5 45
1.93
3.80
5
11.48
6 12
2.53
0.423
N
*)
0. 111
6
10.91
3 00
0.56
6.27
*)
7
12.57
4 07
1.01
6.40
*)
8
10.87
5 60
2.47
0.147
0.85
6.64
3 68
12.07
g
Deviation
in
0. 156
*)
mass b a l a n c e
0 131
5 4
0 024
-0 3
0 026
0 6
o 107
-0 3
0 013
2 3
0 097
_3 o
0 135
1 5
0 004
-0 2
0 126
1 7
n i t r o u s a c i d vapour found i n t h e gas sample.
Table
2
The amount of nitrogen
presence
of water
oxides
vapour
and nitric
acid
vapour
found after
partial
oxidation
of NO in the
w i t h t h e 3160 cm
at known N 0
2
a b s o r p t i o n peak o r c a l c u l a t e d
from t h e e q u i l i b r i u m c o n s t a n t
concentration.
In gas m i x t u r e s c o n t a i n i n g n i t r o g e n o x i d e s and water vapour a l s o
nitrous
a c i d vapour c o u l d be d e t e c t e d , e s p e c i a l l y a t h i g h N O - c o n c e n t r a t i o n s compared t o
the NOg c o n c e n t r a t i o n .
REFERENCES
1. Saltzman, B.E. and Cuddeback, J . E . , Anal.
2. Saltzman, B.E. and Burg, W.R., Anal.
3. A l l e n , J.D. and P h i l ,
M., J. Inst.
Chem. , 1975, 47, 1.
Chem., 1977, 49, 1.
Fuel,
1973, 46, 123.
4. L i e v e n s , F., Rapp. Cent. Étude E n e r g . N u c l . B.L.G., 1973, 480.
5. Forweg, W.,
V.D.I.Ber.
(Ver. D t s c h . I n g . ) , 1974, 24, 247.
6. Campani, P., Fang, C.S. and P r e n g l e , H.W.,
Appl.
Spectroscopy,
1972, 26,
372.
7. Guttman, A., J. Quant. Spectrosc.
Radiât. Transfer,
1961, 2, 1.
8. E n g l a n d , C. and C o r c o r a n , W.H., Ind. Eng. Chem. Fundam. , 1974, 13, 373.
9. E n g l a n d , C. and C o r c o r a n , W.H., Ind. Eng. Chem. Fundam., 1975, 14, 55.
10. F o n t a n e l l a , J.C., O f f i c e s N a t i o n a l d'Etudes
1974,
e t de Recherches
Aérospatiales
Note t e c h n i q u e no. 235.
11. B o d e n s t e i n , M. and Boës, F., Z. Physik.
Chem., 1922, 100, 68.
12. T e c h n i c o n A u t o - A n a l y z e r I I , I n d u s t r i a l Method No. 230-72A/Tentative 1974.
13. H i s a t s u n e , I.C., J. Phys. Chem., 1961, 65, 2249.
14. Ashmore, P.G. and T y l e r , B.J., J. Chem. Soc,
1961, 1017.
15. Wayne, L.G. and Y o s t , D.M., J. Chem. Phys.,
1951, 19, 41.
16. Karavaev,
Journal
1962,
36,
M.M, and S k v o r t s o v , G.A., Russian
17. Waldorf, D.M. and Babb, A.L., J. Chem. Phys.,
18. Nonhebel,
of Physical
Chemistry,
566.
1963, 39, 432.
G., Gas P u r i f i c a t i o n P r o c e s s e s f o r A i r P o l l u t i o n C o n t r o l , Newnes-
B u t t e r w o r t h s , London, 1972.
H o f t i j z e r , P.J. and Kwanten, F.J.G., A b s o r p t i o n o f n i t r o u s g a s e s .
19. Jones, L.H., Badger,
R.M. and Moore, G.E., J. Chem. Phys.,
1951, 19,
1599.
41
4. THE ABSORPTION OF N 0 / N 0
2
2
INTO DILUTED AND CONCENTRATED NITRIC ACID
4
4.1 INTRODUCTION
The
absorption of nitrogen dioxide into n i t r i c
for
the production of n i t r i c
a c i d solutions i s very
important
a c i d and t h e subsequent s t a c k gas problems. In t h e
l i t e r a t u r e many i n v e s t i g a t i o n s can be found c o n c e r n i n g t h e a b s o r p t i o n o f NO^/
NgO^ gas m i x t u r e s
i n t o water. K i n e t i c d a t a c o n c e r n i n g t h e a b s o r p t i o n o f
nitrogen dioxide into n i t r i c
the d e s i g n o f i n d u s t r i a l
a c i d a r e , however, o f much g r e a t e r importance f o r
a b s o r b e r s . No r e l i a b l e
d a t a were found i n t h e
literature.
In t h i s Chapter
t h e a b s o r p t i o n mechanism o f HO^/H^O^
d i l u t e d and c o n c e n t r a t e d n i t r i c
d e s c r i b e d wetted
HNOg-HgO w i l l
acid i s investigated
gas m i x t u r e s
into
i n the p r e v i o u s l y
w a l l column. Moreover t h e e q u i l i b r i a d a t a o f t h e system NgO^-
be c r i t i c a l l y d i s c u s s e d .
4.2 REVIEW OF LITERATURE
4.2.1
A b s o r p t i o n o f NOg/NgO^ i n t o aqueous s o l u t i o n s
The major r e a c t i o n by which n i t r i c
absorbers
t o produce d i l u t e d n i t r i c
a c i d i s formed i n i n d u s t r i a l n i t r i c
a c i d can be r e p r e s e n t e d by t h e r e a c t i o n o f
NO„, which i s i n e q u i l i b r i u m w i t h NO.,
2N0
In
2
( N 0 ) + H 0 •* HN0
2
4
2
w i t h water.
+ HNOg
3
acid
(1)
a c i d s o l u t i o n s t h e n i t r o u s a c i d may decompose:
(2)
3HN0„ X HNO„ + 2N0 + H 0
o
The
o v e r a l l r e a c t i o n can then be w r i t t e n a s :
3N0
42
2
( N 0 ) + H 0 •> 2HN0
2
4
2
3
+ NO
(3)
Reaction
(3) may a l s o p r o c e e d
can be produced
formation
i n t h e gas phase and n i t r i c
[1,2,3]. L i t t l e
i s known c o n c e r n i n g
a c i d vapour o r m i s t
the n i t r i c
a c i d mist
i n t h e gas phase.
Detournay and J a d o t
[4] found
t h a t under normal c o n d i t i o n s t h e n i t r i c
f o r m a t i o n i n t h e gas phase may be n e g l e c t e d compared t o t h e n i t r i c
formation i n the l i q u i d
Equi
acid
phase.
libria
The most important
overall
e q u i l i b r i u m f o r the n i t r i c
a c i d formation
r e a c t i o n ( 3 ) . T h i s e q u a t i o n determines
P
K
2
3
P
°2
P
H
(4)
N
2°
value o f the e q u i l i b r i u m constant
The
i s g i v e n i n T a b l e 1.
same d e f i n i t i o n may be a p p l i e d t o t h e heterogeneous e q u i l i b r i u m and
then P„„_
and P
HNU3
a r e t h e vapour p r e s s u r e s o f HN0_ and H „ 0 over t h e l i q u i d
"20
o
phase. The vapour p r e s s u r e o f HNOg and HgO over n i t r i c
taken
acid
o f t h e n i t r o u s gases:
P
NO
HNO
p
1
i s g i v e n by t h e
t h e maximum n i t r i c
c o n c e n t r a t i o n t h a t c a n be o b t a i n e d a t a g i v e n c o m p o s i t i o n
The
acid
£
a c i d s o l u t i o n s may be
from t h e data o f t h e b i n a r y system HNOg/HgO. T h e r e f o r e
i t i spractical to
define the f o l l o w i n g e q u i l i b r i u m constants:
\-¥
5
p
N0
2
2
P
HNO
K
="p-
p
6
H
(6)
1
2°
Measurements o f t h e e q u i l i b r i u m c o n s t a n t K
s t r e n g t h and t h e temperature can be found
According
to Carberry
g e n e r a l l y accepted
nitric
[5] K
Pi
p
as a f u n c t i o n o f t h e n i t r i c
i n the l i t e r a t u r e
acid
[5,6,7].
and K
s h o u l d be based on N-0. which i s
P5
t o be t h e a c t i v e s p e c i e s d u r i n g t h e a b s o r p t i o n i n t o
diluted
acid.
\
p
N
Carberry
(
3/2
?
)
2°4
[5] c o r r e l a t e d s e v e r a l l i t e r a t u r e d a t a and found
that K
p
was i n -
dependent o f t h e temperature.
43
Equilibria
Equilibrium
H N 0
3N0 + H 0 ( g ) X 2HN0„(g) + NO
2
2
3
Q
o
K
N
3
°
l
P
d
1
(bar )
, „
-9
= 1.75 x 10
1 n
=
P
constant
Reference
,4644, '
exp <-=—)
r
,
[39]
Q l
.P
N 0
H
2
2°
P
2N0
2
$ N 0
2
K
4
= exp ( ^ S L
=
p
2
_
2 1
.
2 4 4
)
[37,38]
P
NO 2
P
NgOg
NO, + NO $ N , 0 ,
2
Z
K
3
d
= =
NO
p
N0
P
2
Table
2
1
Equilibrium
constants
K
4
P
=
-6
4723
exp ( — — )
,.,
[1J
2
2
p
NO' NOg'
of the reactions
[37]
2
H N 0
NO„ + NO + H.O X 2HN0„
*
q
4869
= 41.82 x l o " ' exp (-^-)
= 0.185 X 10
p
HO
of nitrogen
oxides
log K
P
= 7.412 - 20.28921 x w + 32.47322 x w
2
- 30.87 x w
3
(8)
7
i n which w i s t h e weight f r a c t i o n o f HNO^.
The
and
vapour p r e s s u r e s o f HNOg and H^O were measured by Vandoni and Laudy [8]
can be d e s c r i b e d
Mechanism
The
by t h e Margules-Duhem e q u a t i o n
of NO^/Ncfi^ absorption
absorption
into
r a t e and mechanism o f N 0
water have been s t u d i e d by s e v e r a l
accepted that
aqueous
2
which i s i n e q u i l i b r i u m w i t h N^O^ i n t o
authors
generally
i n t h e water. A f t e r d i s s o l u t i o n
a c i d . The f o l l o w i n g
r e a c t i o n scheme
this:
2N0 (g)
2
J N 0 (g)
(9)
N 0
4
t N 0 (£)
(10)
2
N
2°4
2
4
2
+ H
2° *
equilibrium
H N 0
3
4
+
H N 0
2
r e p r e s e n t e d by e q u a t i o n ( 9 ) i s so r a p i d l y e s t a b l i s h e d
may be assumed t h a t NgO^ and N 0
other
[1,10-18,23]. I t has been
and p h y s i c a l l y d i s s o l v e s
the NgO^ r e a c t s w i t h water t o form n i t r i c
The
solutions
i f a NO^/NgO^ gas m i x t u r e i s absorbed i n t o aqueous s o l u t i o n s NgO^
i s the a c t i v e species
represents
[9].
2
are continuously
i n equilibrium
w i t h each
[19]. A p p l y i n g t h e t h e o r y o f mass t r a n s f e r w i t h a r a p i d pseudo
order r e a c t i o n
i n t h e l i q u i d phase t h e a b s o r p t i o n
I f NOg/NgO^ i s absorbed i n t o aqueous s o l u t i o n s
phase mass t r a n s f e r i s a l s o i m p o r t a n t . N 0
continuous e q u i l i b r i u m
2
that i t
first
r a t e can be w r i t t e n a s :
from an i n e r t gas stream t h e gas
and N 0
2
4
are transferred, i n
w i t h each o t h e r , from t h e gas b u l k t o t h e g a s - l i q u i d
interface.
The
v a l u e o f k as d e f i n e d
concentration
laboratory
o f water. V a l u e s o f
absorbers. Table 2 gives
t a b l e i t can be c o n c l u d e d t h a t
one
i n e q u a t i o n (12) does i n c o r p o r a t e
^ )/"^J?, ^
o r
w a
ter
t h e molar
have been measured w i t h
a review o f t h e s e i n v e s t i g a t i o n s . From t h i s
the v a l u e s o f H
JkD„
2 4'
agree r a t h e r w e l l
with
another.
The
r e a c t i o n r a t e c o n s t a n t k was c a l c u l a t e d from t h e s e v a l u e s w i t h t h e
45
10
3
H
X
„ \/kD„
N 0 V
I
2
k sec
Method o f measurement
4
2
kmol/m
20°C
Caudle
.sec.bar.
25°C
and Denbigh [11]
30°C
20°C
25°C
30°C
1.09
506
absorption
Wendel and P i g f o r d [13]
0.57
138
absorption
Dekker [14]
1.09
506
absorption
Kramers e t a l [15]
0.76
0. 88
250
330
absorption
C o r r i v e a u [18]
0.56
136
absorption
Kameoka and P i g f o r d [16]
0.68
195
absorption
0.92
361
absorption
1.0-1.1
>490
absorption
H o f t i j z e r and Kwanten [1]
Gerstacker
D i s c u s s i o n at r e f . [15]
M o l l [20]
267
l i q u i d N0
i n j e c t i o n i n t o water
2
T r e i n i n and Hayon [21]
300+100
Komiyama and Inoue [16]
f l a s h photolysis
194
—9
D
2
25°C "»
m / s at
= 1.41 X 10
2°4,1
H
3
=1.29
kmol/m
.bar at 25°C
desorption
Kramers e t a l [15]
J
2 4
Table
2
Comparison
of literature
data concerning
the absorption
of N 0.
p
into
water
diffusion coefficient
and Henry c o e f f i c i e n t
found by Kramers e t a l [ 1 5 ] . The
agreement between t h e r e a c t i o n r a t e c o n s t a n t d e r i v e d from t h e a b s o r p t i o n
measurements i s r a t h e r poor.
Moll
[20] i n j e c t e d l i q u i d N^O^ i n t o water and t h e r e a c t i o n r a t e c o n s t a n t k
agreed w e l l w i t h t h e v a l u e measured by Kramers e t a l [15] w i t h l a m i n a r j e t
experiments.
T r e i n i n and Hanson [21] found r o u g h l y t h e same v a l u e s by means o f
flash photolysis.
In i n d u s t r i a l a b s o r b e r
d e s i g n t h e v a l u e o f H„ „ i/kD. i s much
N 0 V
2
more important
4
I
than t h e r e a c t i o n r a t e c o n s t a n t . H o f t i j z e r and Kwanten [1] found
the f o l l o w i n g e q u a t i o n f o r water:
760
l
It
0
g
V o . l /
2 4
5
*
(3° - 75°C)
= " 0.53 -
(13)
i s known t h a t t h e a b s o r p t i o n r a t e o f N^O^ d e c r e a s e s w i t h i n c r e a s i n g
acid
s t r e n g t h . T h i s may be a t t r i b u t e d t o t h e d e c r e a s e o f t h e Henry c o e f f i c i e n t
increasing
" f r e e " water becomes r e l a t i v e l y
s m a l l i n more c o n c e n t r a t e d n i t r i c
No r e l i a b l e d a t a can be found
the n i t r i c
a c i d s t r e n g t h on t h e H
L o n g s t a f f and S i n g e r
nitric
acid [1].
i n the l i t e r a t u r e concerning the i n f l u e n c e of
i / kD. v a l u e s .
N2O4V
Jo
[24] found t h a t i f N 0
2
gas i s i n c o n t a c t w i t h 60%
a c i d u n r e a c t e d N^O^ may be p r e s e n t i n t h e l i q u i d phase and t h a t t h e
n i t r o u s a c i d c o n c e n t r a t i o n may be n e g l e c t e d . The r a t i o C
/(C
N
in
with
i o n i c s t r e n g t h and a d e c r e a s e o f k as t h e molar c o n c e n t r a t i o n o f
t h e l i q u i d phase as a f u n c t i o n o f t h e n i t r i c
2°4,J>
acid strength
H N 0
+C
)
2,Jl
20 ,Jo
given i n F i g .
N
4
10
0 5
100
50
1
Fig.
1
The distribution
of the nitric
of ^2^4
aoid
m
N0
^
3
n
^
^trie
acid
solutions
as a
function
strength.
47
From t h i s
"2^4 ^
a
s
m
x
l
f i g u r e i t can be c o n c l u d e d
u
*
r
e
s
into n i t r i c
that the p h y s i c a l absorption o f N0 /
2
a c i d s o l u t i o n s becomes important
fornitric
acid
s o l u t i o n s above 55% and t h a t t h e r e a c t i o n o f N „ 0 . w i t h water may be n e g l e c t e d .
z 4
4.2.2 NCvj/NgO^ a b s o r p t i o n i n t o c o n c e n t r a t e d n i t r i c
The
a b s o r p t i o n o f NO„/N 0 i n t o n i t r i c
£
£ 4
important
of
found c o n c e r n i n g
nitric
a c i d s o l u t i o n s o f more than
f o r the p r o d u c t i o n o f d i l u t e d n i t r i c
concentrated n i t r i c
acid.
acid solutions
a c i d as w e l l as t h e p r o d u c t i o n
In t h e l i t e r a t u r e v e r y l i t t l e
the absorption o f N0 /N 0
2
a c i d . Atroshchenko and Kaut
2
gas m i x t u r e s
4
i n f o r m a t i o n can be
into
concentrated
[26] i n v e s t i g a t e d t h e a b s o r p t i o n o f NOg/NgO
i n t o 70 - 98% HNOg and found t h a t t h e a b s o r p t i o n proceeds p u r e l y
T h i s f i n d i n g was a l s o c o n f i r m e d
55% i s
by Karavaev and V i s l o g u z o v a
l i t e r a t u r e no i n f o r m a t i o n can be found
concerning
physically.
[25]. In t h e
t h e a b s o r p t i o n mechanism.
Equilibrium
The
s o l u b i l i t y o f NgO^ i n t o c o n c e n t r a t e d n i t r i c
for
the design of i n d u s t r i a l
absorbers.
a c i d s o l u t i o n s i s v e r y important
In t h e l i t e r a t u r e o n l y v a l u e s o f t h e
t o t a l vapour p r e s s u r e o f t h e system N^-HNOg-HgO were found
o r d e r t o study
t h e s o l u b i l i t y o f NgO^ i n t o n i t r i c
[27,28,29,30]. In
a c i d s o l u t i o n s i t was assumed
t h a t t h i s t o t a l vapour p r e s s u r e can be d e s c r i b e d a s :
P
=
tot
P
HNO3
+
P +
H 0
P
2
+
N0
P
N 0
2
2
4
+ P „ + P
N 0
NO
2
(14)
3
P„„ and P „ _ a r e v e r y low above c o n c e n t r a t e d n i t r i c a c i d s o l u t i o n s and t h e r e NO
N 0
*
f o r e they may be n e g l e c t e d . The vapour p r e s s u r e s o f HNO^ and HgO were taken
2
3
from t h e d a t a o f t h e b i n a r y system HNO^-HgO measured by Vandoni and Laudy [ 8 ] .
2
With a i d o f t h e e q u i l i b r i u m c o n s t a n t (K
= P
/ H '
equilibrium
2
2 4
2
P
o
f
t
h
e
n
N
2N0
2
X N 0
2
the vapour p r e s s u r e s o f N 0
If
(15)
4
N
2
and
i t i s assumed t h a t m a i n l y
2
0
4
2
0
N
were c a l c u l a t e d .
4
i s present
i n t h e l i q u i d phase
31,32,33] t h e Henry c o e f f i c i e n t H „ „ s h o u l d be d e f i n e d a s :
2°4
N
N
° 2°4 I
48
[27,29,30,
Henry c o e f f i c i e n t s
(H„ _ ) were c a l c u l a t e d
2°4
from
t h e t o t a l vapour p r e s s u r e
data
N
o f Klemenc and Rupp [28] and t h e r e s u l t s a r e g i v e n i n F i g . 2 and F i g . 3. From
t h e s e f i g u r e s i t can be c o n c l u d e d
t h a t t h e Henry c o e f f i c i e n t
i s independent o f
t h e amount o f NgO^ i n t h e l i q u i d phase. A t s m a l l NgO^ c o n t e n t s some d e v i a t i o n s
20
15
10
—v—
V
o
u
o
a.
û
o
CM
~~ A.
A
Q
as a function
o
A
A
5
°/o N
Hjy
0
°C
o
S
2
V
o
o
6
Fig.
V
A
10
2
0
4
ri
A.
A
12 5 °C
25
15
b y w e i g h t in
of the FS^O^ content
°C
20
HNO3
in 16 N HNO^.
(V: 0°C; 0: 12.5°C; A: 25°C) [28].
From t h e t o t a l vapour p r e s s u r e d a t a measured by W e i n r e i c h
Karavaev and Yarkovaya
the temperature
[27]
and
[29] Henry c o e f f i c i e n t s were c a l c u l a t e d as a f u n c t i o n o f
and t h e a c i d s t r e n g t h (see F i g . 4 ) . Some own experiments
c a r r i e d out t o i n v e s t i g a t e t h e Henry c o e f f i c i e n t more d i r e c t l y . N i t r i c
were
acid of
75% was s a t u r a t e d w i t h n i t r o g e n gas c o n t a i n i n g 5-20 volume % o f N 0 by means o f a
2
s a t u r a t o r a t a p r e s s u r e o f 1.04 b a r . The gas phase was a n a l y s e d f o r i t s
content w i t h i n f r a r e d
phase was determined
spectroscopy
0
2
4
_
(Chapter 3 ) . The NgO^-content i n t h e l i q u i d
by i n j e c t i n g a l i q u i d
NaOH s o l u t i o n and then
method
N
sample o f 50 y l i n t o a 10 ml 0.8 N
a n a l y s i n g f o r n i t r i t e content with a c o l o r i m e t r i c
[ 3 4 ] . I t was found t h a t t h e measured Henry c o e f f i c i e n t
was
independent
49
o f t h e p a r t i a l p r e s s u r e o f N^O^ i n the gas phase. The e x p e r i m e n t a l
summarized
results are
i n T a b l e 3.
30
25
o
°c
20 -
15
-o
o
o
o
12 • 5 °C
oo o
!
io
o
A
E
A
ft-
C
°C
25
3
5
10
%> N 0
2
Fig.
3
H
n
as a function
4
of the N„0. content
"24
(V;
50
15
b y weight in HN0
0°C; 0: 12.5°C; A: 25°C) [28].
20
25
3
in 19 N nitric
acid.
0-30
Fig.
4
H
- as a function
2 4
of the temperature
and n i t r i c acid
concentration.
a
a
Weinreich
[27]
25% by weight
of N 0
4
in 75% HN0
ff 0^
in 75% HN0
g
+
Weinreich
[27]
20% by weight
of
0
Weinreich
[27]
10% by weight
of N 0
A
Klemenc and Rupp [28]
•
Karavaev
0
This
and Yarkovaya
g
2
4
3
3
in 75% HNO^
[29]
work
From F i g . 4 i t can be seen t h a t t h e r e i s a r a t h e r good agreement w i t h t h e
r e s u l t s d e r i v e d from t h e vapour p r e s s u r e d a t a .
I t s h o u l d be noted t h a t at a r a t h e r h i g h N^O^ c o n t e n t i n t h e l i q u i d
and low temperatures
liquid
i m m i s c i b i l i t y may o c c u r
The heat o f s o l u t i o n o f N O
phase
[31,32,33].
i n t o n i t r i c a c i d s o l u t i o n s can be c a l c u l a t e d
51
from:
d i n (H
_
)
AH
g
_*JL =
d
( 1 7 )
(^)
R
i n which T i s t h e a b s o l u t e temperature, R t h e gas c o n s t a n t and A H
s o l u t i o n a t t h e temperature c o n s i d e r e d
( t a k e n as n e g a t i v e ) .
g
t h e heat o f
In a f i r s t
a p p r o x i m a t i o n AHg may be assumed t o be c o n s t a n t over a s m a l l range o f
temperature. The r e s u l t s
for different n i t r i c
acid strengths are given i n
T a b l e 4.
From t h i s t a b l e i t can be seen t h a t t h e heat o f s o l u t i o n o f NgO^ i n t o
nitric
acid solutions
i s rather constant f o r d i f f e r e n t
Temperature (°C)
H
™24
kmol/m .bar
7 0
0 0106
- 0 0611
25.2
6 3
0 008
- 0 05
35.2
4 9
0 006
- 0 04
45.3
3 4
0 003
- 0 007
3
Henry coefficient
of the
% HN0
52
bar P
20.3
Table
4
strengths.
Measured range
3
Table
acid
Heats
of N 0
2
4
in 75% n i t r i c acid
as a
function
temperature
AH
3
g
(kJ/kmol N 0 )
2
65
- 23 2 X i o
3
70
- 25 3 X i o
3
75
- 27 2 X i o
3
19 N
- 25 3 X i o
3
of solution
of N 0.
p
into
concentrated
4
n i t r i c acid
solutions
4.3
EXPERIMENTAL
The
experimental
apparatus
i s schematically presented
s o l u t i o n s were pumped t o an overhead
wetted
2. About 0.05% by weight o f an a l k y l
i n order t o e l i m i n a t e r i p p l e s o f the l i q u i c
f i l m . The f i l m h e i g h t h was c o r r e c t e d f o r t h e end e f f e c t
o f t h e s u r f a c e a c t i v e agent
experiments
acid
r e s e r v o i r and then f e d by g r a v i t y t o t h e
w a l l column d e s c r i b e d i n Chapter
s u l p h o n a t e was added t o t h e l i q u i d
i n F i g . 5. N i t r i c
caused by t h e p r e s e n c e
and t h e e f f e c t i v e f i l m h e i g h t h' was i n t h e s e
13.7 cm and 34.6 cm. N i t r o g e n d i o x i d e ( b o i l i n g p o i n t : 21.2°C) was
s u p p l i e d from
condensation
a cylinder
immersed i n a water b a t h a t 47.5°C. To a v o i d
t h e p i p e s c o n t a i n i n g pure n i t r o g e n d i o x i d e were heated
r e s i s t a n c e heating wire
(Pyrotenax
i n s u l a t e d w i t h g l a s s wool. The N 0
2
with
L t d . , Hebburn-on-Tyne, England) and
gas stream
was meted w i t h a n e e d l e
valve
immersed i n a t h e r m o s t a t i c a l l y c o n t r o l l e d water b a t h o f 50°C and then mixed
w i t h a n i t r o g e n gas stream.
the wetted
mixture
The gas m i x t u r e was l e d i n c o - c u r r e n t f l o w
w a l l column i n t h e same way as was d e s c r i b e d i n Chapter
l e a v i n g t h e wetted
w a l l column was scrubbed
through
2. The gas
w i t h an a l k a l i n e hydrogen
p e r o x i d e s o l u t i o n t h e remove t h e n i t r o g e n o x i d e s b e f o r e v e n t i n g i t t o t h e
atmosphere. The n i t r i c
a c i d l e a v i n g t h e wetted
stainless steel vessel.
In t h e experiments
w a l l column was s t o r e d i n a
w i t h 63% and 78% t h e n i t r i c
s o l u t i o n was s t r i p p e d w i t h n i t r o g e n t o remove t h e d i s s o l v e d N„0^
nitric
a c i d . The s t r i p p e d n i t r i c
w i t h 25% and 40% n i t r i c
was
acid
from t h e
a c i d was then r e c y c l e d . In t h e experiments
acid, the n i t r i c
a c i d l e a v i n g t h e wetted
w a l l column
drained o f f .
The
i n - and o u t g o i n g
l i q u i d were a n a l y s e d f o r t h e i r NgO^ and/or HNOg
c o n t e n t by i n j e c t i n g a l i q u i d
sample (50 - 250 y l ) i n t o a 10 mol 0.8 N NaOH
s o l u t i o n . A f t e r reaction the n i t r i t e
method
c o n t e n t was determined
[34]. Note t h a t w i t h t h i s method HN0
2
with a c o l o r i m e t r i c
and N 0 ^ i n t h e n i t r i c
2
acid
samples can not be d i s t i n g u i s h e d . The o u t g o i n g gas streams were a n a l y s e d f o r
t h e i r NOg,
N
2
0
4
and NO c o n t e n t w i t h i n f r a r e d s p e c t r o s c o p y
concentration o f N0
N
2
and ° 4
2
e s t a b l i s h i n g a mass b a l a n c e
i
n
t
n
e
i n g o i n g gas stream
around t h e wetted
(Chapter 3 ) . The
was c a l c u l a t e d by
w a l l column.
53
to
Fig.
5
air
The
experimental
nitric
acid
for
8: rotameter;
less
bath;
54
column;
5: stainless
6: vessel
steel
for
the NO^
and
NO absorption
experiments
into
solutions.
1: wetted wall
vessel;
set-up
2: stripper;
steel
alkaline
9: overhead
filter;
14: needle
vessel
hydrogen
for
stripped
peroxide;
reservoir;
12: cyclone;
valve;
3: scrubber;
13:
15: infrared
4: stainless
nitric
10: flow
gas
acid
7: calibrated
sample
solution;
glass
controllers;
thermostatically
steel
11:
controlled
cell.
pipe;
stainwater
4.4 RESULTS
4.4.1 The a b s o r p t i o n o f N 0
2
4
into diluted n i t r i c
acid
A c c o r d i n g t o t h e model NO^ and NgO^ a r e t r a n s f e r r e d ,
w i t h each o t h e r ,
of a N 0 / N 0
2
2
solutions
i n continuous
from t h e gas phase t o t h e g a s - l i q u i d
equilibrium
i n t e r f a c e . The d i f f u s i o n
m i x t u r e i n the gas phase was r e g a r d e d as t h e d i f f u s i o n o f one
4
f i c t i t i o u s component Q d e f i n e d as:
C
X 2,g
Q =
+
2 C
N
(
n
2 4,g
The gas phase d i f f u s i o n can under o u r measured
G r a e t z model
8
)
c o n d i t i o n s be d e s c r i b e d by t h e
(Chapter 2) and t h e d i f f u s i o n r a t e o f N 0
the g a s - l i q u i d
1
4
2
from the gas phase t o
i n t e r f a c e p e r u n i t o f s u r f a c e a r e a can be w r i t t e n as:
n
Q
The d i f f u s i o n c o e f f i c i e n t o f t h e f i c t i t i o u s component Q as a f u n c t i o n o f D
2
and D
N
D
Q
^
was d e r i v e d by Dekker [ 3 5 ] .
= D
2°4
_
Jl l ++ 8K
8K P„
P„ .. ++ ./
,/"
1 + 8K
+
N
D„„
N0
/
P
= 1.36 x 10
-5
(20)
P
2
Q,o
2
o
m /s a t 20 C and 1.0132 b a r
2
5
D
2
= 0.96 x 1 0 ~ m /s
a t 20°C and 1.0132 b a r
2 4
i n which P_ . i s t h e p a r t i a l
Q,i
tis
the p a r t i a l
p r e s s u r e o f N0„ + 2N„0„ a t t h e i n t e r f a c e and P„
t2 2 4
Q,o
p r e s s u r e i n t h e m i d d l e o f t h e wetted w a l l column. The
N
2
°4
r e a c t s w i t h water:
N
2°4
+
H
H N 0
2 ° ~*
+
3
H N 0
(
2
2
1
)
Decomposition o f t h e n i t r o u s a c i d produced a c c o r d i n g t o
4HN0
2
•* 2N0 + N 0
2
4
+ H 0
2
(22)
55
does not take p l a c e r a p i d l y i f i t s c o n c e n t r a t i o n
i s low. NO i s very
poorly
s o l u b l e i n aqueous s o l u t i o n s and i t can e a s i l y d i f f u s e i n t o t h e gas phase.
Under o u r e x p e r i m e n t a l
c o n d i t i o n s no NO c o u l d be d e t e c t e d
i n f r a r e d spectroscopy.
Therefore
HNOg c a n be n e g l e c t e d
with
i t may be assumed t h a t t h e d e c o m p o s i t i o n o f
[1,13,14,15,16,35]. A p p l y i n g
a r a p i d pseudo f i r s t
order
i n t h e gas phase w i t h
t h e theory
o f mass t r a n s f e r
r e a c t i o n i n t h e l i q u i d phase t h e a b s o r p t i o n
r a t e p e r u n i t o f s u r f a c e a r e a based on t h e p e n e t r a t i o n t h e o r y
can be w r i t t e n
as:
T h i s equation
was
holds only
The
1 [36]. In accordance w i t h
t h i s equation i t
temperature r i s e n e a r t h e i n t e r f a c e as a r e s u l t o f t h e a b s o r p t i o n was
c a l c u l a t e d with
(-AH
A,
T
= _
2
The
i f kx »
assumed t h a t N^O^ i s t h e a c t i v e s p e c i e s d u r i n g t h e a b s o r p t i o n .
[36]:
-
2
P
AH )
S_
C
H
N
p
2°4
V ——
P
N
(24)
2°4,i
heat o f s o l u t i o n o f NgO^ i n t o aqueous s o l u t i o n s and t h e heat o f r e a c t i o n
were t a k e n from t h e d a t a o f M o l l
[20]. W i t h i n
t h e measured c o n d i t i o n s i t can
e a s i l y be shown t h a t t h e temperature r i s e was s m a l l enough t o be n e g l e c t e d
(< 0 . 2 ° C ) . The p a r t i a l
pressures
of N 0
2
4
c a l c u l a t e d from t h e measured a b s o r p t i o n
on t h e g a s - l i q u i d i n t e r f a c e were
r a t e s , equation
(19) and e q u a t i o n (20)
by means o f an i t e r a t i o n p r o c e d u r e . The measured a b s o r p t i o n
of P
was p l o t t e d f o r 25% n i t r i c
r a t e as a f u n c t i o n
a c i d i n F i g . 6 and f o r 40% n i t r i c
acid i n
N
2°4,i
F i g . 7. T h i s s h o u l d g i v e a s t r a i g h t
l i n e through the o r i g i n with
a slope of
i s the a c t i v e
H„N2O4V
„\/kD . From these f i g u r e s i t c a n be c o n c l u d e d t h a t N 0
s p e c i e s d u r i n g t h e a b s o r p t i o n . A l e a s t - s q u a r e method gave t h e s l o p e o f t h e
2
straight
line
and t h e s e v a l u e s
are given
From t h i s t a b l e i t can be seen t h a t
nitric
a c i d s t r e n g t h . F o r low n i t r i c
4
i n T a b l e 5.
h
N 2
q
decreases with i n c r e a s i n g
a c i d c o n c e n t r a t i o n t h i s i s m a i n l y caused
by t h e d e c r e a s e o f H, „ w i t h i n c r e a s i n g i o n i c s t r e n g t h . A c c o r d i n g t o H o f t i j z e r
N2O4
and Kwanten [ l ] t h e i n f l u e n c e o f t h e i o n i c s t r e n g t h on H
can be d e s c r i b e d
2°4
with:
(H
)
2 4 nitric
= (H
acid
)
exp (- 0.075 I)
2 4 water
NQ
where I i s t h e i o n i c s t r e n g t h d e f i n e d by
56
(25)
n
2
I = i I
. -
(Z.
1
From F i g . 8 i t can
be
Kwanten [1]
is valid
nitric
large
acid
pseudo f i r s t
(26)
C.)
1
seen t h a t
for n i t r i c
deviations
order reaction
acid strength since
the
a p p r o x i m a t i o n proposed by
a c i d c o n c e n t r a t i o n s up
o c c u r . T h i s may
rate
o f NgO^
caused by
the molar c o n c e n t r a t i o n o f the
into n i t r i c
the
and
Above
fact that
constant k decreases with i n c r e a s i n g
r e l a t i v e l y s m a l l i n more c o n c e n t r a t e d n i t r i c
coefficient
be
Hoftijzer
to about 25%.
25%
the
nitric
" f r e e " water t e n d s to become
a c i d . Moreover the
a c i d decreases with i n c r e a s i n g
diffusion
nitric
acid
strength.
57
•
bar
P
Pig.
N
7 The absorption
driving
2
0
4
, i
rate
*
1
,
2
P
N 0
of N 0
2
4
2
, i (a)
into
40% HNC> at 20° C as a function
3
of the
force.
(h> = 0.346 m; x = 0. 722 sec; 0: P
Q
l
; A: P
U
H
m
N
n V
2
k D
o
+ P
Q
lV
h 4,i
u
2 4,i
f t ) .
Z,%
m
au
Reference
4
x 10^ kmol/m .bar
Kramers e t a l [15]
water
0 76
25% HN0
3
0 49 + 0 03
this
work
40% HNOg
0 16 + 0 02
this
work
H o f t i j z e r and Kwanten [ l ]
Table
58
5
Q
ykD^ values
as a function
of the nitric
acid
strength
at 20 C
1 0
H
n
n V kDo
2 4
(A;
values
Kramers
et al
as a function
[15]j
approximation
Some a u t h o r s
experiments
of
Hoftijzer
the
and
proposeu
[10,13,14,35] observed
nitric
r e l a t i v e l y h i g h molar c o n c e n t r a t i o n s o f N^O^.
a c i d m i s t was
observed.
Kwanten
by Hoftijzer
c o n c e r n i n g the a b s o r p t i o n o f NgO^
here no n i t r i c
nitric
strength.
[1];
0:
and
a c i d mist
this
worl
Kwanten
[1]).
during t h e i r
i n t o water e s p e c i a l l y
D u r i n g t h e experiments
The n i t r i c
phase does not seem t o be v e r y important
acid
as was
acid
at
presented
formation i n the
found by Detournay and
gas
Jadot
[4]-
4.4.2
The
Previously
a b s o r p t i o n o f N„0„ i n t o c o n c e n t r a t e d n i t r i c
2 4
i t was
l y and t h a t NgO^
acid
solutions
d e r i v e d t h a t t h i s a b s o r p t i o n p r o c e s s proceeds
may
be assumed t o be the a c t i v e
d i f f u s i o n o f N0„ and N O
s p e c i e s . The
purely physical-
gas
phase
can be d e s c r i b e d by the Graetz-model a c c o r d i n g t o :
59
The
p h y s i c a l absorption process o f N 0
2
experimental
J
N
The
2°4
i n t o t h e l i q u i d phase can under o u r
4
c o n d i t i o n s be w r i t t e n as:
= 2(H
2°4
N
temperature
P
2°4,i
- C
2°4,*,o
N
—
)V
N
T
(28)
T
r i s e near t h e i n t e r f a c e as a r e s u l t o f t h e p h y s i c a l a b s o r p t i o n
was c a l c u l a t e d w i t h [ 3 6 ] :
-AH
AT =
PC
H
p
N 0„
24
o
V —
P
N.O^
24,i
< >
29
a
v
I t was found t h a t w i t h i n t h e measured c o n d i t i o n s t h i s temperature
o
neglected
(< 0.2 C ) . T a b l e 6 and T a b l e 7 g i v e t h e e x p e r i m e n t a l
results.
p r e s s u r e s o f NgO^ on t h e i n t e r f a c e were c a l c u l a t e d from e q u a t i o n
equation
(20) w i t h an i t e r a t i o n p r o c e d u r e .
function of the d r i v i n g
f o r c e (H„ „ P„ _
N 04 N 0
2
straight
l i n e through
2
r i s e may be
Partial
(27) and
The measured a b s o r p t i o n r a t e as a
4
i
- C„ _
N 0
2
) s h o u l d be g i v e n a
4 )
£
) 0
the o r i g i n .
From F i g s . 9, 10, 11 and 12 i t can be c o n c l u d e d
that N 0
2
4
i s the active
s p e c i e s d u r i n g t h e a b s o r p t i o n . With e q u a t i o n (28) t h e t h e o r e t i c a l a b s o r p t i o n
r a t e s were c a l c u l a t e d and compared w i t h t h e measured a b s o r p t i o n r a t e s . The
d i f f u s i o n c o e f f i c i e n t o f N_0„ i n t o n i t r i c a c i d s o l u t i o n s was c a l c u l a t e d w i t h
2 4
t h e r e l a t i o n o f Wilke and Chang [ 3 6 ] .
D„ „
= 0.88 x 1 0 ~
2°4,£
9
2
m /s
f o r 78% HN0„ a t 20°C
N
D
N
„
= 0.77 x 1 0
2°4,il
- 9
3
2
m /s
f o r 63% HNO, a t 20°C
3
From T a b l e 6 and T a b l e 7 i t can be c o n c l u d e d
d e s c r i b e s t h e experiments
60
fairly
well.
t h a t t h e proposed
a b s o r p t i o n model
Exp.
P
T
c
N
2°4,£,o
3
3
xlO
kmol/m
bar
35, 1
1. 093
20
35 2
1 ,093
20
0. 346
1 .58
85
3. 05
0. 346
1. 58
85
3 05
NO
2,o
2 4,o
2 4,i
N
N
2°4
6
2
xlO
kmol/m .sec
2°4
6
2
x l O kmol/m . s e c
measured
penetration theory
bar
bar
bar
0 ,0458
0 ,0214
0, 0148
1.11
1, 21
0. 0673
0, 0463
0. 0297
2.39
2 50
0 11
37. 1
1 040
20
0. 346
2 ,40
85
2. 61
0 ,0165
0 ,00279
0. 00231
0.11
37. 2
1 040
20
0, 346
2 40
85
2 61
0 ,0379
0 ,0147
0 0107
0.58
0 70
37 3
1, 040
20
0. 346
2 40
85
2, 61
0 ,0556
0 ,0316
0 0212
1.31
1 44
37 4
1 040
20
0, 346
2 40
85
2 ,61
0 .0664
0 ,0451
0 0296
1.80
2 03
37 5
1 040
20
0. 346
2 40
85
2 61
0 ,0759
0 ,0590
0 0376
2.34
2 60
3 33
0 ,0712
0 ,0241
0 0180
2.13
2 12
3 33
0 ,0931
0 ,0411
0 ,0281
3.92
3 .41
0 .85
2 .85
31 1
1 070
30
0, 137
0 .513
65
31 .2
1 ,070
30
0. 137
0 .513
65
34 1
1 .110
30
0 346
0 .986
65
3 ,10
0 .0538
0 .0137
0 0105
0.88
34 .2
1 .110
30
0 346
0 .986
65
3 , 10
0 .0975
0 .0450
0 ,0323
2.64
36 .1
1 .064
30
0 .346
1 .77
65
2 , 82
0 .0190
0 .00172
0 ,00164
0.03
0 .04
36 ,2
1 .064
30
0 ,346
1 .77
65
2 .82
0 .0436
0 .0090
0 .00745
0.35
0 ,44
36 . 3 1 .064
30
0 .346
1 .77
65
2 ,82
0 .0723
0 .0248
0 ,0191
0.98
1 .23
36 .4
1 ,064
30
0 ,346
1 . 77
65
2 .82
0 .0915
0 .0397
0 .0289
1.69
1 .90
36 .5
1 .064
30
0 346
1 . 77
65
2 ,82
0 .111
0 .0582
0 ,0413
2.49
2 .75
Table 6
N0„ absorption
experiments
into
63% HNO^
Exp.
P
c
T
h'
T
m
C
P
N0„
2,o
K~0. .
2 4,10,0
3
3
P n
2°4,o
P n
2°4,1
N
2°4
N
6
2
xlO
kmol/m .sec
measured
2°4
6
2
xlO
kmol/m .sec
p e n e t r a t i o n theory
bar
bar
bar
3 9
0 0676
0 0468
0 0187
4 10
4 05
210
3 9
0 1006
0 1034
0 0377
8 47
8 24
2 15
210
3 6
0
0644
0 0424
0 0194
3 02
3 74
0 346
2 15
210
3 6
0 0740
0 0561
0 0236
4 02
4 58
0 346
2 15
210
3 6
0 0847
0 0734
0 0269
5 52
5 24
0 346
2 15
210
2 6
0 0323
0 0107
0 00570
0 84
1 07
0 00648
1 11
1 37
0 00107
0 13
0 15
xlO
bar
C
m
sec
12 1
1 075
20
0 346
1 71
210
12 2
1 075
20
0 346
1 71
13 1
1 051
20
0 346
13 2
1 051
20
13 3
1 051
20
14 1
1 051
20
kmol/m
14 2
1 079
20
0 346
1 71
210
2 6
0 0348
0
15 1
1 067
20
0 346
2 15
210
2 75
0 0122
0 00151
0124
15 2
1 087
20
0 346
1 71
210
2 75
0 0126
0 00163
0 00110
0 17
0 17
19 1
1 063
20
0 137
0 43
210
3 35
0 0541
0 0299
0 0136
5 47
5 85
19 2
1 063
20
0 137
0 43
210
3 35
0 0785
0 0631
0 0223
146
2 55
0 0746
0 0264
0 0138
4 28
146
2 55
0 0981
0 0456
0 0217
7 06
5 99
2 35
2 49
5 48
18 1
1 039
30
0 137
0 61
18 2
1 039
30
0 137
0 61
12 0
9 67
3 76
17 1
1 014
30
0 137
0 91
146
2 55
0 0638
0 0193
0 01127
17 2
1 014
30
0 137
0 91
146
2 55
0
0 0469
0 0242
5 41
16 1
1 053
30
0 346
2 15
146
3 64
0 0106
0 000533
0 00040
0 08
16 2
1 053
30
0 346
2 15
146
3 64
0
0
0 00335
0 31
0 41
16 3
1 053
30
0 346
2 15
146
3 64
0 0786
0 0293
0 0170
1 93
2 45
146
3 64
0 0582
0 0161
0 0104
1 02
1 47
146
3 64
0
0 0473
0 .0246
3 17
3 59
16 4
1 053
30
0 346
2 15
16 5
1 053
30
0 346
2 15
Table
7 NO^-absorption
experiments
into
78% nitric
0994
0310
0999
acid
00456
-—
N 0
,i
"
N 0
N 0 ,i
*
'2 NO
2
4
2
. l,o /
4
n
N 0
2
(o)
4
bar
2
Fig.
9
The absorption
driving
rate
4
,i
K
z
of N^O^ into
N0
2,i
NU
C
^
, l.o '
4
H
N 0
2
4
63% HNO^ at 20 C as a function
of the
N„0„ .
2 4,i
P
N 0
The absorption
driving
rate
4
,i
N 0 ,i
2
4
"
C
N
A :
P
+
N„0,
2 4,i
05
2
0
,l,o/ N 0
2
* '2 N0 ,i
1
(o)
H
4
-
P
2
of N^O^ into
C
4
N 0 .l,o/
2
4
H
N 0
2
(
i
)
4
63% HN0 at 30 C as a function
of the
3
force.
(h> = 0.346 m; T = 1.77 sec; 0: P
m
2,1
/H
U
0
p
°N„0
N„0j
2 4,l,o
2^4
H
N
2
10
2
~ N 0
^N 0 '
2 4,l,o
2 4
P
Fig.
0
L
force.
(h' = 0.346 m; T = 2.40 sec; 0: P
F
~ N
_
2 4,%,o
-C
2
/n
r
4
>
l
; A :P
/H
2
4
> * > °
2
4
2
+
4
>
l
2 4
63
0 02
N 0
P
2
P
N 0 .i
2
Fig.
11 The absorption
driving
4
,i
rate
4
" N 0
0 04
C
2
*
of N^
4
1 /
2
P
4
NO
, l.o /
H
N 0
2
(o)
4
.I " N Q
,l,o/ N 0
C
s
into
2
H
4
2
<
A
)
4
78% HN0 at 20° C as a function
3
of the
force.
Ch' - 0.346 m; T - 2.15 sec; 0: P
N
-NO /2 - C
2,i
2°4,lJ W
N
64
H
2°4,i
' X°4,l,o W
/H
\ 0
4
J
4.5
CONCLUSION
The
a b s o r p t i o n o f NOg/NgO^ i n t o d i l u t e d n i t r i c
pseudo f i r s t
a c i d i s accompanied by a r a p i d
o r d e r r e a c t i o n between N^O^ and water. From t h e a b s o r p t i o n
measurements i t can be c o n c l u d e d
that with increasing a c i d strength the values
approximation
decrease o f t h e s o l u b i l i t y o f N „ 0 ,
s t e n g t h . Above 25% n i t r i c
The
innitric
t h i s i s mainly
a c i d s o l u t i o n s t h i s approximation
a b s o r p t i o n o f NO^/NgO^ gas m i x t u r e s
caused by t h e
acid with increasing
ionic
i s not v a l i d .
into concentrated n i t r i c
(> 63%) can be c o n s i d e r e d as pure p h y s i c a l p r o c e s s .
acid
I t c a n be c o n c l u d e d
that
N^O^ i s t h e a c t i v e s p e c i e s d u r i n g t h e a b s o r p t i o n p r o c e s s . The s o l u b i l i t y o f
N^O^ i n c o n c e n t r a t e d n i t r i c
a c i d s o l u t i o n s was c a l c u l a t e d from t h e t o t a l
p r e s s u r e d a t a o f t h e system NgO^HNOg-HgO, and i t can be c o n c l u d e d
law
that
vapour
Henry's
i s valid.
REFERENCES
1. H o f t i j z e r , P.J.
and Kwanten, F.J.G., A b s o r p t i o n o f n i t r o u s gases i n :
G. Nonhebel, Gas p u r i f i c a t i o n p r o c e s s e s
Butterworths,
London, 1972, p. 164.
2. Goyer, G.G., J. Coll.
3. England,
f o r a i r p o l l u t i o n c o n t r o l , Newnes-
Sci. , 1963, 18, 616.
C. and C o r c o r a n ,
W.H., Ind. Eng. Chem. Fundam. , 1974, K3, 373.
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5. C a r b e r r y , J . J . , Chem. Eng. Soi., 1959, ¡3, 189.
6. Theobald,
H. , Chemie-Ing. -Teohn. , 1968, 1_5, 763.
7. Tereshchenko, L.Ya., Panov, V.P., P o z i n , M.E. and Zubov, V.V., J. Appl.
Chem. USSR (Engl.
Transl.),
1968, 41, 1995.
8. Vandoni, R. and Laudy, M., J. Chim. Phys.,
9. A u n i s , G., J. Chim. Phys.,
1952, 49, 99.
1952, 49, 103.
10. Chambers, F.S. and Sherwood, T.K., Ind. Eng. Chem., 1937, 2J3, 1415.
11. Caudle,
P.G. and Denbigh, K.G., Trans.
Far. Soo., 1953, 39, 39.
12. P e t e r s , M.S. and Holman, J . L . , Ind. Eng. Chem., 1955, 47, 2536.
13. Wendel, M.M. and P i g f o r d , R.L., A.I.Ch.E. Journal,
1958, 4, 249.
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16. Kameoka, Y. and P i g f o r d , R.L., Ind. Eng. Chem. Fundam., 1977, 16, 163.
17. Denbigh, K.G. and P r i n c e ,
A . J . , J. Chem. Soo., 1947, 790.
65
18. Sherwood, T.K., P i g f o r d , R.L. and W i l k e , C.R., Mass T r a n s f e r ,
McGraw-Hill,
1975.
19. C a r r i n g t o n , T. and Davidson, N., J. Phys.
Chem. , 1953, 57, 418.
20. M o l l , A . J . , PhD T h e s i s , Washington, 1966.
21. T r e i n i n , A. and Hayon, E., J. Am. Chem. Soa., 1970, 92, 5821.
22. Komiyama, H. and Inoue,
H., J. Chem. Eng. of Japan,
1978, 11, 25.
23. Counce, R.M., Master T h e s i s , U n i v e r s i t y o f Tennessee,
24. L o n g s t a f f , J.V.L.
25. Karavaev,
1974,
Chem. USSR (Engl.
Transl.),
1001.
26. Atroshchenko,
1958,
and S i n g e r , K., J. Chem. Soa. , 1954, 2610.
M.M. and V i s l o g u z o v a , V.G., J. Appl.
47,
K n o x v i l l e , 1978.
31,
V . l . and Kaut, V.M., J. Appl.
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1967,
M.M. and Yarkovaya,
Allg.
Chem., 1930, 194, 51.
V.A., J. Appl.
Chem. USSR (Engl.
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40, 2340.
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M.M. and Bessmertnaya,
A . I . , The Soviet
Chemical
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1969,
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D e l f t , 1958.
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Chem., 1922, 100, 68.
W.F., J. Am. Chem. Soa., 1942, 64, 48.
5. THE OXIDATION AND ABSORPTION OF NO BY NITRIC ACID
5.1 INTRODUCTION
The
oxidation
o f NO t o N 0
by c o n c e n t r a t e d n i t r i c
2
a c i d s o l u t i o n s may be o f
importance f o r the p r o d u c t i o n o f c o n c e n t r a t e d n i t r i c
concentrated n i t r i c
nitrogen
o x i d e s from t h e t a i l
attractive properties.
gas o f n i t r i c
Firstly,
e a s i l y o x i d i z e NO t o N0 .
2
p h y s i c a l l y very well
acid plants,
i t i s a very strong
Secondly, N 0
2
dissolves
a c i d . In a d d i t i o n
a c i d may be used as a s c r u b b i n g l i q u i d
4
f o r t h e removal o f
since
i t has two
o x i d i z i n g agent and i t can
which i s i n e q u i l i b r i u m
into concentrated n i t r i c
with
In t h i s C h a p t e r the mechanism and the k i n e t i c s o f t h i s o x i d a t i o n
nitric
acid are investigated
to obtain
a b s o r b e r s . Furthermore some p r e l i m i n a r y
o f NO i n t o 40% n i t r i c
acid solutions
gathered i n the concentrated region
N0 ,
2
acid.
by 63%-78%
data f o r the design o f i n d u s t r i a l
experiments c o n c e r n i n g the
absorption
a r e c a r r i e d out t o check i f t h e r e s u l t s
apply a l s o t o t h e d i l u t e d system.
5.2 PROPOSED MECHANISM
The
r e a c t i o n o f NO w i t h n i t r i c
a c i d i s presented with the f o l l o w i n g
overall
reaction:
NO + 2HN0
This
3
reaction
acid solutions
t 3N0
2
+ H 0
i s the r e v e r s e d
investigate
t h i s phenomenon
stream c o n t a i n i n g
that
walls
r e a c t i o n of a c i d formation. Concentrated
o f 55%-80% have a c o n s i d e r a b l e
T h e r e f o r e , t h i s r e a c t i o n may
gas
(1)
2
take p l a c e
nitric
a c i d vapour p r e s s u r e .
i n t h e gas phase [ 1 ] . In o r d e r t o
e x p e r i m e n t s were c a r r i e d out by p a s s i n g a
1% NO o v e r a 65% n i t r i c
acid solution.
c l o s e t o t h e g a s - l i q u i d i n t e r f a c e . Furthermore, l a r g e
N0
2
nitrogen
I t was o b s e r v e d
t h e produced water vapour condensed i n t h e gas phase on t h e g l a s s
brown c o l o u r e d
nitric
reactor
amounts o f t h e
were found i n t h e gas phase. Tereshchenko e t a l [2]
67
i n v e s t i g a t e d t h e o x i d a t i o n o f NO i n a n i t r o g e n gas stream
s o l u t i o n s o f 60% t o 80% i n a b u b b l i n g apparatus
by n i t r i c
acid
and found t h a t t h e o x i d a t i o n
r a t e was c o n t r o l l e d by gas phase d i f f u s i o n o f NO from t h e gas b u l k t o t h e g a s liquid
i n t e r f a c e . These o b s e r v a t i o n s can be e x p l a i n e d by r e g a r d i n g t h e gas
phase r e a c t i o n between NO and n i t r i c
a c i d vapour as i n f i n i t e l y
f a s t . The
r e a c t i o n may t a k e p l a c e i n a s m a l l r e a c t i o n zone o r i n an a s y m p t o t i c
r e a c t i o n plane very c l o s e to the g a s - l i q u i d
which p r o c e e d
if
1 Absorption-oxydation
The
and
[11,12],
model
r e a c t i o n i s a c t u a l l y much more c o m p l i c a t e d than e q u a t i o n
(1) s u g g e s t s ,
i t may p r o c e e d v i a a mechanism composed o f t h e f o l l o w i n g s t e p s :
k
l
NO + HNOg
HN0
+ N0
(2)
2
= 0.2 - 9 m /kmol.sec a t 298°K
HN0
2
produced
HN0
g
2
+ HN0
= 6 x 10
3
a c i d vapour
[4,5,6].
2
+
3
[4,5,6].
reacts very r a p i d l y with n i t r i c
k
68 k
2
3
k
The
c a s e on a
(see F i g . 1 ) . R e a c t i o n s
i n a r e a c t i o n zone may be t r e a t e d as though they are i n s t a n t a n e o u s
t h e r e a c t i o n zone i s n o t t o o l a r g e
Fig.
interface
2N0
2
- 9 x 10
+ H 0
(3)
2
3
3
m /kmol.sec a t 300°K
[4,5,9],
The
r e a c t i o n rate constant k
constant
the
o f the
assumed t h a t
the n i t r o u s
J
2C
HN0
HNO
phase r e a c t i o n i s
/Z„
D
.
HNO,
J, 1
i n which J„„„ i s the
HNUg
a r e a i f the
D
reaction
the
i s not
N0
the
+ NO
2
NO
+ N0
From the
be
reactions
+ H0
X
2HN0
X
N 0
2
2
literature
react with n i t r i c
2
g
From the
+ HN0
that
2
2
be
It
an a u t o c a t a l y t i c
catalytic reaction
2
HN0
2
+ HN0
and
N 0
2
D
.
by
the
and
(4)
HN0„
, 1
O
e v a p o r a t e d per
surface
condition
(2)
unit
retained
reaction. Using
the
f o r instantaneous
i s too
slow. Near
water vapour are p r e s e n t ,
i n the
reaction
and
zone:
^0^
reactions
produced by
(5) and
these
to, r e s p e c t i v e l y , equation
(6)
are
reactions
(3)
and
(7)
2
concluded that
the
gas
phase o x i d a t i o n
o f NO
by
nitric
under t h e s e c i r c u m s t a n c e s i t seems
reaction.
the
l i q u i d phase o x i d a t i o n
o f NO
by d i l u t e d
which o n l y n i t r o u s a c i d p r o d u c e s , i s a l s o an
[14,15,16,30]. P r e s e n t l y ,
NOg
t
%
(6)
gas
2
for
3
mechanism t o c o n f i r m
2N0
i t i s formed.
(5)
c o n c e r n i n g k i n e t i c s and
phase r e a c t i o n . The
that
It i s
2
(2) . The
s h o u l d be n o t e d t h a t
(< 25%)
O
./
reaction
important
a c i d vapour a c c o r d i n g
above i t can
acid solutions
HNO,
[3,7,8,10,13] i t i s known t h a t
2N0
3
are
a c i d vapour i s a very complex r e a c t i o n , and
t o be
2C
shown t h a t
which i m p l i e s
following
as
condition
i n the neighbourhood o f the
i t can
much f a s t e r than r e a c t i o n
N 0
N0,o
i n s t e a d o f becoming d e p l e t e d
fulfilled,
the
a c i d which would be
i n t e r f a c e , however, l a r g e amounts o f N0
therefore
C
NO
amount o f n i t r i c
r e a c t i o n r a t e c o n s t a n t k^
found
rate
[11,12]:
O
o f NO
i t was
instantaneously
a c i d i n t o a gas,
/Ik.
concentration
i t s bulk value C
reaction
phase c o u l d be n e g l e c t e d .
J°mO,
,/
3
gas
acid reacts
t r a n s i e n t e v a p o r a t i o n of n i t r i c
i n s t a n t a n e o u s gas
times h i g h e r than the
p r i m a r y r e a c t i o n . With i n f r a r e d a n a l y s i s
amount o f n i t r o u s a c i d vapour i n the
therefore
For
i s about 10
2
however, too
the
little
conditions
produced i s i n e q u i l i b r i u m w i t h
for
nitric
auto-
i s known
instantaneous
NgO^.
(8)
4
69
T h i s e q u i l i b r i u m i s e s t a b l i s h e d very
t h a t N0„ and
A
rapidly
At the g a s - l i q u i d i n t e r f a c e o n l y N 0
2
nitric
4
5.3
EXPERIMENTAL
The
experiments we
(Chapter 2 and
5.4
c a r r i e d out
C h a p t e r 4 ) . The
for nitrogen
o x i d a t i o n was
b u l k and
The
1.
i n the equipment which was
previously
and
and
o u t - g o i n g gas
liquid
a mass b a l a n c e around the
found t h a t
the d e v i a t i o n was
described
were
N0-
l e s s than
5%.
RESULTS
to t h e proposed model t h e o x i d a t i o n
phase d i f f u s i o n from the gas
r a t e o f NO
i s c o n t r o l l e d by
gas
b u l k t o t h e r e a c t i o n zone o r r e a c t i o n p l a n e .
Gas
phase mass t r a n s f e r i n t h e wetted w a l l column t a k e s p l a c e by
d i f f u s i o n only
concentration
C
i n the r a d i a l d i r e c t i o n and
change o f NO
i n the
C„„
NO,o
gas
-a
—
4 I
n=l
a
exp
t h e r e f o r e the
phase can
fractional
be w r i t t e n
as:
The
it
(
)
(9)
Gz„„
NO
n
phase d i f f u s i o n c o e f f i c i e n t
r e l a t i o n o f Chapman-Enskog (I>
given
gas
molecular
2
1
oo
=
N0
of NO
= 1.98
e x p e r i m e n t a l r e s u l t s f o r 78%,
i n F i g s . 2,
oxidation
3 and
r a t e f o r 78%
c o n t r o l l e d and
4.
and
in nitrogen
-5
2
x 10
63%
and
m /s
57%
at 20 C and
nitric
From t h e s e f i g u r e s i t can
63%
nitric
was c a l c u l a t e d u s i n g
o
change o f NO
concentration
o f NO.
concentration
change o f NO
s u g g e s t s , t h a t the
From T a b l e s 1 and
should
be
2 i t can
1.0132 bar)
acid solutions
be c o n c l u d e d t h a t
a c i d i s c o m p l e t e l y gas
agrees w i t h the proposed model. A c c o r d i n g
f r a c t i o n a l concentration
70
the
(see Chapter 4 ) .
in-going
I t was
the
oxidation
According
The
from
to the g a s - l i q u i d i n t e r f a c e .
dissolves physically into
o x i d e s c o n t e n t and
established.
MATHEMATICAL MODEL AND
NO
gas
a c i d , i n which i t i s h i g h l y s o l u b l e
proposed model i s p r e s e n t e d i n F i g .
analysed
i t i s assumed
N„0
d i f f u s e i n c o n t i n u o u s e q u i l i b r i u m w i t h each o t h e r
¿1 4
r e a c t i o n zone o r r e a c t i o n p l a n e t o the
concentrated
[17]. T h e r e f o r e ,
[24].
are
the
phase d i f f u s i o n
t o e q u a t i o n (9)
independent o f the
be
the
the
inlet
c o n c l u d e d t h a t the f r a c t i o n a l
tends t o i n c r e a s e w i t h i n c r e a s i n g P„_
. This
NO, o
r e a c t i o n i s very
complicated.
71
Fig.
3
Fractional
concentration
number for 63% nitric
72
change of NO as a function
acid
(0: 20°C; A : 30°C; —
of the
equation
Graetz(9)).
'Or
Fig.
4
Fractional
concentration
number for
67% nitric
change of NO as a function
acid
(0: 20°C;
equation
of the
Graetz-
(9)).
73
Exp..
h
h
xlC >
6
m
T
sec
T
°C
p
o
c
bar
3.
m /sec
kmol
N
m
P
P
NO, o
bar
N0
P N
2
bar
2°4
bar
P N
2°3
bar
2 V
\
°4
xlO
kmol
6
°N0
C
N0,o
N
2° /
m. s
3
4
2
1.1
5 44
0 346
1 26
20
1 093
6 5
0 0419
0 0284
0 00825
0 000268
5 .25
0.327
1.2
5 44
0 346
1 26
20
1 093
6 5
0 0866
0 0388
0 01541
0 000845
11. 50
0.365
1.3
5 44
0 346
1 26
20
1 093
3 8
0 1274
0 0509
0 02648
0 00177
18. 92
0.396
1.4
5 44
0 346
1 26
20
1 093
3. 8
0 1683
0 0599
0 0367
0 00252
24. 71
0.363
1.5
5 44
0 346
1 26
20
1 093
3 8
0 2099
0 0689
0 0486
0 00395
31. 73
0.396
1.6
5 44
0 346
1 26
20
1 093
3 8
0 0433
0 0248
0 00628
0 000258
5 28
0.349
2.1
7 38
0 346
1 03
20
1 115
2 2
0 0410
0 0224
0 00515
0 000265
5 64
0.420
2.2
7 38
0 346
1 03
20
1 115
2 2
0 0782
0 0304
0 00947
0 000750
11 34
0.458
2.3
7 38
0 346
1 03
20
1 115
2 2
0 1075
0 0373
0 0143
0 00125
15 73
0.452
2.4
7 38
0 346
1 03
20
1 115
2 2
0 1596
0 0479
0 0235
0 00245
23 08
0.466
0 193
0 0560
0 0321
0 00362
27 66
0.486
0 0264
0 00749
0 000578
9 93
0.508
2.5
7 38
0 346
1 03
20
1 115
2 2
3.1
11 05
0 346
0 79
20
1 148
10 6
0 0626
3.2
11 05
0 346
0 79
20
1 148
10 6
0 0989
0 0340
0 01184
0 00117
15 28
0.509
3.3
11 05
0 346
0 79
20
1 148
20 7
0 1247
0 0390
0 01553
0 00180
17 85
0.538
3.4
11 05
0 346
0 79
20
1 148
20 7
0 1553
0 0454
0 0211
0 00256
23 95
0.527
37 0
0 1764
0 0487
0 0243
0 00314
28 88
0.531
8 35
0.270
3.5
11 05
0 346
0 79
20
1 148
5.1
3 45
0 346
1 71
20
1 087
2 5
0 0811
0 0469
0 0223
0 000708
5.2
7 38
0 346
1 71
20
1 087
2 5
0 1362
0 0636
0 0413
0 00148
14 47
0.247
9.1
7 38
0 137
0 43
20
1 069
3 3
0 1631
0 0435
0 0194
0 00317
36 41
0.649
7 38
0 137
0 43
20
1 069
3 3
0 0788
0 0288
0 00851
0 00102
17 60
0.651
9.2
Table
1
Experimental
r e s u l t s of N0-oxidation
by 78% HNO
E X P
-
h
*l
xlO
3
m
'
m
T
T
sec
P
°C
P
c
bar
xlO
kmol
N0,o
bar
P
P
N0,
bar
P
NO
J
N n
bar
N
bar
W
/SeC
Ul
_^N0_
6
xlO
kmol
NO, o
W
m3
20 1
n
m2
0
0 0104
0 000635
0 0498
0 0253
0 00224
0 0150
0 00231
0 000182
0 0232
0 00549
0 000523
5 87
0 677
0 0752
0 0285
0 00832
0 00102
9 98
0 689
0 1124
0 0363
0 0135
0 00197
14 07
0 701
5 06
0 1410
0 0387
0 0153
0 00269
17 9
0 714
1 93
0 0259
0 0233
0 00555
0 000194
2 23
0 467
3 17
0 137
0 75
20
1 060
1 11
0 0593
0 0318
20 2
3 17
0 137
0 75
20
1 060
22 1
7 69
0 137
0 42
20
1 087
1 11
0 1208
5 06
0 0263
22 2
7 69
0 137
0 42
20
1 087
22 3
7 69
5 06
0 0483
0 137
0 42
20
1 087
5 06
22 4
7 69
0 137
0 42
20
1 087
5 06
22 5
7 69
0 137
0 42
20
1 087
24 1
7 69
0 346
1 05
20
1 103
5 25
0 489
12 43
0 541
2 69
0 669
24 2
7 69
0 346
1 05
20
1 103
1 93
0 1085
0 0479
0 0235
0 00169
8 19
0 471
26 1
4 15
0 346
1 58
20
1 091
2 30
0 0309
0 0311
0 00992
0 000244
1 53
0 369
26 2
4 15
0 346
1 58
20
1 091
2 30
0 1277
0 0648
0 0429
0 00247
7 16
0 433
Table
2
Experimental
results
of the NO-oxidation
by 63% HNO
In some experiments w i t h 63% n i t r i c
oxidation
investigated.
it
a c i d the i n f l u e n c e
r a t e o f NO f o r e q u i m o l a r i n l e t
o f N0
2
on t h e
q u a n t i t i e s o f NO and N 0
2
was
The e x p e r i m e n t a l r e s u l t s a r e g i v e n i n T a b l e 3 and from t h i s
can be c o n c l u d e d t h a t
the i n f l u e n c e o f N0
change o f NO i n t h e gas phase i s o f minor
The
oxidation
r a t e o f NO by 57% n i t r i c
on t h e f r a c t i o n a l
2
importance.
acid solutions
phase d i f f u s i o n c o n t r o l l e d , and t h e o x i d a t i o n
liquid
is
the
phase (see
i s not c o m p l e t e l y gas
a l s o takes place
i n the
F i g . 4 ) . Under t h e s e c i r c u m s t a n c e s t h e gas phase r e a c t i o n r a t e
very low n i t r i c
2
a c i d vapour p r e s s u r e . At more d i l u t e d n i t r i c
was found i n t h e gas phase, and under t h e s e c o n d i t i o n s
takes place
only
i n the l i q u i d
phase. I t s h o u l d be noted t h a t
cannot e x i s t i n d i l u t e d n i t r i c
f i n a l product
The
liquid
2N0
+
acid
phase o x i d a t i o n
HN0
(< 40%),
overall
(< 40%)
N0
2
and/or N 0
and i n t h i s case n i t r o u s
2
4
acid i s
+
3
H 0
2
o f NO by d i l u t e d n i t r i c
a c i d c a n be p r e s e n t e d
reaction:
•* 3HN0
(10)
2
Some e x p e r i m e n t a l r e s u l t s w i t h 40% n i t r i c
a c i d a r e g i v e n i n T a b l e 4.
I f t h e t h e o r y o f mass t r a n s f e r w i t h a r a p i d pseudo f i r s t
liquid
acid
the r e a c t i o n
[14,15,16].
with the following
the
partially
t o o slow t o be c o n s i d e r e d as i n s t a n t a n e o u s , a f a c t which may be caused by
no N 0
the
table
concentration
phase may be a p p l i e d
the absorption
order r e a c t i o n i n
rate per unit surface
a r e a can
be w r i t t e n a s :
1
provided that
The
kx »
absorption
straight
line
regression
rate
(J„„)
NO
p l o t t e d as a f u n c t i o n o f P
. should give a
NO, 1
through t h e o r i g i n w i t h a s l o p e
o f J J Q ^ ^ D ^ (see F i g . 5 ) . With
H
a n a l y s i s t h e s l o p e was found t o be:
HJJQ ^/~ki>£
The
1.
oxidation
=
5
2
2.81 + 0.15 x 1 0 ~ kmol/m .bar.sec a t 20°C
o f NO i n t o 5-25% n i t r i c
acid solutions
(12)
seems t o be a u t o c a t a l y t i c
[14,15,16,25,26,27,28,29,30]. A b e l e t a l [14,25,26,27,28] proposed the f o l l o w i n g
reaction
HN0
76
3
scheme:
+ HN0
2
"* N 0
2
4
+ H 0
2
(13)
model
measured
Exp.
N„0„
2 4,o
N0„
2,o
NO,o
p
P
P
p
C
N0
P N
2
2°4
°N0,o
bar
bar
bar
bar
C
C
N0
V Q,i
C
Q,o" Q,i
bar
N0
°N0,o
C
X
10
C
V Q,i
Q,o
J n
2°4
Q,i
6
X
10
6
2
2
kmol/ra
kmol/m .s
1.90
0.490
0.610
1.76
0.67
7.03
0.490
0.610
7.68
0.62
4.76
0.490
0.610
4 . 85
9.78
0.490
0.610
0.53
40.1
0.0199
0.0171
0.00296
0.0293
0.00871
0.482
40.2
0.0704
0.O4O3
0.0164
0.0574
0.0334
0.546
0.0213
0.523
0.0455
0.544
0.69
41.1
0.0484
0.0285
0.00819
0.0458
41.2
0.0941
0.0486
0.0239
0.0671
Table
3
Influenae
of N0 on the oxidation
g
of NO by 63% HNO y
.s
(T - 20°C, T = 1.05 sea,
10 bar).
10.62
N 0
2
+ 2N0 + 2H 0
4
2
*
4HN0
In t h i s r e a c t i o n mechanism
is
established
overall
reaction
(13) i s r a t h e r
very r a p i d l y . The i n i t i a l
reaction
and f i r s t
slow, w h i l e e q u i l i b r i u m (14)
formation r a t e o f n i t r o u s a c i d o f the
(10) was found t o be f i r s t
acid concentration
concentration
(14)
2
order with respect
order with respect
to the n i t r i c
to the nitrous
acid
[15,16]. Furthermore t h e o v e r a l l r e a c t i o n r a t e was found t o be
independent o f t h e p a r t i a l p r e s s u r e o f NO [15,16]. From t h e above i t can be
concluded that within
not
valid
t h e measured c o n d i t i o n s
f o r the oxidation
t h i s a u t o c a t a l y t i c behaviour i s
o f NO by 40% n i t r i c
acid solutions.
g
<J> x 1 0
£
5
h'
T
p
C
HN0
2, o
X
l
2
°
3
N0
3
N0
2
kmol/m . s e c .
3,
m /s
m
sec.
1.21
0 136
0.284
2.65
0 0402
1.40
1.21
0 136
0.284
2.65
0 1038
2.72
0.97
bar
kmol/m
1.21
0 346
0.722
1.92
0 0302
1.21
0 346
0.722
2.62
0 0578
1.91
1.21
0 346
0.722
2.62
0 0870
2.58
1.21
0 346
0,722
2.62
0 109
3.14
1.21
0 346
0.722
2.62
0 129
3.59
0.722
2.62
0 0274
0.88
0 346
1.21
Table
4
Experimental
(T = 20°C; P
78
results
of the absorption
-1.16 bar).
rate
of NO into
40% HN0
Fig.
5
The absorption
rate
of NO into
40% nitric
acid
(0: x =0.722
sec; A ;
x = 0.284 sec).
UOp/IlpO^ diffusion
from the reaction
A c c o r d i n g t o t h e proposed model N 0
plane
2
and N^O^, which a r e i n c o n t i n u o u s e q u i -
l i b r i u m w i t h each o t h e r , d i f f u s e from t h e r e a c t i o n p l a n e t o t h e gas b u l k and t o
the g a s - l i q u i d
physically
i n t e r f a c e . At t h e g a s - l i q u i d
i n the concentrated n i t r i c
i n t e r f a c e only N„0 d i s s o l v e s
& 4
acid. For a quantitative description o f
these d i f f u s i o n processes the c o n c e n t r a t i o n o f N0
2
and N^O^ on t h e r e a c t i o n
p l a n e s h o u l d be known. I t s h o u l d be noted t h a t t h e d i s t a n c e from t h e r e a c t i o n
p l a n e t o t h e g a s - l i q u i d i n t e r f a c e (6 ) v a r i e d i n o u r experiments from 0 -4
3 x 10
m (moving boundary).
c o n c e n t r a t i o n decrease o f N0
liquid
N 0
2
4
2
From t h e s e r e s u l t s
and NgO
i t can be d e r i v e d t h a t t h e
from t h e r e a c t i o n p l a n e t o t h e g a s -
i n t e r f a c e i s s m a l l (< 5 % ) , and t h e r e f o r e t h e c o n c e n t r a t i o n o f NOg and
on t h e r e a c t i o n p l a n e was assumed t o be e q u a l t o t h a t a t t h e i n t e r f a c e .
The c o n c e n t r a t i o n o f N 0
mass b a l a n c e around
situation
- 3D,
NO
Q = N0
[18,19].
3C,
NO
and N 0
2
Q
D
8r
2
2
Q
+ 2N 0
2
4
on t h e r e a c t i o n p l a n e i s c a l c u l a t e d
from a
t h e r e a c t i o n p l a n e by assuming a q u a s i - s t a t i o n a r y
3r
- 2D
(15)
4
i n which t h e s o l u b i l i t y m i s d e f i n e d as:
79
C N
m
=
=
2
For
H N
2°4,£,i
~
0
2 4
2T~
16
< )
4,g,i
small values of the contact
b o t h be c o n s i d e r e d
time t h e gas phase and t h e l i q u i d phase may
t o be i n f i n i t e l y
deep, and t h e r e f o r e t h e l o c a l mass f l u x can
be c a l c u l a t e d from t h e p e n e t r a t i o n
theory.
The l o c a l mass f l u x o f NO from t h e gas b u l k
N
NO " " NO T I = m,o^k
D
c
R-6
For
zero
initial
and N^O^
N0
2
from t h e r e a c t i o n p l a n e
=
Q
If there
- D
i n t h e gas phase t h e l o c a l mass f l u x o f N 0
i s N0„ p r e s e n t
2
t o gas b u l k becomes:
/ D
-—•
3r
^
Q
(17)
f
concentration
3C
N
t o the r e a c t i o n plane i s :
I
= C
Q, i
V—
(18)
K
TTt
i n t h e i n - g o i n g gas stream, t h e l o c a l mass f l u x N
'
can
0,
4
be w r i t t e n a s :
N
Q
"
(
C
Q , i -
C
Q , o
)
V
19
i ?
The l o c a l mass f l u x o f N 0
2
< >
4
i n t h e l i q u i d phase can be d e s c r i b e d by:
3C
V.
=
• V,« "
=
L a ,
V,,,
" V m . . * ^ * *
(20)
A f t e r s u b s t i t u t i o n i n t o t h e mass b a l a n c e
3C
the f o l l o w i n g equation
V — — = (C
- C
)¥ —
t 2 ( 1 C„ „
NO.o' TTt
Q,i
Q V
TTt
2°4,g,i
N
D n
(21)
are only v a l i d
plane
than t h e c o n c e n t r a t i o n o f N 0
D
N
X
These e q u a t i o n s
i s higher
- C „
) x
2°4,£,o
2°4 I
* >
4
coefficient
i s obtained:
i f the concentration of N0
2
2
on t h e r e a c t i o n
i n t h e gas phase. The d i f f u s i o n
was c a l c u l a t e d as was d e s c r i b e d i n C h a p t e r 4. With t h e known
2
e q u i l i b r i u m constant K„ = P
/P„„ , t h e c o n c e n t r a t i o n s o f N0„ and N„0„ on
"2
N2O4 NO2
2
2 4
the r e a c t i o n p l a n e can be c a l c u l a t e d from e q u a t i o n (21) by means o f an i t e r a t i o n
Q
80
p r o c e d u r e . Note t h a t t h e s e
value.
concentrations
a r e independent o f the c o n t a c t
I f the gas phase may n o t be c o n s i d e r e d
t o be i n f i n i t e l y
c o n c e n t r a t i o n o f N0„ and N „ 0 . on t h e r e a c t i o n p l a n e o v e r t h e e f f e c t i v e
2
2 4
h e i g h t h' was c a l c u l a t e d from an o v e r a l l mass b a l a n c e :
2
o o l
a i T
3<j) (1 - 41
—
exp (- - 5 — ) )
n=l a
Gz
n
NO
g
= <j) (C
2
g
8TT(R - 6 ) h'(m C
concentrations
>
1
U
U
2 4,)l,o
obtained
with
The
The
obtained
with
c o n d i t i o n s l e s s than 8% from
and NgO^ from t h e r e a c t i o n p l a n e
2
those
t o t h e gas b u l k can
2
a TT
- * \ - <- <*->
.
oo
,
x
n=l
Q,i
a
n
Q
v a l u e s o f C„ - C„ ,/C
- C„ . were c a l c u l a t e d from t h e C„ . o f t h e
Q
Q,.t Q,o
Q,i
Q > _
1
t h e o r e t i c a l model a c c o r d i n g t o e q u a t i o n
experiments.
(22) and t h e measured v a l u e C
In F i g . 6 and F i g . 7 t h e t h e o r e t i c a l
compared f o r i n i t i a l
r e a c t i o n plane
concluded
The
and t h e measured v a l u e s a r e
t o t h e gas b u l k
i s given
amounts o f N 0
2
penetration
N
o
2 4
i n Table
4
which were p h y s i c a l l y
3. From t h e above i t can be
absorbed i n t o t h e n i t r i c
acid
c o n d i t i o n s be d e s c r i b e d w i t h t h e
theory.
=
2
< V o
. " V o
m
4
2 4,g, i
,
>' V* - iITT
f T ^
4
2 4,J!,,o
measured a b s o r p t i o n r a t e s were compared w i t h
values
r a t e o f NOg from t h e
t h a t t h e experiments agree r a t h e r w e l l w i t h t h e model.
s o l u t i o n s can under o u r e x p e r i m e n t a l
J
from t h e
z e r o c o n c e n t r a t i o n o f NO^ i n t h e gas phase. The i n f l u e n c e
o f NOg i n t h e i n - g o i n g gas stream on t h e d i f f u s i o n
The
< >
T7t
with:
- C_
Q,o
22
Zl '
*
equation (21).
d i f f u s i o n of N0
—
G„
2
o f NOg and NgO^ on t h e r e a c t i o n p l a n e
(22) d e v i a t e under o u r e x p e r i m e n t a l
film
2
°°
i
a i r
) ( 1 - 4E
- r exp ()) +
'°
n=l a
Gz„
.
n
Q
)Y
W
2 4,g,i
equation
be d e s c r i b e d
C
Q
-C
W
The
Q
_
time
deep, an average
according
to equation
r a t e s and the t h e o r e t i c a l
zero N O concentrations
^
concentration
(24).
(24)
the t h e o r e t i c a l l y
predicted
In F i g . 8 and F i g . 9 t h e measured
predicted absorption
rates are p l o t t e d f o r i n i t i a l
i n t h e gas phase. The i n f l u e n c e o f i n i t i a l
i n t h e gas phase on t h e N 0
3. From t h e above i t can be c o n c l u d e d
2
4
absorption
absorption
r a t e i s given
t h a t t h e measured N 0
2
4
N0
2
o
i n Table
absorption rate i s
r a t h e r w e l l p r e d i c t e d by t h e model.
81
Fig.
6
The diffusion
for
82
78% nitric
of iVOg and N 0
4
acid
from
the reaction
(0: 20°C; A : Z0°C;
plane
equation
to the gas bulk
(22)).
10
0-5h
O
U
o
u
o
IU
0
G
Fig.
7
The diffusion
for
63% nitric
of N0
2
acid
z
Q
05
0-10
, red
and Nfl^ from the reaction
(0: 20°C; A: 30° C;
plane
equation
to the gas
bulk
(23)).
83
84
85
5.5 DISCUSSION
In the proposed mechanism a few assumptions were made which w i l l be d i s c u s s e d
i n more d e t a i l .
a) A c c o r d i n g t o t h e p r o p o s e d model water vapour i s produced on t h e r e a c t i o n
plane very c l o s e to the g a s - l i q u i d
interface.
l e a v i n g t h e wetted w a l l column was
a n a l y s e d f o r i t s water vapour c o n t e n t . I t
was
found t h a t
In some e x p e r i m e n t s t h e gas phase
the amount o f water vapour i n t h e gas phase c o u l d be n e g l e c t e d .
T h i s i m p l i e s t h a t a l l t h e water vapour produced condenses on the n i t r i c
liquid
film. Nitric
a c i d d i f f u s e s from t h e l i q u i d
film
i n t o t h i s t h i n water
l a y e r and water d i f f u s e s from t h e i n t e r f a c e i n t h e n i t r i c
c o n c e n t r a t i o n g r a d i e n t s may
acid
acid film.
These
have some i n f l u e n c e on t h e s o l u b i l i t y o f NgO^.
can be shown t h a t t h e average t h i c k n e s s o f t h i s l a y e r i s v e r y s m a l l
_7
(6
< 3 x 10
m). The c o n c e n t r a t i o n g r a d i e n t o f water i n t h i s l a y e r
layer
It
was
r o u g h l y c a l c u l a t e d under s t a t i o n a r y c o n d i t i o n s w i t h :
DH 0
2
AC
H
2°
(25)
H
layer
2°
i n which J
i s t h e c o n d e n s a t i o n r a t e o f water vapour. From t h i s v a l u e i t was
-2
3
c a l c u l a t e d t h a t AC
< 10
kmol/m under o u r e x p e r i m e n t a l c o n d i t i o n s . T h i s
H2O
2
i m p l i e s t h a t t h e a c i d s t r e n g t h i n t h e t h i n l a y e r may
equal to the s t r e n g t h i n the n i t r i c
acid l i q u i d
a l s o be assumed t o be
film.
b) In the c a l c u l a t i o n s t h e i n f l u e n c e o f a temperature change n e a r t h e
i n t e r f a c e as a r e s u l t
o f heat o f r e a c t i o n , heat o f c o n d e n s a t i o n o f t h e water
vapour, heat o f m i x i n g o f the condensed water and n i t r i c
e v a p o r a t i o n o f the n i t r i c
was n e g l e c t e d
NO + 2HN0g
<->
3N0
2HN0
->
2HN0
H0
N
Table
86
2
2°4
5
(g)
<*>
acid
f
N
Heat effects
N
1
=
38.6 X 10
3
(g)
AH
2
=
78.6
AH
3
AH
AH
(A)
2
->
AH
H0
2°4
2°4
<«>
<*>
near the
interface
Reference
298 1
2
*
2
H0
(g)
2
3N0
(£)
into n i t r i c
(see T a b l e 5 ) .
A H
3
a c i d , heat o f
a c i d and heat o f s o l u t i o n o f NgO^
3
J
[23]
X io
3
J
[23]
= -44.2
x io
3
J
[23]
= -85.9
x io
3
4
J
[23]
= -25.3
X 19
3
S
J
t h i s work
The
temperature
change near
t h e i n t e r f a c e was c a l c u l a t e d from
a heat b a l a n c e by
assuming t h a t t h e heat o f m i x i n g may be n e g l e c t e d and t h a t a l l heat
o n l y c o n t r i b u t e t o a temperature
AH
< 1
*
A H
2
+
A H
3
+
^
A H
change i n t h e l i q u i d phase near t h e i n t e r f a c e .
4
) C
N0,o
yf%0,
P
+
effects
V
(26)
pc
P
v
i n which y r e p r e s e n t s t h e f r a c t i o n o f t h e NO^ produced
which i s c o n v e r t e d t o
N^O^. W i t h i n t h e e x p e r i m e n t a l c o n d i t i o n s t h e temperature change near t h e i n t e r o
o
face variei
f a c e v a r i e d from - 0.6 C t o 0 C. T h i s was found t o be s m a l l enough t o be
neglected.
5.6 CONCLUSIONS
The
be
o x i d a t i o n o f NO by c o n c e n t r a t e d n i t r i c
acid
(63-78%) can be c o n s i d e r e d t o
an i n s t a n t a n e o u s gas phase r e a c t i o n i n a r e a c t i o n zone o r on a r e a c t i o n
plane very c l o s e to the g a s - l i q u i d
i n t e r f a c e . P r e s e n t l y too l i t t l e
c o n c e r n i n g t h e mechanism and k i n e t i c s t o prove
criteria
i s known
t h i s hypothesis using the
f o r instantaneous r e a c t i o n s .
I t was found
t h a t Danckwerts' s o l u t i o n s f o r i n s t a n t a n e o u s
irreversible
r e a c t i o n s i n t h e l i q u i d phase can a l s o be a p p l i e d t o gas phase r e a c t i o n s . The
NOg and ^ 0 ^ produced, which a r e i n c o n t i n u o u s
e q u i l i b r i u m w i t h each o t h e r
d i f f u s e from t h e r e a c t i o n zone o r r e a c t i o n p l a n e t o t h e gas b u l k and t o t h e
gas-liquid
i n t e r f a c e . At t h e i n t e r f a c e o n l y NgO^ d i s s o l v e s p h y s i c a l l y
concentrated n i t r i c
a c i d . The mathematical
d i f f u s i o n p r o c e s s e s was found
The
model p r e s e n t e d
into the
to d e s c r i b e these
t o be i n good agreement w i t h t h e experiments.
a b s o r p t i o n o f NO by 40% n i t r i c
phase and under t h e s e c i r c u m s t a n c e s
a c i d s o l u t i o n s takes place i n the l i q u i d
n i t r o u s acid i s the f i n a l
p r o d u c t . The
a b s o r p t i o n r a t e can be d e s c r i b e d by the t h e o r y o f mass t r a n s f e r w i t h a r a p i d
pseudo f i r s t
order r e a c t i o n i n the l i q u i d
phase.
REFERENCES
1. Dohnalek, R. and V e s e l y , S., Neth. A p p l . 6401801, 1965.
2. Tereshchenko, L.Ya., Panov, V.N. and P o z i n , M.E., J. Appl.
Chem. USSR (Engl.
87
Tränst.), 1972, 45, 241.
3. K a i s e r , E.W. and Wu, C.H., J. Phys. Chem., 1977, 81, 1701.
4. K a i s e r , E.W. and Wu, C.H., J. Phys. Chem., 1977, 81, 187.
5. S t r e i t , G.E., W e l l s , J.S., F e h s e n f e i d , F.C. and Howard, C . J . , J. Chem.
Phys.,
1979, 70, 3439.
6. McKinnon, I.R., Mathieson, J.G. and W i l s o n , I.R., J. Phys. Chem., 1979, 83,
1979.
7. Wayne, L.G. and Y o s t , D.M., J. Chem. Phys.,
8. Chan, W.H., Nordstrom,
before
the Division
p. 251-253, A p r i l
1951, 19, 41.
R.J., C a l v e r t , J.G. and Shaw, J.H., Paper
of Environmental
Chemistry
American
Chemical
presented
Society,
4-9, 1975, New York.
9. England, C. and C o r c o r a n , W.H., Ind. Eng. Chem. Fundam., 1974, 13, 373.
10. England, C. and C o r c o r a n , W.H. , Ind. Eng. Chem. Fundam., 1975, 14^, 55.
11. Danckwerts, P.V., G a s - L i q u i d R e a c t i o n s , M c G r a w - H i l l , London, 1970.
12. A s t a r i t a , G., Mass T r a n s f e r w i t h Chemical R e a c t i o n , E l s e v i e r
Publishing
Company, Amsterdam, 1967.
13. V l a s t a r a s , A.S. and W i n k l e r , C.A., Can. J. Chem., 1967, 45, 2837.
14. A b e l , E. and Schmid, H., Z. Physik.
Chem., 1928, 132, 55.
15. Schmid, G. and Bahr, G., Z. Physik.
Chem., 1964, 41, 8.
16. U s u b i l l a g a , A.N., PhD T h e s i s , U n i v e r s i t y o f I l l i n o i s ,
17. C a r r i n g t o n , T. and Davidson, N., J. Phys.
U.S.A., 1962.
Chem., 1953, 57, 418.
18. H i s a t s u n e , I.C., J. Phys. Chem., 1961, 65, 2249.
19. T e c h n i c o n A u t o - A n a l y z e r I I , I n d u s t r i a l method No. 230-72A/Tentative 1974.
20. H i k i t a , H. A s a i , S. and Takatsuka, T., Chem. Eng. J., 1972, 4, 31.
21. Van de Vusse, J.G., Chem. Eng. Sei., 1966, 21, 631.
22. Dekker, W.A., PhD T h e s i s , D e l f t , 1958.
23. F o r s y t h e , W.R. and Giauque,
W.F., J. Am. Chem. Soc. , 1942, 64, 48.
24. R e i d , R.C., P r a u s n i t z , J.M. and Sherwood, T.K., The P r o p e r t i e s o f Gases and
L i q u i d s , M c G r a w - H i l l , 1977.
25. A b e l , E. and Schmid, H., Z. Physik.
Chem., 1928, 134, 279.
26. A b e l , E., Schmid, H. and Babad, S., Z. Physik.
Chem., 1928, 136, 135.
27. A b e l , E., Schmid, H. and Babad, S., Z. Physik.
Chem., 1928, 136, 419.
28. A b e l , E. , Schmid, H. and Babad, S., Z. Physik.
29. A b e l , E., Schmid, H. and Römer,
30. Axente,
2057.
88
E., Z. Physik.
Chem., 1928, 136, 430.
Chem., 1930, 148, 337.
D., L a c o s t e , G. and Mahenc, J . , J. Inorg.
Nucl.
Chem., 1974, 36,
6. AN ABSORPTION MODEL FOR THE DESIGN OF A DILUTED NITRIC ACID ABSORBER AND
METHODS TO DECREASE THE NO CONTENT IN TAIL GASES
x
6.1
INTRODUCTION
A l t h o u g h many i n v e s t i g a t i o n s can be
found
absorption of n i t r o g e n oxides into n i t r i c
still
not w e l l u n d e r s t o o d
a c i d . T h i s i s m a i n l y due
and NgO^
liquid
i n absorbers
i n the l i t e r a t u r e c o n c e r n i n g
f o r the p r o d u c t i o n o f d i l u t e d
a l l p l a y an important
r o l e i n the a b s o r p t i o n p r o c e s s
nitric
t o NOg.
T h i s o x i d a t i o n i s an u n u s u a l
temperature
In
coefficient
t h i s Chapter
as was
i n both
shown by B o d e n s t e i n
an a b s o r p t i o n model, based
the p r o d u c t i o n o f d i l u t e d n i t r i c
acid.
ABSORPTION MODEL FOR
The
overall reaction
3N0
2
THE
negative
on g e n e r a l c h e m i c a l
absorbers
In a d d i t i o n , v a r i o u s methods o f
gases o f n i t r i c
PRODUCTION OF
acid
DILUTED NITRIC ACID
f o r the a c i d f o r m a t i o n i n the a b s o r p t i o n column can
(N 0 )
2
4
+
H0
%
2
a given composition
2HN0
3
+
be
NO
the maximum a c i d c o n c e n t r a t i o n t h a t can be
v e r y p o o r l y s o l u b l e i n aqueous s o l u t i o n s ,
+
0
2
•*
2N0
(1)
o f the n i t r o u s gases ( C h a p t e r 4 ) . The
where i t r e a c t s w i t h m o l e c u l a r
2N0
reaction
with:
T h i s e q u i l i b r i u m determines
at
can
be b r i e f l y d i s c u s s e d .
6.2
presented
the
[1].
d e c r e a s i n g the c o n c e n t r a t i o n s o f n i t r o g e n o x i d e s i n t a i l
plants w i l l
NO^
i n the gas phase o x i d i z i n g
r e a c t i o n w i t h an apparent
e n g i n e e r i n g c o n s i d e r a t i o n s , i s d e r i v e d f o r the d e s i g n o f i n d u s t r i a l
for
NgO^,
a c i d as w e l l as n i t r o u s a c i d
be formed i n b o t h phases. Oxygen i s n o r m a l l y p r e s e n t
NO
nitric
t o t h e f a c t t h a t v a r i o u s n i t r o g e n o x i d e s NO,
and the gas phase. Furthermore
the
a c i d s o l u t i o n s , the mechanism i s
2
and
NO
obtained
produced
i s t r a n s f e r r e d t o t h e gas
is
phase,
oxygen.
(2)
89
The
r e a c t i o n r a t e o f t h i s o x i d a t i o n can be e x p r e s s e d
-dt-
The
k
•
P
by [ 1 - 5 ] :
(
N0 \
r e a c t i o n r a t e constant
k increases with decreasing
3
)
temperature. The r e v e r s e
r e a c t i o n may be n e g l e c t e d under t h e c o n d i t i o n s p r e v a i l i n g
i n the absorption
column. E s p e c i a l l y a t t h e t o p o f t h e a b s o r p t i o n column where t h e p a r t i a l
pressure
o f NO i s low, t h e r e o x i d a t i o n r a t e o f NO i s s m a l l . As a f i r s t
a p p r o x i m a t i o n t h e o x i d a t i o n o f NO can be c o n s i d e r e d
step i n the absorption
The
t o be t h e r a t e
determining
process.
NO produced has a c o n s i d e r a b l e
i n f l u e n c e on t h e a b s o r p t i o n
rate of
NgO^ i n t o water and d i l u t e d a c i d . T h i s e f f e c t may be due t o t h e f o r m a t i o n o f
N
HNOg and 2 ^ 3
N0
2
NO
The
i
n
t
+
NO
+
N0
nitric
n
e
*=
+
as
P
H 0
2
2
h a s e :
%
2HN0
X
N 0
a c i d formation
2
2
3
(g)
(4)
(g)
(5)
i n t h e gas phase seems t o be o f minor
under t h e c o n d i t i o n s p r e v a i l i n g i n t h e a b s o r p t i o n column
constants
of reactions
concentrations
to
the high
of N 0
2
and HN0
3
solubilities
A first
[ 6 , 7 ] . The e q u i l i b r i u m
(4) and (5) were g i v e n i n C h a p t e r 4 ( T a b l e 1 ) . The
2
a r e s m a l l under e q u i l i b r i u m c o n d i t i o n s but due
and t h e r a p i d e s t a b l i s h m e n t
t r a n s f e r o f NgO^ and HN0
neglected.
2
o f these
from t h e gas phase t o t h e l i q u i d phase can n o t be
an a b s o r p t i o n model which i s s c h e m a t i c a l l y p r e s e n t e d
i n F i g . 1. In t h e
from t h e gas phase
t h e l i q u i d phase. In t h i s work a model i s s e t up i n which t h e NgOg t r a n s f e r
and
t h e HNOg t r a n s f e r a r e b o t h taken i n t o account. The model i s based on:
a) D i f f u s i o n o f N 0
and NgO^ from t h e gas b u l k
2
k
=23
N0
=
N 0
2
2
b) T r a n s f e r o f
liquid
N
2
0
4
RT
4
N
0
2
to the g a s - l i q u i d interface
2k
g,NO
J
90
was
[ 8 ] . More r e c e n t l y H o f t i j z e r and Kwanten [7] proposed
m a t h e m a t i c a l d e s c r i p t i o n they n e g l e c t t h e t r a n s f e r o f N „ 0
The
e q u i l i b r i a , the
attempt t o d e s c r i b e such a complex a b s o r p t i o n p r o c e s s
done by Andrew and Hanson
to
importance
4'
N
2°3
_
(P
N0
a
n
d
N
2
H N 0
- P
) +
N0 .>
2>
2
f
r
o
m
t
n
e
e> 2°4
—
(P
- P
)
RT
2 4,
2°4,i
N
g a s - l i q u i d i n t e r f a c e to the
phase.
r e a c t s w i t h water t o produce n i t r i c
a c i d and n i t r o u s a c i d , as was
(6)
3HN0
-HN0 >2NO»
2
3
H 0
2
LIQUID - BULK
Fig.
1 Absorption
discussed
model according
i n Chapter 4. A c c o r d i n g
to Hoftijzer
and Kwanten [?].
to Corriveau
[9] t h e N^O^ r e a c t s r a p i d l y w i t h
water i n the l i q u i d phase t o produce n i t r o u s a c i d .
N
2HN0„
«2°
2°3
(7)
T h i s r e a c t i o n may be c o n s i d e r e d
t o be a r a p i d pseudo f i r s t
order
r e a c t i o n . The
n i t r o u s a c i d formed i n t h e gas phase d i s s o l v e s p h y s i c a l l y i n t o t h e s o l u t i o n .
The a b s o r p t i o n
J
N0
°2
=
2
J
N
r a t e can then be w r i t t e n as:
N„0. =
"2"4
2 P
+
N"2"4,i
4 P
"2"4
HNO„
+
/«HNO
2, i
2
Vo„
/ V o
2 3,i
2 3
A
(8)
In a b s o r p t i o n
columns f o r the p r o d u c t i o n
o f d i l u t e d a c i d the l i q u i d
phase may
be assumed t o be n e a r l y s a t u r a t e d w i t h NO and under t h e s e c o n d i t i o n s t h e
reverse
H
r e a c t i o n becomes i m p o r t a n t . The a b s o r p t i o n
=
2
v
"
4
2
\ '
p
» o
2
A
v « i f o
(
1
•
(
^
r a t e i s then r e p r e s e n t e d
)
2
/
3
3.
i H
HN0 ^P
2
P
4
P
P
• N0, i • N 0 ^ . • H 0 ,
2
k„(l-(-^)
by:
)
1/6
)
2
kD„
8. 1/3
(1 - ( ~ )
)
K
(9)
91
Values o f the e q u i l i b r i u m constants K
p
, Kp , K
p
and Kp
were g i v e n i n Chapter
4 ( T a b l e 1 ) . The v a l u e o f p\ i s d e f i n e d a s :
P
P
NO,i
N 0
2,i
Note t h a t e q u a t i o n
N0 /N 0
2
2
(10)
3
(9) may o n l y be a p p l i e d t o d i l u t e d a c i d . The a b s o r p t i o n o f
i n t o c o n c e n t r a t e d a c i d s h o u l d be c o n s i d e r e d t o be p u r e l y p h y s i c a l .
4
c) T r a n s f e r o f NO from t h e g a s - l i q u i d
J
In
N0
=
J
3 N0
=
2
~wT^
( P
N0,i
P
" N0
i n t e r f a c e t o the gas-bulk.
}
t h e gas b u l k t h e r e o x i d a t i o n o f NO w i t h oxygen t a k e s p l a c e [ 1 - 5 ] .
It may be assumed t h a t t h e gas phase i s s a t u r a t e d w i t h water. The water vapour
p r e s s u r e as a f u n c t i o n o f t h e a c i d s t r e n g t h can be taken
HN0,-H 0 measured by Vandoni and Laudy
from
t h e b i n a r y system
[10] . The v a l u e s o f H
f u n c t i o n o f t h e a c i d s t r e n g t h were g i v e n i n Chapter
be found
•ƒ kD„ as a
4. L i t t l e
i n f o r m a t i o n can
i n the l i t e r a t u r e concerning the values of H
_ and H
%ƒ kD„.
HN0
N 0 V
and Neusser [12] determined t h e HN0 vapour p r e s s u r e above n i t r o u s a c i d
H
m i n
3
n
2
d
H
H
2
n
Abel
3
2
s o l u t i o n s . V a l u e s o f H_„ „
were c a l c u l a t e d from
t
Hi\L)
Theobald
The
t h e e q u i l i b r i u m measurements o f
2
[13] c o n c e r n i n g t h e heterogeneous system n i t r i c
a c i d / n i t r o u s gases.
partial
p r e s s u r e s o f HN0_ i n t h e gas phase were c a l c u l a t e d from P
, P„„ ,
£
NO
NO2
P _ and t h e e q u i l i b r i u m c o n s t a n t K_ . The v a l u e s o f H „ .
as a f u n c t i o n o f t h e
n 0
P4
HNO2
a c i d s t r e n g t h thus found a r e about t w i c e t h e v a l u e measured by A b e l and Neusser
tI
TI
2
[12]
(see F i g . 2 ) . T h i s discrepancy r e q u i r e s f u r t h e r i n v e s t i g a t i o n . Values of
H„ „ U kD. were c a l c u l a t e d from
2°3
N
V
t h e a b s o r p t i o n measurements o f H o f m e i s t e r and
l
Kohlhaas
[14]. The r e s u l t s a r e g i v e n i n T a b l e 1. C o r r i v e a u [9] used a l a b o r a t o r y
a b s o r b e r c o n t a i n i n g f i v e wetted spheres t o i n v e s t i g a t e t h e a b s o r p t i o n r a t e o f
N„0„ i n t o water. From T a b l e 1 i t can be seen t h a t t h e v a l u e o f H „ _ \/ kD„
*s 3
"2O3*
"
r e p o r t e d by C o r r i v e a u [9] i s much lower than t h a t o f H o f m e i s t e r and Kohlhaas
[14].
No i n f o r m a t i o n was found
i n the l i t e r a t u r e concerning the i n f l u e n c e of
the n i t r i c
a c i d s t r e n g t h on t h e v a l u e s o f H
\l kD.. As a f i r s t a p p r o x i m a t i o n
N 0
N 2 O 3 »
*
t h i s i n f l u e n c e may be c a l c u l a t e d from t h e d e c r e a s e o f H „ _ w i t h i n c r e a s i n g
N 0
i o n i c s t r e n g t h . I t i s c l e a r t h a t more work i s needed t o o b t a i n r e l i a b l e d a t a
2
3
2
92
3
liquid
method o f
2
kmol/m
Hofmeister,
Corriveau
Table
1
Kohlhaas
[14]
[9]
5 x
1.58
Comparison
water
of
literature
x
10~
water
laminar j e t
10~
water
wetted
data
concerning
(see a l s o H o f t i j z e r
a) h i g h p a r t i a l
b) low
the
absorption
spheres
of N^O^
into
at 25.0°C
From the proposed model i t can be c o n c l u d e d
i n c r e a s e d by
measurement
.s.bar
t h a t the a b s o r p t i o n r a t e w i l l
be
and Kwanten [ 7 ] ) :
p r e s s u r e s o f the n i t r o g e n o x i d e s ;
temperatures
i n both
phases;
c) h i g h degree o f o x i d a t i o n o f the n i t r o g e n o x i d e s ;
d) l a r g e g a s - l i q u i d
interfacial
area.
93
F o r a s i m p l i f i e d mathematical
nitric
6.3
model o f the a b s o r p t i o n column i n the
a c i d p r o d u c t i o n the r e a d e r i s r e f e r r e d t o the l i t e r a t u r e
METHODS TO
DECREASE THE
NO
diluted
[16,17],
CONTENT IN TAIL GASES OF NITRIC ACID PLANTS
x
Tail
gases o f n i t r i c
o x i d e s and
tail
a c i d p l a n t s c o n t a i n between 100
and
3000 ppm
e f f e c t on the ecosystem an e f f e c t i v e removal o f N0^
the e m i s s i o n l e v e l
i s 1.5
kg NO
i s n e c e s s a r y . At
( c a l c u l a t e d as N0 )
£t
o
X
p l a n t s a l e v e l o f 400
ppm
will
c o u n t r y t o c o u n t r y . F o r new
the l o c a l
n i t r o g e n oxides content
present
per ton a c i d f o r
p l a n t s i n the U n i t e d S t a t e s . T h i s i s e q u i v a l e n t t o about 200
depending on
of nitrogen
gases o f some v e r y o l d p l a n t s even more. Because o f i t s harmful
ppm.
For
new
existing
be r e q u i r e d . In Europe t h e l i m i t v a r i e s
p l a n t s a l i m i t o f 400-500 ppm
may
from
be assumed,
s i t u a t i o n . P r e s e n t l y s e v e r a l methods t o d e c r e a s e
i n these t a i l
the
gases a r e known i n the l i t e r a t u r e [15,18].
T a b l e 2 g i v e s a r e v i e w o f the most important
methods.
Extended a b s o r p t i o n
(water s c r u b b i n g )
•— Wet
.H 0
process
2
2
scrubbing
I—HN0, s c r u b b i n g
[19,20,21]
[22]
[23-37]
NO
abatement
—
Dry
process
Adsorption
[41-43]
. N o n - s e l e c t i v e r e d u c t i o n [44-49]
S e l e c t i v e r e d u c t i o n [15,44,45,46,
50,51]
Table
2
Methods
plants
94
to decrease
the NO^ content
in tail
gases of nitric
acid
6.3.1
Wet P r o c e s s e s
6.3.1.1 Extended a b s o r p t i o n
Increasing
tail
[19,20,21]
of the absorption
gases o f n i t r i c
oxides i n these t a i l
volume d e c r e a s e t h e n i t r o g e n
oxides content i n
a c i d p l a n t s . The degree o f o x i d a t i o n o f t h e n i t r o g e n
gases i s about 0.5. In t h e l i q u i d phase m a i n l y HN0 i s
2
produced.
NO
+
N0
o
+
¿
The
2HN0 (£)
•>
2
HN0
(12)
^
HNOg may be decomposed
3HN0
The
HO
2
+
3
2N0
NO produced i s v e r y
partially:
+
H 0
(13)
2
poorly
s o l u b l e i n aqueous s o l u t i o n s , hence i t i s
t r a n s f e r r e d t o t h e gas phase where i t r e a c t s w i t h oxygen. At the t o p o f t h e
a b s o r b e r t h e r e o x i d a t i o n r a t e o f NO w i l l be very
o f NO i s s m a l l . T h i s
required
for a high
implies that
a relatively
slow as t h e p a r t i a l
large absorption
degree o f o x i d a t i o n . The extended a b s o r p t i o n
r a t h e r o f t e n a p p l i e d i n new p l a n t s . By working a t a p r e s s u r e
a b s o r b e r and by c o o l i n g t h e a b s o r p t i o n
tail
method i s now
o f 12 b a r i n t h e
system w i t h water t h e N 0
x
content i n the
gas may be reduced t o 200 ppm. Even i n e x i s t i n g p l a n t s extended
can be a p p l i e d , p r o v i d e d
(Fig.
that the pressure
r e s u l t i n g weak a c i d becomes t h e f e e d
absorption
i n t h e main a b s o r b e r i s n o t t o o low
3 ) . The extended a b s o r b e r i s p o s i t i o n e d down stream r e l a t i v e l y
e x i s t i n g a b s o r b e r . Condensate i s c o o l e d
pressure
volume i s
and e n t e r s
t o an
t h e extended a b s o r b e r . The
f o r t h e main a b s o r b e r ( F i g . 3 ) . I f t h e
degree o f t h e o x i d a t i o n o f NO i s low such a p r o c e s s i s not e c o n o m i c a l due t o
the
large absorption
6.3.1.2 H O
A 2
The
tail
volume
scrubbing
p r o c e s s [22]
gas o f t h e a c i d a b s o r b e r
following overall reactions
NO
2N0
required.
+
+
N0
2
3H 0
+
2H 0
2
2
•* 2HN0
(A) i s s c r u b b e d w i t h H 0
2
2
(see F i g . 4 ) . The
occur:
2HN0
+
3
2H 0
+
HgO
(14)
(15)
95
o
NH
2
3
1
HpO
A
2 .r
0°/. H N O ,
Fig.
3
Simplified
flow
extended
absorption.
A: converter;
sheet for the production
B: cooler/condenser;
1: feed to converter;
ppm N0 ;
x
nitric
x
to
C: absorber;
6: water;
nitric
acid
D: extended
2: 10% NO; 3: NO oxidized
5: 200-400 ppm N0 ;
acid
of diluted
with
absorber.
to NO^; 4: 2000-600
7: weak nitric
acid;
8: 60%
bleacher.
HoO
u
2
2
H 02
N0
2
60 % HNO3
Fig.
4
Simplified
A: acid
flow
absorber;
1: feed to acid
B; U^O^
absorber;
4: weak nitric
acid with
H0;
to acid
0
96
sheet of the N^O^ scrubbing
0
7: water
process
[22],
scrubber.
2: 2000-4000 ppm N0 ;
x
unreaated
absorber;
3: 200-400 ppm N0 ;
x
H^O^; 5: recycling
8: 60% nitric
acid
E^O^; 6:
to
fresh
bleacher.
The
tail
gas treatment
t a k e s p l a c e at ambient temperatures
and
at a p r e s s u r e
e q u a l t o t h a t i n the a c i d a b s o r b e r . The weak a c i d l e a v i n g the s c r u b b e r
c o n t a i n i n g some u n r e a c t e d H O
¿1
r e a c t i o n equations
(14) and
e n t e r s the top o f the a c i d a b s o r b e r .
experiments
H„0
solutions.
was
(15)
i t can be seen
t h a t t h i s p r o c e s s overcomes the
i n the extended
a b s o r p t i o n method.
were c a r r i e d out t o i n v e s t i g a t e the a b s o r p t i o n r a t e o f NO
I t was
found t h a t H O
into
decomposed r a t h e r r a p i d l y as soon as i t
a c t i v a t e d by r e a c t i o n . The m o l e c u l a r oxygen produced
liquid
From the
2
c h e m i c a l and p h y s i c a l l i m i t a t i o n s which e x i s t
Own
and
diffused
from
the
phase t o the gas phase. In the gas phase the oxygen i s r a t h e r i n -
e f f e c t i v e . The
loss of H 0
2
by d e c o m p o s i t i o n
2
i s a s e r i o u s disadvantage
of
this
process.
6.3.1.3 N i t r i c
acid scrubbing
a) D i l u t e d n i t r i c
[23-40]
a c i d scrubbing
[23-29,40]
The Humphreys/Glasgow and Bolme p r o c e s s uses a 30% n i t r i c
scrubbing l i q u i d
stream
[23,24].
from t h e a c i d a b s o r b e r
which N0„ and NO
NO
+
2N0
The
A simplified
N0
+
2
HNOg
H0
+
2
+
H0
2HN0
*
2
N0^
enters scrubber
at ambient temperature
scrubber
(A) i n
(16)
3HN0
(17)
2
c o n t a i n s 150-250 ppm
o
NO
i s r e g e n e r a t e d by h e a t i n g t o 70 C and
c o o l e d and r e c y c l e d t o the a c i d a b s o r b e r . The
can a l s o be
absorbed
are known i n l i t e r a t u r e
b) C o n c e n t r a t e d
S0LN0X n i t r i c
and
. The
nitric
CW
bar).
acid leaving
the
s t r i p p i n g w i t h a i r o r steam.
N0^
produced
is
main advantage o f t h i s p r o c e s s i s
recovered. S e v e r a l v a r i a n t s of t h i s
process
[25-29].
nitric
acid
scrubbing
[30-37]
a c i d p r o c e s s o f Ugine Kuhlmann produces weak a c i d o f 60-63%
and c o n c e n t r a t e d n i t r i c
a c i d o f 80%
[30,31,32], An
SOLNOX-process i s the d i s s o l u t i o n o f NgO^
dissolution
gas
2
Under t h e s e c o n d i t i o n s the n i t r o u s a c i d i s decomposed. The
t h a t NO
as
according to
p r e s s u r e i n the s c r u b b e r i s about the same as i n the a c i d a b s o r b e r
A f t e r s c r u b b i n g the gas
The
acid solution
i s g i v e n i n F i g . 5. The
c o n t a i n i n g 2600 ppm
can be absorbed
+
flow sheet
important
i s a l s o the method f o r c l e a n i n g the t a i l
sheet
i s presented
first
e n t e r s a precondenser
s t e p i n the
into concentrated n i t r i c
i n F i g . 6. The combustion gas
acid;
gas. A s i m p l i f i e d
l e a v i n g the c o n v e r t e r
(A). In the precondenser
the gas
this
flow
(1)
i s cooled with
97
3
1%
Fig.
5
Simplified
flow
sheet
of
the
Humphreys/Glasgow
and
Bolme process
[23,
24] .
A: n i t r i c
1:
tail
tail
acid
gas
gas
recovered
scrubber;
of acid
(200 ppm
NO^
acid
from
NO^);
4: 30%
column.
2: 30% n i t r i c
n i t r i c acid
absober;
acid
solution;
containing
6: stream
or air
c o o l e d weak a c i d . The
the l i q u i d
c o o l e d gas
circuit
a r e both c o o l e d t o 0°C w i t h c o l d b r i n e . In t h i s way
a c o n c e n t r a t i o n o f about 62-63%. The
d i s s o l v e d i n t o a 80% n i t r i c
p r e s s u r e o f about 8 b a r . The
% o f NgO^
98
acid
gas
a c i d . The
acid solution
l e a v i n g the t o p o f the a b s o r b e r
lowered
c o l d , dry
2
o f -10
t o 200
concentrated n i t r i c
ppm
the
leaving
(C) i n which N 0 / N 0
o
s o l u t i o n at a temperature
T h i s can be e a s i l y
water o r d i l u t e d n i t r i c
nitric
nitric
o x i d i z e d gas then e n t e r s a p h y s i c a l a b s o r b e r
NOg.
5:
and a p o r t i o n o f the c o o l e d weak
the c o - c u r r e n t condenser has
ppm
acid;
regeneration
pass t o the c o - c u r r e n t condenser (B) where
the gas phase. The
about 600
treated
nitrous
for
water vapour i s removed from
fully
3:
liquid.
the precondenser
the gas and
absorber;
to main acid
of scrubbing
circulating
B: regeneration
2
4
and
is
t o 0 C and
a
(5) c o n t a i n s
by s c r u b b i n g i t w i t h
a c i d c o n t a i n i n g 10-30
e n t e r s r e a c t o r (D). In the r e a c t o r t h e d i s s o l v e d NgO^ i s c o n v e r t e d
o
o
a c i d w i t h water and a i r at 60 -80 C and a p r e s s u r e o f 8 b a r .
wt
to
N
2°4
+
H
+
2°
i0
2
"*
Under t h e s e c o n d i t i o n s
p a r t o f the
reactor
a l s o Chapter 1).
Fig.
6
column; D:
i s produced
[38,39]. The
partially
SOLNOX p r o c e s s can
B: co-current
from converter
condenser;
to physical
80% nitric
strong
a c i d i n the
lower
r e c y c l e d t o the p h y s i c a l a b s o r b e r .
be
found i n the
condenser;
literature
[33-37]
[30],
C: physical
acid;
80% nitric
acid;
6: 60% nitric
8: gas containing
11: unbleached
13: bleached
to precondenser;
3: weak nitric
absorber;
acid containing
bleacher;
(18)
3
absorption
reactor.
1: gas stream
nitric
0
flow sheet of the SOLNOX process
A: precondenser;
stream
N
i s b l e a c h e d and
(see
current
H
no NO
P r o c e s s e s s i m i l a r t o the
Simplified
2
4: cooled
acid;
weak acid;
7: bleached
NO^ to physical
10-30% by weight
60% nitric
2: gas stream
acid
N^Q^S
absorber;
10: 60% nitric
to reactor;
to co5: NO^
and
gas
cooled
9: 80%
acid
12: air to
to
reactor;
acid.
99
6.3.2 Dry p r o c e s s e s
6.3.2.1 A d s o r p t i o n
NO
x
[41-43]
can be removed and r e c o v e r e d from n i t r i c
a d s o r p t i o n on m o l e c u l a r
capacity f o r N0
2
a t ambient temperatures,
nitric
2
gas streams by f i x e d - b e d
s i e v e s . M o l e c u l a r s i e v e s show a h i g h a d s o r p t i o n
v e r y low. In t h e p r e s e n c e
o x i d a t i o n o f NO t o N 0
acid t a i l
but t h e a d s o r p t i o n c a p a c i t y f o r NO i s
o f oxygen t h e m o l e c u l a r s i e v e s can c a t a l y z e t h e
which i s adsorbed
on t h e m o l e c u l a r
sieves. T a i l
a c i d p l a n t s c o n t a i n water vapour and water vapour w i l l
T h i s decreases
gases o f
f i r s t be adsorbed.
t h e a d s o r p t i o n c a p a c i t y f o r NOg. An e m i s s i o n l e v e l o f 50 ppm N 0
can be o b t a i n e d . The adsorbed
N0
2
i s periodically
desorbed
and r e c y c l e d t o t h e
a c i d a d s o r b e r . The d e s o r p t i o n p r o c e s s t a k e s p l a c e a t a temperature
o
150-250 C. A s i m p l i f i e d flow sheet i s p r e s e n t e d i n F i g . 7.
1
o f about
3
4
2
Fig.
7
Adsorption
acid
process
for the removal
of NO^ from
tail
gases
of n i t r i c
plants.
A: fixed
bed adsorption
column;
B: regeneration
of the
adsorption
column.
I: tail
gas
gas from n i t r i c
containing
acid
absorber
50 ppm N0^; 3: recovered
4: gas for regeneration
Non-selective reduction of n i t r o g e n oxides
100
( C H , CO, Hg, naphta,
4
N0^ to n i t r i c
of the adsorption
6.3.2.2 N o n - s e l e c t i v e r e d u c t i o n p r o c e s s e s
a r e d u c i n g agent
(2000 ppm NO^); 2: treated
acid
tail
absorber;
column.
[44-49]
i s c h a r a c t e r i z e d by t h e r e a c t i o n o f
etc.) with N0
x
and oxygen i n t h e
x
p r e s e n c e o f a c a t a l y s t . Noble m e t a l c a t a l y s t s are used based on P t , Pd and
d e p o s i t e d on a s u i t a b l e i n e r t
if
carrier.
a l l t h e oxygen i s removed. T a i l
In t h i s p r o c e s s N 0
gases o f n i t r i c
can o n l y be
i s needed f o r the r e d u c t i o n o f NO^.
r i s e o f the t a i l
gas more energy
e x p a n s i o n t u r b i n e . Van den B l e e k and Van den Berg
e x p l a i n i n g why
be b r i e f l y
3%
content. T h i s implies
x
heat e v o l v e d by r e a c t i o n can be r e c o v e r e d i n a waste heat b o i l e r .
due t o the temperature
reduced
a c i d p l a n t s c o n t a i n about
oxygen which i s an orde o f magnitude h i g h e r than the N 0
t h a t a l a r g e amount o f r e d u c i n g agent
x
Rh,
The
Furthermore,
can be r e c o v e r e d at the
[46] put forward a h y p o t h e s i s
t h e s e p r o c e s s e s are n o n - s e l e c t i v e r e l a t i v e t o oxygen. T h i s w i l l
reviewed. The
r e a c t i o n s o c c u r r e d can be p r e s e n t e d as
follows:
cat
N0
2
NO
in
+
Red
-*
NO
+
Red
cat
•*
N
N0
which Red
its
+
Red 0
(19)
+
Red 0
(20)
2
2
and Red 0 r e p r e s e n t a r e d u c i n g agent
(CH^, H ,
2
o x i d a t i o n p r o d u c t , r e s p e c t i v e l y . A c c o r d i n g t o Van
Berg
[46] , N 0
2
main r e a s o n why
NO
The NO
+
0
2
produced
i s easily
NO
reduced t o NO
i s not reduced
cat
•+
N0
selectively
naphtha)
den B l e e k and Van
and
den
(20) i s very slow.
i s due t o the
The
reaction
(21)
2
by r e a c t i o n
oxygen. In t h i s way
while reaction
CO,
(19) w i l l be e a s i l y
and q u i c k l y o x i d i z e d w i t h
the r e d u c t i o n c y c l e has t o s t a r t
a g a i n consuming
another
q u a n t i t y o f r e d u c i n g agent. Only when a l l o f the oxygen i s consumed r e a c t i o n
(20) becomes i m p o r t a n t .
6.3.2.3 S e l e c t i v e r e d u c t i o n p r o c e s s e s
[15,44,45,46,50,51]
A s e l e c t i v e r e d u c t i o n of n i t r o g e n oxides i n t a i l
p o s s i b l e w i t h NH^
gases o f n i t r i c
acid plants i s
as a r e d u c i n g agent. T h i s saves a l a r g e amount o f r e d u c i n g
agent. The r e d u c t i o n can be c a r r i e d out i n a f i x e d bed at a temperature o f
o
o
200 -500 C u s i n g a c a t a l y s t based on P t , Pd, Rh o r metal o x i d e s such as V„0_,
z 5
Fe 0 , Cr 0
and CuO [51]. In t h i s way the NO
c o n t e n t i n t h e s e t a i l gases can
Z
o
Z
o
X
be e a s i l y d e c r e a s e d t o 200 ppm.
result
Van
is
The
temperature
rise
i n the t a i l
gas
as a
o f the heat e v o l v e d from the r e d u c t i o n r e a c t i o n s i s s m a l l (about 2 0 ° C ) .
den B l e e k and Van
den Berg
[46] p o s t u l a t e d why
s e l e c t i v e r e l a t i v e t o oxygen. The
a r e d u c t i o n of N0
x
with
NH^
f o l l o w i n g r e a c t i o n s are considered i n
t h e i r hypothesis:
101
N0
2
+
NH
3
+
NH
3
cat
-
2/3
"
"NH N0
2/3
"
4
cat
NO
+
cat N 0
->
^
2
"NH N0
+
^0
(21)
c a t NgO
4
+
+
H
R
2
(
°
2
2
)
2
On
the c a t a l y s t
s u r f a c e they assume t h e f o r m a t i o n o f n i t r a t e o r n i t r a t e - l i k e
complexes as i n t e r m e d i a t e p r o d u c t s . These n i t r a t e o r n i t r a t e - l i k e
decompose t o N
and N O . In t h i s way no NO i s produced,
complexes
and which o f c o u r s e
i m p l i e s t h a t t h e r e o x i d a t i o n o f NO a c c o r d i n g t o r e a c t i o n
(20) can n o t t a k e
place.
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Du Pont
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Shneerson,
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Buck, B.J. and Matthews, W.G., E n v i r o n . Symp. P r o c e e d i n g s , Washington, 1976,
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43. J o i t h e , W., B e l l , A.T. and Lynn, S., Ind. Eng. Chem. Process.
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Oxides,
The C a t a l y t i c C h e m i s t r y
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Plenum P r e s s , New York, 1975.
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F r e i t a g , W. and P a c k b i e r , M.W.,
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S e a r l e s , R.A., Paper p r e s e n t e d a t t h e 2nd I n t e r n a t i o n a l C o n f e r e n c e
on t h e
C o n t r o l o f Gaseous S u l p h u r and N i t r o g e n Compound E m i s s i o n , U n i v e r s i t y o f
S a l f o r d , England, 1976.
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1979, F e b r u a r y , 117.
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I n t e r n a t i o n a l Symposium on The C o n t r o l o f S u l p h u r and o t h e r Gaseous
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104
A p p e n d i x 1. THE A D D I T I V I T Y OF RESISTANCES FOR MASS TRANSFER IN A WETTED WALL
COLUMN
1. INTRODUCTION AND GENERAL THEORY
In c h e m i c a l e n g i n e e r i n g d e s i g n t h e a d d i t i v i t y o f i n d i v i d u a l mass t r a n s f e r
resistances
derived
for gas-liquid
from
1
K
k
og
originally
1
mk.
(1)
I
g
It s h o u l d be n o t e d
steady
systems i s o f t e n a p p l i e d , which was
the two-film theory:
t h a t t h e use o f t h i s r u l e
s t a t e t r a n s f e r a t a l l times
i s based
on t h e assumption
i n both phases and e q u a t i o n
of
(1) w i l l
hold
t r u e i f t h e f o l l o w i n g two c o n d i t i o n s a r e met:
1. The d i s t r i b u t i o n c o e f f i c i e n t m must be a c o n s t a n t o r known as a
f u n c t i o n o f t h e t r a n s f e r r e d component
i n the l i q u i d
phase.
2. No o t h e r r e s i s t a n c e may be p r e s e n t o t h e r than t h o s e e x p r e s s e d by
k
1/mk^
The
and V g -
gas phase mass t r a n s f e r c o e f f i c i e n t
and t h e l i q u i d phase mass t r a n s f e r
c o e f f i c i e n t may vary w i t h t h e c o n t a c t time o f renewable s u r f a c e s o r may
over a f i n i t e
surface. Equation
the mass t r a n s f e r
K
og,local
In p r a c t i c e
(1) may then be a p p l i e d
k
g,local
(2)
mk„ ,
I,local
s i n g l e phase mass t r a n s f e r
I f , f o r i n s t a n c e , t h e s i n g l e mass t r a n s f e r c o e f f i c i e n t s
the c o n t a c t time o f renewable s u r f a c e s we get f o r t h e time average
transfer
values of
coefficients.
i t i s customary t o d e f i n e average
coefficients.
f o r the l o c a l
vary
vary with
mass
coefficients:
T
£
K
k
o &,local
T
d
t
(3)
105
and
T
ƒ"*
k
=
,
g,local
dt
(4)
-
g
The
t r u e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t
(K
&
with:
og,true
) can be
calculated
T
—
ƒ K
,
og,local
o
—
dt
i
—
~
r
+ —
mk„
>
o k ,
_
g,local
~
«
(5)
,
t,local
og,true
T
In
T
c h e m i c a l e n g i n e e r i n g d e s i g n the average o v e r a l l mass t r a n s f e r c o e f f i c i e n t i s ,
however, o f t e n d e r i v e d from the a d d i t i v i t y o f i n d i v i d u a l
average mass t r a n s f e r
resistances:
K
In
1
I T
+
—
—
k
mk„
g
£
og, addition
—
general equation
equation
1
r •
o
k
t
g.local
—
T
1
r —
•
i mk„ ,
,
dt
+
o
(5) i s not e q u a l t o e q u a t i o n
(5) i s e q u a l t o e q u a t i o n
c o n d i t i o n s are
1
1
dt
I,local
T
(6). King
[1] p o i n t e d out t h a t
(6) o n l y i f the f o l l o w i n g a d d i t i o n a l
fulfilled:
3. The mass t r a n s f e r c o e f f i c i e n t s o f the gas phase and the
phase must not
4. The
local
interact.
v a l u e o f mk„/k
A<
gas-liquid
King
[1] found
must be c o n s t a n t
r e v e r s e d the t r u e average o v e r a l l mass t r a n s f e r
( 6 ) . I f , however, h i g h l o c a l
values of k
the
interface.
d e v i a t e under c e r t a i n c o n d i t i o n s by about 20%
equation
at a l l p o i n t s o f
g
t h a t i f h i g h l o c a l v a l u e s o f k^ tend t o c o i n c i d e w i t h low
v a l u e s o f k^ and
may
liquid
local
coefficient
from the v a l u e o b t a i n e d w i t h
v a l u e s o f k^ c o i n c i d e w i t h h i g h
the d e v i a t i o n w i l l be r a t h e r s m a l l . The
local
d e v i a t i o n i s a function of
g
the r a t i o mk„/k , and a maximum appears at mk„/k
=
1.
Both s i n g l e phase mass t r a n s f e r c o e f f i c i e n t s can v a r y i n h e r e n t l y w i t h
same power o f age o r d i s t a n c e a l o n g the i n t e r f a c e ,
all
f o u r c o n d i t i o n s f o r the a d d i t i v i t y o f phase r e s i s t a n c e s are
In
fulfilled.
C h a p t e r 2 s i n g l e phase mass t r a n s f e r c o e f f i c i e n t s were i n v e s t i g a t e d i n a
wetted w a l l column, i n which a c o - c u r r e n t l a m i n a r flow o f a f a l l i n g
and
106
the
and under t h e s e c o n d i t i o n s
a gas
core with a f l a t
liquid
v e l o c i t y p r o f i l e c o u l d be e s t a b l i s h e d . I t was
film
found
that
t h e l i q u i d phase mass t r a n s f e r can be d e s c r i b e d
conditions with the penetration
described
vary
and
theory.
locally
and not w i t h the same power o f age a l o n g
t h i s phenomenon i m p l i e s
considered
described
The gas phase mass t r a n s f e r can be
w i t h t h e s o l u t i o n o f t h e G r a e t z - p r o b l e m . The t r a n s f e r c o e f f i c i e n t s
t o be i n f i n i t e l y
a r e f u l f i l l e d . At l a r g e
t h e gas phase as w e l l as t h e l i q u i d phase may
deep. Both mass t r a n s f e r c o e f f i c i e n t s can be
with the penetration
resistances holds
the g a s - l i q u i d i n t e r f a c e ,
that not a l l f o u r c o n d i t i o n s
Graetz-numbers i t was found t h a t
be
under o u r e x p e r i m e n t a l
theory
and t h e r e f o r e
t h e a d d i t i v i t y o f phase
[1,2].
In t h i s s e c t i o n t h e a d d i t i v i t y o f i n d i v i d u a l phase r e s i s t a n c e s
previously
described
i n the
wetted w a l l column (Chapter 2) i s s t u d i e d . The t r u e
average o v e r a l l mass t r a n s f e r c o e f f i c i e n t
s o l u t i o n and i s compared w i t h t h e v a l u e
i s c a l c u l a t e d from a n u m e r i c a l
obtained
from e q u a t i o n ( 6 ) .
2. RESULTS
In C h a p t e r 2 a wetted w a l l column was developed i n which a c o - c u r r e n t
flow o f a f a l l i n g
liquid
film
and a gas c o r e w i t h a f l a t
be e s t a b l i s h e d . The flow model and t h e c o o r d i n a t e
The
laminar
velocity profile
system a r e g i v e n
i n F i g . 1.
f o l l o w i n g a d d i t i o n a l assumptions have been made f o r t h e a b s o r p t i o n
1. The a b s o r p t i o n
i s purely
of the absorption
may be
could
model:
p h y s i c a l and heat e f f e c t s as a r e s u l t
neglected.
2. A l l t h e r e l e v a n t p h y s i c a l p r o p e r t i e s
remain c o n s t a n t
during the
absorption.
3. D i f f u s i o n i n t h e gas and l i q u i d phase t a k e s p l a c e o n l y i n r a d i a l
direction.
4. At t h e i n t e r f a c e e q u i l i b r i u m e x i s t s between t h e gas and t h e
liquid.
The
d i f f u s i o n p r o c e s s can be d e s c r i b e d
Liquid
phase
by t h e f o l l o w i n g
z
v
(7)
s
1
Gas
phase
v
equations:
s
with the following i n i t i a l
3C
+
(8)
r
and boundary
conditions:
107
h = 0
S
r > R -
C
=
£
C„
(9)
,o
h=0
r < R - 6
h > 0
C
t
r = 0
h>0
The
c
r
h>0
The double
penetration
An
asymptotic
if
the gas
transfer
phase may
gas
the p e n e t r a t i o n
=
and
calculated
= D
m C . = C . .
g,i
fc.,i
model (asymptotic
solution)
a l s o be
equations
considered
phase and
i n the
can
t o be
be
infinitely
i
with
o f i n d i v i d u a l phase r e s i s t a n c e s
can
are
be
(
+
2m^
Hoornstra
1
5
)
D^/TTT
[5] d e s c r i b e d t h e p h y s i c a l a b s o r p t i o n
of c h l o r i n e
model.
solution
(7-14) have been a p p r o x i m a t e d by
[3,4]
between the gas
and
i n the
analytical
concentration
and
the
then s o l v e d by
finite
d i f f e r e n c e s according
a Gaus-Seidel
iteration
i n t e r f a c e f o r h = 0 where no
liquid.
liquid
Therefore
the
s o l u t i o n o f the p e n e t r a t i o n
theory
profiles
first
[ 4 ] . The
zero
the
gas
concentration
solubility
i n the
liquid
i n the
gas
s t e p by means o f
fractional
change o f the t r a n s f e r r e d component i n the gas
for i n i t i a l
to
procedure.
equilibrium exists
concentration
phase were approximated i n t h e
o f the Graetz-number and
108
mass
described
i
A problem a r i s e s at the
calculated
deep. The
then be
with:
Crank-Nicolson
the
(14)
then the t r u e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t
Berg and
phase and
(13)
found at l a r g e Graetz-numbers
l i q u i d phase can
i n t o benzene by means o f t h i s double p e n e t r a t i o n
Equations
6
-tt- -
r = R - 6.
f
2 ^ D /TTT
Numerical
3C
Ç
D
theory.
og,true
den
_ 6
R
f o u r c o n d i t i o n s f o r the a d d i t i v i t y
fulfilled
Van
(12)
Je, o
are:
s o l u t i o n o f these
i n the
(11)
= C „
3C
h>0
K
0
ic
interface conditions
All
=
g
—
(10)
g.o
3C /3r
- + c o
r
= C
g
phase
was
phase as a f u n c t i o n
(see F i g . 2 ) . From t h i s
f i g u r e i t can
liquid
jfiim]
gas
5 f
/
Fig,
Fig.
1
2
Flow model and coordinate
The fractional
system.
concentration
gas phase as a function
of
change of a transferred
TT/GS
for D./D
x,
(
Numerical
solution,
—
component in the
= 0.0001.
g
Double penetration
model).
109
be
seen t h a t t h e a s y m p t o t i c
s o l u t i o n (double p e n e t r a t i o n
model) i s o n l y
valid
at Graetz-number l a r g e r than 100. The t r u e average o v e r a l l mass t r a n s f e r
c o e f f i c i e n t was c a l c u l a t e d from t h e n u m e r i c a l s o l u t i o n by e s t a b l i s h i n g a simple
mass b a l a n c e around t h e wetted w a l l column. Based on t h e l o g a r i t h m i c mean
d r i v i n g f o r c e between t h e i n l e t
C (h))
g
1> (C
g
g,o
K
A
V
and o u t l e t t h e mass b a l a n c e can be w r i t t e n as:
g,o
m
/
og
v g
h)
(16)
'l,o
B,o
In
C (h)
L e
A comparison o f t h e t r u e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t w i t h t h e
average o v e r a l l mass t r a n s f e r c o e f f i c i e n t
i n d i v i d u a l phase r e s i s t a n c e s
c a l c u l a t e d from t h e a d d i t i v i t y o f
i s o n l y p o s s i b l e i f a l l mass t r a n s f e r c o e f f i c i e n t s
are based on t h e same d r i v i n g f o r c e . F o r t h e l i q u i d phase i t was found
under c e r t a i n c o n d i t i o n s
theory.
t h e mass t r a n s f e r can be d e s c r i b e d
that
by t h e p e n e t r a t i o n
The average Sherwood number based on t h e l o g a r i t h m i c mean d r i v i n g f o r c e
between t h e i n l e t
and t h e o u t l e t can be w r i t t e n
2
In
Fo„
CO
O
Z
e
,2
, V n
«- n=l
X
p
(
"
as [ 6 ] :
(17)
2
Fo )
n
t
11
i n which
(18)
The
eigenvalues
0
m , t h e c o e f f i c i e n t s A and t h e f u n c t i o n s F a r e g i v e n i n
n
n
n
T a b l e 1.
In C h a p t e r 2 i t was found t h a t t h e average gas phase Sherwood-number can be
described
by the s o l u t i o n o f t h e G r a e t z - p r o b l e m .
2
Sh
= - —
g
The
V
values
The
In I
n=l
^
a
2
n
of a are given
n
(
a
IT
\
(19)
exp
Gz
i n T a b l e 1 o f C h a p t e r 2.
d e v i a t i o n o f t h e t r u e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t
compared t o t h e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t
a d d i t i v i t y o f i n d i v i d u a l phase r e s i s t a n c e w i t h e q u a t i o n s
obtained
c a l c u l a t e d f o r mk„/k a 1 at which t h e d e v i a t i o n i s pronounced.
y» g
110
from t h e
( 1 9 ) , (17) and (6) was
For
initial
plotted
in
z e r o gas c o n c e n t r a t i o n
the c a l c u l a t i o n o f t h e t r u e
about 5 °/oo t h i s d e v i a t i o n
c a l c u l a t i o n of the l i q u i d
logarithmic
be
average o v e r a l l mass t r a n s f e r
i s that
k„
This
=
c o e f f i c i e n t , based on t h e
the t h i c k n e s s o f the l i q u i d
to define
an average l i q u i d
2V —
(20)
been based on an a r i t h m e t i c a l
from t h e p e n e t r a t i o n
driving
average o v e r a l l mass t r a n s f e r
(19) and e q u a t i o n
(20).
t h e o r y and i t has
force.
c o e f f i c i e n t following
o f t h e a d d i t i v i t y o f t h e i n d i v i d u a l phase r e s i s t a n c e s
equation
f i l m must
phase mass
follows:
e q u a t i o n can be d i r e c t l y d e r i v e d
The
coefficient i s
The d i s a d v a n t a g e i n t h e
phase mass t r a n s f e r
mean d r i v i n g f o r c e ,
c o e f f i c i e n t , as
(see F i g . 3 ) . Because t h e i n a c c u r a n c y
i s not r e l e v a n t .
known. T h e r e f o r e i t i s u s e f u l
transfer
i n t h e l i q u i d phase t h e d e v i a t i o n i s
o f t h e TT/Gz-number
as a f u n c t i o n
from the a p p l i c a t i o n
was c a l c u l a t e d
I t s h o u l d be n o t e d t h a t
from
i n t h i s case the
s i n g l e phase mass t r a n s f e r c o e f f i c i e n t s have n o t been based on t h e same d r i v i n g
f o r c e . The l i q u i d
driving
phase mass t r a n s f e r
f o r c e , w h i l e t h e gas phase mass t r a n s f e r
logarithmic-mean d r i v i n g
The
c o e f f i c i e n t i s based on an a r i t h m e t i c a l
true
force.
average o v e r a l l mass t r a n s f e r
n u m e r i c a l s o l u t i o n , was t h e r e f o r e
a logarithmic-mean d r i v i n g force
coefficient, calculated
defined as:
(C
In
g,o
of t h i s true
individual
phase r e s i s t a n c e s
2 1 and f o r z e r o i n l e t
was p l o t t e d
measured c o n d i t i o n s ,
there
a r e added, l e a d i n g
from t h e a d d i t i v i t y o f
as a f u n c t i o n
gas c o n c e n t r a t i o n
ir/Gz-numbers. T h i s
unequal d r i v i n g f o r c e s
_
c o e f f i c i e n t obtained
From t h i s f i g u r e i t can be seen t h a t
creases at increasing
m
average o v e r a l l mass t r a n s f e r c o e f f i c i e n t w i t h t h e
average o v e r a l l mass t r a n s f e r
mk./k
M >
- ^ )
m
g
deviation
with the
based on a c o m b i n a t i o n o f an a r i t h m e t i c a l and
*g «g,o .î«»,.ï
>- " V ° - V
g
og
The
c o e f f i c i e n t i s based on a
o f t h e ir/Gz-number f o r
i n the l i q u i d
phase ( F i g . 4 ) .
i s a positive deviation,
deviation
which i n -
i s caused by t h e f a c t
that
to a systematic e r r o r . Within the
however, t h i s d e v i a t i o n
i s s m a l l enough t o be n e g l e c t e d .
Ill
1-05
1 00
0 95
0 0 5
0-1
TI
Gz
Fig.
3
The deviation
of the additivity
mass transfer
0.0001; Fo
in a wetted wall
of individual
phase resistances
column as a function
for
of tt/Gz (D^/V^ =
= 0.01).
i
(Average
overall
driving
force.)
mass transfer
coefficient
based on a
logarithmic-mean
105
100
o
0
95
_L_
0 0 5
0-1
Tt
Gz
Fig.
4
The deviation
of the additivity
mass transfer
in a wetted wall
of individual
phase resistances
column as a function
for
of TT/GZ (D„/D J6
g
0.0001).
(Average
overall
arithmetical
112
mass transfer
coefficient
and a logarithmic-mean
driving
based on a combination
force.)
of an
F
n
n
1
2.26313
1.33823
0.393429
2
6.29782
-0.54556
-0.118857
3
10.30802
0.35893
0.067046
4
14.31325
-0.27211
-0.045787
5
18.31657
0.22113
0.034377
6
22.31892
-0.18732
-0.027320
7
26.32070
0.16313
0.022551
8
30.32213
-0.14488
-0.019128
9
34.32432
0.13060
0.016559
10
38.32519
-0.11908
0.014565
Table
1
Eigenvalues
3. CONCLUSIONS
The
s i n g l e phase mass t r a n s f e r c o e f f i c i e n t s
i n a p r e v i o u s l y developed
wetted
w a l l column do not v a r y w i t h t h e same power o f t h e c o n t a c t time o f renewable
s u r f a c e s . Under t h e s e c o n d i t i o n s t h e a d d i t i v i t y o f i n d i v i d u a l phase r e s i s t a n c e s
f o r mass t r a n s f e r does not h o l d and d e v i a t i o n s may o c c u r .
In o r d e r t o study t h i s d e v i a t i o n , a t r u e average
c o e f f i c i e n t was c a l c u l a t e d from
a numerical
o v e r a l l mass t r a n s f e r
s o l u t i o n , which was compared w i t h
the v a l u e o b t a i n e d from t h e a d d i t i v i t y o f i n d i v i d u a l phase r e s i s t a n c e s .
be c o n c l u d e d
I t can
t h a t t h e d e v i a t i o n i s s m a l l enough t o be n e g l e c t e d .
REFERENCES
1. K i n g , J.C., A. I. Ch. E. Journal,
1964, 10, 671.
2. S z e h e l y , J . , Chem. Eng. Sai. , 1965, 20, 141.
3. C r o f t , D.R. and L i l l e y ,
D.G., Heat T r a n s f e r C a l c u l a t i o n s U s i n g
Finite
D i f f e r e n c e E q u a t i o n s , A p p l i e d S c i e n c e P u b l i s h e r s L t d . , London, 1977.
4. Crank, J . , The Mathematics o f D i f f u s i o n , C l a r e n d o n
P r e s s , Oxford, 1975.
5. Van den Berg, H. and H o o r n s t r a , R. , Chem. Eng. J., 1977, 12!,
6. Brauer,
191.
H., S t o f f a u s t a u s c h , S a u e r l a n d e r A.G., Aarau, 1971.
113
NOMENCLATURE
roots o f the equation
thermal
J (a ) = 0
o n
2,
m /sec
diffusivity
area of i n t e r f a c e
coefficient
concent r a t i o n
kmol/m
3
kg/m
specific
Joule/kg.
heat
d
diameter
D
diffusion
F
K
2,
ra /sec
coefficient
funct ion
n
F o u r i e r number
Fo
g
gravitational
Gz
Graetz
number
Graetz
number c o r r e c t e d f o r t h e i n a c t i v e f i l m
Gz
red
film
acceleration
length or co-ordinate
m/sec
height
o f l e n g t h i n flow
d i r e c t ion
m
h'
effective
Ah
inactive
H
Henry's law c o n s t a n t
3
kmol/m .bar
heat o f r e a c t i o n
Joule/kmol
heat o f s o l u t i o n
Joule/kmol
3
k-ion/m
2
kmol/m .sec
AH
film
film
length
m
length
m
r
AH
s
ionic
strength
I
absorption
r a t e per u n i t of s u r f a c e
area
J
Bessel
f u n c t i o n of the f i r s t
k i n d and z e r o
Bessel
f u n c t i o n of the f i r s t
k i n d and f i r s t
order
J„
reaction rate
gas
og,addit ion
constant
sec
phase mass t r a n s f e r c o e f f i c i e n t
liquid
_og
order
m/sec
phase mass t r a n s f e r c o e f f i c i e n t
overall
mass t r a n s f e r c o e f f i c i e n t
overall
gas phase mass t r a n s f e r c o e f f i c i e n t
from t h e a d d i t i v i t y o f i n d i v i d u a l
m/sec
based on gas s i d e
average mass t r a n s f e r
resistances
K
og,true
K
solubility
114
m/sec
true o v e r a l l
equilibrium
m/sec
derived
gas phase mass t r a n s f e r c o e f f i c i e n t
constant
(= C. ,/C
.)
A,l g , l
m/sec
bar
eigenvalues
m(T)
q u a n t i t y o f gas absorbed p e r u n i t o f s u r f a c e
a f t e r contact
n
time
area
I
kmol/m
2
kg/m
number
N
local
P
pressure
N0„
mass
kmol/m .sec
bar
2N 0
2
distance
Sh
flux
4
in radial direction
m
r a d i u s o f wetted w a l l column
m
gas
o
Joule/kmol. K
law
constant
Sherwood number
t
time
T
temperature
sec
surface
°K
v e l o c i t y o f the l i q u i d
mass flow
film
r a t e o f the gas
co-ordinate
m/sec
kg/sec
of length across
flow d i r e c t i o n
m
v a l e n c i e s o f ions
GREEK SYMBOLS
the
layer
f r a c t i o n o f NO^ c o n v e r t e d
t o ^^0^
thickness
o f the l a m i n a r
thickness
o f f i c t i t i o u s water l a y e r
distance
at
instantaneous
reactions
viscosity
2,
m /sec
3
density
kg/m
contact
time
flow
liquid
film
from r e a c t i o n p l a n e t o g a s - l i q u i d i n t e r f a c e
kinematic
gas
liquid
sec
rate
flow
rate
3.
m /sec
3,
m /sec
SUBSCRIPTS
c
wetted w a l l column
f
liquid
g
gas phase
film
i
gas-liquid interface
&
liquid
local
local
phase
values
115
o
inlet
Q
N0
r
reaction
s
liquid
2
+ 2N 0
2
4
plane
surface
SUPERSCRIPTS
b u l k average
116
or mixing
up v a l u e
; —
•
— - -
-
!
S T E L L I N G E N
1. B i j g a s - v l o e i s t o f c o n t a c t b e s t a a t
gasfase
in
de
t u s s e n een
v l u c h t i g e component u i t de v l o e i s t o f f a s e en een
sprake
zijn
van een
o n e i n d i g s n e l mag
(Dit
reactant
i n d i e n de r e a c t i e a l s
worden beschouwd.
Proefschrift)
dig
gepaard gaande met
s n e l l e r e a c t i e i n de v l o e i s t o f f a s e t u s s e n een o p g e l o s t
tant
i n de v l o e i s t o f f a s e z i j n
een
(Dit
i n p r i n c i p e tevens
een
onein-
gas en een
reac-
g e l d i g voor systemen waar-
v l u c h t i g e component u i t de v l o e i s t o f f a s e o n e i n d i g s n e l r e a g e e r t i n
de g a s f a s e met
3. De
r e a c t i e v l a k i n de g a s f a s e
algemene r e l a t i e s b e t r e f f e n d e g a s a b s o r p t i e
bij
i n de
gasfase.
E r kan
2. De
de m o g e l i j k h e i d dat r e a c t i e o p t r e e d t
een
d a a r i n aanwezige r e a c t a n t .
Proefschrift)
s t e r k t e van
verdund s a l p e t e r z u u r a l s w a s v l o e i s t o f voor de v e r w i j d e r i n g
van n i t r e u z e n u i t a f g a s s e n
t i e g r a a d van NO
wordt voor een
en de o p l o s b a a r h e i d
van
groot
deel bepaald
door de
oxida-
salpeterigzuur i n salpeterzuur.
"V.
4. Het
ondergronds v e r g a s s e n
van k o o l i s o n g u n s t i g
vanuit reactorkundig
oog-
punt .
5. Het
succes
van
i n n o v a t i e - g e r i c h t e onderzoekprogramma's aan
en h o g e s c h o l e n z a l s t e r k afhangen van
om
6.
de n o d i g e i n f o r m a t i e en gegevens t e
De k w a l i t e i t
van
schikbare
verstrekken.
researchwerkzaamheden i s
minimum n i v e a u o n a f h a n k e l i j k van
financiële m i d d e l e n .
(New Scientist,
de b e r e i d h e i d van het b e d r i j f s l e v e n
i n groepsverband uitgevoerde
boven een b e p a a l d
1979, 84_ (1176),
91)
universiteiten
de g r o o t t e van
de
be-
7. De samenleving zou een b e t e r i n z i c h t hebben i n de wetenschap en t e c h n i e k
i n d i e n met name j o u r n a l i s t e n en T . V . - p r e s e n t a t o r e n
h e i d en bekwaamheid op d i t t e r r e i n
8. De s t e l l i n g n a m e
heersen
een g r o t e r e
deskundig-
bezaten.
d a t i n h e t " v r i j e " Westen v r i j h e i d van m e n i n g s u i t i n g zou
b e r u s t op een v o o r o o r d e e l .
9. De r e g e r i n g i s met name door de Wet op de I n v e s t e r i n g s r e k e n i n g
verantwoordelijk
(WIR) mede-
voor de h u i d i g e z u i v e l o v e r s c h o t t e n i n ons l a n d .
10. " M a c r o b i o t i c i " kunnen i n bepaalde o p z i c h t e n beschouwd worden a l s " l u x e
wilde
11.
beesten".
Hoogbouw i s l a a g - b i j - d e - g r o n d s .
Delft,
12 maart 1980
J.B.
Lefers