Separation of H2S from N2 by selective permeation through polymeric membranes
by Robert Leo Heyd
A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE
in Chemical Engineering
Montana State University
© Copyright by Robert Leo Heyd (1976)
Abstract:
Vinylidene fluoride films modified with several different chemicals, and several commercially
available films were tested for the separation of H2S from H2S-N2 mixtures. Flux and separation
factor values were obtained over a temperature range from 23°C to 125°C with an operating pressure of
500 psig.
Microscopic examination of the surfaces of modified vinylidene fluoride membranes indicated that
uniform polymer films could be produced by allowing polymer solutions to stand for 15 days to insure
complete dispersion of the polymer along with de-gassing the solution before pouring it onto a flat
plate to form membrane films.
The following chemicals were added as modifiers to vinylidene fluoride: sulfolene, 3-methyl sulfolene,
1-methyl-2-pyrrolidinone , morpholine, monoethanol amine, triethanolamine, diisopropanol amine, and
monoisopropanol amine. Good separation of H2S from a 5% mixture of H2S in N2 was obtained using
3-methyl sulfolene, morpholene, monoisopropanol amine, and diisopropanol amine. For example, a
film modified with 10% 3-methyl sulfolene gave a permeate, containing 33% H2S at 50°C.
The commercially available polymer films tested included: dimethyl silicone, silicone polycarbonate
copolymer, poly vinyl fluoride, polyamide, heat stabilized polyamide, polysulfone and
polyethersulfone. Very high flux values and good separation were found with several of these
polymers.
Strong trends toward increasing flux with increasing temperature, and an increasing percentage of H2S
in the permeate gas with increasing temperature were observed.
Separation factor values for H2S, determined using a 5% mixture of H2S in N2, ranged from less than
.1 to 12.9. Flux values ranged from less than 10-5 m3(STP)/m2 hr to .727 m3(STP)/m2hr.
Tests conducted with a feed gas containing .27% H2S in a H2S/N2 mixture showed that permeate flux
decreased as the percentage of H2S in the feed gas decreased. No definite trend in the change of
separation factor with feed gas composition was observed. STATEMENT OF PERMISSION TO COPY
In p r e s e n tin g t h i s
t h e s is i n p a r t i a l f u l f i l l m e n t o f th e requirem ents'
f o r an advanced d e g r e e .a t Montana S ta te U n i v e r s i t y ,
L i b r a r y s h a l l make i t
I agree t h a t th e
f r e e l y a v a i l a b l e f o r i n s p e c t io n . . I f u r t h e r agree
t h a t p e rm is s io n f o r e x t e n s iv e c o p yin g o f t h i s t h e s is f o r s c h o l a r l y
purposes may be g r a n te d by my m a jo r p r o f e s s o r , o r , in h i s absence, by
th e D i r e c t o r o f L i b r a r i e s .
I t i s u n d erstoo d t h a t . a n y cop yin g o r
p u b lic a tio n o f t h is th e s is f o r fin a n c ia l
w i t h o u t my w r i t t e n p e r m is s io n .
S ig n a tu r e
D a t e _____ 3 , . / J rV b
.
g a in s h a l l n o t be a llo w e d
SEPARATION OF H2S FROM N2 BY SELECTIVE PERMEATION
' ,THROUGH POLYMERIC MEMBRANES
by
ROBERT LEO HEYD
.
A t h e s i s s u b m itte d i n p a r t i a l f u l f i l l m e n t o f
th e re q u ire m e n ts f o r th e degree
of
MASTER OF SCIENCE
in
Chemical E n g in e e rin g
Approved:
C h a irp e rs o n , Graduate Committee
Heaa, m a jo r uep^cm em ;
Graduate Dfean
MONTANA STATE UNIVERSITY
Bozeman, Montana
December, 1976
iii
■ACKNOWLEDGMENT
The a u th o r wishes t o thank th e e n t i r e s t a f f o f th e Chemical
E n g in e e rin g Department a t Montana S ta te . U n i v e r s i t y f o r t h e i r c r i t i c i s m s
and s u g g e s tio n s w hich le d !to th e c o m p le tio n o f t h i s p r o j e c t .
S p e c ia l thanks goes t o D r. F. P. McCandless f o r h i s a d v ic e ,
a s s is t a n c e , and encouragement t h ro u g h o u t t h i s
in v e s tig a tio n .
thanks are extended t o Mr. S i la s Huso and Mr. James T i l l e r y
A ls o ,
fo r th e ir
h e lp in t r a c k i n g down v a r io u s equipm ent p ieces needed d u r in g th e course
o f t h i s re s e a rc h w o rk.
The a u th o r i s in d e b te d t o th e N a tio n a l Science Foundation f o r th e
fin a n c ia l
s u p p o r t which helped t o make t h i s research p r o j e c t p o s s i b l e .
The a u t h o r wishes t o thank h is p a re n ts f o r t h e i r encouragement
and s u p p o r t.
F i n a l l y , thanks goes t o S h erry Greene f o r t y p in g t h i s t h e s i s .
iv
TABLE OF CONTENTS
.
VITA . . . ........................... ....
fase
..................................................... .....................
ACKNOWLEDGMENTS...........................- ....................... ;
......................... ......................
LIST OF T A B LE S .................. ...........................................i
.....................
I
. . .
LIST OF FIGURES................................
ABSTRACT . . . . . . A . . .
ii
iii
v iii
ix
.■■......................
x ii
I.
'INTRODUCTION AND PURPOSE.............................................................................
I
II.
REVIEW OF THE LITERATURE...........................................................................
3
COMMERCIAL PROCESSES FOR CONCENTRATING HgS...............................
3
COMMERCIAL PROCESSES FOR CONVERTING H9S TO
ELEMENTAL SULFUR............................................ .... . ...............................
5
LABORATORY SCALE HgS REMOVAL PROCESSES........................................
7
USE OF MEMBRANES FOR GAS SEPARATIONS............................................
IO
MEMBRANES FOR SEPARATING HgS FROM GAS MIXTURES......................
12
III.
IV .
.................................................• . ;
THEORETICAL BACKGROUND.
14
NATURE OF THE TRANSPORT PROCESS.......................................
14
TRANSPORT THROUGH A GASEOUS. FILM.
.................................................
14
. TRANSPORT THROUGH A POLYMERIC MEMBRANE........................................
14
POLYMER CHEMISTRY ASPECTS . . . . . . . . . . . . . . . .
18
TEMPERATURE EFFECTS .........................................................
20
DEFINITION OF THE SEPARATION FACTOR . ............................ . . . .
22
EXPERIMENTAL. EQUIPMENT, MATERIALS AND PROCEDURES......................
23
DESCRIPTION OF EQUIPMENT..........................
23
,
'
V
TABLE OF CONTENTS (C ont)
Page
1.
PERMEATION CELL............................................ ...................................
23
2.
CONSTANT TEMPERATURE' ENCLOSURE AND
TEMPERATURE MEASURING EQUIPMENT............................................
26
3.
GAS MIXTURE STORAGE. AND FEEDING EQUIPMENT......................
27
4.
PURGE GAS SYSTEM COMPONENTS..................' ..................................
29
5.
PERMEATE RATE MEASUREMENT EQUIPMENT.- ...............................
29
6.
GAS COMPOSITION ANALYSIS EQUIPMENT...................... ' . . .
30
7.
MEMBRANE SURFACE AND COMPOSITION ANALYSIS
EQUIPMENT...........................• ...............................................................
30
MATERIALS.................................................'....................................................
.
31
EXPERIMENTAL PROCEDURE................................... ' .....................................
*34
1.
PREPARATION OF GAS MIXTURES......................................................
34
2.
CALIBRATION OF BAS' CHROMATOGRAPH.........................................
34
. 3.
CALIBRATION OF EXHAUST GAS ROTAMETER ................................
35
4.
MEMBRANE MANUFACTURE ..................................................... . . . ..
35
5.
MICROSCOPIC EXAMINATION OF MEMBRANE SURFACES . . . .
40
6.
TEST FOR THE PRESENCE OF RESIDUAL SOLVENT
USING INFRARED SPECTROSCOPY.....................................................
41
7.
PREPARATION FOR A TEST R U N .......................
41
8.
TEST RUN PROCEDURE.......................................................................
42
EXPERIMENTAL RESULTS AND DISCUSSION.
. . ........................................
44
MODIFIED VINYLIDENE FLUORIDE FILMS .................................................
44
I.
MATERIALS TESTED................................ ' . ........................ ..
. .'
44
vi
TABLE OF CONTENTS (C ont)
Page „
V I.
2.
RESULTS OFMICROSCOPIC ANALYSIS...............................................
45
3.
THE D A T A ........................................ ... ................................................
60
4 . . DISCUSSION r FILMS .WITHNO MODIFIER ADDED..........................
69
5.
DISCUSSION - FILMS WITH MODIFIERADDED................................
6.
EFFECT OF TEMPERATURE...........................• ........................... ..
'74 .
.
79
COMMERCIALLY AVAILABLEPOLYMER FILMS . . ......................................
80
1.
MATERIALS TESTED.................................................■........................
80
2.
THE DATA
...................................................................
81
3.
DISCUSSION........................................................................................
81
4.
EFFECT OF TEMPERATURE............................... ' ................................
88
EFFECT OF FEED GAS COMPOSITION..........................................................
90
1.
MATERIALS TESTED ...........................................................................
90
2.
THE D A T A .............................................................................................
90
3.
DISCUSSION........................................................................................
91
SUMMARY OF BEST RESULTS..................................................... ..................... ■
97
ANALYSIS OF ERRORS ................................
99
. . . . . . . . . . . .
CONCLUSIONS AND RECOMMENDATIONS...................... ....
. ............................
101
CONCLUSIONS..............................................
101
RECOMMENDATIONS.............................................................................................
102
1.
MODIFIED VINYLIDENE FLUORIDE FILMS. . . . . . . . . . .
102
2.
COMMERCIAL F I L M S ..............................................................
103
. . .
v ii
TABLE OF CONTENTS (C o n t.)
Page
3.
ALL FILMS.
.
104
V I L REFERENCE FOOTNOTES.
106
V I I I . APPENDIX........................
HO
IX .
TABLE OF NOMENCLATURE
111
BIBLIOGRAPHY . . . .
113
v iii
LIST OF TABLES
Table
V -I
V -2
V -3
V-4
Page
Summary o f T est R e s u lts f o r M o d ifie d V in y lid e n e
F lu o r id e F i l m s .........................................
61
Summary o f T e s t R e s u lts f o r C o m m ercially A v a ila b le
Polymer F i lm s ...................... ' ....................■......................................................
82
Comparison o f Flux and S e p a ra tio n Values f o r Two
D i f f e r e n t Feed Gas C o m p o s itio n s .............................................................. ,
92'
Summary o f Best S e p a r a tio n R e s u lts
. . . . . . . . . . . . .
98
ix
LIST OF FIGURES
F ig u re
III-I
IV -I
IV -2
IV -3
IV -4
IV -5
IV -6
V -I
V -2
V-3
V-4
V-5
Page *
C o n c e n tr a tio n P r o f i l e s o f Component a in th e
Membrane and i t s V i c i n i t y . . . .....................................................
15
S i m p l i f i e d Diagram o f Perm eation Equipment ............................
24
Perm eation C e ll Diagram.
.............................................’ ....................
Constant Temperature E n clo su re - High Tem perature.
. . .
25
28
C a l i b r a t i o n o f Gas Chromatograph - Sample
Volume vs. Peak W eight x A t t e n u a t io n f o r
N itr o g e n and Hydrogen S u l f i d e ............................
36
C a l i b r a t i o n Curve f o r Gas Chromatograph - Area
P e rce n t o f HgS vs. Volume P e rcen t o f H g S ...............................
37
Rotameter C a l i b r a t i o n Curve - Flow Rate (STP)
v s . Rotameter R e ading...........................................................................
38
S u rfa ce View o f Membrane C o n ta in in g 15% S u lf o le n e
a t SOOx M a g n i f i c a t io n - Membrane Formed Im m ediate ly
fro m Polymer S o l u t i o n ...........................................................................
46
Cross S e c tio n o f Membrane C o n ta in in g 15% S u lf o le n e
a t 600x M a g n i f i c a t io n - Membrane Formed Im m ediate ly
from Polymer S o l u t i o n ...........................................................................
47
' S u rfa ce View o f Membrane M o d ifie d w i t h 12% S u lf o le n e
a t 400x M a g n i f i c a t io n - Polymer S o lu t io n De-gassed
F o llo w in g a 24 Hour S o lu t io n P e rio d . . . . ...........................
50
S u rfa ce View o f Membrane M o d if ie d w i t h 10% D i i s o p r o ­
panol amine a t 700x M a g n i f i c a t io n - Polymer S o lu t io n
De-gassed F o llo w in g a 24 Hour S o lu t io n P e r io d ...................... ...
51
S u rfa ce View o f Membrane M o d ifie d w i t h 10% I - M e t h y l 2 - P y r r o l id in o n e a t 400x M a g n i f i c a t io n - Polymer
S o l u t i o n De-gassed F o llo w in g a 24 Hour S o lu t io n P e rio d .
52
X
LIST OF FIGURES (C ont)
F ig u re
V -6
S u rfa ce View o f Membrane M o d if ie d w i t h 5%
T r ie th a n o la m in e a t 700x M a g n i f i c a t io n - Polymer
S o lu t io n De-gassed F o llo w in g a 24 Hour S o lu t io n
P e r i o d ..................................................................... I . . . .
V-7
-
V-8
V-9
V-IO
V -Il
V - 12
V-13
V - 14
V - 15
V - 16
Page
53
S u rfa c e View o f Membrane M o d ifie d w i t h 10%
T r ie th a n o la m in e a t 400x M a g n i f i c a t io n - Polymer
S o lu t io n De-gassed F o llo w in g a 15 Day S o lu t io n
P e r i o d ...................... : ........................................................... .......................
55
S u rfa ce View o f Membrane M o d ifie d w i t h 10% M o rp h o lin e
a t 400x M a g n i f i c a t io n - Polymer S o lu t io n De-gassed
F o llo w in g a 15 Day S o lu t io n P e rio d . . . ......................................
55
S u rfa c e View o f Membrane M o d ifie d w i t h 10% T rie th a n o la m in e
a t 400x M a g n i f i c a t io n - Polymer S o lu t io n De-gassed
F o llo w in g a 15 Day S o lu t io n P e rio d . . ..................................... .
57
S u rfa ce View o f Membrane M o d ifie d w i t h 10% Mono­
is o p ro p a n o l amine a t 400x M a g n i f i c a t io n - Polymer S o lu t io n
De-gassed F o llo w in g a 15 Day S o lu t io n P e r i o d ...........................
58
S u rfa ce View o f Membrane M o d ifie d w i t h 10% Mono. is o p ro p a n o l amine a t IOOOx M a g n i f i c a t io n - Polymer
S o lu t io n De-gassed F o llo w in g a 15 Day S o lu t io n P e rio d . .
59
S e p a ra tio n F a c to r v s . Tem perature f o r Some M o d ifie d
V in y li d e n e F lu r o id e F i l m s ....................................................................
65
S e p a ra tio n F a c to r v s . Temperature f o r A lkan o la m in e
M o d if ie r s i n V in y lid e n e F l u o r i d e .......................
66
. F lu x v s . Tem perature f o r Some M o d ifie d V i n y lid e n e
F lu o r id e F i lm s .................................................
67
F lux v s . Tem perature f o r A lka n o la m in e M o d if ie r s in
V i n y lid e n e F lu o r id e
...........................................................................
68
R e s u lts o f I n f r a r e d S p e c tro p h o to m e te r Tests on
Unm odified V in y lid e n e F l u o r id e F i lm s ................................... .
70
.
xi
LIST OF FIGURES (C ent)
F ig u re
V - 17
Page
S e p a ra tio n F a c to r v s . Temperature f o r Unm odified
V in y li d e n e F l u o r id e F i lm s .................. ...............................................
72
F lu x v s . Temperature f o r Unm odified V in y lid e n e
F l u o r id e Film s ........................... ............................... ......................
73
S e p a r a tio n F a c to r v s . Tem perature f o r C om m ercially
A v a i l a b l e F ilm s ........................................ ................................................
84
V - 20
F lu x vs. Temperature f o r C o m m ercially A v a ila b le F i lm s . .
85
V-21
E f f e c t o f Feed Gas Com position on S e p a ra tio n For
V i n y li d e n e F lu o r id e w i t h 10% 3-M ethyl S u lf o le n e
M o d i f i e r ............................... ...................................................................
93
E f f e c t o f Feed Gas C om position on Flux For
V i n y li d e n e F lu o r id e w i t h 10% 3-M ethyl S u lf o le n e
M o d i f i e r .................. ..................... .... . . ............................................
94
E f f e c t o f Feed. Gas C om position on S e p a ra tio n f o r
S i l i c o n e P o ly c a rb o n a te Copolymer . . .
...................................
95
E f f e c t o f Feed Gas Com position on F lu x f o r S i l i c o n e
. Polycarbonate. Copolym er............................... .... . ...........................
96
V - 18
V-19
V-22
V-23
V -24
x ii
ABSTRACT
V in y lid e n e f l u o r i d e f i l m s m o d if ie d w i t h s e v e ra l d i f f e r e n t c h e m ic a ls ,
and s e v e r a l c o m m e rc ia lly a v a i l a b l e f i l m s were t e s t e d f o r th e s e p a ra tio n
o f HgS fro m H g S - ^ m i x t u r e s . F lu x and s e p a r a tio n f a c t o r values were
o b ta in e d o v e r a te m p e ra tu re range from 23°C t o 125°C w i t h an o p e r a tin g
p re s s u re o f 500 p s ig .
M ic r o s c o p ic e x a m in a tio n o f th e s u r fa c e s o f m o d if ie d v i n y l i dene
f l u o r i d e membranes in d i c a t e d t h a t u n ifo rm polymer f i l m s c o u ld be p r o ­
duced by a l lo w in g polym er s o l u t i o n s t o sta n d f o r 15 days t o in s u re
com plete d i s p e r s io n o f t h e polym er a lo n g w i t h d e -g a s s in g th e s o l u t i o n
b e fo r e p o u rin g i t onto a f l a t p l a t e t o form membrane f i l m s .
The f o l l o w i n g chem ica ls were added as m o d if ie r s to v i n y l idene
flu o r id e :
s u l f o l e n e , 3-m e thyl s u l f o l e n e , 1 - m e t h y l - 2 - p y r r o l id in o n e ,
m o rp h o lin e , monoethanol amine, t r i e t h a n o l a m i n e , d i is o p ro p a n o l amine, and
monoisopropanol amine.
Good s e p a r a t i o n . o f HgS from a 5% m ix t u r e o f
HgS in Ng was o b ta in e d u s in g 3-m e thyl s u l f o l e n e , m orpholene, mono­
is o p ro p a n o l amine, and d i is o p ro p a n o l amine.
For example, a f i l m m o d ifie d
w i t h 10% 3-m e thyl s u l f o l e n e gave a permeate, c o n t a in in g 33% HgS a t 50°C.
The c o m m e rc ia lly a v a i l a b l e polym er f i l m s t e s te d in c lu d e d :
d im e th y l s i l i c o n e , s i l i c o n e p o ly c a rb o n a te copo lym er, p o ly v i n y l f l u o r i d e ,
polya m id e , hea t s t a b i l i z e d polya m id e , p o ly s u lfo n e and p o l y e t h e r s u lf o n e .
Very high f l u x va lu e s and good s e p a r a tio n were found w i t h s e v e ra l o f
th e s e polym e rs.
S tro n g tre n d s tow ard in c r e a s in g f l u x w i t h in c r e a s in g te m p e ra tu re ,
and an in c r e a s in g percen tag e o f HgS i n th e permeate gas w i t h in c r e a s in g
te m p e ra tu re were o b se rve d.
Separation fa c to r values f o r HgS, determined using a 5% mixture of
HgS in Hg, ranged from less than .1 to 1 2 . 9 .
Flux values ranged from
less than 10-5 m3(STP)/m2 hr to .727 m3(STP)/m2hr.
Tests conducted w i t h a feed gas c o n t a i n in g .27% HgS i n a HgS/Ng
m ix t u r e showed t h a t permeate f l u x decreased as the percen tag e o f HgS
in th e feed gas decreased.
No d e f i n i t e t r e n d in th e change o f s e p a r a tio n
f a c t o r w i t h feed gas c o m p o s itio n was o b se rve d.
INTRODUCTION AND PURPOSE
The s e p a r a tio n o f HgS from o t h e r components in a gas stream i s
im p o r t a n t i n a number o f a re a s , p a r t i c u l a r l y in th e u t i l i z a t i o n o f th e
fo s s il
f u e l s n a t u r a l gas, p e tro le u m , and c o a l.
Hydrogen s u l f i d e gas is h i g h l y t o x i c , c o r r o s i v e , and a c a t a l y s t
p o is o n .
These p r o p e r t ie s n e c e s s it a t e i t s
removal from gas stream s.
N a tu ra l gas must be "sw e e te n e d ", t h a t i s , hydrogen s u l f i d e must
be removed b e fo r e i t can be d i s t r i b u t e d by p i p e l i n e .
Hydrogen s u l f i d e produced d u r in g r e f i n i n g from t h e s u l f u r p re s e n t
i n crude o i l
is removed t o p re v e n t c a t a l y s t p o is o n in g and c o r r o s io n .
When coal
is g a s i f i e d , r e g a r d le s s o f th e s p e c i f i c process used,
most o f th e s u l f u r in t h e coal i s c o n v e rte d t o hydrogen s u l f i d e .
T h is
hydrogen s u l f i d e must be removed from th e g a s i f i e r e x i t gas because i t
q u i c k l y d e s tro y s th e a c t i v i t y o f m e th a n a tio n c a t a l y s t s .
As p e tro le u m s u p p lie s become more s c a rc e , coal g a s i f i c a t i o n i s
expected t o become a m a jo r i n d u s t r y .
I t has been e s tim a te d ( I ) t h a t
t o produce c le a n f u e l e q u iv a l e n t t o 20% o f c u r r e n t U. S. o i l
consumption
fro m co a l w i l l r e q u i r e 80 g a s i f i c a t i o n p la n t s each p ro d u c in g 250
3
m i l l i o n f t /d a y o f gas.
Each p l a n t would consume about 12,000 to n s /d a y
o f h ig h q u a l i t y e a s te rn coal and produce about 400 t o n s / d a y ( 8 .4 x 10^
SCF/day) o f hydrogen s u l f i d e .
Because most o f t h e coal g a s i f i c a t i o n
processes c u r r e n t l y under
development o p e ra te t o produce a p r o d u c t gas under p r e s s u r e , the
d r i v i n g f o r c e f o r a membrane s e p a r a tio n process i s a lr e a d y p r e s e n t.
.
- 2 T h is i s a b ig advantage o f a membrane s e p a r a tio n process o v e r conven­
tio n a l
l i q u i d a b s o r p tio n hydrogen s u l f i d e removal proce sse s.
Membrane
s e p a r a tio n processes a ls o have th e advantage o f r e q u i r i n g much le s s
equipm ent than l i q u i d a b s o r p tio n p ro c e s s e s .
In an a tte m p t t o c o n t r i b u t e t o th e u l t i m a t e goal o f a membrane
s e p a r a tio n system f o r hydrogen s u l f i d e removal t h i s work was conducted.
The s p e c i f i c o b j e c t i v e s o f t h i s
1.
research were as f o l l o w s :
To t e s t a number o f d i f f e r e n t p l a s t i c i z e r s
i n v i n y l idene
f l u o r i d e t o d e te rm in e perm eation r a t e and s e l e c t i v i t y f o r HgS.
2.
To t e s t v a r io u s c o m m e rc ia lly a v a i l a b l e p o ly m e ric f i l m s to
d e te rm in e p erm eation r a t e and s e l e c t i v i t y f o r HgS.
3.
To d e te rm in e th e e f f e c t o f te m p e ra tu re on s e l e c t i v i t y f o r HgS
i n th e v a r io u s membranes,
4.
To d e te rm in e th e e f f e c t o f te m p e ra tu re
on p erm eation r a te
in th e v a r io u s membranes.
5.
To d e te rm in e th e e f f e c t o f feed gas c o m p o s itio n on perm eation
r a t e and s e l e c t i v i t y f o r HgS f o r m a t e r i a ls w hich appear
p ro m is in g .
REVIEW OF THE LITERATURE
Hydrogen s u l f i d e removal te c h n o lo g y in c lu d e s a l a r g e number o f
commercial processes alo n g w i t h s e v e ra l re c e n t l a b o r a t o r y developments.
Much w ork has been done on membranes f o r gas s e p a r a t i o n s , i n c lu d in g
some s t u d ie s d e a lin g w ith .h y d r o g e n s u l f i d e .
A.
COMMERCIAL PROCESSES FOR CONCENTRATING HgS
.
.
The l a r g e s t number o f commercial processes f o r c o n c e n t r a t in g
hydrogen s u l f i d e gas in v o lv e a b s o r p tio n o f HgS i n t o s o l u t i o n f o llo w e d
by a r e g e n e r a tio n s te p w hich se p a ra te s th e HgS fro m th e a b s o rb in g
s o lu tio n .
A lka n o la m in e s o l u t i o n s are used as t h e a b s o rb e n t i n many p rocesse s.
The G ir b o t a l Process (2 ) and th e A d ip Process (3 ) use aqueous s o l u t i o n s
o f monoethanol amine, d ie th a n o la m in e o r t r i e t h a n o l a m in e as a b s o rb e n ts .
The HgS r e a c ts w i t h th e amines t o form compounds which may be broken
down by h e a t.
The r e a c t i o n form i s :
RNH2 + HgS
RNH3HS
The s o l u t i o n s are re g e n e ra te d by b e in g h e a te d .
T h is d r iv e s o f f the
HgS t o fo rm a c o n c e n tr a te d HgS stream and an amine s o l u t i o n which is
r e c y c le d .
The SNPA-DEA (4) Process uses a c o n c e n tra te d aqueous s o l u t i o n
o f d ie th a n o la m in e as th e s o l v e n t .
Two commercial processes have been developed w h ich use an
a l k a n o l amine combined w i t h o t h e r c h e m ica ls as th e a b s o rb e n t s o l u t i o n .
In th e Amisol Process (5 ) th e s o l v e n t c o n s i s t s o f m e th a n o l, an a d d i t i v e .
- 4 and an o r g a n ic base (d ie th a n o la m in e o r t r i e t h a n o l a m i n e ) .
has th e advantage o f h ig h lo a d in g c a p a c it y and
te m p e ra tu r e .
T h is s o l v e n t
a low r e g e n e ra tio n
The S u l f i n o l Process (6 ) makes use o f an o r g a n ic s o l v e n t ,
s u l f o l a n e ( te tr a h y d r o th i.o p h e n e d io x id e ) , mixed w i t h an a lk a n o la m in e and
w a te r .
Sim ultaneous chem ical and p h y s ic a l a b s o rp tio n o f HgS takes
p la c e u s in g t h i s s o l v e n t .
An'aqueous s o l u t i o n o f t h e p r im a r y a lk a n o la m in e HO-CgH^-O-CgH4 -NH2
( t r a d e named d ig ly c o la m in e ) i s th e s o l v e n t used in th e F lu o r Econamine
Process ( 7 ) .
A number o f processes have been developed w hich use carbon ate
s o l u t i o n s f o r HgS a b s o r p t i o n .
ty p e i s th e
The most w i d e l y used process o f t h i s
B e n f i e l d Process ( 8 ) .
Hydrogen s u l f i d e
i s removed by
c o n t a c t w i t h a potassium carb o n a te s o l u t i o n c o n t a i n in g B e n f i e l d
a d d i t i v e s a t h ig h p re s s u re (100 - 2000 p s i g ) .
The s o l u t i o n
g en erate d by s t r i p p i n g a t a tm o sp h e ric p r e s s u r e .
the
B e n f i e l d Process, known as th e H i-P u re
d i f f e r e n t c a rb o n a te s o l u t i o n s i n s e r i e s .
is re -
•
A m o d ific a tio n o f
Process (9 ) uses two
The advantage o f t h i s process
i s t h a t . i t produces a p ro d u c t gas c o n t a i n in g an e x tre m e ly low co n ce n tra
t i o n o f HgS (1 -2 0 PPM).
The F lu o r S o lv e n t Process (10) uses an
anhydrous o r g a n ic compound, p ro p y le n e c a rb o n a te , f o r HgS rem oval.
T h is
s o l v e n t is re g e n e ra te d s im p ly by p re s s u re le tdo w n o f t h e r i c h s o l v e n t ,
w i t h o u t th e a p p l i c a t i o n o f h e a t.
Two processes are a v a i l a b l e w hich use a potassium s a l t s o l u t i o n
- 5 t o absorb HgS.
used.
A l k a z id
In th e A l k a z id Process (11) two types o f a bso rb ents are
1D IK 1 (p o ta ssiu m s a l t o f d im e th y l amino a c i t i c a c i d ) i s
used f o r th e s e l e c t i v e a b s o r p tio n o f HgS.
A lk a z id
1M1 (potassiu m s a l t
o f methyl amino p r o p i o n i c a c i d ) is used f o r sim u lta n e o u s removal o f
COg and HgS.
A potassium s a l t s o l u t i o n c o n t a i n in g a s t a b l e c a t a l y s t i s
Usedi f o r a b s o r p tio n o f HgS in th e Catacarb Process ( 1 2 ) .
O the r commercial a b s o r p tio n processes in c lu d e th e P u r is o l Process,
th e R e c tis o l
Process and th e S e le xo l S o lv e n t Process.
P h y s ic a l
a b s o r p tio n o f HgS in N - m e t h y l - p y r r o l idone i s used in th e P u r is o l
Process ( 1 3 ) .
The R e c tis o l Process (14) uses a b s o r p tio n in cooled
m e th a n o l, and th e S e le xo l S o lv e n t Process (15) i s based on th e a b s o r p tio n
o f HgS i n d im e th y l e t h e r o f p o ly e t h y le n e g l y c o l .
B.
COMMERCIAL PROCESSES FOR CONVERTING HgS TO ELEMENTAL SULFUR
The most common method f o r c o n v e r t in g HgS to s u l f u r i s th e Claus
Process ( 1 6 ) .
In t h i s process, gas c o n t a i n in g hydrogen s u l f i d e is
fe d t o a r e a c t i o n fu rn a c e where i t
i s burned w i t h a i r .
P a r t o f th e HgS
i s o x i d iz e d t o SOg w hich r e a c ts w i t h th e re m a in in g HgS t o form
e le m e n ta l s u l f u r a c c o rd in g t o th e Claus r e a c t io n :
2HgS + SOg
-+
3S + 2HgO
T h is process can o n ly be used on a gas stream w i t h a h ig h c o n c e n tr a t io n
o f HgS (minimum HgS c o n c e n t r a t io n i s about 15%).
T h e r e f o r e , Claus u n i t s
a r e o f t e n used t o t r e a t th e c o n c e n tr a te d gas streams produced in l i q u i d
- 6 a b s o r p tio n HgS removal u n i t s .
The e x i t gas le a v in g a Claus u n i t c o n ta in s more HgS than re c e n t
e n v iro n m e n ta l r e g u l a t io n s a l lo w ( 1 7 ) .
T h e r e f o r e , a l a r g e number o f
add-on systems f o r rem oving th e HgS fro m Claus Process t a i l
been developed.
The m a jo r t a i l
gas cle anup systems a r e :
gas have
th e Beavon
Process (18) which uses h y d ro g e n a tio n f o llo w e d by tre a tm e n t w it h
sodium c a rb o n a te s o l u t i o n , th e IFP Process (19) in which HgS and SOg
are r e a c te d s t i o c h i o m e t r i c a l l y t o ele m e n ta l s u l f u r , th e SCOT Process
(20) i n w h ich a l l
s u l f u r compounds are c a t a l y t i c a l l y c o n v e rte d t o HgS
and then removed by a b s o r p tio n w i t h an a l k a n o l amine s o l u t i o n , th e
S u lf r e e n Process (2 1 , 22) which uses an a c t i v a t e d carbon o r alumina
c a t a l y s t t o c o n v e r t HgS t o elem e ntal s u l f u r , th e CBA Process (23)
w h ich a ls o uses a c a t a l y t i c process t o c o n v e rt HgS t o s u l f u r , th e
T re n c o r M Process (24) w hich uses a p r o p r ie ta r y . a q u e o u s s o l u t i o n o f m e th y ld i e th a n o l amine to absorb HgS, and th e Catabon Process (25) in
w h ich a complexed p o l y v a l e n t metal, io n o x i d iz e s HgS t o s u l f u r .
In th e H o lm e s - S tr e t fo r d Process (26) HgS is c o n v e rte d t o elem ental
s u l f u r by b e in g scrubbed w i t h an a l k a l i n e s o l u t i o n c o n t a i n in g a
vanadium s a l t and an a n th ra q u in o n e d e r i v a t i v e .
The HgS i s co nve rted to
e le m e n ta l s u l f u r w h i l e th e vanadium s a l t i s reduced.
The Takahax Process (27) uses an a l k a l i n e sodium carb o n a te
s o lu tio n
(pH = 8 . 5 ) c o n t a in in g 1 ,4 - n a p h th o q u in o n e , 2 - s u lf o n a t e as a
redox c a t a l y s t .
Hydrogen s u l f i d e r e a c ts w i t h sodium c a rb o n a te to.
- 7 form sodium b i s u l f i d e and sodium b i c a r b o n a t e .
The b i s u l f i d e
is o x i d iz e d
by t h e c a t a l y s t t o produce f i n e p a r t i c l e s o f s o l i d s u l f u r .
The Konox Process (28) i s based on th e a b s o rp tio n o f hydrogen
s u l f i d e w i t h a s t r o n g i r o n o x id e o x i d i z i n g a g e n t.
a c c o rd in g t o th e f o l l o w i n g r e a c t i o n :
vi
’ 4Na2 Fe O4
An a l k a l i
+
SH3S
.
iii
4NaFe02
->
S u l f u r i s produced
+
4NaOH
- !
+
4H20
+
6S
a rs e n a te s and arse n i te s s o l u t i o n i s used t o scrub H2S
from feed gas in th e Giammarco V etrocoke Process ( 2 9 ) .
t h i o a r s e n i t e i s produced in th e a b s o r p tio n s t e p .
Sodium
The t h i o a r s e n i t e i s
c o n v e rte d t o m o n o th io a rs e n a te in an o x i d i z i n g column.
An a i r - b l o w n
o x i d i z i n g column is used t o decompose th e m o n o th io a rse n a te t o elem e ntal
s u l f u r and a r s e n i t e .
The S u lf o x Process (30) uses an aqueous ammonia s o l u t i o n t o scrub
H2S fro m th e feed gas s tre a m .
The r i c h s o l u t i o n i s warmed, mixed
w i t h a i r and passed o v e r a c a t a l y s t w hich d i r e c t s th e o x i d a t i o n o f
th e m ix t u r e to y i e l d m o s tly s u l f u r , a c c o rd in g to th e r e a c t i o n :
NH4HS
+
l/2 0 2
+
S
+
H2O
+
NH3 .
The s u l f u r forms a s e p a ra te m olten phase -which i s w ith d ra w n as p r o d u c t.
C.
LABORATORY SCALE HgS REMOVAL PROCESSES. .
P h illip s
Petroleum Company has t e s t e d m o le c u la r s ie v e s f o r H2S
- 8 removal
(3 1 ).
In a m o le c u la r s ie v e d e s u l f u r i z e r HgS i s adsorbed and a
s im u lta n e o u s r a t e - l i m i t e d , a d s o r b e n t - c a t a ly z e d r e a c t i o n . o f HgS w i t h '
any a v a i l a b l e COg t o form ca rb o n yl s u l f i d e o c c u r s .
The r e v e r s i b l e
r e a c t i o n in v o lv e d i s :
i
HgS
+
CO2
t
COS
+
HgO
When ca rb o n y l s u l f i d e appears i n th e e x i t gas, th e m o le c u la r s ie v e u n i t
must be re g e n e ra te d .
Five c o m m e rc ia lly a v a i l a b l e m o le c u la r sie v e s were
t e s t e d u s in g feed gas c o n t a i n in g 30-160 PPM HgS.
The s ie v e which gave
t h e b e s t r e s u l t s had a t r u e HgS lo a d in g o f . 8 8 * l b HgS/lb s ie v e .
■A d s o r p tio n o f . HgS on s o l i d s u r fa c e s has been s t u d ie d by a number
o f in v e s tig a to rs .
a d s o rb e n t ( 3 2 ) .
A c t iv a t e d carbon a c ts as a c a t a l y s t as w e ll as an
In th e presence o f f r e e oxygen HgS i s c a t a ly z e d to
ele m e n ta l s u l f u r a c c o rd in g to th e f o l l o w i n g r e a c t i o n :
a c t i v e carbon
I HgS
+
1 /2 Og
-*■
HgO
+
S
A d s o r p tio n o f HgS on manganese d io x id e and a c t i v a t e d carbon has been
s t u d ie d a t t h e U n i v e r s i t y o f I l l i n o i s
(3 3 ).
Several a c t i v a t e d carbons
were t e s t e d a lo n g w i t h manganese d i o x id e im pregnated on saw dust.
Both
th e e f f i c i e n c y and c a p a c it y o f th e MNOg-sawdust were found t o be b e t t e r
th a n th o s e o f any o f t h e a c t i v a t e d carbons t e s t e d .
A d s o r p tio n o f
hydrogen s u l f i d e on A m b e rly s t A26 io n exchange r e s in has been s tu d ie d
(3 4 ).
Rate curves and s o r p t i o n is o th e rm s were d e term ined f o r t h i s
/
- 9 system.
Removal o f HgS u s in g an o r g a n ic s o l u t i o n w hich c o n v e rts the HgS
to c r y s t a l l i n e s u l f u r has been s t u d ie d a t th e U n i v e r s i t y o f New
Brunsw ick ( 3 5 ) .
I t was found t h a t th e b e s t l i q u i d phase system c o n s is te d
o f . a m ix t u r e o f 83 volume p e rc e n t e t h y le n e g l y c o l monoethyl e t h e r
(C e llo s o lv e ),
i
15% w a te r and, 2% d i b u t y l amine.
The w a te r was found t o
a c t as a c a t a l y s t , s in c e no r e a c t i o n o c c u rre d in w a t e r - f r e e m i x t u r e s .
The d i b u t y l amine a llo w e d n u c l e a t io n to proceed t o produce g r a n u la r
s u lfu r.
The U. S. Bureau o f Mines has developed th r e e s i n t e r e d m a t e r i a ls
ca p a b le o f removing HgS fro m p ro d u c e r gas a t IOOO0F t o 1500°F ( 3 6 ) .
The
abso rb e n ts a re m ix tu re s o f f e r r i c o x id e and f l y a s h , f e r r i c o x id e and
pumice s t o n e , and red mud (a f e r r i c o x id e - c o n t a i n in g re s id u e from
b a u x it e p r o c e s s in g ) .
Red mud has t h e g r e a t e s t c a p a c it y , a b so rb in g
16 w e ig h t p e r c e n t s u l f u r a t 1000°F, 24% a t 1250°F and 45.1% a t 1500°F.
A number o f HgS removal processes have r e c e n t l y been p a te n te d .
U. S . 3,50 2 ,4 2 8 (37) covers HgS removal w i t h N -m ethylethan.olam ine i n
2 ,2 d im e th y l-l,3 -d io x o la n e -4 -m e th a n o l.
U. S . 3 ,716,62 0 (3 8 ) d e s c rib e s
a process f o r p u r i f y i n g a gas c o n t a i n in g HgS o r a m e rca pta n,
wherein
th e gas i s c o n ta c te d w i t h a s o l u t i o n o f io d in e and an amine i n an o r g a n ic
s o lv e n t.
Removal o f a c id gases from m ix tu re s by washing w i t h N-
s u b s t i t u t e d e -c a p ro la c ta m s i s d e s c r ib e d i n U. S. 3,653,809. ( 3 9 ) .
U. S. 3 ,4 0 9 ,5 2 0 (40) d e s c rib e s an e l e c t r o l y s i s process f o r removing
- I O - 7
HgS fro m a hydrocarbon gas m ix t u r e .
D.
USE OF MEMBRANES FOR GAS SEPARATIONS
A l a r g e number o f membrane systems have been developed f o r gas
s e p a ra tio n s .
In o r d e r to e f f e c t a s e p a r a tio n th e membrane m a te r ia l
a llo w s one component o f a m ix t u r e t o permeate a t a g r e a t e r r a t e than
th e o t h e r s .
The source o f membrane s e l e c t i v i t y i s u s u a l l y e i t h e r
p r e f e r e n t i a l s o l u b i l i t y o f th e d e s ir e d component o r a lo w e r r e s is t a n c e
t o t r a n s f e r f o r one component than f o r o t h e r s , o r both ( 4 1 ) .
A commercial i n s t a l l a t i o n f o r th e r e c o v e ry o f hydrogen from a
r e f i n e r y gas m ix t u r e u s in g h o llo w f i b e r s o f dacron p o l y e s t e r has been
i n o p e r a t io n s in c e 1969 ( 4 2 ) .
•
•
The use o f a membrane systems t o s e p a ra te h e liu m fro m n a t u r a l gas
has r e c e iv e d c o n s id e r a b le a t t e n t i o n .
S te rn e t . a l . (43) s t u d ie d a
l a r g e number o f polym er m a t e r i a ls w i t h h e liu m , n i t r o g e n , and methane
m ix t u r e s .
S e p a ra tio n f a c t o r s f o r He-Ng and He-CH^ systems were r e p o r te d
f o r 16 polymers i n c l u d i n g s i l i c o n e r u b b e r , t e f l o n FEP, p o ly s t y r e n e ,
p o l y v in y l c h l o r i d e , p o ly e t h y le n e , p o l y v i n y l
f l u o r i d e , m y la r and sa ra n .
Large s c a le t e s t s have a ls o been conducted on th e use o f c e l l u l o s e
a c e t a t e f i l m s f o r th e s e p a r a tio n o f h e liu m from n a t u r a l gas ( 4 4 ) .
The s e p a r a tio n o f oxygen from a i r u s in g an
has been s t u d ie d ( 4 5 ) .
t o 32.6%.
e th y l c e llu lo s e f ilm
In a o n e -s ta g e process oxygen c o u ld be e n ric h e d
11
L a b o r a to ry s c a le s e p a r a tio n in f o r m a t i o n has been r e p o r te d f o r a
la r g e number o f p o lym e r system s..
B rub a ke r and Kammermeyer (46)
prese n te d data f o r p o ly e t h y le n e w i t h f o u r gas systems (He-Og,
COg-Hg-Og-Ng, SOg-Og-Ng= NHg-Hg-Ng), T r if iu o r o m o n o c h lo r o e th y le n e w i t h
two gas systems (COg-Hg-Og-Ng, NHg-Hg-Ng) and c e l l u l o s e a c e ta te
b u t y r a t e w i t h one gas system (COg-Hg-Og-Ng)..
f o u r polymer m a t e r i a ls
■ p a ra -x y ly le n e )
McCandless (47) t e s te d
( c a p r o la c ta m , d a c ro n , p a ry le n e C p o ly (monochloro
and p o ly im id e ) w i t h a b i n a r y gas system o f carbon
monoxide and hydrogen.
T a j a r and M i l l e r (4 8 ) r e p o r te d data f o r th e p erm eation o f COg,
Og and Ng i n a f o u r component membrane system composed o f p o l y e t h y l enim ine, p o l y v i n y l b u t y r a l , epoxy and w a t e r .
T h is membrane system was
found t o be q u i t e s e l e c t i v e f o r COg o v e r Og and Ng.
P e r m e a b ilit y
c o e f f i c i e n t r a t i o s on t h e o r d e r o f 30:1 o r g r e a t e r were r e p o r te d .
M o d ifie d v i n y l i dene f l u o r i d e membranes have been used in two
s tu d ie s .
S e ib e l and McCandless (49) used v i n y l idene f l u o r i d e m o d ifie d
w i t h s u l f o l a n e f o r t h e s e p a r a tio n o f SOg from Ng.
ra n g in g fro m 30 t o 100 were o b t a in e d .
Z a va le ta
S e p a ra tio n f a c t o r s
(50) s t u d ie d a number
o f chem ical m o d if ie r s f o r v i n y l i d e n e f l u o r i d e in o r d e r t o e f f e c t the
s e p a r a tio n o f SOg from Ng.
The m o d if ie r s t e s te d in c lu d e d :
s u lfo le n e ,
3 -m e thyl s u l f o l e n e , p - ami no s u l f o l e n e , phenyl s u l f o n e , 4 , 4 - s u l fo n y l
d ip h e n o l, and t e t r a h y d r o - 3 - thiophenam ine 1 , 1 - d i o x i d e .
Membranes
c o n t a i n in g 18 w e ig h t p e r c e n t s u l f o l e n e gave th e b e s t r e s u l t s .
Membranes
- 12 o f t h i s c o m p o s itio n a llo w e d a permeate stream c o n t a in in g 95.5 volume
p e r c e n t SOg to be o b ta in e d from a feed stream c o n t a in in g 5.6% SOg.
E.
MEMBRANES FOR SEPARATING HgS FROM GAS MIXTURES
There i s some i n f o r m a t i o n in th e l i t e r a t u r e c o n c e rn in g membrane
s e p a r a tio n s o f HgS.
U. S. 3 ,819,80 6 (51) has been.awarded t o Ward,
Salemme and Mayer f o r an im m o b iliz e d l i q u i d membrane w hich i s used to
s e p a ra te HgS fro m a gas m ix t u r e .
The d i s s o c i a t i o n o f HgS i n w a te r
p ro c e e d s ^ a c c o rd in g t o th e f o l l o w i n g r e a c t i o n :
. H2S
<-
H+
+
HS"
By fo rm in g an im m o b iliz e d l i q u i d membrane from an aqueous s o l u t i o n
c o n t a in in g s p e c i f i c a n io n s , th e a c id t o HS" t r a n s f e r so p ro v id e d
g r e a t l y in c re a s e s th e t r a n s p o r t o f HgS across th e membrane from the
v a lu e expected based on d i f f u s i v i t y and s o l u b i l i t y .
A r e c e n t paper prese n te d by Matson and Kimura (52) d iscussed
h i g h l y permeable and s e l e c t i v e p o ly m e ric membranes f o r HgS removal
w h ich are based on th e p r i n c i p l e o f f a c i l i t a t e d t r a n s p o r t .
Chemical
r e a c t i o n augments t h e d i f f u s i o n process th ro u g h th e f o l l o w i n g r e a c t i o n :
H2S
+
CO3=
Z - HS"
+ HCO3"
Robb (53) r e p o r te d th e p e r m e a b i l i t i e s o f a number o f g a s e s , in c lu d in g
hydrogen s u l f i d e , i n a d i m e t h y l - s i l i c o n e r u b b e r c o n s i s t i n g o f d im e th y l
- 13 s ilo x a n e p lu s 33% by w e ig h t s i l i c a
fille r .
The p e r m e a b i l i t y o f HgS in
th e f i l m was s i g n i f i c a n t l y g r e a t e r than t h a t o f th e a tm o s p h e ric gases
n it r o g e n and oxyge n.
is
The v a lu e o f th e p e r m e a b i li t y c o n s ta n t f o r HgS
1000 x 10 ^ ccgas (RTF) cm/sec cm^cmHgAP compared t o 28 x 10 ^
ccgas (RTP)cm/sec cm cmHgAP f o r n i t r o g e n and 6 x 10- y cc g a s (RTP)cm/sec
2
cm cmHgAP f o r oxygen (RTF i n d ic a t e s room te m p e ra tu re and p r e s s u r e ) .
Dim ethyl s i l i c o n e membranes w hich have much g r e a t e r p e r m e a b i li t y
t o HgS than a tm o s p h e ric gases have been developed by General E l e c t r i c
(5 4 ).
o
P e r m e a b ili t y o f HgS i s r e p o r te d as 840 x 10- ^ c c g a s (RTP)cm/sec
1
_g
cm cmHgAP compared t o 25 x 10
2
ccgas (RTP)cm/sec cm cmHgAP f o r n it r o g e n
_ Q
and 50 x 10
p
ccgas (RTP) cm/sec cm cmHgAP f o r oxygen. .
The p e r m e a b i l i t y o f s e v e ra l commercial f i l m s t o hydrogen s u l f i d e
gas was s t u d ie d by Heilm an, e t . al
(5 5 ).
The f i l m s t e s t e d in c lu d e d
Nylon 6 p o lya m id e , c e l l u l o s e a c e t a t e , ru b b e r h y d r o c h l o r i d e , c e l l u l o s e ,
p o ly e t h y le n e , p o ly v i n y l b u t y r a l , p o l y v i n y l t r i f l u o r o a c e t a t e ,
p o l y v i n y l i dene c h l o r i d e and e t h y l c e l l u l o s e .
m y la r ,
THEORETICAL BACKGROUND
A.
NATURE OF THE TRANSPORT PROCESS 1
'
The p erm eation o f a gaseous m ix t u r e th ro u g h a p o ly m e ric m a te r ia l
in v o lv e s t h r e e t r a n s p o r t sta g e s ( F ig u r e I I I - l ) .
These stages a r e :
B.
(1 )
T r a n s p o r t from th e gas m ix t u r e t o th e membrane s u r fa c e .
(2 )
T r a n s p o r t th ro u g h th e membrane.
(3 )
T r a n s p o r t from th e membrane t o th e permeate s tre a m .
TRANSPORT THROUGH A GASEOUS FILM
In gas-phase p e rm e a tio n , t r a n s p o r t t o the.membrane s u r fa c e and
from th e membrane s u r fa c e p ro v id e s n e g l i g i b l e r e s is t a n c e compared to
t h a t o f t r a n s p o r t th ro u g h th e membrane ( 5 6 ) .
The c o n c e n t r a t io n g r a d ie n t
between th e h ig h p re s s u re s id e o f th e membrane and th e b u lk gas can
be g r e a t l y reduced by m a in t a in in g a h ig h gas f lo w r a t e across th e h ig h
p re s s u re s id e o f th e membrane, as was done in t h i s s t u d y .
TRANSPORT THROUGH A POLYMERIC MEMBRANE
The b a s ic e q u a tio n d e s c r ib in g membrane d i f f u s io n
s te a d y s t a t e i s
tra n s p o rt at
(57):
Na = - D
T h is e q u a tio n d i f f e r s
-
(I)
-UTm
from th e P i c k 's f i r s t law e q u a tio n by
a d d i t i o n o f th e second te rm due t o th e presence o f th e membrane.
In
15 -
PERMEATE
FIGURE I I I - I .
CONCENTRATION PROFILES OF COMPONENT a IN THE
MEMBRANE AND ITS VICINITY
- 16 gaseous d i f f u s i o n th e second term i s u s u a ll y n e g l i g i b l e
(5 8 ).
N e g le c tin g t h i s term y i e l d s
3.C
N
a
- D
a
(2 )
Tz
which i s th e m ath em a tical e x p r e s s io n o f P i c k 's f i r s t law f o r steady
s ta te d iffu s io n .
Since th e c o n c e n t r a t io n i s a f u n c t i o n o n ly o f d is ta n c e through
t h e membrane, whole d e r i v a t i v e s can be used.
N
a
= -D
fa
(3 )
dZ
I f an average v a lu e o f th e d i f f u s i v i t y i s used. Equation 3 can be
i n t e g r a t e d s u b j e c t t o th e f o l l o w i n g boundary c o n d it io n s :
(4)
t o g iv e
(5)
d o in g th e i n t e g r a t i o n r e s u l t s
in th e e x p re s s io n
- N a U 1 - Z 2 ) = - D (C a l - Ca 2 )
'
(Z 2 - Z1 ) r e p r e s e n ts th e th ic k n e s s o f th e membrane, I ,
e q u a tio n s can be w r i t t e n :
(6 )
so th e f o l l o w i n g
17 -
(7 )
or
( 8)
The s o l u b i l i t y o f a gas i n a polym er a t e q u i l i b r i u m can be '
d e s c r ib e d by a H e n ry 's Law e x p r e s s io n , where th e c o n s ta n t S r e p re s e n ts
th e s o l u b i l i t y o f th e gas i n th e polym er.
(9)
Since th e p erm eation process i s g e n e r a ll y s lo w , th e use o f t h i s
e q u i l i b r i u m r e l a t i o n s h i p between th e c o n c e n tr a t io n o f sorbed gas and
t h e p a r t i a l p re s s u re a t th e i n t e r f a c e can be used.
S u b s t i t u t i n g th e H e n ry 's Law e x p re s s io n i n t o e q u a tio n 8 r e s u l t s
i n th e e x p r e s s io n :
(
I f s o lu b ility
10)
i s assumed t o be a f u n c t i o n o n ly o f te m p e ra tu r e ,
and both membrane s u r fa c e s are a t th e same te m p e ra tu r e , then
( 11 )
and
(1 2 )
18 The p a r t i a l
o f th e t o t a l
p re ssu re s a t th e i n t e r f a c e s can be expressed in terms
p re s s u re s :
(1 3 )
%
Making t h i s
= pZ^a2
s u b s titu tio n y ie ld s :
„
OS (P1(Xa i )0 - P2[ya;, ) 0 )
d
(M )
L
From t h i s e x p r e s s io n i t
i s c l e a r l y seen t h a t th e f l u x o f gas
th ro u g h a membrane is dependent on s o l u b i l i t y and d i f f u s i v i t y o f th e
gas i n th e membrane.
add p l a s t i c i z e r s
One o f th e o b j e c t i v e s o f t h i s re s e a rc h i s to
to p o ly m e ric membranes in o r d e r to in c re a s e s o l u b i l i t y
and th e r e b y in c re a s e th e f l u x o f t h e d e s ir e d component (H g S ).
D.
POLYMER CHEMISTRY ASPECTS
Gases are t r a n s p o r t e d th ro u g h a p o l y m e r i c . nonporous membrane by
means o f d i f f u s i o n .
As can be seen fro m Equation 14 th e process depends
on t h e s o l u b i l i t y o f th e p e n e tr a n t in th e membrane as w e ll as the
m o b ility
o f th e p e n e tr a n t m o le cu le in t h e polym er m a t r i x .
A t a g iv e n moment a p e n e tr a n t m o le cu le can be v i s u a l i z e d as
o ccu p y in g a v a ca n t s i t e e x i s t i n g between a d ja c e n t polymer, c h a i n s .
The
p e n e tr a n t procee ds, under th e in f l u e n c e o f a c o n c e n t r a t io n g r a d i e n t ,
fro m one p o s i t i o n t o a n o th e r and e v e n t u a ll y achieves a f i n i t e
jump in
)
— 19 —
th e d i r e c t i o n imposed by th e c o n c e n t r a t io n g r a d i e n t .
The o v e r a l l m otion
o f a p a r t i c l e c o n s is ts o f a s e r ie s o f s te p s o r d i f f u s i o n a l jumps.
Gases a re unable t o permeate th ro u g h c r y s t a l l i n e r e g io n s o f a
polym er because polym er chains in th e s e re g io n s are t i g h t l y l i n k e d ,
le a v in g no vacant s i t e s f o r gas m o lecu les t o move i n t o .
C r y s ta llin e
re g io n s have s e v e ra l o t h e r n e g a tiv e e f f e c t s on th e r a t e o f gaseous
d i f f u s i o n th ro u g h a p o ly m e r.
The t h r e e p r i n c i p l e e f f e c t s a re :
r e d u c t io n i n th e a v a i l a b l e polymer volume f o r d i f f u s i o n ,
th e
th e t o r t u o s i t y
in v o lv e d in b y -p a s s in g c r y s t a l l i n e a r e a s , and th e decrease i n th e
m o b i l i t y o f amorphous c h a in segments as a r e s u l t o f t h e presence o f
. c r y s t a l l i n e re g io n s w hich may a c t as c r o s s l i n k s .
At
th e g la s s t r a n s i t i o n te m p e ra tu re o f a polym er t h e r e i s a
change in p h y s ic a l p r o p e r t i e s o f t h e p o lym e r.
Above th e g la s s t r a n s i t i o n
te m p e ra tu re t h e r e i s in c re a s e d m o le c u la r freedom .
c h a in segments are f r e e t o v ib r a t e , and t w i s t .
That i s ,
m o le c u la r
Movement o f p e n e tr a n t
m olecu les th ro u g h th e polym er i s much e a s i e r above th e g la s s t r a n s i t i o n
te m p e ra tu re .
The a d d i t i o n o f p l a s t i c i z e r s o r m o d if ie r s t o a polym er has s e v e ra l
e ffe c ts .
v a p o r.
P la s tic iz e rs
in c re a s e th e d i f f u s i v i t y o f a g iv e n gas o r
The in c re a s e i s a t t r i b u t e d t o th e in c re a s e in . p o ly m e r segmental
m o b i l i t y as a r e s u l t o f lo w e r cohe sive f o r c e s between c h a in s ( 5 9 ) .
!
■«
M o d if ie r s a ls o lo w e r th e g la s s t r a n s i t i o n te m p e ra tu re by a l lo w in g f o r
in c re a s e d m o b i l i t y o f polym er c h a in segm ents.
S o l u b i l i t y o f gases o r
/
- 20 vapors m ay-a lso be g r e a t l y in c re a s e d by t h e a d d i t i o n o f m o d i f i e r s ,
p a r t i c u l a r l y when th e gas o r vapor i s s o l u b l e i n th e m o d i f i e r .
The
n e t e f f e c t o f m o d i f i e r a d d i t i o n i s a marked in c re a s e i n th e gas
p erm eation r a t e .
E.
TEMPERATURE EFFECTS
As can be seen from E q uation 14:
DS( W
o
- lV v U
th e e f f e c t o f te m p e ra tu re on perm eation f l u x w i l l
th e manner i n which d i f f u s i v i t y
te m p e ra tu re i f
(D) and s o l u b i l i t y
depend p r i m a r i l y on
(S) depend on
th e o p e r a t in g p ressures remain c o n s t a n t .
The tem p era ture,
dependence o f d i f f u s i v i t y conforms w i t h an
A r r h e n i u s - ty p e r e l a t i o n s h i p
(6 0 ),
-E/RT
D = D0 e
By m easuring d i f f u s i v i t i e s
(15)
a t s e v e ra l tem p e ra tu re s an A rrh e n iu s
p l o t o f InD vs 1/T y i e l d s th e a c t i v a t i o n energy f o r d i f f u s i o n o f th e
p e n e tr a n t i n th e polym er ( 6 1 ) .
I t i s c l e a r , th e n , t h a t in c r e a s in g th e te m p e ra tu re o f a perm eation
system w i l l
cause th e d i f f u s i v i t y t o in c re a s e e x p o n e n t i a l l y w i t h
a b s o lu te te m p e ra tu re .
For s o r p t i o n o f gases i n t o s o l i d polym er system s, s o l u b i l i t y
a ls o f o l l o w s an A r r h e n i u s - ty p e r e l a t i o n s h i p ( 6 2 ) ,
“ 21 —
n -Ah/RT
S = S0 e
(1 6 )
where Ah is th e a p p a re n t h e a t o f s o lu t io n .
t h i s dependence on te m p e ra tu re is
A n o th e r means o f e x p re s s in g
in term s o f th e H e n ry 's Law c o n s ta n t
(63),
o
-W R T
H = H0 e
(17)
T hus, in c re a s in g te m p e ra tu re in c re a s e s gas s o l u b i l i t y
in an
u n m o d ifie d polym e r f i l m .
•
. However, f o r m o d ifie d f ilm s to w h ich l i q u i d m o d ifie r s have been
added, th e te m p e ra tu re e f f e c t on s o l u b i l i t y
is more c o m p lic a te d .
The s o l u b i l i t y o f gases in l i q u i d m o d ifie r s u s u a lly decreases as
te m p e ra tu re is
in c re a s e d .
S ince th e gases a re much more s o lu b le in
th e m o d if ie r than in th e base p o ly m e r, i t
is p o s s ib le t h a t t h i s e f f e c t
ou tw e ig h s th e in c re a s e d s o l u b i l i t y o f gases in th e p o lym e r.
f l u x c o u ld a c t u a lly decrease w ith
T h e re fo re ,
in c re a s in g te m p e ra tu re .
For a m o d ifie d polym e r f i l m , d i f f u s i v i t y
in c re a s e s as te m p e ra tu re
in c re a s e s , w h ile th e s o l u b i l i t y term is a cte d on by o p p osin g e f f e c t s .
For u n m o d ifie d f i l m s , d i f f u s i v i t y
in c re a s e s and th e s o lu b ii
However, s tu d ie s w i t
s o l u b i l i t y and d i f f u s i v i t y
in c re a s e s as te m p e ra tu re
t y o f th e gases in th e polym er a ls o in c re a s e s .
p o ly e th y le n e (6 4 ) have shown t h a t both
.re dependent on c r y s t a l l i n i t y .
S o lu b ility
was fo u n d to be d i r e c t l y p r o p o r tio n a l t o th e volume p e rc e n t amorphous
m a te r ia l p re s e n t.
I t is re a so n a b le to assume t h a t t h is c r y s t a l l i n i t y
22 -
dependence w i l l
be t r u e f o r o th e r system s in c lu d in g th e f ilm s te s te d in
t h i s re s e a rc h .
T h e re fo re , d iffe r e n c e s between th e te m p e ra tu re -p e rrn e a te -
f l u x r e la t io n s h ip f o r d i f f e r e n t f ilm s may be due to d iffe r e n c e s
in
c r y s ta llin it y .
Because d i f f e r e n t gases have d i f f e r e n t va lu e s o f a c t iv a t io n energy
f o r d if f u s io n and h e a t o f s o lu t io n , th e va lu e s o f d i f f u s i v i t y and
s o l u b i l i t y c o n s ta n ts w i l l
change a t d i f f e r e n t . r a t e s w ith te m p e ra tu re .
T h e re fo re , a change in te m p e ra tu re w i l l
change th e c o m p o s itio n o f th e
perm eate gas.
F,
DEFINITION OF THE SEPARATION FACTOR
The lo c a l s e p a ra tio n f a c t o r a
o f component a in a gaseous m ix tu re
is d e fin e d as:
V
(1-y a)
(1 7 )
V crV
I f p e r f e c t m ix in g can be assumed, th e lo c a l s e p a ra tio n f a c t o r aQ
has a c o n s ta n t v a lu e f o r any p o in t in th e membrane and is id e n t ic a l to
th e o v e r a ll s e p a ra tio n f a c t o r a.
The s e p a ra tio n fa c to r ,w h ic h is analogous to th e " r e l a t i v e
v o la tility "
in d i s t i l l a t i o n , g iv e s an id e a o f th e s e l e c t i v i t y o f a
membrane w ith re s p e c t to a component o f a gaseous m ix tu r e .
The h ig h e r
th e s e p a ra tio n f a c t o r , th e h ig h e r th e s e l e c t i v i t y f o r th e d e s ire d
com ponent.
■
EXPERIMENTAL EQUIPMENT, MATERIALS, AND PROCEDURES
A.
DESCRIPTION OF EQUIPMENT
The arrangem ent o f th e e x p e rim e n ta l equipm ent used in t h is
p e rm e a tio n s tu d y is shown s c h e m a tic a lly in F ig u re IV - 1 .
c o n s is ts o f th e f o llo w in g p a r ts :
The equipm ent
th e perm eation, c e l l , a c o n s ta n t
te m p e ra tu re e n c lo s u re and te m p e ra tu re m easuring in s tr u m e n ts , gas m ix tu re
s to ra g e and fe e d in g e q u ip m e n t, purge gas system com ponents, permeate
r a te measurement eq u ip m e n t, gas c o m p o s itio n a n a ly s is eq u ip m e n t, and
equipm ent f o r a n a ly z in g th e s u rfa c e and c o m p o s itio n o f membranes.
A d e s c r ip tio n o f th e in d iv id u a l components f o llo w s :
(I)
P erm eation C e ll
A diagram o f th e pe rm e a tio n c e ll
is shown in F ig u re IV -2 .
I t is
f a b r ic a t e d fro m two s t a in le s s s te e l b la n k .fla n g e s w h ich a re 5 /8 in c h
t h ic k and 4 .5 in ches in d ia m e te r.
fla n g e .
A c a v it y is machined in t o each
On th e h ig h p re s s u re s id e th e c a v ity a llo w s fe e d gas to
c ir c u l a t e and move a cro ss th e membrane s u r fa c e .
a porous s t a in le s s s te e l d is k covered w ith f i l t e r
th e c a v it y to s u p p o rt th e membrane.
paper is p la ce d in
The membrane is se a le d between
two t e f l o n g a ske ts and th e p erm eation c e l l
e q u a lly -s p a c e d b o lt s .
On th e permeate s id e
is clamped s h u t by e ig h t
The g a s k e t o p e n in g s , w h ic h . d e te rm in e th e exposed
O
membrane s u r fa c e , have an area o f 2 0 .3 cm .
therm oco uple w e ll on th e h ig h p re s s u re s i d e . •
The c e ll has a tu b u la r
0
(I)
Feed Gas C y lin d e r
Tem perature E n clo su re
(8 ) P ressure Gauge
(II)
R otam eter
T e flo n Tube
(2 ) P ressure R e g u la to r
(5 ) Tubing C o il
(1 8 ) Check V alve
(6 ) Perm eation C e ll
(9 ) Back P ressure R e g u la to r
(1 2 ) Sam pling Septum (Perm eate)
(1 5 ) O il Seal
(3 ) M e te rin g Needle Valve
(7 ) High Speed Fan
(1 0 ) Sam pling Septum (V ent Gas)
(1 3 ) Length Measurement
(16) Purge Gas C y lin d e r
(14) C a lib ra te d
(1 7 ) Low P ressure R e g u la to r
(1 9 ) B lo ck Valve
FIGURE IV -1 .
(4 ) C o nsta nt
SIMPLIFIED DIAGRAM OF PERMEATION EQUIPMENT
To Purge Gas
^ System
Permeate to
Measurement
High P ressure
Feed Gas
High P ressure
Gas to Vent
(I)
Membrane
(2 )
(4 )
F i l t e r Paper
T h e rm is to r and Thermocouple
(5 )
(3 )
Porous S ta in le s s S te e l D isk
FIGURE IV -2 .
PERMEATION CELL DIAGRAM
Gaskets
- 26 (2 ) . C o n sta n t Tem perature E n clo su re and Tem perature M easuring Equipment
The c o n s ta n t te m p e ra tu re e n c lo s u re is c o n s tru c te d fro m a s e c tio n o f,
18 in c h d ia m e te r asbe stos p ip e 12 in ch e s h ig h . . The p ip e w a ll th ic k n e s s
is 3 /4 in c h .
The bottom , o f th e e n c lo s u re is se a le d w ith a 1 /4 in ch
asbe stos b o a rd , and th e b o ttom and s id e s o f th e e n c lo s u re a re lin e d
w ith 3 /4 in c h t h ic k f ib e r g la s s in s u la t io n .
The I id o f th e e n c lo s u re
c o n s is ts o f 3 /4 in c h in s u la t io n sandwiched between 1 /4 in c h asbestos
b o a rd s.
To in s u re a t i g h t se a l th e in s u la t io n and lo w e r board have a
d ia m e te r equal to th e in n e r d ia m e te r o f th e e n c lo s u re w h ile th e to p
asbe stos board has a d ia m e te r equal t o th e o u te r d ia m e te r o f th e c o n s ta n t
te m p e ra tu re e n c lo s u re .
Gas lin e s e n te r th e e n c lo s u re th ro u g h grooves
c u t in th e to p s u rfa c e o f th e asbe stos p ip e s e c tio n .
h o le s are c u t. in th e to p o f th e e n c lo s u re .
Two 1 /4 in ch
A therm om eter is placed
in one, and th e o th e r p ro v id e s access to a s ilic o n e ru b b e r septum w hich
a llo w s samples o f th e .p e rm e a te gas to be ta ke n w ith o u t d is t u r b in g th e
c o n s ta n t te m p e ra tu re e n c lo s u re .
The perm eate l i n e le a ve s th e e n c lo s u re
th ro u g h a 1 /4 in c h h o le in the. s id e o f th e e n c lo s u re .
For runs above room te m p e ra tu re , h e a t is p ro v id e d by two 500 w a tt
h e a te rs p la c e d in th e bottom o f th e e n c lo s u re .
The h e a te rs a re covered
w ith a p ie c e o f asbestos board to s h ie ld th e perm eation c e l l and feed
lin e s fro m d i r e c t exposure to th e h e a te r s .
The in p u t c u r r e n t to one
o f th e h e a te rs is c o n t r o lle d by a P o w e rs ta t (The S u p e rio r E le c t r ic
Company) v a r ia b le tr a n s fo r m e r, w h ile th e in p u t to th e o th e r h e a te r is
.
- 27 c o n t r o lle d by a T he rm istenp Model 63 t h e r m is to r te m p e ra tu re c o n t r o ll e r
(Y e llo w S p rin g s In s tru m e n t Company) th ro u g h a n o th e r P o w e rs ta t v a r ia b le
tra n s fo r m e r.
The system is diagrammed in F ig u re IV -3 .
The th e r m is to r
.
probe and an ir o n - c o n s ta n ta n th e rm oco uple f o r te m p e ra tu re measurement
a re mounted in th e th e rm o w e ll s e c tio n o f a te e lo c a te d a t th e gas i n l e t
to th e h ig h p re s s u re s id e o f th e pe rm e a tio n c e l l .
The the rm o co u p le is
connected to a Speedomax Type G (Leeds and N o rth ro p ) c h a r t r e c o rd e r.
A h ig h speed 24 w a tt fa n ( Pamotor Model 4500) is mounted on th e asbestos
b a f f le t o c i r c u l a t e a i r and in s u re u n ifo rm te m p e ra tu re in th e e n c lo s u re .
A m ercu ry therm om eter is used to d e te rm in e th e te m p e ra tu re o f the
a i r in s id e th e e n c lo s u re . 3
(3 )
Gas M ix tu re S torage and Feed Equipment
O
The fe e d gas m ix tu re s a re s to re d in 285 f t
(STP) s iz e gas c y lin d e r s
(N a tio n a l C y lin d e r Gas D iv is io n o f Chemetron C o rp o ra tio n ) .
Flow is
c o n t r o lle d u s in g a Matheson Model B16 c o r r o s io n - r e s is t a n t p re ssu re
r e g u la t o r .
The r e g u la t o r p re s s u re is s e t j u s t above th e d e s ire d
o p e ra tin g p re s s u re .
A Grove M ig h ty M ite back p re s s u re r e g u la t o r
lo c a te d downstream o f th e membrane t e s t c e l l
o p e ra tin g p re s s u re .
is used to s e t th e
O p e ra tin g p re s s u re is checked u s in g a Matheson
Company p re s s u re gauge lo c a te d j u s t u p s tre a m .o f th e back p re ssu re
r e g u la t o r .
Gas f lo w r a te is d e te rm in e d u s in g a c a lib r a t e d ro ta m e te r
( F is c h e r and. P o rte r Company) p la ce d a f t e r th e back p re s s u re r e g u la t o r
Xxxxxxxv
7777
XXXXXXX\ XXXXX
(I)
C e ll
Feed Gas
(2 ) H eaters
(6 ) Thermal
(9 ) Thermometer
I n s u la tio n
(1 1 ) Exhaust Gas
(1 4 ) Tem perature Recorder
(1 6 ) Power S upply
FIGURE IV -3 .
(4 ) Fan
(5 ) Perm eation
(7 ) C o n sta n t Tem perature E nclosu re
(1 0 ) Thermocouple Probe
(1 3 ) V a ria b le T ra n sfo rm e rs
C o n t r o lle r
(3 ) Heat Exchange C o il
(1 7 ) Asbestos B a ff le
(8 ) Permeate
(1 2 ) T h e rm is to r Probe
(15) Therm istem p Tem perature
(1 8 ) Purge Gas
CONSTANT TEMPERATURE ENCLOSURE — HIGH TEMPERATURE
- 29 and is s e t u s in g a W hitey m ic ro -m e te rin g v a lv e w ith a v e r n ie r h a n d le .
To in s u re u n ifo rm gas te m p e ra tu re , a 60 f o o t c o il o f 1 /8 in c h s t a in le s s *
s te e l tu b in g is connected in t o th e fe e d l i n e j u s t upstream o f the
membrane s u r fa c e .
A ll tu b in g in th e fe e d system is
1 /8 in c h s t a in le s s
s te e l.
(4 )
Purge Gas System Components
To remove a l l n itr o g e n fro m th e perm eate l i n e , and th e re b y
in c re a s e th e a ccu ra cy o f perm eate c o m p o s itio n m easurem ents, a purge
gas system is used.
The purge gas system is shown in F ig u re IV -1 .
It
in c lu d e s a gas c y lin d e r ( Matheson Company) equipped w ith a Matheson
p re s s u re r e g u la t o r and a m e te rin g v a lv e .
Purge gas is fe d fro m th e
c y lin d e r th ro u g h a check v a lv e , a b lo c k v a lv e , and then in t o the
perm eate l i n e .
(5 )
Both h e liu m and carbon d io x id e a re used as purge gases.
Permeate Rate Measurement Equipment
The r a te o f p e rm e a tio n is d e te rm in e d by tim in g th e movement o f a
column o f pump o i l
(VanWaters & Rogers) in s id e a c a lib r a t e d 4 f o o t
s e c tio n o f t e f lo n tu b in g (C hem plast In c . C hem fluor S p e c ia l FEP T u b in g ) .
The tu b in g is c a lib r a t e d by d e te rm in in g th e w e ig h t o f w a te r re q u ire d to
fill
a g iv e n le n g th o f tu b in g .
The volume p e r u n i t le n g th o f th e
tu b in g has been d e te rm in e d to be .034 cc/cm .
The o i l
is moved in to
re a d in g p o s it io n by d ra w in g gas o u t o f th e permeate l i n e th ro u g h a
s ilic o n e ru b b e r septum w ith a .5 cc gas s y r in g e .
30 (6 )
Gas C o m po sition A n a ly s is Equipment
Gas samples a re taken th ro u g h s il ic o n e ru b b e r septums p laced in
th e fe e d and perm eate lin e s u s in g a .5cc " p r e s s u r e - lo k " s y rin g e
( P r e c is io n Sam pling C o r p o r a tio n ) .
,
Samples a re an a lyze d u s in g a V a ria n Aerograph S e rie s 1400 therm al
. c o n d u c t iv it y gas chrom atograph coup led w ith a S a rg e n t Model SR c h a r t
re c o rd e r.
The chrom atograph column is a 6 f o o t s e c tio n o f 1 /8 in c h
s t a in le s s s te e l tu b in g packed w ith Porapak Q-S p a ckin g (W aters
A s s o c ia te s , I n c . ) .
The f o llo w in g o p e ra tin g c o n d itio n s a re used:
Column Tem perature - I l S 0C
' D e te c to r Tem perature - I lO 0C
C a r r ie r Gas, Flow - 17.6 m l/m in
D e te c to r C u rre n t - 150 m i l l i amperes
C a r r ie r Gas - Helium
The chrom atograph is c a lib r a t e d u s in g d i f f e r e n t volumes o f pure
n itr o g e n and hydrogen s u l f i d e .
I t has been found (65). t h a t f o r t h i s system d u p lic a te ana lyses o f
gas samples o f known c o m p o s itio n s a re a c c u ra te to abo ut ±1% o f th e
a b s o lu te amount p re s e n t.
(7 )
Membrane S u rfa ce and C o m po sition A n a ly s is Equipment
A C a rl Z e iss m u lt i- o c u la r l i g h t m icroscope is used to examine th e
s u rfa c e s o f a l l
v i n y l i dene f lu o r id e membranes f o r h o le s o r o th e r d e f e c t s .
The m icroscope a llo w s m a g n ific a tio n s o f 4 0 x, IOOx, 200x and 400x to be
-3 1
-
used.
Many o f th e m o d ifie d v in y lid e n e f lu o r id e membranes have been
examined u s in g a scann ing e le c tr o n m icroscope ( I . S . I . Model SMS 2-2
"Super 1 1 ", m an u fa ctu re d by I n te r n a t io n a l S c i e n t i f i c
In s tru m e n ts , In c .).
The e le c tr o n m icroscope g iv e s c le a r e r images and a llo w s h ig h e r
m a g n ific a tio n to .b e used.
A ls o , photographs are made th ro u g h the
e le c tr o n m icroscope to p ro v id e a re c o rd o f th e s u rfa c e , c h a r a c t e r is t ic s
o b se rve d .
To check f o r th e presence o f r e s id u a l d im e th y lform am ide s o lv e n t
in th e v in y lid e n e f lu o r id e membranes, samples o f a membrane made w ith
no m o d if ie r a re te s te d u s in g an in f r a r e d s p e c tro p h o to m e te r (Beckman
In s tru m e n ts Model IR 5A ).
Membrane samples are mounted in s t i f f
paper h o ld e rs f o r t e s t in g .
B.
MATERIALS
The f o llo w in g is a l i s t o f th e m a te r ia ls used in t h i s
■ I.
re s e a rc h :
V in y lid e n e F lu o r id e - used as th e base polym er f o r many o f
th e membranes te s te d in t h is s tu d y .
V in y lid e n e f lu o r id e
r e s in is
c o m m e rc ia lly a v a ila b le under th e tradenam e o f K yn a r, Grade 301 from
Pennw alt C o rp o ra tio n .
2.
No- a n tio x id a n ts a re used in th e r e s in .
D im e thylfo rm am ide - used as a s o lv e n t f o r v in y lid e n e f lu o r id e
and th e m o d if ie r to form a s o lu tio n from w h ich membranes a re c a s t.
Baker grade d im e th y lfo rm a m id e was purchased fro m J . T . - Baker Chemical
- 32 Company.
3.
S u lfo le n e - purchased fro m P h i l l i p s
P etroleum Company in
I l b . c o n t a in e r s .
4.
1 - M e th y l- 2 - P y r r o li d i none - purchased fro m J. I .
Company in 500 g b o t t le s .
5.
Baker Chemical
I
1
M onoethanolam ine - o b ta in e d fro m Union C a rbide C o rp o ra tio n
in 32 f l u i d ounce c o n ta in e r s .
6.
T rie th a n o la m in e - o b ta in e d fro m Union C a rbide C o rp o ra tio n
in 8 f l u i d ounce b o t t le s .
7.
3 -M e th yl S u lfo le n e - purchased from P h i l l i p s
P etroleum
Company in I lb c o n ta in e r s .
8.
M o n o iso p ro p a n o lam ine - o b ta in e d fro m Union C a rb id e C o rp o ra tio n
in 8 f l u i d ounce c o n ta in e r s .
9.
D iis o p ro p a n o l amine - o b ta in e d fro m Union C a rb id e C o rp o ra tio n
in 8 f l u i d
10.
ounce c o n ta in e r s .
M o rp h o lin e - o b ta in e d fro m Union C a rbide C o rp o ra tio n in 8
f l u i d ounce c o n ta in e r s .
11.
Hydrogen S u lf id e - p u r if ie d g ra d e , purchased fro m th e
Matheson Company in c y lin d e r s c o n ta in in g 22 pounds.
12.
N itro g e n - la b o r a to r y g ra d e , purchased fro m Chemetron C orpora­
t io n in c y lin d e r s c o n ta in in g 2 8 5 .c u b ic f e e t (S TP ).
13.
H elium - la b o r a to r y g ra d e , purchased fro m
in c y lin d e r s c o n ta in in g 285 c u b ic f e e t (S TP ).
Chemetron C o rp o ra tio n
- 33 14.
Carbon D io x id e - in d u s t r i a l g ra d e , purchased from Chemetron
C o rp o ra tio n in c y lin d e r s w ith a n e t w e ig h t o f 20 pounds.
15.
P o ly s u lfo n e f i l m
- compound P-1700 p o ly s u lfo n e f i l m
(.0 0 2 in c h
t h ic k ) was o b ta in e d fro m Union C a rb id e C o rp o ra tio n .
16.
D im ethyl S ilic o n e f i l m
- unbacked f i l m
was purchased from
G eneral E l e c t r i c C o rp o ra tio n in s ta n d a rd I f o o t by I f o o t squares w ith
a th ic k n e s s o f I m i l .
17.
t h is f ilm
S ilic o n e P o ly c a rb o n a te Copolymer f i l m
- tradenam ed MEM-213,
was purchased from G eneral E l e c t r i c C o rp o ra tio n in s ta n d a rd
I f o o t by I f o o t squares w ith a th ic k n e s s o f I m il.
18.
P olyam ide f i l m
- two d i f f e r e n t ty p e s o f t h is
f i l m , tradenamed
C apran, were o b ta in e d fro m A l l i e d Chemical C o rp o ra tio n .
sh e e ts I m il t h ic k were te s te d a t low te m p e ra tu re s .
Capran 80 f i l m
19.
Capran 77C
Heat s t a b iliz e d
I m il t h ic k was te s te d a t h ig h e r te m p e ra tu re s .
P o ly e th e rs u lfo n e f i l m
- was o b ta in e d from I . C . I .
U n ite d S ta te s ,
In c . in shee ts h a vin g a th ic k n e s s o f 2 m il.
20.
P o ly V in y l F lu o r id e f i l m
- DuPont "T e d la r " f i l m
100AG30UT was o b ta in e d in 8 -1 /2 in c h by 11 in c h shee ts
21.
F i l t e r Paper - Whatman q u a l i t a t i v e number 5.
22.
M asking ta p e - H ig h la n d brand u t i l i t y
in r o l l s
23.
number
.001 in c h t h ic k .
grade ta p e
was purchased
3 /4 in c h w ide x 60 y a rd s .
Rate M easuring F lu id - pump o i l
Company, C a ta lo g Number 54996.
fro m Van W aters & Rogers
~ 34 C.
EXPERIMENTAL PROCEDURE
(1 )
P re p a ra tio n o f Gas M ix tu re s
The gas m ix tu re s o f H2S-N2 a re pre p a re d as f o llo w s :
A gas c y lin d e r is evacuated u s in g a vacuum pump.
The c y lin d e r is
!
p re ssu re d up w ith H2S to th e re q u ire d p re s s u re .
N itro g e n is then added u n t i l th e chosen f i n a l p re s s u re is reache d.
The c y lin d e r is then p la ce d c lo s e to a h e a te r f o r one week to in s u re
t h a t com ple te m ix in g w i l l
ta k e p la c e .
The m ix tu re s a re found to be
v e ry u n ifo rm in c o m p o s itio n based on p e r io d ic a n a ly s is w ith th e
c a lib r a t e d gas chrom atograph.
(2 )
C a lib r a t io n o f Gas Chrom atograph
The Porapak Q-S column used f o r a n a ly s is o f H2SZN2 m ix tu re s is
c a lib r a t e d u s in g d i f f e r e n t volume samples o f pure hydrogen s u l f i d e
and n itr o g e n .
The samples a re ta ke n w ith a. .5 cc .."p re s s u re -!o k "
s y rin g e , ( P r e c is io n Sam pling C o rp o ra tio n ) th ro u g h a s ilic o n e ru b b e r
septum mounted on a low p re s s u re r e g u la t o r a t 10 p s ig ..
Samples are
a n a lyzed u s in g a V a ria n A erograph Model 1400 low volume th e rm a l
c o n d u c t iv it y gas chrom atograph cou p le d w ith a S a rge nt Model SR c h a r t
re c o rd e r.
The sample t e s ts a re re p e a te d s e v e ra l tim e s to in s u re
r e p r o d u c i b i l i t y , w h ich was found to be v e ry good.
The H2S and N2
peaks re co rd e d on th e re c o rd e r c h a r t paper are th e n co p ie d on a Xerox
c o p ie r u s in g s ta n d a rd co p y in g paper (4024 dual purpose w h ite ) and th e
- 35 r e s u lt in g peaks a re c u t o u t and t h e i r w e ig h t de te rm in e d u s in g a M e tt le r
p r e c is io n ba la n ce ( s e n s i t i v i t y to I x IO "4 g ra m s ).
t h i s c a lib r a t io n
data as a f u n c t io n o f th e p ro d u c t peak w e ig h t x
chrom atograph a tte n u a t io n .
lin e a r .
F ig u re IV -4 shows
The chrom atograph response is q u ite
W eight x a tte n u a tio n values' are d eterm ined f o r bo th components
in H2S m ix tu re s ra n g in g fro m 0 to 100% H2S a t 5% in t e r v a ls
d a ta on F ig u re IV -4 .
u s in g th e
For exam ple, f o r 10% H2S, th e w e ig h t x a tte n u a tio n
f o r .05 cc H2S and .45 cc N2 a re d e te rm in e d fro m th e g ra p h .
Area
p e rc e n t H2S is then c o r r e la te d w ith volume p e rc e n t H2S by c a lc u la t in g
.
th e v a lu e o f H2S w e ig h t x a tte n u a tio n d iv id e d by th e t o t a l w e ig h t x
a tte n u a tio n v a lu e .
The r e s u lt in g c a l ib r a t io n cu rve is shown in
F ig u re IV -5 .
(3 )
C a lib r a tio n o f Exhaust Gas R otam eter
The ro ta m e te r is c a lib r a t e d u s in g a w et t e s t m ete r a t room
te m p e ra tu re (2 3 ° C ).
C o rre c tio n s a re made to a ccou nt f o r th e in c re a s e
in h u m id ity o f th e gas.
The c a lib r a t io n c u rv e is shown in F ig u re IV - 6 ,
to g e th e r w ith th e e x p e rim e n ta l d ata used in i t s
d e te r m in a tio n .
'
(4 )
Membrane M anufacture
The two main ste p s in membrane m a n u fa c tu rin g are fo rm a tio n o f th e
membrane s o lu t io n and c a s tin g o f th e s o lu t io n to form a f i l m .
The membrane s o lu tio n is form ed as f o llo w s :
A p y re x bea ker is c a r e f u ll y cle aned and weighed on a M e tt le r
— 36 -
Tem perature = 23°C
A tm o sp h e ric P re ssure=
2 5 .3 i n . Hg
O
N itro g e n
□
Hydrogen S u lfid e
I_ _ _ _ _ _ _ _ _ _ I
•05
.1
I_ _ _ _ _ _ _ _ _ I
.15
.2
»
.25
I_ _ _ _ _ _ _ _ _ I_ _ _ _ _ _ _ _ _ _ t
.3
.35
.4
I
.45
Sample Volume (CC)
FIGURE IV -4 .
CALIBRATION OF GAS CHROMATOGRAPH-SAMPLE
VOLUME v s . PEAK WEIGHT x ATTENUATION FOR
NITROGEN AND HYDROGEN SULFIDE
.5
- 37 -
Hydrogen S u lf id e Area P e rce n t (%)
FIGURE IV -5 .
CALIBRATION CURVE FOR GAS CHROMATOGRAPH - AREA
PERCENT OF HgS v s . VOLUME PERCENT OF HgS
low Rate
( L it e r s
(S T P )/H ou rl
- 38 —
R otam eter Reading
FIGURE IV -6 .
ROTAMETER CALIBRATION CURVE-FLOW
RATE (STP) v s . ROTAMETER READING
-39 ba la n ce ( s e n s i t i v i t y
in t o th e b e a k e r.
I x 10 ^ g ) .
The membrane m o d if ie r is then weighed
The a p p ro p ria te amount o f v i n y l i dene f lu o r id e is
th e n weighed o u t on w e ig h in g paper and added to th e b e a ke r.
c o m p o s itio n o f th e membrane on a s o lv e n t - f r e e b a s is is
The
d e te rm in e d by
th e r e l a t i v e amounts o f v in y l I dene f lu o r id e and m o d if ie r added to th e
b e a k e r.
D im e thylfo rm am ide s o lv e n t is added in th e r a t i o o f 5 .7 cc
d im e th y lform am ide p e r gram o f v i n y l id e n e f l u o r id e .
The m ix tu re is
s t i r r e d w ith a chem ical s p a tu la f o r abo ut te n m in u te s .
b e a ke r is t i g h t l y covered w ith p o ly e th y le n e f i l m
Then, th e
and p la ce d on a h o t­
p la te s e t f o r low h e a t u n t i l com plete d is s o lu t io n is a c h ie v e d .
The
covered m ix tu re is then a llo w e d to sta n d a t room te m p e ra tu re f o r a
p e rio d w ith o c c a s io n a l s t i r r i n g
polym e r in th e s o lv e n t.
to in s u re com plete s o lu t io n o f the
T h is p e rio d is
15 days f o r . t h e membranes whose
t e s t ; 'r e s u lt s a re p re se n te d in t h i s t h e s is .
The 15 day s o lu t io n p e rio d was found to be ne ce ssa ry based on th e
r e s u lt s o f m ic ro s c o p ic e x a m in a tio n o f membrane s u r fa c e s .
E le c tro n
p h o to m icro g ra p h s showed t h a t by a llo w in g 15 days f o r polym e r to go
c o m p le te ly in t o s o lu t io n membranes w ith u n ifo rm h o le - fr e e s u rfa c e s were
produced.
Membranes form ed fro m polym er s o lu tio n w ith no w a itin g p e r io d ,
and w ith a 24 h o u r
w a it in g p e r io d , had rough s u rfa c e s and many o f them
c o n ta in e d h o le s .
F o llo w in g th e w a it in g p e r io d , a membrane f i l m
polym e r s o lu tio n as f o llo w s :
is form ed from th e
I
- 40 A g la s s p la te is
prepa red by p la c in g la y e rs o f m asking tape
a lo n g th e f o u r edges o f th e p la te to form ra is e d edges.
ta p e form a r id g e abo ut 2 m il h ig h .
c le a n e d .
S ix la y e rs o f
The p la te is th e n c a r e f u ll y
N e xt, th e polym e r s o lu t io n is uncovered and p la ce d in a chamber
connected t o th e "house vacuum" system (16 in ches Hg vacuum) f o r 30
m in u te s .
T h is is done to de-gas th e s o lu t io n .
The s o lu t io n is then
poured o n to th e g la s s p la te and e v e n ly d is t r ib u t e d by d ra w in g a g la s s
ro d a cro ss th e p la te w h ile th e rod re s ts on th e m asking ta p e .
The g la s s
p la te is n e x t p la ce d in an e l e c t r i c d r y in g oven a t 120°C f o r 30 m in u te s .
F o llo w in g removal fro m th e oven, th e p la te is a llo w e d to cool to room
te m p e ra tu re b e fo re th e membrane is s t r ip p e d o f f .
(5 )
M ic ro s c o p ic E xa m in a tio n o f Membrane S u rfaces
Each m o d ifie d v i n y l i dene f lu o r id e membrane is checked f o r le a ks o r
o th e r d e fe c ts by exam in ing s e v e ra l areas o f th e membrane under a l i g h t
m icro sco p e .
In o rd e r to g e t a b e t t e r view o f th e membrane s u rfa c e ,a n d as a
f u r t h e r check f o r le a k s , s m a ll sample pie ce s o f s e v e ra l o f th e membranes
have been examined u s in g a sca n n in g e le c tr o n m ic ro s c o p e .
Some samples
ta ke n fro m membranes a f t e r th e y had been te s te d were a ls o viewed u s in g
th e e le c tr o n m icro sco p e .
\
- 41 (6 )
T e s t f o r th e Presence o f R esidual S o lv e n t U sing I n f r a r e d S p e c tro scopy_________________________________________________
A v in y lid e n e f lu o r id e membrane produced w ith no m o d if ie r was te s te d
u s in g an in f r a r e d s p e c tro p h o to m e te r in o rd e r to check f o r th e presence
o f r e s id u a l d i m e th y lform am ide s o lv e n t .
The c a rb o n yl group in d im e th y l-
form am ide produces s tro n g a b s o rp tio n a t a w a vele ngth o f 6 .0 5 m ic ro n s .
T h e re fo re , th e presence o f d im e th y lfo rm a m ide c o u ld be d e te c te d by
c h e c k in g f o r a b s o rp tio n in t h is
re g io n .
Samples w h ich had been d r ie d
by h e a tin g f o r 24 hours in a vacuum chamber were a ls o te s te d to check
f o r r e s id u a l s o lv e n t.
(7 )
■ ■
P re p a ra tio n For a T e s t Run
The f o llo w in g ste p s a re taken b e fo re a t e s t run is s t a r t e d :
i)
The membrane is mounted in th e t e s t c e l l w ith th e membrane
s u rfa c e t h a t was in c o n ta c t w ith th e g la s s p la te fa c in g th e
h ig h p re s s u re s id e o f th e c e l l ,
ii)
The t e s t c e l l
e n c lo s u re .
is mounted in t o th e c o n s ta n t te m p e ra tu re
Feed,, e x h a u s t, perm eate and purge lin e s are
connected to th e c e l l .
The t h e r m is te r te m p e ra tu re probe and
th e rm oco uple a re p la ce d in th e therm ocouple w e ll on th e
h ig h p re s s u re s id e o f th e membrane,
iii)
The c o n s ta n t te m p e ra tu re e n c lo s u re is covered and i t s
te m p e ra tu re is b ro u g h t to th e d e s ire d le v e l by s e t t in g th e
te m p e ra tu re c o n t r o l l e r .
A te m p e ra tu re r e c o rd e r is used to
- 42 m o n ito r te m p e ra tu re .
iv )
A i r is
purged o u t o f th e perm eate s e c tio n o f th e ap p a ra tu s
by ope nin g th e v a lv e s c o n t r o ll in g th e flo w o f purge gas in t o
th e system .
The f r e e end o f th e permeate li n e
o i l b a th to p ro v id e a l i q u i d s e a l.
is p la ce d in an
Purge gas is fe d th ro u g h
th e perm eate system a t a p re s s u re o f abo ut 5 p s ig u n t i l
c h ro m a to g ra p h ic a n a ly s is o f th e gas in th e perm eate li n e
in d ic a te s t h a t no n itr o g e n is p re s e n t.
The flo w o f purge gas
is th e n stopped by c lo s in g th e b lo c k v a lv e in th e purge l i n e .
(8 )
T e s t Run P rocedure
A t e s t run in c lu d e s th e f o llo w in g p ro c e d u re s :
i )
The v a lv e c o n t r o ll in g th e flo w o f fe e d gas in t o th e system
is opened and th e m ic ro -m e te rin g v a lv e is a d ju s te d to s e t
th e flo w r a te o f fe e d gas a t th e d e s ire d v a lu e .
o f 2 lite r s
ii)
A flo w r a te
p e r h o u r is used in a l l ru n s ,
O il is drawn in t o th e perm eate l i n e by rem oving gas.
T h is is
done by in s e r t in g a .5 c c 's y r in g e in t o th e septum in th e
permeate li n e and draw ing o u t gas sam ples.
th e perm eate l i n e to
O il is drawn in t o
a p o in t as c lo s e as p o s s ib le to th e
t e s t cel I .
iii)
The p erm eation r a te o f gas is measured by tim in g th e movement
o f o i l o u t o f th e permeate l i n e .
These measurements a re
-
43 -
re co rd e d and used to compute perm eate f l u x ,
iv )
Permeate samples a re taken by in s e r t in g a .5 cc s y rin g e in t o
th e septum in th e perm eate li n e and draw ing o u t gas sam ples.
The samples a re a n a lyze d u s in g th e c a lib r a t e d gas chrom atograph
.' -. .When chrom atograph o u tp u t shows th re e samples to be th e same
and th e measured permeate r a te is c o n s ta n t, s te a d y s ta te is
assumed.
S te rn and h is c o lle a g u e s , u s in g a s im il a r a p p a ra tu s , re p o rte d th e
a ccu ra cy o f p e rm e a tio n measurements to be on th e o rd e r o f ±5% (6 6 ) .
EXPERIMENTAL RESULTS AND DISCUSSION
A.
MODIFIED VINYLIDENE FLUORIDE FILMS
(I)
M a te r ia ls T ested
M o d ifie r s were chosen f o r t e s t in g p r im a r ily on th e b a s is o f HgS
s o l u b i l i t y as dem onstrated by use in HgS a b s o rp tio n processes re p o rte d
in th e l i t e r a t u r e .
I t is a ls o nece ssary t h a t a m o d if ie r be s o lu b le in
d im e th y lform am ide and have a b o ilin g
p o in t h ig h enough t h a t i t w i l l
n o t be d riv e n o f f a t th e 120°C te m p e ra tu re used in th e membrane d r y in g
s te p .
The f o llo w in g ch e m ica ls were te s te d as m o d ifie rs in v i n y l idene
f lu o r id e membranes in o rd e r to im prove th e membranes' perm eation
c h a r a c t e r is t ic s w ith re s p e c t to hydrogen s u l f i d e :
- S u lfo le n e
- 3 -M e th yl s u lfo le n e
r 1 - M e th y l- 2 - p y r r o li d i none
- M o rp h o lin e
' - M onoethanolam ine .
- T rie th a n o la n im e
- D iis o p ro p a n o la m in e
- M onoisopropanolam ine
A l l membranes a re form ed fro m d im e th y lfo rm a m ide s o lu t io n s w ith a
m o d if ie r c o n c e n tra tio n o f 10 w e ig h t p e rc e n t used f o r a l l
th o s e c o n ta in in g s u lfo le n e .
f ilm s excep t
12 w e ig h t p e rc e n t s u lfo le n e is used.
The membrane s o lu t io n is a llo w e d to sta n d f o r 15 days b e fo re
c a s tin g to in s u re com plete s o lu tio n o f th e v in y lid e n e f lu o r id e
polym e r in th e s o lv e n t.
Membrane s o lu tio n s a re de-gassed by being
-4 5
-
p la ce d in a vacuum chamber a t 16 in ches Hg vacuum f o r 3 0 .m in u te s .
F ilm s a re form ed by p o u rin g th e polym er s o lu tio n onto, a g la ss
p la t e , th e edges o f w h ich have been ra is e d w ith la y e rs o f m asking ta p e ,
and p la c in g th e p la te in an oven f o r 3 0 m inutes a t 120°C to d r iv e o f f
th e s o lv e n t.
(2 )
R e s u lts o f M ic ro s c o p ic A n a ly s is
A t th e s t a r t o f th e p r o je c t s e v e ra l membranes were made up b y .
d is s o lv in g v in y lid e n e f lu o r id e and m o d if ie r in th e s o lv e n t by h e a tin g
th e m ix tu re on a h o t - p la t e and p o u rin g th e s o lu tio n o n to a prepa red
g la s s p la te as soon as a homogeneous polym er s o lu t io n was form ed.
R e s u lts o f t e s t runs made u s in g th e se membranes were v e ry in c o n s is te n t.
D if f e r e n t membranes w ith th e same c o m p o s itio n gave v a s t ly d i f f e r e n t
t e s t r e s u lt s .
To a id in e x p la in in g membrane b e h a v io r, s u rfa c e c h a r a c t e r is t ic s
were checked u s in g a sca n n in g e le c tr o n m icro sco p e .
Exxon Research
and Development Company (Baytow n, Texas) p ro v id e d e le c tr o n p h o to ­
m icro g ra p h s o f a v in y lid e n e f lu o r id e f i l m
m o d ifie d w ith 15% s u lfo le n e .
F ig u re V - I is a s u rfa c e vie w o f th e f i l m
m a g n ifie d 500x and F ig u re V-2
is a c ro s s s e c tio n vie w a t 600x m a g n ific a tio n .
As can be c le a r ly seen
in th e p h o tom icrog rap hs,the.m em b ra ne s u rfa c e is v e ry uneven
m u lt ip le h o le s .
and c o n ta in s
The cro s s s e c tio n vie w shows' t h a t th e ho le s extend
th ro u g h th e membrane.
The la r g e s t h o le in F ig u re V - I has a lo n g e s t s id e
le n g th o f abo ut 2 .6 x IO^ cm ( 2 .6 x IO5 a n g s tro m s ).
T h is is s e v e ra l
- 46 -
FIGURE V - I .
SURFACE VIEW OF MEMBRANE CONTAINING 15%
SULFOLENE AT 500 x MAGNIFICATION - MEMBRANE
FORMED IMMEDIATELY FROM POLYMER SOLUTION
- 47 -
FIGURE V -2.
CROSS SECTION OF MEMBRANE CONTAINING 15%
SULFOLENE AT 600x MAGNIFICATION - MEMBRANE
FORMED IMMEDIATELY FROM POLYMER SOLUTION
- 48 o rd e rs o f m agnitude la r g e r than th e s iz e o f HgS and Ng m o lecu les
(a ro und 10 a n g s tro m s ).
Even w ith th e o b vio u s h o le s p re s e n t, th e se membranes had n o t.
le a k e d , b u t had h e ld gas a t a p re s s u re o f . 500 p s ig .
p o s s ib le e x p la n a tio n s f o r t h i s f a c t .
F ir s t, i t
There a re two
is p o s s ib le t h a t th e
h ig h , vacuum p re s e n t in th e e le c tr o n m icroscope causes r e s id u a l s o lv e n t
to b o il o f f ,
le a v in g h o le s in th e polym er s t r u c t u r e .
Thus, holes t h a t
a re n o t p re s e n t when th e f ilm s are te s te d can be found when e le c tr o n
p h o to m icro g ra p h s a re made.
Second, i t
is
p o s s ib le t h a t under high
p re s s u re th e t h ic k areas o f th e membrane a re com pressed, c lo s in g o f f
th e h o le s .
T h is phenomenon w ould le a v e a n o n u n ifo rm s u rfa c e w hich
c o u ld a cc o u n t f o r th e in c o n s is te n t i n i t i a l
s e p a ra tio n r e s u lt s .
I t was assumed t h a t th e e le c tr o n m icroscope vacuum d id n o t cause th e
h o le s , because checks o f samples w ith a l i g h t m icroscope showed no
d iffe r e n c e s between samples w hich had been te s te d w ith th e e le c tr o n
m icroscope and th o se w hich had n o t.
Two main p o s s ib le causes f o r ,the h o le s a re p re s e n t.
th e v e ry s lo w r a te a t w h ich polym ers go in t o s o lu t io n , i t
F i r s t , due to
is p o s s ib le
t h a t polym er s w e llin g in th e s o lv e n t had o c c u rre d b u t t h a t s o lu tio n
w a s n 't com ple te when th e membranes were c a s t.
The upper s u rfa c e shown
in F ig u re V-2 appears to be composed o f polym er spheres lo o s e ly jo in e d
to g e th e r .
T h is c o u ld be due to in c o m p le te s o lu t io n .
T h a t i s , th e
membrane may be form ed fro m many s e p a ra te masses o f s w o lle n polym er
jo in e d to g e th e r .
s t ir r in g
49 -
Second, gases tr a p p e d .in th e polym er s o lu t io n d u rin g
and p o u rin g w ould be d riv e n o f f in th e d ry in g s te p .
t h i s b o i l i n g - o f f o f tra p p e d gas r e s u lt s
Perhaps
in h o le s in th e membrane.
In an a tte m p t to e lim in a te th e se e f f e c t s , membrane s o lu tio n s were
a llo w e d to s ta n d f o r 24 hours b e fo re p o u rin g to a llo w tim e f o r a l l th e
polym e r to go in t o s o lu t io n , and membrane s o lu tio n s were de-gassed by
b e in g p la ce d in a vacuum chamber a t 16 in ches Hg vacuum f o r 30 m inutes
j u s t b e fo re th e membranes were c a s t.
E le c tro n ph o to m icro g ra p h s were
made on s e v e ra l membranes form ed in t h is manner.
F ig u re V-3 is a
s u rfa c e view o f a membrane m o d ifie d w ith 12% s u lfo le n e .
W h ile th e re
a re two s m a ll h o le s p re s e n t, i t s h o u ld be noted t h a t th e r e a re a ls o
s e v e ra l d e p re s s io n s in th e s u rfa c e w h ich c le a r l y do n o t e xte n d a l l th e
way th ro u g h th e membrane.
The membrane s u rfa c e a ls o appears more
u n ifo rm th a n t h a t shown in F ig u re V -I .
F ig u re V-4 shows a membrane
m o d ifie d w ith 10% d i is o p ro p a n o l amine a t 700x.
A g a in , h o le s a re p re s e n t,
b u t t h e i r s iz e is much s m a lle r than th o s e found in F ig u re V - I .
No
h o le s a re p re s e n t in a s u rfa c e p h o to m icro g ra p h o f a membrane c o n ta in in g
10% 1 - m e th y l- 2 - p y r r o lid in b n e a t a m a g n ific a tio n o f 400x ( F ig u re V -5 ).
A view o f a membrane c o n ta in in g 5% tr ie th a n o la m in e m o d if ie r a t a
m a g n ific a tio n o f 700x is shown in F ig u re V -6 .
A g a in , no h o le s are
p re s e n t and th e s u rfa c e appears q u ite u n ifo rm .
The s te p s w hich had been ta ke n to im prove th e membrane s u rfa c e
c h a r a c t e r is t ic s appear to have been p a r t i a l l y s u c c e s s fu l.
By a llo w in g
- 50 -
FIGURE V -3 .
SURFACE VIEW OF MEMBRANE MODIFIED WITH
12% SULFOLENE AT 400x MAGNIFICATION POLYMER SOLUTION DE-GASSED FOLLOWING A
24 HOUR SOLUTION PERIOD
51 -
FIGURE V -4 .
SURFACE VIEW OF MEMBRANE MODIFIED WITH
10% DIISOPROPANOLAMINE AT 700x MAGNIFICATION POLYMER SOLUTION DE-GASSED FOLLOWING A 24 HOUR
SOLUTION PERIOD
- 52 -
FIGURE V -5 .
SURFACE VIEW OF MEMBRANE MODIFIED WITH
10% 1-METHYL-2-PYRR0LIDIN0NE AT 40Ox
MAGNIFICATION - POLYMER SOLUTION DE-GASSED
FOLLOWING A 24 HOUR SOLUTION PERIOD
FIGURE V-6.
SURFACE VIEW OF MEMBRANE MODIFIED WITH 5%
TRIETHANOLAMINE AT 700x MAGNIFICATION POLYMER SOLUTION DE-GASSED FOLLOWING A 24
HOUR SOLUTION PERIOD
- 54 th e membrane s o l u t i o n t o s ta n d f o r a p e r io d o f t im e , a more u n ifo rm
polym e r f i l m
i s produced.
T e s t runs a tte m p te d w i t h the se membranes showed t h a t th e y were
g a s - t i g h t , even though holes were p r e s e n t in some samples.
Data from
th e s e runs and th e runs made on f i l m s poured im m e d ia te ly were d is c a rd e d
because o f th e p o s s i b i l i t y t h a t th e presence o f holes had a ffe c te d e x p e rim e n ta l r e s u l t s .
In o r d e r t o f u r t h e r in s u r e com plete s o l u t i o n o f th e polym er,
membrane m ix tu re s were a llo w e d t o stand f o r 15 days w i t h o c c a s io n a l
s tir rin g
b e fo r e membranes were c a s t .
Membrane s o l u t i o n s were a ls o
de-gassed in a vacuum chamber a t 16 in ches Hg f o r 30 m inutes b e fo r e
membranes were c a s t .
Al I data on m o d if ie d v i n y l idene f l u o r i d e membranes recorded in
t h i s s tu d y a re f o r t e s t runs made on membranes prepa red u s in g a 15 day
w a i t i n g p e r io d .
The membranes were checked, u s in g a l i g h t m ic ro s c o p e , f o r holes
and o t h e r s u r fa c e d i s c o n t i n u i t i e s b e fo r e t e s t i n g .
Poor r e s o l u t i o n w i t h
th e l i g h t m icroscope l i m i t e d th e u s e fu ln e s s o f t h i s p ro c e d u re .
E le c t r o n p h o tom icrog rap hs o f s e v e r a l o f th e f i l m s
f o r which data
a re re co rd e d i n t h i s s tu d y are shown i n F ig u re s V-7 t o V - l l .
No h o le s a re p r e s e n t, and a v e r y u n ifo rm s u r fa c e i s seen in
F ig u re V-7 w h ich shows a f i l m
c o n t a i n in g 10% monoethanol amine a t 400x.
S i m i l a r s u r fa c e c h a r a c t e r i s t i c s are p r e s e n t in F ig u r e V-8 which shows
- 55 -
FIGURE V -7 .
SURFACE VIEW OF MEMBRANE MODIFIED WITH
10% TRIETHANOLAMINE AT 400x MAGNIFICATION POLYMER SOLUTION DE-GASSED FOLLOWING A 15
DAY SOLUTION PERIOD
- 56 -
FIGURE V -8 .
SURFACE VIEW OF MEMBRANE MODIFIED WITH
10% MORPHOLINE AT 400x MAGNIFICATION POLYMER SOLUTION DE-GASSED FOLLOWING A
15 DAY SOLUTION PERIOD
- 57 -
FIGURE V -9 .
SURFACE VIEW OF MEMBRANE MODIFIED WITH
10% TRIETHANOLAMINE AT 400x MAGNIFICATION POLYMER SOLUTION DE-GASSED FOLLOWING A 15
DAY SOLUTION PERIOD
58 -
FIGURE V - 10.
SURFACE VIEW OF MEMBRANE MODIFIED WITH
10% MONOISOPROPANOLAMINE AT 400x
MAGNIFICATION - POLYMER SOLUTION DE-GASSED
FOLLOWING A 15 DAY SOLUTION PERIOD
- 59 -
FIGURE V - I l .
SURFACE VIEW OF MEMBRANE MODIFIED WITH
10% MONOISOPROPANOLAMINE AT 400x
MAGNIFICATION - POLYMER SOLUTION DE-GASSED
FOLLOWING A 15 DAY SOLUTION PERIOD
- 60 a film
c o n t a i n in g 10% m o rp h o lin e a t 400x.
The sm all d e p re s s io n shown
i n F ig u re V-8 does n o t appear t o extend th ro u g h th e membrane.
film
The
c o n t a i n in g 10% t r i e t h a n o l a m in e shown in F ig u re V-9 e x h i b i t s a v e r y
u n ifo rm s u r fa c e w i t h a f i n e g r a in s t r u c t u r e .
d i s c o n t i n u i t i e s are p r e s e n t.
No h o le s o r la r g e s u r fa c e
F ig u re V-IO shows a f i l m
monoisopropanol amine a t a m a g n i f i c a t i o n o f 400x.
c o n t a i n in g 10%
The s u r fa c e is v e r y '
rough and c o n ta in s s e v e r a l d i s c o n t i n u i t i e s which may. extend thro ugh th e
membrane t h ic k n e s s .
w hich shows f i l m
A rough s u r fa c e i s a ls o p re s e n t i n F ig u re V - I l
c o n t a in in g 10%.m onoisopropanol amine a t IOOOx.
This
h ig h m a g n i f i c a t i o n p h o tom icrog rap h c l e a r l y shows t h a t no holes are
p r e s e n t, however.
(3)
The Data
In o r d e r t o d e te rm in e th e e f f e c t o f m o d i f i e r a d d i t i o n on membrane
p e rm eation r a t e and s e l e c t i v i t y ,
permeate f l u x va lu e s and s e p a ra tio n
f a c t o r s were d e term ined o v e r a te m p e ra tu re range from 23°C t o 125°C.
Each s e r ie s o f t e s t runs was conducted i n o r d e r o f in c r e a s in g tempera­
t u r e u s in g th e same f ilm . s a m p le .
Al I runs were conducted u s in g a 5%
m ix t u r e o f HgS i n Ng a t a p re s s u re o f 500 psig,.
t e s te d were 2 m il t h i c k .
A l l th e membranes
The data are p resen ted i n T ab le V - I and shown
g r a p h i c a l l y in F igures V-12 t o V-15.
TABLE V- I .
P ressure = 500 p s ig
SUMMARY OF TEST RESULTS FOR MODIFIED VINYLIDENE
FLUORIDE FILMS
Feed = 5% HgS i n Ng
Flow Rate = 2 l i t e r s / h o u r
F ilm Thickness = 2 m il
None ( a f t e r d r y in g )
S e p a ra tio n
F a c to r
Percent HgS
i n Permeate
23
1.3 x IO " 5
.3
50
1.9 x IQ "*
3 .2
14.6
75
4 .8 x IO "4
3.7
16.2
6 . 1
24.4
6 .6
25.7
x IO "3
100
2 .1
125
5.8 x I O " 3
1 .6
23
NO PERMEATE
50
1 .0
X IO "4
3.4
15.3
50
1 .1
4 .4
18.7
75
4 .2 x IO "4
5.4
22.3
100
2 .1
x IO " 3
6 .1
24.4
100
2 .2
x IO " 3
6 .8
26.5
125
8 .2
X
5.8
23.3
125
6 .6
X
6 .2
24.5
ro
CO
I
O
O
r—l
r —I
12% S u lf o le n e
F lu x I M2Hr j
X
None ( b e fo re d r y in g )
[ M3 (STP))
I-—1
O
I
-F*
M o d i f i e r _____________
Temperature
( 0C)
23
5 .3 x IO "2
.9
4 .7
23
4 .9 x IO "2
.9
4 .6
TABLE V - I (c o n tin u e d )
M o d i f i e r ______________
Temperature
( 0C)
12% S u lf o le n e ( c o n t . )
10% 3-M ethyl S u lf o le n e
10% 1 - M e t h y l - 2 - P y r r o l i d i none
M3 (STP) \
F lu x \ M3Hr
/
S e p a ra tio n
F a c to r
Percent HgS
in Permeate
50
4 .0 x I O ' 2
1.1
5.6
50
4 .3 x IO "2
1.0
5.2
50
4 .5 x IO "2
1 .0
5.2
75
3 .3 x I O ' 2
1.2
5.9
75
2 .6 x IO " 2
1.4
7.1
100
2 .3 x I O ' 2
2.2
10.4
100
1.9 x I O ' 2
2 .3
11.0
125
2 .7 x IO "2
2 .8
12.8
125
2 .8 x IO "2
3.7
16.3
125
2 .9 x I O ' 2
3.7
16.4
23
3 .7 x 10
5 .6
22.8
50
4 .3 x 10
9 .4
33.1
75
1.6 x 10
9 .4
33.0
100
4 .7 x 10
6 .8
26.3
125
1.0 x 10
5.5
22.4
23
5 .2 x IO "4
.1
.5
50
8 .2 x IO "4
.1
.6
75
1.0 x IO "3
.5
2.5
100
2 .3 x I O " 3
5.7
23.1
TABLE V - I (c o n tin u e d )
M o d ifie r
Temperature
____________________( 0C)
10% 1 - M e t h y l - 2 - P y r r o l i d i none
( c o n tin u e d )
10% M o rp h o lin e
10% Monoethanolamine
10% T r ie th a n o la m in e
/ m3 (STP)\
Flux
M3Hr
I
S e p a ra tio n
F a c to r
Pe rcen t HpS
in Permeate
2 .3 x I O ' 3
6 .0
24.0
8 .3 x I O " 3
5.9
23.7
125
7 .1 x IO "3
5.5
22.6
23
1.7 x I O ' 5
3 .0
13.6
50
2 .3 x I O ' 4
7.2
27.5
75
9 .4 x IO "4
7.0
27.0
100
3.0 x IO " 3
8 .0
29.7
125
9.1 x IO " 3
6 .6
25.9
23
2 .5 x IO "3
.7
3.4
50
5.5 x IO " 3
1.1
5.4
75
1.0 x I O ' 2
1.5
7.3
100
1.1 x I O ' 2
2.3
10.9
125
1.7 x IO "2
2 .8
13.0
23
2 .1 x I O " 3
.1
.4
23
2 .6 x I O ' 3
.1
.3
50
2 .5 x 1 0 "3
> .l
.1
75
1.6 x IO "3
1.3
6.2
100
3 .3 x - 1 0 ' 3
5.1
21.0
125
8 .5 x 1 0 ' 3
7 .2
27.4
100
TABLE V - I (c o n tin u e d )
M o d ifie r
10% M onoisopropanolamine
10% Di is o p ro p a n o l amine
Temperature
( 0C)
/ m3 ( s t p ) \
F lu x I M2Hr
I
S e p a ra tio n
F a c to r
P ercent HgS
in Permeate
23
2 .6 x I O ' 5
3.9
' 17.0
50
2 .8 x I O ' 4
6 .4
25.3
75
1.4 x IO "3
8 .9
31.9
100
3 .0 x I O ' 3
7.1
27.6
125
6 .3 x IO " 3
6 .7
26.2
23
7.1 x IO " 5
3 .3
14.8
50
4 .3 x I O ' 4
4 .2
18.0
50
4 .0 x I O '4
4 .2
18.1
75
9 .4 x I O ' 4
7.4
28.0
100
2 .1 x IO " 3
7 .6
28.5
125
8 .7 x I O ' 3
5 .6
22.7
- 65 -
A
A
□
■
12%
10%
10%
10%
X
No M o d i f i e r
25
S u lf o le n e
3-M e thyl S u lf o le n e
1 - M e t h y l - 2 - P y r r o l id in o n e
M o rp h o lin e
50
75
100
125
Tem perature ( 0C)
FIGURE V - 1 2 .
SEPARATION FACTOR v s . TEMPERATURE FOR SOME MODIFIED
VINYLIDENE FLUORIDE FILMS
- 66 -
O
©
10% Monoethanolamine
10% T r ie th o n o la m in e
V
10% Di is o p ro p a n o l amine
V
X
10% M onoisopropanolamine
No M o d i f i e r
Temperature ( 0C)
FIGURE V - 13.
SEPARATION FACTOR v s . TEMPERATURE FOR ALKANOLAMINE
MODIFIERS IN VINYLIDENE FLUORIDE
F lu x (M0 (STP)ZM4Hr)
- 67 -
12% S u lf o le n e
10% 3-M ethyl S u lf o le n e
10% 1 - M e t h y l - 2 - P y r r o l i d i none
10% M orp holine
No Modif i e r
Tem perature ( 0C)
FIGURE V - 14.
FLUX v s . TEMPERATURE FOR SOME MODIFIED VINYLIDENE
FLUORIDE FILMS
Flux (MJ (STP)/M^Hr)
- 68 -
O
•
v
V
10%
10%
10%
10%
Monoethanolamine
T r ie th a n o la m in e
Di is o p ro p a n e I amine
M onoisopropan ola m ine
X No M o d i f i e r
25
50
75
100
125
Tem perature ( 0C)
FIGURE V -15.
FLUX v s . TEMPERATURE FOR ALKANOLAMINE MODIFIERS IN
VINYLIDENE FLUORIDE FILMS
- 69 (4 )
D is c u s s io n - Film s W ith No M o d i f i e r Added
A t room te m p e ra tu re u n m o d ifie d v i n y l i dene f l u o r i d e f i l m
has a
s e p a r a tio n f a c t o r le s s than one w hich i n d ic a t e s t h a t N2- i s c o n c e n tra te d
i n th e permeate.
However,’ above room ..temperature u n m o d ifie d f i l m
produces a permeate c o n t a in in g more H2S than i s p re s e n t i n th e feed
(see T ab le V - l ) .
................
These r e s u l t s su g g e st t h a t d i m e t h y lform am ide, th e s o l v e n t used i n
membrane m a n u fa c tu re , i s
i t s e l f a m o d i f i e r t h a t g iv e s t h e membrane
some s e l e c t i v i t y f o r H2S.
Gas s o l u b i l i t i e s
i n d im e th y lfo rm a m id e have been r e p o r te d (67) as
5 .6 g H2SZlOO g d im e th y lfo rm a m id e a t 25°C compared t o
d im e th y lfo rm a m id e a t 20°C.
.007 g N2ZlOO g
Thus, based on s o l u b i l i t y , d im e th ylfo rm a m id e
i s expected t o a c t as a m o d i f i e r .
To check f o r t h e presence o f r e s id u a l dimet h y l form am ide, membrane
samples were t e s t e d u s in g an i n f r a r e d s p e c tro p h o to m e te r.
The carbon yl
group i n d im e th y lfo rm a m id e absorbs s t r o n g l y a t a w a vele ngth o f 6 .0
m ic r o n s .
T h e r e f o r e , a b s o r p tio n a t t h i s w avele ngth i n d i c a t e s th e presence
o f d im e th y lfo rm a m id e s in c e th e v i n y l i d ene f l u o r i d e polym er does n o t
c o n t a in any ca rb o n y l g roup s.
F ig u r e V-16.
The r e s u l t s o f t h i s t e s t are shown in
The s l i g h t amount o f a b s o r p tio n a t 6 .0 m icrons in d ic a t e s
t h a t a sm a ll amount o f d im e th y lfo rm a m id e i s p re s e n t i n th e membrane
sample.
A d r y in g s te p has been t r i e d i n o r d e r t o in s u r e removal o f
- 70 -
y; l. I — TI I IL
-Tl . I;:. I r. rI I
»
— —
—-—
-JIT ITIT ; TI I-t JC- Cl- LI TLfj
—
—- - - . . .
— — — —
—
... —
- —
I
—
——
——
—
- **
j —
—
—
——
—
TI — TLE
—IT
A fter
B e fo re
D r y i ng
j
■
'd
—
IC77—I
...
— ...
—
—
- *•
-iri1t i T —C
l I- .I I. . . . - IT I. C
TI."—T
Zz:1
— —
•—
—
~\
- •
__
—- * -- --TI
I
. Il
TI
t - — —• - •- — "i
-I
~■
-I
C
T
T
—
C
L r'—
V
T
X -y
— —— -- I -J —-- XE!TI I
(—
—— —— ——
T
u
TI I
~ lI-' - —
y
rr
Tl!
I
-
\
r\
I
TT
- ——
- -- —
—r. I LI -T
C
'—
——
—
—
Carbonyl
A b s o rp tio n
U
—-
—
TI
I T
__
__
—... - •
__
.
Cl
C
r—
...
_
-
I
-r
tr - L I I'
TTIII TI I TI — T
■
— T
.
LiCl
TC
—
~ ;i
C- T
L-■ ——- -- I
T“H I Li T
■
—
-
Iii::
E
E
L-
__
-—
—
-
— — —
"I
"I
I
—
TI : —
—-- —
C
l. —
5
Wavelength (m ic ro n s )
FIGURE V-16.
U-
—
I
—
-■TI I
—
I—::
L
r—
TILT TH
■
H - F i- -.r
1
--
6
Wavelength (m ic ro n s )
RESULTS OF INFRARED SPECTROPHOTOMETER TESTS
ON UNMODIFIED VINYLIDENE FLUORIDE FILMS
- 71 r e s id u a l s o l v e n t .
A membrane sample was d r ie d f o r 24 hours a t a
te m p e ra tu re o f IOO0C i n a vacuum'- chamber a t 16 inches Hg vacuum.
S p e c tro p h o to m e te r t e s t s on a sample o f th e d r ie d m a t e r i a l show no
a b s o r p tio n o f i n f r a r e d r a d i a t i o n w i t h a 6 .0 m icron w a v e le n g th .
T h is
in d i c a t e s t h a t th e d i m e t h y lformamide c o n c e n t r a t io n has been reduced
t o a le v e l below t h a t w h ich c o u ld be d e te c te d by th e i n f r a r e d s p e c t r o ­
pho tom eter (see F ig u re V - 1 6 ) .
The r e s u l t s o f t e s t s made on u n m o d ifie d v i n y l idene f l u o r i d e f i l m
a f t e r d r y in g are shown i n F ig u re s V-17 and V-18.
The d r ie d f i l m
t h e u n d rie d f i l m .
d r ie d f i l m
e x h i b i t s lo w e r f l u x va lu e s a t low te m p e ra tu re s than
A t room te m p e ra tu re no gas permeated th ro u g h the
i n a run l a s t i n g s e v e ra l h o u r s .
However a t 125°C f l u x values
s l i g h t l y h ig h e r than those found i n u n d rie d f i l m are found f o r th e
d r ie d f i l m .
!
As can be seen i n F ig u re V-17, t h e r e appears t o be an in c re a s e in
s e p a r a t io n i n th e f i l m
a f t e r d ry in g .
P a r t o f t h e a p p a re n t in c re a s e
may be s im p le data s c a t t e r . . I t i s a ls o p o s s ib le t h a t th e vacuum
d r y in g s t e p causes changes i n th e polym er s t r u c t u r e o f th e membrane.
The d r y in g s te p a p p a r e n t ly does n o t remove a l l
s o l v e n t p re s e n t in th e membrane.
th e d im e th ylfo rm a m id e
However, th e decrease in f l u x a t low ,
te m p e ra tu re and th e r e s u l t s o f s p e c tro p h o to m e te r t e s t s i n d i c a t e t h a t
some o f th e s o l v e n t i s removed by d r y i n g .
- 72 i
O
B e fo re D ry in g
□
A f t e r D ryin g
Temperature ( 0C)
FIGURE V-17.
SEPARATION FACTOR vs. TEMPERATURE FOR UNMODIFIED
VINYLIDENE FLUORIDE FILMS
10
T
O
B e fo re D ryin g
□
A f t e r D ry in g
T
100
125
-3
l u x (Mj (STP)ZNi Hr)
10
-2
Ox DH
- 73 -
10
10
-4
-5
25
50
75
Tem perature ( 0C)
FIGURE V - 18.
FLUX v s . TEMPERATURE FOR UNMODIFIED VINYLIDENE
FLUORIDE FILMS
- 74 M o d ifie d membranes were n o t t r e a t e d u s in g a vacuum d r y in g s te p
because o f t h e l i k e l i h o o d t h a t th e m o d i f i e r would be removed along
w i t h r e s id u a l d i m e t h y lformamide s o l v e n t .
I t must be re c o g n iz e d t h a t th e presence o f r e s id u a l d i m e t h y lformamide has an im p o r t a n t e f f e c t on membrane s e l e c t i v i t y .
The t e s t s
made on u n d rie d v i n y l idene f l u o r i d e membranes w i t h o u t m o d i f i e r a d d i t i o n
a re used as t h e . b a s e - l i n e f o r d e te r m in in g m o d i f i e r e f f e c t i v e n e s s .
Comparison o f m o d if ie d f i l m s w i t h th e u n m o d ifie d f i l m
a llo w s th e e f f e c t
o f r e s id u a l s o l v e n t t o be c a n c e lle d o u t .
(5 )
D is c u s s io n - Film s W ith M o d i f i e r Added
1.
S u lf o le n e - A la r g e number o f t e s t s were made u s in g t h i s
m o d i f i e r in o r d e r t o check o u t t h e r e p r o d u c i b i l i t y o f th e t e s t equipment
used in t h i s
r e s e a rc h .
As can be seen i n Table V - I t h e r e i s some data
s c a t t e r , b u t on th e whole r e p r o d u c b i l i t y i s q u i t e good.
The s u l f o l e n e -
m o d ifie d membrane g iv e s th e h ig h e s t f l u x values o f any o f th e m o d ifie d
v i n y l i d e n e f l u o r i d e f i l m s t e s t e d , b u t s e p a r a tio n i s much le s s than t h a t
o b ta in e d w i t h u n m o d ifie d f i l m .
A t room te m p e ra tu re th e permeate
c o n ta in s le s s HgS than th e f e e d .
film
The b e s t s e p a r a tio n f a c t o r f o r t h i s
i s 3 .7 a t 125°C compared t o 6 .6 f o r th e u n m o d ifie d f i l m
2.
a t 125°C.
3 - Methyl S u lf o le n e - Film s w i t h t h i s m o d i f i e r e x h i b i t the
b e s t o v e r a l l perform ance o f any m o d if ie d f i l m
c o n t i n u a l l y w i t h te m p e ra tu re
and, a t a l l
te s te d .
F lu x in c re a s e s
te m p e ra tu re s , f l u x values are
a t l e a s t t w ic e as h ig h as those o b ta in e d f o r u n m o d ifie d v i n y l i d e n e
- 75 flu o rid e .
3 -m e thyl s u l f o l e n e membranes are v e r y s e l e c t i v e f o r HgS.
A t room te m p e ra tu r e , w h il e most f i l m s g iv e le s s HgS in th e permeate
than th e fe e d , membranes w i t h 10% 3-m e thyl s u l f o l e n e e x h i b i t a s e p a r a tio n
f a c t o r o f 5 .6 (22.8% HgS i n t h e p e rm e a te ).
T h is i s t h e b e s t room
te m p e ra tu re s e p a r a tio n observed f o r v i n y l idene f l u o r i d e membranes.
At
50°C and 75°C f i l m s m o d if ie d w i t h 3 - m e th y l.s u lf o le n e produce th e b e s t
s e p a r a tio n observed w i t h v i n y l i d e n e f l u o r i d e membranes.
c o n t a i n in g 33%.HgS i s fo u n d .
Permeate
A t IOO0C and 125°C s e p a r a tio n decreases
s lig h tly .
3.
1 - M e th y l- 2 - p y r r o l i d i none - A t low te m p e ra tu re s t h i s m a t e r ia l
e x h i b i t s f l u x values s e v e ra l tim es as h ig h as those observed f o r
u n m o d ifie d f i l m , b u t s e p a r a tio n i s v e ry p o o r.
A t 25°C, 50°C, and 75°C,
th e permeate c o n ta in s much le s s HgS than does th e fe e d .
A t IOO0C
s e p a r a t io n in c re a s e s m arke d ly b u t remains le s s than t h a t observed
f o r u n m o d ifie d f i l m .
A t b oth IOO0C and 125°C f l u x i s s l i g h t l y h ig h e r
th a n t h a t observed f o r u n m o d ifie d f i l m , b u t s e p a r a tio n i s s t i l l
le s s
than t h a t o b ta in e d w i t h u n m o d ifie d f i l m .
4.
re s u lts .
M o rp h o lin e - Tests w i t h t h i s m o d i f i e r g iv e v e r y p o s i t i v e
Flux va lu e s in c re a s e c o n t i n u a l l y w i t h te m p e ra tu re and are
s l i g h t l y h i g h e r than th e values found f o r u n m o d ifie d v i n y l i d e n e
flu o r id e .
S e p a ra tio n i s much b e t t e r w i t h t h i s m o d i f i e r than w i t h
u n m o d ifie d f i l m ,
p a r t i c u l a r l y a t te m p e ra tu re s below 1250C.
m o r p h o lin e - m o d ifie d f i l m
A t 50°C
has a s e p a r a tio n f a c t o r o f 7 .2 (27.5% HgS i n
- 76 th e permeate) compared t o 3.2 f o r u n m o d ifie d f i l m .
The maximum
s e p a r a tio n o b ta in e d w i t h t h i s m o d i f i e r occurs a t IOO0C ( s e p a r a t io n
fa c to r= 8 .0 ).
5.
amine
Honoethanolamine - D e s p ite th e w idespread use o f monoethanolas a l i q u i d a b s o rb e n t f o r HgS removal i t does n o t work w e ll as
a membrane m o d i f i e r .
Membranes made w i t h t h i s m o d i f i e r have a brown
c o l o r i n d i c a t i n g p o s s ib le d e c o m p o s itio n o f th e m o d i f i e r .
F lu x values
o b ta in e d u s in g monoethanol amine as a m o d ify in g age nt a re much h ig h e r
than th o s e o b ta in e d w i t h u n m o d ifie d f i l m .
For example, a t room
te m p e ra tu re th e m o d if ie d f i l m
has a f l u x two o rd e rs o f magnitude
g r e a t e r than u n m o d ifie d f i l m
( 2 .5 x IO- ^ M^(STP)/M^Hr compared to
1.3 x 10
M0 (STP)ZM^Hr).
However, a t a l l
te m p e ra tu re s above room
te m p e ra tu r e ,.s e p a r a tio n i s much le s s than t h a t o b ta in e d w i t h no
m o d ifie r.
W h ile s e p a r a tio n does in c re a s e w i t h te m p e ra tu r e , i t
much le s s than s e p a r a tio n i n th e u n m o d ifie d f i l m .
in c re a s e s
A t 125°C permeate
fro m t h e membrane m o d if ie d w i t h monoethanol amine c o n t a in s 13% HgS
compared t o 25.7% HgS i n th e permeate from u n m o d ifie d f i l m .
In summary,
th e presence o f monoethanol amine m o d i f i e r in c re a s e s f l u x b u t g r e a t l y
decreases s e p a r a t io n .
6.
•
T r ie t h a n o la m in e - Membranes c o n t a i n in g t h i s m a t e r i a l a ls o have
a brown c o l o r i n d i c a t i n g p o s s ib le deco m p o sitio n o f t h e m o d i f i e r .
At
lo w e r te m p e ra tu re s (23°C and 500C.) t h i s m o d i f i e r g iv e s r e s u l t s s i m i l a r
t o monoethanol amine - h ig h f l u x b u t v e r y low s e p a r a t io n .
However, a t
- 77 h ig h e r te m p e ra tu re s f l u x values are j u s t above th e va lu e s found f o r
u n m o d ifie d f i l m s and s e p a r a tio n i s c lo s e t o t h a t o b ta in e d w i t h no
m o d ifie r.
A t 125°C th e t r i e t h a n o l a m in e - m o d i f i e d membrane has a b e t t e r
s e p a r a tio n f a c t o r th a n t h a t found f o r u n m o d ifie d f i l m .
h i g h e r te m p e ra tu re s t h i s
Perhaps a t
in c re a s e i n s e p a r a tio n o v e r t h a t o b ta in e d
w i t h u n m o d ifie d f i l m would c o n t in u e .
7.
D iis o p ro p a n o la m in e - Film s w i t h t h i s m o d i f i e r g iv e ve ry
p o s itiv e r e s u lts .
F lu x va lu e s in c re a s e w i t h te m p e ra tu re and remain
above th e values determ ined f o r u n m o d ifie d f i l m .
A t te m p e ra tu re s
below 125°C s e p a r a tio n i s much b e t t e r than t h a t a chieved w i t h no
•m o d ifie r.
The maximum s e p a r a tio n f a c t o r achieved w i t h t h i s m o d i f i e r
i s 7 .5 (28.5% HgS i n th e permeate) a t IOO0C.
S e p a ra tio n d e c lin e s a t
125°C t o a v a lu e o f 5 .6 (22.7% HgS i n th e permeate); a v a lu e le s s than
t h a t o b ta in e d w i t h no m o d i f i e r .
Di is o p ro p a n o l a m in e -m o d ifie d membranes
appear t o be b e s t s u i t e d f o r low te m p e ra tu re a p p l i c a t i o n .
A brown
c o l o r i s observed in th e membrane m a t e r i a l a f t e r t e s t i n g , i n d i c a t i n g
p o s s i b l e therm al d e c o m p o s itio n .
8.
m o d ifie r
t o 75°C.
M onoisopropanolamine - Membranes w i t h monoisopropanol amine
give v e r y good s e p a r a t i o n , p a r t i c u l a r l y a t te m p e ra tu re s up
A t room te m p e ra tu r e , a s e p a r a tio n f a c t o r o f 3 .9 i s observed
(17% HgS i n th e perm e a te ).
The b e s t s e p a r a tio n f a c t o r found f o r t h i s
membrane i s 8 .9 (31.9% HgS i n t h e permeate) a t 75°C.
s e p a r a tio n d e crea ses.
At a ll
Above 75°C
te m p e ra tu re s th e f l u x observed f o r t h i s
- 78 m a te ria l
is s l i g h t l y above t h a t observed f o r u n m o d ifie d membranes.
Thus, a t a l l
te m p e ra tu re s, membranes w i t h m onoisopropanol amine m o d i f i e r
have h i g h e r f l u x values and b e t t e r s e p a r a tio n than u n m o d ifie d membranes.
A comparison
o f th e m o d if ie d f i l m s t e s t e d shows t h a t 3-m ethyl
s u l f o l e n e , m o rp h o lin e , m onoisopropanol amine, and d i is o p ro p a n o l amine
g iv e th e b e s t s e p a r a tio n o f HgS from Ng.
Good s e p a r a tio n values c o u ld be th e r e s u l t o f s e v e ra l f a c t o r s .
The f i r s t i s th e s o l u b i l i t y o f HgS i n th e m o d i f i e r which r e s u l t s in
in c re a s e d s o l u b i l i t y o f t h e gas i n th e m o d if ie d f i l m .
Based s t r i c t l y
on s o l u b i l i t y c h a r a c t e r i s t i c s , a l l o f t h e m o d if ie r s t r i e d
.
should have
g iv e n good r e s u l t s .
The in c re a s e d d i f f u s i v i t y o f HgS i n th e m o d if ie d f i l m may be
p a r t i a l l y r e s p o n s ib le f o r t h i s
in c re a s e d s e p a r a tio n o f HgS by some o f
th e m o d if ie d f i l m s .
A t h i r d im p o r t a n t f a c t o r t h a t may have r e s u lt e d i n in c re a s e d
f l u x and s e p a r a tio n w i t h some o f th e m o d ifie d f i l m s i s decreased
c r y s ta l!in ity .
Unm odified v i n y l id ene f l u o r i d e i s r e p o r te d t o be about
68% c r y s t a l l i n e
(6 8 ).
o f th e f ilm
is
Thu s, i n th e u n m o d ifie d f i l m o n ly about 32%
u s e fu l f o r gas p e rm e a tio n .
By re d u c in g th e c r y s t a l l i n i t y
o f th e v i n y l i d e n e f l u o r i d e some m o d if ie r s would g iv e h i g h e r perm eation
ra te s .
S e p a ra tio n c o u ld a ls o be e f f e c t e d due t o in c r e a s in g
polym er c h a in m o b i l i t y w h ic h ^ would reduce the t o r t u o s i t y o f the path
_ 79 -
f o ll o w e d by gas m o lecu les in moving th ro u g h th e p o ly m e r.
The g la s s t r a n s i t i o n te m p e ra tu re o f v i n y l i dene f l u o r i d e has been
found t o be -40°C ( 6 9 ) .
S ince a l l
t e s t runs were conducted a t a
te m p e ra tu re w e ll above t h i s , t h e e f f e c t o f m o d if ie r s lo w e r in g th e g la s s
t r a n s i t i o n te m p e ra tu re i s n o t im p o r t a n t i n t h i s s t u d y .
For t e s t s
made n e a r th e g la s s t r a n s i t i o n te m p era ture, t h i s f a c t o r c o u ld be o f
*
•
..
m a jo r im p o rta n c e , however.
(6 )
E f f e c t o f Temperature
R e s u lts f o r a l l m o d if ie d v i n y l idene f l u o r i d e f i l m s
show a s t r o n g
t r e n d tow ard in c r e a s in g f l u x w i t h in c r e a s in g te m p e ra tu re .
film
The o n ly
t h a t d id n o t f o l l o w t h i s t r e n d w a s . t h a t c o n t a in in g 12% s u l f o l e n e
m o d ifie r.
A p p a r e n t ly , as te m p e ra tu re in c re a s e s th e in c re a s e i n d i f f u s i v i t y
and gas s o l u b i l i t y
s o lu b ility
in th e polym er tend t o outw e igh th e decrease i n gas
in th e m o d i f i e r .
F ig u re s V - 14 and V-1& which are p l o t s o f f l u x v s . te m p e ra tu r e ,
show th e t r e n d tow ard in c r e a s in g f l u x w i t h in c r e a s in g te m p e ra tu r e .
A t h i g h e r te m p e ra tu re s (75°C and above) many o f th e p l o t s o f f l u x vs.
te m p e ra tu re
w i t h f l u x p l o t t e d on a l o g a r i t h m i c s c a le a re l i n e a r ,
i n d i c a t i n g t h a t f l u x v a r ie s n e a r ly e x p o n e n t i a l l y w i t h te m p e ra tu re .
There i s a ls o a t r e n d tow ard i n c r e a s i n g 's e p a r a t i o n w i t h in c r e a s in g
te m p e ra tu r e .
T h is tr e n d i s p a r t i c u l a r l y w e ll
i l l u s t r a t e d by th e r e s u l t s
o b ta in e d u s in g s u l f o l e n e and monoethanol amine as m o d i f i e r s
(see
-
80 -
F ig u re s V - 12 and V - 1 3 ).
S e p a ra tio n va lu e s f o r s e v e r a l o f th e f i l m s go th ro u g h a maximum.
T h is i s t r u e f o r monoisopropanol amine,, d iis o p r o p a n q la m in e , 3-m ethyl
s u l f o l e n e and m o rp h o lin e .
I t is
i n t e r e s t i n g t o note t h a t th e se are
a ls o th e f o u r m o d if ie r s w hich g iv e th e b e s t o v e r a l l s e p a r a tio n r e s u l t s .
B.
COMMERCIALLY AVAILABLE POLYMER FILMS
(I)
M a t e r i a ls Tested
Tests t o d e te rm in e permeate f l u x and s e l e c t i v i t y f o r hydrogen
s u l f i d e were conducted u s in g th e f o l l o w i n g f i l m s :
- Dim ethyl S i l i c o n e - m anufacture d by General E l e c t r i c C o rp o ra tio n
- S i l i c o n e P o ly c a rb o n a te Copolymer - membrane MEM-213 m anufactured
by General E l e c t r i c C o rp o ra tio n
- P o ly V in y l
F lu o r id e - DuPont " T e d la r " f i l m number 100AG30UT
- Polyamide - "Capran" 77C f i l m m anufactured by A l l i e d Chemical
C o r p o ra tio n
- Heat S t a b i l i z e d Polyamide -
"Capran" 80 f i l m m anufacture d by
A l l i e d Chemical C o rp o ra tio n
- P o ly s u lfo n e - Compound P-1700 f i l m m anufactured by Union
C a rb id e C o rp o ra tio n
- P o ly e t h e r s u lf o n e - m anufacture d by I . C . I .
I n c o r p o r a te d
\
U n itd d S t a t e s ,
-
(2)
81 -
The Data
T e s t runs were made t o d e te rm in e permeate f l u x va lu e s and
s e p a r a tio n f a c t o r s a t te m p e ra tu re s ra n g in g from 23°C (room te m p e ra tu re )
t o 125°C.
A ll
■
runs were conducted w i t h an upstream gas p re ssu re o f
500 p s ig u s in g a 5% m ix t u r e o f hydrogen s u l f i d e in n i t r o g e n .
The
data are p re se n te d i n Table V-2 and shown g r a p h i c a l l y in F ig u re s V-19
and V-20.
(3 )
D is c u s s io n *3
I.
D im ethyl S i l i c o n e - T h is m a t e r ia l g iv e s th e h i g h e s t room
te m p e ra tu re f l u x v a lu e o f any o f th e m a t e r i a ls t e s t e d . . F lu x values
3 o r d e r s o f magnitude g r e a t e r than th o s e found f o r a l l o t h e r commercial
f i l m s e xce p t s i l i c o n e p o ly c a rb o n a te copolym er are ob se rve d .
A t room
te m p e ra tu re th e perm eate.gas c o n ta in s f o u r tim es as much HgS as the
fe e d (20% HgS i n th e permeate compared t o 5% HgS i n th e f e e d ) .
Due t o
th e v e ry h ig h permeate f l u x , c o n c e n t r a t io n p o l a r i z a t i o n may h in d e r th e
s e p a r a tio n perform ance o f t h i s m a t e r i a l .
T hat i s , because o f th e r a p id
r a t e a t w h ich HgS i s p e rm e a tin g , th e c o n c e n t r a t io n o f HgS a t th e membrane
s u r fa c e may be s i g n i f i c a n t l y le s s than th e b u lk feed c o n c e n tr a t io n
o f 5% HgS.
Even when gas i s fe d a t th e h ig h e s t r a t e p o s s i b l e w i t h
th e a v a i l a b l e equipment ( - 5 l i t e r / h o u r )
th e e x i t gas fro m th e high
p re s s u re s id e o f th e membrane c o n t a in s o n l y about 3.1% HgS.
In t e s t s a t room te m p e ra tu re t h e membrane m a t e r i a l l a s t s o n ly
TABLE V-2.
P ressure = 500 p s ig
Polymer F ilm
Dimethyl S i l i c o n e
S i l i c o n e P o lyca rb o n a te
SUMMARY OF TEST RESULTS FOR COMMERCIALLY
AVAILABLE POLYMER FILMS
Feed = 5% HgS in Ng
T h ic k ­
ness
I m il
I m il
Temperature
( 0C)
Polyamide
F lu o r id e
I m il
I m il
/M3 (STP))
I M3Hr
F lu x
I
S e p a ra tio n
F a c to r
7 .3 x I O " 1
23
2 .8 x I O " 1
8 .9
32.0
23
2 .8 x I O ' 1
13.1
40.9
50
3 .5 x I O " 1
7.0
27.0
75
4 .1 x I O " 1
7.6
28.5
-
-
4 .7
Membrane
Ruptures
P e rce n t H
in Permea1
23
50
100
P oly V in y l
Flow Rate = 2 l i t e r s / h o u r
20.0
Membrane
Ruptures
23
No Permeate
23
No Permeate
50
2 .2 x I O ' 4
6 .2
24.6
75
1.3 x I O ' 3
9 .4
33.2
100
4 .7 x IO " 3
12.9
40.5
23
6 .9 x I O '4
.1
.7
50
1.3 x IO " 3
.1
.5
TABLE V-2 (C o n tin u e d )
Polymer F ilm
Heat S t a b i l i z e d
Polyamide
P o ly s u lfo n e
T h ic k ­
ness
I m il
2 m il
Temperature
( 0 C)
2 m il
F l u x I M3Hr
S e p a ra tio n
F a c to r
Pe rcen t H
in Permea
23
1.4 x IO "3
.2
1.0
23
7.1 x I O '4
.1
.3
50
1.1 x I O ' 3
.1
.4
75
3 .2 x IO "4
.7
3.4
100
8 .0 x IO "4
3.2
14.3
100
5.1 x IO "4
5.1
21.2
125
2.1 x I O ' 3
12.9
40.4
150
4 .8 x I O ' 3
13.5
41.6
23
1.4 x I O ' 4
12.7
40.0
50
9.1
32.4
50
4 .0 x IO "4
-4
4 .2 x 10 4
7.4
28.0
75
8 .3 x IO "4
9.7
33.9
100
2.1 x I O ' 3
6 .8
26.5
125
P o le t h e r s u lfo n e
/M3(STP))
-
Membrane
Ruptures
23
1.5 x I O '4
.1
.7
50
2 .5 x I O ' 4
4 .2
18.0
75
6 .8 x IO "4
9 .3
32.8
100
1.2 x IO "3
8 .3
30.3
125
1.1 x IO "3
7.0
26.8
- 84 -
Dim ethyl S i l i c o n e
S i l i c o n e P o ly c a rb o n a te Copolymer
Poly V in y l F lu o r id e
Polyamide
Heat S t a b l iz e d Polyamide
P o lysu l tone
P o ly e th e r s u l fone
Temperature ( 0C)
FIGURE V-19.
SEPARATION FACTOR v s . TEMPERATURE FOR COMMERCIALLY
AVAILABLE FILMS
- 85 -
▲
Dimethyl S i l i c o n e
&
S i l i c o n e P o ly ca rb o n a te
v
Poly V in y l
Flux (Mj (STP)ZhfHr)
■vx Scale Change
Scale Change xs.
__
Z
© Polyamide
O Heat S t a b i l i z e d
I
D P o ly s u lfo n e
B
F l u o r id e
Polyamide
P o ly e t h e r s u l fone
Tem perature ( 0C)
FIGURE V -2 0 .
FLUX v s . TEMPERATURE FOR COMMERCIALLY AVAILABLE
FILMS
- 86 abo ut te n m inutes b e fo r e holes deve lop
in th e membrane.
A t 50°C
th e membrane r u p t u r e s d u r in g s t a r t u p , b e f o r e gas p re s s u re on th e h ig h
p r e s s u re s id e o f th e membrane reaches 500 p s i g .
P o s s ib ly th e t e s t
equipm ent does n o t p r o v id e enough s u p p o r t f o r t h i s v e r y f l e x i b l e f i l m ,
a lth o u g h no problems were encountered w i t h any o f th e o t h e r f i l m s
te s te d .
I t appears t h a t w h i l e t h i s m a t e r ia l g iv e s good f l u x and s e p a r a tio n
v a lu e s i t s
u s e fu ln e s s i s r e s t r i c t e d t o lo w - te m p e r a tu r e , lo w -p re s s u re
a p p lic a tio n s .
. 2.
S i l i c o n e P o ly c a rb o n a te Copolymer - T h is f i l m
h ig h f l u x v a lu e s , and v e r y good s e p a r a t io n .
e x h i b i t s v e ry
A t room te m p e ra tu r e , a
permeate c o n t a in in g 32% HgS i s o b ta in e d from feed gas c o n t a i n in g 5% HgS.
F lu x va lu e s a re n e a r l y 3 o rd e rs o f m agnitude above th o s e observed f o r
a l l o t h e r f i l m s e x c e p t d im e th y l s i l i c o n e .
T e s t runs a t IOO0C are u n s u cce ssfu l because th e membrane r u p t u r e s .
Thus, t h i s f i l m
i s o n ly u s e fu l a t te m p e ra tu re s below 100°C.
f o r low te m p e ra tu re s t h i s f i l m
However,
e x h i b i t s e x c e l l e n t f l u x and good sep a ra ­
t i o n w i t h o u t th e v e ry s h o r t s e r v ic e l i f e
o f d im e th y l s i l i c o n e .
As was th e case w i t h d im e th y l s i l i c o n e ,
th e h ig h permeate f l u x
may cause th e c o n c e n t r a t io n o f HgS a t th e membrane s u r f a c e t o be
s i g n i f i c a n t l y le s s than t h e b u lk c o n c e n t r a t io n .
th e s e p a r a tio n observed t o a v a lu e le s s than i t
f lo w c o u ld be m a in ta in e d a t a h i g h e r r a t e .
T h is e f f e c t would reduce
c o u ld be i f
feed gas
- 87 3.
P o ly V in y l
F lu o r id e - A t room te m p e ra tu re t h i s m a t e r ia l e x h i b i t s
such a slow p erm eation r a t e t h a t no gas permeated in two runs l a s t i n g
s e v e r a l hours even though th e sample t e s t e d was t h i n n e r (.0 0 1 inch
versus
.002 in c h ) than many o f th e o t h e r f i l m s t e s t e d .
P o ly v in y l
f l u o r i d e e x h i b i t s v e ry low f l u x values even as te m p e ra tu re i s in c re a s e d .
S e p a r a tio n in c re a s e s v e r y r a p i d l y w i t h te m p e ra tu re .
The permeate a t
IOO0C c o n t a in s 40.5% HgS w h i l e th e permeate a t 50°C c o n ta in s 24.6% HgS.
No t e s t s were conducted above IOO0C because th e m a t e r ia l
is not
recommended by th e m a n u fa c tu re r f o r use above t h a t te m p e ra tu re .
4.
Polyamide - Runs a t both room te m p e ra tu re and 50°C r e s u l t in
a lm o s t no HgS i n th e permeate ( le s s than 1%).
a t h i g h e r te m p e ra tu re s beause i t
te m p e ra tu re use.
The f i l m was n o t t e s te d
i s n o t recommended f o r c o n tin u o u s h ig h
F lu x values h i g h e r than th o s e found i n most o t h e r
commercial f i l m s are o b se rve d.
5.
Heat S t a b i l i z e d Polyamide - Low te m p e ra tu re runs w i t h t h i s
m a t e r i a l g iv e r e s u l t s s i m i l a r t o th o s e found f o r p o ly a m id e .
Runs a t
23°C, 50°C and 75°C r e s u l t i n a permeate w i t h le s s hydrogen s u l f i d e
than i s p r e s e n t in th e f e e d .
good s e p a r a t i o n .
hydrogen s u l f i d e .
However, runs a t IOO0C and 125°C produce
A t 125°C th e permeate i s found t o c o n t a in 40.5%
S e p a r a tio n is improved s l i g h t l y
permeate) i n a run made a t 150°C.
(41.6% HgS i n th e
There i s d i f f i c u l t y m a in t a in in g th e
te m p e ra tu re o f t h e e n c lo s u r e a t IBO0C w i t h th e h e a te rs a v a i l a b l e , so
no f u r t h e r runs were made a t t h a t te m p e ra tu re .
- 88 6.
P o ly s u lfo n e - A t room te m p e ra tu re t h i s membrane i s p a r t i c u l a r l y
e f f e c t i v e in s e p a r a tin g hydrogen s u l f i d e from n i t r o g e n .
The permeate
strea m a t room te m p e ra tu re c o n ta in s about e i g h t tim e s as much hydrogen
s u l f i d e as th e feed s tre a m .
te m p e ra tu re r u n s .
26.5% HgS.
o b se rve d .
Good s e p a r a tio n i s a ls o noted in h ig h e r
P e rm e a te co m p o sitio n s range between 33.9% HgS and
No tr e n d in th e v a r i a t i o n o f s e p a r a tio n w i t h te m p e ra tu re i s
A t 1250C th e membrane r u p t u r e s , a p p a r e n t ly due to thermal
d e g ra d a tio n o f th e f i l m m a t e r i a l .
b r it t le n e s s o f th e f i l m
7.
T h is i s evidenced by extreme
a f t e r th e 125°C t e s t .
P o ly e t h e r s u l fone - Performance o f t h i s m a t e r i a l
be v e r y dependent on te m p e ra tu r e .
i s found to
A t room te m p e ra tu re th e permeate
c o n ta in s much le s s HgS than th e feed s tre a m .
However, a t 50°C good
s e p a r a tio n i s a chieved and a t 75°C th e b e s t s e p a r a tio n w i t h t h i s f i l m
o ccurs (32.8% HgS i n th e p e rm e a te ).
a t IOO0C and 125°C.
S e p a ra tio n d e c lin e s i n runs made
F lu x va lu e s are below th o s e reco rd e d f o r most
o th e r film s .
(4 )
E f f e c t o f Temperature
R e s u lts f o r a l l
th e commercial f i l m s show a s t r o n g t r e n d toward
in c r e a s in g f l u x w i t h in c r e a s in g t e m p e ra tu r e .
I t i s e xpe cted t h a t f l u x
would in c re a s e w i t h te m p e ra tu re because both d i f f u s i v . i t y and s o l u b i l i t y
o f gases i n polymers f o l l o w an A r r h e n i u s - t y p e r e l a t i o n s h i p .
both d i f f u s i v i t y and s o l u b i l i t y
That i s ,
in c re a s e e x p o n e n tia l I y w i t h te m p e ra tu re .
F ig u re V -2 0 , a p l o t o f f l u x v s . t e m p e ra tu r e , shows th e t r e n d toward
- 89 in c r e a s in g f l u x w i t h i n c r e a s in g te m p e ra tu r e .
For s i l i c o n e
p o ly ­
c a r b o n a t e c o p o lym e r, p o l y s u l to n e , and p o ly v i n y l f l u o r i d e th e p l o t s
w i t h f l u x on a l o g a r i t h m i c s c a le v s . te m p e ra tu re are l i n e a r ,
t h a t f l u x v a r ie s e x p o n e n t i a l l y w i t h te m p e ra tu re .
in d ic a tin g
The p o l y e t h e r s u lf o n e
data c o u ld be expressed as a s t r a i g h t l i n e , a llo w in g f o r some data
s c a tte r.
Heat s t a b i l i z e d polyam ide and u n tr e a t e d polyam ide behave ve ry
s i m i l a r l y a t room te m p e ra tu re and 5 0 0C.
However when h e a t s t a b i l i z e d
polyam ide is t e s t e d above 5 0 ° C . i t e x h i b i t s much lo w e r f l u x values than
would be e x p e c te d .
A p p a r e n t ly , th e hea t s t a b i l i z e d m a t e r i a l i s changed
enough t o behave d i f f e r e n t l y a t h ig h te m p e ra tu re s .
T hat i s , a t high
te m p e ra tu re s (above 50°C) th e h e a t s t a b i l i z e d polyam ide a c ts l i k e a
c o m p le te ly d i f f e r e n t m a t e r ia l
than u n tr e a t e d polyam ide .
D i r e c t comparison o f f l u x va lu e s f o r t h e commercial f i l m s t e s te d
i s n o t p o s s ib le because d i f f e r e n t f i l m
te s ts .
However i t
th ic k n e s s e s were used in th e
i s expected t h a t f l u x would be i n v e r s e l y p r o p o r t io n a l
t o membrane t h ic k n e s s .
The e f f e c t o f te m p e ra tu re on s e p a r a tio n v a r ie s w i d e l y f o r th e
commercial f i l m s t e s t e d (see F ig u re V - 1 9 ) .
Two o f th e f i l m s ,
p o l y v i n y l f l u o r i d e and h e a t s t a b i l i z e d polya m id e , e x h i b i t a c o n tin u a l
in c re a s e i n s e p a r a tio n w i t h te m p e ra tu r e . . A
te m p e ra tu re where
.
maximum s e p a r a tio n takes p la c e i s observed w i t h p o ly e t h e r s u l fo n e .
The
b e s t s e p a r a t io n f o r p o l y e t h e r s u l fone (32.8% H^S i n th e permeate) occurs
- 90 a t 75°C.
S i l i c o n e p o ly c a rb o n a te copolym er and p o l y s u l tone do n o t
f o l l o w a gen eral t r e n d .
Up t o 7 5 °C, th e s e p a r a tio n a ch ie ve d w it h
p o l y s u l fone a c t u a l l y decreases w i t h te m p e ra tu re .
A t room te m p e ra tu re o n ly d im e th y l s i l i c o n e , s i l i c o n e p o ly c a rb o n a te
and p o l y s u l fone f i l m s produce a permeate stream w i t h more hydrogen
s u l f i d e than n i t r o g e n .
W ith th e o t h e r commercial f i l m s n it r o g e n i s
c o n c e n tr a te d i n th e permeate a t room te m p e ra tu re .
C.
EFFECT OF FEED GAS COMPOSITION
( -1)
M a t e r i a ls Tested
Because tim e d id n o t a l lo w a l l
f i l m s to be t e s t e d u s in g a d i f f e r e n t
feed gas c o m p o s itio n , two f i l m s were s e le c te d to be t e s t e d w i t h a feed
gas c o n t a i n in g .2.1% HgS.
The two m a t e r i a ls chosen w ere:
1.
V in y lid e n e f l u o r i d e m o d if ie d w i t h 10% 3-m e thyl s u l f o l e n e
2.
S i l i c o n e p o ly c a rb o n a te copolym er - General E l e c t r i c C o rp o ra tio n
MEM-213
These f i l m s were chosen f o r f u r t h e r t e s t i n g because th e y e x h i b i t e d
good s e p a r a tio n i n t e s t s c o n d u c te d 'w it h 5% HgS feed gas.
(2 )
The Data
T e s t runs were made to d e te rm in e permeate f l u x v a lu e s and
s e p a r a tio n f a c t o r values w i t h a feed gas c o n t a in in g
v i n y l id ene f l u o r i d e
.27% HgS.
A
f i l m m o d if ie d w i t h 3-m ethyl s u l f o l e n e was t e s te d
a t te m p e ra tu re s ra n g in g from 23°C(room te m p e ra tu re ) t o 125°C.
S ilic o n e
p o ly c a rb o n a te copolym er was t e s t e d a t te m p e ra tu re s r a n g in g from 23°C to
75°C.
Al I runs were conducted u s in g an upstream gas p re s s u re o f
500 p s ig .
The d ata are pre se n te d i n Table V-3 and shown g r a p h i c a l l y i n
F ig u re s V-21 to V-24.
' ..
i
I
(3 )
D is c u s s io n
'I.
V in y lid e n e f l u o r i d e f i l m m o d if ie d w i t h 10% 3-m e thyl s u l f o l e n e :
As i s seen i n F ig u re V-21, th e s e p a r a tio n f a c t o r values found u sin g .27%
HgS feed gas are much le s s than th o se c a l c u la t e d f o r 5% HgS feed gas.
In th e
.27% feed gas t e s t s , t h e h i g h e s t s e p a r a tio n f a c t o r i s 4 .5 (1.2%
HgS i n th e permeate) w h i l e th e lo w e s t s e p a r a tio n f a c t o r found in t e s t s
w i t h feed gas c o n t a i n in g 5% HgS i s 5 .5 (22.4% HgS i n th e perm eate).
The e f f e c t o f te m p e ra tu re on s e p a r a tio n does n o t f o l l o w a u n ifo rm
p a t t e r n as feed gas c o m p o s itio n i s changed.
For exam ple, a t 50°C, the
h i g h e s t s e p a r a tio n f a c t o r va lu e w i t h 5% HgS feed gas and th e
lo w e s t v a lu e w i t h
.27%.H^S fe e d gas a re r e p o r te d (see F ig u re V -2 1 ).
F lu x va lu e s d e c l in e i n t e s t s w i t h
r e p o r te d u s in g 5% HgS feed gas.
.27% HgS feed gas from those
As can be seen i n F ig u re V-22, the
e f f e c t o f te m p e ra tu re on f l u x does f o l l o w a u n ifo rm p a t t e r n as feed
gas c o m p o s itio n i s changed.
T h is i s seen i n th e s i m i l a r i t y i n th e
shape o f th e curves shown in F ig u re V-22.
2.
S i l i c o n e p o ly c a rb o n a te copolym er : S e p a ra tio n f a c t o r values
found u s in g .27% HgS feed gas a re h ig h e r than th o s e f o r 5% HgS feed
gas a t 23°C and 5CPC (see F ig u re V - 2 3 ) .
A t 75°C a h i g h e r s e p a ra tio n
TABLE V -3.
COMPARISON OF FLUX AND SEPARATION VALUES FOR
TWO DIFFERENT FEED GAS COMPOSITIONS
10% 3-M ethyl S u lfo le n e M o d i f i e r
Thickness = 2 m il
(M3 (STP)^
Temperature
(°C
)
Fl ux M2Hr
5% Feed
.27% Feed
S e p a ra tio n F a c to r
5% Feed .27% Feed
P e rcen t HgS
in Permeate
5% Feed
.27% Feed
23
3 .7 x I O ' 5
2 .7 x IO " 5
5.6
3.5
22.8
.9
50
4 .3 x I O '4
7 .8 x I O " 5
9.4
.9
33.1
.2
75
1.6 x IO "3
5 .6 x IO "4
9 .4
3 .7
33.0
1.0
100
4 .7 x IO "3
2 .6 x IO " 3
6 .8
4 .5
26.3
1.2
125
1.0 x IO "2
7 .3 x I O " 3
5.5
3 .7
22.4
1.0
VO
no
S i l i c o n e P o ly c a rb o n a te Copolymer
23
50
75
.
Thickness; = I m il
• 2 .8 x I O " 1
2 .0 x I O ' 1
8 .9
10.4
32.0
2 .7
3 .5 x I O " 1
2 .7 x I O " 1
7.0
9 .5
27.0
2 .5
4 .1 x I O " 1
3 .0 x I O " 1
7.6
6 .0
28.5
1.6
- 93 -
5% H0S Feed Gas
•
.27% H0S Feed Gas
Temperature ( 0C)
FIGURE V -2 1 .
EFFECT OF FEED GAS COMPOSITION ON SEPARATION FOR
VINYLIDENE FLUORIDE WITH 10% 3-METHYL SULFOLENE
MODIFIER
lu x (Mj (STP)ZM^Hr)
- 94 —
5% H0S Feed Gas
.27% H0S Feed Gas
Tem perature ( 0C)
FIGURE V-22.
EFFECT OF FEED GAS COMPOSITION ON FLUX FOR VINYLIDENE
FLUORIDE WITH 10% 3-METHYL SULFOLENE MODIFIER
S e p a ra tio n F a c to r
- 95 -
O
5% H9S Feed Gas
.27% H0S Feed Gas
25
50
75
100
125
Tem perature ( 0C)
FIGURE V -23.
EFFECT OF FEED GAS COMPOSITION ON SEPARATION FOR
SILICONE POLYCARBONATE COPOLYMER
I ux (M0 (STP)ZM4Hr)
- 96 -
O
5% H0S Feed Gas
.27% H0S Feed Gas
Temperature ( 0C)
FIGURE V-24.
EFFECT OF FEED GAS COMPOSITION ON FLUX FOR SILICONE
POLYCARBONATE COPOLYMER
- 97 f a c t o r i s p re s e n t f o r 5% HgS fe e d g a s .
W ith .27% HgS feed gas a t
room te m p e ra tu re a permeate c o n t a i n in g 10 tim es as much HgS as th e feed .
is
produced.
Due t o th e r a p id perm eation r a te s o b ta in e d w i t h t h i s
p o ly m e r, a n i t r o g e n b u i ld u p a t th e membrane s u r fa c e ( c o n c e n t r a t io n
p o l a r i z a t i o n ) may have h in d e re d th e s e p a r a tio n p ro c e s s , p a r t i c u l a r l y
w i t h th e 5% HgS feed gas.
F lu x v a lu e s u s in g .27% HgS feed gas are le s s than th o s e r e p o r te d
u s in g 5% HgS fe e d gas.
The s i m i l a r i t y i n t h e shape o f t h e curves i n
F ig u re V - 24 shows t h a t th e e f f e c t o f te m p e ra tu re on f l u x i s n e a r ly
in depe nden t o f feed gas c o m p o s itio n .
In summary, i t
appears t h a t f l u x decreases u n i f o r m ly as. th e
percen tag e o f HgS i n th e feed gas stream d e c l in e s .
Based on these
r e s u l t s , however, no g e n e r a li z a t i o n s c o n c e rn in g th e e f f e c t o f feed gas.
c o m p o s itio n on s e p a r a tio n f a c t o r va lu e s are p o s s i b l e .
D.
SUMMARY OF BEST RESULTS
Table V-4 p re s e n ts a summary o f t h e b e s t s e p a r a tio n r e s u l t s
o b ta in e d i n t h i s s tu d y u s in g a 5% HgS feed gas.
As can be seen i n Table V -4, s e v e ra l o f th e c o m m e rc ia lly a v a i l a b l e
polym er f i l m s show prom ise f o r use i n l a r g e s c a le HgS removal s y ste m s.
For low te m p e ra tu re a p p l i c a t i o n s
(around room te m p e ra tu re ) p o ly s u lfo n e
and s i l i c o n e p o ly c a rb o n a te copolym er f i l m s g iv e good s e p a r a t i o n .
S i l i c o n e p o ly c a rb o n a te copolym er has th e added advantage o f v e r y h ig h
f l u x v a lu e s .
For h i g h e r te m p e ra tu re s (IOO0C - 150°C) h e a t s t a b i l i z e d
-
- 98 TABLE V -4.
SUMMARY OF BEST SEPARATION RESULTS
Feed = 5% HgS i n Ng
F ilm M a t e r ia l
Pressure = 500 p s ig
S e p a ra tio n
F a c to r
Heat S t a b i l i z e d
Polyamide
13.5
Heat S t a b i l i z e d
Polyamide
P o ly V in y l
% H2S in
Permeate
4 1 .6
150
12.9
4 0 .4
125
12.9
4 0 .5
100
P o ly s u lfo n e
12.7
4 0.0
23
P o ly s u lfo n e
9 .7
3 3.9
75
9 .4
3 3.2
75
V i n y lid e n e F l u o r id e
10% 3-M e th yl S u lf o le n e
9 .4
33.1
50
V in y li d e n e F l u o r id e
10% 3 - Methyl S u lf o le n e
9 .4
33.0
75
P o ly e t h e r s u lf o n e
9 .3
3 2.8
75
P o ly s u lfo n e
9 .1
32.4
50
S i l i c o n e P o ly c a rb o n a te
8 .9
32.0
23
V i n y lid e n e F l u o r id e
10% Monoisopropanola m in e
8 .9
31.9
75
P o ly V in y l
F lu o r id e
F l u o r id e
.
Temperature
(°C )
- 99 polyam ide and p o l y v i n y l f l u o r i d e f i l m s p r o v id e good s e p a r a t i o n .
Heat
s t a b i l i z e d polyam ide i s somewhat h in d e re d by r e l a t i v e l y low f l u x values .
compared t o o t h e r commercial f i l m s .
Se vera l o f th e m o d if ie d v i n y l i d e n e f l u o r i d e f i l m s t e s t e d a ls o g iv e
good s e p a r a tio n r e s u l t s .
Tests w i t h 3 -m e thyl s u l f o l e n e m o d i f i e r produce
a permeate c o n t a i n in g 33% H2S fro m a 5% H2S feed a t 50°C and 75°C.
F lu x va lu e s o b ta in e d u s in g 3-m ethyl s u l f o l e n e m o d i f i e r are comparable
t o those o b ta in e d f o r most o f th e commercial f i l m s .
is n o t p o s s ib le in a l l
used in t e s t s .
cases because d i f f e r e n t f i l m
th ic k n e s s e s were
Film s c o n t a i n in g monoisopropanol amine m o d i f i e r also
e x h i b i t good s e p a r a tio n a t 75°C.
Film s m o d ifie d w i t h m o rp h o lin e and
d iis o p ro p a n o la m in e p r o v id e good s e p a r a tio n a t
E.
D i r e c t comparison
a ll
te m p e ra tu re s .
ANALYSIS OF ERRORS
The r e l i a b i l i t y o f th e measurements made d u r in g t h i s s tu d y could
be in f lu e n c e d by s e v e ra l p o s s ib le sources o f e x p e rim e n ta l e r r o r .
The
most im p o r t a n t p o s s ib le e r r o r sources a re :
I.
Changes i n th e te m p e ra tu re o f th e gas i n the permeate l i n e
o u t s id e th e c o n s ta n t te m p e ra tu re e n c lo s u r e .
a ll
I t i s assumed in
c a l c u l a t i o n s t h a t permeate gas i n th e permeate l i n e
o u t s id e th e c o n s ta n t te m p e ra tu re e n c lo s u re i s a t room
te m p e ra tu re .
I f t h i s gas i s i n f a c t a t a te m p e ra tu re above
room te m p e ra tu re th e p erm eation r a t e values re c o rd e d would be
h i g h e r than a c tu a l
v a lu e s .
-
2.
100
-
Changing r e s is t a n c e o f th e f l u i d i n th e permeate l i n e .
more f l u i d
As
i s pushed o u t o f th e permeate l i n e by permeate
g a s . t h e r e s is t a n c e caused by t h e f l u i d
i n t h e l i n e decreases.
T h is e f f e c t i s i n s i g n i f i c a n t when perm eation r a te s are h ig h ,
b u t c o u ld a f f e c t r e s u l t s f o r runs w i t h low perm eation r a t e s .
3.
N o n -u n ifo rm f i l m t h ic k n e s s .
V a r i a t i o n s i n f i l m , th ic k n e s s
between d i f f e r e n t membranes and on d i f f e r e n t areas o f th e
same membrane co u ld cause e r r o r s i n measured f l u x v a lu e s , s in c e
f l u x decreases as membrane th ic k n e s s in c r e a s e s .
4.
V a r i a t i o n in membrane polym er s t r u c t u r e .
F o rv in y lid e n e
f l u o r i d e membranes even sm all changes i n d r y in g te m p e ra tu r e ,
d r y in g t im e , and tim e i n s o l u t i o n b e fo re c a s t in g c o u ld cause
-changes in c r y s t a l l i n i t y o r change th e polym er s t r u c t u r e i n
o t h e r ways.
Any change i n polymer s t r u c t u r e would be
expected t o in f l u e n c e th e values o f f l u x and s e p a r a tio n f a c t o r
reco rd e d d u r in g t e s t r u n s .
CONCLUSIONS.AND' RECOMMENDATIONS
A.
CONCLUSIONS
1.
Exa m ination o f th e s u r fa c e s o f m o d if ie d v i n y l idene f l u o r i d e
membranes u s in g a sca n n in g e l e c t r o n m icroscope i n d i c a t e s t h a t
polym er s o l u t i o n s must be a llo w e d t o stand f o r 15 days and be de­
gassed b e fo r e membranes are c a s t in o r d e r t o in s u r e u n ifo rm
. s u r fa c e c h a r a c t e r i s t i c s .
2.
The a d d i t i o n o f a m o d i f i e r i n which HgS i s s o l u b l e t o v in y li d e n e
f l u o r i d e membranes can r e s u l t in a s i g n i f i c a n t in c re a s e i n HgS
s e p a r a tio n o v e r t h a t o b ta in e d w i t h u n m o d ifie d f i l m . '
3.
Of th e m a t e r i a ls t e s t e d as m o d i f i e r s , 3-m ethyl s u l f o l e n e , m o r p h o lin e ,
m onoisopropanol amine and d i is o p ro p a n o l amine a l l g iv e v e r y good
s e p a r a t io n r e s u l t s .
Using 10% 3-m e th yl s u l f o l e n e a t 50°C a 5%
m ix t u r e o f HgS can be c o n c e n tr a te d t o 33% HgS.
4.
Permeate f l u x and s e p a r a tio n show a s t r o n g tendency t o in c re a s e
w i t h i n c r e a s in g te m p e ra tu re .
5.
Several o f th e commercial f i l m s t e s t e d p r o v id e good s e p a r a tio n
o f HgS fro m Ng.
A t room t e m p e r a t u r e . p o l y s u l f o n e , s i l i c o n e
p o ly c a rb o n a te copolym er and d im e th y l s i l i c o n e f i l m s are e f f e c t i v e
f o r c o n c e n t r a t in g HgS.
p o ly v in y l
A t te m p e ra tu re s o f IOO0C and above
f l u o r i d e and h e a t s t a b i l i z e d polyam ide f i l m s e x h i b i t
h ig h s e p a r a tio n f a c t o r s .
For exam ple, a t 125°C a 5% m ix t u r e
of
HgS can be c o n c e n tr a te d t o 40% HgS u sin g a f i l m o f h e a t s t a b i l i z e d
p o ly a m id e .
.
- 102 6.
Low ering th e amount o f HgS i n t h e feed gas stream causes permeate
f l u x t o decrea se.
No g e n e r a l i z a t i o n s are p o s s ib le co n c e rn in g
th e e f f e c t o f feed gas c o m p o s itio n on s e p a r a tio n based on the
r e s u lts o f t h i s s tu d y .
i
I
B.
RECOMMENDATIONS
(I)
'M o d i f i e d V inyT idene F l u o r id e Films
More work sh o u ld be done to d e te rm in e th e polym er s t r u c t u r e o f
m o d if ie d v i n y l i dene f l u o r i d e membranes.
F u r t h e r e l e c t r o n m icroscope
s t u d ie s o f membrane s u r fa c e s c o u ld be used t o d e te rm in e how membrane
m a n u fa c tu rin g ste p s in f l u e n c e membrane s t r u c t u r e .
For example,
membrane samples formed by d r y in g a t d i f f e r e n t te m p e ra tu re s could be
checked u s in g th e e l e c t r o n m icroscope t o see i f s u r fa c e c h a r a c t e r i s t i c s
a re changed. . C r y s t a l l i n i t y measurements s h o u ld be made t o dete rm in e th e
e f f e c t o f m o d i f i e r a d d i t i o n on c r y s t a l l i n i t y .
x - r a y d i f f r a c t i o n methods.
T h is c o u ld be done u s in g
Techniques t o dete rm in e th e amount o f
m o d i f i e r w hich i s a c t u a l l y p r e s e n t in t h e membrane s h o u ld be i n v e s t i g a t e d .
Such a te c h n iq u e c o u ld a ls o be u s e fu l
in d e te r m in in g w h e th e r o r n o t
th e m o d i f i e r i s l o s t fro m th e membrane as tim e p ro g re s s e s .
Since many
o f t h e m o d if ie r s used c o n t a in n i t r o g e n , perhaps q u a n t i t a t i v e a n a ly s is
methods f o r n it r o g e n c o u ld be used.
■■
O th e r te c h n iq u e s f o r membrane f o r m a t io n , s h o u ld be i n v e s t i g a t e d .
Such te c h n iq u e s in c lu d e d r y in g a t low p re s s u re and low te m p e ra tu re and
removal o f th e s o l v e n t by d i f f u s i o n i n t o a n o th e r l i q u i d
i n which th e
- 103 polym er and m o d i f i e r are n o t s o l u b l e .
These methods may produce
f i l m s w i t h d i f f e r e n t polym er s t r u c t u r e s .
Such te c h n iq u e s m ig h t a ls o
a l lo w more m o d i f i e r t o be r e t a in e d in th e membrane than h ig h te m p e ra tu re
d r y in g does.
The e f f e c t o f th e amount o f m o d i f i e r added t o t h e membrane shou ld
be s t u d ie d .
T hat i s , t e s t s s h o u ld be run on membranes formed from
s o l u t i o n s c o n t a i n in g q u a n t i t i e s o f m o d i f i e r both above and below ten
w e ig h t p e r c e n t.
I t i s p o s s ib le t h a t s e p a r a tio n c o u ld be g r e a t l y
improved by in c r e a s in g th e amount o f m o d i f i e r i n th e membrane.
O th e r m o d if ie r s s h o u ld be t e s t e d i n th e search f o r a system which
p r o v id e s both good f l u x and s e p a r a t i o n .
Compounds r e l a t e d t o those
t e s t e d i n t h i s s tu d y w hich c o u ld be s t u d ie d in c lu d e P-ami nophenyl
s u l f o n e , phenyl s u l f o n e , N-methyl m o r p h o lin e , d i e th a n o l amine and
t r i i s o p r o p a n o l amine.
A n o th e r p o s s ib le re s e a rc h area suggested by t h i s work i s t o add
tho se p l a s t i c i z e r s w h ich were found t o work b e s t t o polymers o t h e r than
v i n y l idene f l u o r i d e .
Polymers t h a t c o u ld be used in c lu d e those o f
v i n y l f l u o r i d e , v i n y l i d e n e c h l o r i d e and v i n y l c h l o r i d e .
(2 )
Commercial Films
In c o n t in u in g th e search f o r a p o ly m e ric membrane m a t e r i a l which
p r o v id e s good f l u x and s e p a ra tio n , o t h e r c o m m e rc ia lly a v a i l a b l e f i l m s
s h o u ld be t e s t e d .
Among th e many p o s s i b i l i t i e s
are m a t e r i a l s such as
p o ly p r o p y le n e , p o l y s t y r e n e , p o ly im id e , and p o l y v i n y l c h l o r i d e .
- 104 I t i s p o s s ib le t h a t s e p a r a tio n c o u ld be improved by adding th e
m o d if ie r s w hich gave t h e b e s t r e s u l t s w i t h v i n y l idene f l u o r i d e t o th e
polym er f i l m s which a ls o gave good s e p a r a tio n r e s u l t s .
I t i s expected
t h a t such a dual system would g iv e v e r y good r e s u l t s .
(3)
A ll
F ilm s
Tests s h o u ld be made a t o t h e r o p e r a t in g p re ssu re s t o determ ine th e
e f f e c t o f p re s s u re on f l u x and s e p a r a t i o n .
S p e c ific a lly , te s ts a t
p re s s u re s o f 100 p s i g , 300 p s ig and 700 p s ig i n a d d i t i o n t o those
a lr e a d y conducted a t 500 p s ig would a l lo w p re s s u re e f f e c t s t o be
e x p lo r e d .
The need f o r t e s t runs a t h i g h e r te m p e ra tu re s i s suggested by
th e r e s u l t s o f t h i s s t u d y .
For many o f t h e m a t e r i a ls t e s t e d , s e p a r a tio n
in c re a s e s c o n t i n u a l l y w i t h te m p e ra tu r e .
Runs a t h ig h e r tem peratures
w ould a l lo w t h i s t r e n d t o be f u r t h e r e x p lo r e d .
Membrane l i f e
s t u d ie s s h o u ld be conducted to d e te rm in e i f f l u x and
s e p a r a tio n v a lu e s change w i t h th e age o f th e membrane. These t e s t s are
p a r t i c u l a r l y im p o r t a n t f o r m o d if ie d v i n y l i d e n e f l u o r i d e membranes because
th e m o d i f i e r c o n t e n t may decrease w i t h t im e .
P a r t i c u l a r l y a t high
te m p e ra tu r e , th e m o d i f i e r may s lo w l y work i t s way o u t o f th e polymer
s t r u c t u r e - and e v a p o ra te .
The f i l m s which g iv e t h e b e s t s e p a r a tio n r e s u l t s s h o u ld be t e s t e d
u s in g a gas m ix t u r e h a v in g a c o m p o s itio n s i m i l a r t o t h a t o f th e gas
stream le a v in g a coal g a s i f i c a t i o n
re a c to r.
For exam ple, th e f o l l o w i n g
- 105 gas co m p o s itio n s have been r e p o r te d (70) f o r raw gas le a v in g a KoppersT otzek Process g a s i f i c a t i o n r e a c t o r (volume percentage on a d r y b a s i s ) :
Western Coal
58.68
CO2
7.04
.
.
Coal
E astern Coal
55.38
55.90
. 7.04
7.18
32.86
34.62
35.39
N2
1.12
1.01
1.14
H2S
.28
1.83
.35
COS
.02
.04
PO
H2
t—*
CO
Illin o is
By making t e s t s on a s im u la te d g a s i f i e r e x i t gas more in f o r m a t i o n on
membrane s e p a r a tio n perform ance c o u ld be g a th e re d .
REFERENCE FOOTNOTES
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3.
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5.
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6.
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7.
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8.
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9.
H. E. Benson and R. W. P a r r i s h , Hydrocarbon P r o c e s s ., 53, 81-82
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10.
Jim W a ll, e d . , Hydrocarbon P r o c e s s . , 54, 89 ( A p r i l , 1975).
11.
Ib id ., p. 85.
12.
Ib id ., p. 87.
13.
Ib id ., p. 92.
14.
Ib id ., p. 93.
15.
Ib id ., p. 94.
16.
Jim W a ll , e d . , Hydrocarbon P ro c e s s . , " 5 2 , 107 ( A p r i l ,
17.
David K. Beavon, Chem. E n g r . , 78, 71 (December 13, 1971).
18.
Ib id .
19.
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20.
J. E. Naber, J. A. Wesselin g h , and W. G roenendaal, Chem. Engr. P ro g r.
69, 29-34 (December, 1973)..
1975).
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Helmut K r i l l
and Klaus S t o r p , Chem. E n g r , , 80, 84-85 ( J u l y 23, 197 3).
22.
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23.
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24.
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25.
John C. D a v is , Chem. E n q r . , 79, 68 (May 15, 1972).
26.
A. J . Moyes, O il Gas J.., 22, 56-58 (September 2, 1974).
27.
Jim W a ll, e d . , Hydrocarbon
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28.
Takeshi K a sai, Hydrocarbon
P r o c e s s . , 54, 93-95 (F e b r u a r y , 1975).
29.
Jim W a ll, e d . , Hydrocarbon
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30.
R. E. C onser, O il
31.
M. R. C in e s , D. M. H a s k e ll and C. G. Houser, Chem. E n g r. P r o g r . ,
72, 89-93 (A u g u s t, 1976).
32.
W. D. L o v e t t and F. I .
(May, 1974).
C u n n i f f , Chem. Engr. P r o g r . , 70, 44-45
33.
John L. Hudson e t a l . ,
(March, 1974).
E n v ir o n . S c i . T e c h n o !. , 8 , 238-243
34.
A y a lu r S. Vaidyanathan and Gordon R. Youngquis t , I n d . Eng. Chem.
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35.
I s h v a r l al G. D iah, Anthony B. P o n te r, and L e s l i e W. S h e m ilt ,
I n d . Eng. Chem. Process. Des. D e v ., 11, 458-461 ( J u l y , 1972).
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20, 93-96 ( F e b r u a r y , 1970).
;
37.
A. P. G e lb e in , W. G. L lo y d , and B. J . L u b e r o f f , U. S. P a ten t
3 ,5 0 2 ,4 2 8 (March 24, 1970).
38.
A. Deschamps, S. Frankow ia k , and P. R e n a u lt, U. S. P a te n t 3 ,716,62 0
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Gas J . , 72, 67-70 ( A p r i l
I,
1975).
1974).
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108 -
39.
K. Wehner e t a ! . ,
U. S. P a te n t 3,65 3 ,8 0 9 ( A p r i l 4 , 1972).
40.
P. W. B o lm e r, LI. S. P a te n t .3,409,520 (November 5 , 1968).
41.
R obert E. Lacey, Chem. Eng. P r o q r . , 79, 60 (Septem ber, 1972).
42.
Sun-Tak Hwang and Karl Kammermeyer, Membranes in S e p a ra tio n s
(New Y o rk:
John W ile y and Sons, 1975) p. 459.
43.
S. A. S te r n e t aI .,. In d . Eng. Chem., 57, 53 ( F e b r u a r y , 1965).
44.
Hwang, p. 461.
45.
Ib id ., p. 76.
46.
David W il li a m B rubaker and Karl Kammermeyer, In d . Eng. Chem., 46,
733-739 ( A p r i l , 1954).
47.
F. P. McCandle s s , In d . Eng. Chem. Process
( O c to b e r, 197217“
48.
Joseph G. T a j a r and I r v i n g F. M i l l e r , A . I . C h . E . J . , 18, 78-83
( J a n u a r y , 1972).
49.
Dennis R. S e ib e l and F. P. McCandle s s , In d . Eng. Chem. Process Des.
D e v ., 13, 76-78 ( J a n u a ry , 1974).
50.
Ronanth Z a v a le t a , " S e le c t iv e Perm eation Through M o d if ie d V in y lid e n e
F lu o r id e Membranes", Ph.D. T h e s is , Montana S t a t e U n i v e r s i t y ,
Bozeman, Montana, 1975.
51.
W. J. Ward, R. M. Salemme, and J . F. Mayer, U. S. P a te n t
3,81 9 ,8 0 6 (June 25, 1974). •
52.
S. L. Matson and S. G. Kim ura, Paper No. 9d p re s e n te d in 8 1 s t
N a tio n a l A . I . C h . E . M e e tin g , A p r i l 12, 1976, Kansas C i t y , M i s s o u r i .
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— 109 55.
W il li a m Heilm an, e t a l . ,
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56.
Hwang, p. 11.
57.
I b i d , p . 20.
58.
Ib id ,
59.
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V. T. S t a n n e t t , "S im p le G a s e s " , . in D i f f u s i o n in P o lym e rs, J. Crank
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65.
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Gas Tech nology, September 10-14 , 1973.
*•
p. 21.
J . A p p l . Polymer S c i . , 7, 2041 (1 9 6 3 ).
APPENDIX
TABLE OF NOMENCLATURE
C
C o n c e n tr a tio n
D
D iffu s iv ity
D0
C o e f f i c i e n t i n A rrh e n iu s e x p re s s io n f o r d i f f u s i v i t y
E
A c tiv a tio n
h
A p parent h e a t o f s o l u t i o n
H
H e n ry 's Law C onstant
H0
C o e f f i c i e n t i n A r r h e n iu s e x p re s s io n f o r H e n ry 's Law C o nsta nt
L
Membrane th ic k n e s s
N
Mass f l u x
P
T o ta l p re s s u re
P
P a r t i a l p re s s u re
R
Gas c o n s t a n t
S
S o l u b i l i t y o f gas o r vapo r in polymer
S0
C o e f f i c i e n t i n A rrh e n iu s
T
A b s o lu te te m p e ra tu re
x
Mole f r a c t i o n in feed
y
Mole f r a c t i o n in permeate
z
Length c o o r d in a t e
energy f o r d i f f u s i o n
S u b s c r ip ts
a
Component a
b
B u lk stream
m
Membrane p r o p e r t y
?
e x p re s s io n f o r s o l u b i l i t y
- 112 S u b s c r ip t s
(c o n t.)
0
In te rfa c e
1
High p re s s u re s id e o f perm eation
2
Low p re s s u re s id e o f p erm eation c e l l
Greek L e t t e r s
a
S e p a ra tio n f a c t o r
c e ll
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