SuccessfullySpecify
Three-Phase
Separators
Hereis a stepwise
procedure
for
designing
liquid/Iiquid/vapor
separutors.
WayneD.Monnery
andWilliam
Y.Svrcek,
University
of Calgary
t is often necessaryto separatetwo
immiscible liquids, the light and
heavy phases,and a vapor. A typical example in petroleum refining
is the separationofwater, and a hydrocarbon liquid and vapor. Little has
been published on three-phase (liquid/liquid/vapor) separarion, with
most information available only in
corporate design files. This article
attempts to alleviate this situation by
covering the basicsofthree-phase separator design. The authors provide a
Step-by-Step procedure and worked
out examples. Further, the examples
offer guidanceon making assumptions
for the calculations.
section calm to promote the separation.
There are different variations of horizontal three-phasevapor-liquid separators. The liquid separation section is
usually a variationof a deviceto provide
interface level control, which may
include a boot or a weir. A boot typically is specifiedwhen the volume of heavy
liquid is not substantial(< l5-207o of
total liquid by weight), while a weir is
used when the volume is substantial.
Thesehorizontal separatorsare illustrated in Figure 2. The bucket-and-weir
type design is used when interface level
control may be difficult, such as with
heavy oils or when large amounts of an
emulsionor a paraffin are present(1).
Selectingthree.phareseparatore
$tokos'lawappliee
As with two-phase designs, threephaseunits can be either vertical or horizontal, althoughthey typically are horizontal (seeFigures l and2). The vertical orientation.Figure 1, is only used if
there is a large amount of vapor to be
separatedfrom a small amount of the
light and heavy liquid (< l0-207o by
weight). Unfortunately, there are no
simple rules for separator selection.
Sometimes,both configurationsshould
be evaluatedto decide which is more
economical. Further, the available plot
space(footprint) may be a factor.
The design of three-phaseseparators
is similar to their two-phase counterparts, except that the liquid section differs. For the vertical type, a baffle commonly keeps the liquid separation
Separatinga vapor from a light liquid
(two-phaseseparation)has been covered
in a previous article (2) and will not be
discussedhere. However, all necessary
information for performing this part of
the calculation is provided here. The following discussioncoversthe separation
of light and heavy liquids.
The flow of rising light droplets in
the heavy liquid phase or settling heavy
dropletsin the light liquid phaseis con- ,
sidered laminar and is soverned bv
Stokes'law:
rt -
v T -
1,488g,fi(p,- pr) ( l )
18F
where 1,488convertsviscosityof the
pRoGREss. SEpTEMBER
CHEMTcAL
ENGTNEERTNG
rgga . 29
MULTIPHASE FLOW
For horizontal separators with a
is more diffigiven diameter,the heights of the light
cult (requires
and heavy liquids are assumedso that
more time) to setthe cross-sectionalarea can be calcutle the droplets
lated. With the vapor disengagement
out of the continarea set by guidelines, the lengths
Wire mesh
uous phase with
required by holdup requirements and
the greater visvapor/liquid separationare calculated.
cosity, since U. is
+ D Feed
Then, with the assumedheights of the
lower. Practically
nozzle
1 -i n .
light and heavy liquids and calculated
speaking, U, is
vent
Maximum
values of settling velocities, the set*
typically limited
liquid
tling
times are calculated.
in calculations
H"ls"rg"
I level
The
actual residence times for the
i
n
.
/
m
i
n
to 10
light and heavy liquids are subsemaximum.
Baffle
quently calculated and comparedwith
For vefiical sep-*
Holdup
A
^.[the required settling times. as in the
arators,the diameLishtliq.
Lioht 'rvertical case. If the residence times
for
required
ter
tiq"uiO +r'-\
Baffleto provide
are not greater than the required setvapor disengagenozzle l H '
c a l m i n gz o n ei s
times, then either the diameter
tling
is
calculated
recommended
ment
lnterface
should
be increased or. for a given
previous
in
our
as
A-A
Section
heavyliquid
Hrl
diameter, the length should be
article (2). In sizincreased(liquid separationis coning a separator,the
trolling). In the subsequentdesign
heightsof the light
I
procedures, the laner approach is
and heavy liquids
Heavy
used, along with the procedures disare assumed, and
liquidnozzle
cussed in our previous paper for
the settling velocivapor/liquid separationr2).
ties and settling
I Figure l.Vertical three-phase separalors are used with high
The following desi-rn procedures
times are then
vapor loadings
and heuristics are a result of a review
calculated.
of literature sources and accepted
The residencetimes of the light and
continuous phasefrom lb/(ft)(s) to cP.
guidelines.
design
industrial
heavy liquids are determinednext. For
Simplifying Eq. 1 and converting
procedures
are predesign
Horizontal
the liquids to separate,the residence
the units of the terminal settling
types
separator
for
the
four
sented
greater
liquid
must
be
time of the light
velocity to in./min from fUs results in:
shown in Figure l. The horizontal
than the time required for the heavy
design procedures incorporate optidroplets to settle out of the light liquid
2.0615l xl}-s|f,QH-pL)
mizing the diameter and length by
phase; and the residence time of the
rr
,r=T
minimizing the approximate weight
heavy liquid must be greater than the
of the shell and heads.To add a degree
time required for the light liquid
of conservatismto the design,the volheavy
liquid
(2)
droplets to rise out of the
ume
available in the headsis ignored.
=
not
satisphase.
are
(1
Ifthese
conditions
whereDo is in microns micron
6
liquid
fied,
then
x
in./min.
10
feet)
and
U7,
3.28084
separationis conEq.2 maybe rewrittenas:
trolling and the
vessel diameter
kt(Pu - Pr)
rt
must be increased.
(3)
u
r=---T
Holdup time for
liquids must be
where
addedto residence ...
xl\-sfi .
121
ks=2.0615t
oi iausti,
time. The height
89
or causiic
of the vertical
Values of ft, are given for some sys0is3'
89
Furfural
89
0.i63
Water
ketone
Methylethyl
three-phase sepatems in Table 1.
0.163
8g
Water
sec-Butyl
alcohol
rator is calculated
From Eqs. 1-3, it can be seenthat
0.163
89
Methylisobutylketone Water
manin
the
same
the settling velocity of a droplet is
0.163
89
Water
Nonylalcohol
ner as for-the twoinverselyproportional to the viscosiphasecase.
ty of the continuousphase.Hence, it
Vapor
nozzle
i
I
I
t
=
=
!
E
:
{
I
il
_T
J
30 .
1994.
SEpTEMBER
pRocREss
ENGTNEERTNG
CHEMIcAL
3. Interfacecontrol with weir
Feed
inlet
l,v
Min.12in.
Min.12in.
Heavy
liquid
outlet
Light
liquid
outlet
* l N , *l L N I * ',l^n.
l
* * u,n.l
2. InterJacecontrol with boot
4. Bucketand weir
Vapor
outlet
Vapor
outlet
Lightliquidholdu/surge
Light Heavy
liquid
outlet
-t
|
I
L,
I
Heavyliquid
outlet
-t
Light Heavy
liquid
outlet
-t
-l
lLzlLslL4l
Note:N= 112d7,1+6in.
drv= Nozzledia'
t - L -
I Figure 2. Basic designs of horizontal three-phase separators,
Vertical design.procedur€
Refer to Figure 1 for dimensions:
1. Calculate the verlical terminal
vapor velocity:
Pv
U- = ylPt rtt2
Pv
(4)
Calculate the K value, using one of
the methods in Thble 2 and set Uu =
A.l5Ur for a conservativedesign.
2. Calculate the vapor volumetric
flow rate:
WV
Q v = 3,600pu
(s)
3. Calculate the vessel internal
diameter,Dur:
140-,\"'
D""=l#
|
\nu, )
(6)
If there is a mist eliminator, add 3-6
in. Io D, to accommodatea support
ring and round up to the next 6-in.
increment to obtain D; if there is no
mist eliminator,D = Dur.
4. Calculate the setting velocity of
the heavy liquid out of the light liquid
using Stokes'law (the maximum is 10
in./min):
kt(Pn - Pr)
vr tH L -ltr
0\
where ft, is obtained from Table 1 or is
calculated(seeEq. 3).
5. Similarly, calculate the rising
velocity of the light liquid out of the
heavy liquid phaseusing Stokes' law:
ur ru . J-- k t ( P r - P t )
ILH
(8)
6. Calculate the light and heavy
liquid volumetric flow rutes, Q* and
QnL:
o
w-r .L-_
w-"LL
(9)
60pL
-Wrt
n
unr- 60pa
(10)
7. Assume Hr = I ft (minimum)
and calculate the settling time for the
. sEprEMBERrggr
. 31
CHEMTCALENGTNEERTNGpRocREss
MULTIPHASE
(0- .0rl\
o,in{
^ =(t.+aeal\/oo
a,
r,
lI--T-/\----G-/
) Equation13
- 0.6973=2.14ftJs
Ur =0' 313 53.95
P, psig
I EquationEl
-*
Qv=
=
:
=
-=
415.000lb/h
=165.32ft'ls
3.600s/hx 0.69731Vff
lEquation E2
^
1TVT4STFIs
--irISHs
- r0.13ft
r-\o=
V
I
lEquation
2
ou -
E3
16.5000lb/h = 5.10ft'lmin
60 minthx 53.95lbtftt
a EquationE4
"=,{*t#d)
vnr=ffi
heavy liquid droplets to settle through
this distance(12 is a conversionfactor
for ft to in.):
-
l2H,
----------=
(11)
U,,
8. Assume Hn = | ft (minimum)
and calculate the settling time for the
light liquid droplets to rise through
this distance:
t
,LH
LzH ,
-- ---:
( 12 )
TI
uLH
9. If there is a baffle Plate, calculate the area:
a. Calculate(P. - P).
b. Assume 11* (use 9 in. as a minimum) and calculate Ht+ H^.
c. Use Figure 3 to obtain G.
d. CalculateAr:
S e eE q . ( l 3 ) i n t h eb o x .
e. Assume Wo = 4 in.
f. Calculate WD/D.
32,
= 0.35ft'lmin
lEquation E5
whereD"is inft.
= 3.28084
x 10{ft
Note:1 micron
t
,HL_
l,3o0lb/h
g. Use Table 3 to determineAo lA.
h. CalculateA= (nl4)D2.
i. Calculate Ar.
j. Selectthe larger value ofAr.
k. Calculate the area of the baffle
plate - settling area for the light liquid;A.-A-Ao.
10. Calculate the residence time
of each phasebased on the volumes
occupied by the light and heavY
phases:
s*=w
(I4a)
- HuAu
A
vHL-
(14b)
H.=+
If eLL < tu, or Iur 1 trr, \tctease
the diameter and repeat the procedure
from Step 7 (liquid separationis controlling). Note that An= A.
11. Calculate the height of the
light liquid above the outlet (holdup
( 1s )
Check this r-alueu'ith that assumed
in Step 9b to ensurethat the assumed
value is reasonable. ft- surge is not
specified, calculate the surge height
basedon surgetime:
(eu* eo)r,
Hs=\---TL
Qrt
. CHEMICALENGINEERINGPRoGRESS
sEprEMBERlggd
basedon the required holduP
(16)
The minimum is 6 in.
12. Calculate the vessel height
using the guidelines:
He= 6 in. minimum.
H aio= h d. + greater of (2 ft or /1t
+ 0.5 f0.
Ho=0.5D or a minimum of:
3 6 i n . + V z d r( w i t h o u t m i s t
eliminator), or
24 in. + t/zd* lwith mist eliminator):
10,000
24in.
c
o_
o
F
c
9,000
-o
G
8,000
G
r.i
High liquid levelaboveinterface
/= 4.0184a8
. = _ J . 9 t 6 4tt
= -i.801705
= :l 1.i5348
pr- pu,lblft3
t. \, to H/D+
= HID
= \/.\r
= 0.00153756
' = 16.787101
= 1.299201
: - i.2.923932
. = 1.1.353518
'= 11.844824
- = -36.999376
: = 10.529572
: = 9.892851
I Figure 3. G is found from the downconterallowableflow.
the time lbr the
light liquid to
rise out of the
heavy liquid
phase:
trH=(12)(1.0)
ll.lJ=6.9min
9. Calculate
2 . C a l c u l a t et h e r a p o r '\ r r l u r r - t r i "
iili ti-:,r-r' :'1:ri- i1f-;1:
r
_ r _ i l , ) <
r l n t l l l _ i l ) S
flow rate,seeEq. El in the bor.
3. Calculatethe vesselinner diamit'
lL.
. . \ : > . r 1 .H
eter,seeEq. E3 in the box.
' t -. = I ) i n . .H r + H n = 2 4
= Hn+ H, + HR+ O^* Ufii
n,
= 10.5fi.
Use
1J
i
n
.
L
'
.
:
r
g
F
r
,
r L r r-e1 .G = 9 . 8 0 0g p h / f t z .
.a
- jr
4 . C a l c u l a tl h
e c : e t t l i n \ge l o c i l r, r l
{. =
_ c a l i i t ' r r 6m
0 i n i h )( 5 . 1 0
- o.-1-i
r ii' 9..\00gph/iir = 0.25 ft2
the heavyliquid out of the iight liqLrid
..\...ll ti ll
It' a mist eliminatorpad is used,
p h a s e .U s i n g T a b l e l , f t s = 0 . 1 6 3
= J jp..
-,-riionalheightis addedas shownin
1
l
r
1
=
Then:
\\ .,D
xl 1 0 . 5=) 0 . 0 3 1 7
: - - l r eI .
Um = 0.163(62.11 53.95)/0.630
= 2 .1 1 i n . / m i n
E x a m p l e1
5. Calculatethe settlingvelocitl of
Sizea verticalseparatorwith a baf'the light liquid out of the heavyliquicl
': :lltc
and wire-meshmist eliminaphase:
: i,.r separatethe mixture given in
U r a = 0 . 1 6 3 ( 6 2 . 1-1 5 3 . 9 5 ) / 0 . 7 6 1
= l.l4 tn.lmin
. -,rleJ. The operatingpressureis 165
'..,:, rrild it is
necessaryto have a
6. Calculatethe light and hearr
Jr'(rcarbon
liquid holduptime of 25
Iiquid volumetric flow rates. see Eq.
:: and a surgetime of only 5 min.
E4 and E5 in the box.
1oa,r=
Ar+A, ft3ls
1. Calculatethe vertical terminal
7 . A s s u m eH r = 1 f t a n d c a l c u l a t e
- .iitr'. Using Table 2, calculateK
the tirne fbr the heavy liquid to sertle
Q , -- vr ,u : . 60
- ' i t h e Y o r k D e m i s t e re q r r r t i o n s . out of the light liquid phase:
Tt:2
,fF;
. . . E q . E 1 i n t h eb o x .
4u,
t n , . =( 1 2 )( 1 . 0 )l 2 . l 1 = 5 . 7m i n
, : n JL ' 1= 0 . 7 5x 2 . 1 4= 2 . 0 5f t l s .
8. AssumeHn = 1 fi and calculate
:.luivalent
expressions,
suchasH',/D
^=#6
. SEPTEMBER
CHEMICAL
ENGINEERING
PROGRESS
1994.
33
MULTIPHASE FLOW
. Using Table 3, AD/A = 0.0095:
1= (nl4)(10.5 ft)2 = 86.59 ftz
A, = (0.0095)(86.59ft2)= g.g21r2
. Use A, = 0.82 ft2.
A, = 86.59- 0.82 = 85.11 ft2
10. Calculate the residencetime of
eachphase:
0r. = (1.0 fI) (85.77 112115.10
ft3lmin = 16.8min
lnr = (1.0 f0 (86.59 ftz)10.35
ft3lmin = 241.4 min
11. Calculate the height of the light
liquid above the outlet, based on
holdup:
tlo = (5.10 ft3imin) (25 min)/85.77
ftz = 1.5 ft
/1s = (5.10 + 0.35)(ft3/min)(5
min)i86.59 ft2 = 0.31 ft
Use I/r = 0.5 ft.
12. Calculate d, according to
Table 5:
)"= Qrl(Qr + Q) = (5.10 +
0 . 3 5 Y ( 5 . 1 0+ 0 . 3 5 + 1 6 5 . 3 2x 6 0 ) =
0.0006
Use Eq. E6 (see box) to calculate
Pr
drum
F Fuelgasknock-out
fuelgaslinebetween
NLt andhighlevelshutdown
ZOft.ilug in theincoming
drum
G.FlareKnock-out
20to 30minto Hll
factors(optionall:
Multiply
bythefollowing
Personnel
Factor
Instrumentation
Factor
Experienced
1.0
WellInstrumented
1.0
Trained
1.2
Instrumented
Standard
1.2
Inexperienced
1.5
PoorlvInstrumented
1.5
p,=##
rbtft3
x 53.es
. +ffi x62.It= 54.55
l EquationE6
=r.rrr,
)'''
1994.
SEpTEMBER
Horizontaldesign procedure:
no boot or weir
1. Calculate the vapor volumetric
flow rate, Qu, using Eq. 5.
I EquationE7
34 .
pu = pLX+ pll - l.) = (54.55)
(0.0006) + (0.6913) (l - 0.0006) =
0.730
Qu = 165.32+ (5.10 + 0.35)/60=
165.4I ft3/s
Use Eq. E7 (seebox) to calculated*
dN > 2l in.; use dt = 24 in.
Calculate Ho'.
Ho = 0.5 (10.5)= 5.25ft or
Ho = 24 + 2412= 36 in. = 3.0 ft
(minimum)
Use 11, = 5.5 ft. From Figure 1, I1t
= I ft and s = 0.5 ft. Calculate Ilur:
Hsw='/,(2.0 f0 + 2 ft = 3 ft
Set 11, = 0.5 ft. Final dimensions:
D = 10.5 ft, HH= I.0 ft^ HL = 1.0 ft,
Hn= 1.5 fr, Ho = 0.5 ft. HB.,= 3.0 ft,
andHo = 5.5 ft. Add 1.5ft for the mist
eliminator.
Hr= l4'0 ft
HID=14.0i10.5=1.3
Add 2 ft to H, (Hp = 2.0 ft, Ho =
7.0 f0 so that HlD = 1.52 (HlD
shouldbe in the rangeof 1.5 to 6.0).
pRocREss
ENGTNEERTNG
CHEMTCAL
2. Calculate the light and heavy
liquid volumetric flow rates, Q' and
using Eqs. 9 and 10.
O711,
3. Calculate the vertical terminal
velocity, Ur, using Eq. 4. (selecta K
r alue from Table 2) and set Uu =
0.75Ur.
4. Select holdup and surge times
t-romTable 6 and calculate the holdup
and surgevolumes,V, atd Vr, (unless
surgeis otherwisespecified,suchas a
>lus volume):
Vo= To Q,
(18)
Vs=TsQr
(19)
5. Obtain an UD from Table 7 and
initially calculate the diameter
accordingto:
(20)
Calculate the total cross-sectional
ilea:
A _ TED'
4
(2r)
6. Set the vapor space height, 11,
, rhe larger of 0.2D or 2 ft; I ft if
:.-,ereis no mist eliminator. Using
1 ,/D in Table 3, obtainAr/A, andcal--llateAu.
7. Set the heightsof the heavy and
;ht liquids, Hrrand Hr,
8. Find (AnL+ AL)lAr, using (I1",
- fl -tlD inTable 3,andcalculateA*
9. Calculatethe minimum length to
-- -,rmmodatethe liquid holdup/surge:
V, +V,
r _
L _
tr-Au_ (our*,4rr)
(22)
Qrr=
I = H,./L't
Uw= Qr/Av
()L\
'=
-"{
trrQ*
tntQ,
e*'(h-Av-o*)
12. Calculatethe minimum length
required for vaporiliquid separation:
Lum= UvlQ
(2s)
13. If L 1 Lr,,,r, then set L = Lr,*
(here, vapor/liquid separation controls). This simply results in some
extra holdup and residencetime. If I
11 Lr,*, then increase Hu and recalculate Au, and repeat, starting from
Step 9. If L > L*n, the design is
acceptable for vapor/liquid separation. If L )) Lr,,.,(liquid holdup controls). I can onlr,be reducedand l.rrr.
increasedif H,, is reduced. F1, ma1'
only be reducedif it is _ureater
than the
minimum specifiedin Step 6. (With
reduced 11* recalculateAu and repeat
the procedurefrom Step9.) Note: For
this and other calculations, "much
greater than" (>>) and "much less
than" (<<) mean a variance of greater
than 20Vo.
14. Calculate the settling velocities
of the heavy liquid out of the light liquid phaseand the light liquid out of the
heavyliquid phase,U". and U.o, using
Eqs. 7 and 8 (find ft, from Thble 1).
15. Calculatethe settling times of
the heavy liquid out of the light phase
and the light liquid out of the heavy
phase:
tur= 12 (D - Hv- HH)IUHL (26)
(21)
16. Calculate the residencetimes
of the light and heavy liquids:
Q3)
Ca]culate the actual vapor
Q,,
velocity:
0ur= AorLlQu,
| [.
1 ?| q r
/
17.If gHL< tLHor 0,-, < t", then
increasethe vessellength (liquid separationcontrols):
tro= 12 HHL/ULH
10. Calculate the liquid dropout
-:a:
I\ A ', - A
, -. A 1u , l\ L .
'
(28)
(30)
18. CalculateUD.If UD << 1.5,
decreaseD (unlessit is alreadyat its
minimum), and if UD >> 6.0 then
increaseD; repeatfrom Step 5.
19. Calculate the thickness of the
shell and headsaccording to Table 8.
20. Calculate surface area of the
shell and headsaccordingto Table 8.
21.Calculate
t h e a p p r o x i m a tvee s sel weight accordingto Table 8.
22. Increaseor decreasethe vessel
diameter by 6-in. increments and
repeat the calculations :until the UD
ratio ran-eesfrom 1.5-6.0.
23. Using the optimum vesselsize
(minimum wei-eht).calculatethe normal and high liquid levels:
Hau= D - H,
( 31 )
Anrr= (Am+ Arr) + Vo/L
(32)
Obtain 11r.. using Table 3 with the
value of A*ro/Ar.
Horizontaldesign procedure:
heavy liquid boot
1. Calculate the vapor volumetric
flow rate, Q, using Eq. 5.
2. Calcrilate the light and heavy
liquid volumetric flow rates, Q* and
Q n r , p e rE q s . 9 a n d 1 0 .
3. Calculate the vertical terminal
velocity, Ur, using Eq. 4 (the K value
comes from Table 2) and set Uu =
0.15 Ur.
4. Select holdup and surge times
from Table 6 and calculate the holdup
and surge volumes, V, and Vr, from
Eqs. 18 and 19 (unlesssurgeis other-
pRocREss. SEpTEMBER
CHEMTcAL
ENGTNEERTNG
rgs+ . 35
?=
:.::
:-=
:-:
-.
::
MULTIPHASE FLOW
,,,:
wise specified,such as slug volume).
5. Obtain UD from Table 7 and
initially set the diameter according to:
/ l?\
s
I
lt
14
F
F
7
E
F
F
Then calculate the total cross-sectional area,A, using Eq. 21.
6. Set the Yapor spaceheight, F1*
to the larger of 0.2D or 2 ft (l ft if
there is no mist eliminator). Using
HutD in Table 3, obtainAv/Ar and calculate Au.
7. Set the light liquid heights in the
vessel and boot, HrN and Hrrt.
8. Calculate the cross-sectional
area of the light liquid above the bottom of the vessel,Ar., using Hrtt/D
in Table 3.
9. Calculatethe minimum length to
accommodatethe liquid holdup/surge:
f
"-
\
:
-
V.,
+V"o
n
Ar-Au-ArN
1994.
SEpTEMBER
Approximate
'=(t+
(34)
10. Calculate the liquid droPout
time, $, using Eq. 23.
1L. Calculate the actual vaPor
velocity, Uuo,using Eq.24.
12. Calculate the minimum length
required for liquid/vaporseparation.
Lrr*, usingBq.25.
13. If L 1 Lr,*, then set L = Lr,*
(vapor/liquid separation controls).
This simply results in some extra
holdup and residence time. If I
11Lr,*, then increaseH, and recalculateAr, thenrepeatfrom Step9.If L>
Lr,*, the design is accePtable for
vapor/liquid separation.If L >> LMrN,
liquid holdup controls. I can only be
reduced atd L*,* increased if I1u is
reduced.Humay only be reducedif it
is greater than the minimum specified
in Step 6.
With reduced Hu, recalailate A,
and repeat from Step 9.
14. Calculate the settling velocity
of the heavy liquid out of the light liquid phase, U"., using Eq. 7 (obtain ks
from Table 1).
15. Calculate the settling time of
36 .
DishedHeads
the heavy liquid out of the light liquid
phase:
tm= 12 (Hrrn + D - H)IUHL Q5)
16. Calculate the residencetime of
the light liquid:
ou=
(,q,,-e,)r
eu
(36)
Note: This volume of light liquid
ignores the light liquid volume in the
boot.
I7. If 1LL < to, then increase the
vessellength (liquid separationcontrols):
19. Calculate the thickness of the
shell and headsaccording to Table 8.
20. Calculate the surface area of
the shell and headsaccording to Table
8.
21. Calculate the aPProximate
weight of the shell and heads according to Table 8.
22.Increaseor decreasethe vessel
diameter by 6-in. increments and
repeat the calculations until 1,/D
rangesfrom 1.5-6.0.
23. With the optimum vessel size
(minimum weight), calculate the normal and high liquid levels:
Hnu= D - Hu
(38)
Attrr= Arru + VrlL
(39)
(37)
18. CalculateUD.If UD << 1.5
then decreaseD (unlessit is already at
a minimum) and if UD >> 6.0 then
increaseD; rcpeatfrom SteP5.
PRocREss
ENGINEERING
cHEMICAL
Determine H*rrusingTable 3 from
A*r/Ar.
24. Design the heavy liquid boot:
Set the height of the heavY liquid,
Hur; calculate the rising velocity of
the light liquid out of the heavy liquid
phase,Uru, using Eq. 8 (find ft, from
Table 1); set U, - 0.15 ULHicalculate
the heavy liquid boot diameter:
Then calculate the settling time of the
light liquid out of the heavy liquid
phase:
ttr=
l2Hu1/Us1
(4r)
Calculate the residencetime of the
heavyliquid:
B
A --ndH*
"ut
4en
(42)
the light liquid comparlment using
Eq. 44 or read it from Table 9.
Htu= 0.5D + 1
where D is in feet andHrrrin inches (round up to nearestin.). If D < 4.0
ft, then Hrm= 9 in. Using HoytD in
Table 3, CalculateArrr.
8. Calculate the weir heisht:
Hw=D-Hv
If eHr < tru, then
diameter.
1. Calculate the vapor volumetric
flow rate, Qu, using Eq. 5.
2. Calculate the light and heavl'
liquid volumetric flow rates, Qu and
per Eqs.9 and 10.
Q11y.as
3. Calculate the vertical terminal
vapor velocity, Ur, using Eq. 4 (find K
tiom Table 2) and set Uv = 0.l5Ur.
4. Select holdup and surge times
tiom Table 6, and calculatethe holdup
and surge volumes, V, and V., from
Eqs. 18 and 19 (unlesssurgeis otherir ise specified,suchas a slug volume).
5. Obtain UD from Table 7 and
initially calculate the diameter
accordingto:
(43)
t.
{
ft
I
u
n
(4s)
the boot
Horizontaldesign prooedure:
weir
1
(44)
Then calculatethe total cross-sectiona r e aA
. . . u s i n gE q . 2 l .
"l
6. Set the vapor spaceheight.Hu.
ro the larger of 0.2D or 2 ft (1 ft if
lhere is no mist eliminator). Using
H../D in Table 3, obtainAu/A, and cal;ulateAu.
7. Calculatethe low liouid level
If Hw< 2 ft, increaseD, and repeat
the calculationsfrom Step 6.
9. Calculate the minimum length
of the light liquid compartment to
accommodate holdup/surge, 1,. in
Figure 2 :
Vn+V,
Ar- Ar.- AuL
(-+6)
Round to the nearest 7: ft. The
minimum for Lr= d, + 12 in.
10. Set the interface at the height
H,*12, obtaining the heights of the
heavy and light liquids, Hrrand Hrr.
11. For the liquid settling compartment,calculatethe cross-sectional area of the heavy liquid, using
Hr/D in Table 3 and calculate the
cross-sectionalareaof the light liquid
from:
Arr=Ar- A,- A*
(4',7)
12. Calculate the settling velocity
of the heavy liquid out of the light liquid phase,Uur, andthe light liquid out
of the heavy liquid phase, U"r, using
Eqs 7 and 8 (find ft, from Table 1).
13. Calculate the settling times of
the heavy liquid out of the light liquid
phase and the light liquid out of the
heavy liquid phase:
(48)
tur= l2Hr1/U*
tru= l2Hrx/Uro
(4e)
14. Calculate minimum I, to facilitate liquid-liquid separation as the
larser of:
( t,,o-, trrQrr\ (50)
, A,, I
L1 =maxl:ff
\
nur
Round to the nearest % ft.
15. Find l,:
L= Lt+ Lz
( s 1)
16. Calculate the liquid dropout
time, Q, using Eq. 23.
17. Calculate the actual vapor
velocity. U,,.,.using Eq.24.
18. Calculatethe minimum length
required for vapor/fliquid separation,
1.,rr,..using Eq. 25.
19. lf L ( Zr'.r- then set L = L*r
(vapor/liquid separation controls).
This simply results in some extra
holdup and residence time. If L <<
Lr,*, then increase Hr, recalatlate A,
and repeat the calculations from Step
6. If L > L*,r, the design is acceptable
for vapor/liquid separation. If Z >>
.Lrr, (liquid separation and holdup
control), Z can only be reduced and
Lr,* increased if Hv is reduced. //u
may only be reduced if it is greater
than the minimum specifledin Step 9.
With reduced Hr, recalculate A, and
repeat from Step 9.
20. CalculateUD.If UD << 1.5,
then decreaseD (unlessit is already at
a minimum) and repeat from Step 6.
If UD >> 6.0, then increase D and
repeat from Step 5.
21. Calculatethe thicknessof the
shell and headsaccording to Table 8.
22. Calculate the surface area of the
shell and headsaccordins to Table 8.
pRoGREss. SEpTEMBER
CHEMTCAL
ENGTNEERTNG
1994. 37
MULTIPHASE FLOW
there is no mist eliminator). Using
Hr/D inTable 3, obtainAu/Arand calculateAu.
7. Calculatel,:
23. Calculate the approximate vessel weight according to Table 8.
24. Increase or decreasethe diameter by 6-in. incrementsand repeatthe
calculations untiT UD ranges from
1.5-6.0.
25. With the optimum vessel size
(minimum weight), calculate normal
and high liquid levels:
(o,,e,,+o-,e-,1
-,'.''.]
f
L l -
Hnu=D-H,
(52)
Atrrr= Arrr+ V,1L,
(53)
Obtain Hr'
ANLL|Ar.
Horizontaldesign procedure:
bucket and weir
1. Calculate the vapor volumetric
flow rate, Qu, using Eq. 5.
2. Calculate the light and heavy
liquid volumetric flow rates, Q, and
Qnr,perEqs.9 and 10.
3. Calculate the vertical terminal
vapor velocity, Ur, using Eq. 4 (flnd K
from Table 2) and set Uv= 0.75 Ur.
4. Select residencetimes for light
and heavy liquids, 0r, and 0o.. For
sour water stripper feed drums, 0o. =
g,, =
,=(
+(e,,0,,re,,,O,,(s4)
0J0n(r1r)
Then calculatethe total cross-section. . u s i n gE q . 2 1.
a l a r e aA
6. Set the vapor spaceheight, 11u,
to the larger of 0.2D or 2 ft. (1 ft if
235'ooolblh
=343.5'7ft3/s
3.600s/hx 0.190lb/ff
I EquationE8
o,,=
45'ooorblh -=18.52ft'lmin
60min/hx 40.5lb/ft'
I Equation E9
o-, =
8. Calculate the liquid dropout
time, Q, usingEq. 23.
9. Calculatethe actualvapor velocUuo,usingEq.24.
10. Calculate the minimum length
required for vapor/liquid separation,
L*,r, using Eq. 25 .
ll.If Ll 1 Lr,*, then set Lt = Luru
(vapor/liquid separation controls).
This simply results in some extra
holdup and residencetime. lf Lt <<
LM,N,th.enincreaseIlr,. recalculateAu
and repeat the calculations from Step
1. If Lt ) Lr,*, the design is acceptablefor vapor/liquidseparation.
12. Calculatethe light liquid layer
thicknessbased on the heavy liquid
settlins out:
60 min for reflnery service,or 10-15
min for chemical-plant service. For
amine regeneratorfeed drums, 0o. =
10-15 min.
5. Obtain UD from Table 7 and
initially setthe diameteraccordingto:
using Table 3 with
7'5oolb/h
=2.y2ft'lmin
60 min/hx62.0lblfr'
tutQrt
A,,
-
Ur = 0' 175
I Equation811
7 r . 8 0 " ' ' = 11'15
ft' usel1'0ft
-, ,-- ( 4 r 2xt.7
\0.6ru xr/i)
I EquationE12
4=ffi=14.54ft
- Equation813
I Equation810
2.0 min x2.02ft3hn1n=0.45ft
s.%ft
I Equation814
trrQo - 2.0 min x 18.52ft3lmin =2.41
rs.02ff
ALL
I Equation E15
ff ls = 4.83ftls
Q u = 343.51
rr
ure=
T
7r.08
te
I Equation816
38 .
1994.
SEPTEMBER
PROGRESS
ENGINEERING
CHEMICAL
155)
Ar-4,
ft
w,D,
tlni
Nomenclbture
prop€rengli
"'
:, =;il;:::T:1",,-*lllllJli,
= v o l u m e t r i cf i o w . f l r / s o r l t r / m i n
- vessem
l l l e r i a l s t r e s sv a l u e .p s i
0
S
lne
=
^-
liquidabor.e
vessel
bor-
ASu = 5psqif. gravity differenceberween
:#, ?:light
==,1'l
l: = :-i;iliill,1l11;1",, "'*ftJil],."',0,'o
'7\
=
0,"0,",,
"'"'.'"
il,,l,l,1l;;
l,'.;l:l
I i g h ra n d h e a v yl i q u i d r
TC
TH
tut
TH
=
*'
"
iiii,r;
i:i::';i'J'
=;5.,ff::;l
TS
= \ u r g et r m r .m i n .
o u t o l l i g h t I i q u i d 'm i n
= ressel
diemeler.
n
"r'llu'
D
of Western
degreesin
ttu
3' =llJli';,ilr,.ormicrons
1s
t . .f r
D , = r 3 p r l r ' J i s e n g a g e m edni a
=
t
e f f i c i e n c yd. i m e n s i o n -
Y:,
;::]o.oLoint
0.00128e'(AsG)Dpr
r-i6,
u
V,'
,.hereD" is in microns.
13. Calculate the differenc- rl
:rsht betweenthe light and he;:,,.,
quid weirs:
i
o. r
_\H=H
--\r r l l - X l
lro + r,)9,,
= h e i g h r .f t
= a l l o r l a b l eb o r i z o r r n lv c l o c i t l . f r l s
= s e t t l i n qr e l o c i t vu f h e r v r l i o u i d
d r o p l e i so L r o
r f l i g h r l i q u i d .i n . t m i n
u,., = r i s i n g v e l o c i t )o f l i g h r l i q u i d d r o p l e r s
H.. = liquid level above baffle, in. or ft
HB,. = liquid height from abor.ebaffle to
H;,
leed nozzle.ft
= disengagementheight. fi
H.
= htrldupheisht. fi
H-
= t e i , s h tl r . . = l i ; . : : i : : : : : = : :
Un
o u t o f h e a v yl i q u i d ,i n . i m i n
= mixture velocity, tVs
Up
= boot velocitr'.in.lmin
f
= l c f r n i l r Jilg l 6 . i 1 _ 1
r i.. ' .c r i n . / r r i n
= \ i - L p i \rre l . r c i t r .f L l :
L
ii li
: , z z : : .: '
,
a,
= ---
.-.-:.:.
:'
H,,;
H,,H,..
- - - :-- - : - - - . . ' - _ - .
H,,,u = :::::::::-::::,
::HP.
= heiSi:t:i,'::r iiir:
Hs
=
g
= totai vertical separarorheight.ii
= vapor disengagementareaheighi. ir
=
l\:.:::\. !-P
nv
=
:::.:::.lbr'ft]
H,,,
= weir height. ft
=
:, :: j irLrloui time- s
fle. 11
irrroe heiohr
ri
=
::-::
-
::'.:::,i:. liquid ifaction
ailrdenaa time. min
= h e i g h ld r f l e r e n c eb e l i i e e nl i g h i u n J
(58)
\ssume 1., is the largerof D/l2
l in.
t 6 , Design the heavy liquid comrnt: Set the top of the heavy liq. - i r ' = D - H v - A 1 1 ;a s s u m ea
:r \Lrr_se(typically, 5-15 min);
: HLL is about 6 in. below the
:r,ght and LLL is about 6 in.
.h- bottomof the vessel.Using
: v,trh Hrrr/D and Hr,,y/D, ca7i . ...andAr_r...
- L L l . i t eL . , :
= baffle liquid toad.gph/fr)
H
heavy iiquid weirs. in.
= S t o k e s l' a u .t e r m i n a lr e l o c i r r c o n -
fts
(Ar,'r_ Arr,')
l : .
6
vHL
i: -.
1 - 1D
. e s i g nt h e l i g h t l i q u i d b u c k e r :
': the top of light liquid u eir'= D
, i l s s u m et h e b o t t o n rr s a t 0 . 1 l 5 D l
. . r t t i e a h o l d u p / s u r g e( t 1 , p r c a l l r .
' . -i min.); assume
HLL is 6 in.
, rrl the weir height andLLL is 6 in.
i e the bottom of the bucket.
. r,c Table 3 with HrrrlD and
D. calculate Au' and Arr.r.
-ulateL,:
_
= c f a \ i t a r i o n acl o n s t a n tJ, 2 . 1 7 f r , / s r
:
Pu l
T
-'
r p r e l so u l
)lr'-:li. l';L
I
- , ; i
s r a n t .r i n . / m i n )cr P r i t l b / f r r
= t e r m i n a lr e l o c i t yc o n s L a n fl .r , i s
;JL:
:.::: l;;iri,l ;evei
K
L, L;
i:i:i .;;iriti
L
= vessellengrh.ft
LLL
: , , . r , r q l i i i e re l
L*,*
= v a p o r , 4 i q u isde p a r a r i o m
n irrrrnum
1/
l
;'':liilr,r-Hj
I
_
(r, + rr)9,,
(5e)
lAorr- Arrr)
17. CalculateL = L, + Lr+ L. + L^.
1 8 . C a l c u l a t eU D . I f U D < < 1 . 5 .
-:.
.\ lI
r
I
',:a.tI
I li.','i,l l-. ^l
then decreaseD trndlepeatfi'onl Step
5 Ii UD >> 6.0. then increaseD and
repeatfi'om Step-5.
19. Calculatethe thicknessof the
shel1and headsaccordingto Table8.
20. Calculate tl're surf'acearea of
pRocnEss. sEpTEMBER
cHEMTCAL
ENGTNEERTNG
1994. 39
MULTIPHASE FLOW
w h e r eP: = d e s i g p
n r e s s u raen dD = d r u md i a ,
shell and headsaccordingto Table 8.
21. Calculatethe approximatevessel weight according to Table 8.
22. Increase or decreasethe diameter by 6-in. incrementsand repeatthe
calculations until UD ranges from
t.5-6.0.
Example2
Design a three-phasehorizontal
separatorwith a weir to separatethe
mixture in Table 10. The operating
pressure and temperature are 25 psig
and 100, respectively,and it is necessary to have a liquid holdup and surge
time of 15 min.
1. SeeEq. E8, box, p. 38
2. SeeEqs.E9 and E10, box, p. 38
3. K = 0.175 (the Gas Processors
Suppliers'Associationvalue in Table
2 was divided by 2 since there is no
mist eliminator).Because of Horizontal
S e eE q . E 1 1 ,b o x , p . 3 8
U v = 0 . 7 5 x 2 . 5 5 = 1 . 9 1f t l s
4. Holdup + surgeas specified= 15
min.
Vu+ V, = (15 min) (18.52ft3lmin)
= 211.80 ft3
Assume 10 min holdup, 5 min
surge.
5 . A s s u m eU D = l . l .
SeeEq. El2, box, p. 38
Ar=n/4 (11.0f|'z = 95.03ft2
6. Since the mass rate of vaoor is
1_
"-2xl7M
55xI32
about827oof theloading,setF1yto be
much greaterthan the minimum.
AssumeHv = 0.70D= (0.70)(1
1.0f0
= 7.70 ft. Using Table 3, Av/Ar =
Av= 71.08ft2
0.148,
7 , H r r r =( 0 . 5 ) ( 1 1 .+07) = I 2 . 5 i n . ,
use13in.
Hrr/D = 13/(11.0
x 12)= 0.098
= 0.051
UsingTable3, ALLrj/Ar
Aur= (0.051X95.03
fP) = 4.95112
8. H, = 11.0- 1.10= 3.30ft
9. SeeEq.El3, box,p. 38
Usel,r = 15.0ft.
= 1.65ft
10.HHL- Hrr= 3.3012
= 0.150
ll. HH//D= 1.65111.0
FromTable3, AH/A, = 0.094
Aur= (0.094X95.03
ft2)= 8.93ft2
- 8.93= 15.02
Au=95.03- 71.08
ftl
12.FromTable1, ks= 0.333
- 40.5)/0.24
=
Uur= (0.333)(62.0
29.83in./min
Use 10in./min(maximum)
- 40.5)10.682
=
Uro= (0.333)(62.0
10.50in./min
Use 10in./min(maximum)
13, tHL = Q2 in./ft)(1.65ft)ilo
in./min= 1.98min, use2.0mins
tr, = tnr = 2.0 min
14. SeeEqs.E14 and E15. box,
p .3 8
Uselr = 3.0ft.
1 5 .L = 3 . 0+ 1 5 . 0= 1 8 . 0f t
'7.10
16.0 =
ftll.gI ftls = 4.03s
17.SeeEq.El6, box.p. 38
*l|rc= 0.307in.
I Equation 817
. .-= = , = 0 . - -x8l=! f8 1-10?. 1? - - * r , 1=60 . 4 9i5n .
'rH
x 55
2 x 1 7 . 5 0x00 . 8 5
I EquationE18
we11.
t =
ff'ltff#(on.at
rt2
+2x r0r.88
n')= ' 7.s20tb
I EquationE19
40 .
sEpTEMBER
1994.
18. LurN = (4.83 ftls)(4.03 s ) =
19.5fr
19. Since L 1 Lr,*, set -L = 19.5 ft
(setZ, = 16.0ft, lr = 3.5 ft)
20. UD = 19.5/11.0= L78
21. Assume dished heads per
Table11.
AssumeE = 0.85
Use SA-516 70 carbon steel,
design temperature= 650o
S = 17,500psi; from Ref. (3).
Corrosionallowance= Vrsin.
P=25+30=55psig
SeeEq. E17, box on this page.
Use r, - 7s in.
SeeEq. E18, box on this page.
Use /" = Vz in; use t = Vzin.
2 2 . A s = ? I ( 1 1 . 0f 0 ( 1 9 . 5 f 0 =
673.81ftz
Au= (0.842)(11.0fO'z= 101.88ft2
23. SeeEq. E19, box on this page.
24. In this example, calculations
were performed for only one diameter. However, nearly the minimum
UD corcespondedto a diameter of
11.0 fU therefore. the next diameter
should be smaller, resulting in a
laryer UD. Also, calculationsshould
be performed using a diameter of
I 1 . 5f t .
25. For the light liquid compartment:
Hnrr= Hw= 3'3 ft - 3 ft, 4 in'
A x L r = 4 . 8 5+ 1 8 5 . 2 0 1 1 6=.106 . 4 3 f P
= 0.713
A*t/Ar= 16.43/95.03
Using Table 3, HNLrt/D= 0.229
Htrrr= (0.229)(11.0)= 2.52 ft - 2
ft, 6 in.
HLLL= 13 in'
Comment: Due to the small
amount of heavy liquid and large
amount of vapor, a better design
would have used a boot. A vertical
vessel should be comoared. as
pRocnEss
CHEMTCAL
ENGTNEERTNG
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