Conceptual Design of Reactive Distillation
S. T. Tung; X. G. Shen; Y. C. Cheng; C. C. Yu
Dept of Chem. Eng.National Taiwan University
Outlineo Background-Reactive distillationo Conceptual design-relative volatility rankingn Motivationn Process studiedn Steady-State Design Proceduren Results and comparisonsn Conclusions
o Scale-upo References
RDRD-- the beginning and the turning pointthe beginning and the turning point- Early development (Keyes, 1932;
Leyes and Othmer, 1945)- Eastman Chemical Methyl Acetate
(Agreda and Partin, US Patent 4435595,1984)
- After MeAc
1 reactor + 9 distillation columns 1 RD column
Clear AdvantageClear Advantage
RDRD-- why it workswhy it works
For reversible reaction, A=B, one can breakthe equilibrium conversion by removing B.
Remark: We have two functions, reaction/separation, in one unit.
Multifunctional unitMultifunctional unit-- more examplesmore examples
RDRD-- potential problempotential problemConflict in the separation and reaction temperatures: different process windows.
RDRD-- a likely solutiona likely solutionSeparators with side-reactors
RDRD-- potential applicationspotential applications
RDRD-- potential applicationspotential applications--contcont
RDRD-- potential applicationspotential applications--contcont
RDRD-- potential applicationspotential applications--contcont
RDRD-- potential applicationspotential applications--contcont
RDRD-- potential applicationspotential applications--contcont
RDRD-- potential applicationspotential applications--contcont
l Limited real world applications– MTBE, ETBE, TAME– EG– MeAc, EtAc
l Design– A large variety of flowsheets exists– Seems to vary from case-to-case (lack
of a systematic approach)l Control
– Seems to be very nonlinear– Very little was known
l Simulation– Very difficult to converge
reactive section
stripping section
rectifying section
Current statusCurrent status
Completely Different Completely Different FlowsheetsFlowsheets for for EtAcEtAc
Condenser
Stripper
Decanter
RDColumn
Organic Reflux
Aqueous Product
Feed to Stripper
Condenser
Reboiler Reboiler
Heavyreactant
Acetate
Lightreactant
Decanter
(1) (2) (3)
flowsheet of a localcompany
18
RDRD-- where to place the reactorwhere to place the reactor
If a reactive zone is present in a RD, where should we put it?
It depends on relative volatility ranking!!!
19
RDRD-- some notation in designsome notation in design
“neat” design
A + B = C + D1 1 0 0
1-x 1-x x x
“excess reactant” designB excessA + B = C + D1 r 0 0
1-x r-x x x
Literature Review
Feasibility analysis6,14-17
conventional multi-unit10,11
excess18
Design single RD columnno excess (neat)
distillation column with external side reactor12
Control2,5,13,19-22
Scale-up3,4,7
The problemo For a generic exothermic
reversible reaction
where the reactants are intermediate boiling between the two products, which is the ideal situation for reactive distillation.
o What will be happened if we sort the relative volatilities in a different manner from the conventional case.
A B C D+ +�
LLK LK HK HHK No. Configuration* Type**
4
65
12
3
10
1211
78
9
16
1817
1314
15
22
2423
1920
21
(3)
(3)(2)
(1)(1)
(2)
(3)
(3)(2)
(1)(1)
(2)
(5)
(6)(6)
(4)(5)
(4)
(5)
(6)(6)
(4)(5)
(4)
A
CD
C
B
B
DB
DC
B
BC
DC
D
B
CD
C
A
A
DA
DC
A
AC
DC
D
C
BD
B
A
A
DA
DB
A
AB
DB
D
D
BC
B
A
A
CA
CB
A
AB
CB
C
Ir
Ir
IIIr
IIr
IIr
IIIr
Ir
Ir
IIIr
IIr
IIr
IIIr
IIIp
IIp
IIp
Ip
IIIp
Ip
IIIp
IIp
IIp
Ip
IIIp
Ip
* 6 distinct possible ranking** 3 process types (I-III) with the reatant or product as the lightest one
Nomenclature
o There are 24 (4!) possibilities which can be simplified to 6 configurations, and then we can classify them into 3types according to the ranking of relative volatilities for the reactants and products
General Classificationo Notations: One-zone: product is lightest IP ; reactant is lightest Ir
Two-zone : product is lightest IIP ; reactant is lightest IIrAlternating: product is lightest IIIP ; reactant is lightest IIIr
one zone
product is lightest
two-zone
Type I with the lightest product or reactant
Type II with the lightest product or reactant
Type III with the lightest product or reactant
Process Studied (Forward reaction)
o Reversible liquid-phase reaction
o Forward and backward specific reaction rates
o Taking (KEQ)366 equal to 2 with the forward reaction rate 0.008 kmol s-1 kmol-1 at 366 K.
o Ideal vapor-liquid equilibrium is assumed with constant relative volatilities.
A B C D+ +�
/FE RTF Fk a e−= /RE RT
R Rk a e−=
366 366 366( ) ( ) / ( )R F EQk k K=
Physical Properties (Forward reaction)
3862386238623862BVP
10.9611.4512.3413.04AVPVapor pressure constantsa
HHKHKLKLLK
8/4/2/1relative volatilities(LLK/LK/HK/HHK)
6944heat of vaporization (cal/mol)-5000heat of reaction (cal/mol)0.004Backward (kB)0.008Forward (kF)specific reaction rate at 366 K
(s-1)
17000Backward (EB)12000Forward (EF)activation energy
(cal/mol)
lnPSi=AVP,i-BVP,i/T where T is in Kelvin and PSi is the vapor pressure of pure
component i in bar.
Steady-State Design Procedureo Place the heavy reactant feed (NFheavey) on the top of the reactive
zone and specify the light reactant feed (NFlight) several trays apart from the upper feed tray .
o Fix a number of reactive trays (Nrxn).
o Guess the tray numbers in the rectifying section (NR) and stripping section (NS)
o Go back to 3 and change NR and NS until TAC is minimized.
o Go back to 2 and vary Nrxn until TAC is minimized.
o Go back to 1 and find the feed tray locations (NFheavey & NFlight) until TAC is minimized.
Process Ip : LK + HK ó LLK + HHK
o Ranking of relative volatilities
o Real system example:
o Optimization variables:Ø Number of rectifying stagesØ Number of reactive stagesØ Number of stripping stagesØ Feed location of heavy reactantØ Feed location of light reactant
LK HK LLK HHKDimethyl carbonate 2 phenol 2 methanol + Diphenyl carbonate
+ ⇔
(Fukuoka et al.; 1990)
Effects of design variables Ip : LK + HK ó LLK + HHK
0 1 2 3 4 5 6 7 8 9240
260
280
300
320
340
360
380
TAC
($10
00/y
r)
NR (NS)
Nrxn=15 Nrxn=16 Nrxn=17
8 9 10 11 12 13 14 15250
260
270
280
290
300
TAC
($10
00/y
r)NFlight
NFheavy = 18 NFheavy = 17 NFheavy = 16 NFheavy = 15 NF
heavy = 14
Profiles Ip : LK + HK ó LLK + HHK
0 5 10 15 20 250.00.10.20.30.40.50.60.70.80.91.0
condenserreboiler
A B C D Ri/Rt
Ri /
Rt
mol
e fr
actio
n
Tray Number
NFBNFA
Configuration Ip : LK + HK ó LLK + HHK
12.6 mol/s
NS=4
N =4
Nrxn=16
16
11
B
12.6 mol/s A
A: 0.045 B: 0.005C: 0.95D: 0.00
12.6 mol/s
A: 0.005 B: 0.045 C: 0.00D: 0.95
12.6 mol/s
VS=30.4 mol/s
R=26.4 mol/s
Dc=0.6915 (m)TAC=254.170 (103/yr)
steam
Process Ir : LLK + HHK ó LK + HKo Ranking of relative volatilities
o Real system example:
o Optimization variables:Ø Number of top reactive stagesØ Number of bottom reactive stagesØ Number of separation stagesØ Side draw location of productsØ Feed location of 2nd columnØ Number of total stages of 2nd column
LLK HHK LK HK(Choi and Hong, 1999)
methanol lactic acid water + methyl lactate
+ ⇔
Effects of design variables Ir : LLK + HHK ó LK + HK
24 25 26 27 281575
1580
1585
1590
1595
1600
1575
1580
1585
1590
1595
1600 Nrxn,bot=1
r
TAC
($10
00/y
r)
Nsep(separation trays)
Nrxn,top=2 Nrxn,top=3 Nrxn,top=4 Nrxn,top=5
11 12 13 14 15 16 17399400401402403
1570
1580
1590
1600
1610
TAC capital cost operatinf cost
NSD(side draw location)co
st ($
1000
/yr)
1170
1180
1190
1200
1210
cost
($10
00/y
r)
Profiles Ir : LLK + HHK ó LK + HK
0 5 10 15 20 25 300.00.10.20.30.40.50.60.70.80.91.0
side draw
condenserreboiler
A B C D Ri/Rt R
i / R
t
mol
e fr
actio
n
Tray Number
NFBNFA
2nd column profile Ir : LLK + HHK ó LK + HK
0 5 10 15 20 25 30 35 40 450.00.10.20.30.40.50.60.70.80.91.0
condenserreboiler
A B C D
mol
e fr
actio
n
Tray Number
NF
Configuration Ir : LLK + HHK ó LK + HK
Process IIp : HK + HHK ó LLK + LK
o Ranking of relative volatilities
o Optimization variables:Ø Number of reactive stagesØ Number of rectifying stagesØ Feed location of heavy reactantØ Feed location of 2nd columnØ Number of total stages of 2nd column
Effects of design variables IIp : HK + HHK ó LLK + LK
3 4 5 6 7360
370
380
390
400
360
370
380
390
400
0
TAC
($10
00/y
r)
NR
Nrxn=4 Nrxn=5 Nrxn=6 Nrxn=7
0 1 2 3 4119.8
120.0
120.2
120.4363
364
365
366
TAC capital cost operating cost
NFheavy
cost
($10
00/y
r)243
244
245
246
247
cost
($10
00/y
r)
Profiles IIp : HK + HHK ó LLK + LK
0 2 4 6 8 100.00.10.20.30.40.50.60.70.80.91.0
condenserreboiler
A B C D Ri/Rt
Ri /
Rt
mol
e fr
actio
n
Tray Number
NFA&NFB
2nd column profile IIp : HK + HHK ó LLK + LK
0 5 10 15 20 25 30 35 40 45 50 550.00.10.20.30.40.50.60.70.80.91.0
condenserreboiler
A B C D
mol
e fr
actio
n
Tray Number
NF
Configuration IIp : HK + HHK ó LLK + LK
Process IIr : LLK + LK ó HK + HHK
o Ranking of relative volatilities
o Real system example:
o Optimization variables:Ø Number of reactive stagesØ Number of stripping stagesØ Feed location of light reactantØ Feed location of 2nd columnØ Number of total stages of 2nd column
LLK LK HK HHK(Chopade and Sharma, 1997)
formaldehyde ethanol ethylal + water
+ ⇔
Profiles IIr : LLK + LK ó HK + HHK
0 5 10 15 20 250.00.10.20.30.40.50.60.70.80.91.0
condenserreboiler
A B C D Ri/Rt
Ri /
Rt
mol
e fr
actio
n
Tray Number
NFA NFB
2nd column profile IIr : LLK + LK ó HK + HHK
0 5 10 15 20 25 30 35 40 45 50 55 600.00.10.20.30.40.50.60.70.80.91.0
condenserreboiler
A B C D
mol
e fr
actio
n
Tray Number
NF
Configuration IIr : LLK + LK ó HK + HHK
Process IIIp : LK + HHK ó LLK + HK
o Ranking of relative volatilities
o Real system example:
o Optimization variables:Ø Number of rectifying stagesØ Number of reactive stagesØ Number of stripping stagesØ Feed location of light reactantØ Feed location of heavy reactant
LK HHK LLK HK
methanol acetic acid methyl acetate water+ ⇔ +
Effects of design variables IIIp : LK + HHK ó LLK + HK
46 47 48 49 50 51 52162
163
164
165
166320
322
324
326
328 TAC capital cost operating cost
Nrxn
cost
($10
00/y
r)
156
158
160
162
164
166
168
cost
($10
00/y
r)
2 3 4 5 6 7 8 9 10 11 12 13320
322
324
326
328
330
332
320
322
324
326
328
330
332
321
321
NR=1 N
R=2
NR=3
TAC
($10
00/y
r)
NS
41 42 43 44 45 46 47 48 49315
330
345
360
315
330
345
360
r
TAC
($10
00/y
r)
NFlight
NFheavy= 57 NFheavy= 56 NFheavy= 55 NFheavy= 54
Profiles IIIp : LK + HHK ó LLK + HK
0 5 10 15 20 25 30 35 40 45 50 55 600.00.10.20.30.40.50.60.70.80.91.0
condenserreboiler
A B C D Ri/Rt
Ri /
Rt
mol
e fr
actio
n
Tray Number
NFBNFA
Configuration IIIp : LK + HHK ó LLK + HK
12.6 mol/s
NS=8
N =2
Nrxn=49
57
46
B
12.6 mol/s A
A: 0.045 B: 0.005C: 0.95D: 0.00
12.6 mol/s
A: 0.005 B: 0.045 C: 0.00D: 0.95
12.6 mol/s
R=27.6 mol/s
VS=31.6 mol/s
Dc=0.7024(m)TAC=320.08(103/yr)
Process IIIr : LLK + HK ó LK + HHK
o Ranking of relative volatilities
o Real system example:
o Optimization variables:Ø Feed location of light reactantØ Feed location of heavy reactantØ Number of reactive stagesØ Number of separation stages
LLK HK LK HHK
methyl acetate water methanol acetic acid+ ⇔ +
Profiles IIIr : LLK + HK ó LK + HHK
0 10 20 30 40 50 60 70 80 900.00.10.20.30.40.50.60.70.80.91.0
condenserreboiler
A B C D Ri/Rt
Ri /
Rt
mol
e fr
actio
n
Tray Number
NFBNFA
Configuration IIIr : LLK + HK ó LK + HHK
Summary (Forward reaction : A+BóC+D)
NS=4
N =4
Nrxn=16
16
11
B
A
C
D
Type Ip : LK+HK LLK+HHK Ip Ir IIp IIr IIIp IIIr
0
500
1000
1500
2000
286%
126%
431%
258%
746%
100%
TAC
($10
00/y
r)
system RD column separation column
TAC Rankings (Forward reaction : A+BóC+D)
Ip IIIp IIp IIIr IIr Ir
0
500
1000
1500
2000
286%
126%
258%
431%
746%
100%
TAC
($10
00/y
r)
system RD column separation column C D
C D
C D
C D
C D
A B
A B
A B
A B
A
C D
B
A B
lowest TAC > >
> > >highest TAC >
α αα α
α αα αα α
α αα α
α αα α
αα
αα αα
> > > >
↓ > > > > > >
> >
Heuristicso H1: It is not favorable to have the reactant as the lightest
component (light component →low temperature).o H2: Prefer the case that the relative volatilities between
two products is large (easy separation for the products).o H3: Prefer the case that the relative volatilities between
two reactants is small (high concentration for both reactants).
Conclusiono The reactive distillation configuration depends on the
ranking of boiling points between reactants and products.o For a quaternary system with second order reversible
reaction , different kinds of feasible process configuration are proposed with all possible relative volatility rankings.
o Heuristics are given to evaluate potential difficulty in the reactive distillation design
o The conclusions drawn is based on the study of the neat process (no excess of either reactants).
A B C D+ +�
ScaleScale--up a RDup a RDn Distillation:
Generally the number of trays, feed tray location, RR, BR remain constant (can be seen from MaCabe –Thiele diagram) while the column diameter is determined from the vapor load.
n Reactor: Hold the residence time constant. So the reactor size increases with feed rate.
n Recycle plant:scale-up the reactor and distillation columns accordingly.
n Reactive distillation: ? ?
VF
τ =
60
Process Studied
•Ideal reactive distillation :
A B C D+ ⇔ +
/F jE RTFj Fk a e−=
/B jE RTBj Bk a e−=
• Reaction rate :
C A B Dα α α α> > >
, , , , ,( )j i i j Fj j A j B Bj j C j DM k x x k x xνℜ = −
61
Process
XD,A=0.00915XD,B=0.00085XD,C=0.99000XD,D=0.00000
XB,A=0.00085XB,B=0.00915XB,C=0.00000XB,D=0.99000
F0A=F0B=45.36kmol/h
62
Optimized Design- Base Case
0 2 4 6 8 10 12 14 16 18 20 22 24 260.0
0.2
0.4
0.6
0.8
1.00.0 0.2 0.4 0.6 0.8 1.0
0.0
0.2
0.4
0.6
0.8
1.0
NF,heavy
x i
stage
Ri//R
XA
XB
XC
XD
NF,light
Ri/R
63
Temperature profiles
0 2 4 6 8 10 12 14 16 18 20 22 24 26
60
70
80
90
100
110
120
130
140
1500 2 4 6 8 10
0
2
4
6
8
10
NF,heavyNF,light
T(o C
)
stage
64
Optimized designs for 5 different production rates
$13,492.2$1,857.8$296.5$56.5$12.5TAC ($1000/yr)
$9,563.6$1,000.8$105.0$10.5$1.1energy
$783.3$87.9$10.6$1.1$0.1Catalyst
$10,346.9$1,088.7$115.6$11.6$1.2Operating cost
($1000/yr)
$5,607,624$1,283,775$294,580$65,955$14,912Heat exchange
$1,146,948$203,636$30,351$5,095$831Column trays
$2,681,288$819,872$217,822$63,841$18,255Column
$3,145.3$769.1$180.9$45.0$11.3Capital cost ($1000/yr)
10313108011311.31.16VS (kmol/h)
8.150.7190.06840.006590.000656Tray holdup (m3)
0.10.10.10.10.1Weir height (m)
9.212.950.9490.3000.0955Dc (m)
2930262625Total no. trays
10/2010/217/207/206/20Nrxn,bot/Nrxn,top
12/1912/198/178/177/18NFA/NFB
9/11/99/12/96/14/66/14/65/15/5NS/Nrxn/NR
12210.81.0270.09880.0101reactive holdup
(kmol)
4536453.645.364.5360.4536FA, FB (kmol/h)
Case 5Case 4Case 3Case 2Case 1
65
Following certain trends
0.01 0.1 1 10 100
0.01
0.1
1
10
100
normalize production rate
FA/FA,BC
V/VBC
DC/DC,BC
MHoldup
/MHoldupBC
TAC/TACBC
66
Implication to scale-up
12
0 768
,
,
,
.
A
ABC
BC
C
CBC
holdup
holdupBC
F PRFV PR
V
D PRDM
PRM
TAC PRTAC
=
=
=
=
=
PR=production rate
67
0 2 4 6 8 10 12 14 16 18 20 22 24 260.000.050.100.150.200.250.300.350.400.450.500.55
NFB
x A
stage
case1 case2 case3 case4 case5
NFA
0 2 4 6 8 10 12 14 16 18 20 22 24 260.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
NFBNFA
case1 case2 case3 case4 case5
x B
stage
0 2 4 6 8 10 12 14 16 18 20 22 24 260.0
0.2
0.4
0.6
0.8
1.0
NFBN
FA
x C
stage
case1 case2 case3 case4 case5
0 2 4 6 8 10 12 14 16 18 20 22 24 260.0
0.2
0.4
0.6
0.8
1.0
NFBNFA
x D
stage
case1 case2 case3 case4 case5
68
Temperature profiles for each case
0 2 4 6 8 10 12 14 16 18 20 22 24 26
60
70
80
90
100
110
120
130
140
150
NFBNFA
T(O
C)
stage
69
1
10
100
1000
10000
100000
1 2 3 4 5
Case
TAC
OptimizedDirect
Little difference in TAC
0.8%0%
0%3.85%
4.98%
70
Direct scale-up
$14298.6$1,929.4$296.5$56.5$12.6TAC ($1000/yr)
$9,839.8$1,069.5$105.0$10.5$1.1energy
$1,014.8$107.1$10.6$1.1$0.1Catalyst
$10,854.6$1,176.6$115.6$11.6$1.2Operating cost
($1000/yr)
$5,684,170$1,328,255$294,580$65,955$14,763Heat exchange
$1,042,565$182,442$30,351$5,095$855Column trays
$2,479,843$747,857$217,822$63,841$18,711Column
$3,068.9$752.9$180.9$45.0$11.4Capital cost
($1000/yr)
10615.91155.519113.4511.3451.134VS (kmol/h)
8.2860.7490.06840.006590.000646Tray holdup (m3)
0.10.10.10.10.1Weir height (m)
9.293.0170.9490.30.095Dc (m)
2626262626Total no. trays
7/207/207/207/207/20Nrxn,bot/Nrxn,top
8/178/178/178/178/17NFA/NFB
6/14/66/14/66/14/66/14/66/14/6NS/Nrxn/NR
124.28711.231.02670.09880.0097reactive holdup
(kmol)
4536453.645.364.5360.4536FA, FB (kmol/h)
Case 5Case 4Case 3Case 2Case 1
Basecase
71
Conclusions
o Investigate the scale-up of reactive distillation.o Direct scale-up is possible with little loss of economical
potentials.o Direct control structure extrapolation is workable.o Little difference between conventional distillation and
reactive distillation as far as scale-up is concerned.
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