Conceptual Design of Reactive Distillation

S. T. Tung; X. G. Shen; Y. C. Cheng; C. C. Yu

Dept of Chem. Eng.National Taiwan University

ccyu@ntu.edu.tw02-3365-1759

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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