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Catalytic Distillation Modelling and Simulation using HYSYS.ProcessEnvironment

Gheorghe BUMBAC1, Grigore BOZGA1, Valentin PLESU1, Vasile BOLOGA1, Ilie. MUJA2 andCorneliu Dan POPESCU2

1University “POLITEHNICA” of Bucharest, Department of Chemical Engineering, 1 PolizuStreet, RO-78126, Bucharest, Romania, Tel/Fax:+40 (0)21 21.25.125, email: cttip@chim.upb.ro2S.N.P. PETROM, INCERP Ploiesti Subsidiary, B-dul Republicii Nr. 291 A, RO-2000, Jud.Prahova, Telefon +40 (0)244 135111, Fax +40 (0)244 198732, email: popescu@serv.incerp.ro

The catalytic-distillation process for the production of t-amyl-methyl-ether (TAME) frommethanol and isoamylenes was simulated by developing the process model as acombination of unit operations from HYSYS operations palette. Geometricalcharacteristics of catalytic-distillation column are those of an industrial pilot plant and theresults of simulation were compared with experimental data. The experimentallydetermined reactions kinetics was applied in the model. UNIQUAC-UNIFAC modelequations were selected for the vapour-liquid equilibrium.The results show that fair agreements between the calculated and experimental data wereobtained.

1. INTRODUCTIONMethyl ethers replace lead compounds in gasoline. One of these ethers obtained by the

etherification of an isoamylenes mixture (2-methyl-1-butene, 2M1B, and 2-methyl-2-butene)with methanol is t-amyl-methyl-ether (TAME).

Catalytic distillation is a suitable technique for TAME synthesis due to thereversibility of the etherification reactions and the difference between the reactants andproducts volatilities. These particularities favours the enhancement of the reactant conversionand the increase of the interphase mass transfer potential.

Despite of the important number of publications in the field of catalytic distillation,relatively few of them concern the industrial application of TAME synthesis.

In this paper we focused on the relevance of the commercial software HYSYS forthe simulation of catalytic distillation problems. To predict the behaviour of TAME synthesisreactor and reactive distillation column HYSYS.Process environment was used. Thesimulation results were compared with pilot plant experimental data. The pilot plant systemconsists mainly from a tubular, fixed bed pre-reactor (TFBR) and a reactive distillation (RD)packed column.

The purpose of this study is: a) to develop a suitable simulation module forheterogeneous RD with HYSYS and b) to apply the model to an industrial applications.

The TFBR is used to bring the reaction mixture near its equilibrium composition. Theadvantage of using a pre-reactor for TAME synthesis is based on the fact that the greatest partof reaction components can react before RD column and the throughput of reaction systemincreases.

HYSYS provides many built-in modules for simulating various processes.Unfortunately the COLUMN subflowsheet environment allows simulation of RD withreactions taking places only in homogeneous phase. In the heterogeneous catalytic distillationprocess the solid catalyst particles are placed into many special packing envelopes, servingalso as vapour-liquid contacting. The reactions occur inside the catalytic package where theliquid contacts the catalyst particles. Then the product flows out of the catalytic zone.Additional separation takes place on the packing placed below and above the reaction sectionof the column.

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In a RD column the reaction and separation actually take place in the differentlocations of the column i.e. reaction on the catalyst pellets of the packaging and separation inthe inert packing. Therefore, the parameters of the liquid residence time or the liquid hold-upon the trays or packings in the heterogeneous process can only be used in the separationcalculation whereas the reaction calculation needs the parameters of contacting time of liquidwith catalyst in the catalytic package. We underline that in the current version of HYSYSthe built-in RD module is not directly suitable for the simulation of the heterogeneouscatalytic distillation process.

To overcome the above problem our study concentrated to develop a model forheterogeneous RD and implement the model in the HYSYS simulation environment. In thismodel, the catalyst space velocity appearing in the reaction system equations represents thecontacting time of liquid with catalyst.

2. Reaction kinetics and thermodynamicsIndustrial processes for TAME synthesis are based on the reversible reactions of

isoamylenes (2M1B and 2M2B) with methanol. The equilibrium conversion of isoamylenesto TAME, at 60°C, is 56% if stoechiometric amounts of isoamylenes and methanol are used 6

and increases slightly with the increase of methanol/isoamylenes ratio. In Table 1 theequilibrium conversion as a function of temperature, for stoechiometric methanol/ isoamylenesmolar ratio, is presented 6.

A typical industrial process for TAME synthesis involves at least 8 components:isoamylenes, n-pentane, i-pentane, methanol, 1-pentene and trans-2-pentene. Since methanolassociates almost all hydrocarbon components into simple and complex azeotropic pairs, thesystem shows strong non-ideal properties.

The property package used to calculate the liquid activities of the consideredcomponents is based on UNIQUAC-UNIFAC model.

Table 1. Equilibrium isoamylenes conversion as a function of temperature

Temperature(oC)

Conversion

50 0.61860 0.56070 0.49180 0.446

The two reactive olefins (2M1B and 2M2B) are contained in the hydrocarbon mixture,resulted as a C5 fraction from Fluid Catalytic Cracking (FCC) unit. In Table 2 thecomposition of the feeding mixture used in the simulated process scheme is presented.Residual TAME is present in this stream from recycled stream. All components from the C5fraction are producing azeotropes with methanol and the composition of these azeotropes ispresented in Table 3. The methanol concentration in these azeotropes increases with pressure.

Table 2. The feed composition.

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Table 3. Azeotrope compositions in TAME synthesis.p=2.5 bar p=4 bar p=5.5 bar

Component 1 Component 2 x1 t, oC x1 t, oC x1 t, oCmethanol 2M1B 0.21 53.76 0.243 69.24 0.268 80.07methanol 2M2B 0.28 58.69 0.31 73.78 0.331 84.69methanol n-pentane 0.295 58.64 0.328 73.96 0.347 85.20methanol i-pentane 0.21 51.22 0.252 66.61 0.280 77.85methanol 1-pentene 0.22 53.64 0.267 69.06 0.283 80.75methanol 2-pentene 0.265 56.73 0.301 72.29 0.322 82.72methanol TAME 0.763 87.56 0.793 102.45 0.802 113.20

The synthesis of TAME from methanol and isoamylenes, catalysed by acid ion-exchangeresin catalyst is a reversible process as shown in following reaction mechanism containing themain and secondary reactions (1÷7):

)1()l(3CH2CH

TAME3CH

|

3CHO|C3CH

)B1M2(Butene1Methyl2

)l(OH3CH)l(3CH2CH

3CH|C2CH −−

−

−−+−−=

)2()l(3CH2CH

TAME3CH

|

3CHO|C3CH

)B2M2(Butene2Methyl2

)l(OH3CH)l(3CHCH

3CH|C3CH −−

−

−−+−=−

B2M2B1M2

)3()l(3CHCH

3CH|C3CH

)l(3CH2CH

3CH|C2CH −=−−−=

amyleneisodi)l(20H10C

B2M2)l(10H5C

B1M2)l(10H5C

−−→+ (4)

amyleneisodi)l(20H10C

B2M2)l(10H5C

B2M2)l(10H5C

−−→+ (5)

ethermethyldi

)6()l(O2H)l(3CHO3CH)l(

OH3CH2

−−

+−−−

alcoholamyltertamylenesiso

)7()l(O12H5CO2H)l(10H5C

−−−

−+

There are three main reactions (reactions 1÷3, one for etherification of 2M1B, one foretherification of 2M2B and an isomerisation reaction between 2M1B and 2M2B) and foursecondary reactions (4÷7). Both etherification reactions, (1) and (2), are exothermic, i.e. theequilibrium conversion of TAME decreases with temperature. The isomerisation reaction atoperation temperature (between 60°C and 120°C) favours the 2M2B formation and thiscomponent will have the greatest concentration in the reaction mixture. From kinetic point of

r1

r2

r3

r4

r5

r6

4

view this situation is not advantageous because a faster reaction (1) is replaced by a slowerone (2).

High temperatures and low methanol concentrations are favourable conditions forisoamylenes’ oligomers formation. On the other hand, excess methanol produces higherdimethyl-ether concentration, whereas t-amyl-alcohol formation is very limited, inequilibrium conditions, due to a very small water concentration. Frequently used catalysts aresulphonic acid ion-exchange resins (Amberlyst 15 or 35, Levatit SPC 118). The kineticmechanism is based on the consideration that methanol and TAME are stronger adsorbed onthe catalyst’s surface, compared to isoamylenes.

According to our knowledge kinetic studies for TAME synthesis were published byMuja et. al 7, Randriamahefa et al 8 , Piccoli and Lovisi , Oost and Hoffmann 3 and Rihko etal. 2 etc.

In this work the TAME synthesis kinetic model of Rihko et al.3 on Amberlyst 16 wasused. The 2M1B and 2M2B consumption rates have the expressions:

⋅−⋅⋅−

+⋅

⋅

⋅−⋅⋅⋅−

=B1M2

B2M2

3B1M25B

MTM

T

B1M2M

T

1B1M2M1B

B1 aa

K1

1ak

aaKK

aaa

K1

1aak

r (8)

⋅−⋅⋅+

+⋅

⋅

⋅−⋅⋅⋅−=

B1M2

B2M2

3B1M25B

MTM

T

B2M2M

T

2B2M2M3B

B2 aa

K11ak

aaKK

aaa

K1

1aak

r (9)

The activation energies for the reactions (1), (2) and (3) are:

and

hgmol

107.0)K343(k;hg

mol125.0)K343(k;

hgmol

286.0)K343(k

mol/kJ6.81E;mol/kJ1.94E;mol/kJ76E

5B3B1B

5kB3kB1kB

⋅=

⋅=

⋅=

===

are reaction rates constants at 343 K.

The equilibrium constants in the above reaction rates as temperature functions are:( )T/2.40413881.8expK1 +−= (10)( )T/3.32252473.8expK 2 +−= (11)

( )T/3.8331880.0expK 3 +−= (12)

( )334T00061.01405.0KK

M

T −⋅−= (13)

The ion exchange capacity Amberlyst 15 is 5 mequiv/g 8,9. The reaction kinetic datahave been verified using pilot plant synthesis of TAME in which the fixed bed reactorpacking of the same catalyst was used. Molar ratio between methanol and isoamylenes in thefeed stream was 1.256.

3. Process flowsheetIn figure 1 TAME synthesis process flowsheet using catalytic distillation is presented.

The C5 fraction is mixed with methanol and the resulting stream is fed to the preliminary

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reactor (IV). In the Table 4 the composition of the mixture at the preliminary reactor exit ispresented.

The resulting product is mixed with a recycled methanol stream and is fed to acatalytic distillation column, with three zones, below the reaction zone. The stripping zone ofthe catalytic-distillation column is simulated as reboiled absorber, a standard operation inHYSYS.

The second part is the reaction separation zone, represented in our model by abackflow cell model (BCM) with forward flow of the liquid and backward flow of the vapourin the reactive part of RD zone. The BCM consist of series of five continuous stirred tankreactors (CSTR) units with the same geometry and size of the individual unit. The third part isanother pure mass transfer unit, representing the rectifying zone of the reactive distillation.This zone is simulated as refluxed absorber a HYSYS standard operation. Both strippingand the rectifying zones are represented as non-catalytic packed columns.

From the computational point of view, each cell of the series was assumed to be at V-L equilibrium, the increase of conversion being calculated as in a CSTR reactor. Herereactions take place in liquid-solid interface, following the kinetic law mentioned above.

System characteristics: The main characteristics of the catalytic distillation columnare: pre-reactor volume: 0.12 m3, stripping zone: 6 theoretical stages, rectifying zone: 3theoretical stages and in cell model there are five CSTRs, a CSTR vessel has 0.02 m3 ofcatalyst. Catalyst particles average diameters considered in the simulation were of 1 mm as inthe pilot plant case. These characteristics are in agreement with those of the pilot plant. Moreexplicit characteristics for stripping and rectifying zones of the RD column are presented inTables 6 and 7.

Table 6. Stripping zone characteristics Table 7. Rectifying zone characteristics

Table 4. Feed conditions for the RDcolumn system

Table 5. Column heat exchangers

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4. Results and discussion:Several results from the simulation of main streams are presented in Table 8. The

isoamylenes conversion in the reactive distillation column and pre-reactor is 80.76 %. Pilotplant scheme is presented in Figure 2. The characteristics of the experimental pilot plant are:pre-reactor volume 120 l; rectifying zone packing height 2 m; stripping zone packing height3.5 m. Feeding condition and thermodynamic regime was the same as in the simulation.

The simulation results with HYSYS for the TAME synthesis reactive distillationmodule set-up, presented in this work, allow drawing the following conclusions:

- From the chemical transformation point of view it is profitable to place the reactionzone as close as possible to the top of the column. However, above the reaction zonea separation zone is needed to separate TAME from the distillate.

- It is recommended to place the column feed bellow the reaction zone in order toensure high concentration for the reactants in this zone (as there are more volatilecompared with the reaction product).

- The best structure for the RD column, obtained from this simulation study, involve 15theoretical plates. As we denoted plates from top to bottom, the best position for thereaction zone are the theoretical plates 3 and 4, and the feed plate is the 5-th plate.The optimal reflux ratio is 2, as result of the trade-off between separation degree andenergy saving.

To describe the flow and fluid phase mixing in the reaction zone, a classical, multi-cellular, model was used, considering the back-flow of the vapour phase. In each cell theconversion increase was calculated considering uniform distribution of the catalyst andvapour-liquid equilibrium.

The results obtained (over 90% conversion, much bigger than the equilibriumconversion) emphasises the advantage of catalytic distillation compared with the classicalscheme, because the chemical transformation is not limited by the chemical equilibrium asresult of continuous separation of the reaction product from the mixture.

Table 8. Simulation results.

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II

I

III

IV

a) process flowsheet

L1 V0

L3

LN

V1

VN-2

VN-1

VN

V3

V2

L4

LN-1

L2

LN+1

n=1

n=2

n=3

n=N-1

n=N

b) cells in series model

Figure 1. Simplified flowsheet for RD column.

Figure 2. TAME Pilot Plant with Catalytic Distillation Column.

Iso-amylenes conversion data (Table 9) are obtained using the pilot plant presented in fig. 2,under the same experimental conditions.

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Table 9. Experimental results for TAME synthesis.

5. Conclusions

This paper presents a theoretical study for the modelling of reactive distillationcolumn operation in TAME synthesis. The simulation procedure is based on a mathematicalmodel considering chemical reaction kinetics for the main reactions and the vapour-liquidequilibrium. Phase contact in the reaction zone is described with the back-flow cell model.The problem statement in HYSYS.Process environment was made considering three zones forthe catalytic distillation column (rectifying, reaction and stripping). Constructive andoperational characteristics of the column are specified as a consequence of the parametricstudy: reaction zone position, feed position and reflux ratio, in order to obtain maximum yieldfor the transformation of C5 reactive olefins in the pilot plant. The simulation results are ingood agreement with experimental data obtained in the experimental pilot plant at SNPPETROM, INCERP Ploiesti subsidiary.

The quality of the results obtained in this paper is limited by the uncertaintyintroduced by the phase hydrodynamics in the reaction zone the phase equilibrium hypothesis.The authors foreseen additional studies in order to better describe phase hydrodynamics, toconsider interphase mass transfer inside the catalyst pallets on process performances.

Nomenclature:ai – activity of component i;Kj – equilibrium constants in TAME synthesis

reactions;kBm – rate constants in reactions (8), (9);

ri – reaction rate;p – pressure;T – absolute temperature, K;R – ideal gas constant.

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