Dynamic simulation of reactive distillation: An MTBE case study

Computers chem. Engng Vol.20, Suppl.,pp. S1619-SI624,1996 Copyright© 1996ElsevierScience Ltd

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S0098-1354(96)00275-X

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DYNAMIC SIMULATION OF REACTIVE DISTILLATION: AN MTBE CASE STUDY STANY SCHRANS,1 SJOERD DE WOLF Shell Research and Technology Centre Amsterdam P.O. Box 38000, 1030 BN Amsterdam AND RICHARD BAUR2 Institut ffir Systemdynamik und Regelungstechnik UniversitEt Stuttgart PfMfenwaldring 9, 70550 Suttgart Dynamic simulations of a 15 stage MTBE reactive distillation column have been performed with the SPEEDUPflowsheeter. The simulations confirm that the column configuration with fixed bottom outflow rate can display steady-state multiplicity. Changes in the column operation (feed flow rate or composition) can cause a jump from one steady state to another or trigger oscillations. The observed steady-state multiplicity remains when the reboiler temperature is fixed rather than the bottom flow rate.

1. I n t r o d u c t i o n The technology of integrating reactors with separators has made significant progress in recent years. In reactive distillation products are made by reaction and distilled in s i t u . These processes often have better conversion and selectivity, and lower capital and operating costs than the corresponding reactor-followed-by-separation technology. Several processes containing reactive distillation columns have now commercial applications, the Eastman Kodak process for the production of methyl acetate, and the catalytic distillation of MTBE being the most well-known examples. Reactive distillation processes may display a variety of dynamic phenomena. ASPENPLUS simulations have resulted in the discovery of steely-state multiplicity in an M T B E reactive distillation column (Jacobs and Krishna, 1993, Nijhuis et al., 1993, Hauan et al., 1995). Experimental evidence for steady-state multiplicity in a pilot TAME reactive distillation column of Neste Oy have been reported by Bravo et al. (1993), while sustained oscillations of process variables have been observed in a lab-scale reactive distillation column for MTBE production (Sundmacher and Hoffmann, 1995). Despite all these indications of dynamic phenomena, most of the modelling and simulations of reactive distillation processes reported in the open literature have been of the steady-state type. Rigorous dynamic simulations are needed to investigate the relevance of the steady-state multiplicity and the process sensitivity to disturbances in operational parameters. Dynamic modelling of reactive distillation processes is thus required for the development of a control scheme (see, for example, Roat et al., 1986), startup and shutdown procedures, and will moreover permit a more robust operation of the column. 2. T h e R e a c t i v e

Distillation

Process

for M T B E P r o d u c t i o n

The process studied in this paper is the reactive distillation of Methyl Tert-Butyl Ether (MTBE). MTBE is an octane booster and clean-air component in gasoline, and is reputed to be the world's fastest growing chemical. The reactive distillation technology for MTBE production has been developed in the early 80's (Smith and Huddleston, 1982). 1Author to whom correspondence should be addressed, e-mail: [email protected]. 2Work performed during a traineeship at the Koninklijke/Shell-Laboratorium, Amsterdam. c~cE

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MTBE is formed in a highly selective reversible exothermic reaction from isobutene and methanol over an acid ion-exchange resin catalyst according to the reaction isobutene + methanol ~ MTBE.

(1)

A rate equation for the reaction over the ion-exchange resin has been proposed by Rehfinger and Hoffmann (1990): ( alB rate = mcatqacidk! × \ aMeOH

1 aMTBE~

Keq a~eOH / '

(2)

where meat is the catalyst loading, qacid the catalyst ion-exchange capacity, and ai the activity coefficient of the corresponding component. The forward reaction rate constant k! and the equilibrium constant Keq have been fitted experimentally by Rehfinger and Hoffmann (1990). Typically, isobutene is one of the components of a C4-raflinate feed. The vapor-liquid equilibrium is highly non ideal (including two binary azeotropes and one ternary reactive azeotrope). No liquid phase separations are known to occur. 3. S t e a d y - s t a t e Results Using ASPENPLUS Jacobs and Krishna (1993) performed steady-state simulations of an MTBE reactive distillation column with 15 trays, 8 of which were reactive. They discovered that when the bottom flow rate is fixed certain column configurations have, besides a high isobutene conversion steady state, also a low isobutene conversion steady state. Hauan et al. (1995) confirmed these results and presented a mechanistic explanation for the existence of the two conversion branches. Even though this steady-state multiplicity in simulations is by now well established, the stability and relevance of these solutions has not yet been investigated with a dynamic simulator. Abufares and Douglas (1995) performed dynamic simulations of an MTBE reactive distillation column (with only 1 reactive tray) with the SPEEDUP flowsheeter but did not report the existence of multiple steady states. The MTBE reactive distillation column used in our simulation study is shown in Fig. 1; it is similar to the one of Jacobs and Krishna (1993) (the inerts being n-butane rather than n-butene). The column contains 2 rectification trays, 8 reactive trays, 5 stripping trays, a total condenser (with fixed reflux ratio and pressure) and a partial reboiler. In the reboiler it is possible to fix either the bottom flow rate or the temperature. The C4 feed is located on the lowest reactive tray, C8, whereas the location of the methanol feed can be varied (and distributed) over the column. The chemical reactions are kinetically limited, and the vapour and liquid are assumed to be in equilibrium. Our steady-state and dynamic simulations were performed with the SPEEDUP flowsheeter. Fig. 2 shows the isobutene conversions in the steady state for different methanol feed locations for fixed bottom flow rate and fixed reboiler temperature. The left graph of Fig. 2 shows the results when the bottom outflow rate is fixed to 674 kmol/h. When the methanol is fed on the trays C7 or C8 multiple steady states are possible; amongst them one with a high and one with a low isobutene conversion. The steadystates with the lower isohutene conversions have large negative reaction rates on the lowest reactive trays, indicating MTBE decomposition on these trays. The right graph of Fig. 2 shows the isobutene conversions for the different steady states when the reboiler temperature is fixed to 410 K. In this case, every methanol feed location results in multiple steady states.

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Figure 1: Reactive distillation column configuration in dynamic simulations. The C4 [eed is always on the lOth tray, C8. The reboiler outflow rate or temperature can be fixed with a PI-controller. Con[IB] (%)

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The behaviour of this reactive distillation column is to a large extent determined by the existence of steady states for different operating conditions. This is a complex problem since many operating variables can be changed. Fig. 3 shows some examples of phase space diagrams when only one of these variables is changed for the column configuration with fixed bottom flow. The dotted vertical line corresponds to the base case column, i.e. the column with methanol fed on C7 and bottom flow given by 674 kmol/h. These diagrams seem to be of a forbidden complexity, and suggest that a variety of dynamic phenomena are liable to occur, making the column ditficult to operate.

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Figure 3: Steady states isobutene conversions for different reflux ratios and catalyst loadings. Fig. 3 also reveals some striking differences between reactive and non reactive distillation. The isobutene conversion (on the high conversion branch) is not a monotonically increasing function of the reflux ratio. Thus, unlike for non reactive distillation, column performance is not optimized at infinite reflux. Similarly, increasing the catalyst loading (or equivalently the catalyst activity) does not necessarily lead to a better conversion. 4. D y n a m i c S i m u l a t i o n s Figs. 4 and 5 show some results of dynamic simulations of this MTBE reactive distillation column with methanol feed located at C7 when the bottom flow rate is fixed to 674 kmol/h. The left graph of Fig. 4 shows the response to a +10 % methanol feed flow pulse between t - 3 h and t -- 4 h. After the disturbance the column returns to the high isobutene conversion steady state. The right graph of Fig. 4 shows the response to a similar feed pulse of 12.5%. As a result of this disturbance, the column jumps from the high conversion operation point to the low conversion operation point. Even though the operating condition at t > 4 h is identical to the initial operating conditions, the column operates in a different steady state.

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Figure 5: Response to a disturbance in the C4-feed composition. Left, a 4 %increase in the isobutene composition, followed by a 5 % increase. Right, a 2.5 % increase in the isobutene composition ( : conversion; . . . . : temperature). of the (74 feed. First, at t = 3 h the mole fraction of isobutene in the C4 feed is increased with 4%, while the total C4 flow rate is kept constant (just as the methanol feed flow rate): the process variables start to oscillate. Next at t = 25 h the isobutene mole fraction in the C4 feed is increased a little more: the isobutene conversion now drops to a steady-state value of ca. 53 %. The right graph of Fig. 5 shows the response of the column to a 2.5 % increase in the isobutene composition of the (74 feed. This change in operating condition results in damped oscillations of the process variables. 5. Conclusions Dynamic simulations of an MTBE reactive distillation column have been performed using the SPEEDUP flowsheeter. Fixing the bottom outflow rate or fixing the reboiler temperature leads to multiple steady states. Dynamic simulations have shown that this reactive distillation column is 'unstable' for certain disturbances of the feed. This work illustrates that, in this case, dynamic simulations should be taken into account during the first steps of the design process in order to avoid dynamic surprises. This work also illustrates the power of dynamic simulations compared to steady-state simulations. 6. List of Symbols

aIB aMeOH aMTBE Keq

kl mcat qacid

Activity coefficient of isobutene Activity coefficient of methanol Activity coefficient of MTBE Equilibrium constant Forward reaction rate constant Catalyst loading of a tray Ion-exchange capacity of the catalyst

1/s kg eq(H+)/kg

7. References A.A. Abufares and P.L. Douglas, 1995, Mathematical modelling and simulation of an M T B E catalytic distillation process using SPEEDUP and ASPENPLUS, Trans IChem., 72, 3.

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J.L. Bravo, A. Pyh~ilahti, and H. J/irvelin, 1993, Investigations in a catalytic distillation pilot plant: vapor/liquid equilibrium, kinetics, and mass-transfer issues, Ind. Engng. Chem. Res. 32, 2220. S. Hauan, T. Hertzberg, and K.M. Lien, 1995, Multiplicity in reactive distillation of MTBE, presented at ESCAPE-5, Comp. Chem. Engng., 19, S-327. R. Jacobs and R. Krishna, 1993, Multiple solutions in reactive distillation for methyl tertbutyl ether synthesis, Ind. Engng. Chem. Res., 32, 1706. S.A. Nijhuis, F.P.J.M. Kerkhof, and A.N.S. Mak, 1993, Multiple steady states during reactive distillation of methyl tert-butyl ether, Ind. Engng. Chem. Res., 32, 2767. A. Rehfinger and U. Hoffmann, 1990, Kinetics of methyl tertiary butyl ether liquid phase synthesis catalyzed by ion exchange resin-I Intrinsic rate expression in liquid phase activities, Chem. Engng. Sci., 45, 1605. S.D. Roar, J.J. Downs, E.F. Vogel, and J.E. Doss, 1986, The integration of rigorous dynamic modeling and control system synthesis for distillation columns: an industrial approach, in Chemical Process Control-CPC III, M. Morari and T.J. McAvoy (Eds), Elsevier, NY. L.A. Smith and M.N. Huddleston, 1982, New MTBE design now commercial, Hydrocarbon Processing, March 1982, 121. K. Sundmacher and U. Hoffmann, 1995, Oscillatory vapor-liquM transport phenomena in a

packed reactive distillation column for fuel ether production Chem. Engng. J., 57, 219.