Modelling reactive distillation

R. Taylor, R. Krishna / Chemical Engineering Science 55 (2000) 5183}5229 5211and/or rate-based models. Two of the three examplesthat illustrate their paper concern RD (ethyl acetateproduction and an MTBE column). The models arecompared to experimental data of Suzuki et al. (1971).The agreement between the pro"les obtained with therate-based model and the data is very encouraging.The authors do, however, demonstrate a sensitivity of thecomputed pro"les to the activity coe$cient model used.A particularly novel feature of their paper is the introductionof Gibbs energy pro"les in the column.In an interesting experimental study Sundmacher andHo!mann (1995) obtained oscillatory behaviour ofa packed laboratory-scale distillation column. The oscillationswere encountered in the MTBE process as well asin the distillation of non-reactive binary mixtures. Itappears likely that these oscillations originate from thehydrodynamics in their packed RD column. The authorsuse an NEQ model for non-reactive distillation thatcompletely neglects mass transfer in the liquid-phase and,by extension, any e!ects due to reaction. The modelequations were solved by a relaxation method, the resultsused to demonstrate multiple steady-states. The NEQmodel does not provide an explanation for the observedoscillations.Sundmacher and Ho!mann (1996) presented a detailedNEQ stage model for vapor}liquid}porous catalystsystems. The model di!ers from that described aboveonly in that the phases are assumed to be in thermalequilibrium with each other. This means that sensibleheat transfer between phases is ignored and that theoverall energy balance replaces the individual phase balances.Mass transfer in the vapor and liquid-phases ismodelled using an explicit approximate solution of theMaxwell}Stefan equations (see Chapter 8 of Taylor andKrishna (1993) for details). The system of equations issolved via a pseudo-relaxation method using the DAEsolver LIMEX cited earlier. The model is used to simulatesome data obtained on a laboratory-scale MTBEcolumn. One of the conclusions from this study is that itis absolutely necessary to account for di!usion and reactionin the porous catalyst (modelled in this paper usingthe catalyst e!ectiveness factor approach developed inother papers from these authors and discussed above).The in#uence of side reactions on RD processes hasbeen investigated by Schoenmakers (1982), Sundmacher,Ho!mann and co-workers (Sundmacher, Uhde &Ho!mann, 1999; Oost, Sundmacher & Ho!mann, 1995;Oost & Ho!mann, 1996). Sundmacher, Uhde and Hoffmann(1999) used both EQ stage (with Murphree e$ciency)and NEQ models to simulate the MTBE andTAME processes. The reactions were handled using bothquasi-homogeneous and heterogeneous methods. Simulationresults were compared to experimental data obtainedin two laboratory-scale columns. A detailed NEQmodel was needed to describe the TAME process, butboth NEQ and the EQ stage (with an e$ciency of 0.8)model could adequately represent the MTBE process. Inthe latter case it was necessary to account for the catalyste!ectiveness along the packing. The authors concludethat it is necessary to account for the side reactions in RDprocess design.Bart and LandschuK tzer (1996) studied the esteri"cationreaction of acetic acid and propanol to propyl acetate,catalysed by an exchange resin in a packed column. Masstransfer across the vapor}liquid interface and betweenliquid bulk and catalyst surface is described with theMaxwell}Stefan equations. The reaction is assumed totake place only at the catalyst surface and is modelledwith an Eley}Rideal kinetic expression. Their model accountsfor axial dispersion along the column, the dispersioncoe$cient being obtained from a Bodensteinrelation that was determined experimentally for theirmodel column.Yu, Zhou and Tan (1997) developed a steady-stateNEQ model that takes into account mass transfer fromthe bulk liquid to the catalyst. Separate rate equationsare written for the vapor}liquid and liquid}solid interfaces.Empirical methods are used for estimating theliquid}solid (and other) phase mass transfer coe$cients.Di!usion within the catalyst phase is ignored. The systemof non-linear equations is solved by a Newton homotopymethod. The solitary numerical example that accompaniesthe paper concerns the reactive stripping processfor the production of Bisphenol-A.Ng, Rempel and their co-workers at the University ofWaterloo (Huang, Podrebarac, Ng & Rempel, 1998a;Huang, Yang, Ng & Rempel, 1998b; Podrebarac et al.,1998a,b) have carried out an in-depth study of the aldolcondensation of acetone to diacetone alcohol (DAA) andmesityl oxide. Experiments were carried out in a pilotscalecolumn with 6 mm Intalox saddles and a reactivesection containing bale-type packing. It was found thatthe DAA production rate was controlled by mass transfer.DAA selectivity was in#uenced by the liquid distribution.Correlations for the solid}liquid mass transfercoe$cients and for the overall mass transfer coe$cientsfor the vapor}liquid transport were developed by Huanget al. (1998a). Their papers also include the developmentof an NEQ model. The model appears to be derived fromthe Krishnamurthy}Taylor model for conventional distillation,although it is interesting to note that the materialbalances are expressed in terms of the mass #ows andmass fractions and include a separate term for the rate ofvaporisation caused by the heat of reaction. Mass transferbetween vapor and liquid-phases is described bya single overall mass transfer coe$cient for each species,a correlation for which was derived for their column.They assume that the rate of reaction is equal to the rateof mass transfer to the catalyst surface and correlationsfor the liquid}solid mass transfer coe$cients were alsodeveloped. Mass transfer in the non-reactive sections wasmodelled using a "lm model for each phase; mass transfer