Modelling reactive distillation

R. Taylor, R. Krishna / Chemical Engineering Science 55 (2000) 5183}5229 5215Fig. 28. In#uence of tray hardware and number of mixing cells on the formation of by-product DEG in the hydration of ethylene oxide to ethyleneglycol (EG). The column con"guration (diameter"1.7 m) is that shown in Fig. 24(a). Details of the simulations are available in Baur, Higler, Taylorand Krishna (1999).Fig. 29. Comparison of the results for conversion of iso-butene obtainedby three di!erent model formulations for taking account ofchemical reactions: (A) pseudo-homogeneous, (B) dusty #uid modelwithout Knudsen contribution, and (C) dusty #uid model with Knudsencontribution. Adapted from Higler, Krishna and Taylor (2000).describe a two-phase model in which di!usion and reactionto and within the solid catalyst is not accounted forby additional equations (and parameters), but their effectsare lumped into a kinetic term that appears in theliquid-phase material balances. The choice of which ofthese two approaches was adopted for their simulationsis not completely clear. The material balances (partialdi!erential equations) are discretised in the spatial dimension(the column height) using the method of lines.The model equations were solved using the equationbasedmodelling environment ABACUS (Allgor, Berrera,Barton & Evans, 1996). The paper by Kreul et al. includesa limited comparison with some unsteady-statedata for a methyl acetate column. The results shown inthe paper suggest that their dynamic model is well able todescribe the product composition transients. The actualexperimental data are not provided in the paper.Kreul, Gorak and Barton (1999b) described an NEQmodel to study batch distillation. In essence, the model isan extension and generalisation of the dynamic NEQmodels of Kooijman and Taylor (1995) for tray columnsand Pelkonen, Gorak, Kooijman and Taylor (1997) forpacked columns. The MS equations were used to modelvapor and liquid-phase mass transfer. The model systemwas implemented and solved using the ABACUS system.The paper includes extensive comparisons of true modelpredictions of experiments performed in a pilot plantcolumn involving the system methanol}acetic acid}methyl acetate}water. Under the conditions in the experimentsthere was essentially no liquid-phase chemicalreaction; vapor-phase dimerisation was, however, takeninto account. The agreement between the predicted andmeasured compositions is exceptionally good.Strictly speaking, reactive absorption falls outside thescope of this review. However, NEQ models as outlinedabove apply more or less unchanged in principle toreactive absorption and RD alike. The only di!erencesbetween the models are the inclusion of di!erent submodelsfor the reaction kinetics and thermodynamicproperties. A nitric acid recovery plant modelled byWiesner, Wittig and Gorak (1996) and by Kenig andGorak (1997) involves nine di!erent reactions and simultaneoustransport and reaction of 10 di!erent species.Schneider, Kenig and Gorak (1999) used the Maxwell}Stefanapproach to model the dynamics of reactiveabsorption processes. Of particular interest in theirmodel is the inclusion of the gradient of the electricalpotential in the Maxwell}Stefan equations and theelectro-neutrality equation that must be satis"ed at allpoints in the liquid due to the presence of ions in theprocesses they model. Rascol, Meyer, Le Lann and