Modelling reactive distillation

5214 R. Taylor, R. Krishna / Chemical Engineering Science 55 (2000) 5183}5229Fig. 26. Details of #ows in and out of multiple cells used to model the hydrodynamics of trayed columns.Fig. 27. The non-equilibrium stage (cell) for heterogeneous catalysedreaction.Fig. 20(a)) was simulated under three di!erent sets ofassumptions:A A pseudo-homogeneous approach, in which the catalystis neglected and the reaction rate is evaluated atliquid bulk conditions.B A case in which the Knudsen di!usion coe$cient islarge, in which case the mass transfer interaction withthe catalyst is ignoredC A case one in which the Knudsen di!usion coe$cientis a factor "ve times smaller than the smallest of theMaxwell}Stefan di!usion coe$cients. The value of theKnudsen di!usion coe$cient is assumed to be thesame for all components.Shown in Fig. 29 is the steady-state conversion of isobuteneas a function of the bottom product #owrate. Themultiple steady-state region has disappeared for thedusty #uid model, and the Knudsen di!usion term hasa substantial in#uence on the results of the dusty #uidmodel simulations. This may be explained by the factthat, in the lower branch of the multiple steady-stateregion, MTBE is consumed in part of the column. Therate of consumption of MTBE is very much in#uenced byadditional mass transfer resistances. Additional masstransfer resistances, therefore result in a higher conversion.This is not the case for the TAME process where thereaction rate is very much lower than it is for MTBE andmass transfer in#uences are not signi"cant. Multiplesteady-states have been found experimentally for an RDcolumn used to produce TAME, but not in the MTBEprocess (Rapmund et al., 1998), appearing to con"rm theresults shown in Fig. 29.5.6. Dynamic NEQ modelsKreul, Gorak, Dittrich and Barton (1998) described anNEQ model for RD in packed columns. Theirs is a dynamicmodel that, as is usual in such models, neglects thehold-up in the vapor-phase. Their paper addresses theproblems of modelling mass transfer and reaction ina vapor}liquid}porous catalyst system. In addition, they