Predicting multiple steady states in equilibrium reactive distillation. 2.Analysis of hybrid systems

Guttinger and Morari (Ind. Eng. Chem. Res. 1998, 38, 1633-1648) developed g raphical methods for the prediction of output multiplicities caused by the reactive vapor-liquid equilibrium in reactive distillation. These methods r est upon the limiting case of reactive columns of infinite length operated at infinite internal flows (infinity/infinity analysis) and are directly ap plicable to systems where the reactions take place in the entire column ("n onhybrid" columns). In this work, the reactive infinity/infinity analysis i s extended to columns with a reactive core ("hybrid" columns) by introducti on of two new procedures. First, necessary and sufficient feasibility condi tions for hybrid column profiles are derived under the assumption that each of the reactive and nonreactive column sections has infinite length. Using these conditions, an "exact" method is formulated where all possible produ cts of such an infinity/infinity hybrid column can be located in the compos ition space (by a continuation of solutions). The existence of multiple ste ady states and the feed region leading to output multiplicities can be pred icted. Second, an "approximate" procedure is proposed to obtain an estimate of the product locations of a hybrid column with finite nonreactive sectio ns. The exact method was applied to an "ideal" reactive system, and a new t ype of multiplicity, which is purely caused by the hybrid nature of the sys tem, is shown. Moreover, the methyl tert-butyl ether (MTBE) reactive distil lation process is analyzed using both methods, and the physical causes of t he MTBE multiplicities are studied in detail. All analytical results are ve rified by simulation.