Dynamic behaviour of reactive distillation columns described by a nonequilibrium stage model

Abstract

In this paper we develop a generic, dynamic, nonequilibrium (NEQ) stage model for reactive distillation (RD) columns. The main features of our model are: (1) use of the Maxwell–Stefan equations for describing mass transfer between fluid phases, (2) solid catalysed reactions are treated using a pseudo-homogeneous liquid phase model with the appropriate pseudo-homogeneous kinetic expressions and (3) a comprehensive set of design correlations, for both trays (sieve, bubble caps) and packings (random dumped, structured), for hold-up and mass transfer have been incorporated into the software package following earlier work (Taylor et al., Comput. Chem. Eng. 18 (1994) 205). We report some interesting dynamic features of RD columns by examining the response of a column for methyl tert-butyl ether (MTBE) synthesis to perturbations in the feed of methanol, iso-butene or n-butene. When operating at a high-conversion branch of the bifurcation diagram, small perturbations are shown to lead to a transition to a low-conversion branch. The NEQ model is shown to be more susceptible to feed perturbations than a conventional equilibrium (EQ) model with constant component efficiencies. The differences between the dynamic behaviour of trayed and random-packed columns are also emphasised in this work. The dynamic behaviour of the column has been interpreted on the basis of the iso-butene recycle flow in the column. It has been shown that the size, type and duration of a feed perturbation causing a transition from one to another steady state depends on the model description, on the column configuration and on the residence time distribution along the column height. Experimental verification of the developed dynamic model is obtained by comparison with the experimental results of Mohl et al. ( Chem. Eng. Sci. 54 (1999) 1029) for synthesis of tertiary amyl ether (TAME).