Comparison of equilibrium stage and nonequilibrium stage models for reactive distillation

Abstract

For modelling reactive distillation columns, two distinctly different approaches are available in the literature: (1) the equilibrium (EQ) stage model, in which the vapour and liquid phases are assumed to be in thermodynamic equilibrium, and (2) the nonequilibrium (NEQ) stage model in which the finite mass transfer rates across the vapour–liquid interface are accounted for. In this paper, these two approaches are compared using two case studies: (a) synthesis of MTBE and (b) hydration of ethylene oxide to ethylene glycol. It is shown that while the phenomena of multiple steady states is exhibited by both modelling approaches, the “window” in which these multiplicities occur is significantly reduced in the NEQ model. It is also shown that in actual column design, some of the steady states calculated by the EQ model cannot be realised due to e.g. flooding or weeping limitations on distillation trays. Another important conclusion that can be drawn from this work is that the hardware design can have a significant influence on the conversion and selectivity. It is concluded that for design of reactive distillation columns we must adopt the NEQ modelling approach.