Multiple steady states in heterogeneous azeotropic distillation sequences

Abstract:
In this paper multiplicity in heterogeneous azeotropic distillation sequences is studied. Two sequences suitable for ethanol dehydration are treated as sample problems and compared. As a basis the -analysis method (Petlyuk and Avet'yan, 1971; Bekiaris et. al.; 1993), which assumes infinite reflux and infinite number of trays, is extended to and applied on heterogeneous azeotropic distillation sequences with the objective to determine steady state bifurcation diagrams from thermodynamic considerations. The -predictions are very different for the two sequences despite their similar structures. In particular, it is predicted that output multiplicity of the azeotropic column, as recently experimentally verified by Müller and Marquardt (1997), can induce output multiplicity of the entire sequence. It is further predicted that output multiplicity can be avoided by the choice of a different sequence structure. Furthermore, -analysis predicts state multiplicities in heterogeneous azeotropic distillation sequences. This phenomenon is also discussed and it is shown that this sequence behaviour in the -case can either be induced by a corresponding single column behavior or by the closing of the sequence. Finally, the implications on column sequences operating at finite reflux and with finite number of stages are investigated by rigorous simulation.

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