Multiple steady states in heterogeneous azeotropic distillation sequences

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 infinity/infinity analysis method (Petlyuk, F. B.; Avet'yan, V. S. Theor. Found. Chem. Eng. 1971, 5, 499-507; Bekiaris, N.; Meski, G. A.; Morari, M. Ind. Eng. Chem. Res. 1993, 32, 2023-2038), which assumes infinite reflux rate and infinite number of trays, is extended to and applied on heterogeneous azeotropic distillation Sequences in order to determine steady-state bifurcation diagrams from thermodynamic considerations. The bifurcation diagrams are very different far the two sequences despite their similar structures. In particular, it is predicted that output multiplicity of the single azeotropic column, as recently experimentally verified by Muller and Marquardt (Ind. Eng. Chem. Res. 1997a, 36, 5410-5418), can induce output multiplicity of one sequence. It is further predicted that output multiplicity can be avoided by the choice of a different sequence structure. Furthermore, infinity/infinity analysis predicts state multiplicity in both heterogeneous azeotropic distillation sequences. This phenomenon in the infinity/infinity case is also analyzed and it is shown that state multiplicity in heterogeneous distillation sequences can be induced either by the corresponding single column behavior as reported by Gani and Jorgensen (Comput. Chem. Eng. 1994, 18, 55) or by closing the sequence. The predicted bifurcation diagrams and multiplicities are substantiated through rigorous simulation of column sequences operating at finite reflux and with a finite number of stages. Finally, the implications of the thermodynamics including the liquid-liquid plait point position are demonstrated to be important for obtaining reliable predictions.