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, 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 for 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 Müller 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, ∞/∞ analysis predicts state multiplicity in both heterogeneous azeotropic distillation sequences. This phenomenon in the ∞/∞ 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 Jørgensen (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.

Operation of Homogeneous Azeotropic Distillation Column Sequences

“…(These kind of multiplicities were also observed for a heterogeneous sequence. 45 ) 3.3. Sequence C: Three Columns with Two Recycles.…”

“…These sets are illustrated in Figure a. (These kind of multiplicities were also observed for a heterogeneous sequence …”

Analysis and Multiple Steady States of an Industrial Heterogeneous Azeotropic Distillation

“…This phenomenon is not only a mathematical feature as it has been experimentally demonstrated. [6][7][8][9] MSS are usually classified in the three following categories: 10,11 (1) input multiplicity occurs when multiple input values exist for a given set of outputs, (2) output multiplicity occurs when multiple output values exist for a given set of inputs, and (3) state multiplicity occurs when multiple output values of internal states exist for a given set of inputs and outputs.…”

∞/∞-Analysis of heterogeneous distillation processes

“…Therefore, it has been systematically used for studying homogeneous and heterogeneous azeotropic distillation (Bekiaris et al, 1993;Müller and Marquardt, 1997;Müller, 1998). Further, it has successfully been applied to processes with more than one unit operation in a flow sheet (see e.g., Laroche et al, 1992;Esbjerg et al, 1998;Güttinger and Morari, 1996;Blagov et al, 2000;Nazanskii et al, 2001;Ulrich, 2002;Ulrich and Morari, 2003;Bonet et al, 2007). The stream table of a process resulting from ∞/∞ -analysis is, despite its inherent strong assumptions, often a good approximation of the stream table obtained from corresponding simulations based on the equilibrium stage model (Ryll et al, 2006;Grützner et al, 2007;Burger and Hasse, 2013).…”