Bekiaris et al. (1993) explained the existence of multiple steady states in homogeneous ternary azeotropic distillation, on the basis of the analysis of the case of infinite reflux and infinite column length (infinite number of trays). They showed that the predictions of multiple steady states for such infinite columns have relevant implications for columns of finite length operated at finite reflux. In this article, experiments are described for the ternary homogeneous system methanol−methyl butyrate−toluene which demonstrate the existence of multiple steady states (output multiplicities) caused by the vapor−liquid−equilibrium. The experiments on an industrial pilot column show two stable steady states for the same feed flow rate and composition and the same set of operating parameters. The measurements are in excellent agreement with the predictions obtained for infinite columns using the ∞/∞ analysis tools as well as with stage-by-stage simulation results. These experiments represent the first published study reporting evidence for the predictions and simulations by various researchers showing that type of output multiplicities in distillation.

Bekiaris et al. (Ind. Eng. Chem. Res. 1993, 32 (9), 2023) explained the existence of multiple steady states in homogeneous azeotropic distillation on the basis of the analysis of columns with infinite reflux and infinite length (infinite number of trays). They showed that the predictions of multiple steady states for such infinite columns have relevant implications for columns of finite length operated at finite reflux. The first experimental verification of the existence of such multiple steady states was published by Güttinger et al. (Ind. Eng. Chem. Res. 1997, 36 (3), 794). Using an industrial pilot column without an automatic control system, they confirmed the existence of two stable steady states for the ternary homogeneous system methanol−methyl butyrate−toluene. That is, two different column profiles occurred for the same operating parameters, feed flow rate, and feed composition. In this paper, experiments for the same ternary system are described which show the existence of a third unstable steady state. The unstable steady state is stabilized with PI control. Furthermore, the transition from an unstable to a stable operating point is demonstrated when the control action is removed.

In this article, multiple steady states are studied in sequences of interlinked columns commonly used to separate azeotropes in ternary homogeneous distillation. More specifically, two major separation schemes were concerned:  the intermediate entrainer scheme and the “boundary separation” scheme. On the basis of the results on multiplicities in single columns for the case of infinite reflux and infinite column length (infinite number of trays) similar criteria for separation sequences were developed. It is shown how to construct bifurcation diagrams on physical grounds with one product flow rate as the bifurcation parameter and how the product paths of a sequence can be located. Moreover, a necessary and sufficient condition for the existence of multiple steady states in column sequences is derived on the basis of the geometry of the product paths. The overall feed compositions that lead to multiple steady states in the composition space are also located. For the intermediate entrainer scheme multiplicities occur for all feed compositions. For the boundary separation scheme multiplicities may disappear when a single column is integrated into a sequence. By use of examples of the two separation schemes, it is shown that the prediction of the existence of multiple steady states in the ∞/∞ case has relevant implications for columns with finite length (finite number of trays) operated at finite reflux.

During the last few years, multiple steady states (output multiplicities) have been discovered for reactive distillation processes, for example, for the production of fuel ethers and for some esterification processes. Using the transformation of Ung and Doherty (Chem. Eng. Sci. 1995e, 50 (1), 23−48), a method is presented to predict the existence of output multiplicities based on the reactive vapor−liquid equilibrium for the limiting case of reactive columns of infinite length operated at infinite internal flows. This graphical method rests upon the ∞/∞ analysis for azeotropic distillation columns by Bekiaris et al. (Ind. Eng. Chem. Res. 1993, 32 (9), 2023−2038). It is directly applicable to systems where the reactions take place in the entire column (“nonhybrid” columns). When all possible profiles and products are located through a bifurcation analysis, qualitative and quantitative predictions are obtained. The region of feed compositions leading to multiple steady states can be constructed graphically by applying a necessary and sufficient geometrical condition. The prediction results are shown to carry over to finite columns by application to the methyl tert-butyl ether process and are verified by simulation.

Güttinger and Morari (Ind. Eng. Chem. Res. 1998, 38, 1633−1648) developed graphical methods for the prediction of output multiplicities caused by the reactive vapor−liquid equilibrium in reactive distillation. These methods rest upon the limiting case of reactive columns of infinite length operated at infinite internal flows (∞/∞ analysis) and are directly applicable to systems where the reactions take place in the entire column (“nonhybrid” columns). In this work, the reactive ∞/∞ analysis is extended to columns with a reactive core (“hybrid” columns) by introduction of two new procedures. First, necessary and sufficient feasibility conditions for hybrid column profiles are derived under the assumption that each of the reactive and nonreactive column sections has infinite length. Using these conditions, an “exact” method is formulated where all possible products of such an ∞/∞ hybrid column can be located in the composition space (by a continuation of solutions). The existence of multiple steady states and the feed region leading to output multiplicities can be predicted. Second, an “approximate” procedure is proposed to obtain an estimate of the product locations of a hybrid column with finite nonreactive sections. The exact method was applied to an “ideal” reactive system, and a new type of multiplicity, which is purely caused by the hybrid nature of the system, is shown. Moreover, the methyl tert-butyl ether (MTBE) reactive distillation process is analyzed using both methods, and the physical causes of the MTBE multiplicities are studied in detail. All analytical results are verified by simulation.