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 spite of significant nonlinearities even in the simplest model, the distillation literature generally takes for granted that distillation columns display relatively simple dynamic behavior. For example, although widely observed in chemical reactors, any instances of periodic oscillations have not yet been associated with models of distillation columns. In this paper we study the steady-state and dynamic behavior of the azeotropic distillation of the ternary homogeneous system methanol−methyl butyrate−toluene. Our simulations reveal nonlinear behavior not reported in earlier studies. Under certain conditions, the open-loop distillation system shows a sustained oscillation (limit cycle). The limit cycles are accompanied by traveling waves inside the column. Significant underdamped oscillations are also observed over a wide range of product rates. To our knowledge, this is the first simulation result reporting the presence of Hopf bifurcation points in open-loop distillation models.

A novel approach is presented for the qualitative analysis of the dynamic behavior of homogeneous azeotropic distillation columns of finite length. The methodology can be used to study the dynamic behavior of steady-state column profiles after perturbations. The key concept is to study the interaction of changes in the total column holdup composition and the shape of the column profile. This allows one to qualitatively determine if the profiles are locally stable or unstable. It is shown that the stability of the steady states is dominantly governed by the external mass balance around the column. This is highlighted by studying the case of infinite internal flows where the internal dynamics of the column become infinitely fast while the external dynamics remain at a slow time scale. The analysis of the local stability of column profiles presented in this work will be extended to construct bifurcation diagrams in the accompanying paper (Dorn, C.; Morari, M. Ind. Eng. Chem. Res. 2002, 41, XXXX).

Studying global mass balances and how they interact with the shape of column profiles, Morari, M. Ind. Eng. Chem. Res. 2002, 41, XXXX) developed a graphical method for a qualitative stability analysis of steady-state profiles in homogeneous azeotropic distillation columns. Based on the qualitative stability analysis, here it is shown how bifurcation diagrams can be constructed containing information on the stability of the equilibria. Fold, Hopf, and homoclinic bifurcations as well as more complex bifurcations can be predicted. The existence of limit cycles is explained and their approximate shape determined. It is explained how the feed composition and the reflux flow rate influence the bifurcation behavior of the column. The results are compiled in two-parameter bifurcation diagrams as well as in projections of a threeparameter bifurcation diagram. The method is applied to predict several new phenomena in distillation, e.g., operating conditions for which the only steady state of the column is unstable and surrounded by a stable limit cycle. All predicted phenomena are confirmed by steady-state and dynamic simulations.