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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.

Summary: A new analysis tool is presented that uses the governing kinetic scheme to predict properties of low‐density polyethylene (LDPE) such as the detailed shape of the molecular weight distribution (MWD). A model that captures mixing details of autoclave reactor operation is used to provide a new criterion for the onset of MWD shouldering. Kinetic effects are shown to govern the existence of MWD shoulders in LDPE reactors, even when operation is far from perfectly‐mixed. MWD shoulders occur when the mean reaction environment has a relatively high radical concentration and has a high polymer content, and is at a low temperature. Such conditions maximize long chain formation by polymer transfer and combination‐termination, while limiting chain scission. For imperfectly‐mixed reactors, the blending of polymer‐distributions produced in different spatial locations has a small effect on the composite MWD. However, for adiabatic LDPE autoclaves, imperfect mixing broadens the stable range of mean reactor conditions, and thereby increases the possibility for MWD shouldering.Polymer MWD produced in an LDPE autoclave reactor by various kinetic mechanisms.magnified imagePolymer MWD produced in an LDPE autoclave reactor by various kinetic mechanisms.