Experimental Study of Multiple Steady States in Homogeneous Azeotropic Distillation

Abstract

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.

A robust approach for finding all well-separated solutions of sparse systems of nonlinear equations

“…There are three steady-state branches: two stable steady-state branches and an unstable branch; this was experimentally verified in an industrial pilot column operated at finite reflux [23,39]. Multiple steady-states can be predicted by analyzing columns with infinite reflux and infinite length [14,38,64].…”

“…However, all steady-state branches were computed either with the AUTO software package [20] or with an appropriate continuation method [39,46,85]. The initial estimates were carefully chosen with the ∞/∞ analysis [14,38], and special attention was paid to the turning points and branch switching.…”

“…The liquid phase activity coefficient is computed from the Wilson equations. The model and its parameters correspond to the Auto model [39], except for the number of stages N and the feed stage location N F = N/2. The specifications are the feed composition (methanol-methyl butyrate-toluene), the reflux ratio, and the vapor flow rate.…”

Multiple Steady-States Analysis and Unstable Operating Point Stabilization in Homogeneous Azeotropic Distillation with Intermediate Entrainer

“…As shown in Figure 11, the details of the basic control scheme are described as follows: (1) Feed is flow-controlled (reverse acting); (2) base levels in C 1 and C 2 are held by manipulating the corresponding flow rate of bottom stream (direct acting); (3) reflux drum level in C 1 is held by manipulating distillate flow rate (direct acting); (4) reflux drum level in C 2 is held by manipulating the makeup benzene flow rate (reverse acting); (5) flow rate of stream REC is ratioed to the feed flow rate; (6) top pressures in both columns are controlled by manipulating the corresponding cooling water flow rate in the condenser (reverse acting); (7) reflux ratios in both columns are fixed; (8) temperatures on stage 38 in C 1 and stage 46 in C 2 are controlled by the flow rates of the heating steam into C 1 and C 2 reboilers, respectively (reverse acting).…”

Strategies for Starting Up Azeotropic Distillation Columns with Multiple Steady States

“…They stated that hysteresis may be experienced during the actual operation. Furthermore, Kienle et al (1995) and Koggersbøl et al (1996) confi rmed the existence of multiple steady states in an experimental way when separating a binary mixture.As regard to azeotropic distillation operations, Güttinger et al (1997) found two stable steady states caused by the vapour-liquid equilibrium for the ternary homogeneous methanol-methyl butyratetoluene system in an industrial pilot column. Besides, Dorn et al (1998) verifi ed the existence of a third unstable steady state and studied the transient behavior of that azeotropic column.…”