This paper focuses on the use of batch extractive distillation for separating homogeneous minimum-boiling azeotropic mixtures, where the extractive agent is a high-boiling, miscible zeotropic component. The effect of column design on the operation is evaluated and a control structure for the batch extractive middle vessel distillation is proposed.
In extractive distillation, the non-volatile solvent is fed continuously to the top of the column in order to be present on all the plates during the separation. In a conventional batch column configuration, this semi-continuous operation has two main disadvantages due to accumulation of the extractive agent: (1) filling of the reboiler, and (2) degradation of the composition profile in the column. The first may cause operational problems and the latter decreases the efficiency of the separation.
One way to overcome these problems is to use a middle vessel column. The middle vessel column arrangement consists of two column sections, a batch rectifier and a batch stripper, connected by a middle vessel. The azeotropic mixture to be separated is charged to the middle vessel and the extractive solvent is fed to the head of the rectifying section. This configuration allows for simultaneous removal of product(s) and recovery of the extractive agent. In addition, the volume of the middle vessel can be kept constant or even decreasing with time. Thus, high separation efficiency can be achieved since the sharpness of the column composition profile is maintained during the distillation run. Furthermore, the middle vessel implementation is potentially more energy efficient than conventional distillation due to the combination of a rectifying-extraction section and a stripping section in one column. Batch extractive distillation with a middle vessel is closely related to the well-known continuous extractive distillation except that the separation is performed in one rather than in two columns, which may imply capital savings for the batch implementation.
We propose a novel control configuration for the batch extractive middle vessel distillation where the liquid flow from the middle vessel is manipulated to control the temperature in the stripping section, achieving a desired purity of the extractive agent. This indirectly adjusts the vessel holdup and there is no need for level control. The extractive agent feed is used to control the purity of the distillate and the reflux is held constant below its maximum value.