Semicontinuous, middle-vessel distillation of ternary mixtures

The operation of a semicontinuous, middle-vessel column (SCMVC) that separates a nearly-ideal ternary mixture is analyzed. MVCs have a large vessel between the rectification and stripping sections. In traditional batch rectification or stripping, a still is charged and distillate or bottoms products are removed, leaving heavy or light concentrates in the still, respectively. In contrast, the middle vessel of an MVC is charged, and, similar to a continuous process, both distillate and bottoms products are withdrawn. The process separates nearly-ideal ternary mixtures into three nearly-pure species, with the intermediate-boiling species concentrating in the middle vessel, and the light and heavy species concentrating in the distillate and bottoms products, respectively. Shortcut methods are adapted to determine the minimum number of trays and minimum reflux ratio. Alternative control configurations are considered to achieve suitable performance throughout the campaign. Two DB configurations are shown to be suitable for this process, with the middle vessel decoupling the composition control loops and permitting dual-composition control with little interaction. Simulations confirm that difficulties encountered when using a DB configuration for the dual-composition control of a continuous column do not occur for a SCMVC.

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