Middle-vessel Distillation Research Articles

Dual Composition Control in Continuous, Middle-Vessel Distillation Columns, with a Draw Stream in the Middle Vessel

The dual composition control problem for a class of continuous middle-vessel distillation, which accepts a draw stream in the middle vessel, is studied in this work. This distillation configuration is advantageous when the separation goal is to recover high-purity light and heavy components from a multicomponent mixture with sufficiently large separation between relative volatilities. By using an estimator of the input–output modeling error, a first-order output-feedback compensator is designed which is shown to be equivalent to a multi-input multi-output proportional–integral (MIMO PI) controller. Tuning issues in terms of two time constants are given, and the ability of the controller to regulate the distillate and bottom product compositions in the face of sampled/delayed measurements is shown via numerical simulations.

Limitations in the Operation and Control of Continuous Middle-Vessel Distillation Columns with a Draw Stream

This paper studies the limitations in the operation and control of a continuous middle-vessel distillation column with a draw stream in the middle vessel. This separation layout can be of interest to cases where the most and less volatile components of fractions of a mixture are desired in high concentrations, as in the processing of some oil streams. It was found that to maintain distillate and bottoms products within specifications, large recycling rates can be required to compensate for the storage effects of the middle vessel. This is because, contrary to the case where a draw stream is not present, in the studied distillation column layout the middle vessel alters material balances. In this form, the middle vessel acts as both a dynamical damper to reduce bottoms and distillate control loops and an accumulator for intermediate fractions. That includes some residuals of nonseparated most and less volatile components. The control objective is to track prescribed product compositions of the most and less volatile components by means of manipulations of the reflux and vapor boilup rates. Given that the middle vessel introduces a sluggish controlled response, the recycling stream from the middle vessel to the column is taken as an additional control input to improve the operation of the column and the control performance. In this way, a rectangular control configuration is obtained, which is regularized (i.e., squared) with a habituating control parameter. It is shown that this habituating control configuration can satisfy the control objectives more easily because it provides a balance between the separation and mixing tasks provided by the distillation column and the middle vessel, respectively.

SOME ASPECTS OF THE OPERATION OF SEMI-CONTINUOUS, MIDDLE-VESSEL DISTILLATION COLUMNS

Some aspects of the operation of a semi-continuous, middle-vessel distillation column that separates a ternary ideal mixture are analyzed. In general, middle-vessel distillation columns have a large vessel between the rectification and stripping sections and are usually operated in a batch or continuous fashion. Recently, Phimister and Seider (2000) presented a novel process that operates in a semi-continuous manner, showing that the process can separate a ternary ideal mixture 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 bottom products, respectively. By means of numerical simulations of middle-vessel continuous and middle-vessel batch processes, in this work we arrive in a natural manner at the middle-vessel semi-continuous process. Due to the fact the operation of semi-continuous middle-vessel distillation requires the use of controlled policies, a dual composition control configuration was designed. The control schemes used in this work are based on stable inverse-dynamics feedback control law by assuming complete knowledge of the process dynamics. As a result, a new operating policy based on refills of the middle vessel is proposed and discussed.

Semicontinuous, middle‐vessel distillation of ternary mixtures

AbstractThe 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.

Distillate−Bottoms Control of Middle-Vessel Distillation Columns

The performance of a middle-vessel distillation column subject to distillate−bottoms (DB) control configurations is analyzed. DB-controlled columns, where the distillate flow rate is manipulated to control the distillate composition and the bottoms flow rate is manipulated to control the bottoms composition, have been labeled as inoperable for continuous distillation. In this paper, two DB-control configurations for a middle-vessel column (MVC) are shown to overcome the deficiencies commonly associated with DB control. In the first configuration, the feed flow rate from the middle vessel is fixed and the flow rate of the heating medium controls the sump level. In the second “on-demand” configuration, the feed flow rate from the middle vessel is manipulated to control the sump level. To demonstrate the effectiveness of these configurations, the distillation of a near-ideal binary mixture is examined.