Operation and control of a high-purity distillation column

A dynamic model for a continuous minimum reflux distillation column is developed for the case of constant relative volatility (CRV) and constant molar overflow (CMO). This model predicts the dynamic behavior of minimum reflux distillation columns and provides a clear description of both the steady state and the dynamic conditions necessary for maintaining high purity operation. Boundaries at which the steady state composition profile in a binary minimum reflux column is indeterminate are identified and analytical expressions predicting the location of these boundaries are established. Two new model based control strategies are presented which provide excellent disturbance rejection for large variations in feed composition. The optimum operating policy for a thermally coupled ternary Petlyuk column is derived. A control strategy for the Petlyuk column is proposed. This policy is based upon an extension of the proposed control strategy for single feed columns. It implicitly varies the internal flow ratios within each countercurrent section of the complex column. This leads to a full, multivariable feedback control structure which suppresses the dynamic interaction plaguing the operation of thermally coupled distillation networks. A continuum model for multicomponent distillation in the minimum reflux limit (Wachter, Ko and Andres, 1988) is used to explore the rich variety of composition profiles found in complex columns. These profiles are divided into two classes: favorable and unfavorable. Favorable profiles, found in both single feed and multiple feed columns, are characterized by low energy usage and relatively simple column dynamics. Unfavorable profiles, found only in multiple feed columns, are characterized by relatively high energy usage and complex column dynamics. A design procedure is presented in which unfavorable profiles are eliminated by changing feed locations or combining feed streams. The effect of mass transfer resistance on the pinch compositions in both countercurrent exchangers and single feed distillation columns is explored. A staged model for the dynamic behavior of these devices is presented and the form of the mass transfer rate expressions is critically examined for self consistency. This examination leads to a new set of restrictions on multicomponent film models for interphase mass transport. The implications of violating these restrictions is examined in both applications to countercurrent adsorption and continuous distillation processes