Control of One- and Two-Column Distillation Systems with Maximum Temperature Limitations

Abstract

Maximum temperature limitations are encountered in distillation columns in which thermally sensitive materials are being separated. The maximum base temperature often results in expensive refrigeration being required in the condenser because of the resulting low condenser temperature. The concept of using two columns instead of one column was discussed in a recent paper (Masel, R. H.; Smith, D. W.; Luyben, W. L. Ind. Eng. Chem. Res. 2013, 52, 5172−5176). Results demonstrated that a two-column process is economically better than the conventional one-column process when the feed contains a significant amount of the light component and when there is a large difference in the normal boiling points between the light and heavy components. The purpose of this paper is to explore the dynamic control issues in the one-column and two-column alternative flowsheets. The dynamic performances of several control structures are evaluated, and effective control schemes are developed that handle large disturbances in throughput and feed composition. The key concept is the need to have a structure in which both the base temperature and bottoms composition are controlled. The temperature must be tightly controlled because of the maximum temperature limitation. To maintain the bottoms composition at a constant temperature (in a binary system), the base pressure must be maintained, not the condenser pressure.