1820 8.19 Distillation: Basic Controls H. L. HOFFMAN, D. E. LUPFER (1970) L. A. KANE (1985) B. A. JENSEN (1995) B. A. JENSEN, B. G. LIPTÁK (2005) INTRODUCTION Distillation is the most common class of separation processes and one of the better understood unit operations. It is an energy-separating-agent equilibrium process that uses the dif- ference in relative volatility, or differences in boiling points, of the components to be separated. It is the most widely used method of separation in the process industries. The distillation process will most often be the choice of separation unless the following conditions exist: • Thermal damage can occur to the product. • A separation factor is too close to unity. • Extreme conditions of temperature or pressure are needed. • Economic value of products is low relative to energy costs. Control involves the manipulation of the material and energy balances in the distillation equipment to affect product composition and purity. Difficulties arise because of the mul- titude of potential variable interactions and disturbances that can exist in single-column fractionators and in the process that the column is a part of. Even seemingly identical columns will exhibit great diversity of operation in the field. Therefore, this section will not attempt to provide control strategies that can be applied to columns in a “cookbook” fashion. Instead, discussion will begin with a basic description of the distillation process and equipment, followed by techniques used to derive a mathe- matical column model. The presentation in this section will then describe meth- ods to evaluate interactions and alternative control strategies; control models used for some product quality, pressure, and feed flow control strategies; and finally some common feed- forward advanced regulatory control strategies commonly used in the regulation of fractionators. The goal of this section is to provide the process control engineer with the tools necessary to design unique control strategies that will match the specific requirements of distil- lation columns. General Considerations Distillation separates a mixture by taking advantage of the difference in the composition of a liquid and that of the vapor formed from that liquid. In the processing industries, distilla- tion is widely used to isolate and purify volatile materials. Thus, good process control of the distillation process is vital to maximize the production of satisfactory purity end products. Although engineers often speak of controlling a distilla- tion tower, many of the instruments actually are used to control the auxiliary equipment associated with the tower. For this reason, the equipment used in distillation will be discussed. V V F L F Q R i or L i T R R or L D L F B Q V Flow sheet symbol © 2006 by Béla Lipták