Basic Distillation Control Philosophy

A malfunctioning control system causes instability. The instability can adversely affect product purity, column capacity, economy, and ease of operation. Instabilities are often transmitted to dovrastream or upstream units, or can amplify small disturbances. In extreme cases, an instability can also lead to column damage or safety hazards. 

As in the case of reboilers and condensers, distillation control is too wide a topic to be adequately covered in a handful of chapters. Entire texts (68, 89, 301, 332, 362) deal exclusively with distillation control. Most of these strike a balance between theory, practice, controls design, and controls optimization. In contrast, the coverage here emphasizes operational aspects what various control schemes can and cannot do, how to put together a control system (not necessarily optimum, but one that works), how to recognize and avoid a troublesome system, what are the ill effects of various poor control schemes, and what corrective action can restore trouble-free operation. 

Computer controls and advanced controls are outside the scope of this text. Although these controls are widespread and can be of primary importance for column optimization, their role in setting stable operation is usually secondary. Advanced controls often enhance column stability, but they seldom assure that the primary stability objectives are met. A troublesome computer control loop can usually be taken off control, and stable (albeit nonoptimum) operation can be restored. On the other hand, an unstable basic control loop usually means an unstable column, even when computer control is used. 

The next four chapters deal with column control practices. This chapter addresses the overall philosophy of column control, while the 

Nevertheless, a lot is known about the overall philosophy of column control. Although this knowledge may fall short of predicting the optimum philosophy, it is usually sufficient for detecting poor and troublesome practices and distinguishing them from good practices. This aspect is of greatest interest to the distillation troubleshooter, superintendent, and operator, and is promoted here. 

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Two physical examples, a continuous blending system and a distillation column, have been used to introduce basic control concepts, notably, feedback and feedforward control. We also motivated the need for a systematic approach for the design of control systems for complex processes. Control system development consists of a number of separate activities that are shown in Fig. 1.10. In this book we advocate the design philosophy that for complex processes, a dynamic model of the process should be developed so that the control system can be properly designed. 

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