Instrumentation and Control Systems

chapter is shown in Figure 1, where V = vapour flow, F = feed flow, R = reflux flow, D = distillate flow, B = bottom flow, x = composition, M = mass hold-up and W and S are cooling water and steam flow respectively. In order to maintain constant separation in the distillation column, it should be well instrumented and controlled. The treatment of instrumentation and control techniques will focus primarily on packed columns or columns with trays.

A number of excellent books and review articles have been written about distillation control. Shin-skey's book (1984) is a very practical one and provides a good introduction to the subject of distillation control, although a detailed explanation of different approaches to control alternatives is always clear to the novice. The book by Buckley et al. (1985) provides a comprehensive treatment of pressure, level and protective controls; control of composition is restricted to a short treatment of composition dynamics in binary columns. The book edited by Luyben (1992) is probably the best starting point: It has been written by several experts in the field of distillation dynamics, instrumentation and control. It provides a comprehensive treatment of distillation models, distillation simulation, identification of distillation processes and selection and comparison of control structures. In addition, several chapters are devoted to particular case studies. The Process Control Instrument Engineers' Handbook (Liptak, 1995) also provides three interesting sections on distillation control: one section discusses basic controls, another section discusses advanced controls and in a subsequent section the subject of relative gain calculations is reviewed. The handbook also provides a wealth of information on instrumentation.

A book which gives a good introduction to batch distillation control is the one by Fisher (1990) and Luyben (1992) gives a good overview. Three review articles should be mentioned as a starting point for further reading: the first one is by Tolliver and Waggoner (1980: 195 references), another is written by McAvoy and Yang (1986: 270 references), and the one by Skogestad (1992) also provides easy-to-under-stand material: it has 206 references.

Because vapour and liquid with a certain energy content are present in the column, basic instrumentation includes measurements of the vapour hold-up (column pressure), liquid hold-ups (column bottom level and reflux drum level) and generally a number of temperatures along the column. In addition, the in-and outgoing flows are usually measured, as shown in Figure 1 .

Since the feed to the distillation column is often fixed by a preceding process, no control valve is shown in this flow, although sometimes the feed pre-heater control valve is used to control the amount of feed that vaporizes. This means that five manipulable flows remain and there are essentially five degrees of freedom for control. However, the vapour and liquid hold-ups have to be controlled, which means that 5—3=2 degrees of freedom remain, which are generally used for composition control.

Distillation columns pose some interesting control problems. First of all, the process is often highly nonlinear. Secondly, there are five variables to be controlled (pressure, P, level, hB, level, hR, composition, xD, composition, xB) and five variables which can be manipulated (flows R, D, B, S and W). How the coupling between these controlled and manipulated variables should be established has been an interesting field of study for many years. In our approach a matrix (Figure 2) is constructed where all variables are listed; for each combination of controlled output and manipulated input the control quality is determined. The control quality can be established on the basic of speed of control, power of control and the requirement of minimal interaction between control loops. The speed of control is related to the period of oscillation of the control loop at the limit of stability. The shorter this period, the higher the speed of control. The power of control relates to the range over which control is effective. For an acceptable combination, the speed of control should be large, as

Basic control











Figure 2 Control matrix, showing controlled and manipulated variables.

Figure 2 Control matrix, showing controlled and manipulated variables.

should the power of control - in other words, the controlled output should respond quickly to changes in the manipulated process input. However, there are not always five variables to be controlled. It could be that there is no strict requirement for the column pressure, in which case it is often optimal to minimize the pressure and open the cooling water valve of the condensor completely. It could also be that there is a strict requirement for the top product composition but no requirement for the bottom composition, in which case there would be an extra degree of freedom.

The issue of column operation, column instrumentation and selection of the right pairing between controlled process outputs and manipulated inputs will be considered in more detail in the following sections.

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