Control of distillation columns

In the previous chapter the procedure for the design of control schemes was discussed. The procedure was illustrated on a reactor with recycle. The selection of appropriate combinations of controlled and manipulated variables was relatively simple, since the interactions were limited. In this chapter the procedure will be applied to a distillation column. This is a unit operation with many interactions between the corrections that are made. Using a basic knowledge of the process dynamics, a basic control scheme is designed. Subsequently, two control schemes will be compared a basic control scheme based on material balance control and a control scheme based on energy balance control. The distillation column can also be used to demonstrate the optimization of the control scheme. The principle is that the control scheme should be designed in such a way, that an objective function can be maximized. 

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The topic of control of distillation columns has been discussed by many authorities with a wide variety of experience [117-120, 237], and is too specialized to be covered in this text. 

It is desirable that the process variable to be monitored be measured directly often, however, this is impractical and some dependent variable, that is easier to measure, is monitored in its place. For example, in the control of distillation columns the continuous, on-line, analysis of the overhead product is desirable but difficult and expensive to achieve reliably, so temperature is often monitored as an indication of composition. The temperature instrument may form part of a control loop controlling, say, reflux flow with the composition of the overheads checked frequently by sampling and laboratory analysis. 

Skogestad, S. Dynamic and Control of Distillation Columns—A Critical Survey. Modeling Identification Control 18 177-217 (1997). 

The process control scheme proposed for the column is based upon similar suggested layouts used for the control of distillation columns (Ref. A8). Modifications were made in accordance with the individual operating characteristics and requirements of this system. Final verification of the proposed scheme is achieved by consulting Refs. A7 and A9. 

A9. P S. Buckley, Techniques of Process Control, Chapter 5 Control of Distillation Columns, R.E. Krieger Publishing, New York (1979). 

Toijala (Waller), K. V. and Fagervik, K. C., "A Digital Simulation Study of Two Point Feedback Control of Distillation Columns," Kern. Teollisuus, 1972, 29, 5. 

Skogestad, S., Studies on Robust Control of Distillation Columns. Ph.D. thesis, California Institute of Technology, Pasadena, 1987. 

I. Development of the Mathematical Model and Algorithm, Rev. Chim., vol. 37, p. 697 A. Woinaroschy, 2007, Time-Optimal Control of Distillation Columns by Iterative Dynamic Programming, Chem. Eng. Trans., vol. 11, p. 253... 

The control of units can be treated following effective structures proven by the industrial practice. However, if a unit is involved in a recycle, the eontrol scheme should be reviewed and adapted. For example, dual-concentration control of distillation columns in recycles can be replaced by more flexible one-point concentration control. 

Step 8. The control of distillation column can be implemented mainly by stand-alone considerations. Liquid inventories in the flash and separation columns have classical structure base flow on level control. The liquid level in the flash drum is kept by the condenser cooling duty. Pressure is controlled by condenser duty in the production and recycle columns, but by vapour distillate in stabiliser. 

Luyben, W. L., "Feedback Control of Distillation Columns by Double Differential Temperature Control, Ind. Eng. Chem. Fund. 8(4), 1969, p. 739. 

Luyben, W. L., "Profile Position Control of Distillation Columns with Sharp Temperature Profiles, AIChE J. 18, 1972, p. 238. 

The basis of distillation is phase equilibrium—specifically, vapor-liquid equilibrium (VLE) and in some cases vapor-liquid-liquid equilibrium (VLLE). Distillation can only effect a separation among chemical components if the compositions of the vapor and liquid phases that are in phase equilibrium with each other are different. A reasonable understanding of VLE is essential for the analysis, design, and control of distillation columns. 

T. L. Tolliver, Control of distillation columns via distributed control systems, in Practical Distillation Control, van Nostrand-Reinhold, 1992, p. 351. 

There is no claim that the material is all new. The steady-state methods are discussed in most design textbooks. Most of the dynamic material is scattered around in a number of papers and books. What is claimed is that this book pulls this material together in a coordinated easily accessible way. Another unique feature is the combination of design and control of distillation columns in a single book. 

This book focuses on the fundamentals of process control of distillation columns. It covers the following topics ... 

Distillation is a mature technology that is well developed and tested by the scientific method. Two comprehensive books have been written by Kisteb on the design and operation of distillation columns. The control of distillation columns has been widely studied for many years, many papers have been written, and several books have been published on the subject such as the one by Buckley, Luyben, and Shunta. There is a large body of information to search and understand in the state of the art of distillation and process control. 

In the control of distillation columns, the distillate composition is often close to 100% light component 1, and the bottoms is close to 100% heavy component 2, so the key impurity separation power can be calculated for control purposes from Equation 3.4. 

When discussing the control of distillation columns, the need to control column temperature was mentioned. The temperature is the controlled variable . What do you alter in order to achieve this (that is, what is the manipulated variable ) A reconsideration of an earlier chapter may be required. 

Skogestad, S. (1997), Dynamics and control of distillation columns a turorial introduction, Trans IchemE75. 

Lupfer, D. E., and M, L. Johnson Automatic Control of Distillation Columns to Achieve Optimum Operation, ISA Trans., April, 1964. 

Doukas, N., Composition Control of Distillation Columns with Side-stream Drawoffs Separating Ternary Mixtures, Ph. D. Thesis, Lehigh University, 1978. 

Use of the Smith and other predictors recently has been shown by Seborg et al. to provide substantially better feedback control of distillation-column terminal compositions than do PI and PID controllers. Further work needs to be done to compare their performance with that achieved by PI or PID controllers with feedforward compensation. 

Enhanced Control of Distillation Columns via On-Line Models... 

Steady-State Control of Distillation Columns, presented at American Institute or Chemical Engineers Meeting, Chicago, 1976. 

Composition control of distillation column with feed forward compensation and decouplers... 

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