Distillation column, control

The primary objective of distillation column control is to maintain the specified composition of the top and bottom products, and any side streams correcting for the effects of disturbances in ... 

Shinskey (1984) has shown that there are 120 ways of connecting the five main parts of measured and controlled variables, in single loops. A variety of control schemes has been devised for distillation column control. Some typical schemes are shown in Figures 5.22a, b, c, d, e (see pp. 234, 235) ancillary control loops and instruments are not shown. 

Distillation column control is discussed in detail by Parkins (1959), Bertrand and Jones (1961) and Shinskey (1984) Buckley et al. (1985). 

Buckley, P. S., Luyben, W. L. and Shunta, J. P. Design of Distillation Column Control Systems (Edward Arnold, New York, 1985). 

Another example is sketched in Fig. 8.3i>. A hot oil stream is used to reboil a distillation column. Controlling the flow rate of the hot oil does not guarantee a fixed heat input because the inlet oil temperature can vary and the AT requirements in the reboiler can change. The heat input Q can be computed from the flow rate and the inlet and outlet temperatures, and this Q can then be controlled. 

Professor Luyben has published over 100 technical papers and has authored or coauthored four books. Professor Luyben has directed the theses of over 30 graduate students. He is an active consultant for industry in the area of process control and has an international reputation in the field of distillation column control. He was the recipient of the Eckman Education Award in 1975 and the Instrumentation Technology Award in 1969 from the Instrument Society of America. ... 

Several control techniques have been developed to compensate for large dead-times in processes and have recently been reviewed by Gopalratnam, et al. (4). Among the most effective of these techniques and the one which appears to be most readily applicable to continuous emulsion polymerization is the analytical predictor method of dead-time compensation (DTC) originally proposed by Moore ( 5). The analytical predictor has been demonstrated by Doss and Moore (6) for a stirred tank heating system and by Meyer, et al. (7) for distillation column control in the only experimental applications presently in the literature. Implementation of the analytical predictor method to monomer conversion control in a train of continuous emulsion polymerization reactors is the subject of this paper. 

FIGURE 19 Typical distillation column control scheme. 

More work has appeared in the chemical engineering literature on distillation column control than on any other unit operation. Books on this important subject date back to the pioneering work of Rademaker et al. (1975), Shinskev (1977), and Buckley et al. (1985). Some of the more recent developments are discussed in Luyben (1992). The longterm popularity of distillation control is clear evidence that this is a very important and challenging area of process control. Most chemical plants and all petroleum refineries use distillation columns to separate chemical components. Distillation is the undisputed king of the separation processes. 

We cannot hope to cover the vast subject of distillation fundamentals and control in a single chapter in this book. Our objective here is to review some of the basic principles about distillation and then to summarize the essentials of distillation column control, particularly as it relates to the plantwide control problem. Many more details are available in the books cited above. In the first section we review some of the important fundamentals about distillation. Sections 6.3 through 6.8 discuss distillation column control mostly from the perspective of an isolated column or column system, This treatment presents what may appear to be a laundry list of control structures for different types of columns. Although we feel this presentation is valuable, particularly for the young inexperienced student or engineer, it is also vital to retain a broader plantwide perspective. Section 6.9 addresses some of these plantwide distillation control issues. In the next section we review the process fundamentals of distillation as they relate to process operation and control. 

One of the challenging aspects of distillation column control is the many limitations imposed on the operation of the column. There are hydraulic constraints, separation constraints, heat-transfer constraints, pressure constraints, and temperature constraints. We recommend the excellent books by Kister (1992 and 1990) on distillation design and operation. 

We briefly discussed in Chap. 2 (Sec. 2.5) one important aspect of distillation column control when one of the products leaving the column is a recycle stream. Should the composition of this recycle stream be controlled Probably not from the perspective of the isolated column because the control loop holding the composition of the other stream leaving the column, which is a product from the plant, could then be more tightly tuned. However, the plantwide control perspective may show that the performance of the reactor can be improved by holding the purity of the recycle stream more constant, and this could result in smaller disturbances to the column. The overall effect may be better product quality control even though the product-quality loop is less tightly tuned. 

There are many other aspects of distillation column control in a plantwide context. One of the earliest studies of these issues is the work of Downs (1992). According to Downs, the control strategy for each unit operation must be developed within the framework of the overall component inventory control structure. 7 He presents several... 

Four alternative control schemes are commonly used for distillation column control, as shown in Figure 3.15 through Figure 3.18, respectively. Scheme 1 directly adjusts the material balance by manipulation of the distillate flow. If the distillate flow is increased, then the reflux accumulator level controller decreases the reflux flow. As less liquid proceeds to flow down to the sump, the sump level controller decreases the bottoms flow a like amount. The separation is held constant by manually setting the reboiler steam flow to maintain a constant energy per unit feed. 

Figure 3.19. Distillation column control—Control plate location.

Figure 3.20. Distillation column control—Temperature sensitivity.

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