DISTILLATION COLUMN CONTROL DESIGN USING STEADY STATE MODELS : USEFULNESS AND LIMITATIONS

Steady state models continue to be powerful and efficient tools for designing control systems for distillation columns. This paper presents a control design procedure and an example application of this technique to an actual column.  Steady state process models have long been used to assist the control engineer in designing control strategies for distillation columns. However, with the large number of industrial columns still operating in manual or with ineffectual controls, there remains a need for sound distillation column control design techniques. We believe that Tolliver and McCune (1978) have made the greatest contribution to the development of this type of design procedure. Two other very good papers on this subject are by Thurston (1981) and Roat, et al (1988). While our procedure is an extension of that proposed by Tolliver and McCune, we have improved the procedure in the following ways: • We advocate that mass flows be used in models versus the previous standard of molar flows. We have determined independently that use of molar flows can lead to incorrect results. A recent review article on distillation column control by Skogestad (1992) confirms these findings. • We also advocate that the actual control structure be enforced when using the steady state simulation to identify a temperature sensor location for composition control. This is accomplished by a careful choice of independent variables when defining the model solution conditions. Tolliver and McCune advocate varying only molar distillate flow regardless of the proposed control structure. This too can lead to incorrect results. • We show that this technique can be used for multicomponent columns to quantify the incremental benefit of composition control using on-line analyzers versus temperature control. This paper deals exclusively with the design of single point composition controls. The vast majority of columns have one sided composition specifications; those in which a single point composition control scheme can keep both top and bottom product compositions at or below limits for a wide range of disturbances. This does not have to be accepted on faith because the design procedure explicitly tests this hypothesis. The predominance of one sided specifications leaves the main incentive for dual point control schemes to be energy savings. In most cases, the energy savings is small and does not justify the added difficulty of implementing and maintaining dual point control. Luyben (1975) presents the potential energy savings for many different types of separations. Additionally, dual point schemes often have significantly longer recoveries from upsets due to interactions between the control loops. We believe it is appropriate to contrast steady state and dynamic models as control design tools. While both tools have a place, we have found that using steady state models coupled with experience and a general knowledge of distillation column dynamics is adequate for many problems and can be more efficient than using dynamic models. For a good development on the rationale behind using steady state models refer to the chapters on Quasi-Static Analysis in Rademaker, et al (1975). One obvious limitation of steady state modeling is that it tells us nothing about the dynamic response, making it difficult to compare the dynamic disturbance rejection capability of alternative control schemes. When we encounter a difficult and important problem we invest the extra engineering time to develop a dynamic model. The ideal design tool would be one that has both steady state and dynamic capabilities. This tool would provide the efficiency of steady-state analysis, but would also have the added benefit of comparing the disturbance rejection capabilities of different schemes with the dynamic model. The combined tool would allow the designer to perform both tasks without requiring an investment in time to develop two different models. A new product soon to be released by Hyprotech, Ltd. will combine steady state and dynamic modeling in one such package. Our design procedure can be best thought of as general approach rather than a single detailed procedure that covers all cases. The procedure must be adapted to each problem because there are many different types of distillation and almost every industrial problem usually has some unique requirement. In this article, we begin by presenting some background material on distillation column control and the use of steady state models. Next, we describe our design procedure in detail. We then conclude by illustrating the design procedure with an actual applied industrial example.