Based on the material balance principle applied to microbial reactions in continuous bioprocesses, the concept of reaction rate control has been developed theoretically. This concept provides a more direct way of controlling biological activities than the control of physical or chemical parameters in practice today. From an analysis of dynamic and steady-state experiments, two control systems for carbon dioxide production rate control during the continuous culture of baker's yeast have been designed and evaluated experimentally. In these control methods, intracellular NADH concentration is used as an immediate indication of the onset of glucose repression. A more sophisticated master controller based on the respiratory quotient can be combined with these control methods. The resulting control system provides a means to indirectly optimize biomass production while preventing ethanol formation in the continuous culture of baker's yeast.

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Two time delay compensation techniques, the Smith predictor and the analytical predictor, are used for bottom composition control of a pilot scale methanol-water distillation column. The closed-loop performance of the two predictor schemes is compared to that for a proportional-integral controller in experimental and simulation studies. The predictors resulted in improved control for both set point and feed flow disturbances. SCOPESignificant time delays can occur in processes due to the presence of recycle loops, distance-velocity lags in fluid flow, and the dead time inherent in many composition analyses. The detrimental effects of time delays on closedloop stability and control system performance are wideIy recognized. Thus, there is considerable motivation for the development of time delay compensation techniques that provide improved control of systems with significant time delays.This investigation provides an experimental evaluation of two time delay compensation techniques, the Smith predictor (Smith, 1957(Smith, , 1959 and the analytical predictor (Moore, 1969; Moore et al., 1970). Both techniques employ a simple dynamic model to predict future outputs based on current information. These time delay compensation techniques and a conventional proportional-integral (PI) con-trolIer are used to control the bottom composition of a pilot scale, methanol-water, distillation column. The same techniques have been evaluated for top composition control in a related study (Meyer et al., 1977).This investigation and the related study (Meyer et al., 1977) provide the first experimental applications of time delay Compensation techniques to distillation column control that have been reported in the open literature. CONCLUSIONS AND SIGNIFICANCEIn general, the analytical and Smith predictor control schemes performed better than a PI controller in controlling the bottom composition of the pilot scale column. The simulation study demonstrated that both time delay compensation techniques can provide significant improvements in regulatory and servo control. Use of the analytical predictor resulted in shorter settling times and lower integral which, in turn, outperformed the PI controller. The ex-C. B. G. Meyer is with Gulf

Two time delay compensation techniques, the Smith predictor and the "analytical predictor", are used to control top composition of a pilot scale distillation column. In each control scheme the predictive model consists of a first-order plus time delay transfer function developed empirically from open-loop data. Experimental and simulation studies are used to evaluate the predictive control schemes and to compare their performance with conventional PI control.

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