Ind. Eng. Chem. Res. 1991,30, 2159-2168 2169 Gavalas, G. R; Megiris, C. E.; Nam, S. W. Deposition of Hz-Perm- selective SiOz Films. Chem. Eng. Sci. 1989, 44 (9),182H835. Hair, M. L.; Hertl, W. Chlorination of Silica Surfaces. J. Phys. Chem. 1973, 77 (17), 2070-2075. Hudson, R. F. The Vapor Phase Hydrolysis of Non-metallic Chlo- rides. Proc. Int. Congr. Pure Appl. Chem. 1947,297-305. Lee, R. W. Diffusion of Hydrogen in Natural and Synthetic Fused Quartz. J. Chem. Phys. 1963,s (2), 448-455. Leiby, C. C., Jr.; Chen, C. L. Diffusion Coefficients, Solubilities and Permeabilities for He, Ne, Ha, and Nz in Vycor Glass. J. Appl. Phys. 1960, 31 (2), 268-274. Nam, S. W.; Gavalas, G. R. Stability of Hz-PermselectiveSiOp Films Formed by Chemical Vapor Deposition. AIChE Symp. Ser. 1989, 85 (2681, 68-74. Reichmann, M. G.; Bell, A. T. Raman Study of the Preparation of SiOrSupported TiOz from Tic&and HC1. Longmuir 1987,3 (l), Shelby, J. E. Molecular Solubility and Diffusion. In Treatise on Materials Science and Technology; Tomozawa, M., Doremus, R. H., Eds. Academic: New York, 1979 Vol. 17, pp 1-40. Tolmachev, V. A.; Okatov, M. A. Investigation of Process for Syn- thesizing Ultrathin Titanium Oxide Layers in Porous Gh. Sou. J. Opt. Technol. 1984,51(2), 104-107. Wong, P.; Robinson, M. Chemical Vapor Deposition of Polycrys- talline AlzOS. J. Am. Ceram. Soc. 1970,53(ll), 617-621. Receiued for review January 29, 1991 Accepted June 10,1991 11 1-116. PROCESS ENGINEERING AND DESIGN Combined Balance Control Structure for Distillation Columns Dae R. Yang,t Dale E. Seborg, and Duncan A. Mellichamp* Department of Chemical and Nuclear Engineering, Uniuersity of California, Santa Barbara, California 93106 A new control structure for distillation columns, called the “combined balance structure” is presented. The combined balance structure consists of a linear combination of material balance and energy balance control (conventional control) that can outperform traditional control structures such as LV, DV, ratio structures, etc. Mathematical derivation of this new structure is based on a rigorous transformation technique. The combined balance method unifies the analysis of distillation column control structures and leads to general relative gain array (RGA) relationships that can be used to compare alternative distillation control structures. The importance of inventory control dynamics and the ability of the new optimum combined balance control structure to outperform other con- ventional control structures are demonstrated by closed-loop comparisons using transfer function representations. Introduction Distillation is one of the most important and widely used separation processes in the chemical and petroleum in- dustries. It is also one of the most energy-intensive op- erations in these industrial plants. For example, a 1982 study reported that distillation accounts for about 3% of the total energy consumption in the United States (Mix and Dweck, 1982). Large potential improvements in distillation control can be obtained by developing better process control strategies. One of the best candidates for improved strategies is the choice of control structure for the distillation column. In the field of distillation control, usually reflux flow rate L or distillate flow rate D (or a ratio of these two) is used for top composition manipulator, and vapor boilup rate V or bottom flow rate B (or a ratio of these two) is used for bottom composition control. These cases represent three general control structures-energy balance control, material balance control, and various forms of ratio control. Energy balance control uses the L and V flow rates as composition manipulators, thus fixing the energy inputs. Material balance control uses product flow rates (D and B) as composition manipulators, thus fixing the overall * Author to whom correspondence should be addressed. ‘Current address: ABB S i c o n Inc., 8001 Savoy Dr. Suite 400, Houston, TX 77036. material balance. Ratio control utilizes a ratio of any two flow rates among those mentioned previously at each end of the column. These control structures perform quite differently, depending on the distillation column charac- teristics. Typically much effort is expended in finding a suitable structure, and a unified approach to deal with the choice of control structure has been unavailable. A wide variety of distillation control structures has been reported in the literature. In addition to using simple flows as composition manipulators, as in the LV and DV structures, ratios between flows often have been suggested. An early example is Rijnsdorp’s suggestion (1965) to use the ratio of reflux flow and overhead vapor flow L/(L + D) as a manipulator for the top composition control. The suggestion was studied experimentally by Wood and Berry (19731, who compared the [(L + D)/L]Vstructure with the conventional LV structure. The book by Rademaker et al. (1975) lists a large number of flow ratio manipulators that have been discussed in the earlier literature. Renewed interest in ratio control returned in the 1980s. Ryskamp (1980) proposed a dual composition control scheme which uses the ratio of the distillate flow to the overhead vapor flow D/(L + D) to control distillate composition and the vapor boilup to control bottom composition. Waller et al. (1988) experimentally compared four structures-L V, DV, [D/(L + D)]V, and [D/(L + D)]V/B-on a pilot-plant column. The choice of manipulators also has been in- vestigated recently by Hiiggblom and Waller (19891, Sko- 0 1991 American Chemical Societv 0888-5885 191 I 2630-2159%02.50 I 0