Ethylene glycol distillation column modeled

Drs. Aditya Kumar and Prodomos Daoutidis, chemical engineers at the University of Minnesota, Minneapolis, have modeled an ethylene glycol reactive-distillation column, showing its dynamic behavior, and addressed the design of a nonlinear model-based controller.

Reactive distillation - the process of simultaneous reaction and distillation in a single unit offers numerous advantages over conventional reactor/separator configurations. In reactive distillation, the reactants/products are continuously separated from the liquid-reaction phase into the nonreactive vapor phase. This feature allows enhanced conversion in equilibrium-limited reversible reactions such as the production of methyl/ethyl acetate, high-product selectivity in the case of multiple competing reactions such as those involving ethylene glycol manufacture, and provides an efficient means of removing or adding heat to the liquid phase. These advantages, the researchers say, have renewed interest in the use of reactive distillation technology to produce chemical commodities. (For more information on reactive distillation, see CEP, Mar. 1992, pp. 43-50.)

The researchers developed a detailed, tray-by-tray model that includes vapor phase dynamic balances. A comparison of the dynamics predicted by this model with that of a conventional model that ignores vapor holdup illustrates the importance of including the vapor phase for an accurate description of the process dynamics, the authors note.

"A steady-state bifurcation analysis for the column yielded the existence of up to five steady states, indicating the presence of strong nonlinearities, even in the case of ideal phase behavior. The column was also found to exhibit non-minimum phase behavior at high purity, which makes the design of effective controllers a challenging problem. Nonminimum phase behavior occurs due to a competition between a favorable reaction in the column and an unfavorable distillation in the reboiler. This may be a common phenomena in reactive distillation columns where the desired product is withdrawn at the bottom from the reboiler. The proposed method provides an approach for addressing the non-lineal control of such columns," Daoutidis says.

The work was supported by a grant from the National Science Foundation. More information about this research appears in the January issue of the AIChE Journal.