PROCESS used the simulation software to quickly execute multiple cases for a three-tower HCl absorption system designed to produce concentrated hydrochloric acid from two production offgas streams with widely differing flow rates and HCl concentrations. The following variables were adjusted in a systematic fashion using simulation to maximize acid concentration, minimize offgas HCl emissions, and minimize operating costs: tower packed heights, recycle flow rates and temperatures, quench/absorption water flow rates, and feed gas and intermediate stream routings. After design was complete, sensitivity analyses were run on the key process variables to determine the “controllability” of the system during changes in operating conditions.