Control of Highly Interconnected Reactive Distillation Processes: Purification of Raw Lactic Acid by Esterification and Hydrolysis

Plantwide control of processes to purify lactic acid by esterification and subsequent hydrolysis are investigated. Two processes are considered and compared: one using methanol and the other using butanol. The alcohol liberated in the hydrolysis column is recycled to the esterification column, and the water liberated in the esterification column can be reused in the hydrolysis column, resulting in highly interconnected process flowsheets. For both processes, designs with one recycle stream (alcohol only) and two recycle streams (alcohol and water) are tested, and for each alcohol and each configuration, both temperature and composition control are tested for all columns (reactive and nonreactive). The results show that processes with only a single recycle stream are somewhat easier to control. For the butanol process, a temperature control structure with only a single composition controller is found to be adequate, while additional composition measurements with constraints on controller outputs are required for good control of the methanol process. For both processes, responses are more symmetric when temperature control structures are used.