STARTUP OF A DISTILLATION COLUMN USING NONLINEAR ANALYTICAL MODEL-PREDICTIVE CONTROL

This paper discusses the application of nonlinear analytical model pre dictive control to distillation column startup between the time when t he trays are hydraulically sealed and the time steady-state operation is reached. A novel startup method is proposed whereby the liquid feed is initially used as reflux to seal the trays by establishing the pla te holdups. The control algorithm utilizes a nonlinear analytical proc ess model to generate an optimal control vector trajectory, which is i mplemented by cascade controllers. The analytical model, based on mate rial and energy balances, additionally utilizes the secondary process measurements resulting in improved controller performance. As the stea dy state is approached, a neighboring optimal control signal is superi mposed on the output from the supervisory level controller to eliminat e fast disturbances entering the column. In the hierarchical control s tructure, the adaptive online optimizer resides in the PC/AT which is networked with PC/XTs for neighboring optimal and cascade controls. A PID type controller with periodic update of set points and a manual st artup have been compared with the present algorithm. Experimental resu lts showed a significant reduction in time as well as cost in the case of the proposed scheme.