Control of Heat Integrated Distillation Employed by Cryogenic Air Separation Using Decentralized Simple PID Controllers

Abstract

Coupling cryogenic air separation plant to industrial processes imposes demand change on the air separation process. Therefore, control of cryogenic air separation plant is a must for stable operation. In this research, we introduce a control scheme for heat integrated distillation which is the main process of cryogenic air separation. The control is achieved via decentralized PID controllers, and its performance is investigated using numerical simulation. Sizing of distillation columns, control valves and heat exchanger were undertaken to simulate industrial air separation plant. In order to verify control system performance, five different control scenarios were studied, including switching between full load to part load, and switching between full production of oxygen to full production of nitrogen, which has not been reported in literature. The simulation results show satisfactory performance of control system facing the disturbing scenarios. However, in severe transition cases (i.e. transition from full liquid nitrogen to full liquid oxygen production), liquid level in the low pressure column base increased excessively and approached safe operating limits. This side effect requires care in controllers tuning, or even introducing level interlocks.