Controller optimisation of a tube heat exchanger

This study seeks to improve the process control of an ammonia/steam heat exchanger by optimising the existing PID controllers. The exit ammonia temperature of the heat exchanger is maintained by the combination of three PID controllers, an ammonia temperature controller, a pressure controller and a steam pressure controller. Originally, the pressure and temperature controllers were de-tuned to prevent interaction, which results in below optimal control. Due to the hazardous materials and operating conditions, we first develop the mathematical models for the heat exchanger and steam. Then simulations are carried out to develop control schemes. Finally a suitable optimisation schemes that could be applied to the existing control hardware, a 'feed forward' with 'dynamic decoupling' strategy was proposed. The control scheme was then programmed into the DCS and plant trials performed to fine tune the control schemes. The system now does not require any operator intervention after starting as it quickly reaches and maintains setpoint without any nuisance alarms.