Design of plantwide control structure of double effect water/lithium bromide absorption chiller

Alternative control structures for a parallel double effect aqueous lithium bromide absorption chiller, operated in two modes of operation, has been designed using Skogastad’s plantwide control procedure. In on-demand mode, two complete decentralized control structures with proportional integral (PI) controllers and one structure with a two inputs – two outputs model predictive controller (MPC) were proposed. Their performance was evaluated using MATLAB-Simulink dynamic simulation. The results varied depended on type of disturbance, but overall, the structure with MPC controller provided slightly better results with less liquid holdup changes and faster setting time in average. For on-supply mode, two self-optimizing control variables obtained from minimum singular value (MSV) and null space method were implemented to a decentralized control structure. In general, both variables were able to reduce majority of loss due to change in optimal process conditions. Nevertheless, the null space control variable provided slightly better results for main disturbances, which are cooling utility temperature and amount of heat input.