A heuristic approach to grade transition strategy of the HDPE slurry process in different operation modes

To satisfy fast changing market demands, polymer plants are forced to operate under frequent grade transition polices which produce a huge amount of off-specification product and waste the time of normal production. In the present study, a model of grade transition is presented for the industrial continuous high-density polyethylene (HDPE) slurry process, which includes two stirred reactors connected in either parallel mode or series mode. The objective of the study is to maximize the economic benefit and minimize the negative environmental impact of the process. Different objective functions are constructed according to the different operation modes. The trajectories of the operating variables can be optimized successfully with the desired polymer quality targets. A heuristic combined strategy is proposed to solve the unstable problem caused by mathematical optimization during the transition between parallel mode and series mode. The problem is solved from three aspects: temperature, operating variables, and material flow. The result indicates that the heuristic combined grade transition takes longer time, but is more feasible in the restriction of the manipulated variables and therefore more effective in controlling the instantaneous properties of the polymer produced.