Experience on advanced multivariable control of a polyethylene plant

This paper describes the process control and monitoring systems and applications of a worldscale Polyethylene Plant producing bimodal polyethylene grades. The emphasis is on the advanced control applications of this plant. This Polyethylene Plant uses the proprietary Borealis Borstar PE technology and a general overview of the process is given. The automation level of this plant is high and a general structure of process computer, DCS, ESD and on-line analyzer systems is explained. The advanced multivariable control of the plant is performed using a process computer system which includes multivariable regulatory controls of reactant compositions and other relevant variables in the polymerization reactors. On-line process model is employed to predict relevant reactor variables. A closed-loop control of product properties such as density and melt flow rate is based on continuously updated process models, online process analyzers and data from the quality control laboratory. It is important to notice that even the results from laboratory analysis are used in closed-loop control without any manual intervention. Comprehensive recipe system is used to span all necessary catalysts and grades. The process computer system also includes real-time applications for performing continuous tracking of production batches and lots. This system is also completely automatic and utilizes the measurements and calculated variables from the plant. The system allows studies to be made about the processing conditions in the reactors and extruders during production of each product lot and recording raw material and catalyst consumption and batch numbers.