Optimization of water-cooled chiller in polyethylene plant

Abstract

This research was carried out to reduce the energy associated with process of cooling solvents for polymerization in Polyethylene (PE) plant. Cooling of cyclohexane to 10oC in a PE plant increased power consumption to 413.3kW and the optimization reduced the power compressor power input by 32.1 percent. Optimization was carried out with the use of Aspen HYSYS version 11.0, where Peng Robinson was used as the fluid package. To optimize, double stage centrifugal compressor was used. Optimum compression ratio of 1.73 and a corresponding intermediate pressure of 800kPa were obtained. A case study was made using variable First stage discharge pressure to determine compression ratio at each stage. A plot of compression ratio against first stage discharge pressure for each stage was used to determine the optimum compressor ratio and intermediate pressure. Another case study to determine the total compressor input at variable compression ratio was done and a plot of total compression power input against the compression ratio at each stage revealed that the total power input of the chiller was 280.5kW. The optimized chiller has an actual coefficient of performance (COP) of 6.24 and a theoretical COP of 8.64 while the efficiency of the chiller was 72.2%. The optimized chiller reduced the energy cost per day from 257.9USD to 154.8USD. The heat duty of the evaporator and condenser were 8377000kJ/h and 9865000kJ/h respectively while the flow coefficient of the J-T valve was 51.57gal/min. The power consumption of the first stage was 121.8kW while the power consumption at the second stage was 158.7kW. The compressor discharge was 1300kPa and 56.89oC. R134a was used as the refrigerant.