Study On Phase Equilibrium And Gas-Liquid Mass Transfer In Ethylene Slurry Polymerization System

High-Density-Polyethylene(HDPE) is a kind of nonpolar engineering thermoplastic with high crystallinity. Due to its excellent corrosion resistance, waterproofness as well as insulation, it has a potential value for commercial application. Slurry polymerization is a common process to produce HDPE. With the increasing demand and price of HDPE, it is essential to expand the capacity of existing plants. During the plants amplification design, phase equilibrium data and gas-liquid mass transfer data of slurry polymerization system are both significant parameters. In the process of polymerization and separation, gas solubility in solvent, gas-liquid mass transfer rate and polymer solubility in solvent are all basic problems. According to the excess existence of low molecular weight polyethylene (oligomer) in the previous reconstruction by using slurry external circulation, this thesis focused on oligomer effect and studied on the phase equilibrium and gas-liquid mass transfer of slurry polymerization system through experiment and theoretical analysis. The innovative results can be summarized as follows:(1) Based on the principle that the transmitted light intensity varied with the solution composition, the cloud points of oligomer and n-hexane system with different composition have been measured under pressure by self-designed laser apparatus. The experiment reults showed that the solubility of oligomer in n-heane increased with temperature and had two different trends. At low concentration, the solubilty changed little with temperature while it changed large at a critical concentration. Crystallized polymer solubility model was applied to correlating with experiment data and conducting sensitivity analysis. It was found that the solubility of oligomer in n-hexane was insensitive to small fluctucation of pressure, but sensitive to the melting temperature and crystallinity.(2) Based on the principle of gas isovolumic absorption, the gas solubilities of two components system(ethylene-n-hexane, propylene-n-hexane, hydrogen-n-hexane, nitrogen-n-hexane) as well as three compnets system(ethylene-n-hexane-oligomer) have been measured by transient gas physical absorption technique. The CHAO-SEA property method and PC-SAFT EOS were applied to calculate the equilibrium data for the two or threee components system respectively. The average absolute relative deviation(AARD) between experiment data and calculated value were 7.25%, 9.04% for two and three components system respectively. The results showed that the solubilities of ethylene, propylene, hydrogen and nitrogen increased with pressure and obeyed Henry's Law under experimental condition. The solubilities of ethylene and propylene decreased with temperature while the solubilities of hydrogen and nitrogen were not obviously affected by temperature. Insoluble oligomer had no effect on the solubility of ethylene. However, the solubility decreased dramatically with increasing the concentration of oligomer solution.(3) Based on the principle of gas isovolumic absorption, the volumetric liquid-side mass transfer coefficients of propylene-n-hexane, ethylene-n-hexane, ethylene-n-hexane-oligomer have been measured. An experiential correlation was established to fit experiment data with ARD 1.14%. The results showed that the volumetric liquid-side mass transfer coefficient of ethylene and propylene slightly decreased with pressure and increased with temperature. Specially, the volumetric liquid-side mass transfer coefficent of ethylene decreased dramatically with increasing the concentration of oligomer solution.