Separation of reacting ethylene-polyethylene mixtures in the industrial production of polyethylene at high pressures is of theoretical and commercial interest. The composition of the phases formed in the reactor and in the separator is highly informative for the manufacture of low density polyethylene. The industrial synthesis of low density polyethylene is carried out in a reactor under high pressure (1000-3000 bar) and at relatively high temperature (150 -300"C).The polyethylene-ethylenemixturein the reactor with stirrer may consist of one or two phases, depending on the molecular mass distribution of the polymer and on the reactor temperature and pressure. At the beginning of polymerization only one phase is present. However, on lowering the pressure and the temperature, two phases appear, i.e. apolymerrich (heavy)phaseand a polymer poor (light) phase. The heavy phase is separated from the reaction mixture when the critical concentration of polymer has been reached. The polyethylene-ethylene mixture leaving the reactor contains approximately 84% of unreacted ethylene, which must be separated from the product by decompression, freed from low molecular mass polyethylene products, and recycled to the process. To reduce the quantity of ethylene retained by the product and the amount of polyethylene in the ethylene rich phase, the pressure and temperature must also be reduced in the separator. Under such conditions, however, significant energy would be required to recompress the purified ethylene on recycling. Thus an optimal operating pressure and temperature have to be specified. It is the purpose of the present study to predict the phase equilibrium of the polyethylene-ethylene mixture under conditions corresponding to the reaction and the separation steps of the industrial production of low density polyethylene.
Calculational ProcedureIn the present study the computational algorithm of Bonner, Maloney and Prausnitz [I] was used to calculate the phaseequilibrium between polyethylene-rich and polyethylene-poor phase, and the corresponding molecular mass distribution. The basic model for phase equilibrium has been presented elsewhere [l -31. Some remarks concerning the calculational procedure, stated previously [4], will be briefly presented and discussed. The calculation requires solution of the following problems : -determination ofthephaseconditionofthesystem, with respect to -the multicomponent character of polymer systems must be taken Difliculties Concerning the determination of phase condition consist primarily in thedefinition ofthestate ofethylene. Some authorsareof the opinion that compressed ethylenecan be treated as aliquid [5,6] in the part of the process considered, others regard it as a gas [7]. The second problem, consisting in the proper choice of thermodynamic theory of the polymer solution, is difficult since conditions are far from ideal. The freevolume theory 181 ischosen,since it is based on present-day knowledge of the liquid state. Finally the multicomponent character of polymer systems is ...
