PDF MODELING OF TURBULENT-MIXING EFFECTS ON INITIATOR EFFICIENCY IN ATUBULAR LDPE REACTOR

The effect of turbulent mixing in the reaction zone of a tubular low-d ensity polyethylene reactor was studied by combining a Langrangian com position probability density function (LCPDF) code with a computationa l fluid-dynamics code. Because the LCPDF code can treat the chemical r eaction terms in a turbulent flow without resorting to moment closures , it is used to describe the temperature and scalar fields of reactant s including initiator and monomer molar concentrations, and the moment s of the molecular weight distribution. The chemical reaction terms ar e efficiently dealt with using a three-parameter chemical lookup table that contains the temperature and composition changes as functions of initiator and monomer concentrations and temperature over a small tim e step. The reaction-rate constants from the study of Lee and Marano ( 1979) are functions of temperature and pressure. The flow fields are o btained using the k - epsilon turbulence model. Because the temporal a nd spatial evolution of all fields in the reactor can be simulated, it is possible to study the effect of the initiator injection location, flow rate and temperature of the monomer and initiator feed streams on polymerization in considerable detail. Moreover, by observing the pro bability distribution of the composition fields, a better understandin g of hot-sot formation is achieved, leading to improved reactor design s.