Mathematical Simulation Of High-Pressure Ethylene Polymerization Process In Tubular Reactor

High-pressure low-density polyethylene (LDPE) is the earliest PE material that achieved its commercial production among the three major polyethylene products. Because of its superior transparency and processability, LDPE is mainly used for film products, which is warmly welcomed in the PE market. Tubular reactor is the most popular process in the production of LDPE because of its simple equipment structure and large capacity. High-pressure ethylene polymerization plant is operated under rigorous conditions of high pressure and temperature, which implies that the production process of LDPE is of high danger. However, there are only very limited supervision and monitoring devices in the LDPE industrial plant, therefore lots of distribution information including conversion, molecular weight and its distribution, phase equilibrium condition, etc which has significant influence on the safe production and product quality can not be obtained. Therefore, by mathematical simulation, this thesis did research on the modeling of high pressure ethylene polymerization process, which mainly includes the following three aspects:(1) A mathematical model of an industrial high-pressure ethylene polymerization tubular reactor was established, including conclusion of kinetic model of free-radical polymerization initiated by peroxide and detailed calculation of physical and transport properties of ethylene and LDPE. By solving this mathematical model the distribution of some important variables along the reactor length were obtained, including temperature of processing stream, reactants'and initiators'concentration, conversion, number average molecular weight, weight average molecular weight and melting index of LDPE. In addition, temperature distribution, conversion and melting index at reactor outlet were compared with industrial data. It shows that the values of these three variables obtained by this model are in good accordance with industrial values, proving the accuracy of this model.(2) Monte Carlo stochastic numerical method is adopted to simulate the molecular weight distribution (MWD) in the LDPE tubular reactor. The influence of different initiator, chain transfer agent concentration and chain transfer to dead chain reaction on molecular weight and long chain branching distribution has been investigated. Different initiator can form the LDPE with different shape of molecular weight distribution curve; propylene which served as chain transfer agent only has influence on the position of MWD curve but no influence on the shape of the curve; the long chain branch distribution is mainly influenced by the chain transfer to dead chain reaction; by comparison of simulation result and GPC data, the accuracy of Monte Carlo simulation method has been proved.(3) PC-SAFT equation of state model has been adopted to calculate the liquid-liquid phase equilibrium of ethylene-polyethylene system. By combining POLYMIX algorithm, the phase equilibrium condition of polydispersed LDPE and ethylene system in industrial plant is calculated. Liquid-liquid phase equilibrium of polymer and solvent of small molecular is mainly influenced by temperature and the molecular weight of polymer; binary interaction parameter in PC-SAFT model also has significant influence on the accuracy of simulation results. By adopting POLYMIX algorithm, the complexity of multi-component phase equilibrium problem has been greatly reduced. According to the simulation results of phase equilibrium condition in industrial tubular reactor, it has been found out that the phase separation tends to occur in the near-wall region and at the end section of reactor, phase separation can also happens when producing product grade of large molecular weight.(4) A supervision strategy aimed at fouling prevention and alarming is proposed by combining reactor simulation, molecular weight distribution calculation and phase-equilibrium calculation, which laid the foundation for the establishment of an on-line fouling prevention software package used in LDPE plant with tubular reactor.