Dvnamic Simulation And Optimization Of Slurry Polyethylene Process Based On Molecular Weight Distribution

Molecular weight and its distribution are extremely important quality indexes in the polymer production process, but the current level of technology and real-time measurement of molecular weight and its distribution cannot be achieved.The dynamic modeling and simulation based on the reaction mechanism are important method for implementing soft measurement. In this study, the polymerization conditions on the ethylene slurry polymerization plant (HDPE) and grade transition strategies for dynamic simulation and optimization are investigated based on the polymerization mechanism, with the molecular weight distribution curve as the target. The paper has achieved the following results:With polymer molecular scale quality index, i.e., the molecular weight and its distribution curve as the target, based on the polymerization mechanism and reaction kinetics with five active sites, the steady-state and dynamic models of the HDPE plant are established, while the analyzed data from industrial plant are used to validate the model. Simulation results of polyethylene molecular weight and its distribution and industrial analysis value are quite consistent. Dynamic model can reflect the dynamic response of the step change of cycle gas H2/C2H4 setpoint.With the polymer average molecular weight and its distribution curve as the target, the ratio of hydrogen to ethylene in recycle gas as the decision variables, steady-state optimization method is used to optimize the operating conditions of the polymer products. The results showed the large deviation between the simulation results with the average molecular weight as the optimizing target and those of plant analysis. Although the average molecular weight of the polymer can reach the desired polymer index, the maximum error between the molecular weight distribution curve from simulation and the desired polymer product can up to 0.12. However, the maximum error is only 0.02 with the molecular weight distribution curve as the target. Therefore, the optimization method with the molecular weight distribution curve as the target is significantly superior to the conventional optimization method with average molecular weight as the target.With the molecular weight distribution curve as the target, the dynamic optimization of grade transition strategy for the single reactor and two reactor series process of the HDPE production are studied. With the hydrogen feed rate as the control variables, the effects of dynamic optimization with different initial time are investigated on the grade transition time and the operating stability. The results showed that:for single-reactor HDPE process, when changing the molecular weight of the polymer from 25121.6 gm/mol to 20000 gm/mol, the range of the optimized grade transition time is from 8 hr to 15 hr, for HDPE process in series reactors, when changing the molecular weight of the polymer from 5239.75 gm/mol to 4500 gm/mol, the range of the optimized grade transition time is from 5 hr to 15 hr. But during the process of optimization, the initial time will affect the grade transition time and the fluctuation range of variables. The longer the initial time, the optimized hydrogen feed rate is closer to the steady-state optimization results, but the longer time required to grade transition. If the initial time is shorter, the smaller the grade transition time required for, but these control variables (hydrogen feed flow rate) as well as the ratio of hydrogen to ethylene in recycle gas, the reactor pressure all oscillate seriously. There is a large difference of hydrogen feed rate between the dynamic optimization and steady-state optimization. Generally speaking, the optimization results will be helpful to the actual production.The hydrogen oscillation operation is used to achieve the bimodal distribution of polyethylene for single-reactor HDPE process, and the influence of hydrogen concentration, oscillating operation period and the oscillation time distribution on polyethylene molecular weight distribution is studied. The results showed that:for the oscillation operation of the low hydrogen concentration, the content of the high molecular weight polymer is bigger, while molecular weight and its distribution index, the molar ratio of hydrogen to ethylene in the circulating gas flow and the reactor pressure all fluctuates smoothly. Changing the oscillation period can regulate the transition zone of the molecular weight distribution, polymer of medium molecular weight distribution can be produced by reducing the period of oscillation. When increasing the proportion of feeding time of the hydrogen, the content of low molecular weight part rises, while the content of high molecular weight portion decreases. Till the proportion of time occupied by the hydrogenation stage increase to a certain value, there may not be a bimodal distribution of the polymer. Therefore, through adjusting the operating conditions of oscillation, the single-reactor process can regulate the polyethylene molecular weight and its distribution.