Modeling And Optimization Of Physical Properties In Ethylene Slurry Polymerization

With the intense market competition and the various demands of polyolefin,the polymer grades have an excess of low-end and a shortage of high-end.It is important to study continuous production of different grades polyolefin switchover.To achieve the optimization of rapid polyolefin grade transition,it is a good way to solve the problem equation oriented,in which the polymer thermodynamic model is focused on.PC-SAFT(Perturbed-Chain Statistical Associating Fluid Theory)equation of state is a suitable physical property model for ethylene slurry polymerization system,but PC-SAFT equation of state has numerous and complex formulas and its iterative calculation is based on sequential method.Contributions of this thesis include the following four aspects.1.The equation oriented mode of PC-SAFT EOS is presented and coded on Matlab.Physical properties of ethylene,hydrogen,two-component system of ethylene and hydrogen and multi-component system containing polymer are calculated and compared with that obtained from Aspen Plus.It is shown that the results are in great agreement with Aspen Plus.Moreover,less convergence steps are used in equation oriented mode than that in sequential modular mode.2.Design an optimization problem of the reactor’s recovery stream and solve it with equation oriented mode on Matlab and sequential modular on Aspen Plus,respectively.The accuracy of both case are similar,while the number of steps and the convergence time are better in equation oriented mode.3.In order to embed the equation oriented program of PC-SAFT into the process model of the ethylene polymerization,the PC-SAFT simultaneous model is established on AMPL(A Mathematical Programming Language),and appears the situation of multiple solutions.After introducing three constraints including basic physical constraint,additional average segment constraint and additional yield constraint,the problem is solved.Meanwhile,the reactor recovery stream is optimized with the optimization model on AMPL,resulting that the model can get the optimum solution with adverse conditions.4.The EOS program is finally embedded into the ethylene slurry polymerization process on AMPL to improve the problem totally equation oriented.Single-reactor process without recycle or tear and single-reactor process with recycle are simulated.The result shows that simulation of single-reactor process with recycle can converge while Aspen Plus can’t.Meanwhile,simulation of the slurry polymerization of ethylene with a wide range of variable condition can converged in equation oriented mode well and efficient.