Modeling And Emulational Optimization Of Dual-loop Polypropylene Process

There are mainly two polypropylene manufacturing processes, liquid-bulk polymerizationand gas-phase polymerization. The liquid-bulk polymerization in loop reactors accounting for35%of all polypropylene production capacity is currently the most widely used process forproducing polypropylene. Operation cycle time of the loop-reactor-liquid-bulk polymerizationcan be up to thousands of hours. In the practical manufacturing process, all materials aresealed and in the fluid state except productions. Establishing a whole process model whichnot only reflects the mechanism of polymerization reaction but also represents actualoperating conditions of devices is helpful to understand the inner law of operating devices, tooptimize devices and processes and to increase economic benefits.The object of this study is the double-loop-reactor polypropylene polymerization device,for which a reasonable simplification of the process is made. The models of operation unitswhich include catalyst preparation, reaction, flash separator, propylene monomer recovery aremade by Aspen Plus. Samples of four grades produced by the study device are collected toconduct high temperature gel permeation chromatography (HT-GPC) measurements and dataanalysis to determine the number of active sites on the model catalyst and polymer yield fromeach active site. Suitable model is screened out from polymerization equations embedded inAspen Plus software to simulate seven involved reaction sets, including catalyst activation,chain initiation, chain growth, chain transfer, catalyst deactivation, and random growth ofchain. Furthermore, according to process conditions of the study device and analysis results ofthe product, combined with reference data, the optimal model is altered to fit kineticparameters for the single-grade-single-site reaction, the multi-grade-single-site reaction, andthe multi-grade-multi-site reaction. Eventually a steady-state simulation model is established.The steady-state model is verified and applied to simulate the process, and the resultsagree well with the production data, which means the model can be used as a guide foroperating and optimizing the device. Sensitivity analysis tools in Aspen Plus are used toinvestigate the influences of flow changes of several components, such as hydrogen andelectron donor, on the production load and indicators. Optimization schemes includingadjustment of the density of reactors, reduction of the content of propane in the flow ofpropylene in reactors are proposed, resulting in reducing the unit consumption of three agents(catalyst, additive assistant and process assistant) and improving the loading of reactors.