| A good model of polymerization process is not only contribute to understand the kinetics mechanism, but also be the basis of reactor design and optimization. For its complexity of kinetics mechanism, simulation of polymerization process become a major branch of polymer reaction engineering.The plant of styrene polymerization studied is five reactors in series (DC205, DC201, DC202, DC203, DC204). The two tank reactors (DC205, DC201) for prepolymerization were simulated respectively with one CSTR. The three reactors for postpolymerization were tower reactors; every reactor was simulated with CSTR+PFR+CSTR+PFR+CSTR in series. Through adjusting the kinetics parameters with a set of industrial data, a model of styrene polyerization on plant scale was developed with Aspen software.Then we find that the simulated results were consistent with steady-state data of plant according to the following evidences: the difference of conversion of styrene was less than 4% and the difference of Mw (weight average molecular weight) of polystyrene was less than 5%, and the molecular weight distribution almost the same. Comparison between the model developed in this article and model built in Aspen indicated that the mechanism of styrene bulk polymerization involves disproportionation termination, combination termnation and gell effect. Moreover, simulated results were compared with unsteady-state data of plant,and the results suggested that the model simulate nosteady-state prcocess very well.The sensitivities of operation parameters (temperature (T), total input flux (Qtotai), weight fraction of styrene in feed flow (v), the ratio of side flow to main flow (Qside/Qtota)) were studied with tool of sensitivity analysis in Aspen. The fluence of Qside/Qtota was almost neglectable; the fluence to conversion of styrene was in following order: v> T > Qtoiai', the fluence to Mw was T > v > Qtotal; and the fluence to molecular weight distribution was T> Qtota\ > v.To maximize the yield of the plant and Mw, optimization of operation parameters was studied, the yield increased more than 10% with optimized operation parameters, in the case of Mw not being changed. The plant could produce polystyrene with Mw up to 320,000,when the conversion of styrene was no less than 60%.Moreover, the dynamics behaviors of the model had been studied with dynamic simulation tool (Dynamic Plus) in Aspen. When the temperature of DC205 increase 5K for two hours, the change of solid content and Mw at outlet of DC205 were very large, while DC204 had changed little; The total feed flux reduced to 80 percent for 1 hour, the influence to all reactors were very little; and the weight fraction of styrene changed from 0.8705 to 0.6 for 1 hour, the influences to the solid content at outlet of all reactors were neglectable, while Mw had changed a large amount, and the Mw at outlet of DC204 returned to steady-state after 17 hours.Some urgent accidents were also simulated during steady-state. When heat media of plant was broken off, polymerization in the top section of DC202 would be out of control after 3 hours; After the feed flow was changed to styrene for 4 hours, DC201 would be out of control; If the feed flow changed to recycled liquor, the plant would come to another steady-state 20 hours later, and the Mw at the outlet of DC204 was 160,000.Finally, an optimized policy of grade changeover on the scale of whole polymerization process had been put forward. The transition time might be shortened 58% as compared with that of practical policy, and the amount of off-specification ploystyrene might been shortened 23%. Increasing the profit could be expected using this policy we suggested. |
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