Studies On Mixing Characteristics Of Polymerization Reactors For Ethylene-tetrafluoroethyl Continuous Copolymerization And Vinyl Chloride Suspension Polymerization

Polymerization reactor is the key equipment in the production of polymers, which has significant effects on the production efficiency, product quality and production cost. The polymer production has been orientated to polymerization using large-scaled or continuous reactors in order to improve the quality stability of products. In connecting with developing of continuous copolymerization technology of ethylene-tetrafluoroethylene (E-TFE), and improving of the mixing of suspension polymerization of vinyl chloride (VC), the experimental and simulation on the mixing characteristics of polymerization reactors were carried on in this thesis. It would provide the basis to the design of continuous reactors and the structural improvement of VC suspension polymerization rectors.Firstly, a cold-flow model device with on-line mixing measuring system was established, to investigate the mixing characteristics of E-TFE copolymerization reactor, and to choose the appropriate propeller. The study on the rheological properties of the ETFE dispersion showed that it was pseudoplastic fluid. The consistency coefficients (K) of ETFE dispersions were increased with the increase of solid content, while the flow indexes (n) remained the same. CMC aqueous solution was chosen as the simulation material due to its pseudoplastic properties similar to ETFE dispersion. After comparing the mixing effect of four different propellers, a wide-blade double ribbon impeller was determined to use in the batch E-TFE copolymerization. The copolymerization process was stable and the obtained ETFE had good particle features.Secondly, the double ribbon impeller was applied in the reactor under continuos processing based on the resuls obtained from the batch experiments. The influences of the rotation direction and rate, feeding rate, viscosity on the torque, lag-time, peak-time, residence time distribution (RTD), the number of equivalent full-mixed reactors were studied. The RTD of the reactor with the double ribbon impeller was similar to a CSTR except a long tail. The feeding rate and viscosity of fluid had no significant effect on the mixing characteristics, and the average residence time was decreased with the increase of feeding rate. An axis channel-flow was observed under the reverse rotation direction. Thus, a flow pattern model of the stirring tank was proposed based on the experimental result:the mixing mostly took place in the blade region, which could be regarded as CSTR; the other area could be regarded as PFR because of its function of circulating.The software of Fluent 3.36 was employed to simulate the mixing and flow field of double ribbon impeller stirring tank under continuous processing. The grid processing was done by Gambit 2.3.16. The simplified blade moved upwards when the impeller rotated and inlet and outlet flow was set at 0.0133m3/s. The mixing condition of CMC solution was simulated. Tthe simulated stirring power and experimental value met well. Also the relation between rotation speed and radial flow was derived, which agreed to the conclusion from the cold-flow model experiment. By changing the rheological index of the simulated input feed, the Metzner constant of the stirring reactor, ks, could be determined to be 12 while the critical Reynolds number in laminar flow region to be 80. If the rheological properties of the product suspension was introduced into the numerical simulation, then the mixing effect of the real feed in stirring tank with double ribbon impeller could also be derived. Compared to CMC solution, the real condition had a stronger disturbance at the upper part of the tank, and radial flow decreased with the rise of rotation speed. Finally, the optimization of a 105m3 reactor of suspension polymerization of vinyl chloride was also discussed. The main problems were the overuse of dispersant and wide distribution of particle size. As a solution to that, the plan of using upturned blades with stronger circulating flow and adding baffles was raised. The research was carried out in a mid-scale cold-flow model. Two impeller pattern and three baffle conditions were involved and relations of Np, Nqc and Ne with Nrc were derived, as well as the correlation of Np, Nqc, Ne and the diameter(d) and width(b) of the propellers. The results showed that adding a couple of short asymmetric triangular baffles or a couple of cylinder baffles could improve the circulation condition and meet the Nc requirements with less Pv. A four-flat-blade turbine(d=1.68m, b=0.36m) rotated at a speed of 3.54r/s with baffle I was determined to be the optimum combination.