Experiment And Modeling Research Of Novel Circulating Reactor In Ethylene Polymerization

Bimodal polyethylene is a kind of engineering thermoplastic with extensively expanded properties. It is now mainly produced by a two-step or multi-step sequential polymerization process, in which the first step is ethylene polymerization and the second step is ethylene-hydrogen copolymerization. The problem with two-step process is that the polymer has low homogeneity and lead to the formation of fish-eyes in the production of films. Meanwhile, if the reacting gas can not be isolated and flows to next reactor, it will influence the polymerization in the next reactor, and the molecular weight distribution of the final product will become lower.First of all, in this article, three new reactors are designed and three patents have been applied, which are separating and recondensing reactor, multi-temperature reacting zone reactor and multizone circulating reactor. Calculations have been made to figure out the size and operating parameters of the separating and recondensing reactor and multi-temperature reacting zone reactor. Besides, the principle of selecting technological conditions is established according to the actual production data.Secondly, the multizone circulating reactor has been researched deeply. The fluid hydrodynamics of multizone circulating reactor is investigated. The influences of different superficial gas velocity and static bed height on the pressure distribution, circulating solid flow rate and the effect of gas barrier in both the bubbling bed and the riser have been investigated. The main achievements are as follows:(1) Besides the gas distributor, the pressure drop in reactor is mainly affected by the solid particle concentration. (2) Both the increase of superficial gas velocity and the polymer mass quantity in reactor will cause the increase of pressure drop. (3) Pressure drops faster in the bubbling bed than in the riser. (4) Employment of elutriation in this design successfully realizes the solid material transportation among the reacting zones. (5) Based on single-channel AE measurement, a regression model of particle mass flowrate in high-speed fluidization is built up by the use of wavelet packet transformation and partial least square(PLS) method. Results show that PLS method based on multi-scale analysis of A E signals is feasible for the detection of particle mass flowrate in high-speed fluidization. (6) Gas chromatography is used to detect the concentration of tracer gas CO2 both in the location of before and after the feeding position of barrier gas. Results show that the adoption of barrier gas can effectively create two different reacting conditions, making it possible to produce bimodal polyethylene.Last but not the least, based on the kinetic mechanism of polyethylene polymerization and the multigrain model, a mathematic model of gas phase polyethylene polymerization in multizone circulating reactor is presented. This model can be used in both the dynamic state and the steady state of the reactor. It can predict the important parameters of product, such as molecular weight and molecular weight distribution, particle size distribution and the reactor production, and so on. After the simulation of the reactor, there are some conclusions as follows:(1) The molecular weight, exponent index and particle size can reach their balance soon, but the production of the reactor causes a long time, which is 4 hours in the simulation. (2) Monomer concentration has the greatest impact on the molecular weight and the molecular weight distribution of product. (3) Concentration of hydrogen takes an obvious effect on the low hydrogen reacting zone, which means it's important to control the hydrogen concentration in this zone strictly. (4) The increase of recycle rate R, will lower the product molecular weight, which goes against of the production of bimodal PE. (5) Catalysts with high activation are more suitable for this reactor.