Experimental and modeling study of ethylene polymerization in gas phase reactors: impact of thermodynamics – THERMOPOLY

Polyethylene is by far the most significant polymer worldwide in terms of volume of production, and demand for this product continues to rise. The size of the market and increasing demand means that producers need to increase capacity either by improving existing processes, or building newer, more efficient processes. For gas phase processes which represent nearly 50% of all production processes, fluidized bed reactors are the equipment of choice at the industrial scale, where the polymer is grown in the form of particles suspended in a flowing gas stream. One of the major operational issues associated with increasing capacity in gas phase reactors is the ability to remove the significant amount of heat produced during the reaction. On the one hand, one needs to avoid dangerous overheating of the reactor, and on the other hand it is necessary to have precise temperature control in order to maintain the quality of the polymer in terms of molecular weight and particle size, minimizing particle aggregation and sintering. A popular approach to control overheating is the so-called “condensed operating mode” where liquid species are injected together with the monomer feed. The temperature and composition of the feed are chosen so the feed is below its dew point, which in turn is below the reactor temperature. Upon entering the reactor the liquefied components vaporize and the latent heat of evaporation helps to cool the system. However, it has recently been demonstrated that the inert species most typically used for this purpose can strongly influence the solubility of all species in the growing polymer particles, and since they also act as plasticizers they can also impact the physical properties of the particles. Despite the wide-spread use of condensed mode cooling, very little is understood about the impact of the inerts on the reaction rate, molecular weight distribution, particle morphology and particle agglomeration.
The aim of this project is therefore to develop a fundamental understanding of the different phenomena observed during condensed mode cooling in ethylene polymerization, and translate this new knowledge into a sophisticated model able to predict the reactor performance. Accordingly, the project is organized into three subsequent tasks covering the different scales typical of the system, from the single particle to the reactor. Namely, the equilibrium partitioning of different species (monomer, comonomers, solvents, inerts) into polyethylene films and particles will be studied experimentally using different techniques. Then, such thermodynamic knowledge will be incorporated into a single-particle model accounting for the reaction and diffusion phenomena as well as the thermal behavior. Such model will be validated by comparison with experimental data obtained using specially designed spherical stirred bed reactors that are well adapted for gas phase polymerization. Finally, taking advantage of computational efficient methods, a comprehensive model of stirred-bed reactor will be developed, accounting for all phenomena affecting the particle size distribution, such as aggregation due to polymer softening related to the plasticization induced by additional species. The model will be validated by experimental data collected in the same type of stirred bed reactor mentioned above. Such detailed model is expected to become a key tool for the design of intrinsically safer, more efficient reaction modes while ensuring precise polymer quality control.