(628g) Modeling and Simulation of Slurry-Phase Catalytic Olefin Polymerization Industrial Loop Reactors: Prediction of Molecular, Morphological and Rheological Polymer Properties

Polyolefins are the most widely used plastics today due to their low production cost, reduced environmental impact, and wide range of applications (e.g., packaging, building and construction, transportation, electrics and electronics, furniture, etc.). It is believed that the degree of technological and scientific sophistication in relation to the polyolefin manufacturing has no equal among other synthetic polymer production processes. Polyolefins are commonly produced in low-pressure catalytic (e.g., Ziegler-Natta (Z-N), metallocenes, etc.) bulk, slurry and gas-phase reactors with current total world capacity exceeding 120 million tons per year. Polyethylene (i.e., HDPE, LDPE and LLDPE) and polypropylene cover 60 % and 40 % of the total polyolefins production, respectively. The annual worldwide polyolefins production growth in the next years is foreseen to be 4-6%, making polyolefin production technology a very active research area.

An industrial slurry-phase catalytic olefin polymerization process usually consists of two jacketed loop reactors in series. The reaction mixture (i.e., monomer(s), solvent, catalyst and polymer) flows through the reactor tube by means of an axial centrifugal pump preferably placed at the bottom of each loop reactor. The reactor's cross-section area is usually uniform and the reactor operates free of any obstruction that can interfere with the circulation of the reaction mixture. An ?O? shape or any similar arrangement (e.g., a vertical double loop) is the most commonly employed reactor design. The first reactor of the series is continuously fed with monomer (e.g., ethylene), co-monomer (e.g., 1-hexene), solvent (e.g., iso-butane, propane, n-pentane, i-pentane, neopentane and n-hexane).

During the polymerization, the polymer particles are gradually collected in the settling legs placed at the lower part of the reactor configuration. The settling legs periodically open to transfer the concentrated slurry (i.e., a highly solids concentration mixture consisting of settled polymer particles, a minor proportion of slurry, sorbed monomer(s) and solvent) produced in the first reactor to the second reactor of the series. In the second reactor of the series, the polymerization may take place under different operating conditions (i.e., pressure, temperature, co-monomer and chain transfer agent concentrations) from the first reactor and is continuously fed with fresh monomer(s), solvent and transfer agent. After the concentrated slurry (i.e., fluff) is removed from the reactor, the polymer particles are separated from the unreacted monomer(s) and the solvent by hot flashing. The solvent is completely recovered due to the high monomer(s) conversion (i.e., 95%-98%) while there is no need for monomer(s) recovery. Finally, the polymer product is dried and pelletized.

In the slurry-phase loop reactor, two phases co-exist, namely, a liquid phase (i.e., consisting of solvent, monomer, co-monomer and transfer agent) and a solid-polymer phase (i.e., consisting of polymer and sorbed quantities of solvent, monomer, co-monomer and chain transfer agent). The fluid circulation pump is designed to provide high flow velocities (e.g., 5-7 m/s) and a very intensive and well-defined mixing pattern of the reaction mixture. Moreover, the resulting turbulent flow provides high heat transfer rates between the water in the jacket coolant and the reaction mixture and significantly lowers reactor fouling caused by particle deposition on the reactor wall.

The process typically produces polyolefin grades with broad or/and bimodal MWD. Typically, polyolefins of high molecular weight and low density are produced in the first loop reactor due to the low concentrations of the chain transfer agent and high concentrations of the comonomer. On the other hand, in the second loop reactor of the series, high concentrations of the chain transfer agent as well as low concentrations of the comonomer result in the production of polymers of low molecular weight and high density. The industrial, slurry-phase cascade loop-reactor series typically operate at temperatures of 70-120^oC and pressures of 30-90 bars. The solids concentration is approximately 45 % w/w.

In the present study, a multi-scale, multi-phase, dynamic model is developed for the determination of the distributed properties (i.e., particle size distribution (PSD), molecular weight distribution (MWD)) and rheological properties of polyolefins produced in an industrial catalytic slurry olefin polymerization cascade loop-reactor series.

The multi-scale description of each loop includes models at four different length and time scales, namely, a kinetic model, a single particle model, a population balance model and a multi-phase reactor model. The reaction medium comprises two phases, namely, a liquid phase (i.e., consisting of solvent, monomer, co-monomer and transfer agent) and a solid-polymer phase (i.e., consisting of polymer and sorbed quantities of solvent, monomer, co-monomer and chain transfer agent). The Sanchez-Lacombe equation of state (S-L EOS) is utilized to calculate the thermodynamic equilibrium concentrations of the various molecular species (i.e., monomer(s) solvent, hydrogen, etc.) in the two phases.

At the molecular level, a multi-site catalytic kinetic model is employed to describe the copolymerization of ethylene with 1-hexene over a heterogeneous Ziegler-Natta (Z-N) catalyst. To calculate the particle growth and the spatial monomer and temperature profiles in a growing particle, the random pore polymeric flow model (RPPFM) is utilized. The RPPFM is solved together with a dynamic particle population balance model, using the postulated multi-site Z-N kinetic scheme, to calculate the dynamic evolution of PSD and MWD in each loop of the cascade reactor configuration.

The operation of the settling legs is modeled as a non-ideal semi-batch reactor. The product withdrawal rate from each loop, through the settling legs, depends on the reactor pressure. To calculate the non-continuous product removal rate as well as the polymer concentration in the outflow stream a comprehensive model for the operation of settling legs is employed. In addition, overall dynamic mass and energy balances for the two loop reactors are derived to calculate the dynamic evolution of the concentrations of the various molecular species as well as the temperature profiles and coolant requirements. Finally, a rheological model based on the reptation and Rouse relaxation theories is employed to calculate the rheological behavior (i.e., melt viscosity versus frequency) of polyolefins in terms of the calculated MWDs.

Numerical simulations are carried out to investigate the effects of the reactor operating conditions (i.e., reactor temperature and pressure, inflow rates, feed composition, etc.) on the dynamic behaviour of the cascade loop-reactors (i.e., start-up, grade transitions) in terms of the reactor productivity and molecular, morphological and rheological properties of the polymer.