Study On Preparation Of Unsupported Metallocene Catalyst Microparticles Using Supercritical Fluid Technology

The RESS process and SAS process are innovative and promising technology to produce microparticles with narrow particle size distribution. The precipitation of fine unsupported metallocene catalyst (Cp2TiCl2) particles using CC>2 by the RESS process and SAS process respectively is investigated in this article.In spite of their numerous advantages, there are several practical problems to be solved in order to use metallocene catalysts widely in industry, such as the difficulty in controlling polymer morphology with soluble homogeneous catalysts, the inability to be used in continuous slurry and fluidized-bed gas-phase processes, significant reactor fouling, and the very large amount of MAO needed to achieve maximum metallocene catalytic activity.Although immobilizing metallocene complexes on a support can overcome these problems to a certain extent, the surface properties of supports strongly influence the behavior of its supporting catalysts, and immobilizing often depresses catalyst activities at some extent. Moreover, the supports influence the processing properties of polymer because of their containment in polymer as additional ash. If unsupported metallocene complex particles can be produced to catalyze the polymerization of olefins, the influence of supports on polymer's properties will be eliminated. It can be supposed that the unsupported fine metallocene catalyst particles will produce the polymers with narrow size distribution and good morphology due to the clone phenomena in size distribution and morphology of catalyst and its polymer.Firstly, the Cp2TiCl2 solubility in supercritical CO2 was measured using the static equilibrium method. The collected Cp2TiCl2 was dissolved in toluene and analyzed by the established colorimetric method. To enhance Cp2TiCl2 solubility in supercritical CO2, toluene was added to form multicomponent system as a cosolvent and its effect on Cp2TiCl2 solubility was investigated. Four models independent of solute thermodynamics properties, including two modified "compressed gas" models, i.e. P-R EOS model and R-K EOS model, and two empirical models, were used to correlate the experimental results. It was found that all these four models have good agreement with experimental data of Cp2TiCl2 solubility in supercriticalCO2 with AARD < 8%. R-K EOS model is best one with respect to AARD and the least parameters, and calculated values of parameters approximate actual value.Secondly, an RESS apparatus was set up with a normal nozzle which could avoid jamming. The RESS experiment for naphthalene-supercritical CO2 had been conducted to clarify the effect of various RESS conditions, such as nozzle structure, sample collecting distance, flow rate and the concentration of solution on the size and morphology of the formed particles. The size of particles increases with increasing the sample collecting distance until 50 mm. The process of crystal growth has strong effect on the particle size and is determinative factor with respect to the process of nucleation at large sample collecting distance. Furthermore, the size of particles collected at large sample collecting distance increases with increasing concentration of solution, whereas, that collected at small sample collecting distance decreases with increasing concentration of solution.The fluid flow in the nozzle was simulated numerically based on a one dimensional time-independent adiabatic flow model. The model considers the nozzle as taper capillary inlet and capillary and SC-CO2 flow as the pure carbon dioxide flow with wall friction in the capillary and isentropic flow in the taper capillary inlet was used to calculate temperature, pressure, density and flow velocity profiles along the expansion path. With the calculated pressure and temperature changes of the pure fluid along the expansion path, the supersaturation ratio profile of the solute in the real mixtures CCVnaphthalene was calculated using solubility model, which was used to calculate nucleation rate, the radius of nucleus and growth rate based on classi...