Gas-liquid mass transfer in a slurry reactor operating under olefinic polymerization process conditions

The equilibrium solubilities, C*, and the volumetric liquid-side mass transfer coefficients, k(L)a, of propylene, ethylene and hydrogen in liquid n-hexane containing up to 30 wt% solid polypropylene powder were obtained in a 4-l agitated batch reactor operated in surface-aeration mode. The data were collected under pressures between 2 and 55 bar, temperatures from 313 to 353 K, mixing speeds from 13.3 to 20.0 Hz and solid concentrations between 0 and 30 wt%. The gas solubilities were calculated using a modified Peng-Robinson equation of state (PR-EOS) and the mass transfer coefficients were determined using the transient physical gas absorption technique. The solubilities of hydrogen, ethylene and propylene in n-hexane were found to obey Henry's law and the values were not affected by the presence of solids. The gas with the closest solubility parameter to that of liquid n-hexane appeared to have the highest solubility. As expected, the mass transfer coefficients of the three gases in liquid n-hexane with and without solids increased with increasing mixing speed. The k(L)a values of hydrogen in n-hexane and slurries were found to slightly increase whereas those of ethylene and propylene slightly decrease with increasing the mean partial pressure of the gas component. The temperature appeared to have no effect on k,a values of hydrogen in n-hexane with and without solids while those bf ethylene and propylene were slightly increased with temperature. The k(L)a values for the three gases increased at low solid concentration (10 wt%) and decreased at high solid concentration (30 wt%). A dramatic decrease of k(L)a values for ethylene and propylene in liquid n-hexane was observed at particular operating conditions (T = 353 K, N = 13.3 Hz, W-s = 30 wt% and P-1,P-m greater than or equal to 5 bar). This behavior was attributed to the high slurry viscosity prevailed under these particular conditions. The k(L)a values of the three gases used in n-hexane were correlated with operating variables using empirical correlations.