Mass Transfer Characteristics in an Agitated Slurry Reactor Operating Under Fischer-Tropsch Conditions

摘要

In this study, the equilibrium solubility, C*, and the volumetric mass transfer coefficient, kLa, were measured for 4 different gases (He, CO, N2 and H2) in two different liquids (Poly-Alpha-Olefins, PAO-8 and C70 Sasol Wax) in a 1-liter agitated reactor operating in a Gas Inducing mode under the typical conditions of Fischer-Tropsch synthesis. The effect of operating variables, including pressure (0.7 - 3.5 MPa), temperature (423 - 523 K), mixing speed (13.3 - 20 Hz), and alumina powder (used as a support for cobalt catalyst) concentration (0 - 50 wt.%) on kLa and C* values were statistically investigated using the Central Composite Statistical design and analysis technique. The data were measured within wide ranges of operating condition covering the industrial range of this Fischer-Tropsch synthesis. The transient physical gas absorption technique was used to determine kLa under the operating conditions used.The C* values for the four gases in the two liquids used appeared to increase linearly with increasing pressure at constant temperature, i.e., obey Henry's Law over the pressure ranges studied. The C* values were found to decrease and then increase with increasing temperature and the effect of temperature on C* was described by an Arrhenius-type equation, where the apparent activation energy of absorption was a function of temperature. The solubility values for the four gases in the two liquids used followed the trend: C*CO>C*N2>C*H2>C*He and the values in Sasol wax were slightly higher than obtained those in PAO-8.The kLa values for the four gases in the two liquids were found to increase with increasing pressure, temperature and mixing speed. The kLa values, however, slightly decreased (20%) at solid concentrations less than 30 wt% and then dramatically decreased (80%) with increasing solid concentrations from 30 to 50 wt%. Under similar operating conditions, kLa values for the four gases were found to be higher in Sasol wax than in PAO-8. This behavior was related to the greater gas-liquid interfacial area created in the Sasol wax due to its lower surface tension and foamability when compared with those of the PAO-8. Statistical correlations were developed to predict the kLa values for the four gases in the two liquids with a regression coefficient (R2) greater than 97%. Also, an empirical correlation using dimensionless numbers was proposed to predict kLa values in gas induction slurry agitated reactors with a regression coefficient (R2) greater than 94%.