Completed Research
Mass Transfer Characteristics in a Slurry Agitated Reactor with Organic Liquid Mixtures under High Pressures and Temperatures
Abdul Karim Ali Alghamdi, MS, 2001
The proper design, modeling and scaleup of Fischer-Tropsch synthesis require, among others, precise knowledge of the factors controlling the overall reaction, including the reaction kinetics and/or gas-liquid mass transfer characteristics. There are several kinetic data available in the literature, however, only few studies on the gas-liquid mass transfer in Fischer-Tropsch products under typical industrial conditions can be found.
The main objective of this study is to obtain the volumetric mass transfer coefficient (kLa) and equilibrium solubility (C*) for gaseous N2, He, H2, and CO in liquid Isopar-M, which is a mixture of i-decanes, representing F-T medium fraction, and N2 in 8 cSt Polyalphaolefin (PAO8), which is a diesel fraction of F-T products in the absence and presence of solid Alumina powder, a typical support for F-T catalyst. These mass transfer characteristics were obtained in wide ranges of temperature (298-473 K), pressure (7-35 bar), mixing speed (800-1200 rpm), and solid concentration (0-50 wt%) in a one-liter agitated reactor operating in gas inducing mode.
The transient physical gas absorption technique was used to measure kLa values for the gases in the two liquids. The Alumina particles were analyzed using Scanning Electron Microscope (SEM). The Sauter mean diameter of the particles was calculated using BioScan Optimas software for various catalyst samples. The Central Composite Statistical Design (CCSD) was employed to investigate the effects of process variables (pressure, temperature, mixing speed, catalyst concentration) on kLa and C* and the data were statistically correlated.
The C* values for the four gases in the two organic liquid mixtures were found to increase with pressure at constant temperature and the data were modeled using Henry's Law. The C* values for N2, He, and CO in Isopar-M and N2 in PAO8, however, decreased with temperature and at a certain turn-around-point, which was dependent on the gas type used, they appeared to increase with temperature. The effect of temperature on the Henry's law constant for the gases in both liquids was modeled using an Arrhenius-type equation. The solubility of gases in Isopar-M followed the trend C*CO > C*N2 > C*H2 > C*He. Also, the C* values of N2 in Isopar-M were greater than hose in PAO8 under similar pressures and temperatures.
The kLa values for the gases in Isopar-M and PAO8 were found to significantly increase with mixing speed and the kLa values appeared to increase with temperature and slightly increase with pressure and level off at high pressure values. For the Isopar-M liquid, there was a slight decrease of kLa for the gases at the highest pressure of 35 bar. The kLa values for the gases showed an increasing trend with catalyst concentration up to 12.5 wt%, then exhibited a gradual decrease from 12.5 wt% to 37.5 wt%, followed by a significant decrease at 50 wt%. It was observed that at 50 wt%, kLa values were almost the same regardless of the working pressure under the same temperature and mixing speed. The kLa values for N2 were found to be the same as those of the equivalent diffusivity CO and kLa values for He and H2 in Isopar-M were also practically the same under similar operating conditions. The behavior of kLa values for the gases in Isopar-M followed their diffusivities as (kLa)H2 >> (kLa)He > (kLa)CO >> (kLa)N2; and kLa data for N2 in Isopar-M were greater than those obtained in PAO8, having higher viscosity.