HYDRODYNAMIC AND MASS TRANSFER PARAMETERS IN LARGE-SCALE SLURRY BUBBLE COLUMN REACTORS (2005)

The design, modeling, optimization and scaleup of slurry bubble column reactors (SBCRs) require, among others, the knowledge of the kinetics, hydrodynamics, and mass as well as heat transfer characteristics in larger-scale reactors, operating under typical industrial conditions. In this study, the hydrodynamic (gas holdup, ƒÕG, bubble size distribution, dB, and the Sauter-mean bubble diameter, d32), gas solubility (C*) and mass parameters (gas-liquid interfacial area, a, and volumetric liquid-side mass transfer coefficient, kLa) were measured for various gases (H2, CO, N2, CH4 and He) in an organic liquid (Isopar-M) in the absence and presence of two different solids (glass beads and alumina powder) in two large-scale SBCRs. The data for the five gases were obtained in a cold SBCR (0.301 m ID) under wide ranges pressures (P = 1-8 bar), temperatures (T = 293-305 K), superficial gas velocities (UG = 0.08-0.20 m/s), and solid concentrations (CV = 0-36 vol.%); and the data for He and N2 were obtained in a hot SBCR (0.29 m ID) under wide ranges pressures (P = 7-25 bar), temperatures (T = 323-453 K), superficial gas velocities (UG = 0.08-0.30 m/s), and solid concentrations (CV =0-20 vol.%). All the experiments and the operating ranges were selected following the Central Composite Statistical Design (CCSD) approach. The experimental data obtained showed that the gas holdup and volumetric liquid-side mass transfer coefficients increased with pressure due to the increase of small gas bubbles holdup; increased with superficial gas velocity due to the increase of the gas momentum; and significantly decreased with solid concentration due to a reduction of small gas bubble population. The gas holdup and volumetric liquid-side mass transfer coefficients were found to increase with temperature due to the decrease of the Sauter mean bubble diameter and increase of the mass transfer coefficient (kL). The gas holdup, however, was found to decrease with temperature when the solid concentration was greater or equal 15 vol.% due to the reduction of froth stability under such conditions.Empirical and back propagation neural network (BPNN) models were developed to correlate the hydrodynamic and mass transfer parameters in BCRs and SBCRs obtained in our laboratory and those from the literature. The developed models were then used to predict the effects of pressure, superficial gas velocity, temperature and catalyst loading on the total syngas holdup and mass transfer coefficients for the Low-Temperature Fischer-Tropsch (LTFT) synthesis carried out in a 5 m ID SBCR with iron oxides and cobalt-based catalysts. The predicted total syngas holdup and mass transfer coefficients appeared to increase with reactor pressure, superficial gas velocity and the number of orifices in the gas sparger. The predicted values, however, were found to decrease with catalyst loading and reactor temperature. Also, under similar LTFT operating conditions (P = 30 bar, T = 513 K, CW = 30 and 50 wt%), the total syngas holdup and mass transfer coefficients predicted for H2/CO ratio of 2:1 with cobalt-based catalyst were consistently lower than those obtained for H2/CO ratio of 1:1 with iron oxide catalyst in the superficial gas velocity range from 0.005 to 0.4 m/s.