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A multiphase-Eulerian, three-dimensional (3-D), computational fluid dynamics (CFD) model was built to investigate the local hydrodynamics of a pilot-scale (0.29 m ID, 3 m height) Slurry Bubble Column Reactor (SBCR). The model was first validated against the gas holdup radial profiles in an air-water-glass beads system obtained in a 0.254 m ID and 2.5 m height column under ambient conditions at various superficial gas velocities by Yu and Kim (Bubble characteristics in the radial direction of three-phase fluidized beds. AIChE Journal 34, 2069-2072, 1988). The model was next validated against the gas holdup radial profile data for N2-Drakeol-glass beads system obtained in a 0.44 m ID and 2.44 m height reactor, including internals, operating under ambient conditions at various superficial gas velocities by Chen et al. (Fluid dynamic parameters in bubble columns with internals. Chemical Engineering Science 54, 2187-2197, 1999). The model was also validated against experimental data obtained in our lab for N2-Fischer Tropsch (F-T) reactor wax-Fe catalyst system obtained in a pilot-scale, Slurry Bubble column Reactor, SBCR (0.29 m ID, 3 m height) under pressures and temperatures up to 25.9 bar and 490 K, respectively. These three validations led to the selection of the turbulence and interphase drag coefficient models, and the optimization of the solution method, mesh size and structure and the step size. Moreover, the inclusion of RNG k-ε turbulence model coupled with the Wen-Yu (Mechanics of Fluidization. Chemical Engineering Progress Symposium Series 62, 100-111, 1966) / Schiller-Naumann (A drag coefficient correlation. Zeitung Ver. Deutsch. Ing 77, 318-320, 1935) drag correlations, and the mass transfer coefficients were found to provide the most accurate predictions of the experimental data. The CFD model was then used to investigate local gas holdup, liquid recirculation, local turbulence intensities, bubble diameters, and solids distribution throughout our pilot-scale SBCR, operating under typical F-T process conditions. The model predictions showed strong liquid recirculation and backmixing near the walls of the reactor, and the solid-phase velocity vectors closely followed those of the liquid-phase. A relatively high liquid turbulence intensities were observed in the vicinity of the sparger upon startup, however, after reaching a steady state, the liquid turbulence intensities became more evenly distributed throughout the reactor. The liquid turbulence intensities were slightly higher near the center of the reactor, and closely resembled the velocity vectors. Also, the Sauter mean bubble diameters increased, whereas the solids distribution decreased with reactor height above the gas distributor. [ABSTRACT FROM AUTHOR]

This paper presents an extensive review of the kinetics, hydrodynamics, mass transfer, heat transfer and mathematical as well as computational fluid dynamics (CFD) modeling of Low-Temperature Tropsch Synthesis (LTFT) synthesis in Slurry Bubble Column Reactors (SBCRs), with the aim of identifying potential research and development areas in this particular field. The kinetic expressions developed for F-T synthesis over iron and cobalt catalysts along with the water gas shift (WGS) reactions are summarized and compared. The experimental data and empirical correlations to predict the hydrodynamics (gas holdup, Sauter mean bubble diameter, and bubble rise velocity), mass transfer coefficients and heat transfer coefficients are presented. The effects of various operating variables, including pressure, temperature, gas velocity, catalyst concentration, reactor geometry, and reactor internals on the hydrodynamic and transport parameters as well as the performance of SBCRs are discussed. Additionally, modeling efforts of SBCRs, using axial dispersion models (ADM), multiple cell recirculation models (MCCM) and computational fluid dynamics (CFD), are addressed. This review revealed the following: (1) Numerous F-T and WGS kinetic rate expressions are available for cobalt and iron catalysts and one must be careful in selecting the appropriate expressions for LTFT. Iron catalyst suffers from severe attrition and subsequent deactivation in SBCRs and accordingly building a costly catalyst manufacturing facility onsite is required to maintain a steady operation of the F-T reactor;(2) Experimental data on the hydrodynamic and transport parameters at high pressures and temperatures, typical to those of actual F-T synthesis, remain scanty when compared with the plethora of studies conducted using air-water systems in small reactors at ambient conditions; (3) Several empirical correlations for predicting the hydrodynamic and mass as well heat transfer parameters are available and one should select those which consider the reactor diameter, gas mixtures and the potential foamability of the F-T liquids; (4) The effect of cooling internals configuration and sparger design on the hydrodynamic and transport parameters, local turbulence, mixing and catalyst attrition are yet to be seriously addressed; (5) The impact of operating variables on the hydrodynamic and transport parameters as well as the overall performance of the SBCRs should be investigated using actual F-T fluid-solid systems under typical pressures and temperatures using a large-scale reactor (> 0.15 m ID) in the presence of gas spargers and cooling internals; (6) Significant efforts are still required in order to advance CFD modeling of SBCRs, particularly those pertaining to the relevant closure models, such as drag, lift and turbulence. Also, cooling internals configuration and the design as well as orientation of gas spargers should be accounted for in the CFD modeling; and (7) Proper validations of the CFD formulations using actual systems for F-T SBCR are needed. [ABSTRACT FROM AUTHOR]

The hydrodynamics (gas holdup, Sauter mean bubble diameter, d32) and the overall volumetric liquid-side mass transfer coefficients (kLa) were measured in a large-scale (0.29 m ID, 3 m high) slurry bubble column reactor (SBCR) for He/N2 gaseous mixtures, as surrogates for syngas, in three different Fisher-Tropsch (F-T) products (liquid paraffins mixture, light F-T cut and heavy F-T cut) in the presence and absence of three different solids (spent iron oxides catalyst, alumina powder and Puralox alumina). The effects of pressure (10-30 bar), temperature (up to 500 K), superficial gas velocity (0.14-0.26 m/s), solid concentration (0-20 vol.%) and gas density on these design parameters were investigated. The experimental data revealed that increasing the reactor pressure or gas density increased the gas holdup and decreased d32, by increasing the population of the small gas bubbles, which increased the overall kLa values for all the gas mixtures used in the three F-T cuts under most of the operating conditions employed. Increasing temperature increased the gas holdup in the three F-T cuts, except for N2-light F-T cut, where the gas holdup values remained almost constant from 400 to 500 K. Increasing the slurry concentration decreased the gas holdup and increased d32, mainly for gaseous mixtures with high He mole fractions, which decreased the overall kLa under all conditions used. Increasing the gas superficial velocities (UG) increased the gas holdup and kLa values, even though d32 was found to increase or decrease with increasing UG. Increasing the He mole fraction in the He/N2 gaseous mixture at constant pressure led to low gas holdup and high d32 which decreased kLa values, and under similar operating conditions, kLa values of He as a single gas were always lower than those of N2 as a single gas. Increasing the He mole fraction in the He/N2 gaseous mixture at constant density, however, was found to have negligible effect on the gas holdup, d32 and subsequently on the overall kLa. The gas holdup, the overall kLa and the population of the small gas bubbles for N2 in the liquid paraffins mixture were greater than those in the light F-T cut. Operating the SBCR with the heavy F-T cut resulted in the lowest gas holdup and the largest gas bubbles size which led to the lowest gas--liquid interfacial area and consequently, the lowest kLa values. Also, under the operating conditions investigated, the behavior of overall kLa for the gases used in the three F-T cuts in the presence and absence of the three solids employed was controlled by that of the gas--liquid interfacial area (a). Using the data obtained, two novel empirical correlations for predicting the gas holdup and the overall kLa for gases specifically in F-T cuts are proposed. [ABSTRACT FROM AUTHOR]

A user-friendly simulator based on a comprehensive computer model for slurry bubble column reactor (SBCR) was used to predict the performance of a conceptual commercial-scale (9-m ID and 50-m height) SBCR for Fischer-Tropsch (F-T) synthesis. Novel correlations for predicting the hydrodynamic and mass transfer parameters; a new relationship between the axial dispersion of large gas bubbles and their average diameter; and up to ten different kinetic rate expressions available in the literature for iron and cobalt-based catalysts were included in the simulator. The effects of operating conditions, including catalyst concentration, pressure, temperature, H2/CO ratio, and superficial gas velocity on the SBCR performance were predicted using the simulator for both types of catalysts. The predictions showed that the performance of the reactor was strongly dependent on the type of catalyst and the kinetic rate expression used. [ABSTRACT FROM AUTHOR]

Abstract: A comprehensive literature search was conducted to obtain the holdup data for different gases in various liquids and slurries using bubble and slurry bubble column reactors operating under wide ranges of conditions in different size reactors provided with a variety of gas spargers. The data were used to develop two novel correlations, one for the total gas holdup and the other for the holdup of large gas bubbles. The total gas holdup correlation is capable of predicting the experimental data within an absolute average relative error (AARE) and standard deviation (σ) of 20%, whereas the correlation of the holdup of large bubbles is capable of predicting the experimental values within AARE and σ of about 25 and 27%, respectively. The novel correlations were used to predict the effects of pressure, temperature, gas velocity, solid concentration, rector size, and distributor type on the holdup of syngas (H2/CO=2) in various slurry bubble column reactors (SBCRs) operating under typical Fischer–Tropsch conditions. [Copyright &y& Elsevier]