Hydrodynamic and Mass Transfer Characteristics in a Large-Scale Slurry Bubble Column Reactor for Gas Mixtures in Actual Fischer–Tropsch Cuts

Abstract

Abstract The hydrodynamics (gas holdup, Sauter mean bubble diameter, d 32) 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 d 32, 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 d 32, 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 d 32 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 d 32 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, d 32 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.