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The effects of gas velocity, system pressure, and catalyst loading on gas holdup of H2, N2, CO, and CH4 in an organic mixture of hexanes were investigated in a 0.316 m diameter, 2.8 m height slurry bubble column reactor operating with a commercial Fischer−Tropsch iron-based catalyst. The data were obtained in the churn-turbulent flow regime with catalyst loading up to 50 wt % and a system pressure up to 8 bar. The hydrostatic pressure head method and the dynamic gas disengagement technique were employed to obtain the gas holdup profile and the values corresponding to different gas bubble sizes in the reactor. The experimental data showed that the gas holdup consists mainly of two classes of gas bubbles, small and large. The gas holdup data for the gases used were found to increase with pressure and superficial gas velocity due to the increase of the volume fraction of the small and large gas bubbles, respectively. The increase of catalyst loading, however, appeared to decrease the gas holdup values, due to the decrease of the volume fraction of the small gas bubbles. Statistical and empirical correlations for gas holdup data were proposed.

A statistical experimental design was employed to study the effects of pressure, temperature, catalyst loading, and mixing speed on the solubilities (P) and volumetric gadliquid mass transfer coefficients (kLa) for H,, N,, CO, CH, and C,H, in a liquid mixture of hexanes containing iron oxide catalyst in a 4-litre agitated autoclave. Statistical correlations for k,a values for the gases used were developed. Mixing speed and solid concentration showed the strongest effects on k,a. At low catalyst concentrations, a maximum in kLa was observed and at concentrations > 37 mass%, kLa decreased by more than one order of magnitude.On a utilise un plan d'experience statistique afin d'etudier les effets de la pression, de la temperature, du chargement de catalyseur et de la vitesse de melange sur 1es solubilites (CC) et les coefficients de transfert de matiere volumetriques gadliquide (k,a) pour H,, N,, CO, CH, et C,H, dans un melange liquide d'hexanes contenant un catalyseur a I'oxyde de fer dans un reservoir agite de 4 litres. Des correlations statistiques ont ete etablies pour les valeurs de k,a pour les gaz utilises. C'est la vitesse de melange et la concentration de solides qui agissent le plus sur kLa. A de faibles concentrations de catalyseur, on a observe un maximum dans k,a et a des concentrations > 37% massique, kLa diminue de plus d'un ordre de grandeur.

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A novel approach to determine the rate-limiting step in the Fischer-Tropsch (F-T) slurry process was developed using a simple computer model. This approach, unlike others, takes into account the water gas shift (WGS) reaction in the calculation of the importance of the gas-liquid mass transfer and makes use of the "singular kinetic path" concept proposed by Espinoza in 1993 which incorporates F-T kinetic expressions from different investigators. The predictions of the computer model showed that CO was the likely limiting reactant when using iron-based catalyst, since it is consumed by Fischer-Tropsch and water gas shift reactions and has lower mass transfer coefficients than those of H 2 . It was also found that in the case of cobalt-based catalyst F-T synthesis is sensitive to the H 2 /CO ratio of the inlet gas and H 2 was the likely limiting reactant, especially when the reactor was supplied with syngas having a H 2 /CO ratio lower than the stoichiometric value. The Fischer-Tropsch synthesis in a slurry phase using the currently available iron-based catalysts and the given operating conditions could be considered a reaction kinetics-controlled process. The reactor performance could be improved by increasing the catalyst activity or catalyst concentration up to a maximum of 37-40 wt. %. In both cases, the reactor would approach a mass transfer-controlled regime, especially at high catalyst concentration which would increase gas bubbles coalescence tendency. The increase of the slurry reactor performance due to the increase of cobalt-based catalyst concentration appeared to strongly depend on the H 2 /CO ratio in the syngas supplied to the reactor.