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This study deals with the effects of electrolyte and particle concentrations on the gas holdup in both the homogeneous and the heterogeneous flow regimes in a slurry bubble column. Gas holdup measurements and video recordings of the bubble behavior were carried out in a 2D slurry column (0.015 × 0.30 × 2.00 m) under ambient conditions. The additions of electrolyte (sodium gluconate, 0.05-0.2 M) and of solid carbon particles (diameter 30 µm, 0.1-1.0 g L -1 ) both lead to a considerable increase in gas holdup. In both cases, critical concentrations exist above which no further increase in gas holdup is observed. The transition from the homogeneous to the heterogeneous regime is not significantly affected by electrolyte but is increased by the presence of particles. Three mechanisms are proposed that might account for the gas holdup increase resulting from particle and electrolyte addition. It is suggested that a layer of carbon particles around the gas bubbles results in a lower average bubble rise velocity. Both the addition of carbon particles and the addition of electrolyte lead to bubble stabilization, a decreased rate of coalescence, and thus a higher gas holdup. It is further suggested that the presence of electrolyte changes the surface tension, leading to smaller bubbles, a lower average bubble rise velocity, and thus a higher gas holdup. The combined addition of electrolyte and carbon particles confirms these hypotheses.

An industrial-size airlift loop redox cycle (ALRC) reactor has been designed for the selective oxidation of alcohols. Selective catalytic alcohol oxidation can produce valuable products for fine chemistry applications. However, the catalyst undergoes a factor of 10 loss in activity within hours as a result of overoxidation. The catalyst can be reactivated by contacting it with a reducing environment. This modeling and reactor design study shows that the ALRC reactor is a suitable option for achieving alternating contact of the catalyst with a reducing and oxidizing environment. It is shown that the superficial slurry velocity and the inlet partial pressure of oxygen mainly determine the reactor performance. A reactor design is developed on the basis of kinetic, hydrodynamic, and economic considerations, in which the catalyst activity remains high during its presence in the reactor. Finally, the ALRC reactor is compared with a stirred-tank reactor process involving alternating gas feed streams. It is shown that the ALRC reactor is much cheaper in terms of reactor operation than the stirred-tank reactor process and, thus, this type of reactor can feasibly be used in the selective oxidation of alcohols while maintaining a high catalyst activity.

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