Gas-liquid mass transfer in an external airlift loop reactor for syngas fermentation

摘要

Converting biomass to useful products through synthesis gas (syngas) fermentation has the potential to replace petroleum based products with biobased ones; however, these process are limited in their application. One of the most significant limiting steps in syngas fermentations is the gas-liquid mass transfer in the bioreactor due to the low solubilities of the major syngas components, CO and H2. Hence, to explore possible solutions for over coming the gas-liquid mass transfer barrier, a non-traditional external airlift loop reactor is considered. This study evaluates the hydrodynamics and gas-liquid mass transfer rates in an external airlift loop reactor with an area ratio of 1:16 operating under different conditions. Two downcomer configurations are investigated consisting of the downcomer vent open or closed to the atmosphere. Experiments for these two configurations are carried out over a range of superficial gas velocities (UG) from UG = 0.5 to 20 cm/s using three aeration plates with open area ratios of 0.66, 0.99 and 2.22%. These results are compared to a bubble column operating under similar conditions. Water quality variations are also investigated over the same range of UG with the downcomer open to the atmosphere. Experimental results show that the gas holdup in the riser does not vary significantly with a change in the downcomer configuration or bubble column operation, while a considerable variation is observed in the downcomer gas holdup. Gas holdup in both the riser and downcomer are found to increase with increasing superficial gas velocity. Test results also show that the maximum gas holdup for the three aeration plates is similar, but that the gas holdup trends are different. The superficial liquid velocity is found to vary considerably for the two downcomer configurations. However, for both cases, the superficial liquid velocity is a function of the superficial gas velocity and/or the flow condition in the downcomer. These observed variations are independent of the aerator plate open area ratio. Gas-liquid mass transfer results indicate that mass transfer rates do vary for oxygen and carbon monoxide gas species. Gas-liquid mass transfer rates are observed to increase linearly with UG in the presence of a surfactant and to increase similarly to riser gas holdup with UG for deionized water and ionic solutions. The gas-liquid mass transfer rates are relatively unaffected by the reactor configuration. The results also show that the addition of a surfactant or ionic compounds has a significant effect on mass transfer, where the surfactant restricts gas-liquid mass transfer and the ionic compounds enhance gas-liquid mass transfer.