Mixing Studies in Loop Bioreactors for Production of Biomass from Natural Gas

The mixing behavior of the gas-liquid phase in three loop bioreactors was investigated. A gas-induced External Airlift Loop Bioreactor (EALB), a forced-liquid Vertical Tubular Loop Bioreactor (VTLB) and a forced-liquid Horizontal Tubular Loop Bioreactor (HTLB) were used for mixing studies as well as biomass production from natural gas. The effect of design parameters, riser to downcomer cross sectional area ratio (Ar/Ad), height to diameter ratio (H/D), length to diameter ratio (L/D) and volume of gas-liquid separator (S); as well as operational parameters, i.e. superficial gas velocity (UsG) and superficial liquid velocity (UsL) on mixing time were studied. It was found that liquid circulation (pumping) had an important effect on mixing time. VTLB, because of providing an effective countercurrent flow between gas and liquid streams, demonstrated the best mixing time performance. HTLB, as the second, provides a moderated mixing time output. EALB, since circulates no forced liquid, presents less mixing ability (gas moves liquid). It was observed from experimental results that mostly superficial gas velocity has an obvious effect on EALB. Accordance to mixing time data, a region that was independent on bioreactor type was explored that happened in high gas superficial velocity. In that zone, mixing time was not reliant on bioreactor variety and varies with the variation of operational and design parameters only. Some empirical correlations for mixing time in terms of Ar/Ad, H/D, L/D, UsG, UsL and volume of gas-liquid separator were obtained and expressed separately which can be used for design and scale up. The best biomass production occurred in the VTLB for gas mixture of 40% methane and 60% air.