Investigation of gas properties, design, and operational parameters on hydrodynamic characteristics, mass transfer, and biomass production from natural gas in an external airlift loop bioreactor

An external airlift loop bioreactor (EALB) was used for production of biomass from natural gas. The effect of riser to downcomer cross sectional area ratio (Ar/Ad), volume of gas–liquid separator, superficial gas velocity (Usgr), and physical properties of gases and their mixtures [υg (μ/ρ) and Dg] were investigated on mixing time, gas hold-up, and volumetric gas liquid mass transfer coefficients (kLa). It was found that Ar/Ad has remarkable effects on gas hold-up and kLa due to its influence on mixing time. Kinematic viscosity (υg) showed its significant role on mixing time, gas hold-up and kLa when different gases used (mixing time changes directly whereas gas hold-up and kLa change indirectly). Moreover, it was found that diffusion coefficient of gas in water (Dg) has remarkable effect on kLa. The volumetric mass transfer coefficients for methane and its mixtures with oxygen (three different mixtures) were determined at different geometrical and operational factors. In average, the rate of oxygen utilization is approximately 1.8 times higher than that of methane. A gas mixture of 25 vol% methane and 75 vol% oxygen was the best gas mixture for biomass production in the EALB. The correlations developed for predicting the mixing time, gas hold-up, and kLa in terms of Usgr, Ar/Ad, volume of gas–liquid separator, and gas phase properties have been found to be encouraging.