Mass-transfer properties of microbubbles. 1. Experimental studies

Synthesis-gas fermentations have typically been gas-to-liquid mass-transfer-limited due to low solubilities of the gaseous substrates. A potential method to enhance mass-transfer rates is to sparge with microbubble dispersions. Mass-transfer coefficients for microbubble dispersions were measured in a bubble column. Oxygen microbubbles were formed in a dilute Tween 20 solution using a spinning disk apparatus. Axial dispersion coefficients measured for the bubble column ranged from 1.5 to 7.2 cm(2)/s and were essentially independent of flow rate. A laser-diffraction technique was used to determine the interfacial area per unit gas volume, a. The mass-transfer coefficient, K-L, was determined by fitting a plug-flow model to the experimental, steady-state, liquid-phase oxygen-concentration profile. The K-L values ranged from 2.9 x 10(-5) to 2.2 x 10(-4) m/s. Volumetric mass-transfer coefficients, K(L)a, for microbubbles with an average initial diameter of 60 mu m ranged from 200 to 1800 h(-1). Enhancement of mass transfer using microbubbles was demonstrated for a synthesis-gas fermentation. Butyribacterium methylotrophicum was grown in a continuous, stirred-tank reactor using a tangential filter for total cell recycle. The fermentation K(L)a values were 14 h(-1) for conventional gas sparging through a stainless steel frit and 91 h(-1) for microbubble sparging. The Power number of the microbubble generator was determined to be 0.036. Using this value, an incremental power-to-volume ratio to produce microbubbles for a B. methylotrophicum fermentation was estimated to be 0.01 kW/m(3) of fermentation capacity.