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 × 10 −5 to 2.2 × 10 −4 m/s. Volumetric mass‐transfer coefficients, K L a , for microbubbles with an average initial diameter of 60 μm 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.