title: Mass-transfer properties of microbubbles. 1. Experimental studies.
abstract: 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 cm2/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, KL, was determined by fitting a plug-flow model to the experimental, steady-state, liquid-phase oxygen-concentration profile. The KL values ranged from 2.9 x 10(-5) to 2.2 x 10(-4) m/s. Volumetric mass-transfer coefficients, KLa, for microbubbles with an average initial diameter of 60 microns 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 KLa 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/m3 of fermentation capacity.
authors.affiliation: Department of Chemical Engineering, Michigan State University, East Lansing 48824-1226, USA.
authors.forename: M D
authors.initials: MD
authors.lastname: Bredwell
authors.forename: R M
authors.initials: RM
authors.lastname: Worden
mesh_terms: D002248:Carbon Monoxide; D003102:Colloids; D004058:Diffusion; D005285:Fermentation; D006859:Hydrogen; D007700:Kinetics; D008433:Mathematics; D008863:Microspheres; D013501:Surface-Active Agents
chemical_list: D003102:Colloids; D013501:Surface-Active Agents; D002248:Carbon Monoxide; D006859:Hydrogen