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Mass‐Transfer Properties of Microbubbles. 2. Analysis Using a Dynamic Model
Author(s) -
Worden R. Mark,
Bredwell Marshall D.
Publication year - 1998
Publication title -
biotechnology progress
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.572
H-Index - 129
eISSN - 1520-6033
pISSN - 8756-7938
DOI - 10.1021/bp970131c
Subject(s) - bubble , sparging , mass transfer , mass transfer coefficient , microbubbles , volume (thermodynamics) , radius , mechanics , chemistry , materials science , thermodynamics , analytical chemistry (journal) , chromatography , ultrasound , physics , acoustics , computer security , computer science
Gas‐to‐liquid mass transfer is commonly the rate‐limiting step in industrial fermentations. Microbubble sparging has been shown to give extremely high volumetric mass‐transfer rates, even for low power‐to‐volume ratios. Microbubbles are surfactant‐stabilized bubbles having a radius on the order of 25 μm. The extremely high surface‐area‐to‐volume ratios of microbubbles can result in rapid changes in their size, internal pressure, and gas composition. Consequently, an unsteady‐state modeling approach is needed to adequately describe microbubble mass transfer. A dynamic model of a single microbubble immersed in an infinite pool of stagnant liquid was developed and solved numerically. The model accounts for mass‐transfer resistances of the surfactant shell and surrounding bulk liquid, bubble shrinkage, changes in the gas pressure and composition inside the bubble, and changes in the liquid‐phase concentration profile of the transferred gas surrounding the bubble. The model was used to explore a variety of dynamic phenomena associated with microbubble mass transfer. The presence of a nontransferred gas in the microbubble was predicted to significantly reduce the mass‐transfer rate, indicating that microbubble sparging is better suited to gases with a high consumable fraction. The instantaneous mass‐transfer coefficient was predicted to change significantly with time, but the time‐averaged coefficient was constant enough to justify the measurement of average mass‐transfer coefficients for microbubbles. Average mass‐transfer coefficients predicted by the model agreed well with values measured experimentally and calculated using literature correlations.