Mass-transfer properties of microbubbles. 1. Experimental studies

Area of Science:

• Biochemical Engineering
• Mass Transfer
• Fermentation Technology

Background:

• Synthesis-gas fermentations are often limited by poor gas-to-liquid mass transfer due to low solubilities of gaseous substrates.
• Enhancing mass transfer is crucial for improving the efficiency of these bioprocesses.

Purpose of the Study:

• To investigate the use of microbubble dispersions for enhancing gas-to-liquid mass transfer in synthesis-gas fermentations.
• To quantify mass-transfer coefficients and interfacial area for microbubble dispersions.
• To demonstrate the practical application of microbubbles in a microbial fermentation.

Main Methods:

• Microbubble dispersions were generated using a spinning disk apparatus and characterized in a bubble column.
• Mass-transfer coefficients (KL) and volumetric mass-transfer coefficients (KLa) were measured using a plug-flow model and laser-diffraction.
• The effectiveness of microbubbles was tested in a continuous stirred-tank reactor for Butyribacterium methylotrophicum fermentation.

Main Results:

• Volumetric mass-transfer coefficients (KLa) for microbubbles ranged from 200 to 1800 h-1, significantly higher than conventional methods.
• Fermentation KLa values increased from 14 h-1 with conventional sparging to 91 h-1 with microbubble sparging.
• An estimated incremental power input of 0.01 kW/m3 is needed to produce microbubbles for fermentation.

Conclusions:

• Microbubble sparging is an effective strategy to overcome gas-to-liquid mass-transfer limitations in synthesis-gas fermentations.
• This technology offers a substantial improvement in mass transfer for microbial processes, as demonstrated with Butyribacterium methylotrophicum.
• The energy requirement for microbubble generation is relatively low, suggesting economic viability.