Formation and coalescence properties of microbubbles

Abstract

Fermentations that involve a gaseous substrate are inherently rate-limited by mass transfer from the gaseous to the liquid phase. Aerobic and synthesis-gas fermentations are primary examples of such mass-transfer-limited fermentations. The traditional approach to enhance gas-to-liquid mass transfer is to increase the impeller rate, thereby increasing the interfacial area available for mass transfer. However, this approach results in a dramatic increase in power consumption, especially for large-scale systems, because power consumption is proportional to the impeller rate to the third power and the impeller diameter to the fifth power. Microbubble dispersions offer an alternative, providing high interfacial area with the potential for low power input. In aerobic fermentations with yeast, the transport of oxygen from a gas to a liquid, reported in terms of k{sub 1}a, was found to be greater by an order of magnitude using microbubble sparging oven conventional air sparging. Microbubbles are surfactant-stabilized, spherical bubbles with diameters on the order of 100 {mu}m. By comparison, gas bubbles found in conventional sparging units range from 3-5 mm. The surfactant layer surrounding the microbubble generates a diffuse electric double layer that acts to repel other bubbles and prevent coalescence. A microbubble dispersion exhibits colloidal properties and is stable enoughmore » to be pumped. This article examines the effects of surfactant type and concentration on microbubble formation, drainage, and stability. Effects of salts on microbubbles dispersions are examined as well. This information is needed to evaluate the utility of microbubble dispersions for biotechnological applications.« less