This proposal basic research on interfacial mass transfer was originated from the successful application by the PI of a technique called "pressure pulsation" in which practical aerobic fermentation and gas-liquid contacting processes. The technique is very effective and intriguing and it has led to this present proposal on examination of its basic mechanisms.
For an aerobic fermentation process carried out in an agitated-sparged fermentor air is flown into a suspension of microbial cells in an aqueous nutrient solution. Usually, the rotating blades of a centrally mounted agitator will mix and also cut inlet air into small bubbles to provide surfaces for oxygen transfer from the bubbles into the liquid to support microbiological activities. The air passes through the liquid by buoyancy to the top and then leaves the fermentor through an exit pipe. Depending on specific conditions, the oxygen partial pressure in the gas changes from about 21% in the inlet stream to anywhere from 16 to 20.5% of the total pressure at the exit. For tall tanks where the bubbles spend a longer residence time in the liquid, the final oxygen partial pressure will be lower; for aerobic fermentation is often a major cost item, second only to the carbon source.
Pressure pulsation (or PP) involves a simple procedure and a simple mechanical set-up. All is needed is to install an On/Off valve on the exit line of the fermentor. This valve will be periodically opened and closed, controlled by an electrical timer. When the valve is closed, pressure inside the fermentor will increase when the valve is open, the pressure goes down. The periodical closing and opening of the exit valve creates pressure pulsation. With such a simple system, PP has been found to be able to increase oxygen transfer and thus enhance the fermentation performance by 20-30%, as observed in preliminary experiments. The current proposal is to further examine the effect and the mechanism of PP in gas-liquid contacting system. When PP is applied to solid phase fermentation where the gas-liquid mixture is replaced by a porous bed of moist solids, the fermentation performance was also increased substantially. In solid phase fermentation, heat dissipation is slow and the high temperature thus created hinders the fermentation performance. PP creates convective flow in and out of the porous beds, enhancing heat dissipation and also air circulation, and thus helps the fermentation process greatly.
This proposed project starts with theoretical analysis, trying to provide theoretical explanations of why PP can enhance oxygen transfer. It is then followed by experimental verification. Several possible mechanisms of enhancement have been postulated in this document. Through the proposed project, a better understanding of PP will result.
Interfacial mass transfer is involved in numerous industrial processes for manufacturing, water treatment, pollution prevention, etc. Even just for aerobic fermentation alone, an improved oxygen transfer often means an improved fermentor productivity because most aerobic fermentation processes are limited by the oxygen transfer capability of their systems. The rate of aerobic fermentation of citric acid, for instance, is known directly proportional to oxygen transfer. Aerobic fermentation is used for producing large varieties of products including antibiotics, enzymes, organic acids, amino acids, vitamins, xanthan gums and other polysaccharides, etc
PP is simple, effective and inexpensive. It can be added to the long list of important research topics of interfacial mass transfer. Besides bioprocesses, it must have many other applications, once its basic mechanisms are better understood and quantified. Another interesting observation shows that pressure pulsation also de-stabilizes foam which often exists gas-liquid contactors. PP will serve as a foam breaker, saving the cost of antifoaming agents and reducing the usual hinderance of mass transfer by antifoams.
