| Bioreactors are becoming more important in the production of biobased products such as proteins, medicines, and renewable fuels. The economic viability of these processes is dependent on the bioreactor's ability to aid the microorganism and provide a friendly environment. One of the important microorganism requirements is proper gas concentrations so that the microorganism has the necessary inputs for proper metabolism. These gas concentrations are obtained and maintained through optimized gas-liquid mass transfer and mixing, also known as hydrodynamics. Other bioreactor responsibilities include damage mitigation and bioreactor volume utilization. A proper bioreactor design should also maximize profitability through ease of use, maintenance, and construction.;This thesis work provides a survey of gas-liquid mass transfer theories, applications, and dependencies in major bioreactor and several novel designs. The major reactor designs include the stirred tank bioreactor, bubble column, airlift bioreactor, and fixed bed bioreactor. Variations of these major designs are also considered such as the slurry bubble column, internal- and external-loop airlift, draught-tube bioreactor, and trickle, packed, and flooded bed bioreactor.;Since the microorganisms used in biological processes are diverse, a best or preferred bioreactor design is not feasible. Rather, bioreactor options can be presented based on the microorganism properties and production scale. Stirred tank bioreactors generally produce the largest gas-liquid mass transfer rates, but they also tend to cause high shear rates and variations, which can be very harmful to microorganisms. The impeller often limits the operating range, scale, process time, especially with non-Newtonian liquids. The bubble column and internal-loop airlift bioreactor have similar gas-liquid mass transfer rates; however, the bubble column has significant backmixing while the airlift bioreactor has lower bioreactor volume utilization. The external-loop airlift bioreactor provides more process and mixing control and generally has lower shear rates, but the attainable gas-liquid mass transfer rate and volume utilization tend to be lower. The fixed bed bioreactors protect and support microorganisms very well. On the other hand, the phase flow rates are much lower than in the other bioreactor designs. In other words, each bioreactor design has important advantages and disadvantages, and the microorganism may very well determine the optimal design.;The bioreactor designs may be described as complementary rather than competitive. Each design and design variation has been implemented to fill a void caused by the original form. This design mentality has led to highly complex bioreactor relationships and the inability to identify the single best bioreactor because that was not the intent. Future research and development can be taken into two different directions. First, a design variation could be approached with the clear intent of superiority for biological processes. Such a device could possible use a mixture of airlift and stirred tank bioreactor attributes. Second, research could be oriented towards the continued niche creation. Each design improvement would be implemented with the intent of improving a certain bioreactor attribute or application with a specific type of microorganism. For example, the fixed bed bioreactor research could investigate new packing that would provide better support and shear protection for very sensitive microorganisms such mammalian cells. |
