Reactor Design for Root Culture
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Design equations for oxygen transfer rates were developed to examine the influence of poor mixing in large-scale reactors for root culture. The experimentally observed condition of plug flow of the gas and axially stratified liquid was compared to the typical bioreactor design case of a well mixed gas and liquid phase. In addition, the effect of hydrostatic pressure was included to consider the case for large-scale systems (up to 10,000 L). Calculations were carried out based on typical operating conditions at a root tissue concentration of 200 grams fresh weight per liter. The kinetics for biological oxygen demand were taken as first order, based on experimentally measured rates of root extension at different dissolved oxygen levels. These kinetics are believed to result from diffusion controlled oxygen transport within the root tissue. Unless hydrostatic head effects were considered, the assumptions of mixing did not affect the predicted oxygen availability (dissolved oxygen) to the roots. This results because the low respiration rates, at a typical gassing rate of 0.2 VVM, do not appreciably alter the gas composition as it passes through the reactor. When hydrostatic pressure is considered, the calculations predict increases in volumetric oxygen uptake rate of about 15% from small to larger scale vessels, independent of the mixing assumptions. However, since roots inhibit mixing, a 33% increase in dissolved oxygen is predicted for the bottom of a large scale reactor as a result of increased oxygen transfer rates. In contrast to a well mixed system, where the hydrostatic dependent differences in oxygen transfer are uniformly distributed, the lack of mixing in a reactor of cultured roots will result in substantial differences in oxygen availability from the top to the bottom of the reactor. Refinements needed for the design equations are discussed including the influence of localized meristematic respiration, and the potential for fluorescent imaging to provide more details into tissue oxygen gradients.
KeywordsHairy Root Root Culture Biological Oxygen Demand Hairy Root Culture Stir Tank Reactor
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