生物反应工程教学课件-Bioprocess Engineering Lesson-6.pptVIP

* Mass balance for oxygen at steady-state: We can use Eq. (7.27) to predict the response of the fermenter to changes in mass-transfer operating conditions. For example, if the rate of cell metabolism remains unchanged but kLa is increased by raising the stirrer speed to reduce the thickness of the boundary layer around the bubbles, the dissolved-oxygen concentration CAL must rise in order for the left-hand side to remain equal to the right-hand side. Similarly, if the rate of oxygen consumption by the cells accelerates while kLa is unaffected, CAL must decrease. * Further, we can deduce some important relationship for fermenter operations. First, let us estimate the maximum cell concentration that can be supported by the fermenter’s oxygen-transfer system. For a given set of operating conditions, the maximum rate of oxygen transfer occurs when the concentration-difference driving force (CAL* ? CAL) is highest, i.e. when the concentration of dissolved oxygen CAL is zero. Therefore, the maximum cell concentration that can be supported by the mass-transfer function of the reactor can be estimated: * Another important parameter is the minimum kLa required to maintain CAL Ccrit in the fermenter. This can also be determined as: Example 7.1 Cell concentration in aerobic culture A strain of Azotobacter vinelandii is cultured in a 15 m3 stirred fermenter for alginate production. Under current operating conditions kLa is 0.17 s?1. Oxygen solubility in the broth is approximately 8 ? 10?3 kg m?3. (a) The specific rate of oxygen uptake is 12.5 mmol g?1 h?1. What is the maximum possible cell concentration? * (b) The bacteria suffer growth inhibition after copper sulphate is accidentally added to the fermentation broth. This causes a reduction in oxygen uptake rate to 3 mmol g?1 h?1. What maximum cell concentration can now be supported by the fermenter? Solution: (a) From Eq.(7.28): (b) Assume that addition of copper sulphate does not affect CAL* and kLa. If qO is reduced