Effect of additives on mass transfer in turbine aeration

Comparative investigation of fine bubble and macrobubble aeration on gas utility and biotransformation productivity

The sufficient provision of oxygen is mandatory for enzymatic oxidations in aqueous solution, however, in process optimization this still is a bottleneck that cannot be overcome with the established methods of macrobubble aeration. Providing higher mass transfer performance through microbubble aerators, inefficient aeration can be overcome or improved. Investigating the mass transport performance in a model protein solution, the microbubble aeration results in higher k_L a values related to the applied airstream in comparison with macrobubble aeration. Comparing the aerators at identical k_L a of 160 and 60 1/h, the microbubble aeration is resulting in 25 and 44 times enhanced gas utility compared with aeration with macrobubbles. To prove the feasibility of microbubbles in biocatalysis, the productivity of a glucose oxidase catalyzed biotransformation is compared with macrobubble aeration as well as the gas-saving potential. In contrast to the expectation that the same productivities are achieved at identically applied k_L a, microbubble aeration increased the gluconic acid productivity by 32% and resulted in 41.6 times higher oxygen utilization. The observed advantages of microbubble aeration are based on the large volume-specific interfacial area combined with a prolonged residence time, which results in a high mass transfer performance, less enzyme deactivation by foam formation, and reduced gas consumption. This makes microbubble aerators favorable for application in biocatalysis.

Nutritional and engineering aspects of microbial process development

Today we use many drugs produced by microorganisms. However, when these drugs were discovered it was found that the yields were low and a substantial effort had to be put in to develop commercially viable processes. A key part of this endeavor was the studies of the nutritional and the engineering parameters. In this chapter, the basic principles of optimizing the nutritional and engineering aspect of the production process are described with appropriate examples. It was found that two critical components of nutritional medium, carbon and nitrogen source regulated the synthesis of the compounds of interest. Rapidly utilizable carbon source such as glucose supported the growth but led to catabolite repression and alternative carbon sources or methods of addition had to be devised. Inorganic nitrogen sources led to undesirable changes in pH of the medium. Organic nitrogen sources could influence the yields positively or negatively and had to be chosen carefully. Essential nutrients like phosphates often inhibited the synthesis and its concentration had to be maintained below the inhibitory levels. On many occasions, trace nutrients like metal ions and vitamins were found to be critical for good production. Temperature and pH were important environmental variables and their optimum values had to be determined. The media were designed and optimized initially with 'one variable at a time' approach and later with experimental design based on statistics. The latter approach is preferred because it is economical, considers interactions between medium components and allows rapid optimization of the process. The engineering aspects like aeration, agitation, medium sterilization, heat transfer, process monitoring and control, become critical as the process is scaled-up to the production size. Aeration and agitation are probably the most important variables. In many processes dissolved oxygen concentration had to be maintained above a critical value to obtain the best yields. The rheological properties of fermentation broth significantly affect the aeration and mixing efficiency. The removal of heat from the large fermentors can be difficult under certain conditions. However, new designs of impellers, availability of sensors to monitor important physiological and process variables and advent of computers have facilitated successful scale-up of fermentation processes.

Self-Cycling Fermentation Applied to Acinetobacter calcoaceticus RAG-1

The self-cycling fermentation (SCF) technique was applied to a culture of Acinetobacter calcoaceticus RAG-1. This method was shown to result in synchronization of the cells, achieving a 77% improvement in cell synchrony over that of the batch case. Cellular occurrences, averaged out by asynchronous batch cultures, were magnified by the temporal alignment of metabolic events brought about by the synchronization associated with SCFs. The cell population doubled only once per cycle, thus establishing an equality between cycle time and doubling time. Parameters of interest were biomass concentration, total bioemulsifier (emulsan) production, cycle time, and residual carbon concentration. Cycle-to-cycle variation of these parameters was, in most cases, insignificant. Repeatability of doubling time estimates (based on 95% confidence intervals) was roughly 7 to 10 times better between cycles in an SCF than between batch replicates. The carbon substrate was completely utilized in all cases in which it was measured, giving this technique an advantage over chemostat-type fermentations. The dissolved-oxygen profiles monitored throughout a cycle were found to be repeatable. A characteristic shape, which can be related to the growth of the organism, was associated with each carbon source. The specific emulsan productivity of SCFs was found to be approximately 50 times greater than that of the batch process and 2 to 9 times greater than that of the chemostat, depending on the dilution rate considered. With respect to specific emulsan production, a 25-fold improvement over that in an immobilized cell system recently introduced was obtained. Thus, SCFs are a viable alternative to established fermentation techniques.

Oxygen transfer rate, respiration and yields in batch and chemostat cultures of Klebsiella aerogenes

The effect was studied of oxygen supply on the changes in total and specific rate of oxygen consumption by the cells, oxygen transfer rate, saturation concentrations of dissolved oxygen and the yields of batch and continuous cultivations. Experiments were done on the microorganism Klebsiella aerogenes CCM 2318 growing on synthetic glucose medium. Continuous cultivations were carried out at dilution rates of 0.96 and 0.178 h-1. The rate of oxygen transfer was determined by the sulphite method and the coefficient KLa was assessed using the dynamic method with a correction for changes in the saturations of dissolved oxygen. A lowered oxygen supply in batch cultivations caused deformations in the course of cell respiration. Comparison of results of batch and continuous cultivations showed that the highest yields Y X/S and Y X/O are attained at low dilution rates without oxygen limitation. Batch cultivations, on the other hand, exhibit the lowest yields and the highest cell respiration levels. In both types of cultivations, a respiration peak was ascertained under the conditions of growth limitation by oxygen.