Enhancement of Interfacial Mass Transfer in Gas-Liquid Contactors

摘要CTS-9818504 Tsao，G./该建议的界面传质基础研究起源于PI成功应用一种称为“压力脉动”的技术，其中实际的好氧发酵和气液接触过程。 该技术是非常有效和有趣的，它导致了目前的建议，在检查其基本机制。对于在搅拌-鼓泡发酵罐中进行的好氧发酵过程，空气流入微生物细胞在含水营养液中的悬浮液中。 通常，中心安装的搅拌器的旋转叶片将混合并且还将入口空气切割成小气泡，以提供用于氧气从气泡转移到液体中的表面，以支持微生物活性。 空气通过浮力穿过液体到达顶部，然后通过出口管离开发酵罐。 取决于特定条件，气体中的氧分压从入口流中的约21%变化到出口处总压力的16 - 20.5%。 对于气泡在液体中停留时间较长的高罐，最终的氧分压会较低;对于好氧发酵，氧分压通常是主要的成本项目，仅次于碳源。压力脉动（或PP）涉及简单的程序和简单的机械设置。 所有需要的是在发酵罐的出口管线上安装开/关阀。 该阀门将定期打开和关闭，由电子定时器控制。 当阀门关闭时，发酵罐内的压力将增加，当阀门打开时，压力下降。 出口阀的周期性关闭和打开产生压力脉动。 通过这种简单的系统，已经发现PP能够增加氧传递，从而将发酵性能提高20- 30%，如在初步实验中观察到的。 本文建议进一步研究聚丙烯在气液接触体系中的作用和机理。 将聚丙烯应用于固相发酵中，用湿固体多孔床代替气液混合物，发酵性能也得到了显著提高。 在固相发酵中，散热缓慢，因此产生的高温阻碍了发酵性能。 PP在多孔床中产生对流，增强了热量的散失和空气的流通，从而对发酵过程有很大的帮助。本项目从理论分析入手，试图从理论上解释PP增强氧气传递的原因。 然后进行实验验证。 本文件中提出了几种可能的增强机制。 通过本项目的研究，可以更好地了解聚丙烯。界面传质涉及到许多工业过程，如制造，水处理，污染防治等。即使只是好氧发酵，改善氧传递往往意味着提高发酵罐的生产力，因为大多数好氧发酵过程都受到其系统氧传递能力的限制。 例如，已知柠檬酸的有氧发酵速率与氧转移成正比。 好氧发酵用于生产抗生素、酶、有机酸、氨基酸、维生素、黄原胶和其他多糖等多种产品。 它可以添加到一长串的重要研究课题的界面传质。 除了生物过程，一旦它的基本机制得到更好的理解和量化，它必须有许多其他的应用。 另一个有趣的观察表明，压力脉动也使泡沫不稳定，泡沫通常存在于气液接触器中。 聚丙烯将作为一种破泡剂，节省消泡剂的成本，并减少通常的阻碍传质消泡剂。

ABSTRACTCTS-9818504Tsao, G./Purdue U.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, etcPP 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.