Enhancement of Interfacial Mass Transfer in Gas-Liquid Contactors

[摘要]曹，G./Purdue u .本文提出的界面传质基础研究源于PI在实际好氧发酵和气液接触过程中“压力脉动”技术的成功应用。这项技术是非常有效和有趣的，它导致了目前关于检查其基本机制的建议。对于在搅拌喷射式发酵罐中进行的好氧发酵过程，将空气送入营养液中的微生物细胞悬浮液中。通常，中央安装的搅拌器的旋转叶片将混合并将入口空气切割成小气泡，为氧气从气泡转移到液体中提供表面，以支持微生物活动。空气在浮力作用下通过液体到达顶部，然后通过出口管离开发酵罐。根据具体情况，气体中的氧分压从入口流的21%左右变化到出口总压的16%到20.5%之间。对于高罐，气泡在液体中停留时间较长，最终氧分压会较低；因为好氧发酵往往是一个主要的成本项目，仅次于碳源。压力脉动（或PP）涉及一个简单的程序和简单的机械设置。所需要的就是在发酵罐的出口管线上安装一个开/关阀。这个阀门将定期打开和关闭，由一个电子计时器控制。当阀门关闭时，发酵罐内压力增加，当阀门打开时，压力下降。出口阀的周期性关闭和打开会产生压力脉动。在这样一个简单的系统中，我们发现PP能够增加氧传递，从而提高发酵性能20-30%，这是在初步实验中观察到的。目前的建议是进一步研究PP在气液接触体系中的作用及其机理。当PP应用于固相发酵时，气液混合物被湿润固体的多孔床所取代，发酵性能也大大提高。在固相发酵中，散热缓慢，由此产生的高温影响了发酵性能。PP在多孔床内外产生对流，增强了散热和空气循环，从而极大地促进了发酵过程。本项目拟从理论分析入手，试图对PP增强氧转移的原因提供理论解释。然后进行实验验证。本文提出了几种可能的增强机制。通过提议的项目，将更好地了解PP。界面传质涉及许多工业过程，如制造、水处理、污染防治等。即使仅仅是有氧发酵，一个改进的氧传递往往意味着提高发酵罐的生产力，因为大多数有氧发酵过程是由其系统的氧传递能力的限制。例如，柠檬酸的好氧发酵速率与氧转移成正比。好氧发酵用于生产多种产品，包括抗生素、酶、有机酸、氨基酸、维生素、黄原胶和其他多糖等，具有简单、有效和廉价的优点。它可以被添加到界面传质的重要研究课题的长列表中。除了生物过程，一旦其基本机制被更好地理解和量化，它必须有许多其他应用。另一个有趣的观察表明，压力脉动也会使泡沫失稳，而泡沫通常存在气液接触器。PP将用作泡沫破泡剂，节省消泡剂的成本，并减少通常由消泡剂传质的阻碍。

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.