Enhancement of Interfacial Mass Transfer in Gas-Liquid Contactors

气液接触器中界面传质的增强

• 批准号: 9818504
• 负责人: George Tsao
• 金额: $ 10.53万
• 依托单位: Purdue Research Foundation
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1999
• 资助国家: 美国
• 起止时间: 1999-06-15 至 2002-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9818504&HistoricalAwards=false
• 关键词: Enhancement; Interfacial; Mass; Transfer; Gas

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.

界面传质的基础研究源于PI对“压力脉动”技术的成功应用，该技术应用于实际的好氧发酵和气液接触过程。这项技术是非常有效和耐人寻味的，它导致了目前对其基本机制的研究建议。对于在搅拌喷雾发酵罐中进行的好氧发酵过程，空气被空运到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提供表面，以支持微生物活动。空气通过浮力通过液体到达顶部，然后通过出口管道离开发酵罐。根据具体情况，气体中的氧分压从入口气流中的约21%变化到出口处总压力的16%至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；因为好氧发酵通常是一项主要成本项目，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。所需要的是在发酵罐的出口管路上安装一个开/关阀。这个阀门将由一个电子定时器控制，定期开启和关闭。当阀门关闭时，发酵罐内的压力会增加，当阀门打开时，压力会下降。出口阀的周期性关闭和打开会产生压力脉动。初步实验发现，在这样一个简单的体系中，PP能够增加氧转移，从而使发酵性能提高20%-30%。目前的建议是进一步研究PP在气液接触体系中的作用和机理。将PP应用于固相发酵，用多孔湿固体床代替气液混合物，发酵性能也有很大提高。在固相发酵中，热量散失缓慢，由此产生的高温阻碍了发酵性能。PP在多孔床层中产生对流进出，增强散热和空气循环，从而极大地帮助发酵过程。本项目从理论分析入手，试图为PP增强氧气传递提供理论解释。然后进行实验验证。本文件假定了几种可能的增强机制。界面传质涉及许多工业过程，如制造、水处理、污染防治等。即使仅就好氧发酵而言，氧传递的改善通常意味着发酵器生产率的提高，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，柠檬酸的有氧发酵速度与氧气转移成正比。好氧发酵用于生产包括抗生素、酶、有机酸、氨基酸、维生素、黄原胶等多种多糖的产品，具有简单、有效、价格低廉等优点。它可以被添加到界面传质的一长串重要研究课题中。除了生物过程，一旦更好地理解和量化了它的基本机制，它肯定还有许多其他应用。另一个有趣的观察表明，压力脉动也会破坏泡沫的稳定性，而泡沫通常存在气液接触器。PP将作为泡沫破碎剂，节省消泡剂的成本，并减少消泡剂通常对传质的阻碍。