Using Microbubbles to Intensify the Performance of Airlift Bioreactors

工业生物技术目前只用于生产少量高价值特种化学品，但有希望能够以可持续的方式满足生产各种产品的需要(更低的环境影响、更好的能源和原材料利用以及更低的废物产生)。然而，阻碍这一目标实现的主要因素之一是，目前的大多数生物反应器设计无法同时实现高生产率和能源效率，这些因素强烈依赖于生物反应器中存在的气体和液体之间的材料交换速度。这是必要的，以便为微生物提供刺激其生物活性并加速生物转化过程的环境。在提高生产率和经济竞争力通常需要的高细胞密度下，这一点尤为关键。 以节能的方式实现高的气体交换率是至关重要的，因为它允许使用更小、更便宜和更安全的反应器，并可以显著提高传质控制的化学反应和生化反应的选择性和产率。这些因素在生产相对低价值产品的大规模生物加工作业中尤其重要，例如：藻类养殖、低价值发酵产品、废水处理以及将天然气生物转化为动物饲料和/或液体燃料。不幸的是，由于天然/工业水流中存在的大多数成分对气体交换产生负面影响，这项任务变得困难起来。 利用能够保持高能源利用效率和最大限度地减少对微生物的损害的特殊设计的气升式生物反应器，气体交换率实现了数量级的提高。这是通过引入处于精细分散状态的气体(微泡)来实现的，这些气体在相之间具有较大的界面接触面积，并利用大多数工业相关流固有的聚结延缓特性来保持相对较长时间的有利状态。在能量利用效率为每千瓦时9公斤氧气的情况下，传质系数高达440h-1。 尽管这样的性能比目前的设计有了很大的改进，但还需要进一步的研究，以确保新的生物反应器设计足够灵活，以提供广泛的生物转化操作所需的最佳流体力学和传质条件。这将提高利用工业生物技术以可持续的方式满足我们对食品和原材料日益增长的需求的可能性，并以有利于环境的方式回收/再利用许多废物流。

Industrial biotechnology is presently used to produce only small quantities of high-value specialty chemicals but holds the promise of being able to meet the need for generating a wide range of products in a sustainable fashion (lower environmental impact, better energy and raw material utilization, and lower waste generation). However, one of the major factors that hinders the achievement of this goal is the inability of most current bioreactor designs to simultaneously achieve high productivity and energy efficiency, factors which are strongly dependent on the rate at which material is exchanged between the gas and liquid phases present in the bioreactor. This is necessary in order to provide the microorganisms with the environment that stimulates their biological activity and accelerates the biotransformation processes. This is particularly critical under the high cell densities usually needed to improve productivity and economic competitiveness. Achieving high gas exchange rates in an energy efficient fashion is critical as it allows for the use of smaller, less expensive, and safer reactors and can significantly increase the selectivity and yield of mass-transfer-controlled chemical and biochemical reactions. These factors are particularly important in the case of large-scale bioprocess operations producing relatively low-value products such as: algae culture, low-value fermentation products, wastewater treatment, and the bioconversion of natural gas into animal feed and/or liquid fuels. Unfortunately, this task is made difficult by the fact that most ingredients present in natural/industrial water streams negatively influence gas exchange. Order-of-magnitude enhancement of gas exchange rates were achieved using specially-designed airlift bioreactors capable of maintaining high energy utilization efficiency and minimizing damage to the microorganisms. This was accomplished by introducing the gases in a finely-dispersed state (microbubbles) which have large interfacial area of contact between the phases, and take advantage of the coalescence retarding characteristics inherent to most industrially relevant streams to maintain this advantageous situation of a relatively long period. Mass transfer coefficients as high as 440 h-1 were thus achieved at an energy utilization efficiency of 9 kg of Oxygen per kWh. Although such a performance represents a significant improvement over present designs, additional investigation is needed in order to ensure that the novel bioreactor design is flexible enough to provide the optimum hydrodynamic and mass transfer conditions needed for a wide range of bioconversion operations. This will improve the possibility for using industrial biotechnology to meet our growing needs for food and raw materials in a sustainable manner, and to recycle/reuse many of the waste streams in an environmentally-beneficial fashion.