Using Microbubbles to Intensify the Performance of Airlift Bioreactors

工业生物技术目前仅用于生产少量高价值的特种化学品，但有望满足以可持续方式生产各种产品的需求（减少环境影响，提高能源和原材料利用率，减少废物产生）。然而，阻碍实现该目标的主要因素之一是大多数当前生物反应器设计不能同时实现高生产率和能量效率，这些因素强烈依赖于生物反应器中存在的气相和液相之间的材料交换速率。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。在通常需要提高生产率和经济竞争力的高细胞密度下，这一点尤其重要。以节能的方式实现高气体交换速率是至关重要的，因为它允许使用更小、更便宜和更安全的反应器，并且可以显著增加传质控制的化学和生物化学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工艺操作的情况下特别重要，例如：藻类培养、低价值发酵产品、废水处理和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都会对气体交换产生负面影响，因此这项任务变得困难。使用专门设计的气升式生物反应器实现了气体交换速率的数量级增强，该气升式生物反应器能够保持高的能量利用效率并最小化对微生物的损害。这是通过引入在相之间具有大的接触界面面积的细分散状态（微泡）的气体来实现的，并利用大多数工业相关流固有的聚结延迟特性来保持这种相对长时间的有利情况。因此，在每千瓦时9千克氧气的能量利用效率下，实现了高达440 h-1的传质系数。*尽管这样的性能代表了对现有设计的显著改进，但是需要额外的研究以确保新颖的生物反应器设计足够灵活以提供宽范围的生物转化操作所需的最佳流体动力学和传质条件。这将提高利用工业生物技术的可能性，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，并以环保的方式回收/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.*************