Using Microbubbles to Intensify the Performance of Airlift Bioreactors

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.