The main objective of this project is to investigate the continuous extraction capabilities of microbubbles in a fermentation reactor operated at 60-65C, to improve cellulosic biofuel production. One of the main issues pertaining to fermentation of sugars to alcohol is the decline in performance of fermentative organisms at high product concentrations, due to the inhibitory effects of the product on the producing organism. This is particularly true with thermophilic bacteria which grow at relatively high temperatures (50-70C). However, some of these bacteria are particularly well suited to growth on renewable, lignocellulosic feedstocks, so an effective way to continuously remove the alcohol from the fermentation broth would make the lignocellulose to ethanol process more economic. In previous studies we have already shown that at high gas flow through rates, using normal (mm) sized bubbles, ethanol can be continuously stripped from the fermentation broth, so removing its inhibitory effects. However, the gas flow rates required are far too high to be practical. When using a gas to strip material from a liquid, or to deliver material (eg oxygen) from a gas to a liquid the most important feature for determining the mass transfer rate is the ratio of bubble surface area to volume. For the same volume of gas, smaller bubbles will have a higher surface area than larger bubbles and should therefore be more effective at stripping ethanol from a solution. However, smaller bubbles could potentially be more damaging to the bacteria, so their overall benefits cannot be assumed. In this project we will develop devices to allow continuous microbubble generation and extraction in a small scale bioreactor, demonstrate its effectiveness in simulated mixtures containing ethanol but no cells and finally investigate its effectiveness for continuous ethanol extraction from fermentations containing bacteria growing at 60-65C. The effects on the bacteria will be monitored and conditions (temperature, bubble size etc) modified to achieve optimal performance.

Technical Abstract:
Perlemax has developed and patented the concept of microbubble generation by virtue of fluidic oscillation which has advantages over current methods of microbubble generation due to its very low power input. This opens up the field of microbubble technology to a much wider range of applications, including addressing processes where mass-transfer is a limitation. We intend to address the issue of ethanol removal from thermophilic (60-65C) fermentation broths to improve cellulosic biofuel production. Batch production of bioethanol by the thermophilic bacterium Geobacillus thermoglucosidasius is limited by moderate (cf yeast) concentrations of bio-ethanol which significantly limits the possibility for process intensification and volumetric productivity. This can be improved by gas-stripping but the volumetric throughput of gas using normal sparger aeration would be impractical in a commercial process. As a practical and economic solution to this problem we will continuously extract fermentation products from the bioreactor by using pre-heated microbubbles using the Perlemax energy efficient microbubble generation technique. Availability of high interfacial area for mass transfer and intense internal mixing within the microbubbles will be key features in this approach.
While the rationale for the approach should be self-evident, the effect of microbubbles on the production organisms needs to be established. Microbubbles could potentially damage bacteria when rupturing at the top of the reactor. Therefore, after an initial optimisation using simulated broths, to establish the useful operating range, we will investigate the physiology of bacteria during experiments and adjust the operating parameters to find the most suitable conditions. Additionally, we will develop a computational model to assist scaling up the process and assess economic viability.

Potential Impact:
As described in proposal submitted to TSB