Summary of Oscillatory Baffled Reactor (OBR) for Enhanced C1 Gas Bio-Conversion for Energy Production and Storage. Innovate UK Project No: P132133
The aims of the proposed research were to:
- investigate / demonstrate efficiency benefits that the OBR technology can bring to the bio-methanation biotechnology processes compared to other reactor designs,
- investigate the effect of OBR technology on the microbial community developed, and
- to demonstrate the overall feasibility of the bio-methanation process with an incorporated OBR, both in terms of productivity and energy efficiency
Experimental work has compared OBR performance with that of Liquid Recirculation Reactors (LRR) previously developed at USW, and industry standard Continuously Stirred Tank Reactors (CSTRs).
- The gas conversion capacities of the three reactors where compared using the same culture. Under standardised conditions across different reactor types the OBR reached a conversion efficiency of 75% (highest to date) and the CSTR an efficiency of 66%. The pump of the LRR was not able to deal with the high gas input rates achieved by the other reactors.
- Data from intermittent operational periods indicate that the enrichment of hydrogenotrophic populations is faster in the OBR than in the other two reactors. A possible reason is the milder agitation of the culture media (less shear forces acting on the microbes).
- The project has delivered significant improvements in the throughputs of gas which had been obtained previously by USW:
- this project identified that long term operation of the LRR design was limited by the liquid / gas circulation method used (i.e. mixing pump): any further increase is only possible using a different gas circulation design;
- the OBR has enabled gas throughputs of 1.5 times the maximum achieved by the LRR, and optimisation of the OBR operation is still taking place, both microbially and to increase the transfer of the feed gas to the liquid phase: it is possible that at this gas throughput the conversion efficiency will still increase, and that further gas throughput as well as gas conversion efficiency will be obtained by a higher working pressure (evaluation still to take place).
Other OBR advantages include the minimisation of foam which is particularly important as high gas flows promote foaming, especially at inoculum enrichment phases with complex organics available in the source of inoculum, and when high carboxylic acids are aimed to be produced. CSTR operations for this application are limited by foam generation.
Regarding the OBR, investigations of the methanation capacity at different amplitudes and frequencies indicated that conversion is still gas diffusion limited. Higher frequencies are needed to reach the methanation limit of the culture. The same applies for the CSTR (higher rotational speeds). With a CSTR however, the increase in impeller rotations in addition to the reactor would lead to further shear on the culture as well as re-solubilising the methane product, which will bring inefficiencies in terms of the gas diffusion.
In regards to the prototype OBR reactors that were applied for the first time to the novel bio-methanation process, it is believed that the limit of gas diffusion rates was reached and that moderate design changes would allow further improvements in performance for this specialist application. For the OBR 1 (ambient pressure) the inability to increase amplitudes/frequency further has been the barrier to further throughput improvements. Testing the performance of the OBR tech at higher pressures i.e. OBR2, is therefore envisaged to be able to increase the ability to achieve higher gas transfer rates and reduce parasitic energy requirements for the unit. Operations at 1.6 bar have indicated that over 20% of greater methane conversion is possible in comparison to OBR1. The impact of further increases in pressure will be evaluated during future research and development.
Professor Sandra Esteves
University of South Wales