The aims of the proposed research were to:
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).
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