Study On Mass Transfer Property And Optimization Of Stirring Form For High Solids Anaerobic Digestion

As an important unit operation in high solids biogas fermentation process, mixing could improve contact between the microorganisms and the biodegradable wastes, and maintain a stabilized anaerobic digestion process. However, as far as now, lack of support of enough experiment results hold back application of mixing technology in biogas engineering for some questions about mixing had not been totally answered: effect of mixing, especially mechanical stirring, on anaerobic digestion process of bioreactors with different D:H ratios, materials flow regime when mixing and the relation of flow regime and mass transfer, comparison of different mechanical stirring forms. So, in view of the above problems, the mass transfer property and the optimization mechanical stirring form were research in this paper with the support of the national plan supportted by science and technology (2008BADC4B15).(1) Mixing had a positive influence on improving anaerobic digestion process and increasing biogas production: the biogas production rate (L·L^-1·d^-1) and the maximum bio-chemical reaction rate of all bioreactors with different D:H ratios increased after the application of mixing, however, the main biogas production location could not be dislocated with mixing.(2) With the mixing intensity enhanced, the totally mixing time of top agitation, tilting agition, and bottom agitation shortened, when the mixing speed at 70rpm, tilting agitation got the shortest totally mixing time– 215s. The Morrill Dispersion Index of top agitation, tilting agitation, and bottom agitation were 23.38, 10.90, and 30.95, and totally mixed bioreactor could hold a better volume efficiency under top agitation conditions.(3) Tilting agitation could gain the off-bottom suspension of materials at a lower mixing speed (40rpm), and the critical suspension speed of top agitation was above that of tilting agitation, and bottom agitation was generally incapable of getting the off-bottom suspension of materials completely at the speed among 50rpm¹30rpm. As for the mixing effects, both top agitation and tilting were better than bottom. Meanwhile, top agitation and tilting agitation were likely to be more effective in terms of scum breaking.(4) Relative to the two TS concentrations of 8.0±0.2% and 2.0±0.2%, anaerobic digestion materials could get a better mixing performance under TS concentration of 4.0±0.2%. In addition, compared to chicken manure, pig manure and cow manure got better mixing performance on the whole.(5) Increase in energy consumption for mixing could promote the mixing performance of materials at different locations of the bioreactor to some extent. Both tilting agitation and bottom agitation had superior material energy consumption per unit volume to top agitation.