Mass transfer enhancement for syngas fermentation
Converting biomass to useful products through synthesis gas (syngas) fermentation has the potential to replace petroleum based products with biobased ones; specifically increasing polyhydroxyalkanoate (PHA) production, a feedstock for bioplastics. A rate limiting step in syngas fermentations is the gas-liquid mass transfer in the bioreactor due to the low solubilities of the major syngas components, CO and H₂. This study compared the power demand and the gas-liquid mass transfer coefficient, k[subscript L]a, of the standard Rushton turbine setup in a continuous stirred tank reactor (CSTR) (T = 0.211) with different impeller designs and schemes using both CO-water and O₂-water systems. Eleven different impeller schemes were tested over a range of operating conditions which were based off the "after large cavity" region (ALC) of the Rushton turbine (D/T = 0.35). It was found that the dual Rushton impeller scheme exhibited the highest volumetric mass rates at all operating conditions; however, it also exhibited the lowest mass transfer efficiency (defined as mass transfer per unit power input) values at all conditions due to high power consumption. Dual impeller schemes with an axial flow impeller as the top impeller showed improved mass transfer rates without dramatic increases in power draw, especially for the A310 and the LS. At high gas flow rates, dual impeller schemes with a lower concave impeller have similar values as the Rushton schemes, but show improved mass transfer efficiency. It is believed k[subscript L]a and efficiency can be further enhanced for the impeller schemes with a bottom concave impeller by using operating conditions beyond the ALC region defined for Rushton impellers, due to the ability of the concave peller to handle higher gas flow rates prior to flooding.