Transfer of ethanol to a solvent during fermentation in the presence of agitation

The aqueous phase mass transfer coefficient (K_aq) for ethanol transfer to the organic phase during fermentation has been successfully correlated with various system parameters in the presence of suspended yeast flocs under agitated conditions. In particular, a semi-empirical correlation of K_aq with the agitation speed of the stirrer, the void space in the broth, and the surface area of dead cells has been developed. In general, it has been found that K_aq increases with the agitation speed of the stirrer and the ratio of the void volume to the dead cell surface area. K_aq increases marginally though with higher incubation time at a given agitation speed of stirrer. The correlation is valid within a period of 12–30 h of incubation time. The agitation speed range is 200–300 rpm, and the ratio of void volume to dead cell surface area ratio ranged from 3.86 × 10⁻⁴ to 1.69 × 10⁻³ m. The fermentation time required was only 25–30 h compared to conventional fermentation systems, where it takes a minimum of 40–48 h. The organic phase mass transfer coefficient (K_org) is 300–500 times higher than K_aq. This is due to the drag of the yeast floc surfaces to the ethanol transferring eddies from the bulk to the interface.