The ability to maintain high cell concentrations and high gas transfer rates in the reactor enhances productivity and reduces required reactor size. Gas-liquid mass transfer can limit the rate of syngas fermentation due to the low solubility of CO and H2 in fermentation medium. The rate of mass transfer (dn/dt) is given as follows:
(10)
where, dn/dt is the rate of mass transfer (mmol/h); kLa is the overall mass transfer coefficient (h-1); Ci is the concentration of the gas in gas liquid interface (mmol/L); CL is the concentration of gas in the bulk liquid (mmol/L) and V is the working volume of the reactor (L). The rate of gas transfer can be increased by either increasing the mass transfer coefficient (kLa) or by increasing the driving force (Ci-CL). The driving force can be increased by operating the reactor at high CO partial pressures