Reversible retrofitting of a stirred-tank bioreactor for gas-lift operation to perform synthesis gas fermentation studies

A retrofit approach enables the non-invasive and reversible modification of a commercially available stirred-tank bioreactor into a gas-lift bioreactor within a few minutes. The gas-lift bioreactor configuration was characterised using a non-coalescent medium at working volume-specific volumetric gas flow rates of up to 20 L L⁻¹ h⁻¹, showing mean gas bubble diameters of up to 0.57 mm, volumetric power inputs of up to 23 W m^-3, gas-liquid mass transfer coefficients of up to 153 h⁻¹ (CO) and up to 122 h⁻¹ (H₂), mixing times (θ₉₀) of down to 0.5 min and gas hold-ups of up to 1.2%. A comparison of autotrophic batch processes with Clostridium aceticum and Clostridium carboxidivorans in the stirred-tank (2.0 L) and the retrofitted gas-lift bioreactor (2.5 L) at identical operating conditions showed differences in the cell dry weight and product concentrations (acetate, ethanol, butyrate, 1-butanol, hexanoate, 1-hexanol) due to gas-liquid mass transfer limitations caused by the reduced power input in the gas-lift bioreactor. CO limitation was demonstrated by a myoglobin-protein assay for CO measurement in the liquid phase. The simple retrofitting of a stirred-tank bioreactor for gas-lift operation thus provided comparative data on process performances of acetogenic bacteria that convert synthesis gas in gas-lift and stirred-tank bioreactors on a lab-scale.