Experimental investigation of $ \mathrm{O_2} $ and $ \mathrm{CO_2} $ mass transfer in bioreactors

Gas-liquid mass transfer of oxygen ( \mathrm{O_2} ) and carbon dioxide ( \mathrm{CO_2} ) was studied experimentally in a stirred bioreactor. Oxygen kLa was measured by the dynamic method using nine combinations of agitation speed and gas flow rate, with two runs performed for each condition. Multiple linear regression explained 93.8% of the variation in kLa (R2 = 0.938), and the model was statistically significant (F = 114.0, p < 0.001). Over the range studied, gas flow rate had a greater effect on oxygen kLa than agitation speed. For the CO2 experiments, dissolved \mathrm{CO_2} concentration was estimated from carbonate hardness (KH) and pH measurements using a simple and low-cost procedure. The U-shaped tubular and ring spargers were then compared under the same operating conditions. The U-shaped tubular sparger gave a slightly higher kLa (0.625 min⁻¹) than the ring sparger (0.59 min⁻¹) and showed a faster initial \mathrm{CO_2} transfer. However, the ring sparger reached a higher final dissolved CO2 concentration, about 1300 mg/L compared with 1050-1080 mg/L for the U-shaped sparger. These results suggest that sparger performance should be considered in terms of both transfer rate and the final dissolved-gas concentration when selecting operating conditions for bioprocess applications.