Efficiently transitioning a process from the laboratory to large-scale production is essential for achieving commercial viability while maintaining product quality, minimizing operational risks, and validating the process before full-scale investment. With this context, this study focuses on optimizing and scaling up butyric acid production through salting-out-induced extractive fermentation. This innovative approach enhances the yield by continuously removing the acid product from the fermentation broth while the fermentation is still taking place. This method prevents product inhibition and reduces downstream processing costs, providing a more cost-effective and environmentally friendly alternative to traditional methods. With the global butyric acid market valued at USD 317 million in 2022 and projected to grow at a compounded annual growth rate of 9.2%, reaching USD 1.12 billion by 2030, this optimized and scaled-up fermentation process is well-positioned to meet the rising demand from industries such as food, pharmaceuticals, and animal feed. To achieve this, the study first optimized a 48-hour flask fermentation by varying parameters such as initial concentrations of FeSO4·H2O (0.0004, 0.0252, 0.05M), KH2PO4 (0.022, 0.111, 0.20M), glucose (5, 15, 25 g/L), and the surfactant Ecosurf™ EH-3 (0, 20, 40% w/w) as extractant. The concentrations of salts FeSO4·7H2O and KH2PO4 were adjusted in the modified Savin’s fermentation medium based on their positive effects on butyric acid recovery from pure acid solutions in previous experiments. To avoid Clostridium tyrobutyricum toxicity, the maximum concentrations were set to 0.05M FeSO4·7H2O and 0.2 M KH2PO4. Using numerical optimization on the experimental results, the recommended conditions were 13.61 g/L glucose, 0.010M FeSO4·7H2O, 0.022M KH2PO4, and 25.36% w/w Ecosurf™ EH-3. Experimental validation yielded a butyric acid yield of 0.4556 g/g, a biomass yield of 1.36 g/g, 93.41% butyric acid recovery, an acid distribution coefficient of 5.03, and surfactant selectivity of 11.38. Then, to characterize the mass transfer and energy requirements of flask fermentation, equations for mass transfer flux and volumetric power input (P/V) were applied based on correlations of Reynolds number (NRe) and modified Newton number (NNe). Under the optimized flask fermentation conditions, the overall volumetric mass transfer coefficient (kLa), P/V, and NRe were 1.57/hour, 38.13 W/m3, and 13,833, respectively, achieved at a 100-rpm agitation rate. For designing a 2-L vessel while maintaining similar mass transfer and hydrodynamic conditions with the flask fermentation, P/V and NRe were used as scaling parameters. The hydrodynamic properties, mass transfer, and design specifications for the scaled-up tank were also computed using dimensionless-number-based correlation models. Among various mixing tanks with different agitators—disc turbines with impeller-to-tank diameter ratios (Di/DT) of 0.65, 0.40, and 0.33, and a propeller with a Di/DT of 0.5—the tank with propeller maintained nearly identical P/V (38.37 W/m3) and NRe (13,016) but at a higher agitation rate of 280 rpm. This configuration resulted in an increased kLa of 25.53/hour, driven by an enhanced interfacial area, improving the mass transfer of butyric acid from the broth to the extractant phase and ensuring efficient extraction rates like those in the flask setup.
