Continuous ethanol production via ultrasound-enhanced yeast sedimentation

Continuous glucose fermentation produces bioethanol at higher volumetric rates than conventional batch or fed-batch systems. The retention of yeast cells via ultrasonic sedimentation in a lab-scale fermenter allowed for shearless, continuous cell upconcentration, and consequently process intensification. The cell separation efficiency of the ultrasonic system was predicted with a Response Surface Model (RSM) developed for yeast cells based on the linear, mixed, and quadratic effects of the operating variables and flow rates (3 levels, 5 variables). The experiments for the RSM calibration were designed via a central composite design. The efficiency model was validated and showed dependency to the Biomass concentration, Power input, and Harvest rate (R²_calibration = 0.92, R²_prediction = 0.83). A lab-scale fermenter fed with yeast growth medium was operated at varying dilution rates (0.1 – 0.6 h⁻¹) based on the RSM to maximize the cell retention efficiency (23%−90%). Yeast cell concentration in the fermenter reached up to 31.5 ± 0.7 g/L while it remained at around 3 g/L in the harvest stream for all the dilution rates tested. The ethanol concentration ranged between 17 and 23 g/L and reached high volumetric productivity (8.8 g/L/h). A control run without ultrasonic sedimentation led to the washout of biomass at a dilution rate of 0.6 h⁻¹. Ultrasonic yeast sedimentation is a promising technology for cell retention and enhanced productivity in continuous fermentation processes.