Liquid–liquid extraction for in situ carboxylic acid recovery via continuous membrane-based emulsion separations

Abstract

The separation and purification of bio-based chemicals from conversion processes often contribute substantially to bioprocessing costs, and for many biochemicals produced through fermentation, solvent-based liquid–liquid extraction (LLE) is a common separations strategy for in situ product recovery (ISPR). Many in situ LLE-based separations for biochemicals are often challenged by emulsion formation between the immiscible aqueous and organic phases. Typically, membrane contactors have been used to overcome emulsion formation, with the aqueous and organic phases coming into contact in the membrane contactor pores, but these unit operations require a large membrane area to compensate for their limited effective interfacial area. In this study, we show that a hydrophobic polytetrafluoroethylene (PTFE) membrane-based emulsion separator (MBES) enables continuous LLE for the recovery of an exemplary fermentation product, butyric acid, with substantially improved throughput relative to a membrane contactor. With a membrane permeate flux of 290 L m⁻² h⁻¹, the overall butyric acid flux of the single-stage MBES system was calculated to be 1450 g m⁻² h⁻¹, which is substantially higher than the 9 g m⁻² h⁻¹ achievable with a membrane contactor. At an equivalent butyric acid throughput, process modeling estimates that MBES-assisted LLE can enable a 55% and 91% reduction in process costs (the sum of CAPEX and OPEX) and greenhouse gas emissions, respectively, compared to a membrane contactor due to a ∼160-fold decrease in the required membrane area. Although membrane fouling from cellular debris led to reduced membrane flux and phase separation efficiency, common fouling mitigation strategies including the incorporation of ultrafiltration and periodic membrane backwashing effectively recovered the membrane performance. Overall, MBES systems can potentially enable continuous LLE processes in bioprocessing separations, including where emulsion formation is a challenge.