Direct medium-chain carboxylic-acid oil separation from a bioreactor by an electrodialysis/phase separation cell

Abstract

Medium-chain carboxylic acids (MCCAs) are valuable platform chemicals with numerous industrial-scale applications. These MCCAs can be produced from waste biomass sources or syngas fermentation effluent through an anaerobic fermentation process called chain elongation. We have previously demonstrated successful approaches to separate >90%-purity oil with several MCCAs by integrating the anaerobic bioprocess with membrane-based liquid-liquid extraction (pertraction) and membrane electrolysis. However, membrane electrolysis without pertraction was not able to separate MCCA oil. Therefore, we developed an electrodialysis/phase separation cell (ED/PS) and evaluated whether it can function as a stand-alone extraction and separation unit. First, we tested an ED/PS cell, which, when evaluated in series with pertraction, achieved a maximum MCCA-oil flux of 1,665 g d-1 per projected area (m2) (19.3 mL oil d-1) and a MCCA-oil transfer efficiency [100%*moles MCCA-oil moles electrons-1] of 74% at 15 A m-2. This extraction system demonstrated a [~]10 times lower electric-power consumption of 1.05 kWh kg-1 MCCA oil when compared to membrane electrolysis in series with pertration (11.1 kWh kg-1 MCCA oil) at 15 A m-2. Second, we evaluated our ED/PS as a stand-alone unit when integrated with the anaerobic bioprocess (without pertraction), and demonstrated that we can selectively extract and separate MCCA oil directly from chain-elongating bioreactor broth with just an abiotic electrochemical cell. We assumed that such a stand-alone unit would reduce capital and operating costs, but electric-power consumption increased considerably due to the lower MCCA concentrations in the bioreactor broth compared to the pertraction broth. Only a full techno-economic analysis will be able to determine whether the use of the ED/PS cell should be as a stand-alone unit or after pertraction.