Abstract
Gas fermentation has rapidly emerged as a commercial technology for the production of low-carbon fuels and chemicals from (industrial) CO and/or CO2-rich feedstock gas. Recent advances in using CO2 and H2 for acetic acid production demonstrated that high productivity and substrate utilization are achievable. However, the costly constant addition of base and the energy-intensive nature of conventional recovery options (e.g. distillation) need to be overcome to drive organic acid production forward. Recently, membrane electrolysis has been presented as a technology that enables for the direct extraction of carboxylates across an anion exchange membrane into a clean and low pH concentrate stream. Continuous in-situ extraction of acetate directly from the catholyte of a microbial electrosynthesis reactor showed that membrane electrolysis allows pure product recovery while improving productivity. Here we demonstrate that the system can be further enhanced through additional input of electrolytic hydrogen, produced at higher energetic efficiency while improving the overall extraction efficiency. A gas-lift reactor was used to investigate the hydrogen uptake efficiency at high hydrogen loading rates. During stable operation acetate transport across the membrane accounted for 31% of the charge balancing, indicating that the use of external H2 can lead to a more efficient use of the extraction across the membrane. By coupling membrane electrolysis with the gas fermentation reactor the pH decrease associated with H2/CO2 fermentations could be prevented, resulting in a stable and zero-chemical input process (except for the CO2). This now enables us to produce more than 0.6 M of acetic acid, a more attractive starting point towards further processing.
Highlights
In recent years, microbial electrosynthesis (MES) has emerged as a promising bioreactor technology for the production of multi-carbon compounds from CO2 and renewable electricity (Rabaey and Rozendal, 2010; Logan and Rabaey, 2012)
This is the highest titer of acetic acid reported so far for MES from CO2 feed
Higher carbon fixation rates (1.48 g L−1 d−1) were observed during stable operation, whereas a maximum value of 3.54 g L−1 d−1 can be reported. These results confirm that an 8 times higher H2 feeding rate and a higher H2 retention time (∼ 1 h by continuous recirculation of the H2 headspace through the fermentation medium) resulted in 2.6–4.1 times higher acetic acid concentration and 2.1–2.7 times higher volumetric productivity compared to our previous studies (Gildemyn et al, 2015, 2017)
Summary
Microbial electrosynthesis (MES) has emerged as a promising bioreactor technology for the production of multi-carbon compounds from CO2 and renewable electricity (Rabaey and Rozendal, 2010; Logan and Rabaey, 2012) This electricity-driven CO2-conversion process uses the cathode of a so-called bio-electrochemical system to supply the reducing equivalents (in the form of electrons and/or H2) for reducing CO2 in the Wood-Ljungdahl pathway (May et al, 2016). Gildemyn and co-workers have already demonstrated the advantages of using membrane electrolysis (ME) for MES This approach can uniquely couple the production and recovery of acetic acid through in-situ product extraction across an anion exchange membrane (AEM) using nothing but an electrical current (Andersen et al, 2014; Gildemyn et al, 2015). Use of CO2 as a raw material for large scale bioproduction will require proper integration of autotrophic biotechnology to fully exploit the intrinsic power of CO2-based bioproduction
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