Our fermentations ran at 20,000 liters. The tanks gave us acids, alcohols, and hydrogen sulfide, and each needed its own way out. Most of what we pumped and heated was water. The product was a small part of it.
We distilled the alcohols. Distillation works because the vapor over a mixture can carry more alcohol than the liquid it leaves. At an azeotrope, vapor and liquid match, and ordinary distillation at that pressure stops gaining. Extractive distillation adds another component to change that balance. [1] We used additives, extractive distillation, and sometimes 3A molecular sieves to get past those limits. Ion exchange pulled out acids, and we consistently recovered acids ranging from acetic through C12.
The hydrogen sulfide took the strangest route. We bled small amounts of air into the headspace, and sulfuric acid collected in a tray. Sewer engineers know related chemistry as a problem: sulfur-oxidizing bacteria on damp concrete turn hydrogen sulfide into sulfuric acid that attacks the pipe. [2] Much microaeration work aims to recover elemental sulfur while cleaning up biogas. [3] We took ours to the acid, collected it, and concentrated it by boiling off water.
The bigger change came in 2010, when we started extracting products while the fermentation was still running. Before that, acids built up in the broth until they slowed the culture making them. The mechanism is the one from Piece 04. Uncharged acid crosses the cell membrane, then dissociates inside. The cell has to manage the released protons and accumulating acid anions. A culture can stall on the acid it makes. Pulling the acids out as they formed helped relieve that inhibition. [4]
With active extraction, we kept cultures running for months at a time. New feed went in and chemicals came out, and we produced much more recoverable product over those runs than we expected. It reminded me of the hydrogen-consuming partners in the rumen. Keeping a product moving out could help production continue upstream.
That changed how I thought about separation. We started out treating recovery as the work after fermentation. By the end, it was part of how the fermentation ran.
Sources
[1] Pla-Franco et al. (2013). Phase equilibria for the ternary systems ethanol, water + ethylene glycol or + glycerol at 101.3 kPa. Fluid Phase Equilibria.
https://doi.org/10.1016/j.fluid.2012.12.022
[2] Chaudhari et al. (2022). Microbiologically Induced Concrete Corrosion: A Concise Review of Assessment Methods, Effects, and Corrosion-Resistant Coating Materials. Materials.
https://doi.org/10.3390/ma15124279
[3] Krayzelova et al. (2015). Microaeration for hydrogen sulfide removal during anaerobic treatment: a review. Reviews in Environmental Science and Bio/Technology.
https://doi.org/10.1007/s11157-015-9386-2
[4] Angenent et al. (2016). Chain Elongation with Reactor Microbiomes: Open-Culture Biotechnology to Produce Biochemicals. Environmental Science & Technology.
https://doi.org/10.1021/acs.est.5b04847