The vacuum is adjusted to the vapor pressure of the alcohol-water solution at the fermentation temperature (30 to 40 C). Volumetric productivity is far better than that of a conventional fermenter, but there is a killing disadvantage of having to recompresses large volumes of vapor so that alcohol can be condensed with normal cooling water instead of expensive cold brine. Furthermore, the large amounts of carbon dioxide generated by fermentation are evacuated and recompressed along with the alcohol and water vapors. A far better design is shown in the next figure.
The fermenter operates at normal pressure so that carbon dioxide escapes; broth is circulated through the flash pot for vaporization of the ethanol. Although this system may seem to have attractive energy economy because metabolic heat is removed as the vapor flashes, initial investment and the cost of pumping the vapors are high. Operating the ethanol fermentation at higher temperatures with thermophillic organisms has better economics in terms of milder vacuum and less recompression because of the higher vapor pressure. Other alternatives for overcoming inhibition by the product are extraction from the fermentation broth with an immiscible solvent and or operating with very dense cultures so that low productivity per cell is compensated by having many more cells.