Methods for increasing the production of ethanol from microbial fermentation

Abstract

A stable continuous method for producing ethanol from anaerobic fermentation of a gaseous substrate containing at least one reducing gas in a liquid nutrient medium; supplying the gaseous substrate to the bioreactor. The free acetic acid concentration in the bioreactor is maintained at less than 5 g/L free acid. This method allows ethanol to be produced in the fermentation broth in the bioreactor at a productivity of greater than 10 g/L per day. Both ethanol and acetate are produced in a ratio of ethanol to acetate ranging from 1:1 to 20:1.

1 . A process for producing one or more alcohols from a gaseous substrate comprising CO, the process comprising the following steps: (a) in a bioreactor, anaerobically fermenting the gaseous substrate to produce one or more alcohols and acetate; (b) culturing a continuously fed liquid nutrient medium to provide a fermentation broth and supplying to said fermentation bioreactor said gaseous substrate comprising carbon monoxide (CO); (c) the ratio of acetic acid to ethanol is less than 0.8:1; (d) producing ethanol in a fermentation broth at a productivity greater than 10 g/L per day; (e) producing both ethanol and acetate in said fermentation broth in a ratio of ethanol to acetate ranging from 1:1 to 20:1. 
     
     
         2 . A process for producing one or more alcohols from a gaseous substrate comprising CO, the process comprising the following steps: (a) in a bioreactor, anaerobically fermenting the gaseous substrate to produce one or more alcohols and acetate; (b) culturing a continuously fed liquid nutrient medium to provide a fermentation broth and supplying to said fermentation bioreactor said gaseous substrate comprising carbon monoxide (CO); (c) producing both ethanol and acetate in said fermentation broth in a ratio of ethanol to acetate ranging from 1:1 to 20:1; (d) wherein the ratio of acetic acid to ethanol is less than 0.8:1; (e) wherein said liquid nutrient medium comprises more or more of the following: Fe, PO 4 , Zn, Mn, B, Co, Cu, Ni, Mo, Se, W, K, Na, Mg, Ca NH 4 , Cl, and S. 
     
     
         3 . A process for producing one or more alcohols from a gaseous substrate comprising CO, the process comprising the following steps: (a) in a bioreactor, anaerobically fermenting the gaseous substrate to produce one or more alcohols and acetate; (b) culturing a continuously fed liquid nutrient medium to provide a fermentation broth and supplying to said fermentation bioreactor said gaseous substrate comprising carbon monoxide (CO); (c) producing both ethanol and acetate in said fermentation broth in a ratio of ethanol to acetate ranging from 1:1 to 20:1; (d) wherein the ratio of acetic acid to ethanol is less than 0.8:1; (e) wherein cell recycle provides increased ethanol productivity by ratio ranging from 1.465 to 1.85. 
     
     
         4 . The method of  claim 1 , wherein said gaseous substrate is selected from the group consisting of (a) carbon monoxide, (b) carbon monoxide and hydrogen, and (c) carbon monoxide, carbon dioxide and hydrogen. 
     
     
         5 . The method of  claim 1 , wherein the gaseous substrate comprises carbon monoxide and hydrogen in a ratio of carbon monoxide to hydrogen ranging from 0.25 to 3.25. 
     
     
         6 . The method of  claim 1 , wherein the gaseous substrate additionally comprises at least one gas chosen from nitrogen and methane. 
     
     
         7 . The method of  claim 1 , further comprising supplying cobalt wherein the cobalt is maintained at a constant concentration. 
     
     
         8 . The method of  claim 1 , wherein the fermentation bioreactor consists of one or more continuous stirred tank reactor (CSTR), immobilized cell reactor (ICR), trickle bed reactor (TBR), bubble column, gas lift fermenter, static mixer, or other device suitable for gas-liquid contact. 
     
     
         9 . The method of  claim 1 , wherein the fermentation bioreactor comprises one or more growth reactors that feed the fermentation broth to a fermentation bioreactor in which some or all of the ethanol is produced. 
     
     
         10 . The method of  claim 1  further comprising: removing the fermentation broth from the fermentation bioreactor; distilling ethanol from the broth; and recovering the ethanol. 
     
     
         11 . The method of  claim 1 , wherein the pH is greater than 4.5.