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) wherein said free acetic acid concentration in said fermentation bioreactor is less than 5 g/L free acetic acid; (d) producing ethanol in a fermentation broth at a productivity greater than 10 g/L per day; (e) and 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) wherein said free acetic acid concentration in said fermentation bioreactor is less than 5 g/L free acetic acid; (d) producing ethanol in a fermentation broth at a productivity greater than 10 g/L per day; (e) and producing both ethanol and acetate in said fermentation broth in a ratio of ethanol to acetate ranging from 1:1 to 20:1; and wherein said gaseous substrate comprises 25-65% carbon monoxide gas.   
     
     
         3 . A continuous method for producing ethanol from the anaerobic fermentation of a gaseous substrate comprising carbon monoxide, the method comprising:
 culturing in a fermentation bioreactor, subject bacteria in a continuously fed liquid nutrient medium to provide a fermentation broth and supplying to said fermentation bioreactor said gaseous substrate comprising carbon monoxide (CO); maintaining a specific rate of CO uptake in said fermentation bioreactor at an amount of 0.3 to 2 mmol CO/gram dry cells weight of bacteria/minute in the fermentation broth after said bacteria achieve a stable cell concentration in said fermentation bioreactor, wherein said free acetic acid concentration in said fermentation bioreactor is less than 5 g/L free acetic acid, producing ethanol in a fermentation broth at a productivity greater than 10 g/L per day; and wherein said gaseous substrate comprises 25-65% carbon monoxide gas.   
     
     
         4 . The method of  claim 3 , wherein the ratio of ethanol to acetate and the ratio of free acetic acid to ethanol is measured after the bacteria achieve a stable cell concentration in the bioreactor. 
     
     
         5 . 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. 
     
     
         6 . 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. 
     
     
         7 . The method of  claim 1 , wherein the gaseous substrate additionally comprises at least one gas chosen from nitrogen and methane. 
     
     
         8 . The method of  claim 3  further comprising supplying calcium pantothenate in an amount ranging from 2 to 50 μg calcium pantothenate per gram of dry cells of bacteria in the fermentation bioreactor. 
     
     
         9 . The method of  claim 3 , further comprising supplying calcium pantothenate wherein the amount of calcium pantothenate ranges from 2 to 25 μg calcium pantothenate per gram of dry cells of bacteria. 
     
     
         10 . The method of  claim 3 , further comprising supplying calcium pantothenate wherein the amount of calcium pantothenate ranges from 10 to 30 μg calcium pantothenate per gram of dry cells of bacteria. 
     
     
         11 . The method of  claim 3 , further comprising supplying calcium pantothenate wherein the amount of calcium pantothenate ranges from 10 to 50 μg calcium pantothenate per gram of dry cells of bacteria. 
     
     
         12 . The method of  claim 3  further comprising supplying cobalt in an amount ranging from 5 to 100 μg cobalt per gram of dry cells of bacteria in the fermentation bioreactor. 
     
     
         13 . The method of  claim 3  further comprising supplying cobalt in an amount ranging from 20 to 50 μg cobalt per gram of dry cells of bacteria in the fermentation bioreactor. 
     
     
         14 . The method of  claim 1 , further comprising supplying cobalt wherein the cobalt is maintained at a constant concentration. 
     
     
         15 . The method of  claim 3 , wherein the free acetic acid concentration after the bacteria achieve a stable cell concentration in the bioreactor is less than 5 g/L free acetic acid. 
     
     
         16 . 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. 
     
     
         17 . 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. 
     
     
         18 . The method of  claim 1  further comprising: removing the fermentation broth from the fermentation bioreactor; distilling ethanol from the broth; and recovering the ethanol. 
     
     
         19 . The method of  claim 3  further comprising purging the bacteria from the fermentation bioreactor. 
     
     
         20 . The method of  claim 1 , wherein the pH is greater than 4.5.