Methods for increasing the production of ethanol from microbial fermentation

Abstract

A stable continuous method for producing ethanol from the anaerobic bacterial fermentation of a gaseous substrate containing at least one reducing gas involves culturing in a fermentation bioreactor anaerobic, acetogenic bacteria in a liquid nutrient medium; supplying the gaseous substrate to the bioreactor; and manipulating the bacteria in the bioreactor by reducing the redox potential, or increasing the NAD(P)H TO NAD(P) ratio, in the fermentation broth after the bacteria achieves a steady state and stable cell concentration in 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 method for producing ethanol, said method comprising:
 culturing an anaerobic, acetogenic  Clostridium ljungdahlii  bacterium in a liquid nutrient medium in a fermentation bioreactor;   supplying a gaseous substrate comprising carbon monoxide to said fermentation bioreactor;   manipulating the supply of said liquid nutrient medium to said fermentation bioreactor;   maintaining a specific rate of CO uptake at an amount of 0.3 to 2 mmol CO/gram dry cells weight of bacteria/minute in said fermentation bioreactor;   maintaining less than 5 g/L of free acetic acid in said fermentation bioreactor;   producing ethanol in said fermentation broth at a productivity greater than 10 g/L per day; and   producing ethanol and acetate in said fermentation broth in an ethanol to acetate ratio of 1:1 to 20:1.   
     
     
         2 . The method according to  claim 1 , wherein said fermentation bioreactor comprises a growth reactor which feeds said fermentation broth to a second fermentation bioreactor. 
     
     
         3 . The method according to  claim 1 , further comprising removing said fermentation broth from said fermentation bioreactor, distilling ethanol from said broth and recovering said ethanol. 
     
     
         4 . The method according to  claim 3 , further comprising recycling water containing acetate from said distilling back to said fermentation bioreactor. 
     
     
         5 . (canceled) 
     
     
         6 . The method according to  claim 1 , wherein said  Clostridium ljungdahlii  is selected from the strains consisting of PETC, ERI-2, O-52 and C-01. 
     
     
         7 . The method according to  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. 
     
     
         8 . The method according to  claim 7 , wherein said gaseous substrate further comprises nitrogen or methane. 
     
     
         9 . (canceled) 
     
     
         10 . The method according to  claim 1 , wherein the pH in said fermentation bioreactor is 4.5 or above. 
     
     
         11 - 12 . (canceled) 
     
     
         13 . The method according to  claim 1 , wherein said specific rate of CO uptake is 0.5 to 1.5 mmol CO/gram of dry cell of bacteria/minute. 
     
     
         14 . The method according to  claim 1 , wherein said liquid nutrient medium comprises 2 to 50 μg calcium pantothenate/grams of dry cell of bacteria produced in said fermentation bioreactor. 
     
     
         15 . (canceled) 
     
     
         16 . The method according to  claim 14 , wherein said liquid nutrient medium comprises 2 to 25 μg calcium pantothenate/grams of dry cell of bacteria produced. 
     
     
         17 . The method according to  claim 1 , wherein said liquid nutrient medium further comprises cobalt in an amount of 5 to 100 μg cobalt/grams of dry cell of bacteria produced in said fermentation bioreactor. 
     
     
         18 . The method according to  claim 17 , wherein said amount of cobalt is 20 to 50 μg cobalt/grams of dry cell of bacteria produced. 
     
     
         19 . The method according to  claim 1 , wherein said fermentation bioreactor is a batch-fed fermentation bioreactor. 
     
     
         20 . The method according to  claim 1 , wherein said liquid nutrient medium supply is manipulated using the liquid nutrient medium feed rate. 
     
     
         21 . The method according to  claim 20 , wherein the liquid nutrient feed rate is increased. 
     
     
         22 . The method according to  claim 20 , wherein the liquid nutrient feed rate is decreased. 
     
     
         23 . The method according to  claim 1 , wherein the supply of liquid nutrient medium is turned off. 
     
     
         24 . The method according to  claim 1 , which is a fed-batch fermentation.