Management of ethanol concentration during syngas fermentation

1. A process for reducing ethanol inhibition in fermentation of syngas comprising:

contacting syngas with a medium inoculated with acetogenic bacteria in a fermentor to provide a minimal cell density of at least about 0.3 grams per liter, providing a startup agitation of about 10 to about 30 Hz, wherein syngas is introduced into the fermentor at a rate to provide a pressure of about 10 to about 50 psig;

ramping up agitation to about 35 to about 50 Hz at a ramping rate of about 5 Hz every 10 minutes;

fermenting the syngas to convert CO to ethanol;

starting to remove cells and medium from the fermentor upon reaching a cell density of about 5 grams per liter or more and an ethanol concentration of more than about 30 g/L in the fermentation;

separating the removed cells and medium to provide concentrated cells and a permeate, wherein the permeate is transferred to a permeate holding tank located in between the fermentor and a distillation column;

delivering the permeate to the distillation column and separating the permeate to provide an ethanol, a fusel oil and a reduced ethanol aqueous stream having less than about 5 weight % alcohol;

recycling a first portion of the reduced ethanol aqueous stream having less than about 5 weight % alcohol to the fermentor;

sending a second portion of the reduced ethanol aqueous stream having less than about 5 weight % alcohol to a reboiler to produce a preheated reduced ethanol aqueous stream and recycling the preheated reduced ethanol aqueous stream to the distillation column; and

calculating a growth factor and controlling the rate of recycling the first portion of the reduced ethanol aqueous stream and the rate of removing cells and medium from the fermentor to maintain the growth factor within 0.01 to about 0.25 , where the growth factor is an increase in an amount of cells (in gram, dry weight) per gram of (parent) cells (dry weight) per hour;

wherein an added aqueous stream is provided to the fermentor when the growth factor is less than a critical growth factor of 0.02;

wherein an aqueous recycle rate is maintained to provide a total H 2 and CO uptake of 0.7 to 2.0 mole/min;

wherein an ethanol concentration of more than about 10 g/L is maintained in the fermentation.

2. The process of claim 1 wherein the reduced ethanol aqueous stream comprises acetic acid.

3. The process of claim 1 wherein CO 2 is removed from the permeate prior to distillation.