The culture broth can contain or not cells of yeast or other ethanol-producing microorganisms. This stream is called fermented wort (especially when sugarcane is the feedstock employed), fermented mash (when cereal grains are used), or sim­ply wine or beer. Besides ethanol and depending on the raw material employed, other substances can be found in a culture broth, such as:

• Nonfermented sugars

• Oligosaccharides resulting from the incomplete saccharification of starch or cellulose

• Ground and spent cereal grains

• Lignin (in dependence on the biomass pretreatment method)

• Other fermentation products like glycerol

• Lactic acid produced from contaminant bacteria

• Small amounts of acetic acid released during hemicellulose hydrolysis

• Dissolved carbon dioxide, salts, and excretion products from microbial cells metabolism

• Other compounds and materials

However, the two main components are water (80 to 90%) and ethyl alcohol. Making use of the higher volatility and lower boiling point of ethanol, the unit operation used as a rule for its separation is the conventional distillation at a pres­sure equal or higher than atmospheric pressure.

Usually, two distillation columns are utilized to elevate the ethanol concentra­tion up to 90 to 92%. Concentrations higher than 95.6% (or 89.4 molar %) are impossible to obtain by conventional distillation due to the similar composition of both the saturated vapor and saturated liquid achieved in the top of the distillation column. This composition is named azeotropic and is one of the main thermo­dynamic limits imposed on ethanol purification process. To produce anhydrous ethanol (99.5% or more), nonconventional separation technologies are required (see Section 8.2).

The technological scheme of the concentration and rectification steps of the ethanol contained in the culture broth is shown in Figure 8.1. During fermentation, carbon dioxide is generated as a result of the microbial metabolism. In the gas outlet stream and, along with CO2, small amounts of volatilized ethanol, as well as even much smaller amounts of water and other volatile substances, can be found. To avoid the ethanol loss, this gaseous stream is fed to a scrubber where a counter-current stream of water absorbs more than 98% ethanol. The scrubber is filled with a plastic-packed bed favoring the contact between the rising gaseous stream and the downward liquid stream. The gaseous ethanol-stripped stream is released into the atmosphere while the liquid stream containing about 2.5% etha­nol is unified with the culture broth coming from fermenter in order to be fed to the distillation column (Wooley et al., 1999).

The first distillation column is called the concentration or beer column. This column has a determined number of plates that can be of various types. It has been suggested that the fixed-valve Nutter-type plates are the most suitable to handle streams containing solids exhibiting a relatively good efficiency (about 48%). The concentrated ethanol stream (35 to 50%) is removed from the column by a side stream. The overhead vapors contain mostly CO2 (approximately 84%), a significant amount of ethanol (12%), and a small amount of water.

The bottoms of this column, called stillage or vinasses, concentrate the non­volatile substances and suspended solids that enter along with the culture broth.

Stillage composition depends on the feedstock employed for fuel ethanol produc­tion. The heat of this bottom’s stream is utilized to preheat the stream feeding the same distillation column. As an example of the location along this column of these streams, the design information for a concentration column corresponding to the biomass-to-ethanol process is provided (Wooley et al., 1999). This column has 32 plates, the feed stream is supplied to the fourth plate from the top, the side ethanol stream is removed from the eighth plate, and the reflux ratio required is 6.1.

The second column or rectification column is fed with the side ethanol stream from the concentration column. The operation of this column allows for a distil­late with 90 to 92% ethanol concentration, which is sent to the dehydration step. The bottoms of this column have a very low ethanol content (less than 1%), being mostly water. For illustration purposes, the design data of the rectification col­umn for the above-mentioned lignocellulosic ethanol process are provided as well (Wooley et al., 1999). The column has 69 plates and an additional feed stream corresponding to the recycle stream from the dehydration step, which is fed into the nineteenth plate from the top. The feed stream from the concentration column is supplied in the forty-fourth plate; fixed-valve Nutter-type plates are employed and the reflux ratio required is 3.2.