Mash is usually centrifuged or settled in order to separate the micro­bial biomass from the liquid and then sent to the ethanol recovery system. Distillation is typically used for the separation of ethanol, alde­hydes, fusel oil, and stillage [9]. Ethanol is readily concentrated from mash by distillation, since the volatility of ethanol in a dilute solution is much higher than the volatility of water. Therefore, ethanol is sepa­rated from the rest of the materials and water by distillation. However, ethanol and water form an azeotrope at 95.57 wt% ethanol (89 mol% ethanol) with a minimum boiling point of 78.15°C. This mixture behaves as a single component in a simple distillation, and no further enrichment than 95.57 wt% of ethanol can be achieved by simple distil­lation [9, 47, 81]. Various industrial distillation systems for ethanol purifi­cation are (a) simple two-column systems, (b) three- or four-column barbet systems, (c) three-column Othmer system, (d) vacuum rectification, (e) vapor recompression, (f) multieffect distillation, and (g) six-column reagent alcohol system [9, 47]. These methods are reviewed by Kosaric [9]. The following parameters should be considered for selection of the industrial distillation systems:

■ Energy consumption (e. g., steam consumption or cooling water con­sumption per kilogram of ethanol produced).

■ Quality of ethanol (complete separation of fusel oil and light compo­nents).

■ How to deal with the problem associated with clogging of the first dis­tillation column and its reboiler because of precipitation or formation of solids. Special design and use of a vacuum may be applied for over­coming the problem in the column. Using open steam instead of appli­cation of a reboiler can prevent clogging of the reboiler, in spite of the increase in amount of wastewater.

■ Simplicity in controlling the system.

■ Simplicity in opening column parts and cleaning the columns.

Of course, lower capital investment is also one of the main parameters in the selection of distillation systems.

Ethanol is present in the market with different degrees of purity. The majority of ethanol is 190 proof (95% or 92.4%, minimum) used for sol­vent, pharmaceutical, cosmetic, and chemical applications. Technical — grade ethanol, containing up to 5% volatile organic aldehyde, ester, and sometimes methanol, is used for industrial solvents and chemical syn­theses. High-purity 200 proof (99.85%) anhydrous ethanol is produced for special chemical applications. For fuel use in mixture with gasoline (gasohol), nearly anhydrous (99.2%) ethanol, but with higher available levels of organic impurities, is used [47].

A simple two-column system is described here, while other systems are presented in the literature (e. g., [9, 47]). Simple one — or two — column systems with only a stripping and rectification section are usually used to produce lower-quality industrial alcohol and azeotrope alcohol for further dehydration to fuel grade. The simplest continu­ous ethanol distillation system consists of stripping and rectification sections, either together in one column or separated into two columns (see Fig. 3.10).

The mash produced is pumped into a continuous distillation process, where steam is used to heat the mash to its boiling point in the stripper column. The ethanol-enriched vapors pass through a rectifying column and are condensed and removed from the top of the rectifier at around 95% ethanol. The ethanol-stripped stillage falls to the bottom of the stripper column and is pumped to a stillage tank. Aldehydes are drawn

Stillage <

Figure 3.10 Two-column system for distillation of ethanol.

from the head vapor, condensed, and partly used as reflux. Fusel oil is taken out from several plates of the rectifying section [9, 47, 82].

With efficient distillation, the stillage should contain less than 0.1% ethanol since the presence of ethanol significantly increases the chem­ical oxygen demand (COD) of wastewater. For each 1% ethanol left in the stillage, the COD of the stillage is incremented by more than 20 g/L. Due to the potential impact of residual ethanol content, therefore, proper control over distillation can greatly affect the COD of stillage [82].