4.1. Distillation Process

In sugar and starch fermentation, other alcohols, aldehydes, ketones, fatty acids, and esters are produced as volatile byproducts, whereas cyclic and heterocyclic compounds are volatile byproducts in lignocellulosic ethanol fermentation ^[29][91]. After the fermentation process is finished, the centrifuged broth is obtained by separating the yeast from the fermented beer. The centrifuged broth containing ethanol at about 5–15 wt.% is passed to the distillation column to remove the water. The distillation column consists of two columns. The first one is called the distillation column, or the beer column. In this column, approximately 50 wt.% ethanol can be achieved. The second column is the rectifying column. Hydrous ethanol (about 93 wt.% ethanol) can be achieved in this column ^[30][31][30,35]. Distillation can remove some impurity from ethanol with increasing ethanol concentration. Furthermore, chemical molecules with low boiling points, or those similar to ethanol, show up in distillate because distillation is ineffective in removing them [17]. For example, volatile impurities (acetaldehyde, acetone, ester, methanol) still show up in distillate. These contaminants result in lower engine efficiency when ethanol is used as fuel ^{[7][8][15][29][32]}[7,8,15,91,242].

4.2. Stillage Recycles

The remaining bottom liquid product after distillation of the ethanol from the beer column is called whole stillage. The whole stillage can contain ethanol up to 0.02 wt.%. Not only ethanol, but also solid particles, such as yeast cells, dissolved matter, and minerals, can be found ^[26][33][26,243]. After removing solid particles through a solid–liquid separation unit (e.g., centrifuge or decanter), the obtained liquid product called thin stillage can be recycled back to different process steps, e.g., fermentation or saccharification, to minimize effluent treatment cost. However, thin stillage recycling can possibly cause some drawbacks, such as the accumulation of lactic acid, minerals, and unutilized substrates ^[26][33][34][26,243,244]. The difference in the type of feedstock affects the impurities in the stillage. When stillage is recycled, it causes different contaminations. In the case of cane molasses feedstocks, whole stillage (without yeast cell separation) can be recycled in the fermentation step [26]. In the case of starch-containing feedstock, 25–75% of the thin stillage can be recycled in the fermentation or saccharification processes [26]. Other feedstocks, such as corn, wheat, and triticale, can be recycled at 75%, 60%, and 60% of thin stillage, respectively ^[33][35][243,245]. In Thailand, produced stillage during ethanol production from molasses or cassava is often treated and converted into methane gas. Stillage can also be distributed to farmers because stillage provides minerals for plants ^[36][37][246,247].

4.3. The Fate of Electrolytes during Distillation

During ethanol distillation, sulfite as sulfur dioxide can be distilled into the final ethanol product. The presence of sulfite in distilled ethanol appears to be a common experience in the distilled spirits industry ^[7][38][7,248]. Zhang et al. ^[39][249] reported that the distillate of chardonnay contained 12% ethanol and 176 mg/L sulfite as SO₂. After two stages of distillation, the concentration of ethanol and sulfite as SO₂ were increased to 69 vol% and 654 ppm, respectively. This phenomenon can be explained with the vapor–liquid equilibria for dilute aqueous solutions of SO₂ as volatile weak electrolyte ^[40][250].

4.4. Dehydration Process

The distillation process produces 95 vol% ethanol, approximately, because of the azeotropic mixture of ethanol and water (95.6 wt.% at 78.15 degrees Celsius). Before mixing ethanol with gasoline, it is necessary to increase the ethanol concentration to 99.3 wt.%, to make anhydrous ethanol. Anhydrous ethanol can be obtained by several dehydration methods such as molecular sieves, azeotropic distillation, and pervaporation. The molecular sieve is most commonly used because it has lower investment costs than pervaporation and requires less steam than azeotropic distillation ^[30][31][30,35]. The most common dehydration methods in Brazil are heterogeneous azeotropic distillation, extractive distillation, and molecular sieve adsorption ^[31][35]. The heterogeneous azeotropic distillation method requires an entrainer to increase separation. Many entrainers, such as benzene, toluene, and cyclohexane can be used to separate ethanol from water ^[31][41][35,251]. However, using an entrainer can cause product contamination ^[42][43][252,253]. Extractive distillation, as an alternative method, requires the addition of a third component to change the relative volatility of ethanol and water. The third component acts as a separating agent, such as ethylene glycol, glycerol, 1,3 diamino pentane, diethylenetriamine, or hexachlorobutadiene. The separating agent and water mixture is obtained at the bottom of the column, which is fed to the second column to recover the separating agent. Anhydrous ethanol is obtained at the top of the extractive column. Compared to azeotropic distillation, this method provides less energy consumption and less ethanol contamination ^[31][35]. In the case of molecular sieve adsorption, there is no requirement to add solvent. Ethanol vapor is fed to zeolite beds. When hydrated ethanol contacts zeolite, water molecules are absorbed. When compared to azeotropic distillation and extractive distillation, molecular sieve adsorption offers lower energy consumption and no chemical contamination ^[31][35]. Pervaporation, a membrane dehydration method, is a relatively new alternative to the dehydration process. While adsorbents need regeneration, membrane separation offers continuous operation and energy saving. Industrial applications of zeolite membranes have been reported ^[44][254].