The purification processes for synthetic anhydrous ethanol and fermented ethanol differ significantly due to the different impurities they contain. The main impurities in the ethanol solution obtained from fermentation include acetaldehyde, acetic acid, and various esters, along with residual solid particles produced during fermentation. In purification, liquid-solid separation is typically achieved first in a primary distillation column, distilling off volatile components and water. Subsequently, the ethanol is further concentrated and impurities removed in a rectification column. Because the acetic acid in the fermentation broth is highly corrosive to equipment, the distillation apparatus must be constructed from corrosion-resistant materials.
Extractive distillation is a special distillation method, typically consisting of a main column (for separating lighter components) and a secondary column (for recovering the extractant). It is suitable for liquid mixtures that are difficult to separate using conventional distillation. Its basic principle is to introduce a third component (called the extractant) into the distillation system, thereby changing the relative volatility of the components in the original mixture and achieving effective separation.
Taking ethylene glycol as an extractant as an example, ethylene glycol has a high boiling point (nearly 200 °C). When added to the ethanol-water system, it disrupts the existing azeotropic system. In the main column, anhydrous ethanol distills from the top, while ethylene glycol and water are pumped from the bottom of the column into the auxiliary column under pressure. The water then distills from the top of the auxiliary column, while the ethylene glycol is discharged from the bottom and recycled back into the main column.
The extractive distillation process used in the crude alcohol refining process differs from conventional methods. Generally, extractive distillation requires an additional extractant recovery step. However, the extractant used in the crude alcohol refining process is water, which is already abundant in the system. Therefore, the extractant does not need to be separated separately and can be discharged along with the bottom residue, entering subsequent distillation units for further processing.
