The separation of ethanol and water is challenging because the two liquids mix completely and form a constant boiling point mixture called an azeotrope. This minimum-boiling azeotrope limits the purity achievable through simple thermal methods. Overcoming this natural barrier requires specialized techniques that either physically remove the remaining water or chemically alter the mixture’s boiling behavior.
Utilizing Standard Distillation Techniques
Standard distillation, including simple and fractional methods, relies on the difference in boiling points between components to achieve separation. When the ethanol-water mixture is heated, the vapor phase becomes richer in the more volatile component, ethanol. This process of vaporization and condensation is repeated across the distillation column, continually increasing the ethanol concentration in the distillate.
This technique works efficiently until the mixture reaches the azeotrope concentration. At standard atmospheric pressure, the ethanol-water azeotrope forms at approximately 95.6% ethanol by mass and 4.4% water. Once this composition is reached, the liquid and vapor phases have the exact same ratio of ethanol to water.
The azeotrope acts as a purity ceiling because, at this constant boiling point, further boiling and condensation cycles will not change the vapor composition. Although pure ethanol boils at 78.5°C and water at 100°C, the azeotrope boils at a slightly lower 78.2°C. Therefore, the concentration of ethanol in the distillate will not exceed the 95.6% limit.
Achieving Anhydrous Ethanol Through Desiccation
To obtain anhydrous ethanol, which has a water content of less than 1%, non-thermal techniques are required to remove the final percentage of water remaining after distillation. These processes rely on highly hygroscopic materials, known as desiccants, to physically or chemically bind the residual water. Molecular sieves, such as crystalline aluminosilicates like zeolites, are a common industrial method.
Specifically, 3A molecular sieves are engineered with pore sizes of approximately three angstroms, large enough to trap small water molecules. The larger ethanol molecules, which are about 4.4 angstroms in diameter, are physically excluded from the pores and pass through. This size-exclusion mechanism allows for the selective adsorption of water directly from the near-azeotropic ethanol vapor or liquid.
Other desiccants include highly concentrated salts or quicklime (calcium oxide). Certain salts, like potassium carbonate, are highly soluble in water but less so in ethanol, encouraging the water to separate into a distinct, denser salt-rich layer (salting-out). Quicklime is a chemical desiccant that reacts directly with water molecules to form calcium hydroxide, effectively removing the water from the ethanol.
Breaking the Azeotrope with Entrainers
For large-scale production of high-purity ethanol, specialized distillation methods introduce a third chemical component called an entrainer. These entrainers alter the vapor-liquid equilibrium of the mixture, eliminating the azeotropic point. Two major techniques utilizing entrainers are azeotropic distillation and extractive distillation.
In azeotropic distillation, a volatile entrainer like cyclohexane or toluene is added, forming a new, lower-boiling ternary azeotrope with the water. This new azeotrope is collected as the distillate, effectively carrying the water out of the system. The nearly anhydrous ethanol is left behind at the bottom of the distillation column, and the entrainer is recovered and recycled.
Extractive distillation uses a high-boiling, low-volatility entrainer, such as ethylene glycol or glycerol. This heavy entrainer is fed near the top of the column and flows downward, selectively interacting with one of the components, usually the water. This interaction increases the relative volatility of the ethanol, allowing it to be separated as the overhead product while the entrainer and water are collected at the bottom.