Breaking the ethanol–water azeotrope: What distillation method is used to obtain ethanol concentrations greater than about 95 percent by volume?

Introduction / Context:Conventional distillation of fermented beer or mash is limited by the ethanol–water azeotrope near 95–96% v/v at atmospheric pressure. Exceeding this concentration is necessary for fuel-grade ethanol and some industrial uses. This question targets the unit operation needed to surpass the azeotrope.

Given Data / Assumptions:

• Azeotropes behave as single pseudo-components in distillation.
• Standard column operation cannot enrich ethanol beyond the azeotropic composition.
• Special techniques are required to shift or break the azeotrope.

Concept / Approach:Azeotropic distillation introduces a third component (entrainer) or exploits pressure-swing to alter relative volatilities and break the ethanol–water azeotrope, allowing purification above 95%. Common industrial alternatives include molecular sieves (adsorption) and pervaporation, but in the context of “distillation” methods, azeotropic distillation is the canonical answer.

Step-by-Step Solution:Recognize the 95% v/v limit from ordinary fractional distillation.Identify the need for an azeotrope-breaking technique.Select “azeotropic distillation” as the appropriate method among the options.

Verification / Alternative check:Industrial flowsheets often apply rectification to 92–95% followed by azeotropic distillation or dehydration (e.g., cyclohexane entrainer or newer dehydration technologies).

Why Other Options Are Wrong:Fractional distillation alone cannot exceed the azeotrope; flash or mere “successive” distillations do not change the azeotropic limit.

Common Pitfalls:Confusing post-distillation dehydration methods (molecular sieves) with “distillation” per se; nonetheless, among distillation choices, azeotropic distillation is correct.

Final Answer:azeotropic distillation