Bioethanol separation — distillation limit: In continuous distillation of fermented broth, what is the maximum ethanol concentration achievable by simple distillation before reaching the ethanol–water azeotrope?

Introduction / Context:Ethanol recovery from fermentation relies on distillation. However, ethanol and water form an azeotrope that caps the achievable purity using ordinary distillation, a critical design constraint in fuel and potable alcohol industries.

Given Data / Assumptions:

• Feed: fermentation beer (typically 7–15% ethanol by volume).
• Unit operation: continuous distillation without entrainers or special dehydration steps.
• Question: the maximum achievable product strength before the azeotrope stops further enrichment.

Concept / Approach:The ethanol–water system exhibits a minimum-boiling azeotrope near 95% ethanol by volume (about 96% v/v at 1 atm). Past this point, the vapor and liquid compositions are identical; no further separation by simple distillation is possible without auxiliary techniques.

Step-by-Step Solution:Recognize distillation enriches the more volatile component (ethanol) until the azeotrope is reached.Recall the azeotropic composition at atmospheric pressure ≈ 95% v/v ethanol.Conclude that simple distillation in continuous stills tops out at ≈ 95%.

Verification / Alternative check:Fuel ethanol plants add molecular sieve dehydration or azeotropic/pressure-swing methods to surpass 95% and reach anhydrous (≥99%) ethanol.

Why Other Options Are Wrong:60–70% are far below the azeotrope and easily exceeded; 99% requires additional dehydration; 80% is not the limiting composition.

Common Pitfalls:Confusing weight percent with volume percent; the azeotrope is commonly cited on a volume basis in industry communications.

Final Answer:95%.