Relative volatility versus pressure: For a typical binary mixture in distillation, how does the relative volatility α generally change with increasing system pressure?

Introduction / Context:Relative volatility α measures the ease of separating two components by distillation. Higher α indicates a larger volatility difference and easier separation. Pressure affects vapor–liquid equilibrium (VLE), which in turn influences α.

Given Data / Assumptions:

• Binary, non-azeotropic mixture over practical pressure ranges.
• No strong associating or pressure-activated complex formation.
• Comparison at the same temperature approach (or along operating range).

Concept / Approach:As pressure increases, vapor-phase non-ideality and saturation pressures shift such that K-values (y/x ratios) of components tend to converge. This convergence usually lowers relative volatility (α = K_A / K_B), making separation more difficult. Hence low-pressure operation often improves α and reduces stages.

Step-by-Step Solution:Define α = (y_A/x_A) / (y_B/x_B) = K_A / K_B.Note that with increasing pressure, K-values often move closer to 1 and to each other.Therefore, α generally decreases as pressure rises.Select 'Decreases' as the most representative trend.

Verification / Alternative check:VLE charts and process simulators commonly show increased column difficulty (more stages/higher reflux) at higher pressures for many hydrocarbon pairs, consistent with reduced α.

Why Other Options Are Wrong:'Increases' is uncommon except for special systems; 'remains the same' ignores VLE pressure sensitivity; 'either/or' is too vague for the general rule-of-thumb expected in fundamentals.

Common Pitfalls:

• For azeotropic/associating systems, trends can deviate; the question seeks the general case.
• Confusing α changes due to pressure with temperature effects along the column.

Final Answer:Decreases