Steam distillation economics: The amount of steam required per unit quantity of distillate in steam distillation can be reduced by adjusting operating conditions.\r Which change decreases steam consumption?

Introduction:
Steam distillation is used to volatilize temperature-sensitive or high-boiling organic compounds by contacting them with steam. The steam rate required per unit of organic distillate depends on vapor pressures at the operating temperature. Reducing steam demand improves energy efficiency and throughput.

Given Data / Assumptions:

• Immiscible steam distillation behavior: total pressure ≈ p_water(T) + p_organic(T).
• Ratio of moles of steam to organic in the vapor ≈ p_water(T) / p_organic(T).
• We compare effects of raising temperature and lowering total pressure.

Concept / Approach:
As temperature increases, the vapor pressure of the organic generally rises faster than that of water, decreasing the ratio p_water/p_organic and thus reducing the steam required per unit organic distillate. By contrast, simply lowering total pressure forces operation at a lower temperature where both vapor pressures are smaller; this often increases the steam-to-organic ratio and does not reduce steam use for a given distillate rate.

Step-by-Step Solution:
Use relation: steam required per organic ≈ p_water(T) / p_organic(T).Effect of raising T: p_organic increases significantly ⇒ ratio decreases ⇒ less steam per unit distillate.Effect of lowering total pressure: operating T drops ⇒ p_organic may drop markedly ⇒ ratio increases ⇒ more steam.Conclusion: raising temperature reduces steam requirement.

Verification / Alternative check:
Design charts for steam distillation show reduced steam-to-feed ratios at higher boiling temperatures for many organics, consistent with the faster rise of organic vapor pressure.

Why Other Options Are Wrong:

• Lowering total pressure: Typically raises the steam/organic ratio.
• Both/neither: Contradict the trend above.
• Increasing reflux: Not a primary lever in direct steam distillation without a rectifying section.

Common Pitfalls:
Confusing total pressure effects with simple distillation behavior; in steam distillation of immiscible systems, the key ratio is of vapor pressures at the operating temperature.

Final Answer:
raising the temperature.