Near azeotropic blend fractionation will contain two saturation pressure valves that are close.

• A. Two valves that are far apart
• B. One saturation valve
• C. Two saturation pressure valves that are close
• D. No valves

Answer: (C)

Azeotropes show a strong dependence of separation on pressure, because the relative volatility of the components changes with pressure. Near the azeotropic point, a small shift in pressure can move the vapor composition enough to enable a different split, so you can progressively separate the components by cycling between two operating states.

Having two saturation pressure controls that are close lets you perform a gentle pressure swing between two nearly identical, easy-to-control states. This small pressure difference is just enough to alter the equilibrium behavior and steer the process away from the azeotropic composition without incurring large energy penalties or drastic temperature changes.

If the pressures were far apart, the swing would be more drastic and harder to manage, and a single valve wouldn’t provide the two distinct equilibria needed. No valves would mean no pressure adjustment at all, offering no chance to break the azeotrope.