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Difficulty: Very hardIndustrial Applications of Separation Methods

In an industrial plant for the fractional distillation of liquefied air, atmospheric air is compressed, cooled to −200∘C-200^\circ\text{C}, and fed into a fractionating column. Given the boiling points of nitrogen (−196∘C-196^\circ\text{C}), argon (−186∘C-186^\circ\text{C}), and oxygen (−183∘C-183^\circ\text{C}), which of the following statements correctly accounts for the sequence and physical state of the components as the liquid air mixture is gradually warmed?

  1. Nitrogen vaporizes first at −196∘C-196^\circ\text{C} because it has the lowest boiling point, followed by argon at −186∘C-186^\circ\text{C}, leaving oxygen as a liquid residue until −183∘C-183^\circ\text{C}.Answer
  2. B
    Oxygen vaporizes first because its boiling point of −183∘C-183^\circ\text{C} is higher than that of nitrogen (−196∘C-196^\circ\text{C}), requiring less thermal energy to transition into gas.
  3. C
    Argon vaporizes before nitrogen because monoatomic noble gases possess higher kinetic energy at −200∘C-200^\circ\text{C} than diatomic nitrogen molecules.
  4. D
    Nitrogen and oxygen vaporize simultaneously at −183∘C-183^\circ\text{C} as a constant-boiling azeotropic mixture, leaving liquid argon at the base of the fractionating tower.

Answer

Nitrogen vaporizes first at −196∘C-196^\circ\text{C} because it has the lowest boiling point, followed by argon at −186∘C-186^\circ\text{C}, leaving oxygen as a liquid residue until −183∘C-183^\circ\text{C}.
The correct answer is the statement noting that nitrogen vaporizes first at −196∘C-196^\circ\text{C}, followed by argon at −186∘C-186^\circ\text{C}, leaving oxygen as a liquid. In fractional distillation of liquefied air, warming the mixture from −200∘C-200^\circ\text{C} causes the component with the lowest boiling point (nitrogen at −196∘C-196^\circ\text{C}) to reach its boiling threshold first and turn into gas at the top of the fractionating column. Argon (−186∘C-186^\circ\text{C}) boils next, while oxygen (−183∘C-183^\circ\text{C}) remains liquid longest and collects near the bottom.

Step-by-Step Solution

1
Analyze the starting condition and temperature change
The mixture starts in the liquid state at −200∘C-200^\circ\text{C} and temperature gradually increases (warming up).
Fractional distillation of liquid air operates by gradually supplying heat to a cryogenic liquid mixture.
2
Compare boiling points of components
Boiling points on the Celsius scale: Nitrogen (−196∘C-196^\circ\text{C}) < Argon (−186∘C-186^\circ\text{C}) < Oxygen (−183∘C-183^\circ\text{C}).
−196∘C-196^\circ\text{C} is a lower temperature than −186∘C-186^\circ\text{C} and −183∘C-183^\circ\text{C}.
3
Determine the phase change order during temperature rise
As the temperature ascends from −200∘C-200^\circ\text{C}, it first reaches −196∘C-196^\circ\text{C} (nitrogen boils), then −186∘C-186^\circ\text{C} (argon boils), and finally −183∘C-183^\circ\text{C} (oxygen boils).
The component with the lowest boiling point (most volatile) boils off first at the top of the fractionating column.

Key Concept

Fractional distillation of liquid air depends on differences in boiling points, where the component with the lowest boiling point vaporizes first.
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Topics

Industrial Applications of Separation Methods