Liquefaction Of Air — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Liquefaction of Air Purified, dry air is compressed and precooled, then allowed to expand (Joule–Thomson throttling in Linde–Hampson, or turbine expansion in Claude) to produce intense cooling that condenses a portion of the air to a cryogenic liquid. The resulting liquid air is separated by fractional distillation to obtain high-purity nitrogen, oxygen, and argon. Frosting/ice forms on cold lines and exchangers; temperatures drop to cryogenic levels. Liquid oxygen appears pale blue and is paramagnetic (attracted by a strong magnet), nitrogen is colorless and boils vigorously at ~77 K, and argon is colorless with an intermediate boiling point. No chemical reaction occurs; cooling arises from real-gas enthalpy decrease on expansion below the inversion temperature (Joule–Thomson effect) and from work-producing expansion (Claude). Latent heats must be removed to condense components (approximate values at bp: N2 ≈ 199 kJ·kg⁻¹, O2 ≈ 213 kJ·kg⁻¹); the process is overall exothermic to the surroundings as compressors supply work that is ultimately rejected as heat. 1. Purification: Ambient air is filtered, CO2 is removed by passage through alkali solution (e.g., KOH/NaOH), and moisture is removed using desiccants (silica gel/molecular sieves) to prevent dry-ice/ice blockages upon cooling. 2. Compression and after-cooling: The clean, dry air is compressed (Linde: ~150–200 atm; Claude: ~40–60 atm) and cooled in water/air coolers back toward ambient, removing the heat of compression. 3. Regenerative counter-current cooling: The high-pressure air is passed through a heat exchanger against returning cold, low-pressure exhaust, lowering its temperature typically to ~200–230 K before expansion. 4. Expansion step for cooling: (Linde) The precooled, high-pressure air undergoes Joule–Thomson throttling through a valve/porous plug to ~1 atm; because T is below the inversion temperature (N2 ≈ 621 K; O2 ≈ 764 K), the gas cools sharply and partly liquefies: ( Air (g) JT at P hi P lo Air (l) + colder Air (g) ). 5. (Claude enhancement) A portion of high-pressure air expands in a turbine/expander producing external work (w out), causing further adiabatic cooling before mixing with the JT-throttled stream, improving efficiency and yield. 6. Collection and separation: The cold two-phase mixture is fed to a distillation column system (double column). Lower-boiling N2 (b.p. 77.4 K) rises and is withdrawn overhead; higher-boiling O2 (b.p. 90.2 K) collects near the bottom; Ar (b.p. 87.3 K) is drawn from an intermediate section and further purified. Assuming Joule–Thomson expansion always cools gases; cooling occurs only below the inversion temperature (true for N2 and O2 at room temperature). Thinking extremely high pressures (≫200 atm) are necessary; typical are ~150–200 atm (Linde) or 40–60 atm (Claude). Mixing up boiling points: N2 (77.4 K) < Ar (87.3 K) < O2 (90.2 K); hence N2 comes off first, O2 last. Forgetting pretreatment: CO2 and H2O must be removed to avoid dry-ice/ice blockages in valves and exchangers. Confusing that a catalyst is involved—this is a physical (cryogenic) separation, no catalyst.