Chemistry 2nd Paper · Classes 11-12 · Chapter 1 · 1.8 to 1.9

Cylinders, Liquefaction of Gases and Nitrogen Fixation by Lightning.

The cooking-gas cylinder holds a liquid, yet the stove gets a gas. The CNG cylinder of a car is packed with real gas at 220 atm. When can a gas be turned into a liquid, and when will no pressure do it? This note answers that, works the cylinder sums, and ends with how a lightning flash feeds the soil.

1. What goes into a cylinder

LPG, liquefied petroleum gas, is mainly a liquid mixture of propane and butane. Cooking gas is 25% propane and 75% butane. Isobutane, butene and a little ethane and propylene come along too.

The gas mixture is cooled and liquefied at 6 atm. A full cylinder holds 85% liquid with gas in the upper 15% of the volume, and the pressure inside is 3 kg cm⁻². LPG has no smell, so a little strongly smelling mercaptan, thiomethanol (CH₃SH) or thioethanol (C₂H₅SH), is mixed in while filling. A leak then announces itself to your nose.

LPG, LNG and CNG are clean fuels: no smoke or ash, though CO₂ and a little CO are released. The calorific value is about 94000 kJ m⁻³ for LPG and about 38000 kJ m⁻³ for LNG and CNG, so LPG is 2.47 times higher.

CNG, compressed natural gas, runs motor vehicles; it is 95-99% CH₄ with the rest C₂H₆, C₃H₈ and C₄H₁₀. At the filling station gas arriving at 2.3-3.0 atm is stored in four stages: 5-7, 7-28, 28-82 and 82-220 atm. At 25°C and 220 atm the cylinder holds 60 L, 90 L or 120-150 L depending on the vehicle.

To make LNG, CO₂, H₂S and heavy hydrocarbons are first removed from natural gas. Then mainly CH₄ is cooled to −160°C at 1 atm until it turns liquid. The pressure stays at 1 atm; the cold does the work.

In cylinder filling, only LPG escapes the gas laws, because a liquid is filled. For CNG and other gases the ideal gas law is applied. Cylinders hold oxygen, acetylene (for cutting and joining iron) and argon (an inert atmosphere in stainless-steel welding).

2. The cylinder sums

Temperature goes into every sum in kelvin, K = °C + 273. The calculator below carries four of the book's examples; change the numbers to try your own.

A 60 L CNG cylinder at 25°C and 220 atm holds n = PV/RT = (220 × 101.325×10³ × 60×10⁻³) / (8.314 × 298) = 539.84 mol. At 25°C, 1 mol = 24.789 L, so 539.84 mol = 13.38 m³; CNG is sold by the m³.

Problem 1.67(a): the moles of CH₄ in a 120 L cylinder at 27°C and 200 atm are n = PV/RT = 200 × 120 / (0.082 × 300) = 975.61 mol; mass = 16 × 975.61 = 15609.76 g. Notice R = 0.082 here (litre-atmosphere) and 8.314 in the sum before (SI); pick R to match the units of P and V.

Problem 1.67(b): the first cylinder withstands 27°C and 200 atm. At 37°C, P = 200 × 310 / 300 = 206.67 atm, above the limit of 200 atm, so it is unsuitable at 37°C. The second withstands 37°C and 50 atm; at 27°C, P = 50 × 300 / 310 = 48.38 atm, below the limit, so the second cylinder is the more suitable one.

3. When a gas can be liquefied

Liquefying a gas needs two conditions. One: bring the temperature below the critical temperature. Two: apply a suitable pressure at that temperature. The further the gas sits below its critical temperature, the less pressure it needs.

The critical temperature T_c is the temperature above which a gas cannot be liquefied however much pressure is applied: 31.1°C for CO₂, 132.4°C for NH₃, −240°C for H₂, −268°C for He. The critical pressure P_c is the minimum pressure needed to liquefy the gas at the critical temperature: 72.9 atm for CO₂, 111.5 atm for NH₃, 12.8 atm for H₂, 2.26 atm for He.

The table (T_c in °C, P_c in atm): He −268, 2.26; H₂ −240, 12.80; N₂ −147.1, 33.50; Ar −122.1, 48.36; O₂ −118.8, 50.14; CH₄ −82.3, 45.80; CO₂ 31.1, 72.90; NH₃ 132.4, 111.50; Cl₂ 143.9, 78.75; SO₂ 157.7, 77.80.

Squeeze a gas. First below its critical temperature, then above it.

A gas with a high critical temperature and strong intermolecular attraction (a large van der Waals a) liquefies easily. So NH₃ (T_c 132.4°C) liquefies easily and He (T_c −268°C) with the greatest difficulty. Now drop each gas-and-temperature card into a box. Once a chip lands, its T_c shows.

4. The Joule-Thomson effect and inversion temperature

The Joule-Thomson effect: when a confined high-pressure gas is suddenly let expand through a porous plug or a hole into a large low-pressure space, its temperature falls. The gas does work against intermolecular attraction, and that uses up internal energy.

Commercially, air is first cooled a little, then cooled further by repeated Joule-Thomson expansion, and finally liquefied at about −200°C and 200 atm.

The inversion temperature is the temperature below which a gas cools on expansion. Above it, Joule-Thomson expansion raises the temperature instead of lowering it. It is −80°C for H₂ and −240°C for He.

H₂ and He are the hardest to liquefy because their inversion temperatures are very low. So H₂ is first cooled with liquid air, and He with liquid H₂, to below the inversion temperature. Then Joule-Thomson expansion and high pressure liquefy them.

5. Nitrogen fixation by lightning

Nitrogen fixation is the process of converting the N₂ of the air into nitrogen compounds and holding it in a usable form.

The first step happens in the sky. At about 3000°C in a lightning flash, the N₂ and O₂ of the air combine to form NO: N₂ + O₂ → 2NO. High-temperature combustion in power plants and engines makes NO as well.

At about 50°C the NO combines with more O₂ to give NO₂: 2NO + O₂ → 2NO₂. With rain water it forms HNO₃: 4NO₂ + O₂ + 2H₂O → 4HNO₃.

Strike a bolt of lightning, then follow the nitrogen of the air all the way down to the soil.

The HNO₃ falls with the rain and reacts with the basic substances lime (CaO) and CaCO₃ in the soil to form soluble nitrate salts: CaO + 2HNO₃ → Ca(NO₃)₂ + H₂O. This is natural N-fixation in the soil. Now put the steps in order.

6. Nitrogen from factories and microbes

The Haber process: N₂ + 3H₂ → 2NH₃, at 500°C and 200 atm, with an Fe catalyst and Al₂O₃. Urea and DAP fertilisers are made from the NH₃.

The Ostwald process oxidises NH₃ over a catalyst (Pt, 900°C): 4NH₃ + 5O₂ → 4NO + 6H₂O. Then 4NO + 2O₂ → 4NO₂ and 4NO₂ + O₂ + 2H₂O → 4HNO₃. The HNO₃ gives nitrate fertilisers such as CAN and Ca(NO₃)₂.

In biological fixation, green algae and the symbiotic bacteria in the root nodules of legumes such as pea, bean and gram absorb atmospheric N₂. More N₂ is fixed this way than by the other two processes. A molybdenum (Mo) coenzyme reduces N₂ to NH₃ and NH₄⁺.

Other soil bacteria oxidise NH₄⁺ first to NO₂⁻ and finally to NO₃⁻. Plants absorb the NO₃⁻ and reduce it to make protein. Animals eat the protein and excrete urea [(H₂N)₂C=O].

Denitrifying bacteria close the loop: they reduce NO₃⁻ to NO₂⁻, N₂O and finally N₂, and send it back to the air. That is why the air stays about 78% N₂. This transformation of nitrogen in nature is called the nitrogen cycle.

7. Check yourself

Three questions in the exam's style. Start with the first.

In the second, match the facts.

Think a moment before choosing the last one.

The whole note on eight cards.

Source: Chemistry 2nd Paper, Classes 11-12 (NCTB), Chapter 1 “Environmental Chemistry”, section 1.8 to 1.9.