How do we liquefy petroleum gas

Liquefied petroleum gas (LPG) is increasingly employed as a fuel. We produce it by applying a huge pressure (10-20 x p ) to the petroleum gas obtained from oil fields. 

Above a certain, critical, pressure the hydrocarbon gas condenses we say it reaches the dew point, when droplets of liquid first form. The proportion of the gas liquefying increases with increased pressure until, eventually, all of it has liquefied. 

Increasing the pressure forces the molecules closer together, and the intermolecular interactions become more pronounced. Such interactions are not particularly strong because petroleum gas is a non-polar hydrocarbon, explaining why it is a gas at room temperature and pressure. We discuss other ramifications later. 

The particles of an ideal gas (whether atoms or molecules) do not interact, so the gas obeys the ideal-gas equation all the time. As soon as interactions form, the gas is said to be non-ideal, with the result that we lose ideality, and the ideal-gas equation (Equation (1.13)) breaks down. We find that pV tiRT. 

When steam (gaseous water) is cooled below a certain temperature, the molecules have insufficient energy to maintain their high-speed motion and they slow down. At these slower speeds, they attract one another, thereby decreasing the molar volume. 

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