Liquefied petroleum gas (LPG) is a mixture of two gases. In everyday life this mixture has a short name - PROPANE. Propane and butane are produced from oil and condensed petroleum gases. In order to keep this mixture in a liquid state, it is stored and transported under the pressure of 1.6 MPa (16 atmospheres). The process of filling the vehicles with propane looks very similar to the gasoline filling process.

Chemical composition of PROPANE-BUTANE mixture

Propane - C3H8

In the fuel mixture butane acts as fuel while propane creates pressure.

Propane-butane gas mixture is 2 times heavier than air.

In fact, the gas has no smell that is why a special odorous substance (odorant) - ethyl-mercaptan – is added into its composition.

Anti-knock index (octane rating) of propane-butane gas mixture is 110 units – it is its advantage over gasoline, whose maximum octane rating is equal to 98 units.
[in brief]
Propane-butane is lighter than gasoline and diesel fuel:
1 l of gas - 0.6 kg
1 l of gasoline - 0.73 kg
1 l of diesel - 0.82 kg

The percentage of propane and butane in the mixture is regulated by the state and depends on climatic conditions. For example, in winter the quantity of propane must be at least 70-80%, whereas in summer - just 40%.

One of the most important properties of propane and butane is the formation (in case of availability of a free surface over the liquid phase) of a two-phase "liquid-vapor" system. "Liquid-vapor" system is formed as a result of the saturated vapor pressure, i.e. the vapor pressure in the presence of liquid phase in the tank. During the tank filling process, the first portions of liquefied gas vaporize quickly and fill its entire volume creating a certain pressure in it. In case of pressure drop the gas evaporates immediately. In the tank, liquefied gas evaporation lasts until the formed liquefied gas vapors become saturated.

This property of propane and butane makes it possible to store gas in insignificant volumes, which is very important.

Let’s consider this example: pressure of the saturated butane vapor is 0.1 MPa at 0°C and 0.17 MPa at 15°C while at the same temperatures the pressure of the saturated propane vapor is 0.59 and 0.9 MPa, respectively. This inequality leads to a significant difference in pressure of the mixture when the propane and butane ratio changes. Pressure grows as the temperature increases, which leads to significant changes in the volume of liquefied gas that is in a liquid state. Consequently, if liquefied gas being in a liquid state completely fills the container and the temperature keeps on rising, then, the pressure will grow rapidly, which could lead to the container destruction.

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