What is LPG?

Published October 18th, 2000 - 02:00 GMT

LPG is the abbreviation for "Liquefied Petroleum Gas". It comes from two general sources. Firstly, it is removed from crude oil and natural gas along with other similar condensible chemicals, often referred to as "Natural Gas Liquids", NGL. 


The second normal source is from the processing of crude oil at refineries where it is generated during the production of other petroleum products such as gasoline, kerosine, gas oil, etc. 


Whatever the source, the major constituents of all LPG are hydrocarbons containing three or four carbon atoms. The normal components of LPG are thus propane (C3H8) and butane (C4H8), butane normally being present as either n-butane or iso-butane. Small concentrations of other hydrocarbons may also be present, e.g. propylene (C3H6), butylene (C4H8), ethane (C2H6) and pentane (C5H12). 


Depending on the source of the LPG and how it has been produced, other components may also be present. 


To ensure that the LPG you receive is of a consistent high quality, detailed specifications are laid down and every batch of LPG is rigorously tested to ensure that it always meets the appropriate specification. 


LPG is a gas at atmospheric pressure and normal ambient temperatures, but it can be liquefied when moderate pressure is applied or when the temperature is sufficiently reduce. 


This property makes LPG an ideal energy source for a wide range of applications, as it can be easily condensed, packaged, stored and utilised. When the pressure is released, the liquid makes up about 250 times its volume as gas, so large amounts of energy can be stored and transported compactly.  


LPG is usually available in one or two forms for standard heating and cooking purposes: commercial propane and commercial butane. Propane has the lower boiling point, forming a vapour at temperatures down to -45 C, so it is the more versatile for general use.  


Butane has a boiling point of -2 C and so cannot be used in cold environments as it will not vaporise as readily as propane.  


LPG can also be used in specialised applications which require a more rigorous specification. Such applications include food processing, aerosol propellants, food processing, as an automotive fuel.  


How Does LPG Work? 

LPG exists as a gas at normal atmospheric pressure and temperature, only existing as a liquid at very low temperatures or under pressure. Normally the gas is stored in liquid form under pressure in a steel container - a cylinder or a tank.  


The container will be full of LPG - liquid with vapour above it. The pressure inside the container will depend on the type of LPG (commercial butane or commercial propane) and the outside temperature. 


When you start to use LPG, some of the pressure in the container is released. Some of the liquid LPG then boils to produce vapour. Heat is needed to convert the liquid to vapour (known as the latent heat of vaporisation). As the liquid boils, it draws the heat energy from its surroundings.  


This is why containers feel cold to touch and why, if there is a heavy off-take, water or ice may appear on the container. When you stop using the LPG, the pressure will return to the equilibrium value for the surrounding temperature. 


Because the pressure of the LPG in the container varies with the surrounding temperature and because it is much higher than is needed by the appliances that use it, it needs to be controlled to ensure a steady supply at constant pressure. This is done by a regulator, which limits the pressure to suit the appliance that is being fuelled. 


Because LPG has to draw heat from the surroundings in order to vaporise, it is important that the correct size of container is chosen which will satisfy the off-take all the year round. 

Source: Shell.  

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