Liquefied petroleum gas (LPG) includes propane, butane, butadiene, isopropane, propylene, and vinyl chloride monomer, which are all by-products of the production of oil and natural gas. LPGs have a variety of uses including as a cooking/heating fuel, refinery feedstock, automotive power, and numerous other plastics and chemicals applications.
another definition of LPG is that LPG is a mixture of commercial butane and commercial propane having both saturated and unsaturated hydrocarbons. LPG marketed in
India shall be governed by Indian Standard Code IS-4576 (Refer Table 1.0) and the test methods by IS-1448.
The Chemistry of LPG
Atoms of hydrogen (H) and carbon (C) combine to form hydrocarbon molecules which can be made up of different numbers of hydrogen and carbon atoms, hence the term
‘hydrocarbon’.
A molecule containing three carbon atoms and eight hydrogen atoms is called propane.
In a like manner, four carbon atoms bonded to 10 hydrogen atoms forms butane:
There are two possible configurations for the butane molecule. The above arrangement consists of a straight C-chain and is called normal butane or n-butane. If the C-chain is branched, it is called iso-butane. Such a re-arrangement of the atoms is known as isomerisation and has no significant effect on the fuel properties.
Hydrocarbons with single carbon bonds are known as saturated hydrocarbons while those with double or triple bonds are unsaturated hydrocarbons. Examples of saturated
hydrocarbons are methane (CH2), ethane (C2H6), propane (C3H8) and butane (C4H10). Unsaturated hydrocarbons include ethylene (C2H4), propylene (C3H6), butylene (C4H8)
and acetylene (C2H2).
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LPG Density:
LPG at atmospheric pressure and temperature is a gas which is 1.5 to 2.0 times heavier than air. It is readily liquefied under moderate pressures. The density of the liquid is approximately half that of water and ranges from 0.525 to 0.580 @ 15 deg. C.
Since LPG vapour is heavier than air, it would normally settle down at ground level/ low lying places, and accumulate in depressions.
Calorific Value (CV) :
All substances which burn generate energy in the form of heat, which varies in quantity with the nature of the substance. The total amount of heat liberated by burning a substance is known as its Calorific Value or CV. It is usually expressed in megajoules per kg (MJ/kg). For LPG, it is 49,6 MJ/kg.
Vapor Pressure :
The pressure inside a LPG storage vessel/ cylinder will be equal to the vapour pressure corresponding to the temperature of LPG in the storage vessel. The vapour pressure is dependent on temperature as well as on the ratio of mixture of hydrocarbons. At liquid full condition any further expansion of the liquid, the cylinder pressure will rise by
approx. 14 to 15 kg./sq.cm. for each degree centigrade. This clearly explains the hazardous situation that could arise due to overfilling of cylinders.
Thermal rate of expansion (expansion and contraction)
The thermal rate of expansion of liquid LPG is about 10 times that of water and since liquids can not be compressed, it is probably the most important property of LPG affecting the way the gas is stored, handled and filled. Storage tanks and portable cylinders filled to allow for an ullage space in the vessel and cylinders must never be filled to more than about 85% of the internal volume.
When the valve of an LPG is opened, the pressure inside the cylinder is reduced and the liquid starts to vaporise (boil) at lower pressure. This vaporisation of the gas causes cooling to occur and the temperature of the gas will decrease. If the gas off-take rate is too high, the gas temperature will decrease to below 0ºC and ice will start to form on the lower outside wall of the cylinder. Because LPG contains propane and butane, with boiling points of –42,1ºC and – 0,5ºC respectively, the mixture begins to separate – propane continues to boil off while the butane remains in liquid form at temperatures below its boiling point of –0,5ºC. To avoid this situation, vaporiser units are used for LPG or pure
propane can be used instead of Handigas (butane/propane mixture). It should be noted that low winter temperatures will aggravate this situation.
Flammability :
LPG has an explosive range of 1.8% to 9.5% volume of gas in air. This is considerably narrower than other common gaseous fuels. This gives an indication of hazard of LPG vapour accumulated in low lying area in the eventuality of the leakage or spillage.
The auto-ignition temperature of LPG is around 410-580 deg. C and hence it will not ignite on its own at normal temperature.
Entrapped air in the vapour is hazardous in an unpurged vessel/ cylinder during pumping/ filling-in operation. In view of this it is not advisable to use air pressure to unload LPG cargoes or tankers.
Combustion :
The combustion reaction of LPG increases the volume of products in addition to the generation of heat. LPG requires upto 50 times its own volume of air for complete
combustion . Thus it is essential that adequate ventilation is provided when LPG is burnt in enclosed spaces otherwise asphyxiation due to depletion of oxygen apart from the formation of carbon-dioxide can occur.
Odour
LPG has only a very faint smell, and consequently, it is necessary to add some odourant, so that any escaping gas can easily be detected.
Ethyl Mercaptan is normally used as stenching agent for this purpose. The amount to be added should be sufficient to allow detection in atmosphere 1/5 of lower limit of
flammability or odour level 2 as per IS : 4576.
Colour
LPG is colourless both in liquid and vapour phase. During leakage the vapourisation of liquid cools the atmosphere and condenses the water vapour contained in them to form a whitish fog which may make it possible to see an escape of LPG.
Toxicity
LPG even though slightly toxic, is not poisonous in vapour phase, but can, however, suffocate when in large concentrations due to the fact that it displaces oxygen. In view of
this the vapour posses mild anaesthetic properties.
