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LPG Composition & the Chemical Properties of Propane

LPG Composition - Components of LPG - LPG Chemical Properties

LPG composition includes propane, butane & isobutane.

LPG composition is primarily propane, butane, isobutane, butylenes, propylene and mixtures of these gases, which are the components of LPG . LPG is composed of liquid or gas (vapour), depending on pressure and LPG gas temperature.

The domestic LPG composition and the commercial LPG composition are typically the same propane, butane or a mixture of the two gases.

LPG – Liquefied Petroleum Gas – is produced during natural gas processing and petroleum refining.

Natural Gas Liquids – NGL – have the same LPG composition and gas temperatures plus a few more gases not normally included in LPG.

The full NGL list of components of LPG includes ethane, ethene, butylenes, propylene, propene, isobutene, butadiene, pentane, pentene and pentanes plus, as well as propane, butane and isobutane.

LPG - Propane Chemical Properties

LPG (propane) has many chemical properties including:

  • LPG (propane) gas boiling point temperature

  • LPG (propane) gas melting/freezing temperature

    Propane boiling point - LPG (Propane) Vaporisation

  • Relative LPG density (Propane Density) - LPG specific gravity (Propane specific gravity)

  • LPG composition - components - constituents

  • chemical formula - molecular formula

  • LPG gas flame temperature

  • LPG gas autoignition temperature

  • flash point

  • vapour pressure

  • odour

  • appearance

  • energy content

  • gaseous expansion

  • combustion formula

  • limits of flammability

  • nomenclature

  • molecular weight

We’ll review these most commonly referenced chemical properties…

LPG (Propane) Chemical Properties Chart

LPG (Propane) Chemical Properties Chart
 LPG - Propane Boiling Point
 -42 °C  or  -44 °F
 LPG Melting - Freezing Point
 -188 °C  or  -306.4 °F
 Specific Gravity of Liquid LPG-Propane
 0.495 (25°C)
 LPG Density Propane Gaseous Density
 1.898 kg/m3 (15°C)  or  0.1162 lb/ft3
 Energy Content of LPG
 25 MJ/L or 91,547 BTU/Gal (60°F)
 LPG Gaseous Expansion
 1 L (liquid) = 0.27 M3 (gas)
 Propane Flame Temperature
 1967 ºC  or  3573 ºF
 Ignition Temperature of Propane in Air  470°C - 550°C (878°F - 1020°F)
 Limits of Flammability
 2.15% to 9.6% LPG/air
 Autoignition Temperature
 470 °C  or  878 °F
 Molecular Weight
 44.097 kg/kmole
  Note: Some numbers have been subject to rounding.

LPG Structure - Chemical Formula of LPG Gas

LPG structure - Propane molecule

LPG structure (propane structure) is as a three carbon molecule with the chemical formula of LPG gas - propane as C3H8. Butane is a four carbon molecule with the formula C4H10. This is also a chemical formula of LPG gas. The illustration shows a model of the LPG-propane molecule structure.

LPG physical structure is as both a liquid, when under pressure, and as a gas.

LPG - Propane Boiling Point

Water boils at 100°C or 212°F, becoming a gas (steam).  

In contrast, LPG (propane) boils at -42°C or -44°F, becoming gas vapour.  

LPG stays liquid because it is under pressure in a gas cylinder. 

As a liquid, it looks a lot like water.

It is colourless and odourless in its natural state. 

LPG Specific Heat Capacity

LPG specific heat capacity is the amount of heat required to change the temperature of a kg or other mass unit of LPG by one degree. Propane is a liquid below -42°C and a gas above that boiling point.

Commercial & Domestic LPG Composition: Which Gas is Present in LPG?

The commercial and domestic LPG composition includes propane, butane and mixtures of these gases. LPG – Liquefied Petroleum Gas – describes flammable hydrocarbon gases.

LPG, liquefied through pressurisation, comes from natural gas processing and oil refining.

In different countries, the commercial and domestic LPG composition can be propane, butane or propane-butane blends.

In Australia, LPG is just propane.

LPG Density - Relative Density of LPG (propane) Gas & Liquid - Specific Gravity of LPG (propane) Gas & Liquid

Relative density of LPG (propane) gas (specific gravity of LPG gas) is 1.55 times heavier than air at 1.898 kg/m3 vs 1.225 kg/m3 for air (both 15°C at sea level). Relative density of LPG (propane) liquid is about half that of water at 495 kg/m3 (25°C) vs 1,000 kg/m3 (4°C) for water.

Specific gravity of LPG (propane) liquid - Relative density of LPG (propane) liquid and water is 0.495 (25°C) and 1.000 (4°C), respectively.

Specific Gravity of LPG Gas - Relative Density of LPG Gas - Density of Propane gas at STP is 1.882kg/m3 at 0°C & 1ATM (0 psig), which is STP (Standard Temperature & Pressure)

LPG Density - Specific Gravity of LPG Liquid - Specific Gravity of Propane Liquid

Density of LPG - PropaneThe relative density of LPG liquid - specific gravity of propane liquid is 0.495 (at 25°C). One litre of propane liquid weighs 0.51kg. The LPG density of Propane is one gallon of liquid propane weighs 4.24 lbs

Unlike water, 1 kilogram of LPG (propane) does NOT equal 1 litre of LPG. Relative density of LPG (propane) liquid or specific gravity of LPG (propane) liquid is about half that of water.  

In Australia, where LPG is propane, 1kg of LPG has a volume of 1.96L. 

The LPG density of 1L of propane liquid weighs 0.51kg. 

The LPG density of propane is 1 gallon of liquid propane weighs 4.24 pounds.

The relative density of LPG liquid - the specific gravity (SG) of LPG liquid –  specific gravity of propane liquid – is 0.495 (at 25°C)

The relative density of LPG (propane) gas is 580.88 kg/m3 (at boiling point)

The The relative density of LPG liquid - the specific gravity (SG) of Butane liquid is 0.601 (at 25°C)

The relative density of LPG gas - butane gas is 601.26 kg/m3 (at boiling point)

Specific Weight of Propane (LPG)

The specific weight of propane (LPG) is defined by the ratio of the weight to the volume. So, the specific weight of propane is 0.51kg per litre (0.51kg/L) or 4.24 pounds per gallon (4.24lbs/gal).

Relative Density of LPG Gas - Specific Gravity of LPG Gas - LPG-Propane is Heavier than Air

The relative density of LPG gas (specific gravity of LPG gas) is 1.898 kg/m3 (at 15°C and sea level). 1 ft3 of propane weighs 0.1162 pounds.

LPG density of gaseous LPG - Butane gas is 2.5436 kg/m3 (at 15°C and sea level)
In contrast, the density of Air is 1.225 kg/m3 (at 15°C and sea level).
So, LPG relative density is heavier than air.
Note that LPG gas is also referred to as LPG vapour, which is the more technically correct term.

10 Important LPG - Propane Facts

1. LPG (or LP Gas) is the acronym for Liquefied Petroleum Gas or Liquid Petroleum Gas.

2. LPG is a group of flammable hydrocarbon gases liquefied through pressurisation.

    They are, in most cases, used as fuel.

3. LPG comes from natural gas processing and petroleum refining.

4. There are a number of gases that fall under the “LPG” label.

    These include propane, butane and isobutane (i-butane), as well as mixtures of these gases.

5. LPG gases are compressible into liquid at low pressures.

6. The common uses for LPG include use for fuel in heating, cooking, hot water and vehicles.

    It is also utilised for refrigerants, aerosol propellants and petrochemical feedstock.

7. LPG is stored, as a liquid, in steel vessels ranging from small BBQ gas bottles to larger gas cylinders and storage tanks. (45kg gas bottles shown)

8. "Wet gas" is a term that is sometimes used to describe LPG, as a result of its liquidity. 
9. An alternate reference for LPG (propane) is as a Natural Gas Liquid NGL.
10. In nature, propane does not occur alone.

LPG = Propane

It is also called LPG Gas, LP Gas, Propane, BBQ Gas, Camping Gas or Autogas.
LPG can be other gases in other countries. 

LPG Gas Temperature: Flame, Boiling Point, Melting/Freezing PointLPG Gas Temperature - Flame Temperature

LPG gas temperature includes LPG gas flame temperature, LPG gas boiling temperature and LPG gas freezing temperature. LPG - liquefied petroleum gas - temperature also affects the gas cylinder pressure.

LPG gas flame temperature (when burned with air) is 1967ºC or 3573ºF.

LPG (propane) gas boiling temperature is -42°C or -44°F, as compare to water at 100°C or 212°F

The LPG boiling point is where liquid propane boils and becomes vapour (gas).

LPG (propane) gas melting/freezing temperature is at -188°C or -306.4°F

So, LPG gas temperature for freezing is a much lower temperature than water, which freezes at 0ºC.

LPG - Propane Pressure Varies with Temperature

As mentioned before, LPG is stored in a gas bottle under pressure. 

The term “pressure” refers to the average force per unit of area that the gas exerts on the inside walls of the gas bottle. 

(LPG Pressure-Temperature Chart shown)

LPG Pressure-Temperature ChartPressure measurement is in kilopascals (kPa) or pounds per square inch (psi).

“Bar” is yet another unit of measure for pressure. 

1 Bar = 100 kPa, so it is metric based but not an SI unit of measure.

LPG pressure can vary based on temperature, as shown in the chart.

The level of fill in the gas bottle comes into play when the LPG is in use, as it affects the rate of vapourisation. 

LPG is a liquefied gas. So, the pressure inside the cylinder will remain the same from full until the vaporistion of the last of the liquid LPG.

Then the pressure will fall, with the use of the last of the LPG vapour.

Odourant Added for Safety

In its natural state, LPG is an odourless gas.
The distinctive smell that people associate with LPG is actually added to it as a safety measure.
Without the addition of an odourant, leaking gas could collect without detection.

Avoid Direct Contact - Cold Burns

Always use caution should to avoid direct exposure, as liquid LPG is cold enough to cause severe cold burns on exposed skin.

Energy Content of LPG - Propane

LPG (propane) contains approximately 25MJ per litre. 
This also converts to 6.9kWh.
One gallon of propane contains 91,547 BTU (60°F).

Calorific Value of Water Gas

The calorific value of the gas used to heat water is no different than LPG used for other applications.
So, once again, 'water gas' contains 25MJ per litre or 91,547 BTU (60°F) per gallon.

LPG Expansion - Gaseous Expansion

Gaseous expansionLPG expansion is 270 times the volume of gas to the volume of liquid. In other words, LPG expansion is to 270 times the volume when it goes from liquid to gas. So, 1L of liquid LPG (propane) expands to equal 270L of gaseous LPG expansion.   

As there are 1000L in a cubic meter (M3), 1L of liquid LPG expands to 0.27M3.

Combustion Formula Equation for LPG - Propane

In the presence of enough oxygen, LPG burns to form water vapour and carbon dioxide, as well as heat.
Formula Equation for Complete Combustion of LPG - Propane:
Propane + Oxygen → Carbon Dioxide + Water + Heat
C3H8 + 5 O2 → 3 CO2 + 4 H2O + Heat
Incomplete combustion of LPG (propane) occurs when not enough oxygen is present.
Incomplete combustion results in the production of water, carbon monoxide, and carbon dioxide:

What's the Equation for Incomplete Combustion of Propane?

Formula Equation for Incomplete Combustion of LPG - Propane:
Propane + Oxygen → Carbon Dioxide + Carbon Monoxide + Water + Heat

2 C3H8 + 9 O2 → 4 CO2 + 2 CO + 8 H2O + heat

LPG Gas Temperature: LPG - Propane Flame Temperature

An LPG (propane) flame burns at 1980°C. 
When it is burning properly, the flame is blue
A yellow or red flame is usually indicative of incomplete combustion.

Limits of Flammability

The lower and upper limits of flammability are the percentages of LPG that must be present in an LPG/air mixture. 
This means that between 2.15% and 9.6% of the total LPG/air mixture must be LPG in order for it to be combustible.

LPG Flash Point Temperature

The flash point of LPG (propane) is  -104°C or -156°F.  
This is the minimum temperature at which propane will burn on its own after ignition.   
Below this temperature, it will stop burning on its own.  
However, if a source of continuous ignition is present, it will burn below  -104°C.

Propane Ignition Temperature in Air - Ignition Temperature of Propane Gas

The propane ignition temperature in air (ignition temperature of propane gas) is when it reaches a temperature between 470°C - 550°C (878°F - 1020°F).  This is also called the Autoignition Temperature of Propane, as explained below.

Propane Autoignition Temperature - Butane Autoignition Temperature

Propane autoignition temperature is 470°C - 550°C (878°F - 1020°F). The propane autoignition temperature is the lowest temperature at which it will spontaneously ignite in air. Butane autoignition temperature is 405°C or 761°F.

Autoignition is when propane or butane ignites without an external source of ignition, like a spark or flame.

The propane and butane autoignition temperature decreases as the pressure or oxygen concentration increases.


LPG gas is heavier than air and will sink to and collect at the lowest point.  
If vented to the outside air, LPG will dissipate with the slightest movement of air. 
With LPG vented into a sealed structure, with no air movement, the LPG gas will collect on the floor. It will rise toward the ceiling, as more LPG enters into the structure.

Molecular Weight for LPG – Propane – Butane – Isobutane

The molecular weight for LPG – Propane – is 44.097 kg/kmole.
The molecular weight for Butane (n-butane) is 58.12 kg/kmole.
For Isobutane (i-butane), the molecular weight is the same as for n-butane at 58.12 kg/kmole.

LPG Vapour (Gas) Use vs. Liquid Use

LPG (propane) supply is either liquid or vapour. 
The difference is in the extraction from supply cylinder or vessel.
Most LPG applications use vapour.  
Appliances such as water heaters, room heaters and cookers all use vapour.  
If these appliances were to have liquid LPG flow to their burners, the result could be a fire or similar safety hazard.  
This is why LPG cylinders should always be upright, so that any gas released is in vapour form only.

6 Things You Didn’t Know About LPG

While you may know some of these facts, chances are you don’t know all of them. 

Have a read and learn more about:

  1. Real LPG Explosions are Really Rare

  2. Simulated Natural Gas from LPG

  3. The Source of LPG

  4. Australian Made Energy

  5. LPG is a Renewable Energy Source

  6. LPG is NOT Coal Seam Gas (CSG)

1. LPG Cylinder Explosions are Extremely Rare + Video

Propane tank peril courtesy of Myth Busters. Hollywood and the media would have you believe that LPG cylinder explosions are a common event. 

In fact, explosions are quite rare and it is quite difficult to even make an LPG cylinder explode on purpose. 

You’ll enjoy watching this Myth Busters Video where they try to make a cylinder explode.

Most gas explosions are the result of gas leaking into a confined space, like a kitchen. 

This is no more likely with LPG than with piped natural gas. 

Often, the gas bottle itself is not even involved in the event, as gas bottles are always stored outdoors.

2. Simulated Natural Gas from LPG

Most people have never even heard of Simulated Natural Gas (SNG) let alone know that it can be made with LPG. 

Mixing vapourised LPG with air produces SNG.

We can SNG use in place of natural gas, as it has near identical combustion characteristics.  

It can be used alone or mixed with regular natural gas. 

There are no changes required in burners, regulators or gas jets.

There are a number of reasons to use SNG:

•  To help meet peak demand when natural gas supplies are inadequate

•  To operate while in preparation for the start-up of a natural gas supply

•  As a stand-by in the event of a natural gas supply disruption

Simulated natural gas has a few names. 

Besides SNG, it is also called propane-air and LPG-air.

3. The Source of LPG

Many people mistakenly think of LPG as a by-product. 

In reality, LPG is a valuable co-product produced from gas fields and crude oil refining.

They process the gas stream from natural gas fields to separate the gases present.

These include methane, ethane, propane, butanes and pentanes.

Impurities are also removed, including water.

The produced gases are each funnelled into their own supply streams. 

They capture propane and butane, the two common types of LPG, and store them in their liquefied form.

The same is true of crude oil refining. 

The refinery process creates many co-products.

The co-products include gasoline, diesel fuel, asphalt base, heating oil, naphtha, kerosene and LPG.

4. LPG is Australian Made Energy

LPG is the only motor fuel in which Australia is self-reliant. 

Unlike both petrol and diesel, for which Australia relies on imports, we produce more LPG than we consume.

Not only is Australia completely self-sufficient in LPG but it is also a net exporter of LPG. 

In 2013, Australia produced 2.3 Million tonnes of LPG.

That satisfied the local demand of 1.5 million tonnes, with net exports of 815,000 tonnes.

5. LPG is now a Renewable Energy Source

LPG is now a renewable energy source

LPG has gone from being a traditional fossil fuel to a new form of renewable energy. 

Scientists have created a genetically engineered version of the common E. coli bacteria.

This version produces propane (LPG).  

So, LPG is now a renewable energy.

The bacteria consume sugar.

With genetic modification, and the help of a couple of enzymes, they make propane.

The propane produced is chemically identical to regular propane. 

6. LPG is NOT Coal Seam Gas (CSG)

There is some confusion over what Coal Seam Gas (CSG) is and what it is not. 

LPG is not CSG.

While CSG may contain various gases, typical CSG is 95% to 97% pure Methane.

LPG is not Methane. 

LPG is Propane.




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The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.