Elgas LPG Gas Blog

LPG Properties & LPG Composition -  What Are the Properties of LPG

LPG Composition - Components of LPG

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 and all with different LPG properties. 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 Properties - What are the Properties of LPG - Properties of Propane and Butane

The properties of LPG (propane and butane) include: LPG is liquid under pressure but becomes gaseous in ambient conditions. LPG vapour is about 1.55 (propane) to 2.08 (butane) times heavier than air. LPG boiling point is between -42°C to -0.4°C, depending on the propane to butane constituent ratio in the LPG gas mixture.

The properties of LPG (propane) are -42°C boiling point, -188°C freezing point, heavier than air density, C3H8 chemical formula, 1967ºC flame temperature, 470°C auto ignition temperature, -104°C flash point, temperature based pressure, Ethyl Mercaptan odour, 2.15% to 9.6% LPG/air limits of flammability and more.

List of LPG Properties

Properties of LPG include:

  • LPG (propane) gas boiling point temperature: -42 °C or -44 °F

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

    Propane boiling point - LPG (Propane) Vaporisation

  • Heavier than Air LPG density (Propane Density) - LPG specific gravity (Propane specific gravity)

  • LPG composition - components - constituents: Propane, Butane & Isobutane

  • LPG chemical formula - molecular formula: C3H8 or C4H10

  • LPG gas flame temperature: 1967 ºC or 3573 ºF

  • LPG-Propane ignition temperature (in air): 470°C - 550°C (878°F - 1020°F)

  • LPG-Propane auto ignition temperature: 470 °C or 878 °F

  • LPG flash point: -104°C or -156°F

  • LPG vapour pressure: 637 kPa @ 27°C (128 PSIG @ 80°F)

  • LPG odour: Ethyl Mercaptan added

  • LPG appearance: clear

  • LPG energy content: 25 MJ/L or 91,547 BTU/Gal (60°F)

  • Gaseous expansion of LPG: 1 L (liquid) = 0.27 M3 (gas)

  • Combustion formula for LPG:  C3H8 + 5 O2 → 3 CO2 + 4 H2O + Heat (complete combustion)

  • LPG limits of flammability: 2.15% to 9.6% LPG/air

  • LPG properties nomenclature

  • Molecular weight of LPG components: Propane is 44.097 kg/kmole - Butane (n-butane) is 58.12 kg/kmole

All of the above are the properties of LPG.

We’ll review these most commonly referenced LPG properties…

LPG 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
 Propane Ignition Temperature in Air  470°C - 550°C (878°F - 1020°F)
 Limits of Flammability
 2.15% to 9.6% LPG/air
 Propane Auto Ignition 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 Gas Chemical Formula

LPG structure - Propane molecule

LPG is comprised primarily of propane and butane, whilst natural gas is mostly methane. LPG is generally a mixture of predominantly propane (C3H8) and/or butane (C4H10), with smaller amounts of other NGL hydrocarbons including ethane, isobutane and pentanes. LPG physical structure is as both a liquid, when under pressure, and as a gas.

LPG structure (propane structure) is as a three carbon molecule with the chemical formula of LPG gas - propane as C3H8. That's 3 carbon atoms and 8 hydrogen atoms making one molecule of propane.

LPG structure (butane structure) is a four carbon molecule with the formula C4H10, with 4 carbon atoms and 10 hydrogen atoms making one molecule of butane. 

These are also chemical formulae (formulas) of LPG gas. The illustrations shows models of the LPG structure of a propane molecule and the LPG structure of a butane molecule.

There are a number of LPG gas chemical formulas (formulae). Ethane chemical formula is C2H6. Propane chemical formula is C3H8. Butane and Isobutane both have the same chemical formula C4H10, as isobutane is an isomer of butane. Pentane (n-pentane) chemical formula is C5H12 but is only a gas over 36.1°C. Heavier hydrocarbons (pentanes plus) are liquids or waxy solids.

LPG structure - butane molecule

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 Gas Temperature: Flame, Boiling Point, Melting/Freezing Point - Liquid Propane TemperatureLPG Gas Temperature - Flame Temperature

LPG gas temperature includes LPG gas flame temperature, LPG gas boiling temperature, LPG gas ignition temperature, LPG gas auto ignition temperature, LPG gas flash point temperature and LPG gas freezing temperature.

  • LPG gas temperature, both propane and/or butane, have adiabatic flame temperature about 1967°C (3573ºF), when burned in air.

  • LPG gas temperature for liquid propane boiling, when it turns to LPG gas, is -42°C or -44°F.

  • LPG gas temperature flash point is -104°C or -156°F

  • LPG gas temperature for ignition in air is 470°C - 550°C (878°F - 1020°F)

  • LPG gas temperature also affects gas pressure, as pressure rises with temperature.

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

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

LPG is composed of liquid or gas (vapour), depending on pressure and LPG gas temperature. The LPG boiling point is the liquid propane temperature at boiling and becomes vapour (gas).

LPG - liquefied petroleum gas - temperature also affects the gas cylinder pressure.

LPG Liquefaction - LPG-Propane Dew Point

The LPG-propane dew point is the temperature at which gas changes into its liquid state, which is more accurately called liquefaction. The conversion of LPG vapour to LPG liquid is called liquefaction, and depends on temperature and pressure of the vapour. The higher the temperature of the vapour, the higher the pressure needed to convert the vapour to liquid.

For Propane vapour at 20°C must be pressurised to about 836 kPa to see it liquefy, and at 50°C, about 1713 kPa pressure is required. The lower the temperature, the easier it is to liquefy the vapour.

For n-Butane vapour at 20°C must be pressurised to about 115 kPa to see it liquefy, and at 50°C, about 510 kPa pressure is required.

For mixtures of Propane and Butane, the liquefaction conditions also depend on the composition of the mix, as well as the temperature and pressure of the vapours.

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

LPG at 1 atm of pressure and 20°C is a gas which is about 1.55 (propane) to 2.08 (butane) times heavier than air. LPG is liquefied under modest pressure of 1,220 kPa (177 psi) at 37.8°C (100°F). LPG liquid propane density is slightly less than 50% that of water at 25°C and almost 60% at -40°C.

LPG density of propane liquid is lighter than water, at about ½ that of water. LPG density of butane liquid is lighter than water, at about 60% that of water.

Relative Density of LPG

Relative density of LPG is measured for both liquid LPG density and gaseous LPG density. The relative density of LPG liquid is compared to water whilst the relative density of LPG gas is compared to air.

Relative density of LPG liquid is about half that of water at 495 kg/m3 (25°C) vs 1,000 kg/m3 (4°C) for water.

Relative density 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 gas is 1.882kg/m3 at 0°C & 1ATM (0 psig), which is STP (Standard Temperature & Pressure), the difference being a lower temperature.

Specific gravity of LPG gas depends on whether it is propane, butane or an LPG gas mixture of the two. Specific gravity of LPG gas as propane is 1.882 kg/m3 (at STP).

Specific Gravity of LPG Gas & Liquid - Propane

Specific gravity of LPG gas goes from 1.5219 kg/m3 to 1.882 kg/m3, depending on the temperature.  Specific gravity of LPG gas is 1.882 kg/m3 at 0°C (32°F) & 1ATM (STP) or 1.5219 kg/m3 at 20°C. So, specific gravity varies with temperature. Specific gravity of LPG liquid is 0.495 at 25°C or 0.585 at -40°C.

Specific gravity of LPG gas is 1.5219 kg/m3 at 20°C (68°F) & 1ATM (NTP - Normal Temperature and Pressure)

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

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

Density of LPG - PropaneDensity of LPG liquid is lighter than water by about half. The relative density of LPG liquid - specific gravity of liquid propane is 0.495 (at 25°C). One litre of propane liquid weighs 0.51kg. The relative density of LPG liquid is one gallon of liquid propane weighs 4.24 lbs

LPG (propane) expands at 1.5% per 5.55°C temperature increase. In other words, the density changes.

So, if the gas bottle was filled by volume on a hot day, it would have less gas, in kg, than a gas bottle filled on a cold day.

That's why it is frequently measured and sold in kg.

Most auto LPG bowsers and tanker delivery trucks have an automatic correction factor so it can be delivered and/or sold in litres.

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

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

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

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

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

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

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 2.48 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 is Heavier than Air - Specific Gravity of LPG Gas - LPG-Propane is Heavier than Air

The LPG density, as gas, is about 1.9 times 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.

Relative density of LPG gas - 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, relative density of LPG gas 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 Pressure Varies with Temperature

As mentioned before, LPG is stored in a gas bottle under pressure. LPG gas pressure varies with temperature. 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 Gas Pressure-Temperature Chart shown)

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

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

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

LPG gas 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 LPG gas pressure inside the cylinder (LPG gas bottle pressure) will remain the same from full until the vaporistion of the last of the liquid LPG.

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

Odourant Added for Safety - Smell in Propane

In its natural state, LPG is an odourless gas.
The distinctive smell in propane that people associate with LPG is actually added to it as a safety measure.
Without the addition of a smell in propane, 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.

LPG Energy Content - Propane

LPG energy content is approximately 25MJ per litre. One gallon of propane has the LPG energy content of 91,547 BTU (60°F).

25MJ also converts to 6.9kWh.

Calorific Value - LPG energy content

The LPG energy content or calorific value of the gas used to heat water is no different than LPG used for other applications.

So, LPG energy content 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 of Propane Equation for Complete Combustion of Propane

In the presence of enough oxygen, the combustion of propane forms water vapour and carbon dioxide, as well as heat.

Combustion of Propane Equation for Complete Combustion of 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.

Equation for incomplete combustion of propane results in the production of water, carbon monoxide, carbon dioxide and heat:

Equation for Incomplete Combustion of Propane

Equation for incomplete combustion of propane - LPG:
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

LPG gas temperature at which a flame burns is 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 - Propane Ignition Temperature

The lowest propane ignition temperature or LPG flash point is  -104°C or -156°F. This is the minimum propane ignition 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).  The propane ignition temperature is also called the Propane Auto Ignition Temperature, as explained below.

Propane Auto Ignition Temperature (Propane Ignition Temperature) - Butane Auto ignition Temperature

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

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

The propane auto ignition temperature decreases as the pressure or oxygen concentration increases.

Dissipation

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.