Liquefied Petroleum Gas

What is the real difference between butane, isobutane, propane and LPG?

All three gases are consider to be LPG – Liquefied Petroleum Gas.

Generally, the acronym LPG and propane are interchangeable, with propane being the preferred term in the United States and LPG preferred in Australia and New Zealand.

In some ways they are almost or exactly the same.

However, there are some important differences that you need to know.

First, some short summary answers:

Butane vs Propane – How do Butane and Propane Differ?

When comparing butane vs propane, the most important dissimilarities are the differences in boiling point and vapour pressure.

Propane has a lower boiling point, at -42°C vs -0.4°C for butane.

So, propane will continue to vaporise – turn to gas – even in colder climates, down to -42°C.

Butane has a lower vapour pressure at a given temperature, being about ¼ that of propane.

This lower pressure is advantageous for some propellant applications.

Butane has a higher energy content by volume.

However, propane energy content is slightly higher by weight but lower by volume.

This seeming inconsistency of butane vs propane is as a result of the two liquid gases having a different specific gravity.

What is Butane?

Butane is a flammable hydrocarbon gas that is liquefied through pressurisation.

Butane (n-butane) is also falls under the category of “LPG”.

It is classified as LPG, along with propane, isobutane and mixtures of these gases.

Butane comes from natural gas processing and oil refining.

Butane is commonly used as a fuel, propellant and refrigerant, as well as a petrochemical feedstock.

The chemical formula for Butane is C₄H₁₀. (Butane molecule model shown)

Butane is supplied to businesses that require Butane, as opposed to propane.

Butane has some specific applications where it has advantages over propane.

n-Butane or n Butane

Both n-Butane or n Butane are just other names for regular butane.

What is Isobutane?

Isobutane (i-butane) is an isomer of butane.

So, it has the same chemical formula as butane —  C₄H₁₀  — but has a different arrangement of its atoms, as you can see in the 3-D model images. (Isobutane molecule model shown)

As with normal butane, isobutane is a flammable hydrocarbon gas that is liquefied through pressurisation.

Isobutane is converted from butane in a process called isomerization.

It also has different physical properties from normal butane (n-butane).

In addition to being used as a fuel, isobutane is commonly used as a refrigerant and a propellant.

Isobutane has very low global warming potential and insignificant ozone depletion potential.

However, its main use is in refineries to increase octane of gasoline and make it cleaner burning.

It is classified as LPG, along with propane, butane and mixes of these gases.

i-Butane, i Butane or Methylpropane

i-Butane or i Butane are just other names for isobutane.

Methylpropane is yet another name for isobutane.

What is Propane Gas?

Propane is a flammable hydrocarbon gas that is liquefied through pressurisation.

It is classified as LPG – Liquefied Petroleum Gas – along with butane, isobutane and mixtures of these gases.

Propane comes from natural gas processing and oil refining.

It is commonly used for heating and cooking.

Propane is the gas that is supplied to virtually all homes and most businesses that purchase LPG in Australia.

LPG is supplied in gas bottles that are either exchanged or refilled on site by LPG tankers.

Large users may utilise bigger LPG storage tanks.

Propane is also frequently used in Autogas, alone or in a propane-butane mix.

LPG goes by a number of names in Australia including LPG, LPG gas, bottled gas, propane, BBQ gas, camping gas and LP gas.

However, no worries, as it’s all the same gas.

The chemical formula for Propane is C₃H₈.  (Propane molecule model shown)

Butane Boiling Point

The boiling point temperature of butane is -0.4°C.

This is significantly higher than propane and can be problematic in colder climates.

Propane Boiling Point

The boiling point temperature of propane is -42°C.

This boiling point temperature is sufficiently low that vaporisation can be achieved in almost all ambient temperature situations, outside of maybe the polar regions.

Butane or Propane Gas – When are they Liquid or Gaseous?

Propane, Butane & Isobutane Properties

This chart shows some of the physical property differences between the three gases.

You can refer back to the chart as we explain the importance of the numbers in the following topics…

One Big Happy LPG Family

Both propane and butane, along with isobutane, are all hydrocarbon gases that fall under the broad label of “LPG”, as they are all liquefied petroleum gases.

They are a group of flammable hydrocarbon gases that are liquefied through pressurisation and commonly used as fuel.

They are also called Natural Gas Liquids (NGLs), along with ethane, pentane and pentanes plus.

Their common distinguishing characteristic is that they can be compressed into liquid at relatively low pressures.

All are used as fuel in combustion, for heat generation, but there are also many other applications for LPG.

The Name Game

What they are called is greatly dependent on what country you are in.

In Australia, we call it LPG but it is propane. 

Autogas in Australia can be either pure propane or propane mixed with butane.

In New Zealand, LPG is almost always a propane and butane mix.

In the USA, they don’t generally use the term LPG.  They just call it “Propane”.

In some countries, like England, you can specifically buy propane or butane.

In other countries, they call it “GPL” or “GLP” instead of “LPG”, as the acronym is based on different languages and syntax. 

For example, in French it is “gaz de pétrole liquéfié” or in Spanish it is “gas licuado de petróleo”.

Energy Content – Myths & Facts

I’ve seen any number of articles saying that butane has more energy content and is, therefore, more economical to use.

Not always true!

It actually depends on the unit of measure used for pricing.

If it is priced by volume — in litres — the butane has about 9% more energy content, with 27.5MJ/L versus 25.3 MJ/L for propane.

However, if it is sold by weight — in kilograms — then propane has about 5% more energy content, with 49.58MJ/kg versus 47.39 MJ/kg for butane.

Why?

Because propane has less density, you get more litres per kilogram, with the difference more than offsetting the lower MJ/L energy content value.

Butane or Propane Gas – Which is Hotter?

The flame temperatures of butane and propane are virtually identical.

Butane burns at 1970°C or 3578°F.

Propane burns at 1967°C or 3573°F.

Butane Combustion

Assuming complete combustion, you get carbon dioxide and water:

2 C₄H₁₀ + 13 O₂ → 8 CO₂ + 10 H₂O + Heat

However, with incomplete combustion you get carbon monoxide and water

2 C₄H₁₀ + 9 O₂ → 8 CO + 10 H₂O + Heat

This would typically occur if the ratio of oxygen to butane was insufficient.

Boiling Point: Turning from Liquid to Gas

Propane and butane have different boiling points — the temperature at which it goes from liquid to gas (vapour).

Propane boils at -42°C whereas butane boils at -0.4°C.

This means you have a huge problem if you try to use pure butane when the temperature drops below freezing.

No Boiling = No Vapourisation = No Gas

So, with butane only, you could find yourself with no gas for your heater and cooking appliances when it gets cold.

In some areas, LPG suppliers provide a mixture of propane and butane to address this problem.

This can work well when there are temperatures both below and above freezing.

However, the mixture in the cylinder can become butane rich if there is too much cold weather, with only the propane vapourising and being used.

Needless to say, propane is the preferred choice for cold weather climates.

What is Butane Commonly Used for?

The most common use of butane is as a heating fuel.

It can be used for cooking, hot water and space heating.

It is also frequently blended into autogas, to fuel vehicles.

There are also commercial and agricultural applications, including the heating of greenhouses.

In non-fuel applications, butane is also commonly used as a propellant in aerosol products and as a refrigerant.

Vapour Pressures & Use as Propellants for Butane or Propane Gas

One of the other important differences between the two gases is vapour pressure.

Vapour pressure is the pressure exerted by the vapour (gas), in equilibrium with the liquid, against the walls of the cylinder or other closed container at a given temperature.

Propane has a much higher vapour pressure than either butane or isobutane.

Propane has approximately 4x the vapour pressure of butane and about 2.75x the vapour pressure of isobutane. (See chart above)

Propane, butane and isobutane are all used as propellants in aerosol products, as they are naturally odourless, non-corrosive and non-toxic.

For obvious reasons, no odourant is added to the LPG when used as a propellant.

Nobody wants a stinky hairspray!

These gases may be used individually or in combinations to achieve the desired pressure.

The lower pressures of the two butanes tend to be favoured for everything from deodorant to disposable cigarette lighters.

When the product label lists “hydrocarbon” as the propellant, it is often butane or isobutane.

LPG gases replaced chlorofluorocarbons (CFCs) as propellants about 30 years ago. 

CFCs were banned because they damaged the ozone layer of the atmosphere.

Different Refrigerant Applications

All three gases are used in refrigeration but for different applications. 

They have different refrigeration uses because of their different thermodynamic properties.

They are used to replace the harmful CFC refrigerants, such as R-12, R-22, R-134a.

High purity propane (>97.5%) is used as a refrigerant, known as R290.

Both n-Butane (R-600) and i-Butane (R600a) are also used as refrigerants.

The three gases can also be mixed to achieve different properties, such as with R-290a, a mixture of isobutane and propane.

Just as with propellants, LPG gases also replaced chlorofluorocarbons (CFCs) as refrigerants, to preserve the ozone layer.

Improved Yields for Greenhouses

Butane is favoured, over propane, for use in greenhouses.

It provides both heat and enriches the atmosphere with CO₂, which aids in plant growth.

While both propane and butane are environmentally friendly fuels, butane does have an extra carbon atom (C₄H₁₀ vs C₃H₈) that results in ⅓ more CO₂ when burned.

Improved yields make it the preferred choice for greenhouse use.

Butane or Propane Gas Availability Around the World

Propane is generally available in most countries.

Butane can be a bit harder to find in some areas.

Nevertheless, in many countries it is available, either as pure butane or blended with propane.

Final Thoughts

For many people, the different gases that qualify as LPG are indistinguishable and never pose an issue.

However, for others they provide the flexibility to use them for various specialised applications.

Either way, LPG is an exceptional energy product.