ELGAS Knowledge Hub

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

It is also an LPG – Liquefied Petroleum Gas – as are butane and propane.

Its main use in refineries, as a gasoline – petrol – additive.

However, it is different in some important ways.

Physical Properties

This chart shows some of the physical properties of isobutane.

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

Note: Some numbers have been rounded.

What is Isobutane? Difference Between n Butane and Isobutane

Difference between n butane and isobutane is that isobutane (i-butane) is an isomer of normal butane (n-butane).

That means 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)

Isobutane is converted from butane in a isobutane production process called isomerization.

As with normal butane (n-butane), isobutane (i-butane) is a flammable hydrocarbon gas that is liquefied through pressurisation.

However, it has different physical properties from normal butane (n-butane).

Isobutane is colourless with a weak petrol odour.

It is both very flammable and gas/air mixtures can be explosive.

Isobutane vapour (gas) is heavier than air.

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

Production Process

Isobutane production is converted from butane (n-butane) in a process called isomerization. The isobutane production process rearranges the atoms into a different molecular configuration.

The component atoms are the same but are arranged in a different geometric structure.

This isomerization happens in something called a butamer unit and includes the use of platinum or another metal catalyst.

In this isobutane production process, only some of the butane is actually converted to isobutane.

After the butamer process, the output mixture goes through a fractionator or deisobutanizer tower that separates the unconverted butane from the isobutane production.

Uses of Isobutane – What is the Use of Isobutane

The main use of isobutane is in refineries, as a gasoline – petrol – additive.

There, isobutane is processed through an alkylation unit to make an alkylate.

It is used to make isooctane, a high octane gasoline component, which increases the octane rating and anti-knock properties of gasoline.

It is rated at 100 points on the octane rating scale.

In addition to being used as a fuel, isobutane is commonly used as a refrigerant (R600a) and a propellant. Isobutane is an excellent refrigerant and eco-friendly minus the ozone harming properties of CFCs.

It can be used as a replacement for R-12, R-22, R-134a, and other chlorofluorocarbon or hydrofluorocarbon refrigerants, in conventional refrigeration systems.

Whilst it is flammable, there have been few problems in the millions of refrigeration units worldwide.

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

Another important use of isobutane is as a feed stock for plastics.

It is used to manufacture propylene oxide for use in making polyurethane plastics.

Another use of isobutane is as a solvent.

Vapour Pressure & Use as Propellants

One of the other important differences between isobutane and the other LPG 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.

Isobutane has about 64% less vapour pressure than propane but about 44% more than butane (at 21ºC).

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

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

Nobody wants stinky aerosol products!

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 isobutane or butane.

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

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

In Refrigerant Applications

All three LPG 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.

Isobutane is used as a refrigerant known as R600a.

Both n-Butane (R-600) and high purity propane (>97.5%), known as R290, are also used as a 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.

Combustion & Limits of Flammability

Isobutane’s limits of flammability are 1.8% to: 8.4%, by volume.

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 isobutane was insufficient.

Boiling Point: Turning from Liquid to Gas

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

Isobutane boils at -11.75°C whereas butane boils at -0.4°C.

This means you have a problem if you try to use pure isobutane when the temperature drops below -11.75°C.

No Boiling = No Vapourisation = No Gas

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

However, propane and butane are less costly so isobutane is rarely used as heating fuel.

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

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

Needless to say, propane is the preferred choice for cold weather climates, as it boils at -42°C.

LPG – Liquefied Petroleum Gas

Both isobutane and butane, along with propane, 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  – NGL, 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.

Final Thoughts

Many people have never even heard of isobutane but it affects their everyday life.

It does so as a petrol additive, to keep their cars from knocking (pre-ignition), and as propellant for hairspray, deodorant and other aerosols.

Isobutane also contributes to a variety of plastic products and solvents.

So, whilst nearly unknown, it is important nonetheless.