Elgas LPG Gas Blog

Is Propane a Liquid or Gas? Liquid Propane vs Gas Propane

Propane is a gas at standard temperature and pressure, but liquefies under pressure for storage. Propane, used as a fuel, is a co-product of crude oil and natural gas processing. Propane is classified as one of the liquefied petroleum gases - LPG gas.

Propane is a liquid if stored under pressure in a gas bottle or larger vessel. It is also a liquid if stored under -42°C.

Propane is a gas when released from pressure at standard temperature and pressure (STP) or normal temperature and pressure (NTP).

Both gas and liquid propane have applications but are not interchangeable. Propane liquid is LPG - Liquefied Petroleum Gas - fuel in its liquid state when under pressure or below its 42°C (-44°F) boiling point. It turns to gas above -42°C or when not under pressure.

Propane is a liquid when it is stored in a pressurised vessel. It is a gas at 0°C (32°F) and 1 atm pressure (STP) so, when released from a pressurised cylinder the propane liquid becomes gas. Propane is stored and distributed as a liquid but typically used as a gas for heating, cooking and vehicle fuel.

The pressure and temperature at which it is stored determines whether you have propane liquid or gas.

Propane is a flammable hydrocarbon with three carbon atoms and a chemical formula of C3H8. It is a gas at 0°C and 1 atm (STP) and liquefies under pressure for storage and transport.  LPG-propane comes from natural gas processing and crude oil refining and is used as fuel for heating, cooking, vehicles, agriculture and industry. It is a liquid under pressure and a gas at standard temperature and pressure (STP).

Is LPG a Liquid or a Gas

Liquid propane is at the botton of the bottle. Propane gas is at the top.

LPG – Liquefied Petroleum Gas – is a liquid and a gas. It is a liquid under pressure or below -42°C (-44°F). It is a gas at 20°C (68°F) and 1 atm pressure (NTP) so, when released from a cylinder the propane liquid becomes gas.

LPG is both a liquid and a gas within the gas bottle.  LPG is gas vapour at the top of the bottle and liquid LPG at the bottom, as shown in the image.

Liquid Propane vs Gas Propane

LPG is liquid when stored in a gas bottle

There is a difference between liquid propane vs gas propane. Propane is a liquid when it is under pressure or below -42°C. Propane is a gas when the pressure is relieved and the temperature is over -42°C. Comparing liquid propane vs gas propane, the applications are also different and not interchangeable.

Most gas BBQ grills use BBQ gas bottles full of liquid propane. It turns back into gas when it is released for use. Portable liquid propane is available almost everywhere whilst natural gas requires a connection to a house with a gas main.

When is LPG Liquid or Gas?
Propane - Butane
LPG (1atm)
Liquid
Vapour (Gas)
Propane
< -42°C
≥ -42°C
Butane
< -0.4°C
≥ -0.4°C

 

Is Propane a Liquid or Gas? Difference Between Liquid Propane & Propane Gas

Propane is a gas at standard temperature and pressure, but liquefies under pressure for storage. Propane, used as a fuel, is a co-product of crude oil and natural gas processing. Propane is classified as one of the liquefied petroleum gases - LPG gas.

You must use the type that the application was designed for.  For example, the difference between liquid propane and propane gas grill is significant. Grills are designed to use gas.  Using liquid propane instead would create a major safety hazard on a gas grill.

What is Liquid Propane

Propane exists in two different forms, liquid propane & propane gas. The storage pressure & temperature determines determines which form of propane you have.

Propane gas can be turned back into liquid propane by increasing the pressure on the propane, without reducing the temperature. Chilling propane gas below -42°C (-43.6°F) will also turn it back into liquid propane.

How Does LPG Work?

LPG - Liquid Petroleum Gas - is stored under pressure, as a liquid, in a gas bottle.  

It turns back into gas vapour when you release some of the pressure in the gas bottle by turning on your gas appliance.

The LPG gas vapour is held in the top of the bottle and the liquid LPG at the bottom, as shown in the image above.

Almost all of the uses for LPG involve the use of the gas vapour, not the liquefied gas. 

Liquid Gas?

LPG - propane - cannot be both a liquid and a gas at the same time.

The change from liquid to gas is known as vaporisation.

LPG Vapour vs Gas

First, let's sort out some of the terminology.

The two terms, vapour and gas, are used interchangeably by most people in reference to LPG.

Vapour (or vapor in American spelling) is the more technically correct term for LPG, as it is in gaseous and liquid equilibrium at room temperature.

It can be turned back into a liquid by increasing the pressure on it without reducing the temperature.

A gas has one defined state at room temperature.

So, vapours are gases however not all gases are vapours.

Same but Different

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

Propane is a flammable hydrocarbon gas with 3 carbon and 8 hydrogen atoms in a propane molecule and is also referred to as natural gas liquids – NGL.

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

LPG liquid and vapour characteristics have some similarities to water and steam. 

Liquid LPG is colder, denser and heavier. 

LPG vapour is warmer and much lighter.

Liquid LPG and vapour LPG both have applications but are not interchangeable when used.  

The choice depends on the purpose. 

In most applications, such as home appliances, LPG is used as vapour (gas). 

However, some LPG fuelled products require it to be in its liquid form, such as with autogas LPG liquid injection systems.

Different Cylinders

45kg vapour cylindersLPG is liquid in a cylinder with an area at the top of the cylinder where it turns into vapour. 

However, the cylinders come in two configurations, Liquid Withdrawal and Vapour Withdrawal

The difference depends on from where the LPG is extracted. 

If drawn from the bottom of the cylinder, it comes out as liquid. 

If drawn from the top of the cylinder, it exits as vapour.

The cylinders are clearly marked as either “Vapour Withdrawal” or “Liquid Withdrawal”. 

In Australia, liquid withdrawal cylinders are also painted with a blue top, to make them instantly recognisable.

(See blue top image below)

Using the wrong type of cylinder for an application could pose a serious safety hazard.

LPG-Propane as a Liquid

LPG liquid withdrawal cylinders with blue topsPropane is a liquid because it is under pressure in a gas cylinder or under -42°C. Propane as a liquid looks a lot like water. Propane liquid is colourless and odourless in its natural state. LPG density or specific gravity is about half that of water, at 0.51.

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

In contrast, LPG boils at -42°C becoming gas vapour. 

Unlike water, 1 kilogram of LPG does NOT equal 1 litre of liquid LPG.

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

Conversely, 1L of LPG weighs 0.51kg.

The distinctive smell of LPG comes from an odorant that is added to LPG, for safety and leak detection reasons. 

Caution should always be used to avoid direct exposure, as liquid LPG is cold enough to cause severe cold burns on exposed skin.

LPG as a Vapour or Gas

Gaseous expansion of LPGLPG becomes vapour at temperatures of -42°C and above.  

LPG expands to 270 times the volume when it goes from liquid to gas.

So, 1L of liquid LPG equals 270L of gaseous LPG.  

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

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 gas vapour is heavier than air and will sink to and collect at the lowest point. 

If LPG is vented to the outside air, it will quickly dissipate with the slightest movement of air.  

Conversely, if LPG is vented into a sealed structure, with no air movement, the LPG gas will collect on the floor and rise vertically as more LPG is vented into the structure. 

Obviously, this would create an extremely hazardous situation.

Are Liquefied Gas and Compressed Gas the Same?

No. As already discussed, some gases, like LPG - butane and propane - is a liquid under pressure.

Other gases, like LNG - Liquefied Natural Gas (methane) - only become liquid when cooled cryogenically.

Still others just become compressed at high pressure but never become liquid as a result of pressurisation.

Know the Difference for Safety

As previously mentioned, 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 possibly be a fire or similar safety hazard. 

This is why you must use the right type of cylinders and LPG cylinders should always be kept upright, so that only vapour is released. 

An upside-down vapour cylinder, or even one laying on its side, could release the LPG as a liquid.

 

 

 

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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.