Demystifying Hydrocarbon Gas Mixtures in Non-Refillable Cylinders

Why does hexane or pentane have less gas pressure in the cylinder than methane at 50% LEL?

The reason why you will see reduced cylinder pressure when using pentane or hexane instead of methane for the LEL component of a standard 4-Gas mixture is that heavier hydrocarbons are more easily liquefied than the lighter hydrocarbons.

Hydrocarbons are organic compounds which are composed of only hydrogen and carbon atoms. Hydrocarbons are used to calibrate LEL sensors. Each hydrocarbon has benefits & detriments when used to calibrate particular sensors, and so the sensor manufacturer will determine their recommendation as to which hydrocarbon is best used to calibrate.

Simple hydrocarbons are commonly known as “C1” to “C10,” based on the number of carbon atoms in the molecule. They are:

The higher the “C-Number,” the “heavier” the hydrocarbon is, and also the easier it is to liquefy. However, there are two other factors to take into consideration when determining the fill pressure of a cylinder containing a “heavy hydrocarbon,” and they are 1) the concentration of the hydrocarbon and 2) the ambient temperature. For our intent and purpose, methane & ethane do not present any problem as they are difficult to liquefy, but when dealing with C3+, we need to be aware of these variables.

Each of the above hydrocarbons in their pure form become liquid at a certain temperature and pressure. However, when they are diluted, they can remain gaseous at lower temperatures and at higher pressures. So, the lower the concentration of the hydrocarbon, the better, for the filling of gas cylinders.

In the same way that water will turn from steam to liquid (and then even to ice) as temperatures drop, so will a hydrocarbon turn from gas to liquid. This same effect happens when pressure is increased. Both temperature and pressure act on the hydrocarbon at the same time. If a hydrocarbon, or any other component of a mixture, become liquefied, they will, in effect, drop out from the gaseous mixture. If any of the mixture is used from the cylinder while one or more components are liquefied, the mixture ratios will be irrevocable altered, and the mixture rendered unusable for calibration.

When we fill a calibration gas mixture, we try to prevent any component of the mix becoming liquefied. We therefore fill to a certain pressure based on mathematical calculations which take into account a “worst case” ambient temperature (our standard procedure is to use 0 degrees Celsius as our “worst case” temperature).

Let’s look at three common examples for a practical comparison:

Mixture 1:

Methane gas does not liquefy except under extreme conditions, so we have no problems reaching full pressure with this mixture.

Mixture 2:

Pentane is heavier than methane, and we do run into compressibility issues at 50% LEL, which equals 0.7% v/v. This is one of the reasons that pentane mixtures are typically 25% LEL, which allows for more pressure before the pentane will liquefy.

Mixture 3:

Hexane is the easiest to liquefy of the commonly used hydrocarbons for gas detection applications. Even at 50% LEL, or 0.6% v/v, hexane liquefies at relatively low pressures. For this reason it is best to keep the LEL value as low as possible.

So, when you are determining the best calibration mixture to use with your particular instrument or sensor, you must weigh the advantages and disadvantages of your hydrocarbon’s concentration against the fill pressure / gas volume which is achievable. CAC Gas can complete the calculations required to determine the fill pressure of your desired mixture.