Water Dew Point vs. Hydrocarbon Dew Point

For those of us who came up in the compressed air industry, transitioning to work in the natural gas industry presents some challenges of terminology.   The term dew point is good example.  In the compressed air industry dew point is always a measurement of water content.  However in the natural gas industry, at least in North America, water content is stated in terms of pounds of moisture per million standard cubic feet (MMSCF).

In the gas industry dew point often refers to hydrocarbon dew point.

According to the source of all truth, Wikipedia:

The hydrocarbon dew point is the temperature (at a given pressure) at which the hydrocarbon components of any hydrocarbon-rich gas mixture, such as natural gas, will start to condense out of the gaseous phase. It is often also referred to as the HDP or the HCDP. The maximum temperature at which such condensation takes place is called the cricondentherm.  The hydrocarbon dew point is a function of the gas composition as well as the pressure.

It’s important to realize that untreated natural gas is a mixture of many different gases.  These gases condense.  This is much different than in a compressed air system, where for practical purposes water is the only condensable.

One determinant of natural gas quality is how much hydrocarbon is present over and above methane.  As more hydrocarbons are present – for example butane, propane, ethane, etc – the heating, or BTU, value of the gas goes up.  Theoretically this makes it more valuable.  But it can also cause problems.  Higher BTU gas increases the likelihood of liquids showing up in unwanted places.  That’s why folks in the gas industry care about hydrocarbon dew point.

Hydrocarbons above a certain molecular weight behave like any liquid, in that they go back and forth between liquid and vapor, depending on pressures and temperatures.  I will not bring out phase diagrams or anything like that.  You’ll have to trust me.  Besides, I long ago forgot what little I knew about them.

Sometimes it’s worthwhile to capture sell these hydrocarbon liquids, if there’s enough.  Therefore, it is important to know what is there and possibly how to change it. 

You’ll sometimes hear high BTU gas referred to as “wet gas.”  Technically this is appropriate, as high BTU gas is prone to condensation.  But the phrase “wet gas” when it’s used this way also invites confusion, because the word wet is usually associated with water.  This is where we as a dryer manufacturer often get pulled into the conversation.  However, applying one of our dryers to “wet” gas (i.e. high BTU gas) does not result in “dry” gas (i.e. low BTU gas).

 This can be especially confusing when the phrase dew point is thrown around.  I often get the question: “By how much do your deliquescent dryers and filters reduce dew point?”  The answer of course depends on what kind of dew point we’re talking about.  Our dryers do not change hydrocarbon dew point.  Our dryer do affect water dew point.  We always need to verify what kind of dew point we’re discussing.

In summary, we do not have equipment for reducing the hydrocarbon content or hydrocarbon dew point of gas, at least not in a predictable or systematic way.  Some liquids will drop out in the dryer, desiccant bed and filter, but we cannot predict how much it will be.  But if someone needs to lower the water dew point of natural gas, we’re the company to speak with.