Hydrocarbon dew point is an essential quality parameter for natural gas. It refers to the temperature at which the first hydrocarbons, usually the heaviest ones, begin to condense out of the gas phase into the liquid phase. That seems simple enough! (For more detailed information go to the Tutorial and Videos page or the Technical Resources page).
Hydrocarbon dew point depends on two things: 1) The composition of the gas, and 2) the pressure. In general, the higher the percentage of heavier hydrocarbons are, the higher the dew point is. The effect of the pressure is a little different though. Let's look at a typical natural gas phase diagram:
The blue line is the "phase envelope". The right hand portion of the blue line is the phase boundary between vapor and liquid phases. In other words, the right hand part is the collection of the hydrocarbon dew points at different pressures. The "cricondentherm" is the highest possible dew point at any pressure. For most pipeline quality gas, the cricondentherm corresponds to a pressure of ~25-35 bar.
So what is the effect of pressure on dew point? For a given gas mix, lowering the pressure lowers the dew point, if pressure is under the cricondentherm pressure. Above the cricondentherm pressure though, increasing the pressure reduces the dew point. This is called the retrograde region.
In future posts, we will discuss why measurement of hydrocarbon dew point (as well as water dew point) is important for natural gas refiners, pipelines, storage facilities, and gas-based power plants. Remember, you can visit Tutorial and Videos or the Technical Resources pages for much more in depth discussions.