Dew point is defined as the temperature at which vapor begins to condense. We see it in action every foggy morning. Air is cooled to its water dew point and the water starts condensing and collects into small droplets. We also see it demonstrated by a cold glass "sweating" on a humid day. The cold glass lowers the air temperature below the water dew point temperature and the water condenses on the sides of the cold glass. Water dew point is relatively simple and easy to predict since it is a single component system. It is easily removed using conventional techniques, primarily TEG (Triethylene Glycol) dehydration units.
Hydrocarbon dew point (HDP) is similar to the water dew point issue, except that we have a multi-component system. Natural gas typically contains many liquid hydrocarbon components with the heavier components found in smaller amounts than the lighter gaseous ends. It is the heaviest weight components that first condense and define the hydrocarbon dew point temperature of the gas. The dew point temperature also moves in relation to pressure.
One of the first questions we are asked by producers with a hydrocarbon dew point issue is:
"How can my hydrocarbon dew point be so high?"
In return, we ask the producer at what temperature does his high-pressure production separator operate? By definition, a production separator separating oil from gas operates at vapor-liquid equilibrium. Therefore, the gas leaving the separator is in equilibrium with the oil. In other words, the gas leaving the separator is at its hydrocarbon dew point that equals the separator operating temperature (and pressure.) If the separator is operating at 100°F, then the gas has a 100°F dew point at separator pressure. As the gas leaves the separator and cools flowing through the piping system, liquids condense and the dew point decreases as the heavy ends condense. The TEG dehydration unit will remove some heavy hydrocarbons, in addition to water, and further reduce the hydrocarbon dew point. At the sales meter, (without a conditioning unit) the hydrocarbon dew point is usually close to the lowest temperature the gas has achieved on the location before it was sampled, at operating pressure.