4 replies to this topic

[Pavankumar]

Dear sir,

All your articles and Explanations are awesome.Let me know what happens to dewpoint temperature when pressure is changed.

[PaoloPemi]

you may find detailed answers in good books discussing phase equilibria,
for a quick answer google for "phase envelope"

[Art Montemayor]

Pavankumar:

The topic of "Dew Point" is often abused when being discussed simply because of a lack of understanding its basic definition.

The term "Dew Point" is formally defined as the temperature at which a vapor forms its first drop of condensed liquid, AT ATMOSPHERIC PRESSURE. However, as happens in most industrial applications, the subject vapor is often found under substantial pressure and, therefore, the related dew point at the process pressure is a different temperature - one much higher. That is why - like other engineering terms such as "standard conditions", density, specific gravity, ppm, etc. - it is very important to ALWAYS SPECIFY THE RELATED BASIC REFERNCE PROPERTY (pressure, in this case).

Another specific item that is required when discussing dew point is WHICH dew point. Do you mean the dew point of any associated water vapor content in the subject gas? Or do you mean the dew point of the basic components (such as a light hydrocarbon mixture)? The former is usually what some careless textbook writers mean; the latter is sometimes the case when one is processing hydrocarbon gases or operating a refinery (as PaoloPemi infers in his/her post). It makes a BIG, BIG difference as to what you mean to state or discuss.

[Pavankumar]

I mean the Dew Point of the associated water vapor content in the Natural gas.

[PaoloPemi]

that may depend from the method you use to solve the water-hydrocarbons equilibria,
several simulators model water as separate phase and since water saturation pressure (fugacity) depends from temperature you get (usually) the same sign (higher pressure -> higher temperature)
I prefer to model water and hydrocarbons with the same EOS (setting specific mixing rules , see GERG methods, or with complex mixing rules as Wong Sandler etc. I use Prode Properties), in that case the resulting behaviour is (generally) more complex.