First, you need to have the vapor/liquid K factors which are listed in a lot of sources, the GPSA Engineering data books being one.

Since you have a gas composition, you have the vapor mol fractions for each component.  To calculate the dew point, you guess a temperature and look up the K factor for each component.  Use that K factor (K = y/x) to calculate the liquid mol fraction for each component given its vapor mol fraction.  

Then add up all the liquid mol fractions.  If the sum is equal to 1.0, you made a great guess for the dewpoint temperature and are finished.  

If the sum of the x for all the components is less than 1.0, you need to try a lower temperature.  

If the sum is greater than 1.0, you need to try a higher temperature.  You continue until your sum of the x = 1.0 OR you sufficiently bracket your answer for what you need.

laydyCR,

See my response to this in the Heat Transfer and Thermo thread.


Regards,

Bob

I've seen a graph determining the dew point of natgas as function of pressure and water content. The ref. was Fredenslund et al. AIChE Journal, 21, 1086, 1975.