Calculate the dew point of natural gas if I know the composition?
Calculate the dew point of natural gas if I know the composition?
(OP)
I am looking into adding more gas to our current scrubber and we don't measure the dew point of our gas, only the composition. Does anyone know how to quickly calculate the dew point of natural gas if I know the composition or a program that will do it for me? (This is a one time thing, would rather not buy a program.)
Thanks in advance!
:)ladyCR
Thanks in advance!
:)ladyCR





RE: Calculate the dew point of natural gas if I know the composition?
Cathy
Biber Thermal Design
www.biberthermal.com
RE: Calculate the dew point of natural gas if I know the composition?
If the gas is water saturated, you can get the partial pressure of the water using steam tables but I don't follow what you have posted.
RE: Calculate the dew point of natural gas if I know the composition?
Try the following link to Quest Consultants:
http://www.questconsult.com/~jrm/thermot.html
They have an on-line dew point calculator using the Peng-Robinson equation of state that should do what you need. I checked it using Peng-Robinson and a commercial simulator for a composition of 20% C1, 20% C2, 20% C3, 20% iC4 and 20%nC4 at 60 F and got the following result:
Commercial Simulator: Dew Point Pressure = 68.9 PSIA
Website simulation: Dew Point Pressure = 474.8 kPa abs = 69.4 PSIA
I'm sure this is close enough for what you need and should be more accurate and less work than using equilibrium ratio data (K-charts).
I hope this helps!
Regards,
Bob
RE: Calculate the dew point of natural gas if I know the composition?
example: say you a have 1.72% mol fraction of water vapor in the mixture and the operating pressure is 1250 PSIA.
The partial pressure of the water vapor is therefore...
pp = .0172 X 1250 = 21.5 psia
From a steam table, you can look up 21.5 psia and see that the dew point would therefore be about 232 deg. F.
RE: Calculate the dew point of natural gas if I know the composition?
The partial pressure equals the mole fraction.
Biber Thermal Design
www.biberthermal.com
RE: Calculate the dew point of natural gas if I know the composition?
Reading the initial thread the question is not about calculating when water droplets form. I believe that ladyCR is looking for the method for calculating the dewpoint of the natural gas stream...which is the point where the first droplet of hydrocarbon forms.
Regards,
Bob
RE: Calculate the dew point of natural gas if I know the composition?
RE: Calculate the dew point of natural gas if I know the composition?
The best solution is to buy an inexpensive ($2,000 one-time fee for an outright purchase) but extremely competent process simulator called PD-Plus from Deerhaven Technical Software (Burlington, MA).
Generally, I would not recommend use of shortcut methods for such work. If you have a one-time problem, send it to a good chemical engineer who can handle it for you. There are many thermodynamic issues to consider that are covered in a fairly lengthy thread related to bubble pressure calculations in another chemical engineering forum in eng-tips.com. That problem is closely related to the one for compuing dew points, except that you also have a water dew point issue. I made a number of points in that thread which you may wish to review.
Best wishes.
RE: Calculate the dew point of natural gas if I know the composition?
However, in case of light hydrocarbons, the use of Ki=yi/xi values taken from DePriester's graphs published by CEP Symposium Series No.7, vol.49, p.41, 1953, re-published in SI units in the CEP vol.74,(4), pp.85-86, April, 1978, or estimated by formulas as recommended by S.I. Sandler in Foundations of Computer Aided Design Vol2, p.83 (AIChE, 1981), may be used as practical and reasonable approximations.
The dew point is estimated by iterative methods selecting K values at different assumed dew point (usually P or T are known) conditions, using yi/Ki, the sum of which must equal 1 for the right (P,T) dew point selection.
RE: Calculate the dew point of natural gas if I know the composition?
1) Create a stream
2) Enter your gas composition
3) Enter an arbitrary flow rate (this doesn't affect
the dew point calculation)
4) Enter either known temperature or pressure
5) Enter 1 on the vapour fraction cell
Either the dew point pressure or temperature (coressponding to 4 above)is calculated.