Calculate the dew point of natural gas if I know the composition? 3

[ladyCR]

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

 

[cbiber]

The dew point is the saturation temperature for the partial pressure of water vapor. From the composition of the gas and the molecular weights of the components you can work out the partial pressure of water in the gas. Then use a table in the back of a thermodynamics textbook to get the dew point (you might have to interpolate).

Cathy

Biber Thermal Design

http://www.biberthermal.com

 

[TD2K]

How, from the composition of the gas and the MW of the components do you get the partial pressure of water vapor?

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.

 

[kbander]

ladyCR,

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

 

[speco]

Here's a simpler method to determine the dew point of water vapor based on the composition of the mixture. The partial pressure of the water vapor can be easily calculated as the vol fraction of water vapor X the total operating pressure:

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.

 

[cbiber]

To clarify some of the above ...
The partial pressure equals the mole fraction.

Biber Thermal Design

http://www.biberthermal.com

 

[kbander]

speco and cbiber,

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

 

[ladyCR]

I think I may have confused some with my question. Initially, I was interested in knowing how to calculate when water droplets form, however after further work on my part I also needed to calculate the dewpoint of the natural gas stream. So...thanks for answering both of my questions.

 

[mirchee]

ladyCR:

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.

 

[25362]

to ladyCR, mirchee's approach using computer programs is known to me from another thread. I must agree with him/her in that programs like the one he/she suggests are available, useful, and sometimes the only chance to reliably estimate VLE conditions for a mixture of chemicals.

However, in case of light hydrocarbons, the use of K_i=y_i/x_i 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 y_i/K_i, the sum of which must equal 1 for the right (P,T) dew point selection.

 

[tad123]

If you have HYSYS program, you could do as following to find the gas Dew Point:
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.