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Mixture Dew Point

dew t vle

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#1 ezralh

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Posted 08 November 2014 - 06:50 AM

How to determine dew temperature of a mixture consists of hydrogen, methane, carbon monoxide and water at 30 bar?  If I use gamma/phi method, I have to first determine the Tsat of each compound. The problem is how to determine the Tsat for hydrogen because 30 bar is beyond the hydrogen critical pressure which is 13.13 bar. 



#2 PingPong

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Posted 09 November 2014 - 06:27 AM

I would assume that with "dew point temperature" it is here meant to be the water dew point temperature.



#3 MrShorty

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Posted 10 November 2014 - 11:00 AM

How to determine dew temperature of a mixture consists of hydrogen, methane, carbon monoxide and water at 30 bar?  If I use gamma/phi method, I have to first determine the Tsat of each compound. The problem is how to determine the Tsat for hydrogen because 30 bar is beyond the hydrogen critical pressure which is 13.13 bar. 

 

Good question. My response would be that you are only partially correct that you need Tsat/Psat to use the gamma/phi method for this kind of thing. Tsat/Psat is needed when your activity coefficients are based on a Raoult's Law standard state -- which specifically refers to the vapor pressures of the pure components. I could see performing these calculations using a gamma/phi method, but I would probably choose a different standard state. Without giving everything away too quickly, what other standard state are you familiar with (that is often very convenient for use in cases where you have supercritical gases dissolved in subcritical solvents)?



#4 RockDock

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Posted 20 November 2014 - 11:00 AM

The first component to condense would be water. That would be your dew point. However, you may actually form a solid before you form a liquid, due to the presence of C1 and CO2. The GPSA handbook gives a correlation for predicting dew point and hydrate temperatures for various mixtures. It is only used as a starting point, though. ProMax will give the most rigorous and accurate calculation for both of those if you are concerned about an actual operating unit.



#5 shan

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Posted 20 November 2014 - 11:15 AM

Put your desired composition into a Hysys stream and specify P = 30 bar and Vapor Fraction = 1.  The calculated temperature is the dew point you are looking for.



#6 serra

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Posted 21 November 2014 - 02:22 AM

"How to determine dew temperature of a mixture consists of hydrogen, methane, carbon monoxide and water at 30 bar? "

 

you have a mixture with (polar) & (supercritical) components at high p,

if you wish to adopt a "gamma/phi method" you may try the procedure suggested by Prausnitz

"see Computer Calculations For Multicomponent Vapor-Liquid and Liquid-Liquid Equilibria"

formula 4.20 , the arbitrary function for uncondensable components,

however I am not sure about the accuracy of calculated values.

 

if you have a decent amount of water probably you will be able to estimate the dew point

from the  partial pressure of water calculated with some accurate correlation,

however this method is not accurate as it doesn't estimates the real

fugacity of water in mixture.

 

A much better approach is based on a EOS,

however if you decide to adopt a EOS you need an accurate alpha function

cause in addition to C1 and CO you have hydrogen (supercritical) plus water,

different simulators have extended EOS which may help with water and H2,

You may try Prode Properties, there is a free version for students here

 

http://www.prode.com...aseenvelope.htm

 

it includes several methods as EOS with extended alpha correlations

and EOS with complex mixing rules (Wong Sandler etc.)

 

you may also look in literature for similar compositions

to see how researchers have solved the problem...






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