partial pressure of mixed flow 1

[mielke]

Assume I have two gases flowing through a pipe that is at constant pressure and is being cooled. Lets say that the first gas completely condenses to liquid, due to cooling, while the other gas is still in vapor form.

Also if we assume at the start that gas a had a partial pressure of 10psia and gas b had a partial pressure of 20psia (giving a gauge/total pressure of 30psia). now once gas a condensed would gas b then have a partial pressure of 30psia (since its at constant pressure) or would something else happen or is this even a real situation???

Thanks

 

[speco]

Mielke,

What you are describing happens in a typical compressor aftercooler. The total pressure essentially remains constant, except for the pressure drop through the aftercooler.

However, in a binary mixture of a condensable vapor and a not condensable gas, you can never totally condense the the condensable vapor. The reason is that the saturation temperature of the condensable portion is constantly decreased as the partial pressure of that component decreases due to condensation.

For an example, assume you have a mixture of 95% air and 5% water (by volume = mol fraction) vapor operating at 100 psia. You are cooling the mixture down to 120 F. If you look at a steam table you will see that the water will start to condense at about 163 F (saturation temp at 5.0 psia), and there will be a partial pressure of 1.6924 psia at the outlet condition. That determines the amount of the vapor that will be left in the mixture after the rest condenses.

Regards,

Speco (

http://www.stoneprocess.com)

 

[Latexman]

Similar question here thread124-264575 .

Good luck,
Latexman

 

[mielke]

thanks speco, but from your example will the partial pressure of the air increase? since the amount(volume) of the water vapor is going down, or will the total or will the total operating pressure go down and the air pressure remain somewhat constant? also i thought that pressure drop in a compressor aftercooler was somewhat negligable, is this correct?

 

[speco]

mielke,

Yes, the partial pressure of the non-condensable component will increase. Remember, the total pressure of the mixture is the sum of the partial pressures.

The pressure drop in an aftercooler is usually a small percentage of the operating pressure. I would be hesitant to use the term "negligible". It needs to be considered, and really depends on the design of the exhanger.

Regards,

Speco