I am designing a process heat exchanger to cool a gas effluent stream consisting of 70% Nitrogen 23% Carbon Dioxide and 7% Water (all mole percent).

The stream is going to enter at 250 degC and 1.51325 bar (i.e. just above atmospheric pressure) and leave at 50 degC.

The stream is being cooled by an aqueous stream which has been specified as having thermal properties of water.

My question is this, I have calculated that the partial pressure of the water in the mixture initially is approximately 0.106 bar. I used the formula Py=p to find this. P being the total pressure of the system, y being the mole fraction of component in the gas mixture and p being the partial pressure.

Is this formulation correct for the partial pressure and will partial pressure remain constant?

Also, is it correct to calculate the vapour pressure and compare this to partial pressure to check if the water in the gas mixture will condense?


1 Answer 1


TL;DR You are correct.

Is this formulation correct for the partial pressure

Yes, your Py=p is correct.

...and will partial pressure remain constant?

Strictly speaking no, because total pressure will not remain constant. Use ideal gas law or one of the real gas laws to find the new pressure of each species at the new temperature. From there find the new total pressure, also compare partial pressure of steam to vapor pressure at 50°C to see if you have condensation.

  • $\begingroup$ in a handbook of heating/ventilating/air conditioning technology (HVAC) you will find charts and tables which will allow you to solve for the dew point of an exhaust stream and determine whether or not condensation will occur. $\endgroup$ Commented Dec 26, 2017 at 22:12

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