Water condensation

[MNDIAS]

Hi!! I have a gas stream at 850ºC with 25% (vol) of water and I would like to calculate the amout of water that condensates at 25ºC and the amount of water that remains in the gasous phase. At 850ºC I know the molar faction and the mass fraction of each component of the gas and the total volume of the gas and at 25ºC I know the vapour pressure of the water. Is there anyone that can give me a hand...I have lost the past two days with this question in my mind...and I could not find the answer.

Another question I have is: "can I consider the vapour pressure of the water to be equal to the partial pressure?"

Thank you very much.
Mário

 

[JimCasey]

Vapor pressure of water vapor at 25C is 3.2E-03 MPa(3.2kpa)
Certainly one can expect if the partial pressure of the water vapor exceeds that pressure the vapor will condense until there is only enough vapor to account for that much partial pressure.

A psycrometric chart may also be useful for your calculations.

 

[Mr2jz]

At 25% by volume and at 850°C temperature, the vapor pressure of water is roughly 197 kPa and if the target dew point is 25°C then water in excess of 3.171 kPa will condense (in ideal heat transfer conditions) dropping the water concentration to 0.4% by volume. These are general values but they should get you in the ball park.

As a rule Psychrometric charts can get the job done but they usually don't have the resolution to get this type of task done at these temperatures.

 

[sshep]

MNDias,

If you can consider any condensible hydrocarbons to be immiscible with water and vis-versa, then if the water condenses it can be treated as a pure liquid water phase. In such a case, the answer to your second question is "yes" if a pure water phase forms (PP water vapor = Psat water). If the partial pressure of water in the vapor is less than the vapor pressure of pure water (PP water vapor < Psat water) then no liquid water phase forms. This pure water assumption is valid for a great many hydrocarbon water mixtures and makes the calculations simple using your steam tables.

This simplifying assumption (liquid water only as a pure phase) is still valid even if a water free hydrocarbon liquid phase also forms, although the condensation of hydrocarbons must be accounted for in the final vapor composition as well. This is done by basic flash calculation on the hydrocarbon components.

If water is distributed between two liquid phases, or if there is only one liquid phase consisting of water and other miscible components, then the calculation is a more complicated VLE or VLLE situation. In these two cases the partial pressure of water in the vapor could be more or less than the vapor pressure of water depending on the system.

What type components are you dealing with? Even a VLLE calculation is a 5 minute job for many components if you have the right simulation tools, but it wasn't time wasted if you learned something.

best wishes always,
sshep

 

[dcasto]

Here is the simplified equation (there is a slight difference as compared to a partial pressure) for water content of gas;


lbs H20/MMSCFD =( 10^(3.5551-0.94283*LOG(Pressure in psig+15)+0.01576*(Temperature Deg F-31)))

You didn't give a pressure in your OP.