How do I get water vapor fraction and pressure? 1

[ausairman]

Hi Guys,

I'm building a numerical Matlab simulation of a thermochemical heat storage device (using zeolites), and need to update the state of water vapor in the device at every time step. My question is, if I have an empty space with water in it, with temperature, pressure, density, etc. all known, how can I get the vapor fraction and pressure of that same boiler after removing x kilograms of water from it (and not replacing it with anything)?

The temperature range is about 50°C - 500°C and pressure should be between 1-20atm.

Ideally, I would love to get an empirical function for Vapor fraction of water - or at least a comprehensive table I can index in my simulation, so far I haven't been able to find anything myself.

Help would be greatly appreciated!

 

[willard3]

Is the control volume constant?, ie, does the vessel change size? Is the vessel open to atmosphere?

I need more information to understand the problem.

 

[ausairman]

The vessel is completely sealed, water mass is removed via sorption and can be considered to simply have disappeared.

The vessel remains constant in size and is completely sealed. The only thing that changes is the mass of water and the total temperature.

So to restate my problem more eloquently:

I have a control volume with water vapor in it, at a certain, known temperature, pressure, volume, mass, water vapor fraction, etc. (everything known, basically).

I then proceed to remove a certain, known mass from the control volume, and increase the temperature of the remaining amount of water by a known amount. Pressure is no longer known, nor is the water vapor fraction. I only know the mass, volume and temperature. I do not know the water vapor fraction (I do have access to the state of the system prior to the removal of the mass of water and change in temperature though, if needed)

I now need to know the state of the system. I suppose my question could be simplified to simply asking how I can calculate water vapor fraction and pressure, if I only know mass, volume and temperature of a body of (saturated or otherwise) water vapor.

The volume is filled with pure H2O, nothing else. I do not know if the water is partially condensed, saturated, or superheated.

Thanks a ton for your help, I hope I've given enough info!

 

[ausairman]

To clarify further, if the water is saturated, the mass of water being removed is considered to be representative of the whole volume. So if the vapor fraction is 40%, and I remove 1kg of water, I will have removed an 400g of steam and 600g of water.

 

[chicopee]

If the final stage is still in the saturated region, then you know the pressure; however the quality of steam in the final stage remains unknown unless the process is isentropic for example.

 

[chicopee]

Here is an JPEG attachment of a basic evaluation of your OP. I use the T-v diagram to clarify the evaluation. There three possible scenarios presented although your OP is for a saturated vapor.
A few years back, mathematical relationships were presented by a researcher in Chemical Engineering. These relationships dismissed the use of steam tables or Mollier's diagram. You may want to research that info.

 

[chicopee]

Chemical Enginnering periodicals dated Sept. and October 1991 in their notebook section "Practical Formulae Calculate Water Properties" will have what you need.

 

[ausairman]

Thanks chicopee,
I can't thank you enough for helping me out with this. I'm assuming the document you added makes use of the empirical relationships you mentioned?

 

[chicopee]

Part I and Part II have the following relationships:
vapor pressure vs temp
boiling point temp. vs pressure
liquid specific volume vs. temp
liquid density vs. temp.
vapor specific volume vs temp.
vapor density vs. temp
liquid and vapor enthalpies vs. temp
and several other relationships.
Regression analysis was used to estimate all these relationships . Data was obtained from Perry's CE Hdbk.
Good luck