Boiling Point Elevation 1

[cawse001]

I need to calculate the change in water vapour pressure for a system at 95 deg C resulting from the boiling point elevation due to dissolved salts.

System is a closed agitated tank at atmospheric pressure into which sulfuric acid is added. Carbon dioxide is evolved and recovered. Hence, I need the water vapour pressure to calculate total vent gas composition and flow.

I have a MetSIM balance, but would like to cross check. Suitable references, tables or rules of thumb for doing this?

 

[c2sco]

Hi,
You could start by considering boiling point elevation due to the dissolved salts. The amount that boiling point is elevated is calculated using the "ebullioscopic constant", and I've found the following info that you might find useful from searching on Google:

"Ebullioscopic constant (or elevation of boiling point constant) - when a solute is added to a solvent the boiling point of the solvent is raised by an amount proportional to the molar concentration of the solute, the constant of proportionality being the ebullioscopic constant."

"The relationship between molecular weight of the dissolved substance, M, and the ebullioscopic constant, k, is given by
M = k(w/eW)
where w is the weight of the solute, W is the weight of solvent (water) and e is the elevation in boiling point."

"The high heat of vaporization causes water to have an anomalously low ebullioscopic constant (i.e. effect of solute on boiling point elevation, 0.51 K kg/mol, c.f. CCl4 4.95 K kg/mol)."

Here, k has units {°C per (mole solute per kg solution)}
You could work out your boiling point elevation, then look up the vapour pressure of water at (95°C minus the BP elevation) and that should be a good starting point since you are close to 100°C.

Regards,
Stuart

 

[m777182]

In some books there is a note that ebullioscopic constant works fine with non electrolyte salts that do not dissociate.
For some common salts there are data about the water vapour pressure over the saturated solutions and this may help you to get the upper limit of the boiling point elevation. The elevation for unsaturated solutions will be less than that.
m777182

 

[c2sco]

I seem to remember from school chemistry that for dissociating materials you need to count all the ions separately, eg if it's NaCl dissolved, you work out the molar concentration of NaCl and double it, for Na2SO4, you triple it, and so on.

 

[m777182]

You can find data about lowering of vapor pressure by salts in aqueous solutions at different concentrations in the Handbook of Chemistry and Physics published by Chemical Rubber Co,Library of Congress Card 13-11056. In edition 52nd (1971-1972) for about 100 inorganic salts at 100deg C on page E1.
m777182