5 replies to this topic

[mbeychok]

Jasp:

You did not state whether the 1 atm pressure is absolute or gauge. I will first assume that it is 1 atm absolute. Also, since you didn't say whether the percentages are by weight or by volume, I will assume that you meant percentages by volume. Finally, I will assume that you are looking for the water dewpoint (rather than the acid dewpoint due to the small amount of SO2 in your gas mixture).

Therefore, for your gas mixture at 1 atm absolute and containing 25.5 volume percent water vapor, the partial pressure of the water vapor is:

(0.255) (14.696 psia) = 3.75 psia (where 14.696 psi = 1 atm)

Using a set of steam tables, the temperature at which water has a vapor pressure of 3.75 psia is 150 °F and that is the water dewpoint of your gas mixture at 1 atm absolute.

If you meant the pressure to be 1 atm gauge, then the absolute pressure is 2 atm and the partial pressure of the water vapor in your gas is:

(0.255) (14.696 psia)(2) = 7.50 psia

And using the steam tables, water has a vapor pressure of 7.50 psia at 180 °F which is the water dewpoint of your gas mixture at 1 atm gauge.

If you would rather use an equation than a set of steam tables:

T = [6997.5 / (14.466 - LN (P*VF))] - 382

where:

T = water dewpoint, °F
P = total gas pressure, psia
VF = volume fraction of water vapor in the gas
LN = natural logarithm (i.e., logarithm to the base e)
P*VF = partial pressure (in psia) of the water vapor in the gas

I would caution you that the acid dewpoint might easily be 50 to 150 °F higher than the water dewpoint.

I hope this helps you.

[siretb]

If you have a figure that is expected to be higher than 100°C it is because you are concerned about the acid dewpoint, not the water dewpoint.
Calculating the xwater dewpoint is straightforward, and you can use the procedure given by Milton.
Assuming volume percent and abs. pressures, I calculate T(water dew)=152°F so about 66-67°C
However if what concerns you is corrosion, then the acid dew point is what you want. In addition to the information, you need to give the fraction of SO2 that is SO3 or sulfuric.
one ppm of sulfuric changes a lot the picture.
with 1% (assumption 0.5%-3% usual) of the SO2 given as SO3 I calculate 116°C acid dewpoint.
usually, for many application the acid dewpoint will be around 110-130°C.With lots of SO2/SO3 it can be much higher.
you MUST know the SO3 (or %SO2 as SO3) to get meningful figures.
other component like HBr, HCl, H3PO4 will alter the acid dewpoint.
But remember, it is the SO3 figure that will dictate the dewpoint, mostly. Not the SO2.
If exact knowledge is needed, measure the dewpoint (cooled mirror method)

[siretb]

It's experience.
Systems containing SO2, some oxygen and water, have a small fraction of SO2 converted as SO3 (order of magnitude is a percent). Even ppm's of SO3 raise the acid dew point a lot, because of sulfuric mist.

The water dewpoint will be under 100°C at atmospheric pressure, but what you are concerned about for corrosion and to some extent fouling is the acid dew point. Do not confuse the two.