Flue Gas Condensation 2

[macmet]

I am hoping someone can help me out. I have a flue gas stream that has the following composition.

vol% mol%
CO2: 10.5% 26%
H20: 28.82% 12%
SO2: 0.006% ~0%
O2: 4.01% 5%
N2: 56.67% 57%

I have searched this site and found this thread (amongst others)

http://eng-tips.com/viewthread.cfm?qid=118571

The section thread has a link to a PDF that is pretty good. However, I do not understand where this line comes from:

yv = 18.6/ 100 +18.6 = 0.157

Can anyone explain to me what that mole fraction line means and if it correct or not? Its seems pretty straightforward to follow this process except for that one line.

 

[speco]

Macmet,

I believe that there is something wrong with your data. Mol percent means volume percent. The other composition method is by weight percent. In a mixed gas, the components with the higher molecular weights will have higher weight fractions than volume fractions.

For a gas including water vapor and other non-condensables, (that is, with only one condensable component) it can be treated as a simple binary mixture of water vapor and everything else. You can use a steam table to find out at what temperature the water vapor will condense based on its mol (volume) fraction of the total mixture and the total pressure of the mixture.

However, when you have H2S present there is a sulfuric acid dew point which is a bit higher than the water vapor dew point. I would check a reference such as B&W's Steam Book for that. The big issue is generally corrosion when any surface of the equipment goes below the acid dew point.

Regards,

speco (

http://www.stoneprocess.com)

 

[macmet]

It could be that my mol% is off. To figure that I went (vol% * MW) / (total moles) but now that I look at that I see this is not correct.

I did also use a steam table to find my dew point and got similar numbers. But, I used the mol% figures I had posted which then would cause the same problem.

If I use a steam table and 28.82% as my mol % than I get a dew point of about 155F. As you mentioned though with the sulphur this would actually be a bit higher. I assume Cl- has the same effect which could explain the corrosion we are seeing quite easily.

Cheers

 

[NGbri]

Since you have 100 parts of dry gas to 18.6 parts of moisture (on a volume basis), you have a total of 118.6 parts.

The calculation represents the amount of water as a mole fraction, or 18.6 parts of water in 118.6 parts of stuff

Mole Fraction = 18.6/118.6 = 0.157 or 15.7% moisture by volume

Brian

 

[JKEngineer]

There have been two mentions of the dewpoint being a "bit" higher in the presence of sulfur gases. I think that is severly understated. If memory serves, at atmospheric pressures, the dewpoint of SO2/SO3 bearing gases is considerably higher than the water dewpoint for the same gas. As in, well above 212F (100C) vs never above it for a pure water/air system.
Jack

Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com