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# Applicability Of Pv = Nrt For Flue Gas And Moisture Content Measureme

flue gas moisture content pv = nrt

5 replies to this topic
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### #1 cheminst

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Posted 17 June 2019 - 06:14 AM

Can anyone please explain the suitability to apply PV = nRT to fluegas since there is high water vapor content and the remaining gases are not air (High CO2 content) ?

I found in this webpage explaining that it can be applied

But I got this question. What if the temperature is reduced to a point where the flue gas is saturated with water vapor.

Are there any equations/ data tables to be applied for flue gases? e.g. coefficient of compressibility.

Please tell me any references with flue gas calculations applying PV = nRT.

Also is there a way to determine the moisture content of a flue gas by flue gas analyzer provided that only O2, CO2, CO, and excess air percentages in flue gas are measured and the fuel is C11 to C16.

Edited by cheminst, 17 June 2019 - 06:18 AM.

### #2 breizh

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Posted 17 June 2019 - 08:00 AM

Hi ,

For flue gas , let you use your favorite search engine , key words  ORSAT flue gases analysis.

Good Luck

Breizh

### #3 cheminst

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Posted 17 June 2019 - 10:06 AM

Breizh:

Thanks for the information about flue gas calculation. Could you also please give an answer to my other question.  The applicability of PV = nRT to flue gas?

### #4 MrShorty

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Posted 17 June 2019 - 11:37 AM

With a few exceptions, just about any gas at 1 atm pressure is fairly well represented by the ideal gas law. I would expect that you can use the ideal gas equation of state for your flue gas and be within tolerable limits for accuracy.

Then you ask about near saturation. At saturation, the gas phase is probably still well represented by the ideal gas law. However, at saturation, there is the possibility that liquid water will be present, and the liquid phase cannot be represented by the ideal gas law. Reducing this to a single "compressibility factor" will depend in large part on how much liquid phase is present. If you are near saturation but no water has condensed, then you can use the ideal gas law. If you are near saturation and a lot of water has condensed, you will need some way to determine how much liquid water is present. Then you can use the ideal gas law to determine what you need for the gas phase, and whatever you are using for the equation of state/density for water to get what you need for the liquid phase, and combine those two calculations however you intend to combine them to get the overall "compressibility factor".

### #5 cheminst

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Posted 18 June 2019 - 12:15 AM

MrShorty:

Thanks for your answer. Also can you tell me about how to calculate the dew point in flue gas.

I found this graph https://www.engineer...es-d_1583.htmlÂ . However there is no reference and since the the dry flue gas composition can be varied significantly this graph doesn't seem reliable.

### #6 PingPong

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Posted 18 June 2019 - 05:49 AM   Best Answer

Composition of dry flue gas is not relevant.

Water dew point is reached when temperature is such that water saturated vapor pressure is equal to the water vapor partial pressure in the flue gas.

All you need is a Steam Table.

For example: if the flue gas contains 10 mol% (vol%) water vapor then water vapor partial pressure is 0.1 atm = 0.101 bar so dew point is 46 oC according to any steam table.