Dew point Vs Water content

[qca2003]

How to convert the result measuring of water / moisture content in the gas phase from pressurise condition ( at various pressure ) to atmospheric pressure ( 760 mmHg ) condition ? Because I have a table correlation "water content to dew point" at the atmospheric condition.

For example : If I measure the dew point in the H2 gas at temperature 45 C and pressure 8 bar, I found the dew point is -4 C. When I look at the table "water content to dew point" at atmospheric pressure ( 760 mmHg ) -4 C is aqual to 4430 vol ppm ( water content ). So How much water content at dew point -4 C and temperature 45 C, pressure 8 bar ?

 

[25362]

When the dew point is -4[sup]o[/sup]C at 1 atmosphere, the
vapour pressure of water/ice at this temperature, will indicate the ppm. In this case 0.00443 atm.

The mass of moisture in the gas will stay constant as long as there is no condensation or vaporization (by contact with liquid water) at any other P,T condition.

 

[qca2003]

Thank you very much 25362, I would like some comment fo your information. I have two kinds table the correlation between dew point and water content. The first table is in the "atmospheric condition ( 760 mmHg )" and the second one is in the "compressed gas". But why if I compare at the same Dew point I found different value of water content from the both table
for example at dew point -20 C, the water content value in the table atmospheric condition is 1257 vol ppm, but in the table compressed gas is 420 vol ppm. I need your comment.

Last time you have inform me some method testing from ASTM ( D-5454,D-1142, D-2713 ), unfortunately,this ASTM is not present in my library, how can I find free ASTM in the internet ?

Best regards,

 

[Mogi]

Dear qca2003,

The reason why the water content in the compressed gas is lower then atm cond for the same temperature is as follow:
The higher the pressure, the water vapor in the gas tend to condense and the amount of water vapor content in the gas reduce. The same goes if u reduce the gas temp at constant pressure. When u reduce the temperature, the water vapor in the gas will condense and leave the gas with small amount of water vapor.

 

[25362]

qca2003, what are the P,T conditions at the higher pressure ?
Concerning the standards you may try

http://www.astm.org

but I think the standards are not free.

 

[Compositepro]

The topics of relative and absolute humidity, moisture content, dew point, etc. can get very complicated if you don't keep in mind the fundamental principles. In this case it is that water, like any liquid, has a well defined relationship between its temperature and its vapor pressure, and the vapor pressure is not affected by other gasses that are present. A one liter container of air at 20C cannot contain anymore water vapor than the vapor pressure of water at 20C. This air would be called saturated and at 100% relative humidity and the dew point would be 20C. If you add more air to this container and double its pressure the dew point and relative humidity do not change but the weight percent of water in air is cut in half.

The trick to understanding the humidity tables is to remember its related to the vapor pressure of water and to work (or think)in units of partial pressure (mmHg) as a chemist would. The tables that are in weight percent are designed for engineers who often need to calculate weights and percentages. Basically you selected the wrong chart to solve your problem if you are working with H2 gas. You need to refer to a vapor pressure curve for water. Dew point tells you what the partial pressure of water vapor is in your sample.

Did you test the dew point at 8 bar or at ambient? If the sample was tested at ambient then the patial pressure of water in your process is actually 8 times higher than the tested sample. It is also possible that water may have condensed in the sampling line if it is not properly heated. In that case the measurements would be very inaccurate.

 

[25362]

To qca2003, coming back to your first question. The vapour pressure of water at the dew point of -4[sup]o[/sup]C is 3.282mm Hg (Lange's Handbook of Chemistry).

The mol fraction at atmospheric pressure (760 mm Hg) would be: 3.282/760=0.004318, and the volumetric ratio to hydrogen: 0.004318/(1-0.004318)=4340 ppm vol.

If the total pressure -when measuring the dew point- is 8 barg (760+8*750=6760 mm Hg), the mol fraction of water in the vapour at -4[sup]o[/sup]C, assuming ideal gas behaviour, would be: 3.282/6760=0.000486, or 486 ppm vol, or thereabouts.

Since the gas is actually at 45[sup]o[/sup]C the moisture is considered "superheated" and apparently starts condensing (dew point) when the gas is (isobarically) cooled down to -4[sup]o[/sup]C.

 

[qca2003]

To 25362, Compositepro and Mogi, thank for your interesting to my questions, one more question :
What is the dew point will be found same value at any other P and T condition, because dew point is dependent on the concentration of water vapor present and on the relative humidity ? ( It means dew point is not change by exchange P and T ).

When we measure dew point and water/moisture content at the operation condition ( P = 8 bar, T = 45 C ) and at ambient ( P = 1 atm, T = 25 C )in the same sample line, but different treatment during measuring. What is the posibility of result measuring ? ( Dew point is same value, but water content is different or dew point and water content both are different ).

To 25362,
Base on your explanation, if the total pressure when measuring the dew point is 8 barg ( 760+8*750=6760 mmHg ).
Why multiply by 750 not 760, please comment.

Best regards,

 

[25362]

1 bar = 750 mm Hg. 1 atm = 760 mm Hg.

 

[Compositepro]

To qca 2003, Stop confusing yourself by thinking in the wrong units. Dew point tells you exactly what the water content of a gas is in grams per liter, for example. Your confusion is that you make no distinction between "water content" in grams per liter or in percent by weight. they are not the same. Grams of water per liter of gas can stay constant while weight percent of water in gas will vary as the gas pressure varies.
Look at a vapor pressure curve for water. At 100C the vapor pressure is 760 mmHg. Any sample of gas that has a dew point of 100C will contain a partial pressure of 760 mm Hg, of water. If the sample pressure is 760 mmHg it contains 100% water. If the sample pressure is 1520 mmHg it contains 50% water by volume. If the other gas is air with a molecular weight of 30 and water has a MW of 18 then the sample is (.5x18)/(.5x18+ .5x30)= 37.5% by weight water. Everything else is derived fron the vapor presure curve using the ideal gas law.