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Water Dew Point


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#1 romianne

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Posted 12 December 2016 - 11:39 AM

Hello. My question is:

 

Water dew point of natural gas is -30 C at 4 MPa, which means 50 kg of water per milion Sm3. What if I have the same gas at 5 MPa? What the dew point will be? Why do I keep getting the same result, 50 kg of water per milion Sm3?


Edited by romianne, 12 December 2016 - 12:22 PM.


#2 Francisco Angel

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Posted 12 December 2016 - 12:19 PM

Dear romianne:

Water dew point, as you say: "Water dew point of natural gas is -30 C at 4 MPa" is the temperature value at which water condensation occurs, at specified pressure, and for a given gas water mixture.

The quantity of water per standard m^3 of gas is called humidity (it can also be measured as kg of water per kg of dry air), as long as no condensation or water addition to the water-gas mixture exists, the humidity will remain constant.

Be careful with which concept you are applying in a given problem.

Best regards.



#3 romianne

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Posted 12 December 2016 - 12:29 PM

But isnt there higher water content at higher pressure? Shouldnt I get more kg of water per Sm3 at 5 MPa than at 4 MPa and therefore a higher dew point temperature? Can you please explain it to me in the simple way, I am really lost :(


Edited by romianne, 12 December 2016 - 12:56 PM.


#4 romianne

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Posted 12 December 2016 - 12:33 PM

I have this but i cannot figure out what the steps are..

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#5 romianne

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Posted 12 December 2016 - 01:39 PM

And also this paper but i still cannot figure it out

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#6 shan

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Posted 12 December 2016 - 02:49 PM

The higher natural gas dew point temperature corresponds to the higher pressure.  If you keep the stream temperature constant at -30C, water condensate will appear when you increase the pressure from 4 MPa to 5 MPa in the condition your natural stream dew point is -30C at 4MPa.  Therefore, the water content in the gas phase is less than 50 kg/MMSm3 due to formation of water liquid phase.



#7 romianne

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Posted 12 December 2016 - 03:24 PM

I have dry natural gas at 5 MPa and 15 C. There is a measurement which says the water dew point of this gas is -30 C, but it is dew point recalculated to 4 MPa; it is not water dew point at real pressure of my system. I found out that at -30 C and 4 MPa, gas is saturated with 50 kg of water per milion Sm3.

I understand that if I increase the pressure, my dewpoint is gonna increase too, but probably at like -28 C. But my system is at 15 C, so I dont have to worry about any condensation and my gas still contents 50 kg of water per milion Sm3. Am I right? My system is closed.

Edited by romianne, 12 December 2016 - 03:36 PM.


#8 Francisco Angel

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Posted 13 December 2016 - 12:03 PM

Romianne:

 

But isnt there higher water content at higher pressure? Shouldnt I get more kg of water per Sm3 at 5 MPa than at 4 MPa and therefore a higher dew point temperature? Can you please explain it to me in the simple way, I am really lost :(

 

From your additional post I understand that the value 50 kg of water per milion Sm3 is the humidity at saturation at 4 MPa and temperature of -30 C.

Consider the equation given in the document you provided:
 

P*Y_i=P°

 

Where P: Absolute pressure, Y_i: Mol fraction of water at gas phase (water content) , and P°: vapor pressure. You must fix two of these quantities to obtain the third value, so your question "Shouldnt I get more kg of water per Sm3 at 5 MPa than at 4 MPa and therefore a higher dew point temperature?" doesn't quite make sense, the response is "it depends", the kg of water depends on what temperature you set, the value of dew point temperature depends on what initial water content you set.

 

Look at what shan says:

 

The higher natural gas dew point temperature corresponds to the higher pressure.  If you keep the stream temperature constant at -30C, water condensate will appear when you increase the pressure from 4 MPa to 5 MPa in the condition your natural stream dew point is -30C at 4MPa.  Therefore, the water content in the gas phase is less than 50 kg/MMSm3 due to formation of water liquid phase.

 

In this case, keeping the temperature at -30C and increasing the pressure from 4MPa to 5 MPa, translates in a smaller water content, you must specify two values (P and T) to get water content.

 

I have dry natural gas at 5 MPa and 15 C. There is a measurement which says the water dew point of this gas is -30 C, but it is dew point recalculated to 4 MPa; it is not water dew point at real pressure of my system. I found out that at -30 C and 4 MPa, gas is saturated with 50 kg of water per milion Sm3.

I understand that if I increase the pressure, my dewpoint is gonna increase too, but probably at like -28 C. But my system is at 15 C, so I dont have to worry about any condensation and my gas still contents 50 kg of water per milion Sm3. Am I right? My system is closed.

 

From you first paragraph, we have:

P       water content  dew point

5MPa   x                  -30C

 

4MPa   50                -30C

 

x must be smaller than 50 kg of water per milion Sm3.

In the second paragraph you have a different comparison:

P       water content  dew point

5MPa   50                 y

4MPa   50                -30C

yes you are right, y will be greater than -30C, you must do the calculation to confirm that "y" will have a safe margin as compared to the operating temperature of 15C.

 

Best regards.



#9 Profe

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Posted 15 December 2016 - 08:28 AM

Hi Romiane
 

About your concern: what exactly is the task. Can you send us the whole problem to understand what you really want?
What I request is only to avoid wasting time because according to the data that you send and what you really have.

In the attached material that is the GPSA regarding the water content in natural gas. By increasing the pressure of a gas, the water content decreases if the temperature is kept constant. The difference is due to the separation of the water by condensation. Separate water must be removed from the gaseous stream or system by suitable equipment.
Attached copy of the page of Figure 20-3 "Water content of hydrocarbon gas by GPSA."

I hope this helps. Do not hesitate to really consult what is the problem or the task you want to solve your worries.

 

 

On the subject of natural gas I have attached two links to download technical information that will be very useful, especially the volume 1 chapter 5

 

http://ucalgary.ca/E...gy - Part I.pdf

 

http://ucalgary.ca/E...y - Part II.pdf

 

Luck

Fausto

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Edited by Profe, 16 December 2016 - 10:45 AM.





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