8 replies to this topic

[HOUTAN]

good day ladies and gentelmen , fellow engineers

 

i want to calculate the water content in a hydrocarbon gas stream leaving a dehydrator , i have an analyzer that gives a reading in terms of water dewpoint down stream of the dehydrator , the question is how to translate a water dew point temperatue into mass of water?

 

gas composition:

65% c1

25%c2

10%c3

flow = 160mmscfd

water content at inlet 40lb/mmscf

Pin = 830psig

Tin = 79 F

 

Pout= 825 psig

Tout= 80 F

Water dew point at exit = -80.8 F

[PingPong]

Based on extrapolation of the well known McKetta & Wehe chart I would guestimate that the maximum water content at

840 psia & -80.8 F is about 0.031 lb/MMscf

 

Feel free to do your own guestimation: http://petrowiki.org..._Image_0001.png

Edited by PingPong, 07 August 2014 - 09:42 AM.

[Art Montemayor]

I think you want to CONVERT a known dew point temperature of “a hydrocarbon gas” into the mass of water contained in a unit quantity of gas.

 

Unfortunately, what PingPong gives you doesn’t do that.  The McKetta & Wehe chart predicts the dewpoint of the gas for a given condition of temperature and pressure.  I understand your query as wanting to convert a known DP value to a water mass content.

 

Additionally, you fail to tell us what your conditions are.  You don’t identify the pressure basis of your dew point reading.  Is your dew point reading based on “standard” atmospheric pressure or is it at the actual, process pressure you cite?  ALWAYS give the pressure basis of your dew point.  We can only guess.  Some dewpoint meters report their DP readings at atmospheric pressure, not at process pressure.

[PingPong]

Unfortunately, what PingPong gives you doesn’t do that.

I do not agree with you.

In my opinion the chart can be used to estimate natural gas water content when a dewpoint temperature at a specified pressure is given.

I assumed that the given dewpoint referred to the given pressure of 825 psig, not to atmospheric pressure.

If the dewpoint of -80.8 F is given for atmospheric pressure then I would guestimate a maximum water content of  0.7 lb/MMscf.

 

We can of course argue about the accuracy of extrapolating the chart,

Moreover the accuracy of the chart below 32 F is limited. See the Warning on the chart:

"Dashed lines are meta-stable equilibrium. Actual equilibrium is lower water content. Angle is a function of composition."

[ankur2061]

Houtan,

 

Refer the link below for water content of natural gas and the attached spreadsheet in the link:

 

http://www.cheresour...of-natural-gas/

 

Regards,

Ankur.

[PingPong]

Ankur,

 

Correct me if I am wrong, but as far as I understand your spreadsheet should not be used for pressures above 500 psia, or temperatures below 0 ^oC, due to use of Raoult's law and vapor pressure of liquid water.

 

Moreover, because in cell B31 the mg/Sm³ value is rounded up to display multiples of 100, the lb/MMscf value in cell B32, calculated from the rounded up value of cell B31, shows similar behavior in displaying only multiples of 6.24 lb/MMscf.

Edited by PingPong, 08 August 2014 - 05:24 AM.

[ankur2061]

PingPong,

 

If your remove the rounding-off for the mg/Sm3 given in the spreadsheet considering the example pressure and temperature the value of the water content returns as 1287 mg/Sm3 (80.3 lb/MMSCF). And yese, the pressure limitation is 500 psia with the temperature range of 0-370°C.

 

I would suggest that the second spreadsheet in the link that I have provided with the title

 

New_Correlation_for_Water_Content_of_Sour_NG.xls

 

be used for water content calculation since I believe the results are more accurate. This sheet can be used for both sweet and sour gas. While providing the inputs considering sweet gas, just input the mole percent of sweet gas as 100 & mole percent for CO2 & H2S as zero in the last worksheet.

 

Regards,

Ankur.

[PingPong]

I understand that I can remove the rounding off, but some users in the past (and future) may not have realised that, so any small values they calculated were off by quite a bit. Would be best if you would update the Sweet Gas spreadsheet yourself in your other topic.

 

I did not look at the Sour Gas spreadsheet this morning, because of its name, that I interpreted to be for sour gas only.

 

For HOUTAN's natural gas with dew point of -80.8 F (-63 ^oC) at 825 psig (5790 kPaa) the Sweet NG spreadsheet would predict a water content of 0.01 lb/MMscf, and the Sour NG spreadsheet 0.04 lb/MMscf.

I earlier guestimated 0.03 lb/MMscf (based on extrapolation of the McKetta-Wehe chart), so I agree with you that the Sour Gas spreadsheet is more accurate in this case.

 

However if the dewpoint of -80.8 F is actually at atmospheric pressure (see Art's remarks), then the Sweet NG spreadsheet predicts 0.6 lb/MMscf, while the Sour NG spreadsheet predicts 3.1 lb/MMscf.

I earlier guestimated 0.7 lb/MMscf (based on extrapolation of the McKetta-Wehe chart), so in that case it seems the Sweet NG spreadsheet is more accurate.

Edited by PingPong, 08 August 2014 - 07:22 AM.

[HOUTAN]

thank you gentlemen for sharing the knowledge , and  MR.ART 

 

the basis of the DP temperature are at the actual process pressure ,

 

so now how can you translate a DP temperature into mass of water ?

 

thank you