10 replies to this topic

[Olaniyi]

Hi all
Need some advice on water dewpoint spec and mositure content of natural gas.

I have a natural gas feed with high H2S (20%) and CO2 (6%). The gas is to be dehydrated to a dewpoint spec of -40degC at 93 bara. The client equates this to a moisture content of 20ppmv, with plans to install a moisture analyser with an alarm set at this point. I have been trying to reconcile these two figures. I've tried to use STFlash to estimate the saturated water content in the feed at the specified temperature and pressure, and then convert it to STP. I keep coming up with a value of around 5 - 6 ppmv. However I've seen in a few reports and literature online where this same spec (-40degC and 93 bara) is also equated to 20 ppmv or 20 ppm mol.

How would you go about converting a dewpoint spec to a water moisture content? I have attached a file of my gas composition and st flash results (dont know if you use or are familiar with it)

Many thanks for any help!
Warm Regards,
Ogeds

Attached Files

•  kashagan_drygas_water content.xlsx   20.34KB   89 downloads

[Art Montemayor]

Ogeds:

I am not familiar with STFlash or its capabilities and uses.
I am, however, familiar with TEG dehydration of natural gas and fixed bed adsorber dryer systems, having designed and operated them for some years.

Drying natural gas at 100 bara with a TEG unit and producing a product gas with -40 ^oC at line pressure is within the expectations of such a unit. This is assuming the feed gas and contactor at 38 ^oC.

I recommend you visit a couple of websites and download some useful information and conversion programs to obtain your answer.

Go to:
http://www.phymetrix..._NaturalGas.htm
Here, you can download the Phymetrix MS-Windows® Moisture Calculator that will allow you to quickly convert between different moisture concentration units as well as easily obtain dew points and partial water vapor pressures at different total pressures.

You can also go to:
http://www.amsystems...Natural_Gas.htm
where there is a moisture equivalent calculator available for your use.

Mitchell Instruments at:
http://www.michell.c...e-downloads.htm
offers a humidity calculator that has a comprehensive selection of humidity units. High and low humidity ranges are covered. It facilitates calculations with extremely dry gases containing only parts per billion (ppb) moisture content - which is, of most relevance to sectors such as semiconductor industry.

I am also attaching some downloadable information for you to study and evaluate according to your needs.

Attached Files

•  Advantica Moisture Measurement Paper.doc   150.5KB   54 downloads
•  ASTM D2029 - Water Content Measurement.doc   40.5KB   63 downloads
•  PhyMetrix Natural Gas Analyzer Notes.pdf   170.2KB   61 downloads
•  Water Content of Natural Gas.doc   89.5KB   77 downloads
•  Replace Glycol Dehydration with Adsorption-EPA.pdf   470.96KB   77 downloads

[ankur2061]

ogeds,

Water or moisture content of natural gas both sweet and sour in units of mg/Sm3 or lb/MMscf can be obtained from the spreadsheets posted at the link:

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

Regards,
Ankur.

[Olaniyi]

Thanks Art and Ankur for your replies.. will go through the references and be back if I have any more questions!

[Olaniyi]

Quick 2nd question (@ thanks again for the replies so far).

@ Ankur,
Trying to use the sour gas spreadsheet...the pressure and temperature inputs are the dewpoint specs right (i.e. in my case 93 bara and -40degC). I ask this because when I put in these values I get negative water content values..?

[ankur2061]

ogeds,

Because at 93 bara and -40°C, natural gas will carry negligible amount of water with it. In other words, gas is almost bone-dry and the empirical equations in the spreadsheet become inaccurate at such low temperatures. You will probably need to use a process simulator such as HYSYS for calculating the water content at such a low temperature.

Note that the water carrying capacity or water-saturation of NG increases with decrease in pressure and increase in temperature.

Regards,
Ankur.

[Olaniyi]

Thats indeed what I have done with STFlash (I didnt want to use unisim (hysis?) because its my understanding that it may not fully account for hydrates..

[PaoloPemi]

as Ankur suggested at those conditions you need an accurate model to get reliable results,
for natural gas including H2S , CO2 and water I have utilized CPA (Cubic Plus Association) with good results at temperatures a bit higher than those reported,
may be with so low water contents you need to tune (VLE data regression) the model, easy to do with some experimental data.
I do not know your software and I can't suggest the thermodynamic model to select,
I work mainly with Excel and I utilize Prode Properties for all thermodynamic calc's,
for natural gas with water Prode includes these packages

ISO 18453
accurate but doesn't include H2S

CPA , Cubic Plus Association
accurate , you can tune parameter to match your experimental data

GERG 2008
accurate, limited possibility to match experimental data

generally you can solve separately water condensation and hydrates formation conditions,
this gives some addtional flexibility.

[Art Montemayor]

Ankur & Paolo:

I believe the O.P. has asked: "converting a dewpoint spec to a water moisture content" . I don't think the problem is in trying to figure out what is an accurate value for the water content in a very sour gas. Consequently, I have thought that what is being asked is how to convert from a dew point readout (in temperature degrees at either atmospheric or line pressure) to a veritable engineering value such as gm of water per m3 of dry gas. I have interpreted this request as one where a dewpoint meter readout is involved, not the design of the TEG system for removal of a certain water content in the sour gas.

Am I missing something here?

[ankur2061]

Art,

Yes, indeed the OP wants to know how to convert a dew-point value to a corresponding water content in sour gas. In my over enthusiasm of sharing water content values as a function of the gas pressure and the corresponding saturation temperature I missed out on the original query by the OP. My apologies for that. I hope we are back on track now.

Regards,
Ankur.

[PaoloPemi]

Art,
I agree that possibly there was some confusion, I read the last posts and considering these points "I have a natural gas feed ...The gas is to be dehydrated to a dewpoint spec of -40degC at 93 bara", "The client equates this to a moisture content of 20ppmv", "I have been trying to reconcile these two figures" I did suppose a difficult to replicate (by calc's) the client's 20 ppmv ,
hence my answer.