Water Content in Natural Gas 2

[berbie78]

Hello Experts,

I am doing simulation for a TEG Dehydration package using HYSYS. I have the natural gas composition which is saturated at 42.3 Deg Celsius @ 36 bar (a). In the composition I could not find the H2O content.

Is there a way to find how much water present in the given composition? Composition of natural gas is attached here.

Please help in finding the water content in the natural gas.

Kind Regards,
Berbie

 

[zdas04]

In Hysys there is a place to either check that the gas is saturated or a place to put in relative humidity. Just have to find it (I haven't run Hysys in many years, but that functionality has to still be there).

David

 

[EmmanuelTop]

One old trick is to mix enough water and your gas @ given pressure and temperature at the Mixer outlet. Put enough water so as to ensure that you have liquid water flowing out from the 2-phase separator downstream of the Mixer Unit Operation. The vapor phase flowing out of the separator is water-saturated at given process conditions (P,T).

Another good resource is GPSA databook where you can find water content of natural gases.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[PaoloPemi]

if you have water in a gas phase you should be able to see the relative amount, differently check VLE settings, possibly for your software all water is in liquid phase (VLE for water and hydrocarbons mixtures is always a difficult matter).
When calculating equilibrium points my software (Prode) permits to specify the fraction (in this case 0.000..1 fraction) for a specified component, this is useful in these cases, perhaps your simulator has a equivalent procedure.

 

[berbie78]

Thank you for your replies. I will try and come back.

I don't understand, why they have not provided the water content in the natural gas. They have given the composition for all other components except water.

 

[katmar]

If there were liquid present under the conditions you have stated it would be virtually pure water, so you can take the activity coefficient of the water as 1.0 with reasonable accuracy. This means you can simply take the vapor pressure of pure water at 42.3[°]C, which is 0.083 bar(a) and divide it by the system pressure to get the molar composition. This makes the molar fraction of water in the vapor phase 0.083/36=0.0023 or 0.23%.

I suppose you should correct this for the compressibility, but that will be a small change. From this manual estimation you can work out how much water you need to add to use EmmanuelTop's trick, and then HYSYS will work out the water content using whatever model you have specified.

Katmar Software - Engineering & Risk Analysis Software

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[srfish]

There is a 1996 article in Hydrocarbon Processing entitled "Quickly Calculate the Water Conteht of Natural Gas".

Another possibility is using the chart in the GPSA Engineering Data Book. It is based on the McKetta-Wehe method published in 1958. This has been put into a computer program at the University pf Tulsa.

 

[maddocks]

Be cautious using the Mcketta chart if your gas is sour - you'll need correction factors using the Wichert corrections. Best way is to saturate in a simulator - make sure that the saturation doesn't change the temperature - depending on how you set it up, sometimes the outlet gas temp changes as a result of the evaporation of the water.

 

[berbie78]

I used the steps given by 'EmmanuelTop' above.

Following are the steps I followed in HYSYS,

1) Used PR as fluid package.

2) Added two streams one without water (with given natural gas flowrate) and one with pure water(added maximum flowrate).Both streams operating at 42.3°C and 26 bara.

3) Used 2-phase separator to separate the phases. Now the separated gas is saturated but the temperature got decreased.

4) I used 'ADJUST' to change the 'Water Stream' temperature to get 42.3°C.

Is it right???

 

[maddocks]

No, you should force the temperature of the stream going into the separator to 42.3 Deg C and then it will be correct.

 

[PaoloPemi]

I am not familiar with your software but in all simulators a isothermal (or similar operation) flah produces a vapor (and liquid) compositions which depend from the selected model (in your case possibly Peng Robinson with Stryjek Vera extension), BIP binary interaction parameters and mixing rules (possibly van der Waals), what I would say is that when at a specified temperature and pressure you don't see a componente (water) in gas composition (i.e. it's all in liquid phase) to move it in gas phase you can only modify the model, BIPs or mixing rules, ideally you can do a data fitting for BIPs but not easy.
The real point is that with PRSV and standard mixing rules (unless you spend time to fit BIPs) you must be prepared to accept a certain error in VLE.
I have a different software (Prode Properties) which instead of Stryjek Vera extension has the three parameters TWU (three parameter per each component), still there are (little) errors for water possibly because of van der Waals rules (water is a polar component), if I set different mixing rules (Huron Vidal or Wong Sandler) and custom BIPs the errors are much lower.
You should be very careful to select the proper thermodynamic options in your software, PRSV can fit reasonably well the vapor pressure of many pure components but still produce unreliable VLE results.

 

[gaswell]

to saturate a stream just add water then set your operating conditions (42.3 Deg Celsius @ 36 bar) in separator, the gas phase will contain your composition.
I agree with Paolopemi, you must be prudent to evaluate water contents in natural gas with PRSV , you can get unreliable results.

 

[MortenA]

You can download a "water saturate" unit operation from the aspentech support web site. You should be aware that the amount of water will depend on your chosen thermodynamic model as PaoloPemi has warned about.

Best regards

Morteb

 

[EmmanuelTop]

You can get exact saturation conditions you need by specifying outlet temperature from the Mixer Unit Op, and leaving the inlet Water stream temperature to be calculated automatically.

http://antwrp.gsfc.nasa.gov/apod/astropix.html