Calculating Gas Composition In Natural Gas Line

Dear

I have natural 36" Natural Gas(NG)  pipeline of 5 kms. Post maintenance works followed by  commissioning with  Nitrogen at 0.2 kg/cm2g. If we commission the 36"pipeline with NG ( C1 - 96.1mol%, c2-2mol%,c3 - 0.1 mol%, co2 -0.345mol% and N2 -1.5mol%), what is the final composition of the NG+N2 mix when pipeline is pressurized upto 40 kg/cm2g. Assume flowrate to consumer is zero. 

Your query seems not clear for me.

Do you mean pressurize Pipeline with Natural Gas?

and the sum of the composition of your NG is not 100%.

Btw, you can calculate by yourself with common equation P1V1/T1 = P2V2/T2. mass original + mass additional = final mass.

Dear Demank

To clarify , say 36" size line of 5kms parked with Nitrogen at 0.2 kg/cm2g at static condition. Now if i introduce NG having above mentioned composition is pressurized from 0.2 kg/cm2g to 40 kg/cm2. What is the resultant composition ?

Not sure exactly what you need help with. Some depends on what accuracy you require, but here's a general view of how I would expect to do this:

1) Compute the amount of N2 in the pipeline. You give pressure and pipeline dimension data, so I assume the intent is to use the PVT properties of N2 (whether from a table or from an equation of state) to take the PVT data and compute the number of moles of N2 in the pipeline.

2) Compute the amount of total gas in the pipeline after adding the NG. Again, it looks like you are starting with PVT data, so you need to use the PVT data and compute moles of total gas.

3) The difference between 1 and 2 is the amount of NG added. Use that to compute the moles of the individual components added.

4) Add the N2 from step 1 to the N2 from step 3 and compute the composition of the gas in the pipeline.

5) Step 2 may have required you to "guess" at the gas composition in order to find the total moles of gas. If so, evaluate your guess with the result from step 4. If you feel so inclined, repeat step 2 with your new, better guess at the composition of the gas and repeat steps 2 through 4 until you are satisfied with the result.

Dear Mr Shorty

Thanks for your explanation. The query is very clear and there is no requirement of accuracy. Requesting for the resultant composition

Hi Mosa ,

You should follow MrShorty's  recommendations and perform your calculation. This is the only way to learn .

Anyway I added my calculation to support your work . 

PS : You need to review the hypothesis and revise the calculation accordingly  (temperature, compressibility factor =1,Perfect law gas ) .

Note : Using Peng Robinson EOS , the results are quite similar . 

Z Ng =0.9166 

Z N2 = 0.9995

C1  93.43 % mol

C2  1.94%

C3   0.1%

CO2 0.34%

N2    4.19 %

 Good luck

Breizh 

Edited by breizh, 21 December 2020 - 04:56 AM.

If you don't need great accuracy you can assume ideal gas behaviour and use partial pressures (and of course perfect mixing).

Assuming the initial N2 is 100% (i.e the air/O2 was all removed) then the partial pressure of the N2 is 1.2 bar abs.  If the line is then pressurised to 41 bar abs with NG the partial pressure of the NG is 41-1.2 = 39.8 bar abs.

The partial pressure of the N2 in the NG is 1.5% of 39.8 or 0.60 bar abs. Total N2 PP is 0.6 + 1.2 = 1.8 bar abs. Relative to the total this is 1.8 x 100 / 41 = 4.4%

This compares quite well with the 4.19% calculated rigorously by breizh.

Dear All

Appreciate for your answers