CNG storage cylinders

[VRIguy]

I must preface this by saying that I am an automotive engineer who finds himself in unfamiliar territory. I am currently working on designing a mobile compressed natural gas (CNG) refueling system. As part of this project I'm being asked to estimate the behavior of our cylinders over the course of a week.

Which leads to my question:

Does anyone have a good formula for the compression of Methane into a storage vessel? I have a table showing pressure vs. stored volume ratios, and I tried to devise the formula from that. I got a rough equation that will work, but it's not as accurate as I'd like.

Any help would be great.

Thanks!

 

[dcasto]

what are you looking for? The horsepower needed to compress the methane into the cylinder. The amount of methane in the cylinder?

Here are some facts. Methane has about 23,000 BTU/pound. 100 cubic feet of methane at standard conditions weights in at 4.25 pounds. There are some equations that can tell you how much 1 cubic foot of methane at 3000 psi will weight in at (its about 11.95 pounds/cubic foot).

 

[MortenA]

It seems like you are trying to develop a small Equation Of Stae for your self.

But if you are only dealing with pure methane (100%) then why dont you just use the 0 order vireal equation (a variation of the ideal gas law you could say) and a table for Z factors based on P/T?

Many data books and perhaps even NIST will give you this:

http://webbook.nist.gov/chemistry/

- search for methane and view "Gas phase thermochemistry data"

Best regards

Morten

 

[VRIguy]

Sorry, I may not have been quite as clear as I'd hoped.

What I'm attempting to get is a means of accurately understanding the quantity of gas (SCF) in a cylinder at any given Pressure (PSIG). I used the data I have, and tried to create a formula. It is a nasty cubic function, but it works. And I would guess that it is about 80% accurate.

The industry standard method of doing this is by using a volume ratio. In other words; at 3600 PSIG, for example, we know that there is 289 SCF of methane per 1 ft^3 of physical space in the cylinder.

I suppose it is possible that this type of equation doesn't exist, and the one I have is as good as I can get. But I was just hoping someone knew where to find it.

Thanks!

 

[dcasto]

Use the nist link above and get the density across a range. the use excels regression routine to get the equation.

ONE big issue is that natural gas has a vert wide range of componets. pipeline specs allow up to 6% CO2 plus N2 and ethen plus up to 10%. Those constituents cahnge all physical prorerties that you are off at least 5% with anything you do.

 

[BigInch]

Assume its all methane with a variation in Specific Gravity accounting for other slight impurities, then disregard the units of time in the flow related variables in these equations and try this,

http://www.lmnoeng.com/Flow/GasFlow.htm

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[paulpaul]

@ VRIguy

First of all, consider that natural gas is a mixture of different compounds. Look for the ISO standard NG composition as vehicle fuel, sorry but I don't remember its number.

To quickly obtain the mass in the cylinder at various pressures I'd rather use the corresponding states equation of state:

pV = mrZT

where r = R (universal gas constant)/mixture molecular weight

Z is the compressibility factor, that you cam obtain from the so called generalized compressibility charts (

https://www.e-education.psu.edu/files/png520/graphics/figure0801.gif)

or by using a CSP correlation like the one of Lee and Kesler.

In both case you need to know:
1. Mixture critical pressure
2. Mixture critical temperature
3. Acentric factor (only for Lee-Kesler correlation)

Those are physical constants you can easily find on the web for each NG constituent. Then you have to mix them up using a simple weighted average, with molar fraction being the weights for critical pressure, temperature and acentric factor.
Once you put this procedure on Excel, it's pretty quick and straightforward and does not require any iterations, only a 2D interpolation. For your purposes (I also work with CNG) I think it's accurate enough.

 

[BigInch]

Unless you're burning "natural gas" from the well head, by the time its getting to a burn point somebody has already extracted all the heavies out of it to sell as LPG somthings, so you're going to be pretty close to a 0.6 SG anyway for burn quality NG. I've only used heavier densities when designing gathering systems.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/