# Methane storage

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28 minutes ago, Moreno said:

Maybe you may make calculation for methane in particular? For example, if we fill some cylinder with LNG and will wait when temperature of LNG will become equal to room temperature. How much pressure on cylinder walls it will exert now?

You need to provide the specifics of the container. I picked an easy value, but all it is is P1V1 = P2V2 and we know what P2 and V2 are: 1 atmosphere  and 22.4 L at 0 ºC and then correct for the temperature (going from 273K to 293K is only ~ 7.3% change)

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3 hours ago, swansont said:

You need to provide the specifics of the container. I picked an easy value, but all it is is P1V1 = P2V2 and we know what P2 and V2 are: 1 atmosphere  and 22.4 L at 0 ºC and then correct for the temperature (going from 273K to 293K is only ~ 7.3% change)

Where did you take it from? When LNG reaches room temperature, I think it's closer to -167 C to +25 C change. Where did you take 0 C from? I think container could have around 50 L volume. Let say initially it was completely filled with LNG. How much pressure will grow, when its content will reach +25 C? What pressure the cylinder walls will experience?

Edited by Moreno
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12 hours ago, swansont said:

That should approximate what would happen for a non-ideal gas.

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4 hours ago, Moreno said:

Where did you take it from? When LNG reaches room temperature, I think it's closer to -167 C to +25 C change. Where did you take 0 C from? I think container could have around 50 L volume. Let say initially it was completely filled with LNG. How much pressure will grow, when its content will reach +25 C? What pressure the cylinder walls will experience?

0 C comes from STP.

36 minutes ago, John Cuthber said:

Better than the one you’ve offered, though.

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20 hours ago, swansont said:

Better than the one you’ve offered, though.

Fair point.
OK imagine I fill a tank with liquid methane at its normal boiling point.

The density is 0.656 kg/litre

And if I close the tank and let it warm up then (ignoring the deformation of the tank)  the density will stay the same, so I need to find the pressure where gaseous (strictly, supercritical) methane has that density at room temperature- say 20C or 293K.
I spent a while trying to find data for that, but I couldn't. I will have a look when I get  back to work, to see if anyone has the data.

On the other hand, if I use pv=nrt well, p=nrt/v

I have chosen v= 1 litre

the mass is 656g

That's 41 moles so n = 41

r = 0.0813 litre bar / k/mol

About 950 Bar (I think- feel free to check).

However, fundamentally, the reason I said it was a way of estimating the pressure badly is that you are using the ideal gas laws- which assume that the compressibility of a gasi is quite large.

However, you are actually considering a supercritical fluid.

Gases are easy to compress because they are mainly empty space.

That's not the case with supercritical fluids where the molecules are nearly "in contact" with one another. If the molecules are "in contact" then you have a liquid

In the case of liquids, the compressibility is tiny- for most  practical purposes water is regarded as incompressible, for example.

So, to get the 656 g of supercritical fluid which (near the critical point) has a density of 0.562 kg/l into a 1 litre tank you need to compress it by a factor of about 85%.

Now, I know it's not the same thing, but to compress water to that extent the pressure needed is huge.

The compressibility is about 0.5% per 100 Bar

And you are trying to compress it by about 15%

Methane is going to be about the same ballpark.

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I found some sort of helpful data
https://www.nist.gov/system/files/documents/srd/jpcrd425.pdf
Page 1155 tells me that at 1000 MPa and 290K the density of methane is 571 g/litre

So, even at ten thousand bar the pressure isn't high enough to raise the density to that of liquid methane at its normal  boiling point.

So, if I have got the arithmetic right, the use of the ideal gas equation gives you an answer that's at least ten fold wrong (and I have no direct data about how much worse it actually is).

At half that pressure the density is about 497 so the density doesn't change very fast with pressure. That, in turn suggests that even at 20KBar the density would be less that that of the liquid

Edited by John Cuthber
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So, if we will fill a cylinder designed to withstand 700 bar pressure (similar to those in Toyota Mirae) with LNG and will left it unattended and with no further refrigeration, will it explode with time?

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2 hours ago, Moreno said:

So, if we will fill a cylinder designed to withstand 700 bar pressure (similar to those in Toyota Mirae) with LNG and will left it unattended and with no further refrigeration, will it explode with time?

It seems LNG only combusts when it constitutes 5-15% of an air/gas mix. Any more or less than those two numbers ignition cannot occur. I presume under stp.

Edited by StringJunky
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2 minutes ago, StringJunky said:

It seems LNG only combusts when it constitutes 5-15% of an air/gas mix.

You don't need to get a combustion process initially to get a cylinder rupture. Even cylinder with pure liquid Oxygen or Nitrogen can get rupture if cylinder walls cannot hold increased pressure any more.

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11 minutes ago, StringJunky said:

I presume under stp.

STP seems an odd thing to presume when the conditions are stated to start at 700 bar and go up.

2 hours ago, Moreno said:

So, if we will fill a cylinder designed to withstand 700 bar pressure (similar to those in Toyota Mirae) with LNG and will left it unattended and with no further refrigeration, will it explode with time?

Not if it is properly designed, the bursting disk will fail and vent the tank safely

https://en.wikipedia.org/wiki/Rupture_disc

However if for some reason that doesn't work (or is not fitted) then yes, the tank will burst- probably violently.

The release of the methane will lead to it mixing with air.
Sooner or later it will be diluted down to a flammable mixture.

It is then a matter of luck if you get a second explosion.

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3 hours ago, John Cuthber said:

However if for some reason that doesn't work (or is not fitted) then yes, the tank will burst- probably violently.

So, how strong tank do you need if 700 bar tank will rupture?!

Edited by Moreno
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7 hours ago, Moreno said:

So, how strong tank do you need if 700 bar tank will rupture?!

We don't know.
The data we have only goes to 10 KBar, Extrapolating from that it looks like you need more than 20 KBar, but we don't know how much more.

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4 hours ago, John Cuthber said:

Extrapolating from that it looks like you need more than 20 KBar

Are you sure in it?!

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8 hours ago, Moreno said:

Are you sure in it?!

No

On 12/9/2019 at 7:55 PM, John Cuthber said:

So, if I have got the arithmetic right, the use of the ideal gas equation gives you an answer that's at least ten fold wrong (and I have no direct data about how much worse it actually is).

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How well does methane dissolve under high pressures in different substances? For example, what methane solubility may we expect in paraffines under 600-700 bar pressures?

Edited by Moreno
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It hardly matters.
There isn't enough room in the tank for the methane- that's why the pressure gets so high.
If you put solvent in the tank there is less room for the methane.

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9 hours ago, John Cuthber said:

It hardly matters.
There isn't enough room in the tank for the methane- that's why the pressure gets so high.
If you put solvent in the tank there is less room for the methane.

Hard to say. There are claims that methane in adsorbent pressurized to 400 bars is capable to achieve 500 v/v density. Certainly it wouldn't be possible without adsorbent. You would be hardly able to compress methane to 500 v/v. http://onlinelibrary.wiley.com/doi/10.1002/ente.201600172/abstract

Possibly a solvent can play role similar to an adsorbent.

Edited by Moreno
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At 400 bar, methane is still acting more or less like a gas. at roughly 40,000 bar, it won't be.

There simply isn't enough space for the molecules.

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