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### Topic: Pressure of liquid gas  (Read 5881 times)

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#### ericandrews

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##### Pressure of liquid gas
« on: June 29, 2005, 02:24:41 AM »
Hi all
Can someone give me the formulas required for calculating pressures of gasses that have liquified ? Eg, Our aerosol can volume is 0.45L, and I add 0.26Kg of DME propellent. I don't know what the formulas are that take into account that at a certain pressure (at say 25 degrees celcius) the gas is liquified and exists as liquid with a headspace containing the vapour.
DME phys properties can be viewed here
http://www.aeropres.com/Flash_Opening/Products/body_products.html
eg formula, MW, vapour pressure, SG, critical pressure/temp etc.
Any help much appreciated, or ideas.
Thanks

#### xiankai

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##### Re:Pressure of liquid gas
« Reply #1 on: July 03, 2005, 03:10:10 AM »
when the compound is at the pressure of its vapour pressure, the equilibrium is constant.

since u want a headspace for the vapour, total conversion of gas to liquid is not neccessary. in that case the pressure required should be the vapour pressure at 25 degrees celsius.

im not sure about the exact pressure, but judging by the values at 21.1 degrees celsius (approx 70 degrees farenheit) and at 37.7 degrees celsius (approx 100 degrees farenheit), the value should be somewhere between 61.3 and 106.0 psig.

1 psi = 689.5 Pa, and psig means that the value given is a measure of how much pressure above atmospheric pressure. in other words, u add in 10 kPa to the value to get the total pressure.

so the vapour pressure would be between 42 300 + 10 000 Pa and 73 100 + 10 000 Pa or 52 300 < x < 83 100 Pa.

it should be more closer to 52 300 seeing that 21.1 celsius is a closer temperature.

i cant find the exact value because all the gas laws require a constant volume, which is of course not present in your case.

should the increment of vapour pressure and temperature be linear, then u can treat it as an ideal gas thus using the equation

P1/T1 = P2/T2
« Last Edit: July 03, 2005, 03:11:26 AM by xiankai »
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#### arnyk

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##### Re:Pressure of liquid gas
« Reply #2 on: July 03, 2005, 02:00:21 PM »
I thought that once the gas was liquified it is no longer an ideal gas and thus the law can't be excercised?

#### xiankai

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##### Re:Pressure of liquid gas
« Reply #3 on: July 03, 2005, 07:14:07 PM »
which is why i added those last 2 sentences of mine
one learns best by teaching

#### ericandrews

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##### Re:Pressure of liquid gas
« Reply #4 on: July 04, 2005, 02:53:36 AM »
Hmm ok if I didn't get an answer I was hoping for a direction to go in, direction is I think I need to learn more on vapour pressures and maybe than some more answers will come. If anyone has any ideas keep them coming

#### eugenedakin

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##### Re:Pressure of liquid gas
« Reply #5 on: July 07, 2005, 10:23:02 AM »
Hello Eric,

I am not sure if this helps, but here is some more information.  Without going into deep details, the system you described has three aspects:
1) does the one pressurized gas condense the other material to a liquid due to pressure
2) if it does condense one gas to a liquid, then what is the final volume of condensed liquid and gas.
3) by knowing the final volume of the remaining gas, you can apply the perfect gas law to determine the final pressure

To assist in determining the pressure to condense, a previous question (and mathematics) was determined in the physical chemistry forum on June 5/2005 titled 'Pressure to condense'.

I hope this helps,

Sincerely,

Eugene
There are 10 kinds of people in this world: Those who understand binary, and those that do not.

#### ericandrews

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##### Re:Pressure of liquid gas
« Reply #6 on: July 11, 2005, 02:06:54 AM »
Thanks mate, I was thinking along the same line but just have to find the time to apply the maths. I will do it soon and compare it to a measured aerosol can.