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Author: Subject: Determining the amount of ethylene in a cylinder
garage chemist
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[*] posted on 30-5-2013 at 09:39
Determining the amount of ethylene in a cylinder


I have recently acquired a cylinder of pure compressed ethylene.
This steel cylinder has been filled from a larger original bottle via a transfer hose and by chilling the evacuated receiving bottle to about -40°C where ethylene is liquid at ca. 13 bar.

I would like to determine how much ethylene I actually have in my cylinder. This is complicated by the fact that, at ambient temperature, the ethylene is only slightly above its critical temperature of 9,4°C. Therefore its pressure-density relationship deviates very strongly from the ideal gas law, with a density several times higher at 50 bar than what extrapolation from its density at STP using the ideal gas law would give.

I'm not versed enough in thermodynamics any more to be sure how to approach this problem. Also, I don't have my thermodynamics textbook at hand.
I think that I would have to calculate the density of the ethylene using the compressibility factor by measuring temperature and pressure of the ethylene cylinder.

The cylinder has a volume of 40 litres, and the pressure is 60 bar at 20°C. The empty weight of the cylinder is, of course, unknown and not stamped onto it.
Now the generalized compressibility factor diagram at Wikipedia (http://en.wikipedia.org/wiki/Compressibility_chart)
gives isothermal curves for the reduced temperatures Tr=1.00 and Tr=1.10 which translate to 9,4°C and 37,6°C for ethylene.
This would mean that I have to warm my cylinder to exactly 37,6°C before measuring pressure and getting a value for Z from the Tr=1.10 curve.

Is there a way to determine the weight of the cylinder contents with reasonable accuracy from the pressure at 20°C?
Does someone have a compressibility factor diagram at hand with Tr values between 1.00 and 1.10 or, perhaps, even specific data for ethylene?

Thanks for any help!
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[*] posted on 30-5-2013 at 09:56


I might be stating the obvious; but have you tried contacting the producer of the cylinder or searching for information about it? I.e. finding the weight of the cylinder.



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[*] posted on 30-5-2013 at 10:01


Quote: Originally posted by garage chemist  

Now the generalized compressibility factor diagram at Wikipedia (http://en.wikipedia.org/wiki/Compressibility_chart)
gives isothermal curves for the reduced temperatures Tr=1.00 and Tr=1.10 which translate to 9,4°C and 37,6°C for ethylene.
This would mean that I have to warm my cylinder to exactly 37,6°C before measuring pressure and getting a value for Z from the Tr=1.10 curve.

How precise do you need this? I'd draw a line exactly between the Tr = 1.00 and Tr = 1.10, call it my estimate for Tr= 1.05, and use that for 23.5oC.

Alternatively, you could remove a measured quantity of ethylene from the cylinder, and use the difference in pressure (before and after) to determine what is left.

[Edited on 30-5-2013 by DraconicAcid]




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[*] posted on 30-5-2013 at 10:05


Quote: Originally posted by Lambda-Eyde  
I might be stating the obvious; but have you tried contacting the producer of the cylinder or searching for information about it? I.e. finding the weight of the cylinder.

That's not possible since it is a cylinder for high pressure gases (it originally held something else), these are never weighed because the contents can simply be determined from the pressure.
It was not me who filled the ethylene into this cylinder, if I had been involved in the filling procedure I would certainly have weighed the cylinder before filling.

@DraconicAcid
I would like to base my calculations on solid literature data and not estimations.

Removing a certain amount of ethylene and weighing it, e.g. equalizing pressure between the large cylinder and an empty 1L bottle and calculating the density at this pressure from the weight increase of the small bottle is definately a possible solution. The only downside is that the empty 1L bottle (rated for 200 bar) will weigh many times more than its contents, over ten times as much, and then the resolution of the electronic scale with a range of over 2kg determines the accuracy of this method. But it's a good idea and I'll keep it in mind.
Ideally, however, it should be possible to determine the cylinder contents without removing any of it.

[Edited on 30-5-2013 by garage chemist]

[Edited on 30-5-2013 by garage chemist]




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[*] posted on 30-5-2013 at 12:39


It's been a long time since I have seen anybody take on a problem like this. I don't have any direct experience with using Z from the reduced property charts. However, I will give you what I was able to find in my library.

In Perry's Chemical Engineers' Handbook,4th ed (1963), p. 3-218, is a generalized compressibility chart by Lydersen(1955). It has Tr lines for 1.02, 1.04, 1.05, etc. Assuming Tr = 1.0375 and Pr = 1.186, I read Z = 0.49. This was estimated in a region of steep change.



[Edited on 30-5-2013 by Magpie]




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[*] posted on 30-5-2013 at 13:03


Quote: Originally posted by garage chemist  

Removing a certain amount of ethylene and weighing it, e.g. equalizing pressure between the large cylinder and an empty 1L bottle and calculating the density at this pressure from the weight increase of the small bottle is definately a possible solution. The only downside is that the empty 1L bottle (rated for 200 bar) will weigh many times more than its contents, over ten times as much, and then the resolution of the electronic scale with a range of over 2kg determines the accuracy of this method. But it's a good idea and I'll keep it in mind.
Ideally, however, it should be possible to determine the cylinder contents without removing any of it.
[Edited on 30-5-2013 by garage chemist]

If you're filling a 1 L bottle at room temp and pressure, you can assume the ethylene removed follows the ideal gas law.




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[*] posted on 30-5-2013 at 14:20


I searched for "density of supercritical ethylene". There's a graph here with a line at 290 K. Estimating graphically, it looks like the density is around 0.2 g / cm3, although there's rather high uncertainty about it since the slope of the graph is quite steep at that point.

Edit: spelling.

[Edited on 2013-5-30 by watson.fawkes]
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[*] posted on 30-5-2013 at 15:20


Seems overly complicated to me.

The horse is already out of the barn, and there are too many unknowns. We do not know the exact volume of the cylinder (40Liters is probably an estimate), and we do not know the weight of the cylinder. Though....you could weigh an apparently identical cylinder as an aid to creating a reasonable guesstimate.

Were I in your shoes, I would rely on the time tested technique of using up that ethylene to see how far it goes.

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[*] posted on 30-5-2013 at 19:39


The cylinder volume of 40 L can be relied on, since it's stamped into the cylinder. After all, high pressure gases are sold in terms of cylinder volume at a given fill pressure, so there's no reason to doubt this.

It seems like I have two options so far:
1. fill a known volume at full cylinder pressure into a tared bottle and weigh it to get the density.
An absolute gravimetric method that is free of thermodynamic calculations, but relies on a good scale with a range big enough to accomodate the weight of the steel cylinder. That is not a big problem.
However, I realized that I will first have to buy a transfer line for the ethylene cylinder and braze it to a high pressure connector that fits the small bottle, since I only have a pressure regulator with maximum outlet pressure of 10 bar for the ethylene cylinder. This would cost additional money.

2. Heat up the entire big cylinder to a temperature at which the compressibility factor can be determined with reasonable accuracy. This won't cost additional money, it's just really cumbersome to place the bottle into a big water bucket and heat it up with a bath thermostat.
I'll probably try this first. Temperature of the bottle won't have to be accurate to 0,1°C since my 200 bar manometer only measures in steps of 10 bar anyway.

I'm still open to new and unconventional suggestions.
Yes I know, it's my own fault that I have to do this since I bought the cat in the bag, so to speak.
But the opportunity was far too good to pass on, since I would otherwise never be able to acquire ethylene for a reasonable price. I also want the cylinder to last many years.





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[*] posted on 31-5-2013 at 04:16


To my mind you can just use van Der Waals Equation
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[*] posted on 31-5-2013 at 14:31


Not to be offensive, I've been known to be a little obsessive myself......But! Is there some reason you need to know exactly how much Ethylene you have?

Seems like you have what you have. You do know very approximately how much that is.

If I really NEEDED to know. I might take an identical, Empty, 40 liter Cylinder, tare it, Couple the two cylinders, open the valves, and let the pressure equalize. Uncouple the second cylinder, weigh it......and, Viola! 2x the weight change, equals the total weight of your Ethylene. Now in two cylinders rather that one.

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[*] posted on 31-5-2013 at 18:23


unless each bottle is hand made different from the last, there has to be a standard weight available some where on the net. with a little effort you can find all kinds of info. if things like the pitch and thread count on a coleman disposable grill tank pressure relief schrader valve can be found, I don't see why you cant find a standard empty canister weight for your type. looking up specs on my N2 tank was easy and they are rated 3000psi. now don't get me wrong I'm not calling any one stupid here just I don't see what makes your tank so special from all the other types available.

what kind of tank is it? other wise you may be able to hook up an adapter( or directly to) from the tank to an or N2 regulator. turn the reg off and attach, open the cylinder valve and get a reading. you said 60 bar @ 20'C, the N2 regulators hold 4000psi are more than capable of the ~870psi.

http://www.ebay.com/itm/New-HTP-Argon-Flow-Gas-meter-Weld-Re...

comes with a flow meter, hose barb attachment and 4000psi rated for a really affordable price. could come in handy. I don't know how hard of a time you would have fitting or adapting this to your cylinder, worth a look though. hope this helps
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[*] posted on 31-5-2013 at 20:44


why nobody use van der wals?
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