Developing Tiny CO2 Cartridge - Questions & Answers

[chrisdarroch]

Hi folks,

I was interested in developing a tiny CO2 cartridge and wanted to ask a few questions of anyone who could/would answer.

The cartridge should be as small as possible and preferably as flat as practicable, but it could simply be cylindrical.

It should contain enough CO2 to fill around 20ml of space at SATP.

Would you estimate that this would mean around 0.1 to 0.05 grams of co2?

Would it be possible to make such a thing?

How small could it get, practically?

If it were made of steel, how thick would the walls have to be?

Could it be made of Aluminium and if so, what thickness would the walls then have to be?

If one were to imagine a continuous tube of steel , full of say liquid CO2, moving along and a press, cutting tiny cartridges out of that tube and crimping the ends closed, would such crimped cylinder ends be able to hold in the pressures generated by the CO2 trapped inside?

I know that some of these questions are rather vague but if you could bear with me and give me your opinion, I would be grateful.

Regards,

Chris

 

[UltrafastPED]

How would you make liquid CO2? It goes directly from solid to gas at STP.

Perhaps you could make a "dry ice" cube tray, and then fill your tubes with those.

You can convert the solid volume to gas volume (STP) by calculating the number of moles of material:
http://en.wikipedia.org/wiki/Molar_volume

Then you can calculate the pressure based on the ideal gas law: it all has to fit in the given volume.

Once you know the pressure you can do further research to discover what kind of container is required, and what type of seal would work to keep it contained.

 

[chrisdarroch]

Thanks Ultrafast,

I am afraid that your reply assumes that I am more technical than I perhaps should be at this point.

I assume that liquid CO2 could be formed by taking the gaseous CO2 then forming a liquid under pressure?

Are you able to work with my lack of technical insight and give me more of a idiots view of your response?

Regards.

 

[UltrafastPED]

If you don't have any technical background, then what are you trying to do?

Here is the pressure required for liquid CO2: http://www.co2info.com/co2.html
At 88 degrees F it cannot exist as a liquid anymore - it will expand to a gas. You need to think about what this means.

 

[chrisdarroch]

I am trying to ascertain whether a component that I require as part of an invention is viable. I am working on the basis that small CO2 filled cartridges of 8g, 12g and 16g are used commonly today for varied applications and I have even found a manufacturer of 0.75 gram cartridges which are 1inch in length and 0.375 in diameter.

I am simply attempting to find out if one could scale these down to contain around 0.1 to 0.05 grams of CO2 which I think would expand to around 20ml in volume at SATP.

The parameters are flexible as the invention is at an adaptable stage right now . The enquiry here is designed to illicit the expertise of those on the site.

I am not completely nontechnical in outlook but it is quicker to find out information from those who are at present more knowledgeable and experienced than I.

I am aware that simple scaling may not be possible but this is why I am asking about the potential thicknesses of steel, aluminium, potential constraints due to the shape of container and crimping to seal such a potential volume of co2.

Am I being too ignorant in my questions?

 

[Q_Goest]

Hi chrisdarroch,

Here is the pressure required for liquid CO2: http://www.co2info.com/co2.html

Per the referenced web page, CO2 remains a liquid at 30 C at a pressure of 71 bar with a critical temperature of 31 C. So yes, CO2 can be put into a steel cylinder at a pressure of 71 bar where it has a density of roughly 670 grams/liter. If it were to warm up, the pressure goes up fairly quickly. At 150 F the pressure is around 190 bar - pretty high but not that bad for small diameter cylinders.

Try googling CO2 cartridge pressure.

 

[chrisdarroch]

So, small diameter cylinders can deal with higher pressures?
Is that given the same thickness of metal casing?
Thanks for the info on the critical temp.
Would you imagine that a crimped end could hold such pressures?

What size of cylinder would you imagine might be possible to contain enough CO2 to expand into a 15-30ml space at SATP? Imagine you were throwing out a ballpark to your family at dinner. : )

It really doesn't have to be held in any kind of crimped container, it could be properly sealed like the 12g cartridges.

Thanks so far, for tolerating my naivety.

 

[Q_Goest]

The container would need to stand up to a few thousand psi but would only be the size of a large vitamin. I calculate you need .00008081 lbm of CO2 which fits in a space of 0.0035 cubic inches. A crimped seal needs to be properly designed. You can't just fold over a piece of metal or clamp it. There are swaging methods that would work but for a pill that small you might as well just machine it out of bar stock and come up with a plug. The hardest part would be getting the correct mass of CO2 inside.

 

[chrisdarroch]

Thank you for that reply Q_Goest. Machining may be a tricky consideration. If my wildest dreams for this invention were to come true then I would imagine mass production of hundreds per minute. I aim to have them produced for fractions of a cent each!

I guess with such high psi then considerations about gas release would become trickier.

I was hoping for a small vitamin in size. If the container were flattened out somewhat, would that increase the required size dramatically do you think?

Would aluminium be practical in your view? I would imagine that it would be easier to pierce.

Would you say that the walls of such a cartridge would have to be the same thickness as those common ones, e.g. for 12g air guns or tyre inflators? Could you guesstimate likely minimum thickness of my cartridge walls in steel and or aluminium?

You have been kind to give of your knowledge and time. I hope that these questions are not irritating to your sensibilities, if so then of course feel free to ignore them. I understand that this may not be the correct forum for such questions.

 

[Q_Goest]

The best shape for containing stresses and minimizing the amount of material used is spherical. If made from aluminum you'd need a sphere aprox. .022" thick. Steel is half that. If cylindrical, thickness doubles, so .044 thick aluminum and roughly .022 thick steel. That's good for 3000 psi per ASME BPV code and assuming a nominal diameter of 1/4", so you might get by with thinner material depending on a final design.

Note that the closer it gets to flat, the thicker it will need to be. Basically, you're just increasing the effective diameter of the part by flattening it which makes it weaker.

To pierce it, I'd suggest creating a weak spot specifically made to allow it to be pierced at that point.

 

[chrisdarroch]

Great stuff Q_Goest. Thank you.

The information you are giving me is definitely helping me decide on the potential efficacy of my idea.

Can I just say, that the dimensions of the overall cylinder is the crucial factor for the application I have in mind.

A 'large vitamin pill' would be too big, unless I could flatten it out somewhat even by the process of elongation.

The thickness of the walls would be a secondary issue relative to the overall cylinder dimensions/shape. Would you allow me a further guesstimate and give me your impression on a potential cylinder regardless of wall thickness, with the emphasis on flattening the cylinder out more or elongating it more, as 1/4" profile would protrude too much in one dimension.

As I may have mentioned I found a cylinder in production which is 0.375" in diameter (website spec: 1" long containing 0.75 grams of CO2 with internal volume of 1.0ml and burst pressure of 7250 psi) that seems not too far from a 'large pill' in size and I would have hoped that since I need to hold a much lesser volume that it would be less than a 'large pill' in dimension and certainly less than 1/4 inch.

Am I too naive in imagining a greater reduction in dimension than you are suggesting?

You kindly gave an opinion on the problem of piercing the cylinder, you suggested creating a weak spot. Can I ask about such an endeavour when the casing is optimised to contain the pressure inside and assuming that the pressure is equal at all points within the container, wouldn't a weak spot rupture?

Would you expect that such small containers could be produced with piercability at a rate of 100's per minute for a fraction of a cent each? I am asking you about the overall likelihood of development for such a spec so please feel free to treat that question with disdain it may deserve.

Thank you again for your tolerance, time and expertise.

 

[Q_Goest]

I don't know if the cost could be cut down that low. I think the rest of your questions are details you'll eventually need to work out but for now, I think you have the information you're looking for. Any final design is going to go through the hands of numerous engineers and all those details can be worked out. Working them out here isn't really feasible.

 

[chrisdarroch]

Thank you for your time Q_Goest. You have been kind.

 

[287J]

I realize this is an old thread and I have just joined the forum to continue this discussion. Hope that is ok.

I have been looking at 8g Mosa "soda chargers" in an attempt to model the the flashing when they puncture. Weighing them before and after puncturing, the CO2 content appears to be slightly above the specification 8g. I also tried to measure the volume inside them by the difference of empty mass and water filled mass, which turned out to be over 10ml.

One sample: 8.19g CO2 and 10.57ml (Pre-punture mass 30.12g, post-puncture mass 21.93g, water filled mass 32.50g). Another sample has 8.24g CO2 and 10.25ml. Others are similar.

Looking at this and the equation of state for CO2 at 28C ambient, the measured data suggested a specific volume of 1.29E(-3) which is much lower than that of saturated liquid at the same temperature of 1.53E(-3). The pressure required to compress the liquid down to the internal volume of the canister will have to be unattainable. Did I miss something or messed up somewhere? I was expecting Pc of around 73 bars.