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Billybobjoe

Wed Sept 24 2008, 01:26AM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

I have a 20lb. (9.1kg) CO2 tank which I can get refilled very cheaply at a local welding supply store. It doesn't have a dip or siphon tube to draw out liquid CO2 but I've managed to get some dry ice out of it by inverting it and attaching a fitting with a very small hole drilled in it (can't remember the exact size but <1mm) and wrapping the apparatus with a rag.

I know there are commercial devices available and the "Frost Stick" seems like something I'd like to have.

But its $200, and it looks like it could be made with a few dollars worth of fittings (sans the pressure washer trigger, which seems very dangerous as it definitely was not designed to handle the near cryogenic temps of liquid CO2 . . .). Thats assuming the device doesn't have any special internal components.

Assumption: The liquid CO2 needs to contact something and it seems that something with a low thermal mass so it cools quickly works best (thats why a rag works to some extent). I do have some ceramic wool used to anneal glass . . .

My question is, what kind of things should I take into consideration if I plan on construct/design a dry ice making nozzle and get the most efficiency possible?

Chris

Wed Sept 24 2008, 01:52AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

I've made dry ice that way in just a wooden box with some small vent holes. The inlet I used consisted of just a pressure gauge, a cheap needle valve, and the CGA fitting itself all connected to a steel pipe tee. Steel and rubber o-rings become brittle at the low temperature of expanding liquid CO2 and dry ice, but remember it doesn't really start to cool down until after it as passed the orifice (in my case the needle valve) to begin expanding. After that it's straight into the expansion/collection chamber. Brass, copper, and aluminum are hardly affected at all even by cryogenic temperatures, so they are fine to use. Just make sure everything leading up to the orifice is rated for 1000psi and that's about all there is to it. If you're using the cylinder upside down be sure it is well anchored so it can't tip over or fly off if something should fail, and use proper protection just in case.

As for making the dry ice production more efficient, you will indeed want low thermal mass and low thermal conductivity in your collector, but this is only a small part. The colder the cylinder itself starts off, the more efficient you will be, so for example cooling it down first in an ordinary freezer will help a lot. If you want to really be efficient in terms of CO2 (close to 100% instead of at most 40%) you will have to either re-compress the CO2 that vents out of your collector, which will require a special high pressure compressor, or condense and crystallize the CO2 directly. The latter can be done with a cold-finger connected to either a sterling or pulse-tube cryo-refrigerator. Neither of these devices are particularly complex, and -78Â°C should not be a hard goal to achieve with a home built one. The most machining required to construct a pulse-tube is a lathe. Commercial models can hit liquid nitrogen temp easily in one stage, and liquid hydrogen temp in one or two stages.

Dr. Slack

Thu Sept 25 2008, 07:19AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Making dry ice from a CO2 cyclinder is as easy as standing on one leg, literally.

Stand on one leg, remove shoe, remove sock, place sock over outlet nozzle, retain with string, open valve. The liquid flash boils as it leaves the nozzle, it doesn't need to contact anything, snow forms in the space downstream of the nozzle, the sock filters the solid from the gas. Invert sock with a gloved hand to get the dry ice out most easily.

Health and safety advice - allow sock to warm to room temperature before replacing on foot

Assuming the low efficiency is OK for a few batches, the cost of the extra CO2 consumed, and the sock, beats building your own Stirling cooler. A small fabric bag (sock) is about as low mass, conductivity and thermal capacity receiver as you will get for low investment. Starting with the CO2 at freezer temperature sounds an excellent idea, it just means you need gloves to handle everything, which for safety is not such a bad thing.

Steve Conner

Thu Sept 25 2008, 10:00AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

You can think of the energy stored in the compressed gas as driving a heat pump that freezes some of it, but couldn't freeze all of it without violating the Second Law.

Chris, how many kW of refrigeration at -78 degrees do you think you'd need to condense the escaping CO2 as fast as it came out? Is it worth it, bearing in mind the cost of the CO2 that powers Dr. Slack's ghetto sock refrigerator?

Chris

Mon Sept 29 2008, 06:18AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

Chris, how many kW of refrigeration at -78 degrees do you think you'd need to condense the escaping CO2 as fast as it came out? Is it worth it, bearing in mind the cost of the CO2 that powers Dr. Slack's ghetto sock refrigerator?

As much or as little as you want, depending on how you do it. For crystallizing CO2 directly with a refrigeration unit, you could feed the CO2 in as slowly as you want. To make the dry ice by flash freezing as we have been mostly discussing, you could always run the vent gas into a large bag to be re-compressed at any rate. You could also use CO2 from combustion or another source which is cheaper than bottled CO2. Other than that you've just got to settle for the losses, which is probably acceptable depending on the quantity you need.

Keep in mind that in many places you can buy dry ice at a grocery store for about half the cost of making it from liquid CO2 if you don't recycle the lost gas. However if you used a method that retains all the CO2, such as re-compressing or condensing the gas, you could make it for about half the cost of dry ice, not including the rather large equipment costs. For example CO2 might cost $1/lb, dry ice might cost $2/lb, and it might take 4lbs of CO2 to make 1lb of dry ice with the flash-freezing method ($4/lb). With nothing more than a sock and the cylinder at room temp, you won't even do that good (expect 10lbs CO2 per lb of dry ice or more), but you might by using a vented insulated box and a pre-chilled cylinder. Collecting and re-compressing the excess gas is how dry ice ends up at retailers for much cheaper than you can make it from bottled CO2 without recycling.

Dr. Slack

Mon Sept 29 2008, 02:37PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

One arrangement that stops short of active cryo machinery but which might be worthwhile is to use the cold "waste" CO2 gas in a counter-current heat exchanger to pre-cool the liquid CO2 prior to expansion, is that what a "frost stick" does? I'm thinking something like 1m or so of brake line to stand the CO2 pressure inside a plastic tube to guide the gas back over it, but maybe the transfer rate with such small simple surfaces would not be worth bothering with, do you need fins or to coil the thin liquid pipe up like a spring? You could vent the gas slower to give more time for heat transfer between the streams, but then heat gain from the environment will limit how slow you can go.

Chris, which of the many grocery stores do you get your dry ice from? I've found the cryo departments at ASDA, Sainsburys, Tesco, Marks'n'Spencer, Waitrose and Morrisons notable by their absence.

Billybobjoe

Mon Sept 29 2008, 09:53PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

Dr. Slack wrote ...

One arrangement that stops short of active cryo machinery but which might be worthwhile is to use the cold "waste" CO2 gas in a counter-current heat exchanger to pre-cool the liquid CO2 prior to expansion, is that what a "frost stick" does? I'm thinking something like 1m or so of brake line to stand the CO2 pressure inside a plastic tube to guide the gas back over it, but maybe the transfer rate with such small simple surfaces would not be worth bothering with, do you need fins or to coil the thin liquid pipe up like a spring?

That seems like something I could do. What about a bent copper tube that loops around and sprays the stream back onto the tube itself to cool it? This could be surrounded by ceramic wool to keep atmospheric heat out and collect the ice.

Cryo refrigeration is very interesting (I'd like to get more info about it but everything I find seems to be theory based . . .) but I can't justify the costs or money saved by building such a device. I'll definitely try cooling the cylinder (should be easier when winter comes . . )

I haven't seen it at any local stores - I'm sure I could find some if I increase my search area but really the whole point of making it is that I can have it anytime I want (it has a nasty habit of disappearing over time . . .).

Chris

Mon Sept 29 2008, 10:20PM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

Using the cold vent gas to pre-cool the incoming liquid is an excellent idea. Industrial systems that liquefy nitrogen and oxygen by adiabatic cooling of gas often include a step similar to that.

I don't know of any stores here that have dry ice, but by friends who live in Missoula can buy it at Safeway for less than $2/lb.

Dr. Slack

Tue Sept 30 2008, 10:57AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Having made the suggestion of cold vent gas to cool the liquid before expansion, I'm getting less and less convinced that it's worthwhile on a small batch system. If the heat exchanger is very small, it won't transfer much heat, and the benefit will be minimal. If it has lots of surface or fins or length, then it's going to have a large heat capacity, take an age to cool down, and the benefit averaged over the whole batch will be minimal. Perhaps thermostat capilliary tube rather than brake line for the liquid? The industrial systems that use such things are continuous flow plant, where the heat capacity is irrelevant. Maybe if you run your whole 20lbs in one go, then a heat exchanger might make a worthwhile difference, but if you only want enough to make a spooky drink then I think the sock rules.

What's your batch size?

Proud Mary

Tue Sept 30 2008, 12:26PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Early patents can often provide the amateur with an accessible way into a subject:

Apparatus for the manufacture of cakes of carbonic snow
United States Patent 1965922 07/10/1934

Dry ice machine
United States Patent 2096584 10/19/1937

Machine for forming solid carbon dioxide US Patent 2217169 October, 1940.

You can download the original patents with drawings as .pdf files free of charge from

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