[bluegoddess]

We've been using dry ice and acetone as a coolant with exceptional results, and the time has come to scale production a bit.  As a result, I'd like to move away from acetone to lower the risk of fire or explosion.  What coolant can I use to get as close to -78C as possible that would be safer?

Open to all suggestions.

PS we'd prefer to continue using dry ice to chill, as installing mechanical chillers would involve a huge overhaul of our buildings out dated electrical system.

[fledarmus]

Generally when you are trying to reduce the risk of fire, you switch to halogenated solvents such as the various freons. Unfortunately the radical scavenging properties that make them so good at preventing fires also make them good at quenching ozone and destroying the ozone layer. Also unfortunately, I don't know what their freezing points are. Trichloroethylene is close to the range you are looking for, at -73C, and it takes a really strong heat source to light it up. (Unlike acetone, which has a flash point of -17C)

[Enthalpy]

Do I understand you mix dry water ice with acetone, which lets ice melt by mixing, hence produces cold?
Then only molecules miscible with water will work, giving me doubts about trichloroethylene.

What about an anti-freeze, to be mixed with your dry ice?
At least the mixture's freezing point can be below -78°C, but this doesn't tell the mixture will attain it:
https://dow-answer.custhelp.com/app/answers/detail/a_id/7214/~/propylene-glycols---freeze-points
(EG means Ethylene Glycol, DEG TEG means Di- Tri-, PG means Propylene Glycol)
None burns, but Mono, Di, Tri Ethylene Glycol shouldn't be swallowed. Tetra is better, Propylene as well. Propylene Glycol is less viscous.

[fledarmus]

Do I understand you mix dry water ice with acetone, which lets ice melt by mixing, hence produces cold?
Then only molecules miscible with water will work, giving me doubts about trichloroethylene.

Dry ice is solid carbon dioxide, not frozen water. The freezing point of acetone is around -95C, the freezing point of carbon dioxide is -78C - when you stir them together you form a slush of frozen carbon dioxide in acetone that can maintain a -78C temperature if your cooling bath is insulated. Almost any solvent with a lower melting point than -78C can be used to make a -78C cooling bath with dry ice.

The lowest temperature you can get with ethylene glycol and water is about -50C at around 70% ethylene glycol. Propylene glycol will get you down to about -60C at 60%. (Oddly enough, I've never seen freezing point data for propylene glycol in water in concentrations greater than 60%).

If you need to get colder than -78C, you usually use liquid nitrogen to cool a liquid with an even lower melting point. The boiling point for liquid nitrogen is -196C, so if you add it to a liquid with a really low melting point (say, ethanol at -116C), you know you're at -116C when the ethanol starts to crystallize. Liquid nitrogen has its own problems to handle, however.

Wikipedia's entry on cooling baths http://en.wikipedia.org/wiki/Cooling_bath links to an interesting article that gets lower temperature water cooling baths from -12C down to -78C using ethanol and ethylene glycol - http://pubs.acs.org/doi/abs/10.1021/ed077p629 - I haven't tried any of those mixtures, since in the quantities I use, acetone is safe enough.

[Enthalpy]

Oops, translation mistake at "dry ice", my apologies...

Nice data sheet about Ethylene Glycol+water there
http://www.meglobal.biz/media/product_guides/MEGlobal_MEG.pdf
its Figure 7 does conceal the freezing point between 55 and 80%wt, or -46°C... Possibly because viscosity is already 230 to 800cP there, little usable in a circuit (a shampoo has 1Pa*s).
Patent 2558030 tells -54°C at 67%wt.

So the query is: stay liquid at -79°C, don't be flammable at room temperature?

If perfluorinated coolants are still allowed (they differ from CFC):
Mp=-88°C http://en.wikipedia.org/wiki/FC-75
Family http://en.wikipedia.org/wiki/Fluorinert

Even without Freons, there are solutions.

A light mineral oil might fit. I didn't find quickly one for -79°C, but these are pour and flash points:
-60°C +230°C Aral Super Tronic 5W40
-60°C +235°C Aral 4T High Energy Motoröl 10W40
-66°C +228°C Castrol Formula Slx 0W-30
so you may find a compromise for lower temperatures with lighter oil.

Polyethylene glycols apparently don't fit, but other synthetic hydraulic or brake fluids maybe. Silicone oils exist as well for very low temperature, don't ignite easily and burn gently:
http://osdir.com/patents/Compositions/Dimethylpolysiloxane-composition-06942820.html
-110°C.

If you can find and afford some special hydrocarbons, their flash point can be above a warm room temperature :
mp °C / fp °C / bp °C
-112 / +46 / +170 5-decene cas=7433-56-9 and 7433-78-5
-89 / good / +203 2,3-Dimethyldecane
-90 / good / +199 2,4-Dimethyldecane
-102 / high / +247 2,4,6-Trimethyltridecane
-112 / high / ? 2,4,6-Trimethyldodecane
-100 / high / +246 Phytane (unhealthy!) and probably Pristane. Expensive?
-79 / +54 / +186 Jp-10 (a cheap missile fuel)

Phosphates look very promising. Tributyl is liquid between -80°C and +289°C and Tripropyl has fp=+113°C. Trioctyl may have mp=-74°C and fp=+218°C. A more branched one, and with varied arm lengths, would improve. Can be synthesized in your lab.