The Chemistry of Refrigerants

Originally posted 2022-07-13

Tagged: chemistry, sustainability

Obligatory disclaimer: all opinions are mine and not of my employer

Refrigerants are gasses that are used in every air conditioning unit in the world. They transfer heat from cold to hot against the natural temperature gradient, by exploiting this One Weird Trick of temperature and pressure dependent phase transitions. There are hundreds of refrigerants known, and yet, we seem to have trouble finding the perfect refrigerant. Fluorinated gases are responsible for 3% of the U.S.’s greenhouse gas emissions.

In this essay I’ll walk you through humanity’s ongoing quest for the perfect refrigerant.

One Weird Trick

When you mix ice at 0° C into a drink at 20° C, you get a drink at 0 degrees with some partially melted ice cubes. That’s pretty weird, if you think about it. Phase transitions like melting and evaporation take up and release a lot of thermal energy - far more than normal heating and cooling of a material. Crucially, the temperature at which the liquid-gas transition happens is pressure-dependent. This allows the refrigeration cycle to compress the gas, forcing condensation (and releasing heat), and then evaporation by expanding the gas (absorbing heat). The ideal refrigerant should have a boiling point that is somewhat below the operating cold temperature. That way, a cheap compressor can provide the moderate (~5 atm) pressures needed to straddle the cold-hot temperature gap.

The coefficient of performance - the ratio of energy transferred to energy used to operate the compressor is typically greater than 1, making it a very efficient way to heat and cool. I won’t get further into the efficiencies of air conditioners and heat pumps - if you want to learn more, this guy is really enthusiastic and will talk for hours about heat pump efficiency.

Another desirable refrigerant property is safety, in the event of a leak. Ammonia, \(\ce{NH3}\) (b.p. -13° C) used to be a popular refrigerant, but it’s a notoriously pungent gas, and not particularly safe to inhale. Propane, \(\ce{C3H8}\) (b.p. -20° C) is another formerly popular refrigerant, with the downside of being highly flammable and explosive in the right mixtures.

It turns out that the space of potential refrigerants is not that large. Most refrigerants have \(\leq 4\) on-hydrogen/fluorine atoms; any more and the boiling point gets too high. (Why are hydrogens and fluorines omitted from this count? Because H/F have extremely low polarizability, and don’t contribute to the Van der Waals forces that help molecules stick together in the liquid phase.)

Based on these criteria, DuPont systematically explored this space and decided that Freons were the way to go. R-12, \(\ce{CCl2F2}\) (b.p. -20° C) is nonflammable and nontoxic. The chlorine and fluorine are similar in electronegativity to oxygen, and therefore, the molecule can be considered “already combusted” and no longer flammable.

The Ozone Hole

Unfortunately, it turns out that carbon-chlorine bonds can be broken by UV light to form chlorine radicals. This wasn’t news to us; free radical halogenation is in fact one of the very very first reactions that organic chemistry students learn. What was news was that a single chlorine radical could catalyze the destruction of 100,000 ozone molecules in the stratosphere. Fortunately, the world agreed to phase out the use of chlorinated hydrocarbons in the Montreal Protocol. Supposedly, Ronald Reagan led the charge here because he blamed his skin cancer on the ozone hole!

Due to ozone destruction, chlorine, bromine, and iodine were no longer acceptable for use in refrigerants. With this additional constraint, the set of allowable atoms shrinks to H, C, N, O, S, and F. HFCs like R-134a, \(\ce{CF3CFH2}\) (b.p. -26° C) a.k.a. were the winners in this generation. Again, fluorine is a key ingredient here thanks to its noncombustible properties.

Greenhouse Warming Potential

It turns out we just can’t have nice things. The C-F bond turns out to absorb IR radiation quite well, and R-134a has a greenhouse effect that is 1300x (!!) more potent than carbon dioxide per molecule. It’s a devil’s bargain: fluorine confers nonflammability but also has a potent greenhouse warming effect. In designing the next generation of HFOs like R-1234yf, \(\ce{CH2=CFCF3}\) (b.p. -30° C), chemists have instead optimized for atmospheric breakdown time, mitigating the overall greenhouse warming potential to 2-4x carbon dioxide. After many excuses from Daimler, parent of Mercedes-Benz, Volkswagen (what is it with these execs?) about the expense of upgrading to HFOs, HFOs are in full use for cars/light trucks in the US/EU. I assume that after the R-1234yf patent expires, it will become cheap enough to deploy for use in residential/commercial HVAC systems as well.

Something to ponder is that it took six years to discover R-1234yf, another four years to run exhaustive safety testing, another five years to gather political support for mandates, and another five years to scale up and finish rolling out to automobiles. Over the next decade, it will likely roll out to adjacent markets. This 30-year process seems fairly representative of new technology rollout, and may even be on the fast side, due to DuPont’s existing expertise and understanding of the problem space, the relatively small search space, and the centralized regulatory mechanisms of the US/EU. Basic research and materials discovery will likely take at least this long to scale to planetary impact, if it ever does.

Eliminating high-GWP fluorinated gases

High-GWP fluorinated gases are emitted to the atmosphere in a variety of ways. It can happen at the manufacturing plant, during improper air conditioner disposal, during a car collision, and even intentionally, in compressed air dusters (!!). Fluorinated gases are apparently also useful as inert gaseous blankets in applications like aluminum refining or electrical transformers. Finding low-GWP alternatives is an important step, but recovering and eliminating existing high-GWP fluorinated gases is also important. Due to the ridiculously high multipliers here, I suspect there is a lot of low-hanging fruit for climate change mitigation in this space.

I did a quick search for how to dispose of fluorinated-gas containing devices, and all I can find on the internet is people pointing fingers at other people saying “somebody local probably knows”. Here’s a startup idea, folks! Just be careful of the cobra effect.

Postscript: The Fluorous Phase

Fluorine is weird. The stability of the C-F bond leads to its chemical inertia - think Teflon. The C-F bond is so unreactive that fluorous solvents won’t even interact with other solvents! This leads to a little-known fact that in addition to oil and water, there is a third mutually immiscible so-called “fluorous phase”, which sounds like a made-up science fiction plot device.

When working with the fluorous phase, fluorous surfactants (molecules with a fluorous end and a hydrophobic/philic end) like PFOA are needed to homogenize reactions involving fluorinated compounds. Since fluorinated compounds are so stable, PFOA bioaccumulates and persists in the environment, and it turns out biology has no idea what to do with these molecules, not unlike the trans fatty acid situation.

Perhaps these toxicity issues could be mitigated with partially fluorinated surfactants (to give oxidative enzymes a handle to solubilize and excrete these molecules, but honestly, I wouldn’t be surprised if at some point we give up on fluorine, the same way we’ve given up on lead. I do hope we find a way to safely use fluorine, because its properties are so unique, and the alternative is probably to revert to propane as a refrigerant. (No, I don’t really think \(\ce{CO2}\) will ever make it as a refrigerant, due to the high pressures necessary!)