# CO2 vs Nitrogen for chilling vessels

#21

I found a pressurized liquid nitrogen dewar set-up at my local gas supplier that you might be interested in (50L capacity?). It looks like they use it for freezing totes of live material (shower head on the end of supply), but I see the possibilities for other uses as well.

In terms of CO2… Are you monitoring the pressure within the jacket using a pressure gauge some how, or do you rely on the PRV to vent pressure without watching psi? Are you running a bizzy setup?

CO2 seems cheaper and more efficient than coolers of ice. If anyone has tips on running CO2 on tanks / heat exchangers It would be much appreciated.

#22

I have been trying to research this topic lately. I want to spew out some stuff and see what you all think.

Pertinent info:

Latent heat of vaporization for CO2= 574 Kj/Kg
Latent heat of vaporization for N2 = 199 Kj/Kg

Latent heat of sublimation for dry ice=571 Kj/Kg

Triple point of CO2= 5.1atm (75 PSIA) and -56.7C
Triple point of N2 =.124atm(1.8 PSIA) and -210C

One of the first things you might notice is that the latent heat of vaporization for CO2 is pretty high, almost 3X that of N2.

Put in other words, one kilogram of liquid CO2 requires almost 3X as much input energy to evaporate as one kilogram of liquid N2.

Or to think about it in yet another way that’s important to us for this topic: CO2 will absorb 3X more heat energy from its surroundings during evaporation than N2, per given quantity of liquid.
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Pressure temperature diagram for CO2:

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Pressure temperature diagram for N2:

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Latent heat of vaporization for CO2= 574 Kj/Kg
Latent heat of vaporization for N2 = 199 Kj/Kg

Latent heat of sublimation for dry ice=571 Kj/Kg

Triple point of CO2= 5.1atm (75PSIA) and -56.7C
Triple point of N2 =.124atm(1.8PSIA) and -210C

If you are trying to take advantage of the phase change properties of these substances, I recommend you become familiar with how to read a pressure temperature diagram (aka phase change diagram). You can wikipedia that right up.

Some of the key takeaways from these diagrams with regard to our particular situation are:

1. Notice how the horizontal line going across at 1 atm on the diagram for CO2 doesn’t slice through the “liquid” section, but the line for N2 does. Since the triple point of CO2 is above atmospheric pressure or 1 atm, you cannot have the liquid to gas phase change at ambient pressure because the liquid can’t exist. In other words, you cannot take advantage of the latent heat of vaporization of CO2 at atmospheric pressure, the only thing possible is sublimation (solid to gas). This is why vessels using the evaporation of LCO2 for cooling must maintain the jacket pressure above about 75 PSIA. Anything less than that and the liquid will flash to dry ice and could cause clogs or flow issues. I’m not sure, but I would guess most LCO2 jackets run at least 100 PSIA to be extra sure they don’t have dry ice issues. This is where implosion dangers enter the scene and you need to be careful. Nitrogen’s triple point is below 1 atm so at ambient pressure you will have evaporation of liquid instead of sublimation of a solid. This means the latent heat of vaporization of LN2 can be taken advantage of without any real pressure build up in the jacket.

2. The latent heat of sublimation for dry ice, 571 Kj/Kg, is pretty much the same as the latent heat of vaporization for LCO2, 574kj/Kg. That means whether you are evaporating liquid CO2 at just above the triple point (75 PSIA) or subliming dry ice at atmospheric pressure for your cooling, you are transfering about the same quantity of heat per a given starting mass of CO2. There is another piece of the puzzle however and that is the temperature at which these phase changes take place. You can see in the CO2 diagram if you follow the line down from the triple point it occurs at -56.7C. If you have your LCO2 jacket pressure set higher than the triple point that means your point on the evaporation curve is further up and the temperature is actually warmer. The sublimation temperature of dry ice at atmospheric pressure is -78.5C. That’s 21.8C colder than the coldest possible temperature at which the liquid can evaporate. Want to inject solvent or do any cooling at -60C or colder? Sorry LCO2 can’t help you.

3. Since the latent heat of vaporization for nitrogen is about 1/3 that of CO2, it will take about 3 times as much nitrogen, by weight, to move the same amount of heat energy as CO2. However, being a true cryogenic substance, the evaporation of liquid nitrogen at atmospheric pressure occurs at -196C. That’s about 3.4X colder than LCO2 at its triple point.

Final thoughts and questions:

• Even though LCO2 has 3X the latent heat of capacity as LN2, does the fact that the evaporation of LN2 occurs at such a drastically lower temperature (about 130C colder) than LCO2 factor in somehow? I remember bizzy had some posts on the old IG page about using LN2 in one of the Exergy tube-in-shells and he was pleased with the result. @710ST can you share any info about this?

• In my opinion, it seems like dry ice provides the best bang-for-the-buck. It can be used at atmospheric pressure, has a very high latent heat capacity and provides significantly lower cooling temperature than LCO2 (-78.5C vs -57C). Although I do understand and acknowledge you cannot exactly use dry ice for a tube-in-shell or tube-in-tube heat exchanger.

• Would it be better from an operational/safety standpoint to use LN2 over LCO2 because there is no risk of implosion from a pressurized jacket, no risk of clogging with dry ice and no need to worry about PRV’s not functioning correctly during operation? Can anyone with experience say anything about the need to monitor PRV’s? Is that a big issue with LCO2? An LN2 jacketed vessel or heat exchanger could just be run open to atmosphere with a control oriface on the outlet.

• I haven’t checked with my local air gas guys yet so I’m unsure of the cost difference between these two gases.

• @Photon_noir can you offer any insight into any of this? Also, I wanted to ask you if you can tell me the reason(s) why CO2 seems to be very unique in the fact that is has a triple point so far above 1 atm but at such a low temperature. It seems like most other substances with a triple point above 1 atm also have a much higher temperature corresponding to the data point. Acetylene is a notable exception that fits with CO2 in that it has a triple point of -81C at 1.2 atm. I notice that due to the triple carbon bond in acetylene it also has a completely linear structure like CO2 so I’m thinking this somehow boils down to molecular structure and electron configuration of CO2 but I’m unsure.

#23

Great write up! I’m going back and fourth between them also. D.I. Ln2, heat exchanger with ln2 or lco2 open ended, or dry ice packed condenser, or dry ice acetone slurry thru heat exchanger. My heads going to explode

#24

thank you @Tech1145!!

#25

Nitrogen has a heat capacity of ~1.039 kJ/kg * K, which means you can effectively account for ~135 kJ/kg of extra energy being dumped into the ethanol by the nitrogen, assuming it actually jumps 130 degrees from input to when it leaves the tank.

On a per-kg basis, that makes LN2 only half as efficient (in heat transfer terms) as CO2.

I’d be much happier having LN2 cooling in my facility for all of these reasons.

#26

Can’t rely on that biological warning system too much with CO2. I’ve heard of cases where people were found dead halfway slumped into a chest freezer with dry ice. Which reminds me, I should probably add sensors to my shopping list…

#27

It’s extremely expensive compared to C02 and it doesn’t have the cooling capacity that lc02 does. I am very interested if someone has tried ln2 for recovery of hydrocarbons, either with a heat exchange of sorts or a tank. But, I don’t think it will be powerful enough. It’s cold enough, but I think it’ll just evap quickly.

Anyone want to educate me on this if I’m way off

#28

Guess since Ln2 Will freeze extract the pumps heatexchanger etc etc to use indirectly make iT less favorable

#29

I think D.I. is the only way to go with ln2.

#30

No not realy for needle valves etc kan make very precise cooling possible
I have My eye on a revco -150C freezer that is Ln2 assited to rip her appart and see how she manages so precise temps

#31

#32

Whoa lol high finally for this country boy ha

#33

Precisely, @Tech1145! That perfectly straight 3-atom combination with double and triple bonds, respectively, creates a “stackable” super-non-polar compound. Basically, the higher pressure forces the molecules together like the little inflexible sticks they are, but when the temperature gets too high, they begin to vibrate and rotate out of “phase” with one another, becoming the more disorganized phase (gas), unless you increase the pressure even more to get them to flatten back out again, where they reside as liquid. When you eventually reach the supercritical point, you end up with a slightly more organized gas phase, so it behaves like a liquid, as you know. It’s definitely a structure-activity relationship, but in a physical sense.
I 2nd the thanks for writing this up! Well done!

#34

Naah. Like in the condensing coil or in the jacket and coil of my tank. If it doesn’t push out as much pressure as lc02 and wouldn’t crush it. But that’s all I can say about it. Besides it might be too cold.

#35

I totally didn’t read this until after I wrote my previous response. Nice read, that was what I was wanting to know/read on the subject

#36

I know a few people using LN2 for recovery. You have to be careful not to freeze your solvent. Yes it can freeze and looks crazy like paraffin wax. Most important part is making sure you have sufficient outlets for the offgassing.

#37

Love the data! For the exergy experiment I took a tube in shell and ran LN2 in it and took the vapor coming out of the outlet and routed it through 2 counterflow coils to make sure we got as much heat exchange as possible. We were able to cool down solution coming out of our falling film at 10c down to -50c but it took more LN2 than I’d like.

#38

You will start to feel dizzy and lathargic from co2. N2 gas is the cheif component of our air you body will not notice anything untill you pass out.
This is what makes N2 a very easy thing to asphyxiate from.

#39

I’ve noticed most people using liquid air gases for refrigeration on things like a tube-in-shells or jacketed tanks usually only have a single inlet port that they inject liquid into.

Do you think it would be much more effective to have multiple inlets that have some sort of spray action to efficiently distribute the liquid?

For example a tube-in-shell with a baffled interior that is having liquid CO2/N2 injected in the bottom shell side port is probably only seeing liquid to gas phase change in a small portion of the bottom section and the rest is cooled by conduction. If the shell side was separated into two or more discrete compartments each with their own inlet/outlet and both hooked up to the CO2/N2 supply the overall efficiency of the exchanger might be significantly increased, depending on its overall length. What do you think?