## Why do they ship with dry ice instead of salted ice?

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ericgrau
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### Why do they ship with dry ice instead of salted ice?

TL;DR: I calculated that salted ice keeps frozen things frozen longer than dry ice. Which makes me wonder why they don't use it instead to ship frozen things. So now I am asking if I made a mistake somewhere below.

I'm doing some calculations for shipping frozen items and something's confusing me. It seems that salted ice outperforms dry ice, so why would anyone ever ship using dry ice? Dry ice is a lot more trouble and more expensive too, right? Btw in case you're wondering about gel packs & other special packs, they are just gelled water. Regardless of wild claims they are exactly the same as ice packs in terms of performance. The advantage is they leak slower. So when I talk about ice packs I am talking about gel packs & etc. too.

Dry ice is far colder and has a higher heat of vaporization (/fusion) than water ice. But this first advantage is actually a disadvantage. The colder the shipping package is, the faster it absorbs heat. Heat transfer is directly proportional to ΔT. At -78.5C it's so extremely cold that heat absorption is 3-5 times faster than a package shipped with ice packs. Its heat of vaporization is only 70% higher. So the higher heat absorption easily trumps this. So the dry ice thaws much faster than water ice. Then once all the dry ice is gone the frozen food rapidly climbs to freezing temperature and thaws normally. Meanwhile the food with salted ice packs is still fully frozen. Because the amount of energy to raise the temperature once the dry ice/water ice thaws is much less than the energy it takes to thaw.

Normally you only ship fresh food with ice packs, not frozen food. Because ice packs have a freezing point of 0 C. And food has a freezing point of about -1 to -4 C. So the frozen food thaws before the ice packs even start to thaw, the ice pack's big heat of fusion is totally useless, and they do little to keep food frozen. That's why the freezing point of your ice packs should be lower than your frozen item; so that the packs melt first not the frozen item. So then comes in dry ice with its -78.5 C freezing point and the food stays completely frozen until all the dry ice is gone. It does a much better job than normal ice packs at keeping frozen things frozen. But you can also get salted ice packs with freezing points of -5C or lower. Why didn't they do this instead? It's not as cold initially, but I think it stays cold longer. Or did I miscalculate something?

Am I missing something? Or should every shipper ditch dry ice and switch to salted ice?

A few figures in case you want to play with the numbers:
Spoiler:
Freezing Temperature of Ice 0 ˚C
Specific Heat of Water 4184 J/(kg˚C)
Specific Heat of Ice 2108 J/(kg˚C)
Latent Heat of Fusion for Ice 335000 J/kg

Dry ice Latent Heat of Vaporization 571000 J/kg
Dry ice Freezing Temperature -78.5 ˚C
Specific Heat of Dry Ice 700 J/(kg˚C)
Specific Heat of Carbon Dioxide 800ish... but effectively zero because it gets vented out of the package.

Styrofoam Thermal Conductivity 0.03 W/(m K)
Last edited by ericgrau on Fri Oct 20, 2017 5:16 am UTC, edited 9 times in total.

Soupspoon
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### Re: Why do they ship with dry ice instead of salted ice?

You seem to have jumped into the middle of a thought process, as if you've already gone through some things and then declared them as worth no more discussion. Or edited some things out. That's the way I read it, anyway. (And I can do the same myself, though I might not notice it myself, at first.)

As I'm unfamiliar with exactly which circumstances you're talking about, I might be wrong but (it seems to me) 'melted' dry-ice seems like a better thing than melted water (and especially salt-water) ice, as the subliming means no water to make porous substances soggier than they should be, nor the same tendency towards corrosive action or to deal with, which makes containment and continued transportation of meltwater less convenient. And then there's weight issues, and/or the potential vor sloshing around (in motion) and undue pooling (at rest).

ericgrau
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### Re: Why do they ship with dry ice instead of salted ice?

TL;DR: I calculated that salted ice keeps frozen things frozen longer than dry ice. Which makes me wonder why they don't use it instead to ship frozen things. So now I am asking if I made a mistake somewhere. Hmm actually I better copy this to post 1.

Bagging the ice in thick plastic keeps the melted ice from getting your shipment wet. Also why they gel it; in case the bag punctures the leak isn't as bad.

Liri
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### Re: Why do they ship with dry ice instead of salted ice?

A few points.

1) shipping times for sensitive reagents or samples is generally overnight at the slowest - duration isn't the main concern
2) a whole lot of things we use either aren't frozen at 0C and/or destabilize at that temperature - there is a big, whacking difference between 0C and -80C
2.5) following on that, we have to keep a lot of stuff at -80C or -20C until the moment we need to use it
3) you might also not be taking into account how excellent the insulation is
4) the cost of dry ice is a negligible concern for biomedical labs, lemme tell ya
4B) I use so much dry ice, and liquid nitrogen
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moiraemachy
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### Re: Why do they ship with dry ice instead of salted ice?

Also, you need a lot of salt to get water to freeze at -20C (about 25% in mass), so it's probably not that cost effective unless in bulk.

Regarding food, usually you want either to refrigerate (coldest possible without ice crystals forming, breaking tissue and changing the texture) or to freeze (the colder the better). Salted ice is in an awkward middle ground. It's also a lot messier than dry ice.

ericgrau
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### Re: Why do they ship with dry ice instead of salted ice?

Medical stuff sure. But what about shipping frozen food like ice cream? With -5 C salted gel packs that are well sealed. Ice creamfreezes at -3 C.

What’s the salt concentration at -5C and are there any downsides vs dry ice? Why the heck would I want -20 C and not -5 C or -10 C?

SDK
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### Re: Why do they ship with dry ice instead of salted ice?

Because even though your food is frozen, the enzymes and whatnot continue to do their thing down to -20 C (0 F).

According to the University of Missouri: "Frozen foods require low storage temperatures because quality continues to change after harvest or slaughter. The higher the temperature, the more rapidly quality deteriorates. For every five-degree increase in storage temperature, changes in quality occur twice as fast."
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### Re: Why do they ship with dry ice instead of salted ice?

Most of the time when I get food delivered (blue apron) they use frozen alcohol packs to keep everything cold, which is probably a better option than salted water, no?
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Mutex
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### Re: Why do they ship with dry ice instead of salted ice?

Do you get to keep the alcohol?

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### Re: Why do they ship with dry ice instead of salted ice?

Mutex wrote:Do you get to keep the alcohol?

It's most likely a mix of ethanol, isopropanol, and methanol. Pure ethanol would be pretty expensive, by comparison.
There's a certain amount of freedom involved in cycling: you're self-propelled and decide exactly where to go. If you see something that catches your eye to the left, you can veer off there, which isn't so easy in a car, and you can't cover as much ground walking.

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### Re: Why do they ship with dry ice instead of salted ice?

Liri wrote:
Mutex wrote:Do you get to keep the alcohol?

It's most likely a mix of ethanol, isopropanol, and methanol. Pure ethanol would be pretty expensive, by comparison.

Judging by the goop that leaked out of one that was punctured, I don't want any part of it.
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ericgrau
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### Re: Why do they ship with dry ice instead of salted ice?

SDK wrote:Because even though your food is frozen, the enzymes and whatnot continue to do their thing down to -20 C (0 F).

According to the University of Missouri: "Frozen foods require low storage temperatures because quality continues to change after harvest or slaughter. The higher the temperature, the more rapidly quality deteriorates. For every five-degree increase in storage temperature, changes in quality occur twice as fast."

Ok but we're talking years vs months for frozen food shelf life. Shipping is days.

freezeblade wrote:Most of the time when I get food delivered (blue apron) they use frozen alcohol packs to keep everything cold, which is probably a better option than salted water, no?

Yeah alcohol works same as salt for declining the freezing point. Maybe it's cheaper and/or it dissolves faster?

It's interesting that at least Blue Apron uses alcohol gel packs. I wonder why Amazon and some other random thing I ordered uses dry ice. Does Blue Apron use the alcohol gel packs for frozen foods or for fresh foods? If it's for fresh then the alcohol may be more of a preservative (which requires less) than a freezing point decliner.

The gelling substance is harmless. And if it's not transparent then it's because they dyed it. You could probably eat the inside of the gel pack if it weren't for the preservative. Even then if it touched my food I'd wipe it off and eat the food anyway.

Liri
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### Re: Why do they ship with dry ice instead of salted ice?

ericgrau wrote:
SDK wrote:Because even though your food is frozen, the enzymes and whatnot continue to do their thing down to -20 C (0 F).

According to the University of Missouri: "Frozen foods require low storage temperatures because quality continues to change after harvest or slaughter. The higher the temperature, the more rapidly quality deteriorates. For every five-degree increase in storage temperature, changes in quality occur twice as fast."

Ok but we're talking years vs months for frozen food shelf life. Shipping is days.

You're undercutting part of your own argument, then, with regards to salted water or frozen alcohol being preferred for longevity of coolness.

Even outside of science applications, I can't think of any reason why one wouldn't opt for the colder option, even if only for "better safe than sorry". If your ice cream freezes just a couple degrees above your cold packs, heterogeneity of the sample could lead to some uneven melting as local temperatures rise above the freezing point. Plus dry ice is cheap. Liquid nitrogen, even, costs less than milk or bottled water.
There's a certain amount of freedom involved in cycling: you're self-propelled and decide exactly where to go. If you see something that catches your eye to the left, you can veer off there, which isn't so easy in a car, and you can't cover as much ground walking.

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### Re: Why do they ship with dry ice instead of salted ice?

(As an aside to the easy availability of liquid nitrogen, this-night-just-gone I was at a place with a bar, and a door behind it (obviously to where the beer kegs/etc were deployed to feed the bar-top pumps) marked with both "Warning: Nitrogen in this room" and "Warning: Carbon Dioxode in this room". No word about Oxygen, though, or Argon... The lack of the former sounds most disturbing. And it also had a Halloween-themed ersatz 'hazard' decorative tape 'discouraging' entry except at your own risk, obviously in keeping with various other temporary decorations like artificial cobwebs and plastic pumpkins. Which, strangely, suggested mixed messages about whether one should take the former dire warnings seriously, at least once you had already gone to the effort of vaulting the bar counter itself...)

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### Re: Why do they ship with dry ice instead of salted ice?

That's kind of the point, isn't it? If a nitrogen tank leaks the oxygen can leave the room, and you will just pass out and die.
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ericgrau
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### Re: Why do they ship with dry ice instead of salted ice?

Liri wrote:Even outside of science applications, I can't think of any reason why one wouldn't opt for the colder option, even if only for "better safe than sorry". If your ice cream freezes just a couple degrees above your cold packs, heterogeneity of the sample could lead to some uneven melting as local temperatures rise above the freezing point. Plus dry ice is cheap. Liquid nitrogen, even, costs less than milk or bottled water.

Because of the part where salted ice stays cold longer. And you can get it at a -10C freezing point if you’re worried that some bits might be a couple degrees warmer. I was looking more to confirm or disprove my calcs that salted ice stays cold much longer than dry ice. Or if there’s some other drawback.

Or put another way, I’m trying to confirm (or disprove) that the shipping weight from dry ice that will survive an x day journey is more than the shipping weight of salted ice that will do the same. This added weight costs much more to ship.

Maybe it’s time to just run an experiment to confirm.

Liri
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### Re: Why do they ship with dry ice instead of salted ice?

Anecdotally, the boxes we receive packed with dry ice don't have any noticeable decrease in volume. Again, you might be underestimating the efficiency of the insulation.
There's a certain amount of freedom involved in cycling: you're self-propelled and decide exactly where to go. If you see something that catches your eye to the left, you can veer off there, which isn't so easy in a car, and you can't cover as much ground walking.

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### Re: Why do they ship with dry ice instead of salted ice?

Thesh wrote:That's kind of the point, isn't it? If a nitrogen tank leaks the oxygen can leave the room, and you will just pass out and die.

That's the point to the (real) warning signs, but it was just the wording. The presence of large amounts of nitrogen in any room (with rare exceptions) is guaranteed, the brevity of the warning signs (not even a generic compressed gases notice, which is probably as relevant as the 'flavours', in the absence of anything with noted flammability to take precedence) piqued my satirical imagination.

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### Re: Why do they ship with dry ice instead of salted ice?

Rather than experiment you could look up what shippers do. One positive for dry ice is the lack of condensation in boxes. Alcohol is a flammable liquid and salt solutions would be subject to whoever mixed them in terms of quality. Or leave you in the position of either using more or less cooling than you might desire if you used standardized packs.

Given the dollars spent on shipping food this is a well investigated area. For a shipper the time in transit is the primary limitation, not the method of chilling. No box you can pack and carry will last sitting in a sorting hub on a weekend. There are multiple expediters in the field. Companies that contract to do this daily. They exist because it is complex.

ericgrau
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### Re: Why do they ship with dry ice instead of salted ice?

I looked into current shipping techniques and it seems to be based on guessing and testing. Most of the techniques are ballpark recommendations and packaging methods. Often shippers do screw up and do things a more expensive way, but at least it still arrives in good shape. Then via trial and error they make moderate improvements to the wasteful excess without causing a failed shipment.

Frozen meat and other food that doesn't melt typically find it acceptable to thaw part way as long as the package stays at freezing temperature, and then the customer refreezes the thawed portion. It's deemed safe as long as the temperature is correct: still ice cold even though partly thawed. They typically use gel(led ice) packs, 32F freezing point I'm sure. Ice cream typically uses dry ice. Mixed frozen goods also use dry ice.

I don't think there's a real science behind it. Sophistication level is roughly equal to warehouse manager knowledge.

I had a hard time finding any gel(led ice) packs with a depressed freezing point for sale. I did finally find one that sells to businesses. I found none available on the consumer level, maybe 1 had it by special request. Some gel packs advertise "colder than ice", which typically means a 28F to 31F freezing point. I don't think people who ship things understand what freezing point depression even means.

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### Re: Why do they ship with dry ice instead of salted ice?

I do think it’s because dry ice is cheap and doesn’t leave any residue. It’s not like it sublimates all that fast — even a styrofoam cooler will keep it for a fair amount of time. I wouldn’t be surprised if it were cheaper than brine solutions — and even if it’s not, it’s much easier to use. There’s no risk of spilling anything onto food. Even if something gets punctured or breaks, you’re not going to be potentially damaging the shipment.

I also wouldn’t be surprised if the fact that it creates a thick CO2 layer is useful to preserve things that need to be cold and will degrade with atmospheric oxygen.

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### Re: Why do they ship with dry ice instead of salted ice?

Just yesterday I ate some frozen shrimp cakes that the package said to store at -18ºC, which requires 260 g/kg for saltwater.

ericgrau wrote:
Specific Heat of Carbon Dioxide 800ish... but effectively zero because it gets vented out of the package.
By what mechanism does CO2 get vented out of the package?

And if it somehow does get vented (despite being heavier than air), it gets replaced with air, which has its own specific heat.
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ericgrau
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### Re: Why do they ship with dry ice instead of salted ice?

gmalivuk wrote:Just yesterday I ate some frozen shrimp cakes that the package said to store at -18ºC, which requires 260 g/kg for saltwater.

ericgrau wrote:
Specific Heat of Carbon Dioxide 800ish... but effectively zero because it gets vented out of the package.
By what mechanism does CO2 get vented out of the package?

And if it somehow does get vented (despite being heavier than air), it gets replaced with air, which has its own specific heat.

CO2 gets vented by the it-will-literally-explode-if-it-doesn't-vent method. Escaping CO2 doesn't get replaced with air because each pound of dry ice sublimates into 10 cubic feet of gas. Only an ounce or two of gaseous CO2 stays in the package, because that's all that will fit. The rest must vent or package go boom. That's part of why dry ice is considered a hazardous substance that most follow proper procedure when being shipped.

Looking at it another way you could say the specific heat is the same but the mass and therefore the thermal mass is extremely low after sublimation. Either way the specific heat of gaseous CO2 is pretty irrelevant.

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### Re: Why do they ship with dry ice instead of salted ice?

The CO2 definitely needs to be vented, and dry ice definitely doesn't last as long as water ice (or saltwater, or gel packs, or whatever). CO2 is also subject to some shipping restrictions and is more expensive than freezer packs. The advantages are that it is much colder and stays dry. All of these have already been brought up in this thread, so I'm not sure what is still a matter of contention. When the lower temperature is necessary or at least worth the cost, dry ice is used. Otherwise, freezer packs are used. Seems like a pretty straightforward calculation.

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### Re: Why do they ship with dry ice instead of salted ice?

Its actually not classified as hazardous, in the EU or the US, only the IATA (International Air Transport Authority) classifies it as hazardous, but since air travel has unique risks, they often take a more "hard line" on various things.

Eg: they have much lower criteria for "flammable" etc. Funnily enough, they never classify things as "hazardous to the environment - I guess because if there an air crash, the last thing to worry about is some water pollution.

You can still take up to 2.5 kg of it in your checked/carry-on baggage though, properly packaged+labelled.

Yeah you can make an explosion of sorts, if you try, but its not all that dangerous a substance.

(I work in chemical safety regulation)

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### Re: Why do they ship with dry ice instead of salted ice?

Reading this thread, a question occurred to me. If you're in a room with dry ice, is there a danger of the CO2 levels getting high enough to be poisonous? Is ensuring that doesn't happen part of the venting process? I've read more than 0.5% CO2 concentration in the air is above safe limits (I think that's when exposed to that for 8 hours) and 7-10% is when it gets lethal.

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### Re: Why do they ship with dry ice instead of salted ice?

I don't know the answer to that but I guess it is less dangerous than most gasses because of how sensitive humans would be to even small increases.

eg. If you entered a room full of Nitrogen you probably wouldn't notice until you passed out, whereas if you entered a room full of Carbon Dioxide you'd probably have a coughing fit and run away screaming.

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### Re: Why do they ship with dry ice instead of salted ice?

The body likely handles raised concentrations of CO2 better (i.e. does something about it, even if it's only potentially distracting involuntary reactions) because it's directly linkable to the inability to purge the body of its CO2 waste, something it always has to deal due to the continual genesis of the gas at the back-end of respiration.

https://en.wikipedia.org/wiki/Hypercapnia#Tolerance

N2 doesn't (under most circumstances) swing in concentrations in either blood or air, so we're more left with the "not enough O2" reaction mechanism from the hypoxia/anoxia alone, and accidents involving nitrogen asphyxiation seem to indicate it creeps up on people without much warning or imparting symptons of suffering, and it has been seriously suggested to use nitrogen as a 'better' method for suicide/euthanasia/execution.

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### Re: Why do they ship with dry ice instead of salted ice?

Breathing in high CO2 is very noticeable, to say the least. When I'm working with it, it's never detectable. Even when I'm blowing on it to make clouds. Or pouring water on it to make bigger clouds.
There's a certain amount of freedom involved in cycling: you're self-propelled and decide exactly where to go. If you see something that catches your eye to the left, you can veer off there, which isn't so easy in a car, and you can't cover as much ground walking.

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### Re: Why do they ship with dry ice instead of salted ice?

The IATA claims dry ice is an explosion, suffocation, and contact hazard, so they must think that it is at least a concern worth regulating. On an airplane with a cabin temperature of 25 °C and a cabin pressure of 0.75 atm, each kg of CO2 gas will have a volume of about 0.74 m3. I don't know what the volume of a typical cargo plane is, but I think a 737 is in the ballpark of 2000 m3. So to reach 0.5% CO2 volume fraction on a 737 would only take like 14 kg of sublimate, which is pretty surprising to me. But keep in mind that we all produce significant quantities of CO2 while breathing, so the plane already needs to be able to vent air properly. If there are 150 people on this hypothetical flight, they are producing 5-6 kg of CO2 per hour already. It's not clear how dry ice sublimation can ever realistically approach that unless a huge fraction of the passengers decided to bring quantities of poorly insulated dry ice in carry-on luggage.

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### Re: Why do they ship with dry ice instead of salted ice?

Dry ice is good because it's dry.

It just sublimates off. No puddles, no leaks, no liquids to keep contained.

The transport people really don't like little surprise "accidents" from wet ice, especially if you're working with potential biohazard scientific samples and they don't know if it's just the refrigerant ice or if some contamination from the inner container may be present.

Dry ice is subject to dangerous goods labelling and safety and transport requirements, even when the real sample of interest within is harmless. For example, venting the outer container to allow safe escape of the CO2 pressure is a big part of that.

It's also significantly colder than wet ice. Even with salt, you're probably not going to stay cold enough for many applications. A freezer is typically about -18 or -20C, colder than wet ice even if salted. And many biological cell stocks and stuff are stored at -80C. Even -20C is just not ideally cold enough.
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