First things first. Safety. At atmospheric pressures, Liquid Nitrogen (Often abbreviated to LN2) is a colourless liquid (although it will usually be emitting a mist formed from moisture in the air condensing), which boils at -195.9 °C (-320.42 °F). This is very cold. Cold enough that it can kill you in a number of different ways. LN2 is a very safe material to work with, if you follow proper safety precautions. It is the most commonly used cryogenic substance in the world, and also the one most often used by amateurs, or professionals who should know what they're doing, but use it in a non-professional manner. As a result, a lot of accidents happen. Should you wish to avoid getting badly hurt, or killed, I suggest you follow proper safety procedures.
One of the simplest ways that LN2 can be harmful is simply by boiling into nitrogen. Nitrogen itself is the main component of air, making up 78% of the Earth's atmosphere. However, when LN2 boils, it produces 700 times the original volume of the LN2, in N2 gas. The problem here is that as it boils, it pushes regular air out of the way, thus creating a space where the concentration of oxygen is lower than normal. Unfortunately, unlike most other gas hazards, there is no warning sign. You won't feel light-headed or anything if there's a lack of oxygen, you'll just pass out if there isn't enough of it. Breathing in anything less than 12% oxygen can be enough to kill a human. Oftentimes someone running in to rescue a coworker who has passed out will pass out as well. This is known as a chain reaction accident.
This usually doesn't become a problem unless you have a great deal of LN2 boiling off at once, such as might happen if you tip over a Dewar (a type of thermos specially designed for cryogenic liquids) full of the stuff. Should that happen, I suggest evacuating immediately, as the room is likely to be rather low on our precious O2 pretty soon. As well, make sure you never take non-trivial amounts of LN2 into an enclosed space. One specific hazard that people have to be reminded of is not to take a Dewar on an elevator.
Also remember that no insulation is perfect. No matter what type of container you store it in, LN2 is eventually going to boil off. Should you store it in a tightly enclosed container, the result will be a lot more gas in the same volume. A quick look at the ideal gas law, PV=nRT, tells us that if we increase the amount of gas in the same space without a corresponding drop in temperature (And how would we drop the temperature, it's already really cold), the result will be an increase in pressure. A big increase in pressure. Left to build up, any tightly sealed container of LN2 will eventually rupture, in a rather violent manner. This is why Dewars come with lids that allow excess gas to vent, and larger scale LN2 containers have pressure release valves that will pretty much constantly be emitting nitrogen gas. So, for the love of God, if you have a thermos full of liquid nitrogen, do NOT screw the lid on.
Because liquid nitrogen is colder than the boiling point of oxygen, if you have a container of liquid nitrogen sitting for a while, liquid oxygen can condense from the air. Should said liquid oxygen pool together, that's a bad thing. Liquid oxygen has the pleasant property of making pretty much anything it comes into contact with flammable. This includes you.
And of course, there are the hazards that are just cold related. LN2 has very low viscosity, which means that if spilled, it will spread quickly over whatever surface it is spilled upon, and also it penetrates fabric easier than water would. Protective equipment worn should be both thick, and loose. If it is tight, you risk having the LN2 trapped against the skin. While human skin can withstand contact with liquid nitrogen for small periods of time, thanks to the Leidenfrost effect, if it comes into extended contact with the stuff, you will be risking severe cold burns. It is advisable to wear full length trousers whose cuffs come down past your shoes / boots, so that no LN2 gets trapped in your shoes, where it will likely cause tissue damage. Oh, and while skin is protected by the Leidenfrost effect, the eyes will not be. My source didn't say why, but were I to venture a guess, I'd say it's because your first instinct would be to blink or close your eyes, which would then trap the LN2 next to the eyeball, with no insulating layer of gas. Small splashes of LN2 or exposure to cold vapours given off by it could cause instant freezing and tissue damage to eyeball. Be sure to wear proper safety goggles.
artman2003 says re liquid nitrogen: What about in the movies where somebody's hand - or worse the whole person - is doused with the stuff and then some bastard takes a hammer to them and smashes them to bits?
Well, ummm first, that'd be a movie. While I suppose it'd feasible, you would have to submerge said hand or person in the stuff for a while. A lot longer than that person will be willing to sit still. Let's face it, if you've already got someone tied down, there's easier ways to kill someone. But I guess it does have that cool supervillain vibe. Good times.
Not only must you be careful with actually getting LN2 on you, you must be even more careful when touching something that's been immersed in it. While some things, such as graham crackers, might become safe to eat after about 10 seconds, a lot of things won't be safe to touch for quite some time. Especially anything metal. Should you touch a pair of metal tongs that you've been using in liquid nitrogen, those things will be very cold, and there will be no Leidenfrost effect to protect you. Your skin will instantly bond to the metal, and will tear away if you try to remove the tongs. Ever seen anyone lick a flagpole in winter? Yeah, it'll be a hell of a lot worse than that. Along the same lines, remove any jewelry, especially rings, when working with the stuff.
Last, but not least, don't swallow Liquid Nitrogen! Ever! I suggest not even putting it in your mouth. A somewhat common trick to make it look like you're blowing smoke is to put a bit of LN2 in your mouth. Should that stuff touch one of your teeth, the thermal stresses it'll induce are capable of cracking them.
That's my quick overview of safety issues when dealing with Liquid Nitrogen. Should you have a job where you are actually working with the stuff, you really should have a full training course on how to deal with it carefully. For a full MSDS, check here.
Try this at home, kids!
OK fine then Mr. Palpz, what can I do with the stuff?
Well, there's a whole lot of fun things you could do with liquid nitrogen. For example:
- You can use it to make ice cream. Just be sure you don't use too much. Let it warm up a bit first.
- Inhale the fumes, and breathe out fog. Just like your favorite dragon or something. You Geek :)
- Use it for pranks. Freeze your friend's stuff and smash it. Stuff that cold tends to be fragile. Your buddy may wonder what happened when they get back to their room to find the (now thawed) remains of a shattered orange all over the place.
- Another good prank (Requiring a lot of cleanup) is to freeze a can of shaving cream, and peel the can itself off of the frozen cream. Then you stick the cylinder of cream somewhere to thaw. It'll expand a great deal when it does. I'd suggest places with easy to clean surfaces, such as your buddy's bathroom.
- Remember what I said about not sealing LN2 in a tightly sealed container above? Here's a good way to demonstrate why. Take some form of durable container to shield you from the shrapnel, such as a garbage can. Fill it about halfway with water. Then take an empty 2 litre bottle, and fill it about a quarter full of LN2. Seal the cap on the bottle, and then quickly toss it into the pail. A few seconds later, the bottle will explode, sending chunks of plastic and water flying up out of the pail.
Hard at work
There are many things you can do with LN2 that are either fun, educational, or both. But that can't be the only reason we've got the stuff around, right? Nope! Liquid nitrogen is used in a wide variety of industrial applications. You know, for grown up people with real jobs. First off, heating up LN2 is by far the easiest and most cost effective method of obtaining pure gaseous nitrogen. The majority of the uses for pure nitrogen involve the fact that it is mostly inert, being a very stable molecule. Some examples of industrial uses of N2 gas include:
- "Blanketing" flammable chemicals, by filling any space in the tanks that they are stored in with nitrogen. This prevents any chance of the chemicals igniting in the tank, no matter how hot they get.
- It can be bubbled through a liquid, which will remove any volatile components present in the liquid, which may be undesirable. This process is called "Sparging".
- Nitrogen is used during some annealing processes, where metals are heated to high temperatures for extended periods of time. Doing so in a nitrogen atmosphere ensures that the surface of the metal will not become oxidized, as can happen when metal at high temperatures is exposed to gas with oxygen in it.
- Pipelines and the like are flooded with nitrogen when they need to be cleared of whatever gas was in them before. Often done before switching the type of gas being transported in a pipeline.
- Some foods are packaged in N2 in order to prevent spoilage during transportation. They call this Modified Atmospheric Packaging.
- Nearly all of the flat glass in the world is manufactured using the float glass process, whereby molten glass is cooled by floating on molten tin, which has a relatively low melting point of 230 °C. This is done in a nitrogen atmosphere to prevent the tin from oxidizing.
- High pressure nitrogen is used as the gas in Gas Assisted Injection Moulding, which is used for the manufacture of stuff like tires.
- Pressurized nitrogen is used as the propellent in dry chemical fire extinguishers.
And then of course, there are always the uses of actual Liquid Nitrogen. In general, they all have to do with cooling stuff down. Shocking, really.
- Flash freezing food. Cools it down for storage and shipment, and also helps prevent bacteria growth when the food does thaw, by displacing the oxygen that would be in the food.
- Shrink Fitting. Metal expands when it's heated, and contracts when it's cooled. If you want to attach something quite securely to a pipe, one way would be to freeze the pipe in LN2 and slip the pipe into a hole that would normally be slightly smaller than the pipe at room temperature. When the pipe warms up, it will expand, and tighten in the hole. This will exert a great deal of pressure, which results in a great deal of friction, and thus a very tight fit. This is an alternative to the more traditional expansion fitting, where the outer component is heated to expand. It is generally better because heating up the outer component can have unwanted effects on the microstructure of the metal, possibly making it weaker.
- LN2 is used for the mist that it gives off, as a result of the cold condensing water in the air it encounters. This is a much safer method than many other types of smoke machines used.
- Cryogenic Grinding. This has something to do with the fact that frozen stuff is more brittle. When you're grinding something, you can grind it to smaller particle sizes, and obtain more regular sizes. As well, the LN2 helps cool down the heat generated by the grinding process, which is often a problem. As well, the inert environment negates the risk of an explosion, which can sometimes occur when grinding organic material. This is often used with plastics, or in the pharmaceutical industry.
- Liquid nitrogen is the refrigerant of choice used in Cryonics, the practice of preserving a recently deceased person at cryogenic temperatures, in the hope that future medical advances will allow the person to be brought back to life. There was, like, a Star Trek: The Next Generation episode about it. So I guess it has gotta work.
- Along the same lines, liquid nitrogen is used to preserve tissue samples, and other things that you wish to keep fresh, or even alive. An excellent example of this is sperm, ova, and fertilized embryos stored by reproductive clinics. They will often use LN2
- Cryogenic Hardening is a unique heat treatment of steel. When certain high carbon or high alloy steels are quenched at room ambient temperatures, some of the Austenite in the structure may remain untransformed into the harder, stronger Martensite. Untransformed Austenite is brittle, and can cause cracking and loss of strength. Plunging the steel into LN2 will complete the transformation, because Austenite is completely unstable at that temperature.
- It is sometimes pumped into oil wells, to increase the pressure down there, and thus force crude oil upwards into our greedy oil consuming hands.
- Doctors utilize liquid nitrogen to "burn" off a wart.
- As well, it can be used to keep supercomputers cool. Some of those can run rather hot.
- RPGeek says re liquid nitrogen : Liquid nitrogen is also used as a quick way to establish a high vacuum, using a device called a 'cryopump'. A cryopump is just a sealed metal cylinder with lots of metal plates inside to provide lots of surface area. When cooled to liquid nitrogen temperatures, most of the gases in the atmosphere will condense on the plates, eventually drawing out most of the air in a sealed vessel. Once the vapour pressure of the air components is reached, the cryo-pump is sealed off from the rest of the system with a valve, and can then safely be brought back to room temperature and vented.
How exactly do we get stuff that cold?
As you can see, it is a rather useful substance. Generally, you'd think that something that requires such careful handling would be rather expensive. But, in this case, you'd be wrong. Liquid nitrogen is rather cheap. Really cheap. Cheaper than Pepsi cheap. You can get it from any company that also sells welding gasses, such as oxyacetylene. The main cost is the equipment needed to safely handle it once you receive it from them. You will need, at least, a Dewar. Mind you, some places would probably be willing to either rent, or lend out the Dewar, with a deposit of course.
The reason it is so cheap is that it's rather simple to make. After all, you get it out of the air. Air is free. The principle used in the majority of LN2 manufacturing is a reversed Stirling engine. From what I understand, the idea is that you compress the air, and then allow it to expand. The energy the gas needs to expand again is supplied by absorbing heat from the gas itself. Repeat this, and then you'll eventually have some liquid forming, which you collect from the chamber.
That's the gist of it, although it gets a bit more complicated because before you start the cooling process, you must remove any water and CO2 in the air, and after you start you also have to remove the oxygen. Thankfully, oxygen condenses at a higher temperature than nitrogen, so you can siphon that off before you cool the remaining nitrogen to liquefy it. The same process also will usually yield liquid argon. This is generally a good thing, as both liquid oxygen and liquid argon are more valuable than liquid nitrogen.
It is a good thing that liquid nitrogen is so easy to manufacture, because there is no doubt that as time passes, it will be used in more and more applications, as we invent nifty new electronics that we need to keep cool, and funky superconductors and the like. The future, my friends, is cold.
Boiling Point @ 1 atm: -195.9 °C
Density @ Boiling Point: 808.6 kg/m3
Specific Heat @ Boiling Point: 2.02 kJ/kg °C
Latent Heat of Vaporization: 199.1 kJ/kg
Melting Point @ 1 atm: -210 °C
Triple Point: -210.1 °C @ 12.5 kPa
Critical Point: -146.9 °C @ 3.4 MPa
Density at Critical Point: 214.9 kg/m3
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