A Liquid Nitrogen Cocktail Party

A long time ago in a galaxy far far away I got to screw around with LN dewars for waveguide pressurization and also extremely high power coax feedline pressurization. You want about 2 or so PSI (varies by system) to keep water out and eliminate all chance of corrosion. The density of liq N2 is such that a small liq tank is the equivalent of a staggeringly large gas N2 pressurization system like the size of a tank car. Also its very hard to explode a liq tank and very easy to explode an ultra high pressure tank by making a mistake with regulators and pipes. A dewar would keep a feedline pressurized for, eh, maybe week, maybe weeks. Some dewars have a better or worse vacuum than others. Boys will be boys and we screwed around with the liq N2 a little. The author got ripped off because in bulk we paid roughly "milk" prices. Supposedly the liq N2 and milk price comparison held for decades at a time in the old days. So his little 20 liter dewar should have cost maybe $20 to fill. Then again, higher energy costs than the 90s, liability insurance, and "we know foodies will pay anything, so lets charge them anything..."

Anyway. Although superficially it sounds shocking that a welding supplier would certify their product as food safe, all their gasses are designed for welding, like welding stainless steel food processing gear. So its safe enough to use anything they sell for industrial use, you'll be OK. If you think they're scared of giving you cooties, they're much more terrified of causing a dairy plant to loose its license. Its clean stuff. Also, they're terrified you or they will "cross the streams" with a O2 supply causing a huge explosion if there's any grease or oil contamination, so you can be absolutely sure nothing oily or burnable ever came near the gear, don't even worry about that class of contaminant. If you think about it, flooding something with N2 specifically to prevent chemical reactions means its not fit for sale unless its ridiculously pure. So it is.

Another chemistry problem is you're looking for something that could end up in the product stream without being filtered out or destroyed by the process gear (or destroying the process gear), so kryptonite is kinda unlikely. Also it has to be liq at liq n2 temps because a gas would blow away and a solid (dirty ice, lets say) would remain stuck to the container. Good luck finding a substance like that. I'm pretty much out of chemistry ideas for a food contaminant.

Don't think of it as an ingredient, think of it as "frozen air". Literally, air so cold that it has frozen. That's extrmeley dangerous because it's cold enough to damage your body very very quickly... but at the same time, when it thaws over time?

It's air.

Just like how when you cook things, you often use "burning air". If you eat something burning? You will destroy your insides. The difference is that freakishly-destructive levels of cold don't ruin food nearly as fast as freakishly-destructive levels of hot. If we blasted your food with a rocket-engine, it would be too hot to eat, but it would also be charcoal. If we blasted your food with liquid nitrogen, it would still look like food and would still be edible after it returns to normal temperature, so there'd be more temptation to try to eat it while it's still lethally cold. Hence the danger.

We got price quotes without telling them the application, so I'm pretty sure we're not paying a "foodie" price, but rather a "you're using a super small dewar and so effectively pay way more" penalty.

Cool stuff. I used to 'play' regularly with LN2 and LHe back in my grad student days (experimental condensed matter physics).

I'm curious how much you paid for the 20 liter dewar. Here in London, just renting a dewar as a one-off and filling with culinary-grade LN2 was amazingly expensive (several hundred pounds). I wanted to do some LN2 demos and also LN2 ice cream at my daughters' party, but instead opted for much simpler and cheaper (and less fun) dry ice.

>>liquid nitrogen is ~700x denser than gaseous nitrogen.

One of the coolest LN2 tricks we used to do at our physics demo days took advantage of this - making bubbles the 'fun way'. Take a big pot of boiling water and add some dish soap. Wearing your protective gear, dump a few liters of LN2 into the water/soap mixture, and see it explosively create 700x its volume in bubbles!

Here's someone doing something similar (he uses room-temp water, not hot, so the bubbles come out 'frozen') https://www.youtube.com/watch?v=X5ZAoHxhihU

On your comment about displacing air - in my actual research (when we weren't making LN2 bubbles) we used alot of Liquid Helium (for reference, LN2 at 77K is about 1/4th room temperature, while LHe at 4.2K is 1/70th room temperature). We'd often have superconducting magnet running with fields up to 10T, which IIRC required nearly 100A of current in the freakishly-thin superconducting magnet wire which was submerged in the bottom of the LHe bath in the cryostat. It's possible that the magnet could 'quench' where part of it goes 'normal', immediately heating up due to the huge current, causing more of the magnet to go normal, causing more heating, which eventually boils off all the LHe, releasing HUGE quantities of He gas into the room. Even something simple like transferring LHe from the storage dewar to our cryostat, or cooling down a cryostat the first time, releases huge amount of He gas into the room. I was glad we had an oxygen meter in the lab (which I demanded our adviser buy, she didn't want to waste the money on it at first).