Liquid Nitrogen

[Sallybeetle]

Can liquid nitrogen be used to supply carrier gas to a GC system? What steps would be required to make this happen?

[chromatographer1]

You would need a container vessel of liquid nitrogen, a means of converting the liquid nitrogen to vapor, and a gas system controlled by regulators. This would be excessively expensive compared to a system of pressurized nitrogen. Why spend several times more for nitrogen than necessary? You would need a system to control the conversion of nitrogen to vapor from the liquid and if this failed...... BOOM !

best wishes,

Rod

[R13]

Theoretically yes.

But as mentioned above, only by using special equipment converting liquid into gas under controlled conditions - specialist built and certified pressure system.
It is economicaly sound only on big scale. In our company it is used to feed N2 into several LC/MS (wich each use around 10-15 l/min) and some other equipment. We have huge liquid N2 tank and convertor station standing outside.

However we anyway use N2 gas bottle for GCs. The system was set up before I started to work here, so I do not know the reason, but it might be that N2 from liquid is too dirty for GC.

[Don_Hilton]

I have used a free standing Dewar of liquid nitrogen (These are delivered by the gas vendor and have pressure regulation on the Dewar.) for a nitrogen supply. The application has been one that has requred liquid nitrogen at about 10 to 20 L/min at about 30 psi. You can obtain Dewars regulated to a pressure of about 250 psi, as I recall, and that should be enough to drive a GC. These commercial Dewars have a pressure building loop built in - an evaporator built into the Dewar, which will maintain pressure as gas is beign drawn off.

For the applictions I have used these Dewars, the gas quality has been fine. I have needed nitrogen with an extremely low dew point. (stuff that would not condense in equipment at liquid nitrogen tempareatures.) Not much distills out of a liquid nitrogen Dewar while there is some liquid in it.

So how do you do this? You install a gas cylinder regulator on the Gas Use connection on the liquid nitrogen cylinder, open the valve and use nitrogen just like it came from the compressed gas cylinder.

It may be worth a discussion with the gas vendor as to the quality of the nitrogen in the Dewar and the cost of ordering liquid nitrogen...

[Johnny Rod]

As Don says, a commercial container has its own gear to control the gas pressure which you can then draw off for using small amounts. The grade would be the next question but really our standard liquid nitrogen here in UK (BOC/Linde) is good enough. You must be using a lot though if cylinders aren't enough to keep you supplied?

[Sallybeetle]

Thank you to everyone for all the excellent input. Your advice is MOST apprectiated. Keep it coming.

I am currently trying to get nitrogen plumbed to fume hoods in 4 - 6 labs, 1 ICP and 3 GCs in one lab, and two GCs in a lab at the opposite end of the building. Since we are expanding our Nitrogen use from a 1 GC / 1 ICP operation to such a larger scale, we thought that a Nitrogen Dewar would be more cost effective and safe in the long run. Moving and storing a large number of tanks around the labs may be more of a safety issue than a main line nitrogen dewar system. And, the cost of several Nitrogen generators appears to equal a dewar system, or even more expensive over years of replacements.

Please correct me if any of my assumptions are incorrect.

[Sallybeetle]

[quote="Don_Hilton"] "For the applictions I have used these Dewars, the gas quality has been fine. I have needed nitrogen with an extremely low dew point. (stuff that would not condense in equipment at liquid nitrogen tempareatures.) Not much distills out of a liquid nitrogen Dewar while there is some liquid in it."

Don Hilton:

For the Nitrogen line branch that feeds the GC systems, I planned on placing a triple trap in line that guards against oxygen, moisture, and hydrocarbons. Do you think that the LN2 dewar will spit out appreciably more moisture, hydrocarbons, or oxygen than zero grade or UHP grade nitrogen cylinders? This is a big concern of mine.

[Sallybeetle]

Questions keep coming to mind on this LN2 subject.

If the LN2 is being distributed throughout out a building, will the gas be extremely cold? Or, will the gas quickly equilibrate to ambient temperature prior to reaching the lab equipment? Is this important when feeding ICPs, GCs, ELSDs, etc.?

[Steve Reimer]

N2 can be distributed as a liquid for 50' or so but the lines must be well insulated. The ones in our lab ran on the order of $100/ft and at the end of a run it can take 10 minutes to get liquid. There is a bleed system to keep the line full of liquid but with it open the 1000 liter tank only lasts a week.

[Don_Hilton]

The boil-off gas from a liquid nitrogen dewar will be at room temperature just a few feet from the dewar.

The gas use for the multiple GC's may be low enough that the dewar would be a waste. Even standing unused for a week or so, they go empty. If the GC's use low flow rates you may come out ahead with a pair of banks of nitrogen bottles and an automatic switchover valve. You gang up enough bottles to last about a week for all the isntruments and the switch over valve will go to the other set when a bank runs low, giving time for gas delivery and a change of gas cylinders.

[Sallybeetle]

Thank you all for sharing your experience in this area.

Also, for the Nitrogen line that feeds the GC systems, I planned on placing a triple trap in line that guards against oxygen, moisture, and hydrocarbons. Do you think that the LN2 dewar will spit out appreciably more moisture, hydrocarbons, or oxygen than zero grade or UHP grade nitrogen cylinders? This is a big concern of mine.

[Don_Hilton]

For the contaminant levels, talk to the LN2 supplier. And do not expect the contaminants to be delivered evenly over time, as you have a distillation going on in the Dewar. Moisture would be essentially non-existant in the gas, as that will remain condensed in liquid nitrogen.

[Johnny Rod]

Sally are you using packed or capillary GC? I'm guessing packed if you're using nitrogen as carrier. For a handful of GCs that you're talking about, you could stil use a manifold with a pair of cylinders instead of going to a liquid system, one large cylinder when full holds around 1000L of gas at STP and with an auto changeover set you get uninterrupted supply (full cylinde takes over from the empty one automatically so you can change out the empty one). If the labs all need line nitrogen for other uses then yes a liquid tank would be a good idea, the usage will be much greater than the GCs anyway.

If it's capillary GC then you'll use such small amounts of gas it's not worth worrying about, in which case switch to helium or hydrogen carrier anyway if you can.

Re. the inline traps, I've always used these (Chrompack or similar) and without them you will most likely see blank peaks on the first run of the day as trace amounts of contaminants collect on the column head. That said we use standard grade helium here and just leave the columns warm (100-150C) when not in use and it's no problem without traps, so just depends on your setup. These are fairly robust phases we use (sieves, PLOT etc.), I wouldn't do the same with PEG/silicone types I don't think.

[Sallybeetle]

Thank you for your input.

The GCs will be capillary. Since this is a new instrument lab, I do not have test methods or know what methods will be developed when the GCs are running.

BTW, I have never used Hydrogen and shy away from it for safety concerns. Do many labs use Hydrogen carrier gas?

[Johnny Rod]

We're totally paranoid about it in my company, but in others I've used it with no problem. Split/purge vents need ventilating safely but really you're using a few mls at a time and in most labs where the air is changed many times per hour, you'll never get close to the 4% lower explosion limit (the GCs I used didn't have dedicated LEVs or anything like that). It saves having to plumb in helium as well unless you'll need that for something. If you're not sure then go for helium, for capillary GC you won't use much anyway. I think hydrogen use isn't so common but I also think that's mainly because people are afraid of it (and on its own it is potentially dangerous) - you've already got it in the FID though so you're almost there anyway.