Hi all,
Some very good comments worth sharing and adding to my previous summary regarding helium recovery in general and liquefaction.
What volume of gaseous Helium per hour do you have to liquify? Probably you collect helium from different spectrometers and further sources. Do you know the minimum rate per hour of gaseous He which is needed to feed such a liquefier on an economic base? Is it worth to think about it if you have three spectrometers on one site for instance? Is it worth to have such systems just for He collection during magnet fills (once a year)?
Helium liquefaction will depend on 2 things. (1) How large is your impure and pure helium gas storage capacity? If your capacity is large you can allow it to build up until you need the liquid. If however, your storage capacity is limited you must consolidate your helium by liquefying it. Given 1L of liquid equals 794 L of gas so it is much more space friendly to store helium in it's liquid phase. If not used immediately these dewars themselves can be attached to the recovery system to mitigate losses.
To put it into perspective, most individuals are familiar with the size of a high pressure gas T-cylinder or sometimes they're called K-cylinders. They are about 5' (1.52m) high and 10" (0.25m) in diameter. Those cylinders can hold about 8 m3 of high pressure helium gas (>2000psi). This amount of gas can be condensed into about 10L of liquid helium. So 1 high pressure T-cylinder equals 10 L of liquid helium.
Our design plan is to capture about 16,000,000 L of gas per year, which means about 44,000 L of gas per day or 55 L of liquid per day. This means we would fill 5.5 T-cylinders everyday . Our impure helium storage will consist of 4 x 16 cylinder "bullpacks" which are manifolded together. Because our initial operations is only expected to capture 50% of the consumed helium our recovery capacity is oversized.
This nominal helium gas flow does not include the higher flow and pressure associated with helium capture from NMR fills. As seen in earlier AMMRL posts we can expect 20%-25% of the helium transfer volume to vent from the helium manifold and be lost or recovered. If we recover that helium and depending on the size of magnet that could mean 5L to 25L of liquid or 4000L to 20,000L of gas contributed to the recovery system with 30min-60min. This is the volume, pressure and rate we are concerned about. Yes, the system can handle nominal flow, but could the system handle these large surges in short time frames? A study being completed this month says yes, however we must coordinate our helium fills through campus so no fills are happening at the same time. Our gasometer, gas bag and recovery compressors will be engaged when surges come down the line so drastic pressure increases do not happen to the recovery system.
In summary, if your gas storage capacity is too low you'll have to liquefy whether you need the liquid or not.
(2) Liquefaction can also happen when the demand calls for it. Again, our demand is about 380L of liquid per week. The system we have purchased is capable of between 17L or 47L per hour depending upon LN2 pre-cooling, so 22 hours at 17L/hour would meet our weekly needs. For pure helium storage there will be 2 bullpacks. Of course because it's almost impossible to recover 100% of the helium you use some is always needed to make up the volume. Because our initial recovery is about 50% we must make up this helium by buying more pure gas in bullpacks. Initially we'll need 2 pure helium bull packs per week.
What we need to figure out is how to balance our liquefaction operations which will a) optimize recovery capacity, b) meet our LHe needs and c) operate all the equipment in an efficient manner. Pumps and compressors operate better and longer if not cycled into and out of operation. As well, cooling and warming of the system should be minimized.
The Cost/Benefit analysis is pretty easy. What is your annual cost of helium? Three magnets of average capacity is probably around 750L/year maximum plus 25% for filling losses and I'll use $10/L for helium for simplicity, so $9375/year on helium. If you have no recovery this is a fixed cost every year.
For three average magnets your boil-off rate would average about 2 L of liquid a day and I'll make your refill rate every 6 weeks. So you'll need 86L + 25% for filling your magnets every 6 weeks, 108L.
The smallest liquefier Linde has does 8L per hour with a price tag around $750,000 for everything (liquefier, recovery compressors, purifiers and recovery piping). The equipment way oversized and would be sitting idle most of the time, allowing bearings and seals to dry and crack. You would save $9375/year if you could achieve 100% recovery which is about $3000 more than what the annual maintenance is on the equipment. So large recovery and liquefaction equipment is not practical.
There are however some very cool (pardon the pun) units by Cryomech
http://www.cryomech.com/index.htm These units recover and condense helium gas directly into an attached dewar. I believe they run about $300,000. A local physicist uses them for his Dilution Fridges. What is important to know is that this system does not have any purification capacity at all. Given the extent of our recovery line, the number of applications and the number of users involved there is a higher probability of contamination in the line so a purification step is absolutely necessary. In smaller laboratories these smaller units may be effective if you can ensure that the recovery system is clean. You also must make sure you have the room to truck the whole system around to fill all the magnets.
Even though the Cryomech system is less expensive say $350,000 including recovery piping. It would take over 37 years to recover the cost with a $9375/year helium budget.
There's definitely a cost and/or usage tipping point that makes the initial capital investment worthwhile. And remember if you are thinking about the larger systems, there is also staff, space, maintenance, and administration to consider.
Deryck
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Deryck Webb - Business and Technical Manager
NANUC - Canada's National High Field NMR Centre
Room 103 Email: deryck_at_nanuc.ca
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University of Alberta Fax: (780) 492-9174
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Received on Wed Dec 01 2010 - 09:14:21 MST