Foam-insulated containers are adequate for some slush baths, but will require more effort to maintain the coolant. Foam-insulated containers should never be used for longor short-term storage of cryogenic liquids. They may be used for cryogenic liquid transport, but there will be significantly more loss of the liquid coolant (even in limited transport) than from a regular Dewar.

Beakers and Flasks. Beakers and flasks are the least effective containers for cryogenic materials because there is no insulation whatsoever. However, if the coolant is only water, ice, or a salt/ice mixture, not much insulation is required. There is little concern for rapid material loss with these coolant solutions because they are easy and inexpensive to replace. In addition, it is (usually) safe to pick up these containers with your bare hands. If any ice forms on the sides of a container, it is simple to use gloves or tongs to pick up the beaker or flask and prevent possible skin damage.

6.2.10 Liquid (Cryogenic) Gas Tanks

Nitrogen, argon, and oxygen can be stored in liquid form in cryogenic gas tanks. As can be seen at the left of Fig. 6.6, they are large containers (about four and one-half feet tall and 20 inches in diameter). These tanks are in fact highly reinforced double-walled Dewars and can maintain the various cryogenic gases in liquid states (at room temperature) with a minimum of bleed-off.

Liquid nitrogen is used almost exclusively as a coolant in the lab. Liquid argon and oxygen are not used as coolants, but they may be used in the lab or in industry when large quantities of these gases are required. The liquid form of the gas occupies much less space than the equivalent quantity of compressed gas. In addition, less time is lost in changing the equivalent number of tanks that would otherwise be required.

There are two major types of cryogenic tanks: one used primarily for liquid dispensing (see Fig. 6.6 and Fig. 6.7) and another used primarily for gas dispensing (see Fig. 6.6 and Fig. 6.10). Both are similar in size, both have rings of sheet metal around their tops to protect their valves from impact, and both have float devices on the top that indicate approximate liquid volume.

Both of the cryogenic tank designs have a tare (net) weight of =230-250 lb, depending on design and manufacturer. When filled with a gas, the tank's weight is considerably greater: With nitrogen its weight is >300 lb, with oxygen its weight is >400 lb, and with argon its weight is >500 lb.12 Unless you have the proper training and equipment, never attempt to move a cryogenic gas tank by yourself. Should one of these tanks be tipped over and rupture, the potential damages and injuries could be extensive.

The Liquid Dispensing Tank. As opposed to high-pressure tanks filled with a highly compressed gas, cryogenic gas tanks hold the liquid form of a gas and are insulated to maintain the cryogenic temperatures necessary to maintain the gas in its liquid state.

Gas port

Liquid port

TOP VIEW

(enlarged from the

Fig. 6.7 The liquid (cryogenic) gas tank for liquid dispensing.

tags are broken off, you have three ways of finding out which of the two ports will deliver gas or liquid, respectively;

1.Call the supplier of the tanks and ask which valve delivers gas and which delivers liquid.

2.See if there are any tanks elsewhere in your building of that type and/or design. Because it is likely that any tanks from the same supplier will be of the same type, it is a safe guess that the valve setup will be similar.

3.Open and close the valve quickly several times to see what comes out. Be sure that the area in front of the valve is free from materials that could be damaged by cryogenic temperatures. The first thing out of the liquid port will most likely be gas because it may take a moment or two for the liquid to arrive. Be sure you have adequate ventilation before doing this activity. Nitrogen and argon can displace air, leaving the user in an oxygen-poor environment. Do not use this technique with oxygen if there are any flames or sparks in the area.

The port in the middle (which may have a small metal clip hanging from it saying "Liquid") dispenses the liquid form of the gas within. To dispense the liquid into a Dewar, first attach a dispensing tube, which is available from your gas supplier. Place the opening of your Dewar over the end of the tube, and then slowly open the valve (see Fig. 6.8). Hissing and gurgling indicate that the liquid gas is pouring out the end of the tube. Always point the open end of the Dewar away from your face and other people when filling so that any splattering cryogenic liq-

uid will not fly into your face. You should wear safety glasses during this and any other operations with cryogenic fluids.

Some labs may have a separate room where liquid nitrogen tanks are stored. [Important: If you have such a room, be sure to leave the door open whenever transferring nitrogen into Dewars. The expansion of the nitrogen can easily leave a closed room oxygen-poor!] This tank setup might service one lab, a whole floor, or an entire building. To transport liquid nitrogen from a storage container to a lab and provide short-term (one day) storage for the lab, there are various designs of transport Dewars (see Fig. 6.9). Transport Dewars are made of stainless steel, copper, plastic, or fiberglass. They are double-walled containers that are evacuated or foam-filled for insulation. The necks of transport Dewars should always be small compared to the size of their bodies. This design helps prevent splashing while transporting and facilitates pouring cryogenic fluids. When used for storage, a Styrofoam ball and pin should be loosely* placed in the neck to limit evaporation.

It is essentially impossible to fill a short or shallow container from a liquid/gas tank because the liquid gas leaves the dispensing tube forcefully (there is limited ability to control the rate of flow), and once the liquid gas hits the bottom of the relatively warm container, it immediately (and forcefully) boils off and out of the container. In a taller container, the liquid gas does not eject itself from the container, but falls back to the bottom. Once back at the bottom, it can cool the container so that newly arriving liquid gas will not boil off. Eventually, incoming liquid gas will collect in a taller Dewar. A short or shallow Dewar can then be filled from the taller Dewar because the liquid can be slowly poured, limiting the amount of resultant ejecting fluids.

The Gas Dispensing Tank. The gas dispensing tank (the body is shown in Fig. 6.6, the top is shown in Fig. 6.10) is designed to provide gas continuously at a delivery pressure between 75 to 175 psig. It can provide a continuous supply of 250-350 cfh (cubic feet per hour) with bursts of up to 1000 cfh. These tanks can

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Attach a liquid dispensing tube before removing liquid

Fig. 6.8 Dispensing liquids from a liquid/gas tank.

If the cap is too snug, the expanding pressure (of the gas evaporating) may cause the cap to be ejected like a rocket or cause the container to explode.

Gas Association) numbering standards (see Sec. 5.1.3) as compressed gases. For oxygen, use a CGA 540 fitting, and for nitrogen or argon, a CGA 580 is required.

To use a gas dispensing cryogenic tank, attach the correct regulator to the gas use outlet. Then, open the gas use valve and pressure building valve. Once the pressure is at least 125 psig, you may then adjust the regulator to the required delivery pressure and dispense the gas as needed. Do not handle any tubing from the tank without some protection because it will be very cold and can severely damage skin.

Cryogenic tanks have limitations that become immediately obvious if gas is withdrawn at a rate faster than the internal coils can maintain: Frost develops on the outlet connections and/or regulator, which decreases gas output. The problem can be resolved with an external heat exchanger. The external heat exchanger is first connected to the gas use valve, and then the regulator is attached to the external heat exchanger.

Other options for gas dispensing cryogenic tanks are manifolds that can connect two to six cylinders together. These manifolds can provide flow rates of 250 cfh (cubic feet per hour), can set up a reserve of gas for uninterrupted flow when changing cylinders, and (with an economizer circuit) can cut loss due to evaporation. For extra-high-capacity gas demands, there are external vaporizing manifolds, which are a combination of the external heat exchanger and manifold setup. n

References

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2.J.P. Hagen and K.L. Barton, "An Inexpensive Laboratory Steam Generator," Journal of Chemical Education, 67, p. 448 (1990).

3.R.Q. Thompson and M. Ghadiali, "Microwave Drying of Precipitates for Gravimetric Analysis," Journal of Chemical Education, 70, pp. 170-171 (1993).

4.D.R. Beltran, E.P. Hervey, and H. Keyser, "Teaching with the Commercial Microwave Oven," J. of Science Teaching, 9, pp. 91-92 (1979).

5.H. Keyser, "Some Uses for the Commercial Microwave Oven in Chemistry," Chemistry, in Australia, 45, p. 44, (1978).

6.A.B. Brown and H. Keyser, "Sample Preparation for Strontium Analysis of Ancient Skeletal Remains," Contributions to Geology, University of Wyoming, 16, pp. 85-87(1978).

7.G.S. Coyne and R. Keys, "The Unintended Melting of Glass in a Microwave Oven,"

Proceedings of the Forty First Symposium on the Art of Scientific Glassblowing, pp. 84-92, 1996.