- Liquid^

nitrogen level

Fig. 7.32 By filling the liquid nitrogen to the top of the Dewar when first cooling the cold trap, the user risks limiting the maximum pressure the system can achieve. However, by keeping the level low in the beginning, the user can keep any frozen material locked up.

because the liquid nitrogen boils off initially very quickly, so there is a reaction to overfill. This overfilling can limit the potential vacuum of a vacuum system and/or create unaccountable pressure blips.

The problem originates because the liquid nitrogen high in the Dewar freezes moisture in the upper regions of the cold trap (see Fig. 7.32). Later, as the liquid nitrogen boils off, the moisture at the top of the trap evaporates (because nothing is keeping it frozen) and the pressure in the system rises, thus becoming a virtual leak (see Sec. 7.6 on Leak Detection), which is very difficult to find because no one thinks to look for a leak within a cold trap.

Fortunately, it is relatively easy to limit the degree of this problem by maintaining a low liquid nitrogen level in the Dewar when starting your system. Later, by maintaining a high liquid nitrogen level, any moisture that is trapped in early on will stay frozen. Unfortunately, that does not allow later collection of liquids to enjoy the same protection. The best defense for this problem is to maintain a high coolant level throughout cold trap use.

This virtually leaking cold trap problem is more likely to occur when first starting a vacuum system, or when a system has been down for an extended time because there is more moisture is in the system. The problem is more difficult to control when you are constantly cycling from atmospheric pressure to a vacuum state because moisture is constantly being brought into the system and the greater amount of liquid nitrogen being boiled off. The problem will always exist if your work creates copious amounts of condensable vapors stopped within the cold trap.

7.4.4 Maintenance of Cold Traps

There are two types of cold trap maintenance: (1) maintaining the liquid nitrogen (or the coolant of choice) at a proper level while the trap is in use and (2) maintaining clean traps to ensure the best throughput.

Maintenance of Liquid Nitrogen. A variety of automatic filling devices have been designed to maintain the liquid nitrogen level within a Dewar. Many of these

devices are available commercially, and many others are sufficiently simple for lab construction. If you use an automatic filling device, try to limit the length of the filling lines.* In lieu of mechanical or electrical automatic filling devices, periodic inspection and manual refilling is always available.

Perhaps more important is limiting the amount of liquid nitrogen lost by boiling off. First, use a good-quality Dewar that is larger in radius than the cold trap by about 2 cm on all sides. All Dewars should be wrapped with tapef to prevent flying glass in case of implosion. If you ever notice frost on the outside of a Dewar, it is likely that the protective vacuum within the Dewar is gone and the void has filled with atmosphere. This occurrence is not uncommon and is caused (typically) by a poor-quality tip-off of the Dewar when it was evacuated.

Additional liquid nitrogen protection can be obtained by placing some insulation on top of a Dewar, which otherwise is exposed to the atmosphere. This insulation can be as simple (yet effective) as cardboard placed over the top of the Dewar, leaving a cutout for the cold trap. Styrofoam, a more efficient insulation material, can also be cut out and placed on the top of the Dewar [see Fig. 7.33(a)]. Alternative approaches include cutting Styrofoam into a cork shape [see Fig. 7.33(b)], or sprinkling crushed-up Styrofoam on top of the liquid nitrogen [see Fig. 7.33(c)]. Each of these approaches has good and bad points: Approach

(a) can be knocked off easily and provides the least amount of insulation. However, it is easy to make and allows for easy examination of the liquid nitrogen level. Approach (b) provides excellent insulation abilities, but is not as easy to

Styrofoam Jl

chips. ^ L L

(a) (b) (c)

Fig. 7.33 Various methods of covering Dewars using Styrofoam. Be sure that whichever approach is used, you allow a route for gases to escape.

*If the filling lines are overly long, any warmer gas (already in the line) may warm the Dewar somewhat before the liquid nitrogen arrives and may cause small pressure blips.

able, difference in how long liquid nitrogen would remain in a Dewar as opposed to no tape or black electrical tape.

make and is more difficult to put into place. It is also more difficult to examine the liquid nitrogen level. Approach (c) is the easiest to make (just crush up some Styrofoam), but can be messy and locating the level of liquid nitrogen can be confusing or difficult. In a high-humidity environment, water vapor can freeze over the crushed-up Styrofoam, making refilling the Dewar with liquid nitrogen very difficult. Regardless of which approach you use, there must be a route for built-up gases to escape, otherwise the plug could be blown out with some force.

Maintain a Clean Trap. During vacuum operation, a trap may become sufficiently filled that emptying the trap is necessary. To empty a trap the lower section must be removed from the system, and to remove the lower section the trap must be vented to the atmosphere. If the trap is not vented, separating the lower removable part of the trap from the upper section could be like separating the Magdeburg hemispheres (see the historical review in Sec. 7.2.3). If brawn is greater than brain, one could damage your system.* Unless there is a reason to vent the entire system, do not. When a system is exposed to the atmosphere, moisture can resaturate the walls of the system, and gases can be resorbed into the vacuum liquids (diffusion pump oils and/or mercury). Extra (wasted) time will be required to return the system to its original vacuum condition. In addition, if your diffusion pump uses hydrocarbon oils, air can destroy the diffusion pump oils (if the oil is hot when the air makes contact) and the oils will then have to be replaced. All this wasted time and money can simply be avoided when first constructing vacuum system with a venting stopcock on the trap side of the system to vent the trap to atmospheric pressure while leaving the rest of the line in a vacuum (see Fig. 7.34).

Incidentally, if your work creates a constant buildup of material in the cold traps, have extra trap bottoms available. This preparation allows you to remove a filled trap bottom and transfer it to a fume hood while immediately replacing another on your vacuum line, thus significantly cutting down the amount of "downtime" on your system.

Following Fig. 7.34, if you need to temporarily remove the base of a trap while a system is in use:

1.Close Stopcocks 1 and 3.

2.Open Stopcock 2.

3.Remove base of cold trap.

4.Replace base of cold trap.

5.Close Stopcock 2.

6.Open Stopcock 3.

7.When the pump quiets down, replace Dewar under cold trap.

8.Open Stopcock 1.

It can facilitate separation to use O-rings on traps rather than standard taper joints because they are always easy to separate in either cold temperature or room temperature.

From system

Fig. 7.34 Using valves or stopcocks to separate the various parts of your system allows you to open sections of your system while maintaining a vacuum in the rest of the system.

Removing the base of a cold trap while it has been in liquid nitrogen can be difficult, especially if the trap uses standard taper joints. The cold temperatures can make (even fresh) stopcock grease sluggish and firm. The easiest way to separate the base from a trap is to let the trap come to room temperature. The joint may also be easier to separate if you aim a hot air gun around the entire circumference of the joint (do not aim the hot air gun at one spot and expect it to heat the other side). If you need to use a hot air gun to soften the stopcock grease, be sure the stopcocks to the system and the mechanical pump are closed to prevent the trapped materials from being re-released to where they shouldn't get into. If you expect to remove a frozen trap often, consider using O-ring joints which are easy to separate (once vented) regardless of temperature.

When shutting down a vacuum system, close off your system from the trap section. That way, as trapped compounds warm up and go into a vapor state, they will not be able to drift into the rest of the vacuum line. You should also vent your pump to the atmosphere. Many pumps do not have adequate check valves near their oil reservoirs. If they are shut off with a vacuum on the vacuum side, the mechanical pump oil can be drawn up into the system. So, to shut down a vacuum system (see Fig. 7.34), it is recommended that you:

1.Close Stopcock 1.

2.Remove coolant (i.e., liquid nitrogen).

3.Turn off Pump.

4.Open Stopcock 2.

7.4.5Separation Traps

All of the traps mentioned so far protect the vacuum line, pumps, pump liquids, and/or the people using the system. There can be other traps on vacuum systems whose function is not for protection, but rather as tools for chemistry. Separation traps fall into this category and can separate a mixed compound into different fractions by using the appropriate freezing temperatures.*