Cold trap
A cold trap is a device with which condensable gas components can be separated from a gas stream. Their purpose is for example the protection of a downstream vacuum pump ( water hammer ); but it can also be used to re- use the solvent and to reduce emissions into the atmosphere.
Cold traps are used in the chemical industry to separate gas flows out of the solvent. In a heat exchanger the gas is cooled so as to condense some components in accordance with the partial pressures of the gas and are separated. At the outlet of the heat exchanger, a droplet separator is used to achieve an effective separation. To reduce the cooling capacity, a gas -gas heat exchanger is usually used to pre-cool the air flowing into the cold trap hot gas by heat transfer from the flowing gas. The outward transfer of the condensate is done automatically by a level control. In vacuum systems can be used either a lock or control the condensate is pumped out.
Cold traps used in laboratories consist of glass or other cold-resistant materials and are similar in construction to a wash bottle. The flask is cooled in order to condense condensable gas components. The cold trap is connected between the vacuum pump and the vacuum system, and immersed in a Dewar vessel, which is connected to a cooling medium such as liquid nitrogen or a freezing mixture of dry ice / acetone is filled, the temperature should be lower than the boiling point of to be condensed vapors / gases ( partial vacuum ) will be the pump pressure. If the pressure in the vacuum apparatus is reduced, any existing solvent and then evaporate as steam or gas is sucked through the cold trap ( suction direction: see picture). The vapors condense and freeze there on account of the low temperature of the cooling medium, for example, 196 ° C (= 77 Kelvin) with the use of liquid nitrogen to the vessel wall. This prevents corrosive or flammable gases entering the pump, which could cause this damage or undesired secondary reaction (fire or explosion possibly even at the pump outlet ).
In the reverse direction cold traps are used to prevent the diffusion of the blowing agent of oil diffusion pump into the container.
Since fills the cold trap in the long run with frozen- solvents, it must be regularly replaced or emptied, otherwise the path from the pump to the apparatus is closed and the vacuum is only maintained between cold trap and pump. In addition, the Dewar vessel with sufficient cooling medium ( eg liquid nitrogen) must always be filled, otherwise the cold trap thaws again and have already collected materials are sucked into the pump.
Before operating sauerstoff-/wasserfrei ( Schlenk technique), a cold trap is also suitable for collecting solvents were evaporated by a vacuum pump. It is advisable to incorporate this additional cold trap in the reverse direction to prevent rapid freezing of the pipe.
In vacuum technology, filled with liquid nitrogen cold traps are used to improve a good vacuum even further. Residual gases condense on the cold trap and the pressure in the recipient is reduced by addition. Such cold traps usually consist of a hollow metallic ' fingers ' extending into the recipient.
Cold trap designs
- Simple cold trap made of glass, NS with 2 standard ground sleeves and 1 core 29 /32, coolant vessel about 500 ml
- About as before, but with spherical ground joints ( a ball, a sleeve )
- Cold trap complete with silver coated vacuum jacket, drain valve, 2 thread GL 18, GL 18 caps with olive, coolant capacity approx 750 ml
- As above, but 2 GL 25
- As above, but 2 GL 32, capacity 1,000 ml
Safety note
A liquid nitrogen (boiling point: -196 ° C = 77 K ) operated cold trap is capable of greater amounts of oxygen (boiling point: -183 ° C = 90K ) to be condensed from the air. This is highly oxidizing in the liquid state, and it can, together with the condensed in cold traps solvents highly explosive mixtures arise. It can also come to the sudden evaporation of oxygen, which may result in breakage of the apparatus. It is to be avoided, therefore, to constantly suck with the vacuum pump air through a cold trap or a liquid nitrogen (or colder cooling media ) cooled trap himself to leave open to the air pump shut off.