Cold trap
A cold trap is a device with which condensable gas components can be separated from a gas flow. It is the simplest condensation pump . Their purpose is, for example, to protect a downstream vacuum pump (liquid hammer); but it can also be used to reuse solvents and reduce emissions to the atmosphere.
Cold traps are used in the chemical industry to separate solvents from gas flows . The gas is cooled in a heat exchanger so that certain gas components condense and are separated according to the partial pressures. A droplet separator is used at the outlet of the heat exchanger in order to achieve effective separation. To reduce the cooling capacity, a gas-gas heat exchanger is usually used to precool the warm gas flowing into the cold trap by transferring heat from the gas flowing out. The condensate is discharged automatically via a level control. In negative pressure systems, either a lock control can be used or the condensate is pumped out.
Cold traps used in laboratories are made of glass or other cold-resistant material and are similar in structure to a wash bottle . The bottle is cooled to condense condensable gas components. The cold trap is connected between the vacuum pump and the vacuum apparatus and immersed in a Dewar vessel, which is filled with a cooling medium such as. B. liquid nitrogen or a cold mixture of dry ice / acetone , the temperature of which should be lower than the boiling point of the vapors / gases to be condensed at pump pressure (partial vacuum). If the pressure in the vacuum apparatus is reduced, the solvents present evaporate and are then sucked through the cold trap as vapor or gas (suction direction: see picture). The vapors condense or freeze there because of the low temperature of the cooling medium, e.g. B. −196 ° C (= 77 Kelvin) when using liquid nitrogen on the vessel wall. This prevents corrosive or flammable gases from entering the pump, which could damage it or lead to undesired secondary reactions (fire or possibly even explosion at the pump outlet).
In the opposite direction, cold traps are used to prevent the diffusion of the propellant from oil diffusion pumps into the recipient .
Since the cold trap fills with frozen solvents over time, it must be regularly replaced or emptied, otherwise the path from the pump to the apparatus is blocked and the vacuum is only maintained between the cold trap and the pump. In addition, the dewar must always be sufficiently filled with cooling medium (e.g. liquid nitrogen), otherwise the cold trap will thaw again and the substances that have already been collected will be sucked into the pump.
If it is necessary to work without oxygen or water ( Schlenk technology ), a cold trap is also suitable for collecting solvents that have been evaporated by a vacuum pump. It is advisable to install this additional cold trap in the opposite direction to prevent the pipe from freezing over quickly.
In vacuum technology , cold traps filled with liquid nitrogen are used to further improve a good vacuum. Residual gases condense on the cold trap and the pressure in the recipient also drops. Such cold traps usually consist of a hollow, metallic 'finger' that protrudes into the recipient.
Cold trap designs
- Simple cold trap made of glass , with 2 standard ground joint sleeves and 1 core NS 29/32, coolant container approx. 500 ml
- roughly as before, but with spherical joints (one ball, one sleeve)
- Cold trap complete with silver mirrored vacuum jacket , drain valve , 2 threads GL 18, caps GL 18 with olives, coolant content approx. 750 ml
- as above, but 2 GL 25
- As above, but 2 GL 32, content approx. 1,000 ml
Security notice
A cold trap operated with liquid nitrogen (boiling point: −196 ° C = 77K) is able to condense larger amounts of oxygen (boiling point: −183 ° C = 90K) from the air. In its liquid state, this is strongly oxidizing and, together with the solvents condensed in cold traps, highly explosive mixtures can arise. The oxygen can also suddenly evaporate, which can lead to the apparatus bursting. It is therefore to be avoided to constantly suck air through a cold trap with the vacuum pump or to leave a cold trap cooled with liquid nitrogen (or colder cooling media) open to the air even when the pump is switched off.
Individual evidence
- ^ Dieter Meschede: Gerthsen Physics . Springer DE, 2010, ISBN 3-642-12894-7 , pp. 133 ( limited preview in Google Book search).
- ^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 752.