Distillation and Reflux heating are common laboratory operations. Distillation becomes necessary when you have to isolate a pure solvent from a mixture of several other solvents based on the differences in their boiling points. The types of commonly used laboratory distillations have been covered in How to select the optimum laboratory distillation technique. In contrast to distillation the process of reflux in involves heating of the reactants in a flask and through condensation the return of condensed liquid back to the heating flask. This operation is useful in preventing loss of solvent thereby increasing the reaction time over which the flask can be heated.
Distillation in comparison to refluxing is a simpler operation and a simple condenser serves the required purpose. Liebig condenser comprises of two concentric tubes which are sealed at both ends. The vapour from the flask moves along the inner tube and condenses due to circulation of water in the outer jacket. The condensed liquid is collected in another container.
Refluxing on the other hand requires a condenser capable of providing a large surface area of cold surface to help condense the escaping vapour and a restriction free passage for its return to the reaction vessel. This has been achieved through several condenser designs which have been developed over the years. In this article some common condenser designs are discussed and their characteristic features are outlined.
Types of Reflux Condensers
Basically condensers fall into two categories, namely, Graham type and coil type. In Graham type of condenser vapour flows through the central tube and condenses along its walls to flow back into the reaction flask. In the coil condenser water as a coolant is passed through the central coil and the vapour moves along the larger outside jacket. Graham type condenser suffers from a major limitation of frequent clogging as the condensed liquid has to return to the reaction vessel through a narrow restricted path.
The Allihn condenser is also known as a ‘bulb condenser’. It consists of an outer water glass jacket. The central tube comprises of a series of bulbs that provide increased surface area for condensation of the vapour. It is an improvement over Graham condenser as the condensed liquid flows down along the bulb walls thereby avoiding blockage to the rising vapour. It also provides a wider bore at the bottom so it is useful for reflux heating when mounted vertically.
Davies is functionally similar to the Liebig condenser but it comprises of three concentric glass tubes. The coolant circulates both in the outer jacket as well as the central tube. The vapour condenses on the inner tubes and flows down into the reflux flask. The condenser is suitable for condensation of vapours over a broad range of boiling points including low boiling volatile liquids.
Jacketed Coil Condenser
It is similar to Davies but instead of a straight inner tube it is a coiled tube. This provides improved performance over the Davies condenser due to increased surface area to facilitate condensation.
A Dimroth condenser has double internal spirals. The coolant inlet and outlet are both at the top. Vapour travels from bottom to top. Such configuration is even more effective than traditional coil condensers.
Fredrichs condensers consists of a spiral internal cold – finger type capillary within a cylindrical housing. Coolant is made to flow through the internal cold finger and vapours rise along the spiral path. Such condensers are suitable for both distillations and reflux reactions.
Now the question arises that with the availability of such a wide range of condensers which is the best option. Your choice should be based on your requirements and you would obviously like a good compromise between performance and price. It is a good decision to buy a condenser that will work both for distillation and reflux reactions.
The Friedrich type proves useful for challenging distillation applications and can be used in a sequence with the Liebig condenser.
Reflux applications are mostly covered with Allihn but Dimroth and double surface coil condensers can prove beneficial for efficient recovery of large quantities of generated vapours and maintain a constant solvent composition in the reflux flask.