A dephlegmator is a device arranged for the partial condensation of a multicomponent vapor stream. The vapor stream flows vertically upwards and the condensate (condensed vapor) runs back down under the influence of gravity. The vapor stream and condensate thus move countercurrently and are in direct contact with each other. In addition to heat transfer between the vapor stream and cooling medium, mass is transferred between the rising vapor and falling condensate. Vapor leaving the device has become concentrated in the more volatile components, while the condensate is richer in the less volatile components. In the industrial rather than laboratory contexts, many writers use the word 'dephlegmator,' only if the device being described is a main unit in the process plant.
If an overhead condenser is used in continuous countercurrent distillation in an industrial setting, it is often designed to receive a multicomponent vapor which flows vertically upwards and is condensed only partially. The condensate forms a reflux of liquid to the distillation column and the remaining vapor has become enriched in the more volatile components. However, the overhead condenser is not a key process unit, but is merely part of a distillation column. Many writers do not, therefore, consider the rather uncommon word 'dephlegmator' appropriate to this subordinate and commonly-encountered device. In its stead, they use 'backward-return condenser' or less informatively, 'partial condenser' or 'reflux condenser.'
In contrast, a particular instance where the importance of the dephlegmator as a heat and mass transfer device is clear (the word is thus used without dispute) is in the separation and recovery of ethene from a cracked gas feed, which contains a significant proportion by volume of light components (hydrogen, carbon monoxide and methane). The heavy component of the feed, ethene, is separated from the light components in a dephlegmator, and the ethene-rich condensate is passed on to a distillation column to remove any remaining methane. An earlier method of recovering ethene involved fractionation of the entire feed stream in a conventional adiabatic distillation column. The introduction of the dephlegmator, constituting a preseparation stage, resulted in vast savings in energy costs. A substantial part of the heat removed in the dephlegmator is transferred to the coolant at a higher temperature than was possible when the fractionation was done entirely in the conventional column. (See also Condensers; Distillation.)