Condensing as Part of the Separation

Everywhere else on this site, the condenser's job is to turn vapor into liquid. Here it also decides what ends up in which product. A total condenser liquefies the entire overhead and hands reflux control to a drum and a pump. A partial condenser liquefies part of the overhead and takes a vapor product as well, which buys you a separation stage but couples the condenser directly into the column's mass balance. A dephlegmator sits on top of the column and generates reflux internally, with no drum and no reflux pump at all. Those are three genuinely different process decisions with different control characteristics, and the right one depends on what the column is being asked to do.

• Total condensers on distillation and fractionation overheads, sized for the full range of vapor composition and pressure the column will actually see across its operating envelope.
• Partial condensers where a vapor product is drawn alongside liquid reflux, rated over the whole condensing curve rather than at a single bubble-point assumption.

• Reflux and knock-back condensers on reactors, batch stills and solvent recovery systems, returning condensate to the vessel while letting light ends and inerts pass to vent treatment.
• Dephlegmators mounted directly on the column for internal reflux generation, designed for countercurrent vapor and condensate flow with adequate margin against flooding.

• Multicomponent and wide-boiling mixtures where the condensing curve is strongly non-linear and a single log mean temperature difference calculation will mislead you.
• Vacuum overhead service, where allowable vapor-side pressure drop is often the binding constraint on the design and drives orientation, tube count and nozzle sizing.