Membrane operations, for instance, membrane reactors, membrane gas separators, membrane contactors, membrane emulsifiers, membrane crystallizers, reverse osmosis, nanofiltration, ultrafiltration, microfiltration, etc. as well the more conventional unit operations, like reactors, distillation columns, adsorbers, absorbers, liquid-liquid extractors, filters, pipelines, etc., are fed by a stream entering the unit. The pressure of this stream (to be treated by the unit operation) at the entering section of each unit operation is defined as feed pressure.
A further specification is required for membrane systems. Often, an auxiliary stream named “sweep” is fed to the membrane unit. This stream is characterized by a different pressure named “sweep pressure”; this pressure cannot be referred to as “feed pressure.”