Chlorine scrubbers fall into two categories. Process scrubbers handle a continuous vent stream. Emergency scrubbers sit idle and must absorb the full contents of a chlorine vessel on demand, without warning, at full rate.

For emergency duty the design case is not steady-state efficiency but transient capacity: the column has to handle a large slug of gas from cold, with the caustic circulation starting from rest. That drives conservative sizing and a bias towards simple, robust internals.