Closed-loop scrubber

A closed-loop scrubber recirculates a freshwater wash dosed with sodium hydroxide to neutralize SO2, with little or no overboard discharge and onboard handling of the bleed-off and residue to a holding tank. It works in zero-discharge zones where open-loop is banned, at the cost of caustic soda consumption, larger tankage, and process complexity. Hybrid systems combine both modes, running open-loop at sea and closing the loop in ports and ECAs with discharge restrictions, which has become the preferred specification for new scrubber installations.