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Wet scrubbers circulate liquid at a liquid-to-gas ratio of 0.7-2.7 litres per cubic metre for packed towers and up to 4 for venturi units. Most of it recirculates; make-up replaces evaporation and blowdown. Blowdown carries the absorbed pollutants and is usually the effluent stream that needs treatment and consent.
Circulation rate
Circulation (L/h) = gas flow (m3/h) x liquid to gas ratio
Make-up and blowdown
Make-up = circulation x (1 - recycle fraction)
Scrubber blowdown is an effluent requiring treatment and discharge consent. Its composition depends on the pollutants absorbed and may be classified as hazardous.
It determines the interfacial area and the driving force for absorption. Too low and the gas is not scrubbed; too high and pumping energy and effluent volume rise without further removal.
Dissolved solids concentration. Beyond saturation, scaling blocks packing and nozzles, and pollutant partial pressure in the liquid rises enough to reduce absorption driving force.