bers is the gas-pressure drop, a measure of the energy applied to atomize scrubbing liquid and create fine droplets for particle impaction. For injector venturi scrubbers, the corresponding criterion is liquid-nozzle pressure drop. Other important design and operating characteristics are the liquid-to-gas ratio, inlet gas temperature (to avoid scrubber-liquid evaporation), solid content of recirculated scrubber liquid, mist eliminator efficiency, materials of construction to avoid corrosion and erosion, particulate loading, and particulate-size distribution. In a properly designed unit, the most-important monitoring and process control measures are pressure drops, liquid and gas flow rates, and liquid blowdown rate (blowdown is used to control solids buildup).

A few designs use steam injection or scrubber-liquid subcooling to enhance flux force and condensation. For those designs, steam-nozzle pressure and scrubber-liquid temperature are additional useful monitoring measures.

Acid Gas Scrubbers

A commonly used APCD for removal of acid gases is a packed-bed absorber. A scrubbing liquid is trickled through a matrix of random or structured packings through which the gas is simultaneously passed, resulting in gas-liquid contact over a relatively large surface area. The scrubbing liquid can be water or an alkaline solution, which reacts with the acid-gas constituents to form neutral salts. The wastewater discharge from the packed-bed absorber is a salt-water brine that must be managed properly. This effluent may contain unreacted acids, trace organics, metals, and other solids removed from the gas stream.

Packed bed absorbers have been used for decades in the United States, primarily in hazardous-waste and medical-waste incineration applications. They have been used in Europe for municipal solid-waste applications. The European installations include duel-stage wet absorbers, in which the first stage is operated with an acidic scrubber liquid and the second stage is operated with an alkaline scrubber liquid. Acid gases, such as HCl, that are highly water soluble are largely collected in the first stage. Acid gases, such as SO2, that are not very water soluble are effectively collected in the second, alkaline stage.

The important design and operating criteria for wet acid-gas absorbers are gas velocity, liquid-to-gas ratio, packing mass transfer characteristics, pH of the scrubbing liquid, and materials of construction (to prevent corrosion).

In recent years, municipal solid-waste and a few larger hazardous-waste and medical-waste incineration facilities have used spray-dryer scrubbers for acid-gas control. The spray dryers use slurries of lime, sodium carbonate, or sodium bicarbonate as the alkaline reagent. The water in the atomized slurry droplets evaporates, cooling the gas, and the alkali particles react with the acid-gas constituents to form dry salts. The salts and unreacted alkali must be captured in a downstream fabric filter or electrostatic precipitator. Dry-injection scrubbers, which use an alkaline reagent without water, have also been used in recent years,

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