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SCRUBBERS

DOI: 10.1615/AtoZ.s.scrubbers

A scrubber is a heat and mass transfer apparatus which implements a direct gas-liquid contact. It is used for gas cooling and, more frequently, for gas cleaning where both suspended particles and other gases are removed from the gas to be cleaned.

Types of scrubber include hollow, packed, centrifugal, and Venturi scrubbers. In the hollow scrubber, the liquid is sprayed as droplets up to a few millimeters in size into a vessel containing the gas, which is to be contacted with the liquid. Liquid dispersion occurs in atomizers arranged on collectors mounted at the top of the vessel. The interacting media are usually in vertical counterflow. Scrubbers are also encountered in which media are in crossflow when the gas flows horizontally and spraying is performed from above. These scrubbers are of rectangular cross section. To avoid droplet carry-over the gas velocity in a hollow scrubber is in the range 1 to 2.5 m/s. If the velocity rises to 5-8 m/s drop pans are mounted at the top of the scrubber. The hollow scrubber affords up to 60-75% dust suppresion, the cleaning coefficient of the main flow from gas impurities attains approximately the same value. The pressure loss in the hollow scrubber is 0.1 to 0.2 kPa.

Packing the scrubber space with cylinders, Rashig rings, or lumps of insoluble matter increases the efficiency of dust suppression and gas absorption up to 85%, but the pressure loss increases by typically 1.5 to 2 times.

Centrifugal scrubbers that use the effect of centrifugal force are an efficient dust collector. They use tangential admission of the dust-laden gas at a velocities up to 25 m/s the dust being captured in a liquid film falling on the walls. The dust-suppresion efficiency attains 90% even for a fine dust with particle diameter Dp = 2 -5 μm.

Venturi scrubbers have received wide recognition in industry. The main component of a Venturi scrubber is a Venturi tube with liquid sprayed into the flow upstream of the tube throat. Typical drop sizes in such units are around 10 mm. Active dust absorption by the liquid droplets occurs as the gas accelerates relative to the liquid in the convergent section of the venturi. A cyclone mounted behind the Venturi tube is used for droplet trapping. Venturi scrubbers enable cleaning of gases containing an extremely fine dust with particle size of the order of 0.1 to 1 μm, such as sublimate and mist, with an efficiency up to 99%. Their pressure loss may exceed that of the hollow scrubber by two orders of magnitude and reach 10 kPa.

The specific spraying rate in all types of scrubbers varies within wide limits, typically from 0.2 to 10 liters of liquid per cubic meter of gas.

REFERENCES

Perry, R. H., Green, D. W., and Maloney, J. O. Eds. Perry's Chemical Engineers Handbook, 1984. McGraw-Hill, New York.

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