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5

Wet and Dry Scrubbing

Lawrence K. Wang, Jerry R. Taricska, Yung-Tse Hung,
James E. Eldridge, and Kathleen Hung Li

CONTENTS
INTRODUCTION
WET SCRUBBERS
DRY SCRUBBERS
PRACTICAL EXAMPLES
NOMENCLATURE
REFERENCES
APPENDIX

1. INTRODUCTION
1.1. General Process Descriptions

The scrubbing process is a unit operation in which one or more components of a gas
stream are selectively absorbed into an absorbent. The term “scrubbing” is used inter-
changeably with “absorption” when describing this process. In wet scrubbing, water is
the most common choice of absorbent liquor. In special cases, another relatively non-
volatile liquid may be used as the absorbent. In dry scrubbing, a dry powder or semidry
slurry are also possible absorbents, depending on the requirements of a given situation.
Scrubbing is commonly encountered when treating flue gas (or some other polluted
gas stream) to control acid gases, particulates, heavy metals, trace organics, and odors.
Often, a scrubbing system is composed of two or more scrubbers in series. This is done
so that an individual scrubber stage can utilize an absorbent specific to a targeted pollu-
tant or pollutants. Higher total removal efficiencies are often possible in a multistage
scrubber system than would otherwise be possible with a single-stage scrubber. An
example of this is commonly found in the rendering industry, where both ammonia and
hydrogen sulfide are produced during normal operations. A scrubber using an acid-based
absorbent liquid is used to remove ammonia from the air. The hydrogen sulfide is then
scrubbed using a caustic solution, sometimes with an oxidizing agent added to the liquid.
In this example, physical or chemical absorption (or both) could occur in the scrubbing
process. If a pollutant is simply trapped (e.g., particulates impinging on water) or dissolved,
then it is a physical absorption process. If the pollutant being absorbed also undergoes a

From: Handbook of Environmental Engineering, Volume 1: Air Pollution Control Engineering
Edited by: L. K. Wang, N. C. Pereira, and Y.-T. Hung © The Humana Press, Inc., Totowa, NJ
197