Canadian Air Systems Co. is a designer and manufacturer of industrial air scrubbing systems including wet dust / fume collectors. Air scrubbing equipment utilizing the principle of wet dust collecting is available in numerous designs utilizing a number of principles and featuring wide variation in effectiveness, initial cost, operating and maintenance expense, space, arrangements and material of construction.

Many industrial processes produce large quantities of gaseous or vaporized contaminants that must be separated from gas stream. Removal of these contaminants is usually achieved through absorption into a liquid or adsorption onto a solid medium. Incineration of the exhaust gas has also been successfully used for the removal of organic gases and vapors. Spray dry air scrubbers are used to absorb and neutralize acidic gaseous contaminants in hot industrial gas streams. They use an alkali spray to react with the acid gases to form a salt. The process heat evaporates the liquid, resulting in a dry particulate that is removed from the gas stream.

Typical industrial applications for spray dry scrubbers are:

* Control of hydrochloric acid (HCl) emissions from biological hazardous waste incinerators.
* Control of sulfuric acid and sulfur trioxide emissions from burning high-sulfur coal.
* Control of sulfur oxides, boric acid, and hydrogene fluoride (HF) gases from glass-melting furnaces.

Spray gas scrubbers conduct four operations:

1) atomization;
2) gas droplet mixing;
3) drying of liquid droplets;
4) removal and collection of a dry product.

These operations are carried out in a tower or a specially designed vessel. The atomizer must disperse a liquid containing an alkali compound that will react with acidic components of the gas stream. The liquid must be distributed uniformly within the dryer and mixed thoroughly with the hot gases in droplets of a size that will evaporate before striking a dryer surface.

In typical spray dryers used for acid gas control, the droplets have diameters from 50 to 200 microns. The larger droplets are of most concern because these might survive long enough to impinge on equipment surfaces. In general, a trade-off must be made between the largest amount of liquid that can be sprayed and the largest droplets that can be tolerated by the equipment.

An important aspect of spray dryer design and operation is the production and control of the gas flow patterns within the drying chamber. Because of the importnace of the flow patterns, spray scrubbers are usually classified on the basis of gas flow direction in the chamber relative to the spray:

- in Cocurrent Scrubber the liquid feed is sprayed with the flow of the hot gas;
- in Countercourent Scrubber the feed is sprayed against the flow of the gas;
- in Mixed Flow Scrubber there's a combined cocurrent and countercurrent flow.

There are several types of atomizers. High-speed rotating disks achieve atomization through centrifugal motion. Nozzles are also frequently used - both centrifugal pressure nozzles and pnneumatic nozzles. In the centrifugal pressure nozzle, energy for atomization is supplied solely by the pressure of the feed fluid. In the two-fluid nozzles, air (or steam) supplies most of the energy required to atomize the liquid. As a result, two-fluid nozzle can produce very fine atomization, particularly with viscous materials.

Spray air scrubbers include metering valves, pumps and compressores, and controls to assure optimal chemical feed and temperature within gas-cleaning system.

For additional information please refer to http://www.nis-co.com/scrubber/Index.html.

Oleg Tchetchel
Engineer of Air Transfer Systems
Canadian Air Systems
http://nis-co.com/contact/Index.html
http://www.nis-co.com/scrubber/Index.html