Scrubbers

views updated

Scrubbers

Scrubbers are air pollution control devices that help cleanse the emissions coming out of an incinerator's smoke stack. Hot exhaust gas comes out of the incinerator duct and scrubbers help to wash the particulate matter (dust) resulting from the combustion out of the gas. High efficiency scrubbers literally scrub the smoke by mixing dust particles and droplets of water (as fine as mist) together at a very high speed. Scrubbers force the dust to move like a bullet fired at high velocity into the water droplet. The process is similar to the way that rain washes the air.

Scrubbers can also be used as absorbers. Absorption dissolves material into a liquid, much as sugar is absorbed into coffee. From an air pollution standpoint, absorption is a useful method of reducing or eliminating the discharge of air contaminants into the atmosphere . The gaseous air contaminants most commonly controlled by absorption include sulfur dioxide , hydrogen sulfide, hydrogen chloride, chlorine , ammonia, nitrogen oxides , and light hydrocarbons .

Gas absorption equipment is designed to provide thorough contact between the gas and liquid solvent in order to permit interphase diffusion and solution of the materials. This contact between gas and liquid can be accomplished by dispersing gas in liquid and visa versa. Scrubbers help wash out polluting chemicals from the exhaust gas by facilitating the mixture of liquid (solvent) and gas together.

A number of engineering designs serve to disperse liquid. These include a packed tower, a spray tower or spray chamber, venturi absorbers, and a bubble tray tower.

The most appropriate design for incineration facilities is the packed tower—a tower filled with one of many available packing materials. The packing material should provide a large surface area and, for good fluid flow, should be shaped to give large void space when packed. It should also be chemically inert and inexpensive. It must be designed so as to expose a large surface area and be made of materials, such as stainless steel, ceramic or certain forms of plastic.

Packing materials come in various manufactured shapes. They may look like a saddle, a thick tube, a many-faceted star, a scouring pad, or a cylinder with a number of holes carved in it. Packing may be dumped into the column at random or stacked in some kind of order. Randomly dumped packing has a higher gas pressure drop across the bed. The stacked packings have an advantage of lower pressure and higher possible liquid throughout, but the installation cost is much higher because they are packed by hand.

Rock and gravel have also been used as packing materials but are usually considered too heavy. They also have small surface areas, give poor fluid flow and at times are not chemically inert.

The liquid is introduced at the top of the tower and trickles down through to the bottom. Since the effectiveness of a packed tower depends on the availability of a large, exposed liquid film, poor liquid distribution that prevents a portion of the packing from being irrigated renders that portion of the tower ineffective. Poor distribution can result from improper introduction of the liquid at the top of the tower or using the wrong rate of liquid flow. The liquid rate must be sufficient to wet the packing but not flood the tower. A liquid rate of at least 800 pounds of liquid per hour per square foot of tower cross-section is typical.

While the liquid introduced at the top of the tower trickles down through the packing, the gas is introduced at the bottom and passes upward through the packing. This process results in the highest possible efficiency. Where the gas stream and solvent enter at the top of the column, there is initially a very high rate of absorption that constantly decreases until the gas and liquid exit in equilibrium.

Scrubbers are used in coal burning industries that generate electricity. They can be used in high sulfur coal emissions because high sulfur coal emits high levels of sulfur dioxide. Scrubbers are also utilized in industrial chemical manufacturing as an important operation in the production of a chemical compound. For example, one step in the manufacture of hydrochloric acid involves the absorption of hydrochloric acid gas in water. Scrubbers are used as a method of recovering valuable products from gas streams—as in petroleum production where natural gasoline is removed from gas streams by absorption in a special hydrocarbon oil.