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Adsorption is similar to using a sponge to soak up water. A porous solid material is used to soak up gaseous air contaminants from the exhaust air. Inside an adsorber, air passes through a layer of materials for pollutants to adhere or "stick." Eventually the adsorbent material becomes saturated or filled with the pollutant and can hold no more. Similar to squeezing a sponge for reuse, the adsorbent material must be refreshed for reuse or disposed.
Activated carbon, silica gel and alumina oxide are common adsorbing agents. The chemical nature of the adsorbent, the total surface area (how porous it is), and pore diameter are given careful consideration before adsorbers are used. If the gas contains particulate matter, the adsorbent bed can become clogged. Some gas is precleaned by a baghouse filter, electrostatic precipitator or cyclone to remove particulate matter before entering the adsorber.
Like squeezing a sponge for reuse, adsorbers can be refreshed too. There are several chemical principles that apply to how adsorbers work. For example, as gas temperature increases, adsorption decreases. As gas pressure increases, so does adsorption. The slower the gas moves through adsorption materials, the more gas removed. These same principles are used to regenerate or refresh saturated adsorbers.
Temperature can be increased to release pollutants from the activated charcoal or the pressure can be decreased. "Steam stripping" uses injected steam to remove pollutants from activated charcoal. Many organic compounds can be condensed, distilled or decanted from captured steam and reused instead of emitted into the air. Charcoal canisters are used to capture emissions from some dry-cleaning machines to remove perchloroethylene, a toxic chemical.
Degreasing, rubber processing, and printing operations sometimes use adsorbers. Toxic or odorous vapors from food processing, rendering plants, sewage treatment plants and many chemical manufacturing process use adsorbers too.
Activated carbon can be made from wood, coal, coconut husks or other nutshells and petroleum byproducts. To activate or make it porous, the material is heated in a chamber with little air. This produces a material with a surface area so great that one gram may have two to five football fields of surface! Activated carbon is often used for controlling organic pollutants such as solvents, odors, toxic gases and gasoline vapors.