How Dry Bed Abatement Works
The best scrubber technology is always driven by the application needing to be abated. These technologies vary with thermal, wet, plasma, and dry abatement which we will focus on for the scope of this article.
Dry Abatement is potentially the most versatile because of its ability to use a variety of medias to target specific hazardous gases. In addition to versatility, these systems usually require little maintenance and offer passive abatement even when power or utilities are lost. Replaceable canisters will have various layers of substrate and resin for maximum efficiency. These materials are chosen based off target gases, flow rates, concentration, and residence time.
The medias use a mixture of adsorption (physical) and chemisorption (chemical) in order to abate hazardous gases. Through these methods, the chemicals are contained in the canister rather than transferring it to another media like wet scrubbing. Using the proper medias can also yield single digit ppm or even ppb discharge, well below the permitted exposure limit.
Chemisorption is typically preferred when possible because it transforms the hazardous gases and can bind these molecules to the surface of the resin. This process is also normally irreversible under typical conditions, so the waste media is safer than the initial hazardous gas. Adsorption, however, is a physical process attaching the molecule to the substrate's surface. A high surface area (porous) media is preferred for this process but has the risk of desorption. Adsorption is usually better for larger molecules that don’t easily react with dry bed media like C4F6 and C3F8.
Either through chemisorption or adsorption, the media in the canister will slowly be consumed during abatement. The active region of abatement, often referred to as the “mass transfer zone” (MTZ) will progress through the canister from the inlet towards the outlet in the general shape of a bullet. Once the tip of the MTZ reaches the outlet, breakthrough will occur and signal that the canister needs to be replaced. This is usually confirmed with a toxic gas monitor and is one of the few abatement technologies that can rely on an accurate reading from a toxic gas monitor.
Dry scrubbing is the preferred choice for several applications including projects that don’t have water or water treatment available, applications with non-water-soluble gases or gases that cannot easily be broken apart thermally, and processes with hydrides that require extremely high destruction efficiency. While each reaction is unique to the gas and resin, an example of a typical hydride reaction would be:
2 AsH3 + 3 O2 → As2O3 + 3 H2O
The above reaction shows the abatement of Arsine, a highly toxic and flammable gas, into arsenic trioxide, a solid. This can be done with oxygen rich media and is a chemical process ensuring complete abatement of the arsine. You will see many reactions yield water as a byproduct and can produce an exothermic reaction. Hydrides can generate a significant amount of heat and require careful monitoring for proper safety.