Desiccants and Sorbents

Desiccants and sorbents are specialized, moisture absorbing minerals or chemicals for product protection as well as moisture, oil or spill absorption. A desiccant packet or desiccant dryer removes humidity that could damage products. Silica gel, calcium sulfate, montmorillonite clay, and molecular sieves are common desiccants. Silica gel desiccant is usually packaged in a semi-permeable dessicant bag. Clay desiccant is made from aluminosilicate clay.

Desiccants and sorbents are related products, but different in important ways. Sorbents recover liquids through absorption, adsorption, or both. Absorbents retain liquid throughout the molecular structure, causing more than 50 percent swelling. Adsorbents are coated by a liquid on the surface (including pores) without swelling more than 50 percent. Sorbents can be divided into three basic categories: natural organic (peat moss, straw, and hay), natural inorganic (clay, perlite, and sand), and synthetic (polyurethane, polyethylene, polypropylene, and cross-linked polymers and rubber materials). In industrial settings, a sorbent pad containing an oil sorbent can be used to catch and contain drips, or to wipe contaminated surfaces.

Desiccants and sorbents differ in how performance is measured. One unit of desiccant will adsorb 3 grams of water vapor at 20% relative humidity, or 6 grams of water vapor at 40% relative humidity at 77°C. To achieve very dry air, a dehumidifier desiccant can be rotated through process air and heated to remove moisture. Most synthetic sorbents can absorb up to 70 times their weight in oil. For cleaning up oil spills, factors to consider include rate of absorption, rate of adsorption, oil retention, and ease of application. Desiccants and sorbents also differ in how they are handled after use. Desiccants can generally be reactivated by heating. Sorbents used to recover oil must be disposed of in accordance with approved local, state, and federal regulations.

Desiccants and sorbents meet standards such as "Standard Methods of Testing Sorbent Performance of Absorbents" (ASTM F716) published by ASTM International (formerly called the American Society for Testing and Materials). Desiccants and sorbents can also meet U.S. government standards such as MIL-SPEC MIL-D-3464E (packaged desiccants) and FED A-A-1280B, FED A-A-1281A,and FED A-A-1282A (oil sorbents).

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