Activated carbons are complex products that are difficult to classify on the basis of their behavior, surface characteristics, and preparation methods. However, some broad classification is made for general purpose based on their physical characteristics. Activated carbon is valuable for a variety of applications; for example, gas and water purification, metal extraction, gold recovery, medicine, sewage treatment, air filters in masks, and filters in compressed air. Additionally, activated carbon is useful for closed space deodorization such as refrigerators and warehouses. Lastly, Ethylene adsorptions prevents premature ripening of fruits and vegetables, and discoloration of sugar, honey, juices, and liquor.
A molecular sieve is a material with very small pores; thus, large molecules cannot be absorbed. Molecular sieve is the best desiccant based on technical performance characteristics. For case water vapor, its ability to adsorb moisture is so pronounced that it can remove trapped H2O molecules from a fully saturated silica gel bead, which in turn changes the silica gel back to its original cobalt blue color. Many molecular sieves are used as desiccants. Some examples include activated charcoal and silica gel.
Pre-packaged desiccants are solids that adsorb water.