In order to avoid any potential confusion in terms, consider an adsorbent as a material that is defined as an insoluble material that retains liquids throughout its structure and expands to more than 50% of its dry volume. An adsorbent retains liquids on its surface – which may include a porous internal structure – and expands no more than 50% of its dry volume. A sorbent is simply a general term encompassing absorbent or adsorbent materials and includes either or both methods of liquid retention. More strict definitions have been established by various standards, but for practical purposes, the definitions above suffice. A simple example of an absorbent is a sponge; a common adsorbent is activated charcoal.
Desiccant chemicals and processes tend to be classified as adsorbent because of their characteristic of using relatively low-level Van der Waal forces at the molecular level to achieve liquid retention. Silicia gel, activated charcoal, and aluminium-based desiccants are some examples of older technologies still in use for air-drying. In low volume applications, they can be used cost effectively, such as gel packs for local humidity control. Cameras and other delicate electronics are often packaged for retail with a silica or similar chemical packet to adsorb any residual moisture remaining in a package before it is sealed after packing.
For industrial high-volume applications, where air is constantly changing, a more durable and practical material is the crystalline molecular sieve. Usually seen as very small beads, molecular sieves are highly adsorbent and can be quickly cleaned and dried for virtually immediate reuse.
Molecular sieves are available with micropores in a number of sizes ranging in the nanometre ranges. Typical micropore sizes range from 3 to 10 Angstroms contained in beads measuring 1.5mm to 5mm.
Depending on the application, it may be desirable to have a visual as well as measured indication of moisture saturation. Some silica gel desiccant air dryers or molecular sieve desiccant bead products change colour noticeably as they adsorb water, providing an instant visual verification of its current state. For example, a neutral near-colourless state when dry can change to a bright orange as it nears saturation.
This can be very useful in field applications for desiccant compressed air dryers. The use of monitored and program-controlled psychrometers or hygrometers is preferred for automated desiccant dryer or dehumidifier applications. Efficient, easily scrubbed and dried molecular sieves can result in significant desiccant dehumidification energy savings by virtue of their durability and rapid processing time, providing a better return on investment and ease of maintenance.