Solid Adsorbents

Solid desiccants are materials with a tremendous internal surface area per unit mass, such as silica gel, molecular sieves, activated alumina, or hygroscopic salts. The capacity of solid desiccants per unit mass, however, is usually less than that of liquids.

• Silica gels are formed by condensing soluble silicates from water or solvent solutions. They are relatively inexpensive and easy to customize. They are available in beads as large as 0.2 in (0.5 cm) in diameter or as fine powders.

• Zeolites are aluminosilicate minerals that occur in nature. They have an open crystalline lattice that attracts and traps molecules, such as water vapor, like objects in a cage. Particular atoms of the material determine the size of the openings or gaps in the cage-like structure, which in turn governs the size of the molecules that can be adsorbed.

• Synthetic zeolites (molecular sieves) are manufactured crystalline aluminosilicates. By controlling the temperature of the thermal manufacturing process and the materials used, the zeolite's structure and surface characteristics can be closely controlled. While more costly than naturally occurring zeolites, this process can produce a more uniform product.

• Activated alumina are manufactured oxides and hydrides of aluminum. Their structures can be controlled by the gases, temperatures, and duration of the thermal manufacturing process.

• Carbons are usually used to adsorb gases other than water vapor, because they have an affinity for nonpolar molecules, such as organic solvents. Carbons have a large internal surface and very large capillaries.

• Solid polymers are long molecules twisted together like strands of string. The sodium ions in these molecules can each bind several water molecules, and the spaces between the strands can also contain condensed water. This polymer's capacity exceeds that of many solid adsorbents, particularly at high relative humidities.

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Guide to Alternative Fuels

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