Molecular sieves, made of porous materials like zeolite, are used to dry or absorb gases and liquids by separating substances at the molecular level. They can absorb up to 24% of their weight and are often used in the petroleum industry. Different types of sieves have different pore sizes and are used for specific purposes, such as insulating glass production or removing poisonous gases from natural gas.
A molecular sieve is used to dry or absorb gases and liquids. Molecular sieves are made of porous materials, usually an aluminosilicate compound called a zeolite, and are used to separate substances at the molecular level. Primarily used as a desiccant or drying agent, these sieves are also often used in the petroleum industry to purify gases.
Just as a sieve might be used to separate rocks from sand, a molecular sieve separates larger molecules from smaller ones. For example, water molecules have a molecular radius of 0.193 nm while an acetone molecule has a radius of 0.308 nm. Pouring an acetone solution into a molecular sieve would cause the smaller water molecules to pass through the sieve while the larger acetone molecules remained, thus drying the alcohol. When two molecules are the same size, molecular sieves can also separate the molecules based on the polarity of the molecules. The more polar molecules will be absorbed as if they were the smaller molecules.
Unlike sieves that separate rocks from sand, molecular sieves don’t allow smaller molecules to simply pass through them. Instead, those molecules are absorbed by the sieve. The empty spaces in the sieve structure, the pores, hold back those smaller molecules, so the effectiveness of a sieve depends not only on the size of the pores but how much empty space is available within the sieve structure.
Molecular sieves can absorb about 24 percent of their weight. Once the water is absorbed into the porous holes and the dried or purified materials recovered, the water can be removed in a number of ways. Because heat does not change a sieve’s molecular structure, water is often removed by heating the sieve to 482 °F (250 °C) for two hours and then allowing it to cool with minimal contact with moist air.
Crystalline in structure, the porous material in a molecular sieve has completely uniform pore sizes. These dimensions, however, differ according to the type of sieve. There are several types, including insulating glass molecular sieves, types 3A–5A, and type 13X.
Insulating glass molecular sieves are specifically used for the production of insulating glass. They prevent the accumulation of water inside the glass surface. Sieves 3A and 4A are universal drying agents. 3As are mainly used to dry or dehydrate hydrocarbons. This is the type of sieve that would be used to dry the acetone in the example above. 4A sieves are used in closed systems, such as electrical components and drug packaging, to ensure dryness.
5A sieves are used to remove unwanted hydrogen sulphide, an extremely poisonous gas, and carbon dioxide from natural gas. These are often used by the oil industry. 13X sieves are used commercially for gas drying only.
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