All molecular sieves are composed of sodium and aluminosilicate, which are grown to form a three-dimensional crystal structure. It’s this crystalline structure that allows molecular sieve to adsorb some molecules while rejecting other molecules that are too large to fit inside the crystal. When A-Type crystals are grown, they initially are all Type 4A, meaning the pore openings are four Angstroms in diameter with an A-Type crystal structure. These 4A crystals are capable of adsorbing molecules that are smaller than 4 angstroms in diameter, such as water, while allowing larger molecules, such as methane, to pass along to the desired product stream.
Smaller Pore Sizes
The pore size of Type 4A can be manipulated, which allows molecular sieve to be used to adsorb, or exclude, certain molecules during refining and purification processes. If 4A crystals are exposed to potassium for instance, an ion exchange can occur, replacing a sodium ion with a larger potassium ion, and creating a smaller pore opening. This ion exchange is how Type 3A molecular sieve crystals are made. Exchanging one ions allows more selectivity for adsorbing water and is critical in processes such as Ethanol Dehydration. Since ethanol molecules are roughly 3.6 Angstroms in diameter, 4A molecular sieve is able to adsorb ethanol, which means that the desired product would be coadsorbed while trying to remove water from ethanol streams. To offer higher efficiency in the dehydration process, Type 3A molecular sieve is commonly used due to its pore size, which enables 3A sieve to adsorb water while rejecting ethanol from entering the crystals.
Larger Pore Sizes
To offer a slightly larger pore opening, calcium is used in an ion exchange to produce Type 5A molecular sieve. A sodium ion is exchanged with the slightly smaller calcium ion, which increases the pore size of the crystal. Type 5A is able to adsorb certain mercaptans while still small enough to block larger hydrocarbons from entering the crystal. These properties allows this molecular sieve to be utilized in LNG and LPG refining to remove sulfur compounds while allowing the desired hydrocarbons to pass through to the product stream. Type 5A is also useful for producing relatively pure streams of hydrogen or oxygen since it is able to adsorb nitrogen, carbon monoxide, and other contaminants.
To create very pure streams of oxygen, a Type 13X crystal is more likely to be used. Type X crystals are shaped differently from Type A crystals are tend to offer much larger pore sizes, about 9 Angstroms in diameter. 13X molecular sieve is often used in air separation process, such as in the production of high purity oxygen. Aside from oxygen production, X type crystals are commonly used in cryogenic distillation processes to deeply dehydrate LNG and LPG streams. It is critical to remove all water from these streams to prevent blockage and freezing in pipelines. There are many variations of Type X crystals with most being Type 13X, although some can be produced to have pore openings about 8 Angstroms in diameter, sometimes referred to as Type 10X.
While all molecular sieves serve the purpose of selectively adsorbing smaller molecules from larger ones, it’s critical to select the right product for your application. Ion exchanges are the key differences in Type A crystals, which offer different capabilities of adsorption, while Type X crystals have a different shape and offer much larger pore openings than Type A crystals.