Activated carbon is known for its high porosity and zeolites are known for their ability to be crafted into highly specialized adsorbents called molecular sieve. Carbon molecular sieve is an adsorbent that fuses the ideas behind both activated carbon and zeolites into one product. Carbon molecular sieve is a product that brings the benefits of both of these products together.
Carbon molecular sieve is made out of coal (the same material most activated carbon is made out of) and it specializes in adsorbing material under 10 angstroms, something activated carbon can not do accurately. The smallest pore size created for carbon molecular sieve is 4A, but it exists in a 5A, and 10A (or 13X) as well.
Carbon molecular sieve specializes in separating oxygen from nitrogen, an important part in natural gas processing. This process is done with a PSA (Pressure Swing Adsorption) device in two phases. The first phase sees the gas enter the PSA generator and the oxygen is adsorbed while the nitrogen passes through because the nitrogen molecules are too large and are used as a separate product. The second phase sees the oxygen slowly released from the sieve at low pressures and thereby regenerating it so that the separation process can be repeated.
Carbon molecular sieve is used in this situation as opposed to activated carbon because the physical size between oxygen (0.28nm×0.40nm) and nitrogen (0.30nm×0.41nm) molecules are so close. The pore sizes on carbon molecular sieve are able to accommodate these small size differences, where as activated carbon would just end up adsorbing both of them.
Molecular sieve isn’t used because it is a polar adsorbent, meaning its surface area attracts other polar molecules. Oxygen is a non-polar molecule and would be attracted to other non polar surfaces.
Carbon molecular sieve is one of the few non-polar adsorbents out there which is why it is chosen over molecular sieve for this application.
In addition to separating nitrogen from oxygen carbon molecular sieve can be used for metal heat treatment, electron production, and as preservative in food products.
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