7 Properties That Determine the Quality of Your Molecular Sieve
A Guide to Determine the Value of Sieve in Ethanol Dehydration
All molecular sieves are not the same. They are not a commodity and the quality varies from manufacturer to manufacturer, therefore it is important to take the time to examine not only the price of molecular sieve but the value.
Virtually all molecular sieve manufacturers measure the same characteristics and properties in molecular sieve, and it is the various measurements of these characteristics that allow you to determine the value of your sieve.
Although this list focuses on determining the value of ethanol grade sieve a lot of these measurements can help determine the suitability of a sieve product for any particular application. Ultimately knowing what makes sieve valuable can make a difficult buying decision less complicated. Listed below are the sieve properties that can help you determine its value.
- Density – Knowing the density (when coupled with water adsorption) allows you to figure out the overall water capacity of a vessel in terms of volume or mass. Higher capacity = more water adsorbed. A more valuable sieve has a higher volumetric capacity.
- Particle size and distribution – Allows for the calculation of pressure drop, fluidization parameters, and critical velocity through the bed which ultimately effects flow rate. A higher quality sieve has a tight distribution with less “tails.”
- Static water adsorption – This refers to the overall capacity of the sieve to adsorb water. (Do not confuse with working capacity which is much less than static capacity and varies with the operation as well as the sieve). For more information on working capacity see my previous article on calculating working capacity. A sieve with a higher static water adsorption capacity is always better.
- CO2 adsorption – This measures how much ethanol is being adsorbed with the water in your dehydration beds. Water and (sometimes ethanol) can be adsorbed by 3A sieve because 3A is made from 4A sieve and as a result the sieve bed will not entirely be made up of 3A. Some of the left over 4A sieve adsorbs CO2 and ethanol therefore the higher the CO2 adsorption rate is the higher the ethanol co-adsorption rate in the bed is. This ultimately reduces the overall working capacity per cycle in an ethanol plant, look for low CO2 adsorption rates.
- Crush strength – This one’s simple, the higher the crush strength the higher the durability of the molecular sieve beads in operation. A higher number here means a higher quality sieve.
- Attrition – This refers to fryability, which is the tendency of the sieve beads to grind up, which produces dust, thus lowering the overall capacity of the bed. A lower attrition number is better.
- Ethanol ΔT (Methanol Delta T) – This is a measurement of the ability of sieve to adsorb ethanol, or a measurement of the co-adsorption characteristics of water and ethanol. If capacity is being taken up by ethanol then the water capacity suffers, which is why a lower number is better.
Feel free to use this list as a guide to determine if the sieve you are currently using or or wish to buy is going to be a quality product. You can find most of, or all of this, information about your sieve by asking your supplier or manufacturer for a certificate of analysis from their quality control department.
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