The Guide To Supporting Your Molecular Sieve Beds
The use of molecular sieve supports such as screens and ceramic balls are very important for the industrial use of molecular sieve. Ceramic balls prevent adsorbent from leaking on the screens that have been placed below them, and the screens keep the molecular sieve separate from the ceramic balls.
In an industrial setting molecular sieve is under a lot of pressure. The large dehydration vessels that molecular sieves are used in subject the molecular sieve to a great deal of weight and temperature related stress.
In order to counteract these stresses, supports and screens are needed in the sieve vessel at the top and at the bottom.
Supporting the Bottom:
At the bottom of the vessel a 6 inch layer of ceramic balls is needed for support. The two layers of ceramic balls will each need 3 inches of space each adjacent layer starting from the molecular sieve should double in size.
For example, if you are using a 4×8 mesh size molecular sieve the first ceramic ball layer beneath it should consist of ¼” balls, and second layer of ceramic balls beneath the first layer should consist of ½” balls. If you are using an 8×12 mesh size, the first layer of ceramic balls should be 1/8” and the second layer should be ¼”.
Loading the support balls on the bottom is usually done (depends on the plant design) through a side manway that should be located on the bottom of the vessel. Ceramic balls should be leveled once they are placed, this can be done using a rake.
Supporting the Top:
At the top of the vessel a 6 inch layer of ceramic balls is also needed. Only one layer of ceramic balls is needed for the top. Ceramic ball size should be either ½” (for 8×12 mesh) or 1” (for 4×8 mesh).
Loading the top is done with either by using small buckets or lifting the boxes/bags of ceramic balls to the top of the vessel. They are then placed evenly on the screen that is covering the molecular sieve. In order to prevent ceramic balls from breaking do not let them fall over 6 feet.
Setting the Screens:
Most support structures designed for molecular sieve can endure a 50-100 PSI pressure drop. The most common designs use I-beams supports that are attached to the vessel walls, the I-beams should be able to move around freely during heating or cooling processes.
When setting up screens at the bottom of the vessel a metal grating is cut so that it is smaller than the vessel diameter and it is placed on top of the I-beam. The gap needs to be covered by rope packing that can withstand temperatures up to 400 degrees Fahrenheit.
Multiple different screens are needed when setting them up in a molecular sieve vessel. On top of the grating a 3-5 mesh screen (¼” to ½” openings) is used first, after that a second screen 10-20 mesh is placed on top of the 3-5 mesh screen.
When setting up screens at the top of the vessel a 10-20 mesh screen is needed. The screen should overlap the vessel wall by 4 inches, this prevents any of the ceramic balls from falling into the molecular sieve.
The use of screens and ceramic balls is very important in a molecular sieve vessel. It can prevent gas surges that move your sieve, an unleveled bed, flow channeling, and a possible early breakthrough.