A Means for Sealing Holddown Plates in Adsorber Vessels
Publication Date: 2016-Mar-16
The IP.com Prior Art Database
PSA and VSA vessels and beds that operate under fluidizing flow regimes must have a holddown device installed atop the adsorbent bed. The holddown device must be free to move axially within the vessel to allow for adsorbent settling. It must also seal against the vessel wall to prevent adsorbent particles from escaping through the gap between the holddown and the vessel wall. This disclosure describes such a seal.
March 16, 2016
Pressure swing adsorption (PSA) and vacuum swing adsorption (VSA) processes are frequently used to separate or purify mixtures of gases. A feed gas is passed through a vessel containing a bed of the adsorbent material (also called molecular sieve), and one or more components of the gas mixture is adsorbed, allowing other components to pass through the bed, and exit at a higher purity. The entire process consists of passing the mixture through the bed for purification, followed by subsequent steps to depressurize the bed, regenerating the bed to purge impurities from it, then repressurizing the bed in order to repeat the cycle.
The adsorbent bed consists of one or more adsorbent materials, selected to adsorb different components of the feed mixture. When multiple materials are used, they are usually segregated from one another within the bed.
The adsorbent may be a granular material, with a particle size ranging from slightly less than 1mm to a few millimeters. Depending on the pressures and flows of the process steps, it is possible that flow of gas through the bed may be sufficient to overcome, for example, any gravitational forces on the particles, and lift or fluidize the adsorbent. When this occurs, the adsorbent particles move relative to each other, with a number of deleterious effects. They can erode each other, creating dust, which clogs the spaces between the particles and increases pressure drop through the bed. Adsorbents segregated from one another in different areas of the bed can also become mixed together.
A bed that operates under conditions that cause lifting must have a holddown device installed atop the bed. The holddown device may consist of a perforated plate covered with a fine screen. The holes allow the process fluid to pass through the plate, and the screen prevents the adsorbent particles from passing through the holes in the plate. The plate is held down with force sufficient to prevent the upward flow from lifting the plate and adsorbent. There are a number of ways to provide the downward force. Methods in common use include compression springs, inflatable bladders, and weights in the form of larger, dense, inert particles loaded atop the plate.
When a holddown is used, care must be taken to seal any gaps between the holddown and the vessel wall. If a gap exists anywhere around the circumference of the holddown, adsorbent particles may blow through the gap and escape from the bed. This disclosure describes such a seal.
It is possible the adsorbent in a bed may settle slightly with time. If this occurs the location of the top surface of the bed will change. It is also possible that changes in temperature may cause thermal expansion or contraction of the adsorbent relative to the vessel. Both of these conditions require that a holddown be able to move axially within the vessel, and still maintain the seal.