Zeochem’s molecular sieves are used in a variety of applications, but the goal is always the same: separating two or more components from each other. This can be done in several different ways, but the most common processes are pressure swing adsorption (PSA) and thermal swing adsorption (TSA). Both of these methods involve using the sieve, regenerating it and then reusing it by taking advantage of the fact that the sieve will adsorb contaminants more strongly in certain conditions than in others (physisorption). However, there are also some important differences between these two methods.
TSA applications make use of the fact that the sieve will adsorb a greater amount of contaminants when the sieve bed temperature is lower. Thus, a gas or liquid stream is fed to the sieve bed at a low temperature for a period of time to adsorb a contaminant onto the sieve and remove it from the stream. The sieve bed is then heated, typically by feeding a hot, clean stream (a gas that isn’t contaminated) to regenerate the sieve for reuse. Next, the bed is cooled down so the adsorption process can start all over again.
TSA applications allow for the greatest use of the sieve capacity because the high temperature regeneration is the most effective at removing adsorbed contaminants. These applications are also best for removing contaminants from the product stream, making TSA with molecular sieves uniquely qualified for the removal of contaminants in certain applications. In most cases TSA is required for the removal of the contaminants CO2 and H2O. TSA is found in cryogenic oxygen plants and natural gas dehydration units.
PSA applications utilize the fact that sieve capacity increases with increasing pressure. In this type of application, the contaminated feed stream is fed to the sieve beds at high pressure for a period of time and then the sieve bed pressure is reduced to regenerate the sieve and remove the adsorbed contaminant. Although this method is not as effective for regenerating the sieve, a vacuum can be added to the regeneration process to further reduce pressure. This process is called vacuum pressure swing adsorption (VPSA) and is used in large oxygen plants, where our Lithium-LSX products are utilized, as well as in ethanol dehydration units.
There are advantages to PSA over TSA. The biggest advantage is keeping the sieve from overheating, which can cause certain components of the feed stream to break down into other compounds. That would cause part of the feed to be lost when it is converted to those other compounds (which may then be considered contaminants in the product stream). Those broken-down components can stick to the sieve and reduce its effectiveness in each cycle. PSA regeneration avoids that problem. Our ZEOX line of molecular sieves are part of the PSA process used for medical oxygen concentrators.
Although there is more than one way to operate regenerable sieve beds, the underlying method remains the same: adsorb contaminants at conditions where they will stick to the sieve very well, regenerate the sieve bed by adjusting conditions to a point where contaminants no longer can adsorb to the sieve and then swing the sieve bed back to the adsorbing conditions and repeat.
-Patrick Purcell