In practical industrial applications, adsorptive separation processes can be generally divided into pressure swing adsorption (PSA) and thermal swing adsorption (TSA). It can be seen from the sorption isotherm of the adsorbent that the adsorbent has a higher adsorption capacity for impurities at high pressure or otherwise lower. At the same time, the adsorption isobar shows the fact that the molecular sieve adsorbs a larger amount of impurities when the temperature is lower at the same pressure. The adsorptive separation process using the former property of the adsorbent is called pressure swing adsorption (PSA), and the one using the latter is temperature swing adsorption (TSA).
Practically , the TSA, PSA or TSA + PSA processes are generally selected in line with the components, pressure of the air source and product gas requirements.
In thermal swing adsorption, the sieve bed needs to be heated, therefore, it is usually used for the purification of trace impurities or elements difficult to desorb due to the features like long cycle period, high investment and high sieve generation. In comparison, with short cycle time, high adsorbent utilization rate, small adsorbent amount and no external heat exchange equipment is required, pressure swing adsorption process is widely applied in the separation and purification of large-volume and mixed feed gases.
In pressure swing adsorption (PSA), the molecular sieve usually adsorbs the easily adsorbable components in the mixed gas at normal temperature and relatively higher pressure so that the components that are not easily adsorbed flow out of one end of the bed. Then the pressure of the adsorption bed is to reduced to normal level, and the adsorbed constituents are desorbed and discharged from the other end of the bed, thereby achieving gas separation and purification, and also regenerating the adsorbent.
But generally, even if the pressure of the seize bed drops to normal, the adsorbed impurities cannot be completely desorbed. At this time, two methods can be used to completely regenerate the adsorbent: First is to flush the bed with product gas to break down the impurities that are more difficult to desorb, the advantage of which is that it can be completed under normal pressure while part of the product gas will be lost. The second is to add a vacuum for regeneration process where impurities are forcibly desorbed under negative pressure, which is commonly known as Vacuum Pressure Swing Adsorption (VPSA). VPSA process has superior regeneration effect and high product gas yield. The disadvantage is that a vacuum pump needs to be included. In the actual application, selection of the above process mainly depends on the composition, volume, product requirements of the feed stream, and also on the set-up capital and site of the factory.