The working principle of VCM pressure swing adsorption.

Basic principle of pressure swing adsorption: Pressure Swing Adsorption (PSA) is a gas separation technology that has seen rapid development due to its advantages such as simple process design, the ability to remove multiple impurity components in a single step, high product purity, operational flexibility, high degree of automation, low operating costs, long lifespan of the adsorbent, reduced investment requirements, and ease of maintenance. It has become one of the main technologies used for air drying, hydrogen purification, small- to medium-scale air separation, as well as the separation and purification of other mixed gases. The so-called pressure swing adsorption method (PSA) is actually an adsorption method that uses reduced pressure for desorption in order to regenerate the adsorbent. The cycle can take place at room temperature, and because the pressure changes very rapidly, the cycle is usually completed in just a few minutes or even seconds. Although the adsorption capacity is not very high, the adsorbent has a high utilization rate, allowing for large processing volumes. The regeneration of pressure swing adsorption adsorbents does not require external heating. Moreover, the thermal conductivity of these adsorbents is usually very low, so the process operates in a nearly adiabatic manner. The cycle time for pressure swing adsorption is also short, meaning that the adsorption heat has enough time to remain available for desorption. The temperature changes in the adsorbent bed caused by adsorption and desorption heats are generally small, allowing the process to be treated as isothermal. Since the porosity of commonly used adsorbents is quite high, and pressure swing adsorption takes place under pressure, the amount of gas contained in the gaps between the adsorbent particles in the bed (commonly referred to as the dead space of the bed) cannot be ignored. During pressure-reduced desorption, as the more strongly adsorbed components are desorbed, the weakly adsorbed components in the dead space of the bed are discharged from the bed, resulting in a loss of light products. The cycle process of the pressure swing adsorption process typically includes the following steps: adsorption, forward pressure reduction, reverse pressure release, flushing or evacuation, pressure equalization and charging, and final pressurization. Here, the role of the main steps in the process is explained using a simple pressure swing adsorption process for obtaining products from the non-adsorbed phase as an example. 1) Adsorption step: The feed gas enters the adsorption bed at the highest pressure in the cycle; components that are strongly adsorbed remain on the adsorbent, while those with weaker adsorption properties are discharged as product. Part of this product gas is used to raise the pressure in another bed layer. The adsorption step is stopped when the adsorption front is still a certain distance away from the product outlet, leaving a portion of the adsorbent unused near the product outlet for use as the adsorption front moves forward during reduced pressure. 2) Forward pressure reduction step: The bed that has completed the adsorption step has its pressure reduced, and the exhaust gas is directed to another bed that has already been cleaned and is at a lower pressure; this connects the outlet ends of the two beds. By the end of this pressure equalization process, both beds have the same pressure, at a higher intermediate level – this is a step for achieving pressure equilibrium. It serves to recover the pressure and product gas from the bed that has completed adsorption, thereby increasing the recovery rate of the product gas. During the equalization step, the adsorption front moves toward the product outlet, but the impurity components have not yet penetrated; therefore, the exhaust gas from this step is essentially pure product gas. 3) Reverse pressure reduction step: Close the valve at the product outlet and open the valve at the feed inlet, allowing the gas within the bed to be discharged in the opposite direction to the feed flow. The pressure inside the bed is reduced from the lower intermediate pressure to the lowest pressure in the circulation system, which is usually atmospheric pressure or negative pressure. In this step, the impurity components adsorbed by the adsorbent desorb and are removed from the bed. Moreover, the gas with a high concentration of residual product components in the dead space near the product outlet flows in the reverse direction, thereby cleaning the bed; this helps to remove the impurity components near the feed inlet from the bed during this step. 4) Flushing or evacuation step: The bed is backwashed at the lowest pressure of the cycle using the exhaust gas from another bed that is currently undergoing the forward feeding step. During the cleaning process, the partial pressure of the impurity components is reduced, causing them to desorb from the adsorbent and be removed from the bed; this also pushes the adsorption front toward the feed end. As a result, by the end of the cleaning process, the regeneration of the adsorbent is essentially complete. Alternatively, a vacuum pump can be used to evacuate the adsorption bed to create a negative pressure, thereby enhancing the regeneration of the adsorbent. 5) Pressure equalization and charging step: The bed that has been regenerated but is at a lower pressure in the cycle must be pressurized to the adsorption pressure before proceeding with the next adsorption step. This pressurization is achieved by using the exhaust gas from another bed that is currently in the depressurization phase, thereby connecting the outlet ends of the two beds together and raising the pressure of the bed from the lowest pressure in the cycle to a lower intermediate pressure. Backflow pressurization can push the adsorption front toward the feed side, ensuring a cleaner product side. 6) Final pressurization step: To increase the bed pressure from a lower intermediate pressure to the adsorption pressure, it is possible to use feed gas for pressurization in the forward direction, or product gas for pressurization in the reverse direction. Reversing the pressurization with product gas can push the adsorption front of impurities toward the feed end of the bed, thereby flattening its concentration front as much as possible, which is beneficial for the subsequent adsorption process; however, this requires a large amount of product gas. The pressurization method can involve pressurizing with feed gas to a higher intermediate pressure and then backpressurizing to the adsorption pressure using product gas, or it can also involve final pressurization from both ends of the bed using feed gas and product gas simultaneously. 7) The bed layer has completed one cycle of operation after the final pressurization step, and is ready for the adsorption operation in the next cycle.

The main process flow for the purification step of this device is the 5-1-3&V/VP process. Each purifier must go through a series of steps in one cycle, including adsorption (A), pressure equalization drop 1 (E1D), pressure equalization drop 2 (E2D), pressure equalization drop 3 (E3D), reverse pressure release (D), evacuation (V), pressure equalization rise 3 (E3R), pressure equalization rise 2 (E2R), pressure equalization rise 1 (E1R), and final pressure increase (FR).