Pressure swing adsorption principle of adsorption dryer

Pressure swing adsorption (PSA for short) is a new gas separation technology. Taking adsorbent molecular sieve as an example, its principle is to separate the gas mixture by using the difference in the "adsorption" performance of molecular sieve on different gas molecules. The adsorption dryer is The drying effect is achieved through "pressure change" (pressure swing adsorption principle). Pressure swing adsorption can not only be used in air drying and purification, but also in oxygen and nitrogen separation, hydrogen purification, etc.

  According to the nature of the gravitational field between the adsorbate and the adsorbent, adsorption can be divided into chemical adsorption and physical adsorption. Pressure swing adsorption belongs to physical adsorption, also called van der Waais (van der Waais) adsorption. It is caused by the gravitational force between adsorbate molecules and molecules on the surface of the adsorbent. This force is also called van der Waais force. Since the molecules on the solid surface are different from the internal molecules, there is a remaining surface free force field. When the gas molecules hit the solid surface, part of them are adsorbed and release the heat of adsorption. Among the adsorbed molecules, only when the kinetic energy of their thermal motion is sufficient to overcome the potential energy of the gravitational field of the adsorbent can they return to the gas phase, so there are always many adsorbed molecules remaining on the solid surface in contact with the gas. Due to the adsorption caused by the intermolecular gravitational force, the adsorption heat is low, close to the heat of vaporization or condensation of the adsorbate, the adsorption and desorption speed are also faster. The adsorbed gas is also easily desorbed from the solid surface, so the pressure swing adsorption is reversible.

  Adsorption dryer pressure swing adsorption principle:

   Adsorption under pressure and desorption under reduced pressure. Due to the short cycle period, the heat of adsorption is too late to dissipate and can be used for desorption. Therefore, the temperature change of the adsorption bed caused by the heat of adsorption and desorption is generally not large, and the fluctuation range is only a few degrees. It can be approximated as an isothermal process. Pressure swing adsorption works The state only changes on an isossorption line. There are several commonly used vacuum adsorption methods, the purpose of which is to reduce the partial pressure of the adsorbed components on the adsorbent, so that the adsorbent can be regenerated.

  A. Depressurization:

   The adsorption bed adsorbs at a higher pressure, and then drops to a lower pressure, usually close to atmospheric pressure, at which time a part of the adsorption group is decomposed and absorbed. This method is simple to operate, the desorption of a single adsorption component is not sufficient, and the degree of regeneration of the adsorbent is not high.

  B. Vacuum:

   After the adsorption bed drops to atmospheric pressure, in order to further reduce the partial pressure of the adsorbed components, a vacuum method can be used to reduce the adsorption bed pressure to obtain a better regeneration effect, but this method increases power consumption.

  C. Flushing:

  Using a weakly adsorbed component or other appropriate gas to pass through the adsorption bed to be regenerated, the partial pressure of the adsorbed component will drop as the flushing gas passes. The degree of regeneration of the adsorbent depends on the amount and purity of the flushing gas.

  D. Replacement:

  Using a gas with strong adsorption capacity to replace the originally adsorbed components from the adsorbent. This method is often used when the product component has a strong adsorption capacity and the impurity component is weak, that is, the product is obtained from the adsorption phase.

  In the pressure swing adsorption process, which regeneration method is used is selected according to the nature of the separated gas mixture components, product requirements, adsorbent characteristics and operating conditions. It is usually implemented by several regeneration methods. For any kind of adsorption, for the same adsorbed gas (adsorbate), in the case of adsorption equilibrium, the lower the temperature, the higher the pressure, and the greater the adsorption capacity. Conversely, the higher the temperature and the lower the pressure, the smaller the adsorption capacity. Therefore, the gas adsorption and separation method usually adopts two cycle processes of temperature swing adsorption or pressure swing adsorption.

   The basic steps of PSA work

  Single fixed adsorption bed operation, whether it is temperature swing adsorption or pressure swing adsorption, because the adsorbent needs to be regenerated, the adsorption is intermittent. Therefore, in the industry, two or more adsorption beds are used to make the adsorption and regeneration of the adsorption beds alternate (or cycle sequentially) to ensure the continuity of the entire adsorption process.

   For the pressure swing adsorption cycle process, there are three basic working steps:

1. Adsorption under pressure

   The adsorbent bed is passed into the separated gas mixture at the highest pressure of the process, in which the strongly adsorbed components are selectively absorbed by the adsorbent, and the weakly adsorbed components flow out from the other end of the adsorbent bed.

2. Decompression and desorption

   According to the performance of the adsorbed components, the aforementioned methods of depressurization, vacuuming, flushing and replacement are selected to regenerate the adsorbent. Generally, decompression and desorption are performed by first reducing the pressure to atmospheric pressure, and then flushing, vacuuming or displacement.

  3. Boost

After the regeneration of the adsorbent is completed, the adsorption bed is pressurized with weak adsorption components until the adsorption pressure is reached. Then the adsorption is carried out under pressure.