Dew point of adsorption dryer and its influencing factors

Dew point of adsorption dryer and its influencing factors

1. Adsorbent Adsorbent is the material basis of the adsorption dryer. The type of adsorbent has a decisive influence on the final dew point of the compressed air being processed. At present, the most commonly used adsorbents in compressed air dehydration and drying are silica gel, activated alumina and molecular sieves. Due to the varying degrees of surface micropores, the effects they can achieve to adsorb water are also inconsistent (as shown in the figure below).

　　2. Adsorption rate During the adsorption process, the adsorption rate has a great influence on the outlet air dew point. Take silica gel as an example. At the beginning of adsorption, the adsorption efficiency is very high, and the outlet air dew point remains unchanged in a larger range, which is the adsorption area of ​​the dryer. When the air continues to flow and the adsorbent process continues, the moisture adsorbed on the surface of the adsorbent particles gradually increases, and the water content of the outlet air begins to rise. When the surface is completely saturated with moisture, the adsorption equilibrium is reached—also called adsorbent "Conversion point".

　　 3. Two-phase contact time The structural parameters of the adsorption dryer mainly refer to the tower diameter D and height H. The influence of these two parameters on the outlet air dew point is comprehensively expressed as the two-phase contact time T of the wet air and the adsorbent. The contact time T between the wet air flow and the adsorbent is determined by the ratio of the height H of the adsorbent bed to the flow rate c of the empty tower: T=H/c

　　4. Inlet pressure. The decrease in working pressure results in an increase in the air flow rate in the tower, a decrease in the two-phase contact time, and an increase in the outlet air dew point.

　　The decrease in working pressure will cause the saturated moisture content to rise, that is, the amount of water entering the dryer increases, and the dew point of the outlet air rises. Therefore, the reduced pressure of compressed air will have an adverse effect on the dew point of the outlet air of the dryer, and an appropriate increase in the pressure of the inlet air will help reduce the dew point of the outlet air.

　　5, intake air temperature

　　 The increase in the inlet air temperature of the adsorption dryer will have a negative impact on the outlet dew point from two aspects:

　　1) According to the adsorption characteristics of the adsorbent, at low temperature, the adsorption capacity is large; at high temperature, the adsorption capacity is small.

　　2) The compressed air with high temperature also contains more saturated water. The compressed air entering the dryer is generally saturated. The higher the temperature, the more water content and the greater the load of the dryer, which will increase the dew point of the outlet air. 6. Adsorption cycle There are two kinds of adsorption cycles in common adsorption dryers. The non-heat regeneration dryer based on pressure swing adsorption belongs to the short cycle type, and all the heating regeneration adsorption dryers belong to the long cycle type. The shorter the adsorption time, the less water the adsorption bed can adsorb at one time, and the greater the adsorption rate of the adsorbent. Generally speaking, the shorter the adsorption period, the better the drying effect.

　　7. Residual water volume For adsorption dryers, adsorbent regeneration is an extremely important link. Ideally, the moisture inside the adsorbent bed must be resolved and taken away during the half cycle of regeneration. This process consumes enough regeneration capacity. If the regeneration capacity is insufficient, there will be residual water in the adsorption bed.

The residual water volume is equivalent to adding an additional adsorption capacity to the adsorbent. After a long time of use, the residual water will accumulate at the end of the adsorbent bed, causing this part of the adsorbent to lose the adsorption capacity in advance, which is equivalent to shortening the height of the adsorbent bed. The contact time between the compressed air and the adsorbent causes the dew point of the outlet to rise.

　　 8. Deterioration of adsorbent Deterioration of adsorbent refers to the weakening of adsorption performance due to fatigue or aging after long use. Specifically, the adsorption "transition point" drops sharply and the outlet dew point rises rapidly.