Discussion on the feed direction of the adsorber

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In the PSA process, axial feeding is generally adopted for the adsorbers, which are usually vertical vessels. Theoretically, the feed to an adsorber seems to be able to flow in from the bottom and out from the top, or in from the top and out from the bottom; however, in practical engineering applications, feeding from the bottom with output from the top is generally chosen, and there are almost no cases of feeding from the top with output from the bottom. There is very little discussion on this issue (over the past few years I have spent a great deal of time collecting domestic and foreign literature on pressure swing adsorption technology, and it seems that only one article indirectly addresses this topic: \"Pulverization of Adsorbent in Large-Scale Pressure Swing Adsorption Auxiliary Columns\" by Zeng Hai from Sichuan Yalian High-Tech Co., Ltd., published in the journal \"China Nitrogen Fertilizer\" in 2014. This article is actually a record of an accident handling process, with little in-depth analysis of various details). In many documents, this topic is either avoided altogether or only superficially analyzed. There is relatively little material available on this topic for two main reasons: first, it is likely that this issue is considered to be established by convention and does not require further discussion ； Second, some companies, thinking they were being \"innovative\" by opting for top-fed systems, experienced accidents but were reluctant to make this public out of fear of embarrassment (from my personal experience, there is often competition and mutual slander among peers in the pressure swing adsorption industry). I worked at Sichuan Asia United for some time, yet I never heard anything about this accident that occurred in the company’s design department, which shows just how tight the information control was; P). Based on my own experience and combined with the chemical engineering theories I have learned, there are mainly two reasons for this: First, bottom feeding can reduce the force exerted on the adsorbent at the bottom, thereby preventing the adsorbent from crumbling. The operating pressure is high in the adsorption state, and there is a certain pressure difference across the bed, which results in a force exerted by the gas flow on the adsorbent. When feeding is done at the bottom of the gas flow, the upward force exerted by the gas flow on the adsorbent can counteract the pressure exerted by gravity on the lower layer of adsorbent from the upper layer, which helps to protect the lower layer of adsorbent and prevent it from breaking or becoming fragmented due to excessive pressure. However, when feeding is done at the top of the gas flow, the gas exerts a downward force on the adsorbent; this force, combined with the gravity of the adsorbent in the upper layers, acts on the adsorbent at the bottom, thereby accelerating the pulverization of the adsorbent in the lower layers. Second, bottom feeding avoids the turbulent disturbance effects of the free space at the top. Due to gravitational settling, a certain amount of free space will inevitably form at the top of the adsorber. When feeding from the top, especially during steps such as final charging, uniform rising, and reverse discharging, the pressure at the top of the tower is always lower than that at the bottom. Moreover, due to the presence of free space at the top, air currents can easily occur there, causing the adsorbent particles to move upward. This leads to collisions and friction between the adsorbent particles as well as between those particles and the walls of the tower, resulting in severe pulverization of the adsorbent at the top of the absorber. During bottom feeding, in steps such as final charging, uniform rising, and reverse discharging, the pressure at the top of the tower is relatively high, creating a downward \"compressing\" effect that helps to prevent the adsorbent at the tower top from shifting unpredictably. Based on the analysis of these two points, it is clear that bottom feeding is indeed superior to top feeding. Furthermore, the bottom feeding method also cannot avoid the problem of airflow disturbances in the upper space; such disturbances can occur during the pressure equalization and gas introduction processes. This is why many design firms emphasize the need to control the flow rate during pressure equalization – primarily to prevent excessive flow rates from causing disturbances in the adsorbent particles at the top of the tower, as well as turbulence caused by too high an inlet flow rate at the top of the tower. Both types of disturbances can lead to the pulverization of the adsorbent. This problem can also occur during normal playback, but under normal playback conditions the pressure is generally lower, the gas density is smaller, and the disturbances generated are also less severe.