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2 ISRN Chemical Engineering V7 V5 V8 V6 12 V3 V4 V1 V2 Figure 1: Schematic design of the first two-column pressure swing adsorption unit and valve sequencing for different steps in the cycle [1]. C1 Feed Blow Purge Press V1, V7 V3 V5, V3 V1 C2 Purge Press Feed Blow V6, V4 V2 V2, V8 V4 the heavy component is accomplished quite fast and since the pressure of the system can be changed rapidly, the time between adsorption and regeneration is balanced. When the concentration is low, the adsorption step may take much longer and other options like temperature swing adsorption (TSA) can be considered [12]. The behaviour of the PSA unit is mainly determined by the adsorbent employed for the separation. However, the engineering of the PSA unit is also an important aspect. In fact, the main task of defining a PSA unit is to select correctly the adsorbent to be employed [103]. After that, all the engineering efforts should be placed in defining an effective strategy to regenerate the adsorbent. Thus, the advances obtained in PSA units can be divided in two main domains: the discovery of new adsorbents (material science) and new and more efficient ways to use and regenerate the adsorbent (engineering). This work provides an overview of PSA processes and its evolution on time. The most important industrial applications of PSA processes will be used to address its technological evolution: air separation and hydrogen purifi- cation. A growing market of PSA, CH4-CO2 separation, will also be used for some specific examples. Although it is not intended to describe the state-of-the-art of materials science, an example of the effect of different adsorbent materials in PSA operation will be provided. Finally, the effect of different regeneration protocols and the reduction of the overall cycle time (Rapid Pressure Swing Adsorption) are discussed. 2. Fundamentals of Pressure Swing Adsorption The essential feature of the PSA is that when the adsorbent is saturated, using a sequential valve arrangement, the feed is stopped and simultaneously the total pressure of the column is reduced. The reduction in pressure results in a partial desorption of all the species loaded in the column, “regenerating” the adsorbent. Since this process was patented after TSA, it was initially known as “heatless” process. The first patent application where PSA technology was described, was presented by Charles Skarstrom for oxygen enrichment [57]. A scheme of the two-column PSA introduced in that patent is shown in Figure 1. In order to operate such unit cyclically, a column experiences a series of “steps”: events like opening and closing valves and changing flowrate direction for example. The sum of all the steps is termed as “cycle”. Even when the process is unsteady, after some cycles it reaches a Cyclic Steady State, CSS. When CSS is achieved, the performance of the cycles of the PSA is constant over time. It should be noted that since this process sometimes involve substantial amount of heat generation, there can be multiple CSS [104]. The four steps of the “Skarstrom cycle” are also shown in Figure 1: feed, blowdown (or evacuation), purge and pressurization. In this cycle, in the feed step, air is fed to the first column (C1) at a pressure higher than atmospheric. The adsorbent initially used (zeolite 5A) is selective to nitrogen, making the exiting stream (after valve V7) richerPDF Image | Advances in Pressure Swing Adsorption for Gas Separation
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