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1. Introduction Adsorption can perform many separations that are impossible or impractical by conventional techniques, such as distillation, absorption, and even membrane-based systems. Lately, applications for adsorption have expanded rapidly because of sharply rising environmental or quality requirements. Likewise, advances in adsorbent technology have made it possible to meet many of those demands. Recently developed adsorbents are now available “off-the-shelf,” and in most cases they can perform satisfactorily. Nevertheless, new adsorbents are constantly being synthesized that have dramatically improved properties which translate into better performance. A new adsorbent may take months or years to perfect, however, so a rule-of-thumb is that there is never enough time to develop a new adsorbent for an urgent new application. Going hand-in-hand with these advances, engineers and scientists have developed a better understanding of the mechanisms of adsorption. In fact, this has led to faster and more accurate simulations and designs of adsorption processes. For the past thirty years, it has been possible to solve the relevant equations numerically. Recently, that ability has been augmented by faster, more accessible machines, as well as models that are able to isolate the relevant effects without being bogged down by too many fitted parameters. This article presents the basic principles of adsorption, emphasizing practical aspects of adsorbent selection. After this introduction is a brief qualitative description of common adsorbents, followed by an overview of important, quantitative adsorbent characteristics, and finally some case studies illustrating the main ideas. There is not sufficient space here to review adsorption technology or adsorber design, although just as much could be said about them. For that matter, most of the topics discussed here are the subject of dozens of technical papers each year. Some subjects are treated in much greater detail by specialized books and/or journals, including adsorption itself, along with carbon, zeolites, reactive polymers, and others. Perhaps the best known applications of adsorption fall in the category of purification, e.g., municipal water treatment to remove traces of pollutants, as well as “taste” or “odor.” Another widespread application, although much smaller in terms of adsorbent consumption, is the pressure-swing air dryer found on semis for their air-brake systems. (Most of us have heard the abrupt, loud “blowdown” of one of these adsorbers and have wondered if one of our tires blew-out. Moments later we realize that instead it was “something” under the truck.) Adsorption is becoming more popular as a unit operation, as a means for separating fluid mixtures. For example, adsorption is used to recover very pure para-xylene from mixed isomers in a well known process called Parex®, offered by UOP. Likewise, pressure swing adsorption (PSA) is commonly used to split very pure hydrogen from refinery off-gases. There exist hundreds, if not thousands, of different applications of adsorption (Yang, 1987), but we are looking at adsorbent selection rather than process selection. Thus, the point of citing the above examples is not just to illustrate the diversity of operations. Rather, it is to highlight the vastly different priorities of adsorbent properties for those applications. All require a degree of effectiveness, usually thought of as high adsorption capacity, coupled with high selectivity. In many cases the adsorption rate and pressure drop are also important; hence, particle size is important. Besides those, nearly every different application has a different set of additional priorities. For example, the main prerequisite for municipal water purification is low cost. Fortunately, activated carbon offers both low cost and high effectiveness and no other adsorbents are close. The choice would be easy, except that there are many activated carbon manufacturers, and each of those typically offers several products. Conversely, there are many potential classes of desiccants for pressure swing air drying, including: activated alumina, silica gel, zeolite, anhydrous calcium sulfate, and even polymeric adsorbents. To be placed on-board a truck chassis, and undergo the vibration, temperature extremes, etc. that are experienced by cross-country rigs, however, adds a few additional constraints. Attrition resistance and protection from oil and exhaust gases are chief among them. We will continue this line of thought below, but in a broader and more objective way that applies to a wide variety of applications and adsorbent materials. 1PDF Image | ADSORBENT SELECTION
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