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Since this article deals with adsorbent selection, it is appropriate to comment briefly on the procedure, instead of just the technology. The first task is to define the purpose of the adsorbent or adsorption system. Until one does that, comparing performance specification or properties is a waste of time. To illustrate, recently I received a phone call from a fellow who loosely described an application. He then said, “We’ve tentatively chosen activated carbon, probably coconut shell-based, because its “standard” toluene isotherms are better than the others. He had mentioned odors and noxious fumes but nothing about aromatic hydrocarbons, so I naturally asked why that particular characteristic was so important. There was a long pause, after which we made progress. The point was that someone selling XYZ Adsorbent had convinced him that it was best because of its toluene adsorption isotherm, without explaining the correlation of that property to his application. It goes without saying that most people who sell adsorbents or equipment, do not make a living by offering unbiased advice. Generally, if they do not leave you with the impression that their products are superior, they have not done their job well. So, the goal of this article is to help you understand what is important, and how to express it, without being swayed by sales tactics alone. The opposite extreme is not much better, which is to assume that any class of adsorbent is a likely match, and that within any class there are several candidates offered by each manufacturer. This approach can lead to a sea of data, so that the initial goal gets lost. It would, however, lead to full employment for laboratory technicians and those skilled at spreadsheets for data analysis. When discussing process alternatives in this article, it is presumed that regeneration in-place is desired. These days it is becoming rare, for several economic, environmental and technical reasons, to propose a system in which the adsorbent is discarded, e.g., in a landfill, after one use. Off-site regeneration or reactivation is still common for activated carbon, however. 2. Adsorbents: Classes, Sources, Properties and Applications This section explains the general characteristics of adsorbents, and why they are important. The following parts deal with classes of adsorbents and several common materials are mentioned, along with a few manufacturers and some typical applications. No attempt has been made to be exhaustive in any lists. Many obvious examples have undoubtedly been omitted. The most important attributes of an adsorbent for any application are: capacity, selectivity, regenerability, kinetics, compatibility, and cost. Rarely will a single adsorbent be optimal in all these respects. Frequently it will be possible to narrow the choice to one or two classes of adsorbents, but that still commonly leaves a vast array of possibilities. To evaluate capacity, selectivity, regenerability, kinetics, compatibility, and cost, a number of different approaches can be taken. First, vendors can be contacted, and if the compound is relatively common, they may be able to provide information quickly. Otherwise, especially for a relatively large application, they may be willing to do measurements. Second, you might use a database, such as one that was mentioned earlier in this article. Third, you might arrange to conduct the measurements, either yourself or by someone else in yourfirm. Fourth,youmightarrangetohavetestsconductedbyanindependentfirm,sincetheycouldoffer an unbiased and cost efficient assessment. Adsorption capacity (or “loading”) is the most important characteristic of an adsorbent. Simply stated, it is the amount of adsorbate taken up by the adsorbent, per unit mass (or volume) of the adsorbent. It depends on the fluid-phase concentration, the temperature, and other conditions (especially the initial condition of the adsorbent). Typically, adsorption capacity data are gathered at a fixed temperature and various adsorbate concentrations (or partial pressures for a vapor or gas), and the data are plotted as an isotherm (loading versus concentration at constant temperature). Adsorption capacity is of paramount importance to the capital cost because it dictates the amount of adsorbent required, which also fixes the volume of the adsorber vessels, and both generally are significant if not dominant. 2PDF Image | ADSORBENT SELECTION
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