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Selectivity is related to capacity, but there are several distinct definitions that will be discussed later in this article. The simplest is the ratio of the capacity of one component to that of another at a given fluid concentration. That ratio generally approaches a constant value as concentration drops towards zero. Of course, the concentrations of interest may not be near zero, so the choice of definitions becomes subtle. The closest analogy is to relative volatility (e.g., in distillation) in that the smaller the value, the larger the required equipment. An ideal situation occurs when the major component is not adsorbed much (so it can be thought of as an inert “carrier”), which leads to a very large selectivity. Some people prefer a bounded selectivity (i.e., 0 to 1), and they employ the inverse of the ratio mentioned above. Thus, it is a good idea to clarify the definition first, or to speak of “good” or “bad” instead of “large” or “small.” All cyclic adsorption applications rely on regenerability, so that the adsorbent can operate in sequential cycles with uniform performance. This means each adsorbable component (adsorptive or adsorbate) must be relatively weakly adsorbed (or physisorbed). The heat of adsorption, which is mentioned later, provides a measure of the energy required for regeneration, and in that regard low values are desirable. Regeneration might be accomplished by a thermal swing, pressure swing, chemical (e.g., by displacement, elution, or supercritical extraction), or sometimes by a combination of those. Displacement would involve introducing a species that adsorbs more strongly than the adsorbate of interest, while elution would entail dissolving the adsorbed material by a solvent that is weakly adsorbed if at all. The chemical methods all require a separate separation operation that may be costly, plus a means must be found for purging the bed of the regenerant. In some cases, regeneration takes place by contacting the adsorbent with a fluid in another phase than is used during loading. This requires draining or displacement which might be time-consuming, so it is avoided whenever possible. The regenerability of an adsorbent affects the fraction of the original capacity that is retained (sometimes called the working capacity), and the time, energy, etc. required for regeneration. Frequently, a short-term loss of working capacity occurs during the first few cycles, often followed by a gradual decay, perhaps over hundreds of cycles, e.g., due to ageing, poisoning, or other causes unrelated to regeneration, that essentially governs the life of the adsorbent. Mass transfer kinetics is a catch-all term related to intraparticle mass transfer resistance. It is important because it controls the cycle time of a fixed bed adsorption process. Fast kinetics provides a sharp breakthrough curve, while slow kinetics yields a distended breakthrough curve. The effect of a distended breakthrough curve can be overcome by adding adsorbent at the product end, or by increasing the cycle time (which reduces the throughput per unit of adsorbent). Both of these options affect the amount of adsorbent required in that the longer the cycle time, the greater the adsorbent inventory. Despite that, kinetics has even been exploited as the basis of adsorptive separations. Perhaps the most common example is the pressure swing adsorption process that splits nitrogen from air using carbon molecular sieve, which relies on fast diffusion of oxygen compared with very slow diffusion of nitrogen. Normally, however, slow diffusion of any adsorbate is a disadvantage. To compensate for slow diffusion, it is also possible to use small particles, but there is a corresponding sacrifice due to increased pressure drop. The common solution to that dilemma is to use relatively large particles and to employ an extra increment of adsorbent. Compatibility covers various possible modes of chemical and physical attack that could reduce the life expectancy of the adsorbent, such as biological fouling or attrition. For example, the adsorbent, binder, and surface groups (depending on the type of adsorbent), should be inert to the carrier or solvent, and should not irreversibly react with (or chemisorb) the adsorbate(s) or contaminants. Likewise, operating conditions such as velocity, temperature, pressure, and vibration should not cause undue disintegration of the adsorbent particles. This could happen by crushing or abrasion, and there are standard methods for measuring those. Cost is perhaps the most subtle characteristic to understand because it may vary from week to week, and from sales rep to sales rep, even for the same exact material. Prices range from $0.30 per pound to $50 per pound for materials that are not particularly exotic. 5PDF Image | ADSORBENT SELECTION
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