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is the solute diffusivity in the fluid, ,P is the particle void fraction, and J is its tortuousity), and for a sphere or cylinder the particle radius (or microcrystal radius), (for a sphere or cylinder shape), and time, t. The relevant grouping is: Deff t/2 . For example, the initial response (Deff t/2< 0.4), and final response (Deff t/>0.4) of a spherical particle to a sudden change of composition, respectively, are approximated by (8) where the C0 , Cf , and Ct represent the initial, final and instantaneous values of concentration averaged over the particle. From these approximations we can see that when Deff t/ is less than 0.001, not much has happened within the particle. Conversely, when it exceeds unity, what was going to happen is largely complete. Thus, when searching for an effective (fast) adsorbent, it is usually a safe bet to choose one having a large diffusivity or small diameter. Other concerns may overrule the selection of small particles, as mentioned later. 4. Adsorbent Selection Criteria: Case Studies We have reviewed the performance criteria, basic properties, and governing equations involved in adsorbent selection. Now, the sole remaining obligation is to answer the question, “how are these actually used?” That is easier said than done, since every application is different. By breaking the field into discrete parts, at least some generalizations can be made. We will focus on the criteria stated at the beginning: capacity, selectivity, regenerability, kinetics, compatibility, and cost. 4.1. Ordinary Adsorption This topic covers situations in which adsorption has “always” been used, for which no other unit operation is deemed suitable, or for which adsorption is the last resort. Examples are water or air purification of every sort, and clean-up of odoriferous or noxious contaminants. Typically, one would buy adsorbent, or in some cases a modular, pre-filled adsorber and install it. Assuming it solves the problem, it is forgotten until the problem is noticed again or until a certain amount of time elapses, when it is replaced. 1. dilute gaseous emissions Let’s take as our first example a process vent containing, say, 100 ppm (vol.) each of MEK, n-hexane, and toluene, at a flow rate of 1 to 10 cfm, and at 80°F. One immediately thinks of activated carbon, with one reservation: compatibility. Ketones in particular have been notorious for causing bed fires in activated carbon systems. Though some new activated carbons are resistant to spontaneous ignition, it is still a good idea to plan for Murphy’s law. Thus, even at this seemingly low concentration and flow rate, it would be prudent to install a deluge system to be interfaced with a CO monitor at the downstream end of the unit. To avoid that complication, other adsorbents might justify consideration, because they are much less likely to spontaneously ignite. Two types are polymeric adsorbents and silicalite. Due to space, we will go on the assumption that activated carbon is acceptable, and it is certainly the least expensive on a single-use basis. To estimate capacity, vendors can be contacted, you might use a database, you might arrange to conduct the measurements, or you might arrange to have tests conducted by an independent firm. At ARI we have an extensive database that is useful for rough estimates of this sort. In Table 4 are some loadings from that source, based on one coal-based (“A”) and one coconut shell-based (“B”) activated carbon. There is a cost differential of 1.5 for carbon “B” over carbon “A.” Taking a closer look at the loadings, it is easy to see why 18PDF Image | ADSORBENT SELECTION
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