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When column performance is analyzed, some engineers feel it is acceptable to use any isotherm that fits the general trend of the data. We have found, in a separate study, that sometimes the precise shape of the isotherm is important, as well as heat effects, mass transfer, and other phenomena. In fact, for the isotherm data just mentioned, only the BDDT isotherm provides reasonable predictions. Actually, the Dubinin- Stoeckli isotherm fit as well and inherently accounts for temperature effects. 3.3. Mixture Equilibria There exist five common means of dealing with mixtures, rather than single adsorbable species. First, easiest, but sometimes disastrously wrong is to pretend that the mixture consists only of the major adsorbable component. The second approach, treating them as independent, is useful and accurate when a nonadsorbing carrier contains very dilute contaminants, and is very easy. Only pure component isotherms are required. Next is a method developed by Tien and co-workers called species grouping. The idea is to deal with a mixture of, say, ten components by identifying two or three (sometimes fictitious) components to represent the entire set. That reduces the complexity, saves time and money, and is fairly accurate if all that is desired is a “ballpark” answer. It requires some pure component isotherm data, in order to know how to group the species. The fourth method is to use one of several empirical isotherm equations which account for “competitive” adsorption of the relevant components. This method requires both pure component and mixture isotherm data. Depending on which equation is selected, the data analysis and fitting are more involved than for pure components, but not enormously so. When luck prevails, the results are compact and relatively simple to use for design or simulation. Examples of the equations are listed in Table 3. Table 3. Multicomponent Isotherm Equations Equation Form ni*=KinMiCi ni*=Ki nMi Ci /(1+3Kj Cj) ni*=(Ki nMi Ci/0i)/[1+3(Kj nMj Cj /0j)] n*=n(KCAi)/[1+3(KC)Aj] iMiiijj ni*=nMi KiCi(3KjCj)Ai-1/[1+3(KjCj)]Ai n*= A C /[1+B C Dii CEi -Di i 3(A C)Di j ] Fourth, is more a field than a concise method, since it embodies so many methods, and they are all grouped together as “adsorbed mixture forms.” Basically, this involves treating the adsorbed mixture (which Name a. Henry’s Law: b. Markham-Benton: c. Schay: d. Yon-Turnock: e. Sips-Yu-Neretnieks: f. Redlich-Peterson-Seidel: (Cref=1 [=] Ci) iii irefi ijj 16PDF Image | ADSORBENT SELECTION
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