Adsorbed isotherms

The 2eohte and the resin adsorbents show different adsorption isotherm characteristics, particularly at higher concentration (51). The resin adsorbent isotherm is slightly concave upward, whereas the 2eohte isotherm is linear, or even slightly concave downward. Resins, therefore, have an advantage in a UOP Sarex operation that involves high feed-soHds concentration. 

The enthalpy of the adsorbed phase ha is evaluated along a path for which the gas-phase components undergo temperature change from r,e( to T and then are adsorbed isothermally, giving... 

The equilibrium adsorption characteristics of gas or vapor on a solid resemble in many ways the equilibrium solubility of a gas in a liquid. Adsorption equilibrium data are usually portrayed by isotherms lines of constant temperature on a plot of adsorbate equilibrium partial pressure versus adsorbent loading in mass of adsorbate per mass of adsorbent. Isotherms take many shapes, including concave upward and downward, and S-curves. Equilibrium data for a given adsorbate-adsorbent system cannot generally be extrapolated to other systems with any degree of accuracy. 

Not all adsorption beds will develop stable MTZs. One requirement for stability (i.e., the MTZ reaches a limiting size) is that the equilibrium line must be favorable. In the case of a single adsorbate isothermally removed from a non-adsorbable component, the curve of loading as a function of composition must be concave downward in the region of loading below the stoichiometric point to be favorable. This effect is described in more detail in Section 7.9. In non-isothermal adsorption it is possible for the temperature effects to cause a favorable isotherm to become an unfavorable equilibrium line. This was discussed previously in the context of the crossover ratio R. 

Except for the two cases mentioned, the predictions from the pure adsorbate isotherms would be, for most practical purposes acceptable. The advantage of being able to predict the phase diagrams from the pure isotherms is that if one wishes to do a screening study the number of isotherms for n adsorbents is n, whereas for the various combinations it is n(n-l) which for a large number of adsorbents could be considerably more work. There are several other sets of experimental data available in the literature. For most, the % formulation works quite well. 

Molar integral heat 2int = 1 mol Integral heat per mole adsorbed Isotherm calorimetry... 

Furthermore, it proved to be a typically hydrophobic adsorbent. The adsorption isotherms of organic compounds showed the rectangular shape with saturation at very low pressures (see Fig. 8) typical of the adsorption on high-alumi-na zeolites. In contrast, ammonia and especially water are much less or practically not adsorbed (isotherms (d) and (e) in Fig. 8). [See the striking difference in the adsorption of water on Na-Y (dotted isotherm e) and dealiuninated Y (continuously traced isotherm e Fig. 8).] This distinctly hydrophobic adsorption behavior of highly dealuminated faujasite indicates that the framework must be virtually free of lattice vacancies necessarily associated with hydrophilic internal silanol groups. 

Figure 8. Adsorption isotherms of four model systems at T = 300 K. Full lines and circles, adsorbate/substrate isotherms dashed lines and diamonds, adsorbate/adsorbate isotherms.

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