Sorbent Selection Equilibrium Isotherms, Diffusion, Cyclic

SORBENT SELECTION EQUILIBRIUM ISOTHERMS, DIFFUSION, CYCLIC PROCESSES, AND SORBENT SELECTION 

The selection of a proper sorbent for a given separation is a complex problem. The predominant scientific basis for sorbent selection is the equilibrium isotherm. Diffusion rate is generally secondary in importance. The equilibrium isotherms of all constituents in the gas mixture, in the pressure and temperature range of operation, must be considered. As a first and oversimplified approximation, the pure-gas isotherms may be considered additive to yield the adsorption from a mixture. Models and theories for calculating mixed gas adsorption (Yang, 1987) should be used to provide better estimates for equilibrium adsorption. Based on the isotherms, the following factors that are important to the design of the separation process can be estimated  

Capacity of the sorbent, in the operating temperature and pressure range. 

The method of sorbent regeneration—for example, temperature or pressure swing—and the magnitude of the required swing. 

The LUB is approximately one-half the span of the concentration wavefront, or the mass transfer zone. The LUB is primarily determined by the equilibrium isotherm (Yang, 1987). A sharp concentration front, or a short LUB, is desired because it results in a high sorbent productivity as well as a high product purity. 

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Chapter 2 provides a simple formula for calculating the basic forces or potentials for adsorption. Thus, one can compare the adsorption potentials of two different molecules on the same site, or that of the same molecule on two different sites. The calculation of pore size distribution from a single adsorption isotherm is shown in Chapter 4. The effects of pore size and shape on adsorption are discussed in both Chapters 2 and 4. Chapter 3 aims to provide rules for sorbent selection. Sorbent selection is a complex problem because it also depends on the adsorption cycle and the form of sorbent (e.g., granules, powder, or monolith) that are to be used. The attributes sought in a sorbent are capacity, selectivity, regenerability, kinetics, and cost. Hence, Chapter 3 also includes a summary of equilibrium isotherms, diffusion steps, and cyclic processes. Simple sorbent selection criteria are also presented. 

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