Adsorption design methods

Fundamental Principles of Adsorption, Theory and Models Adsorption Design Methods and Data Adsorbent Materials... 

Design Methods. Improvements ia the ability to predict multicomponent equilibrium and mass-transfer rate performance will allow significant improvements ia the design of new adsorption systems and ia the energy efficiency of existing systems. 

Thermal swing adsorption (TSA) has been practiced with zeoHte molecular sieves for more than 50 years. In the following sections 1 discuss design methods and provide some reference data so that the design process can be understood. 

Length of Unused Bed. The constant pattern approximation provides the basis for a very useful and widely used design method based on the concept of the length of unused bed (LUB). In the design of a typical adsorption process the basic problem is to estimate the size of the absorber bed needed to remove a certain quantity of the adsorbable species from the feed stream, subject to a specified limit ((/) on the effluent concentration. The length of unused bed, which measures the capacity of the adsoibei which is lost as a result of the spread of the concentration profile, is defined by... 

Fundamental studies on the adsorption of supercritical fluids at the gas-solid interface are rarely cited in the supercritical fluid extraction literature. This is most unfortunate since equilibrium shifts induced by gas phase non-ideality in multiphase systems can rarely be totally attributed to solute solubility in the supercritical fluid phase. The partitioning of an adsorbed specie between the interface and gaseous phase can be governed by a complex array of molecular interactions which depend on the relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate association, the sorption of the supercritical fluid at the solid interface, and the solubility of the sorbate in the critical fluid. As we shall demonstrate, competitive adsorption between the sorbate and the supercritical fluid at the gas-solid interface is a significant mechanism which should be considered in the proper design of adsorption/desorption methods which incorporate dense gases as one of the active phases. 

Many adsorption design approaches assume that adsorption is occurring isothermally. This is a good assumption only when the adsorbable component concentration is low and/or the heat of adsorption is low. There are two simple methods that can be used to determine... 

W. C. Edmister Hydrocarbon Adsorption and Fractionation Process Design Methods—Part 18, Plate Efficiency, Petroleum Engineer, 21 C-45 (1949). 

The sorption of ethane, efoene and propane in silicalite-1 has been studied over foe temperature range 0-50 C. This study is part of a larger programme designed to establish foe preferred zeolite for foe s aration of specific pairs of lower saturated and imsaturated hydrocarixrns and their branched isomos by Pressure Swing Adsorption (PSA) methods. These studies are, of course, also of importance in foe development of a b er understanding of foe fundamentals of hydrocarbon/zeolite sorption interactions. 

Hirose, T. (1991) A simple design method of a new PSA process consisting of both rectifying and stripping sections. Proceedings of the 2nd China-Japan-USA Symposium on Adsorption. 

In the simplest type of adsorption processes in which an adsorption column is used to remove a trace impurity from a process stream, the main requirement for rational design is an estimate of the dynamic or breakthrough capacity of the bed. In such systems the adsorbable impurity is invariably strongly adsorbed with a favorable isotherm and the concentration profile therefore rapidly approaches constant-pattern form. The constant-pattern assumption provides the basis of a very simple design method which permits reliable scale-up from small-scale laboratory experiments. 

The rate of ion exchange depends on mass transfer of ions from the bulk solution to the particle surface, diffusion of the ions in the pores of the solid to the surface, exchange of the ions at the surface, and diffusion of the exchange ions back to the bulk solution. This is similar to adsorption. The differential equations derived are also very similar to those for adsorption. The design methods used-for ion exchange and adsorption are similar and are described in Section 12.3 for adsorption processes. 

There is a tendency for both axial and radial dispersion of mass to occur when a fluid flows through a packed bed. Since the bed diameter is normally far greater than the particle diameter in an adsorption bed, it is common not to have to consider the effects of radial dispersion. Hence the prevalence of plug and axially dispersed plug flow models in rigorous design methods. 

Adsorption The design of gas-adsorption equipment is in many ways analogous to the design of gas-absorption equipment, with a solid adsorbent replacing the liqiiid solvent (see Secs. 16 and 19). Similarity is evident in the material- and energy-balance equations as well as in the methods employed to determine the column height. The final choice, as one would expect, rests with the overall process economics. 

Graphical methods at best are simply illustrative for the student today, but they are occasionally referenced by the process engineer. Extraction, like distillation can be viewed as a stage-wise operation, and hence metliods based on the McCabe Thiele approach briefly described in Chapter 4 have been applied to preliminary design cases. Indeed, both absorption and adsorption are stage-wise operations. 

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