Adsorption process design particle size

Most adsorption separation processes depend on differences in adsorption equilibrium. In such processes the impact of mass transfer resistance is negative, so the design engineer seeks to minimize the effect by reducing particle size and crystal size and by appropriate choice of the operating condi-... 

In most adsorption processes the adsorbent is contacted by the fluid phase in a packed column. Such variables as the particle size, fluid velocity, and bed dimensions determine the pressure drop and have an important impact on the economics of the process since they determine the pumping cost as well as the extent of axial mixing and the heat transfer properties. The hydrodynamics of flow through packed beds have been extensively studied, and detailed accounts may be found in many chemical engineering textbooks. The present review is therefore limited to a brief summary of the principal features of the flow behavior which are important in the design of fixed bed absorbers. 

Separation procedures must be designed with kinetic considerations in mind -- adsorption from a stream of gas is not an instantaneous process. It was stated earlier that the adsorptive capacity of charcoals does not depend on particle size, but it is expected that rates of adsorption will be so dependent. A reduction in the average particle diameter increases the exterior surface (per gram adsorbent), presented to the gas phase and shortens paths through cracks and pores along which adsorbate molecules must diffuse in order to reach adsorption sites in the particle interior. Approach to equi-... 

The degree of bed expansion contributes to the efficiency of fluidised bed/expanded bed adsorption as a composite function of liquid distribution, liquid and particle properties (size, shape and density) and process conditions. Besides being an important design feature, the degree of bed expansion may be used as a quick and simple measure of bed stability.48... 

The necessity of forming zeolite powders into larger particles or other structures stems from a combination of pressure drop, reactor/adsorber design and mass transfer considerahons. For an adsorption or catalytic process to be productive, the molecules of interest need to diffuse to adsorption/catalytic sites as quickly as possible, while some trade-off may be necessary in cases of shape- or size-selective reactions. A schematic diagram of the principal resistances to mass transfer in a packed-bed zeolite adsorbent or catalyst system is shown in Figure 3.1 [69]. 

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