Combining Rate and Equilibrium Concepts Packed Bed Adsorber

3 COMBINING RATE AND EQUILIBRIUM CONCEPTS (PACKED BED ADSORBER) 

The occurrence of a rate process and a thermodynamic equilibrium state is common in chemical engineering models. Thus, certain parts of a whole system may respond so quickly that, for practical purposes, local equilibrium may be 

To illustrate the combination of rate and equilibrium principles, we next consider a widely used separation method, which is inherently unsteady packed bed adsorption. We imagine a packed bed of finely granulated (porous) solid (e.g., charcoal) contacting a binary mixture, one component of which selectively adsorbs (physisorption) onto and within the solid material. The physical process of adsorption is so fast relative to other slow steps (diffusion within the solid particle), that in and near the solid particles, local equilibrium exists 

It is also possible to use the same model even when intraparticle diffusion is important (Rice 1982) by simply replacing the film coefficient with an effective coefficient. Thus, the model we derive can be made to have wide generality. 

We illustrate a sketch of the physical system in Fig. 1.2. It is clear in the sketch that we shall again use the plug flow concept, so the fluid velocity profile is flat. If the stream to be processed is dilute in the adsorbable species (adsorbate), then heat effects are usually ignorable, so isothermal conditions will be taken. Finally, if the particles of solid are small, the axial diffusion effects, which are Fickian-like, can be ignored and the main mode of transport in the mobile fluid phase is by convection. 

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Combining Rate and Equilibrium Concepts (Packed Bed Adsorber) 11... 

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