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                    230                             3. Heterogeneous Processes and Reactor  Analysis


                                                    Table 3.14

                                       Conversion from mesh to cm for particle radius
                                     Mesh                Particle radius (cm)
                                       5                    0.2
                                      10                   0.1
                                      16                   0.06
                                      20                   0.042
                                      30                   0.030
                                      40                   0.021
                                      50                   0.015
                                      60                   0.013
                                      70                   0.011
                                      80                   0.0089
                                      100                 0.0075
                                      120                 0.0063
                                      140                 0.0053
                                      170                 0.0044
                                      200                 0.0037
                                      230                 0.0031
                                      270                 0.0027
                                      325                 0.0022



                    through the pores depending on its size. Consequently a portion of the surface area may
                     ,
                      ,
                    v
                    not be aailable to reactants, or generally to the diffusing species. As discussed earlier ,
                    most of the solids used in catalysis, adsorption, and ion e which e xchange,  ge sur- xhibit lar
                    face areas, are porous. Porosity is a term used to describe the pore space of a material,
                    defined as the fraction of the bulk volume that is occupied by pore or void space.  oid is V
                    the space or interstices between particles.
                      The individual pores may vary greatly in both size and shape within a gi and en solid, v
                    between one solid and anotherAs presented in Chapter 2, the pores can be classified as
                     .
                    (a) macropores, for diameters abo(b) mesopores, for diameters in the range 2–50 e 50 nm, v
                    nm, and (c) micropores, for diameters below 2 nm. It has to be mentioned that in a solid,
                    a wide and continuous range of pore sizes is found, from macropores to micropores. A
                    ace pore can also be an open pore, seen as a ca vity or channel communicating with the surf
                    of the particle, or a closed pore that is not connected to the surf ace.
                      The total pore volume is the total internal volume per unit mass of catalyst. Some of this
                    pore volume may be completely enclosed and thus not available to the reaction molecules.
                    Moreover, the accessible pore volume changes with the size of the dif fusing molecules.
                      The pore-size distribution is the distribution of pore volume with respect to pore size
                    (Figure 3.66). It is an important factor controlling the diffusion of reactants and products in
                    the porous solid and thus an essential property for its characterization. The computation of
                    pore size distribution involves a number of assumptions, and therefore reporting of the data
                    should always be accompanied by an indication of the method used for its determination.
                      An interesting phenomenon about adsorption of gases on solids and ion exchange of ions
                    on resins is swelling. Some porous solids expand on exposure to the vapors of adsorptives.
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