Page 25 - Principles of Catalyst Development
P. 25

CATALYTIC  FUNCTIONS                                              11
                                TABLE 1.3.  Texture Parameters

                  Parameter             Nomenclature           Units

              Surface area             Sg                  m 2 g- 1
              Pore volume              Vg                  cm] g-I
              Pellet density           d p                g(cm 3  of pellet)-'
              Porosity                 8  =  V.dl'
              Average  pore radius     r,  =  (2 V.I Sg)  x  10 3   nm
              Pore size distribution   PSD


                During  formulation,  microparticles  are  compressed,  and grain  boun-
           daries  flow  together to  produce rigid, strong particles.  Interstices between
           the  microparticles  create  a  macropore  structure  with  radii  greater  than
           1.5 nm. Measurement of pore size distribution shows typical bimodal shapes.
           Most  of the  surface  area  is  found  in  the  mesopores,  with  the  combined
           meso- and macropores amounting to approximately 95%  of the total area.
           Important terms  and units  used to characterize texture are given  in Table
           1.3.  Measurement of these  parameters is  discussed  in  detail  in  Chapter 7.


           1.4.  STEPS  IN  THE  CATALYTIC  REACTION

               The catalytic mechanism  extends beyond the  surface to  involve  phy-
           sical diffusion to and inside the particle. Combining these leads to the steps
           shown in  Fig.  1.5.

           1.4.4.  External Diffusion

               First  the  reacting  molecule,  A,  diffuses  to  the external  surface  of the
           particle.  Motion of A through the fluid  outside the particle is  governed by
           external  or bulk diffusion.  The  reader should consult standard  references
           for additional discussion.(2l)  Useful correlations have been found between
           the  mass  transfer  factor,  jD,  and  the  dimensionless  particle  Reynolds
           number:
                                                50 <  N Re  <  1000       ( 1.7)

           where  N Re  =  (2R)G/ /-t,  R  is the particle radius (em), G  is the linear mass
           velocity  (g S-I), and  /-t  is  the viscosity  (poise).
               The mass transfer factor is  given by

                                     .    k  gP (   /-t  )2/3
                                     JD  =- --                            (1.8)
                                          G  pDB
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