Page 23 - Principles of Catalyst Development
P. 23

CATALYTIC  FUNCTIONS                                              9
                             TABLE 1.2.  Common Catalyst  Particles

                     Type                Characteristics

                   Pellets      Made in  high-pressure  press
                               Shape:  cylindrical, very uniform,  rings
                               Size:   2-10 mm diam
                                Use:   packed, tubular reactors
                   Extrudates   Squeezed through holes
                               Shape:  irregular lengths
                                     circular,  star or lobe cross section
                               Use:   packed, tubular reactors,
                                     ebulating beds
                   Spheres      Made by aging  liquid drops
                               Size:   1-20 mm
                               Use:   packed tubular reactors,
                                     moving beds
                   Granules     Fusing and crushing,  particle granulation
                               Size:   8-14 to  2-4 mesh
                               Use:   packed tubular reactors
                   Flakes       Powder encapsulated in  wax
                               Use:   liquid phase  reactors
                   Powders     Spray-dried hydrogels
                               Size:   <100""m
                               Use:   fluidized  reactors,
                                     slurry  reactors


            dramatic result. It would appear that the larger the particles the better. This
            is  true if diffusion is  not a problem. If it  is  a problem, larger particles lead
           to lower conversions,(21)  so  some compromise is  necessary.
                Another parameter of concern to the process designer is the mechanical
            crushing strength of the particle.  If the particle fractures  under the weight
            of the bed or the force of the fluid passing through it, then small fines lodge
           in  interstices  between  larger  particles,  causing  plugging,  uneven  flow,  hot
           spots, and pressure drop.  Fortunately, crushing strength is  fairly  indepen-
           dent of particle size.(22)  Other factors  during  preparation and formulation
           are,  however,  critical. These are treated in  Chapter 6.
                We now turn to the question of texture within the particle, i.e., surface
           area,  pore  shape,  and  size  distribution.  Particles  are  formulated  by
           agglomerating  microparticles  produced  during  a  precipitation  phase,  as
           shown in Fig. 1.4. Approximately 100 Il-m in size, these microparticles consist
           of a complex porous solid. Pores typically range from 1.5 to 15 nm in radius.
           Formerly  termed  micropores,  these  channels  are  now  called  mesopores.
           The name micropores is reserved for those less than 1.5 nm in radius, usually
           found  in zeolites.
   18   19   20   21   22   23   24   25   26   27   28