Page 141 - Principles of Catalyst Development
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CATALYST  PREPARATION                                            129
            lengths by a knife  rotating outside the end plate or allowed to break up as
           it falls  onto a  moving belt on its  way to  the drier.
                Extrudates  are  irregular  in  shape  and  length,  have  higher  porosities
           and  lower  strengths  than  pellets,  but  are  less  expensive  to  make.  Many
            commercial  products are  now available as  extrudates.



           6.S.3.  Spheres

                Hydrogels that age rapidly, such as alumina, silica, and silica-alumina,
            are formed into spheres by a technique similar to the historical method for
            making musket balls. The hydrogel is forced through holes in a plate at the
           top of a column containing oil  in  which the gel  is  immiscible, as  shown in
            Fig. 6.27.  Assuming a spherical shape, the drop falls slowly through the oil,
           hardening as it ages. This process is accelerated by increa:~ing the pH down
           the column with flowing ammonia up the tower. At the bottom, the spheres
           are  separated, dried, calcined, and sieved.  Very  uniform spheres are  used
           in  moving and ebullating bedsy89J






                                                     ALUMINA
                                                      SLURRY

                                                     PERFORATE!)
                                                        PLATE
                                         o
                                           o         OIL
                                         o
                                           o
                                          o          HARDENING

                                                      SPHERES
                                         o
                                          o
                                         o
                                          o
                                         o
                          AMMONIA






                      Figure 6.27.  Production or spht'res  using  the column  method.
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