Page 140 - Principles of Catalyst Development
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128                                                      CHAPTER  6
            cylindrical cavity as shown and a piston applies prescribed pressures. Values
            from 10 to 4 X  103 atmospheres are common, depending upon the compress-
                  2
            ibility properties of the powder. Grains must be deformable since boundaries
            flow  together to  produce  the  finished  pellet.  Factors  such  as  the  ultimate
            tensile  strength  of the  materials,  mesoporosity  of the grains, and  moisture
            content  are  important.  Some  materials,  such  as  kieselguhr,  pellet  easily,
            others require assistance from the addition of small amounts of plasticizers
            and lubricants. Graphite, talc, and stearic acid are used for this purpose. IJS)
                The pellet is  ejected after compression.  It is very uniform in shape and
            dimensions,  has  high  mechanical  strength  and  moderate  porosity.  Fibers
            are sometimes added, for example, polymers, that are burned off to increase
            macro porosity,  and  metals,  to  improve  thermal  conductivity  in  or  out  of
            the  pellet.
                Pelletization is the most expensive method. For continuous production,
            complicated  disk-type  rotating  cylinders  with  staggered  pistons  are  used.
           Complexity of these movements, stress on metal parts due to high pressures,
           and  presence  of abrasive  powders  all  contribute  to  high  equipment  and
           maintenance  costs.  For  a  given  throughput,  larger  pellets  are  generally
            cheaper to  make.


           6.8.2.  Extrudates

                Figure 6.26 shows the formation of extrudates. A slurry of the catalyst
           powder is  fed  from  a  hopper  into  the screw  drive.  Peptizing  agents,  such
           as  nitric  acid,  may  be  added  to  deagglomerate  the  primary  particles  by
           lowering the zeta potential and improve the extrusion process. (I RR) The screw
           forces  the  slurry  through  holes  in  the  end  plate.  Usually  circular in  cross
           section, these holes can also be made in  the shape of lobes or stars. As  the
           ribbon  of  slurry  emerges  from  the  hole,  it  begins  to  dry  and  harden
           sufficiently  to  maintain  its  shape.  The  ribbon  is  either cut  into  prescribed


                               SLURRY




                                                        EXTRUDATE






                               Figure 6.26.  Production  of extrudates.
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