Page 38 - Principles of Catalyst Development
P. 38

24                                                       CHAPTER  2

                        FLUID  FLOW                  HIGH  ACTIVITY
                     Flow  distribution               Chemical  activity
                     Low  pressure  drop              High  specific
                     Mechanical                          active  surface
                        strength                      Porous  pellet




                                          PELLET



                                      LONG  LIFE

                                       STABILITY
                                     Resistent  to
                                        Sintering
                                        POisoning
                                        Fouling

                            Figure  2.1.  Engineering  features  of the  catalyst.
            catalyst for hydrotreating, CoMo/ AI 20], is found in a wide variety of shapes
            and  sizes.  Pellets,  extrudates,  and  spheres,  1-10 mm,  are  all  available,
            depending  on  whether  the  feedstock  is  light  naphtha,  gas  oil,  or vacuum
            residua. (161
                High activity and selectivity is engineered by selecting correct chemical
           components, using preparational methods to give the required surface area,
           and formulating the  pellet to  ensure good access to  active sites.  Achieving
           activity alone is  not sufficient unless balanced with these other requirements.
                Lifetime  stability  requires  resistance  to  deactivation  due  to  sintering
            (loss  of active  surface  through  crystallite  growth),  poisoning  (elimination
           of active sites by strong chemisorption), and fouling (blockage of the pores
           and  surface  by  carbon  and  debris).  Although  also  sensitive  to  process
           conditions,  stability  is  best  achieved  through  addition  of  catalyst  com-
           ponents.
                An interesting example that demonstrates similar chemistry for different
           process  conditions is  found  in  the  reversible  reaction

                                                                          (2.1 )
                Both forward and reverse reactions are catalyzed by the same material,
           i.e.,  nickel.'171  However, the state of the nickel  is  profoundly influenced by
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