Page 122 - Principles of Catalyst Development
P. 122

110                                                      CHAPTER  6







                            DEHYDRATED
                              PELLETS   SALT  SOLUTION   ALKALI  SOLUTION
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                      Figure 6.12.  Steps in  the  precipitation of the active  component.

            Sometimes cooling,  which lowers nucleation and growth rates, is  effective.
            At other times, boiling the solution to introduce turbulence seems to work.
            Usually,  mixing  is  no  problem with  laboratory devices.  Dilute alkali  may
            be added, drop by drop, with rapid agitation to disperse the droplets before
            local concentrations become excessive. Alternatively, both the salt and alkali
            solution  may  be  added  simultaneously  in  a  controlled  manner to  a  well-
            mixed container of water. Mixing is more difficult in commercial operations.
            Vessels  are  larger and  mixing of reagents  takes longer.  This  is  one  of the
            many perplexing problems faced  during scale-up.  (35)
                One effective  solution to  control  uniform  precipitation  is  to  use  urea
            rather  than  conventional alkalis.  Urea dissolves  in  water  but decomposes
            very  slowly  above  90°C.  Appropriate  amounts  of urea  are  added  to  the
            metal salt-support slurry and the mixture heated with stirring. At 90°C, urea
            hydrolyzes and OH groups are formed uniformly throughout the vessel and
           in  the  pores.  Precipitation  takes  place  homogeneously  over  the  surface.
           Since hydrolysis is slow and precipitation rapid, OH groups are consumed
           as  fast  as they are formed and the pH of the solution remains  unchanged.
           Although  requiring longer times, this technique yields  very uniform prod-
           ucts.  Loading is  controlled by the time of reaction. Scale-up also poses no
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