Page 114 - Principles of Catalyst Development
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102                                                      CHAPTER  6
                Promoters, if needed, may be added during the gelation period. Alterna-
            tively,  they  may he deposited,  using the  same  methods  described  later for
            active  components.



            6.2.4.  Washing and  Filtering
                The  next  two  steps,  washing  and  filtering,  are  sometimes  reversed.  A
            satisfactory,  but  time-consuming,  method  is  to  wash  by  decantation.  The
            hydrogel  is  added  to  a  large  volume  of distilled  water  in  a  conveniently
            sized  container  and  the  suspension  thoroughly  mixed.  Upon  standing,
            particles settle slowly,  desorbing foreign  ions  as  they fall.  When  a  definite
            interface  is  visible,  water  is  removed  by  decantation  and  the  process
            repeated.  The  purified  gel  takes  longer to  settle  with  each  washing,  since
            removal  of counterions  increases  the  zeta  potential.  The gel  partly  reverts
            back  to  a  sol,  a  process  known  as  peptization.  Care  must  be  taken  not  to
            wash  too  much  or settling  times  become  too  great.  A  good  method  is  to
            check impurity levels  in  the decantation  water during the operation. When
            washing  is  complete, the  hydrogel  is  filtered.
                If filtering  is  done first,  the  filtrate  is  washed  with successive amounts
            of water  until  a  satisfactory  level  of impurity  in  the  wash  water is  found.
            Again,  care  must  be  taken  not  to  wash  too  much  or the  gel  peptizes  and
            passes  through  the  filter  paper.




            6.2.5.  Drying
                Drying  is  necessary  IJ1  order  to  remove  the  large  volume  of water  in
            the hydrogel. Some collapse of the structure is  to be expected, as shown in
            Fig.  6.7,  but  care  must  be  taken  to  properly  control  drying  operations  if
            high  porosity is  desired.
                Initially,  drying  occurs  through  evaporation  of  moisture  from  the
            outside surface of the  hydrogel.  The  rate of water loss  is  constant and  the
            mass transfer controlled  by temperature,  relative  humidity, flow  rate of air
            over the surface, and size of the filtrate. This process continues until moisture
            content drops to about 50%. The filtrate mass now begins to shrink as most
            of the  external  water disappears.  The  material  is  now  called  a  xerogel.
                Continued moisture loss occurs with a declining rate, in  which evapor-
            ation  is  controlled  by  capillary  forces.  The  saturation  point  decreases  as
            pores become smaller and evaporation slows until water is forced into larger
            pores  by  concentration  gradients.  If  evaporation  occurs  but  removal  of
           moisture is  blocked by smaller pores, then large internal pressures of steam
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