Page 34 - Principles of Catalyst Development
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20                                                       CHAPTER  I
                                     PELLET  RATE,  moles  9- 1  hr- I
                   10 4   ..
                       'Q   on  'Q   .  '"  'Q
                                '0
                   10 3
                    2          Chemica I
                   10
                                 Rate
                               Controlled
                   10
               e
             P[)2
              T   p


                    -2
                   10




                        -6   -~   -4   -3   -2   -I                    4   ~
                       10   10   10   10   10   10      10            10   10
                                     SURFACE  RATE,  moles  9- 1  hr- I
                     Figure  1.8.  Interaction between physical  and chemical  rate  regimes.

                If the surface rate is very low, say 10- moles g -I hr--\ and the diffusion
                                                 6
            index high,  > 1,  then  TJ  =  1 and the surface rate controls.  As  the chemical
            rate increases the curves show decreasing effectiveness at increasingly higher
            diffusion  indices.  Control  passes  to  internal  diffusion  and  becomes  more
            severe the greater the surface rate. Finally, the surface rate becomes so large
            that the pellet rate is  determined only by diffusion  parameters. This  is  the
            regime  for  control  by  external  diffusion.  Thus  it  appears  that  making  a
            catalyst more active reaches a  point of diminishing returns.
                Let  us  demonstrate  Fig.  1.8  with  an  example.  A  process  for  steam
            reforming naphtha is  carried out at H 20/HC = 10, 500°C, and 40 atm total
            pressure. A Nil Al 20 3  catalyst, diameter 4 mm, (J  =  0.6, was tested in a pilot
            unit and gave 75%  conversion at a GHSV of 5000 hr-I. For pressure drop
            reasons,  the  process  designer wants to  double the  size  of the  particle  but
           keep all  conditions the same.  Will  this change the conversion?
                The easiest parameter available is  the diffusion index


                           _(J_ =    (0.6)   =  9.38 x  10-2  =  10- 1   ( 1.30)
                           PTD~  (40)(0.04f

                Unfortunately, the surface rate is not available. This is  usually the case
           unless extensive research  and development data are known.  Working with
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