Page 33 - Principles of Catalyst Development
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CATALYTIC  FUNCTIONS                                             19
            1.5.  DIAGNOSIS  OF  CATALYTIC  FUNCTION


                In  order  to  understand  and  modify  the  functions  of a  catalyst  in  a
            process,  it  is  necessary  to  determine  whether or  not  rates  are  determined
            by physical or chemical steps. Responses to process parameters and catalyst
            adjustments  are  different  for  the  two  regimes.  Diffusional  resistance,  in
            particular,  causes  unexpected  complications.  We  have  seen  how  low
            effectiveness  factors  decrease  conversion  and  disguise  kinetics,  but selec-
            tivity  also can be decreased.(32)  In  addition, poisoning of pore mouth sites
            in  conjunction  with  low  diffusion  results  in  a  much  more  rapid  activity
            decline than otherwise.
                Problems  with  diffusion-controlled  processes  are  eased  by  changing
           catalyst properties such as pore size distribution and particle size or process
           conditions such as  linear velocity. Chemical difficulties are treated through
           modification of the  active components of the catalyst.
                The  remainder  of this  chapter  deals  with  determining  the  dominant
            regime,  physical or chemical,  prevailing under a given set of conditions.


            1.5.1.  Interactions between  Regimes
              Figure  1.8  was  designed  to  demonstrate  interactions  between  physical
           and chemical rates and to serve as  a "first pass" for a process diagnosis.  It
           is calculated for vapor-phase conditions in the temperature range 300-500°C,
           molecular masses from 2 to 500, and "average" values of D eff • As a teaching
           tool it serves its purpose without further justification; as a diagnostic device
           it  is  useful  only  within  these  approximations.  However,  errors  due  to
           deviations  from  these  assumptions  are  not  large  considering  the  range  of
           values. An order of magnitude error in the regime of strong internal diffusion
           is  not  serious  if  the  objective  is  to  decide  whether  further  checking  is
           necessary.
                Procedures  outlined  in  previous  sections  were  used  to  compute  the
           curves in  Fig.  1.8.  Three variables are shown. The abscissa is  the chemical
           rate  applicable  if all  the  surface  is  available.  It is  the chemical  rate  built
           into the catalyst. The ordinate represents values of an index that combines
           all  diffusional  tendencies  of the  particle.  Moving  in the direction of more
           diffusion  control  decreases  this  index.  For  example,  higher  porosity
           increases  Deff  (providing  it  is  not  accomplished  by  decreasing  the  pore
           radius), increasing total pressure decreases DB  (and thus D eff ), and increas-
           ing the  pelIet diameter,  D p ,  lowers  both the effectiveness  factor  and  mass
           transfer  coefficient.  Parameters  on  the  curves  are  pellet  rates.  The  ratio
           between  the  pellet rate  and the  intrinsic chemical  rate  is  the effectiveness
           factor.
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