Solid catalyzed reaction
Characteristics of catalyzed reaction
A catalyst remains unchanged in its amount and chemical
composition at the end of reaction.
A small amount of a catalyst is often sufficient to catalyze a
A catalyst does
3. Chemical reaction controls the rate
Since the progress of the reaction is unaffected by the presence of any ash layer, the rate is proportional
to the available surface of unreacted Core.
4 B rc
1. Diffusion through gas film controls the rate
A(g) + bB(s) Product
Whenever the resistance of the gas film controls the reaction rate,
no gaseous reactant is present at the particle surface. Hence, the
concentration driving force (CAg - CAs) becomes CAg
Experimental methods for finding rates
Any type of reactor with known contacting pattern may
be used to explore the kinetics of catalytic reactions.
Any flow pattern can be used as long as the selected
pattern is known; if it is not known then the kinet
A variety of different solid catalysts used in industry.
Front row - consist of porous substrate material coated or impregnated with
Back row - fine powders with large surface areas
Dr S K Behera, GMRIT Rajam
Prof. Dr.-Ing. Gunther C. Stehr
Associate Professor of Mechanical Engineering
GMIT German Mongolian Institute for Resources and Technology
Lecture Materials Science for Engineers
based on books & materials of
Shackelford, J. F.: Materials Science for Eng
2. Diffusion through ash layer controls the rate
The rate of reaction of A is directly proportional to the rate of diffusion of A to the reaction surface.
No gas film resistance. So, CAg = CAs
Assumption: Rate of disappearance of A = Rate of diffusion o
SCM, when particle size remains constant
Limitations for SCM approach
Not applicable if
Chemical reaction is very slow and particle is porous
There are no clear reaction interface boundary
(intermediate case between SCM vs. PCM)
Fast reaction i
Special characteristics of heterogeneous
The overall rate expressions include mass transfer
terms in addition to reaction kinetics.
Contact surface and its dynamics between the fluid
and solid surface are e
Components of a Typical Heterogeneous
Components of a typical
A. Active phase - metal that provides active sites where the
chemical reaction takes place
Support or Carrier - high surface area oxide which
disperses and st
Deactivation of catalysts
Continuous decrease in the activity of a catalyst with time.
Period could be in the order of seconds (catalytic cracking of
naphthas), months or years (synthesis of ammonia).
Life of a catalyst is of major econom
Performance equations for reactors containing
In this system we follow the changing composition with time and
interpret the results with the batch reactor performance
For each run in a differential reactor, the performance
The effectiveness factor ranges from 0 to 1.
The concentration of reactant is dependent on the dimensionless quantity (mL) called Thiele
Effectiveness factor as a function of Thiele modulus (mL)
Effectiveness Factor ( or )
Mechanism of solid catalyzed reactions
The steps involved in solid catalyzed fluid-phase reactions are as follows:
1. Diffusion of the reactant(s) from the bulk fluid phase to the external
surface of the catalyst through the fluid film external diffusion
Most catalysts do not maintain their activities at the same
levels for indefinite periods.
In some cases, the loss of activity is very rapid (e.g. catalytic
cracking of petroleum naphtha) while in other cases
Catalytic reaction occurs at the fluid-solid interface. So, a large interfacial
area is essential in attaining a significant reaction rate.
In many catalysts, the surface area is provided by the pores.
A typical silica-alumina crack
Selection of a model
1) Progressive Conversion Model (PCM) : Fluid has an access
through a solid particle and reacts uniformly
2) (Unreacted) Shrinking Core Model (SCM): Reaction progresses
as a moving boundary
Particle size remains or
Whole particle sh
Equilibrium Microstructural Development
Stehr, G.: Material Science for Engineers
BMW light metal foundry , manufactu