(165d) Stress History – a Concept for the Description of the Age-Dependent Attrition of Catalysts in Fluidized Bed Systems

In fluidized bed reactors a catalyst is subjected to significant
attrition. This
attrition influences together with the efficiency of the solids
recovery system
the particle size distribution inside the fluidized bed system and thus
the
performance of the reactor. Furthermore the attrition is a major reason
for the
loss of catalyst and may thus become also decisive for the economic
performance
of a catalytic process.

In previous work the attrition of catalyst has been studied in detail
for
catalysts which have undergone attrition inside the fluidized bed for a
long time already and therefore reached a constant attrition rate. It
was
shown already, however, that a fresh catalyst is much more fragile and
shows
up a much higher attrition rate at the beginning than at steady state.
It
took always a considerable long time until a constant attrition rate
was
reached. Therefore the higher attrition rate will significantly
contribute to
the total loss of catalyst.

The higher attrition of the fresh catalyst is due to micro cracks in
the
surface and the roughness of the surface. After some time the particle
is more
rounded and the surface has become smoother due to the attrition and
thus the
resistance against attrition is increased.

In a fluidized bed reactor a particle will be subjected to attrition
due to
different mechanisms in different parts of the system, namely attrition
by gas
jets near the bottom of the fluidized bed, bubble induced attrition in
the
fluidized bed itself and the attrition during the passage through a
cyclone. All
these different attrition mechanisms are described by different
mathematical
models with different time scales. In total they cause the aging of the
particle
and thus the increase of the resistance against attrition.

To be able to summarize the effect of the stresses on the particle
within the
different parts of the fluidized bed system the concept of the
‘stress history’ has been developed, which allows a
uniform treatment of the different attrition mechanisms. This concept
has been experimentally validated and been implemented
into an existing population balance model. The experiments were carried
out with fresh FCC catalyst.

The simulation of the time-dependent variation of the particle size
distribution in the fluidized bed system illustrates the effect of the
age-dependent
catalyst attrition.