Regeneration - Spent Hydroprocessing Catalysts - page 148

In this equation, r c and r s are the rates of the oxidation of carbon and sulfur, respectively, T s
and T g represent temperatures of catalyst pellets and that of gas between the pellets, C ps and d p
the specific heat and apparent density of the catalyst, q c and q s heats generated by combustion
of carbon and sulfur, respectively, h the heat transfer coefficient between the pellet and gas,
and a the equivalent radius of spherical particle. For the heat balance of the gas in the bed
voids, the following equation was derived:

(d T g /d t ) = ( s / e )( T in − T g ) + (1 − e )3 h ( T s − T g )/( eC p d a )

where T in is the temperature of the oxidizing gas entering the bottom of the fixed bed, s is the
air space velocity defined as the ratio of volumetric air flow rate under ambient conditions to
the volume of catalyst bed, e is the bed porosity, d a is the air density and C p specific heat of air.
Assuming that the accumulation terms were very small compared with the other terms; the
above equations were simplified to obtain the following algebraic relations:

T g = T in + [(1 − e ) d p /( C p d s )]( r c q c + r s q s )

T s = T g + [( ad p )/3 h ]( r c q c + r s q s )

The validity of the model was tested by comparison of the calculated results with those
obtained during the experiments. A reasonable agreement between the ignition temperatures
predicted by the model and those estimated experimentally was obtained in spite of the fact
that combustion of hydrogen in coke was not included. In view of much less heat released and
low content in coke, hydrogen combustion contribution to the overall heat release may not be
significant. The necessity for incorporation of this source of heat may be determined on the
basis of hydrogen content in coke.

6.2.4 Characterization of Regenerated Catalysts

Careful control and monitoring of operating parameters emphasized earlier suggests that the
conditions employed during regeneration have decisive effects on the final properties of
regenerated catalysts. In every case, the objective is to approach, as much as possible, the
properties of the fresh catalyst. In this regard, a desirable effect can be achieved by an optimal
combination of temperature and O 2 concentration in the oxidizing medium. Otherwise, the
uncontrolled temperature runaways could not be avoided. Figure 6.17 [401] is presented to
illustrate the adverse effect on the composition of the regenerated CoMo/Al 2 O 3 catalyst, when
temperature exceeded about 600 ◦ C, i.e., the loss of Mo due to sublimation of MoO 3 .Atthe
same time, little change in the content of Co in the regenerated catalyst was observed. It is
shown below that at such temperatures, the surface properties of regenerated catalysts are
affected dramatically.