914
DOBROVINSKAYA et al.
One of the main factors impairing the activity of
the catalysts operating in high-temperature processes
is their caking. It can lead to a decrease in the total
surface area of a support and(or) in the dispersity of
deposited components.
The most important function of the support is to
minimize the rate of growth or migration of crystal-
lites of the active components. Nevertheless, these
processes are inevitable if a catalyst is used at high
temperatures, because the caking of the support grad-
ually diminishes its role as a dispersant, which ad-
versely affects the activity of the catalyst.
To study the thermal stability of the iron-copper
oxide catalysts synthesized, we tested them in de-
composition of real sulfuric-acid wastes from the al-
kylation process for 100 h under conditions con-
siderably more severe than the optimum, which long-
run tests more objective.
Fig. 4. Abradability I of iron-copper oxide catalysts vs.
the working time in sulfuric acid thermolysis. Catalyst:
(1) based on pure -Al O and (2) based on silicated
2
3
-Al O .
2
3
We determined the following optimal parameters of
the process, which provide a nearly equilibrium de-
gree of SSA decomposition to sulfur dioxide at a com-
plete oxidation of organic admixtures: temperature
850 C, fluidization number 3, specific load by SSA
3
500 kg h 1 m , and contact duration 1.4 s.
A test of the chosen catalyst under conditions of
decomposition of SSA formed in alkylation demon-
strated (Fig. 5) that the catalyst activity steeply falls
within the first 15 20 h of operation. Apparently,
the decrease in the activity of this sample is deter-
mined by recrystallization processes occurring in it.
Fig. 5. Degree of decomposition of spent sulfuric acid,
, vs. the working time of a catalyst. Load by the acid
1
3
(kg h
m ): (1) 900, (2) 500. SSA formed in (I) alkyla-
Three stages are commonly distinguished in
the mechanism of caking of oxide supports with a de-
veloped surface [3]. The area of contact between fine
particles increases in the first of these, and new closed
pores appear as a result of overlapping of the newly
formed contacts in the second. Both these stages are
accompanied by an essential decrease in the surface
area accessible to reagents. In the third stage, closed
pores disappear completely. In this stage, which is
most prolonged, the specific surface area changes less
considerably. The experimental data on changes in
the Ssp of the sample under study in thermocatalytic
decomposition of SSA after alkylation are given
below:
tion and (II) production of methyl ethyl ketone.
[1] is of special interest. Experiments demonstrated
that use of an unmodified support for preparing a cat-
alyst for SSA decomposition is unacceptable at all.
As can be seen in Fig. 4 (curve 1), the abradability of
such a sample within the first 20 h of operation was
7.5%. Then the destruction process presumably passed
from the surface into the bulk of grains. As a result, it
was necessary to stop the test ahead of schedule be-
cause of the catalyst transformation into a shapeless
mass. As to a sample prepared on a silicated support
(Fig. 4, curve 2), the loss of substance from it within
the running-in period somewhat increased in com-
parison with that from the support (Fig. 2, curve 2),
probably through loss of a certain amount of active
components. However, the abradability determined
after this period remained unchanged and constituted
0.2% per month.
1
Ssp (mg2 g ) after operation for
(h)
Initial
98.0
20
32.2
100
25.6
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 6 2005