Environmental Engineering Reference
In-Depth Information
time on stream before the N-intermediates are completely converted to hydrocarbons. The
N-intermediates may be at least partly responsible for the increased coke formation in the
downstream reactors.
The study of Al-Nasser et al.
[149]
gave detailed accounts of the selection of the optimal
catalyst bed combinations for the graded system comprising four fixed-bed reactors, which are
part of the commercial atmospheric residue desulfurization (ARDS) process. Thus, every stage
required a different catalyst. In addition, a control of the H
2
S/H
2
ratio between the stages may
be necessary to ensure that the concentration of H
2
S is not in the inhibition region. To certain
extent, this may be achieved by withdrawing a portion of gaseous products between the
reactors and replacing them by a make up H
2
to ensure that the optimal H
2
S/H
2
ratio is
maintained. An option involving scrubbing H
2
S from the gaseous effluent of the reactor before
entering the subsequent reactor may be less practical.
The study of Rana and Ancheyta
[150]
indicated the complex deactivation patterns, which
resulted from different experimental conditions. In this case, the bench scale downflow reactor
and the upflow microreactor were used to study the Maya heavy crude and 50/50 blend on the
Maya crude and diesel oil, respectively. The former reactor could accommodate 10 times more
catalyst (e.g., 100mL/85 g). The experiments in microreactor could not be conducted without
blending the crude. Moreover, the experimental conditions were different, i.e., 653 K, 5.4MPa
and 10 L/h of H
2
in microreactor, compared with 673K, 7.0MPa and 100 L/H of H
2
in bench
scale reactor. The summary of these results (after 120 h on stream) using the CoMo/Al
2
O
3
and
NiMo/Al
2
O
3
catalysts is shown in
Table 4.1 [150]
. The experimental conditions had the most
pronounced effect on hydrodeasphaltization (HDAs). Thus, for the CoMo/Al
2
O
3
catalyst,
significant decrease in the HDAs conversion in the bench scale unit compared with the
microreactor was observed, whereas the opposite trend was observed for the
NiMo/Al
2
O
3
ยท
TiO
2
catalyst. At the same time, for both catalysts, the loss of the HDM, activity
was more evident in the bench scale unit than in the microreactor. These results may be used to
illustrate how the observations and conclusions reached during the catalyst deactivation
studies can be influenced by experimental conditions.
Table 4.1: Catalyst activities in microreactor (MR) and bench scale reactor (BS) [From ref.
150
.
Reprinted with permission].
Catalyst
Conversion (%)
HDS
HDM
HDAs
CoMo/Al
2
O
3
-MR
56
.
6
50
.
0
40
.
0
CoMo/Al
2
O
3
-BS
56
.
6
31
.
2
14
.
9
NiMo/Al
2
O
3
-MR
37
.
8
35
.
4
34
.
0
NiMo/Al
2
O
3
-BS
35
.
6
15
.
7
45
.
7
HDM: hydrodemetallization; HDS: hydrodesulfurization; HDAs: hydrodeasphalting.
