Catalyst Deactivation - Spent Hydroprocessing Catalysts - page 54

4.1 Deactivation Due to Structural Change of Catalyst
During the hydroprocessing of light feeds, the operation lasting several years may be
anticipated. A prolonged exposure to operating temperatures may result in recrystallization of
catalyst causing a change in porosity. In addition, an increase of size of the MoS 2 /WS 2
crystallites in normal direction may occur. This would be an indication of the conversion of the
type I active phase to a more active type II active phase. At the same time, the growth in a
lateral direction would have an opposite effect. In this regard, the studies of Yokoyama et al.
[151] showed that the lateral growth of MoS 2 crystallites in the CoMo/Al 2 O 3 catalyst was
partly responsible for the loss of activity during the hydroprocessing of VGO at about 660 K
and 5.9MPa.
Because the catalytically active site comprises coordinatively unsaturated sites (CUS), it is
essential that their stability during the operation be maintained for a long period of time on
stream. This requires the suitable H 2 S/H 2 ratio to ensure a desirable size of CUS comprising a
sulfur vacancy and SH groups at its proximity [55] . It was established that the number of
vacancies decreased with increasing H 2 S/H 2 ratio. On the other hand, at low H 2 S/H 2 ratios, the
catalyst over reduction may occur. Loss of sulfur from the active phase during the reaction has
been reported to be the main cause of initial deactivation of hydroprocessing catalysts [152] .
Such situation favors the adsorption of N-bases as well as deposition of coke and disfavors
hydrogen activation.
In an effort to simulate deactivation, Tanaka et al. [153] conducted the accelerated aging in the
pilot plant at a higher temperature than that used in commercial units. In the former case, the
activity loss due to the lateral crystal growth was more pronounced than that observed in the
commercial unit operating at lower temperatures for much longer time on stream than that
used during the accelerating aging experiments. This agreed with the observations made by
Gamez et al. [154] who studied the spent CoMo/Al 2 O 3 catalyst used for hydroprocessing of
the mixture of atmospheric gas oil (AGO) and VGO. Thus, only a minor change in
morphology of the MoS 2 crystallites was observed after 12months on stream in a commercial
unit operating at lower temperature than that used by Tanaka et al. [153] during the
accelerating aging. These observations suggest that temperature may be the main parameter
influencing the catalyst recrystallization. It is therefore apparent that the catalyst deactivation
patterns may not be properly identified by accelerating aging.
Promoter segregation from the mixed Co-Mo-S and Ni-Mo-S phases has been reported in
several studies [152,155] . Eijsbouts et al. [156,157] demonstrated that catalyst deactivation by
MoS 2 sintering and segregation of promoters (Ni- or Co-sulfides) was quite extensive in
hydroprocessing units operating at high temperatures.
Usman et al. [158] studied the thermal stability of Co-Mo-S structure by chemical vapor
deposition (CVD) technique. The results revealed that the CoMoS structure was thermally