Environmental Engineering Reference
In-Depth Information
10.2.2 Deactivation
It is believed that the life of catalysts used for dewaxing, as well as those for HYD,
hydrofinishing, and dehazing, which may be integral parts of the overall dewaxing process,
should approach at least two years providing that no unexpected events occurred during the
operation. In most cases, the feed for dewaxing is free of contaminates because it was already
pretreated by hydroprocessing. Therefore, the coke deposition and poisoning by N-compounds
should be the main causes of catalyst deactivation. However, N-bases would have to be
removed down to few parts per million (ppm) of nitrogen in the case that the dewaxing
catalysts contain noble metals. Such catalysts can also be poisoned by H
2
S. Therefore, the
deep HDS and HDN of the feed may be necessary before dewaxing. To a certain extent,
recrystallization of the catalytically active phases to less active phases during the prolonged
exposure to hydroprocessing conditions could occur as well. This could be the cause of
permanent catalyst deactivation.
In spite of rather clean systems being used, catalyst deactivation during HIS/IS could not be
avoided. This was evidenced by declining conversions and selectivity with time on stream
during the HIS of a paraffinic feed obtained from Fischer-Tropsch synthesis free of sulfur and
nitrogen
[587]
. In this case, oxygenates present in the feed were the major contributor to the
activity decline. In the absence of oxygenates, the coke deposition was the main factor
contributing to deactivation, although recrystallization of active phase affecting catalyst
activity could not be ruled out. It is expected that the structure of coke formed during the
dedicated HIS operations will differ markedly from that observed on the spent catalysts used
in hydroprocessing of petroleum feeds. For example, Cowley
[587]
reported that the coke
formed during the HIS of pentenes over an acidic non-zeolitic molecular sieve catalyst was not
aromatic, and did not comprise hydrogen deficient polynuclear aromatics, i.e., hard coke, but
rather paraffinic, olefinic, or polyolefinic structures with the H/C ratio exceeding 1.0. However,
the structure of coke depends on the type of catalyst, and operating conditions. Therefore, it is
possible that the formation of an aromatic coke during the HIS of hydrocarbons occurs as well,
though to a much lesser extent.
The influence of H
2
pressure (from atmospheric to 0.5MPa) on the HIS of n-octane over
Pt/SAPO-5 and Pt/SAPO-11 was investigated by Campelo et al.
[588]
at 648 K. The
deactivation with time on stream was more pronounced for the Pt/SAPO-5 catalyst. However,
gradual increase in H
2
pressure (to 0.3 and 0.5MPa) resulted in an enhancement in catalyst
activity with time on stream, although during the subsequent time on stream, the activity
continued to decline. The activity increase coincided with the decreasing amount of coke
deposited on the catalyst with increasing H
2
pressure. For SAPO-11, the same change in H
2
pressure resulted in a significant increase in the activity. Moreover, no catalyst deactivation
during the entire run (almost 16 h on stream) was observed. It is therefore believed that the
coke formed under conditions of HIS is rather “soft” and can be reversibly removed by an
