Environmental Engineering Reference
In-Depth Information
To ensure safety and environmental acceptance, it is desirable that spent catalyst is de-oiled
and dried prior to unloading. This can be achieved by replacing the flow of the feed with a
lighter fraction while H
2
is still flowing. The absence of discoloration of the washing liquid
caused by dissolution of the catalyst carry-overs may indicate the completion of the washing
stage. The accelerated solvent extraction method used for catalyst characterization has a direct
relevance to catalyst de-oiling
[318,319]
. According to this method, the spent catalyst
de-oiling, using hydrocarbon solvents, is conducted at elevated pressures (e.g.,
10MPa).
Under such conditions, de-oiling was complete within 5min compared with conventional
Soxhlet extraction requiring 6-12 h. This suggests that de-oiling efficiency can be optimized
by operating conditions. After de-oiling, the drying of the spent catalyst can be performed in
the flow of H
2
after the flow of washing liquid was discontinued. The last phase of drying
involves the replacement of H
2
with an inert gas and cooling to room temperature. After
de-oiling, drying, and cooling under inert gas, the safety during catalyst unloading is
ensured.
∼
Some fixed bed reactors, equipped with catalyst dump nozzle, are shown in
Fig. 5.1 [320]
.For
spent catalyst withdrawal, the nozzle is opened under the purge of N
2
. Then, spent catalyst
flows into catalyst bins that were also purged with N
2
. Dry ice (CO
2
) is added to the filled
containers to expel remaining air. The removal of the last amount of spent catalyst may require
the personnel to enter the reactor. It is essential that in such situation the personnel carries all
necessary safety equipment to prevent serious injuries.
Apparently, there is no catalyst unloading procedure that could be commonly adapted by all
refiners. Generally, refineries apply their own procedures unless the catalyst withdrawal is part
of the agreement between the refiners and a partner (e.g., companies involved in regeneration,
transportation, storage, etc.). There may be a need for an approved procedure, which could be
commonly adapted by all refineries. In this regard, several patents describing the catalyst
unloading techniques have been noted
[320,321]
. In every case, the primary focus is on the
reducing a self-heating character of spent catalysts. Otherwise, a spontaneous combustion of
spent catalyst may result in the release of toxic species, such as SO
X
,NO
X
, HCN, NH
3
, etc.
[322]
.
Attempts have been made to develop a pretreatment method that would minimize hazardous
nature of spent catalysts during catalyst unloading and all stages following after. In this regard,
the process developed by Kashima Engineering Co. in Japan enables the catalyst unloading
under air
[323]
. The process passivates self-heating nature of spent catalyst during reactor
shutdown by applying a proprietary mixture of chemicals. The mixture contains compounds
that deposit a film on the surface of spent catalysts. This film slows down oxygen penetration
considerably.
Figure 5.2 [323]
shows a generalized shutdown procedure. Initially, feed-rate is
reduced by about two third, while the reactor starts cooling down. Then, the carrier oil is
introduced to displace the mixture of the feed and products. Once carrier oil is in the total
