Hydrocarbon Processing - July 2022 - 73

Catalysts
through the outlet collector and into the reactor outlet elbow.
This debris must be removed prior to catalyst loading or it will
be pushed through to a downstream reactor or feed/effluent
heat exchanger on startup.
The inert entry crew should take extreme care of multipoint
thermocouple arrays in the top and bottom of the catalyst
beds during unloading of the spent catalyst, as these instruments
are easily damaged.
After removing all catalyst and support material from the
reactor(s), the internal, austenitic stainless-steel surfaces must
be neutralized with an aqueous soda ash solution to protect
against polythionic acid stress corrosion cracking (PASCC).1,2
This failure mechanism is caused by polythionic acids that are
formed by the reaction of sulfide corrosion products [sulfur,
hydrogen sulfide (H2
S), metal sulfides] with oxygen and water.
Normally, TP-347 and TP-321 stainless steel grades are used
for the weld overlay and internals of hydroprocessing reactors.
Austenitic stainless steels are sensitized for PASCC by welding
and/or high temperatures (370°C-815°C). Newer, chemically
stabilized grades of stainless steels resist sensitization for some
time and therefore are less susceptible to PASCC (e.g., TP347AP).
Soda ash neutralization applies to all sensitized austenitic
stainless-steel surfaces, not just reactor internals, that
are exposed to the mix of sulfur, air and moisture (e.g., heater
tubes and some feed/effluent heat exchangers).
When the soda ash wash is complete, the reactor can be
turned over to an air atmosphere. All nitrogen connections
to the reactor(s) must be positively isolated (blinded or
physically disconnected).
Prior to loading, the reactor must be cleaned of all loose
scale, debris and other deposits fouling the internals. Of particular
importance are the outlet collector, distribution trays,
other quench zone trays and the catalyst support grids (profile
wire or mesh screens), which must be 90%-95% clear to
ensure good distribution in the bottom of the catalyst beds.
The inspection requirements of the reactor and its internals
should be detailed in the inspection plan and will include:
* General cleanliness and condition of
the reactor, nozzles and internals
* Reactor internal overlay cracking: spot dye
penetrant testing is utilized to detect cracking in
the weld overlay, especially in areas adjacent to
nozzles and at load-bearing, high-stress areas
* All reactor seams and support skirt-to-shell
seams should be inspected on a set frequency.
The authors emphasize that the shutdown of the unit and
the catalyst unloading are the phases of greatest uncertainty
and hold the highest risk of unplanned events. Therefore,
the completeness of the planning, attention to detail and the
presence of refinery and contract people with experience and
expertise to deal with these potentially schedule-destroying
events cannot be overemphasized.
Part 2 of this article (August 2022) will address the activities
associated with catalyst loading and the restart of the unit.
ACKNOWLEDGEMENT
The authors want to thank Trevor Penny from CR International for the images
and helpful comments.
FIG. 4. Gravity dumping.
LITERATURE CITED
1
The National Association of Corrosion Engineers (NACE) NACE Standard
RP0170-2004, " Protection of austenitic stainless steels and other austenitic alloys
from polythionic acid stress corrosion cracking during shutdown of refinery
equipment, " 2004.
2
American Petroleum Institute (API) Standard 571, " Corrosion and materials:
(Section 5.1.2.1) Polythionic acid stress corrosion cracking (PASCC), "
September 2010.
SHAUN DYKE is an experienced chemical engineer who has
worked in the refining and petrochemical industries for more
than 40 yr in numerous technical, managerial, governance and
consulting roles around the world. He lives in and works from
New Zealand and writes in his spare time. Mr. Dyke earned
a BSc degree (First Class Honors) in chemistry from
Massey University, New Zealand.
NATTAPONG PONGBOOT is an experienced chemical
engineer with extensive experience in refining and
petrochemical technologies, specializing in hydroprocessing
technology. Before partnering with Global R&D, he held
various technical roles with Honeywell UOP, PTT Global
Chemical and SCG Chemicals, with broad exposure to
process and equipment design, refinery operation, plant
troubleshooting and process optimization. Additionally, he is also a technical
instructor for www.chemengedu.com. Mr. Pongboot obtained B.Eng. and M.Eng.
degrees in chemical engineering from Kasetsart University. The author can be
reached at nat.pongboot@gmail.com or nattapong.globalrd@gmail.com.
Hydrocarbon Processing | JULY 2022 73
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