Troubles in radial flow reactors
for ammonia synthesis
In 2000 the Ammonia synthesis plant No.7 reactor was modified. The flow pattern was changed
from axial to radial flow. The modification included a new inner heat exchanger, (122-C1), an
additional one for the outlet (123-C1) and catalyst type. The aim was to achieve a higher
conversion with milder operation conditions, i.e. pressure. During 2003 there were several
problems: low flow rate thru start up exchanger, as well as vibration in the motorized valves at
the inlet and outlet of reactor. Thru years 2004 and 2005 there was a steady increment in
pressure in the synthesis circuit and there are hints of catalyst fine particles carryover. In year
2006, the pressure and abnormal reactor behavior prompted an unscheduled plant stoppage.
Catalyst was unloaded under inert atmosphere and reactor interns are inspected. The company
had no prior experience working under inert atmosphere. Equipment and safety preparation
procedures were prepared. A crack in the welded center screen (inner central tube of synthesis
reactor 105-D) was found. This crack was responsible for catalyst losses. The crack was repaired
and reactor is back to normal operation.
F A Figueroa-Moreno, A M Morales-Herrera,
J A Cruz-Hipolito
Pemex Petroquimica
Introduction capacity is 500,000 MTY. In the year 2000 the
Ammonia synthesis plant No.7 reactor was
T his paper describes the experience of
Pemex Petroquímica on inspecting a
synthesis reactor under inert atmosphere,
unloading the catalyst and repairing the damage
modified, using Haldor’s design. The flow
pattern was changed from axial to radial flow.
The modification included a new inner heat
exchanger, (122-C1), an additional one for the
to the central tube of the synthesis reactor outlet (123-C1) and new smaller catalyst was
installed. The aim was to achieve a higher
conversion with milder operation conditions, i.e.
Background pressure.
Pemex Petroquimica operates four ammonia During maintenance of May – Jun 2003 the start
plants at its Cosoleacaque, Veracruz, Mexico up heater coil was changed due to plugging, and
site. Plant No. 7 was commissioned in 1981. Its during the subsequent start up of the plant in
2010 163 AMMONIA TECHNICAL MANUAL
July of 2003 flow problems in the heater removed from the inside of the 107-F flash
resulted in excessive vibration in MOV-1 and vessel ammonia separator.
MOV-3. Therefore the inlet and outlet of the
coil were opened up for inspection but The plant was shut down on March 14 of 2006.
unfortunately nothing was found that could During inspection of the flash vessels 106-F and
explain the flow problems. 107-F approximately 4,000 kg of catalyst
powder was found.
During the maintenance during April - July in
2004, temperature valves and HCV–3 valve It was observed that during the start up of the
were inspected without a positive response to synthesis reactor 105-D, an abnormal behavior
the abnormal high pressure in the synthesis of the bed 3 took place, even with the same
loop, the vendor was contacted to make a charge no increase of temperature was observed.
technical evaluation and find a possible cause. See Figure 2.
See Fig. 2.
During maintenance in May of 2005 the primary
and secondary flash vessels 107-F and 106-F
were opened and catalyst eroded particles were
found (150 kg. approximately) in 107-F and a
small amount in 106-F.
On June 19 2005 the pressure in synthesis loop
2
was 167 kg/cm and in August of the same year
2
the pressure reached 170 kg/cm . See Fig. 3 for
2
further details. Design pressure is 160 kg/cm .
Photo 2. Catalyst eroded particles.
Pemex knows of similar issues in other
ammonia facilities, caused by damage in center
screen, and it was therefore necessary to take
immediate action and to repair the central
screen.
In order to repair the inside of the synthesis
Photo 1. Found catalyst eroded particles. reactor, the reactor has to be opened. This can
cause oxidation of the catalyst. Risks like
oxidizing the catalyst due to:
During the next inspection in November 2005
catalyst powder, approximately 910 kg, was
AMMONIA TECHNICAL MANUAL 164 2010
Poor control of the oxidation and the The time consumed removing melt catalyst,
temperature inside the reactor. Catalyst repair of the inside parts of the reactor would
could melt creating a big bulk which has to be less. The normal delivery time for a central
be removed with a hammer. This can result tube is 3 or 4 months.
in strong damages to the inside parts of the
reactor. Based on the above the catalyst vendor strongly
recommended to unload the catalyst under inert
While opening the reactor, other companies atmosphere conditions.
have experienced that the top of the 1st and
2nd bed started to burn. As soon the catalyst
is in touch with air the catalyst will start to
heat up and subsequently start burning.
Bigger risk of damage to the other inside
parts of the reactor when any fused catalyst
is removed.
Photo 3. Center Screen
Discharging under inert atmosphere
The Pemex site had no prior experience working
under inert atmospheres. So careful planning
was needed.
Planning
The milestones were defined as follows:
1. Unload the catalyst under inert
atmosphere.
2. Inspection of the reactor internals.
3. Inside parts repair.
4. Loading of fresh catalyst
When doing such jobs, Pemex Petroquimica
Cosoleacaque operates under a common
company Safety system. The safety work
process was administrated strictly according to
company rules. Major PEMEX activities
included:
2010 165 AMMONIA TECHNICAL MANUAL
Unloading procedure for reduced catalyst in
inert atmosphere.
Training of inexperienced people to be able
to work in an inert atmosphere, wearing
complete mask with pipeline of fresh air
(grade D)
Redundant air system
Assuring complete rescue equipment.
Assuring communication equipment (two
way radio)
Regular meetings with vendor’s personnel to
review and improve the safety.
Assuring medical service availability 24/7
Assuring crane availability. Crane available
24/7 in case it should be needed to remove Photo 4. Inside the reactor
an injured person thru the top of the reactor.
Practicing of the withdrawal of an injured
person in closed space.
Nitrogen supply logistics.
Reactor isolation.
Execution
Withdrawal of the heat exchanger 122-C
(cooler inside the reactor 105-D).
Hook up of mechanical suction equipment
and pipeline for interconnection with the
reactor.
Photo 5. Center Screen line
Set up catalyst reception containers.
Establish transportation equipment
Hook up cooling equipment for N2 in order
to control the temperature inside the
synthesis reactor.
“Reunion at the beginning of the journey”
with relevant instructions to people that will
be in closed spaces (catalysis Services and
contractor).
Checklist of Catalysis Services of the
monitoring station for: breathing air supply
“D” type with redundancy.
Photo 6. Catalyst inside the synthesis loop
AMMONIA TECHNICAL MANUAL 166 2010
A life cable holding up for a tripod at the After the complete removal of the catalyst in
top of the reactor in case that an every bed, the reactor was purged with N2
immediate evacuation was needed.
Meeting between Pemex people and Vendor
“Watch person” ready to enter in the to inform that the unload procedure is
reactor in case of emergency. finished in inert atmosphere; blind joints
will be installed in the injection of N2 line
Observations: besides padlocks will be installed in each
valve with label.
Temperature increments in catalytic beds as
a result of the fact that the system is non The Nitrogen supply was interrupted and the
hermetic. reactor vented with air.
Additional injections of N2 were supplied to A rigorous checking of O2 content in every
limit or reduce the increase of the bed took place with equipment of Vendor
temperature in the catalyst. and Cosoleacaque Petrochemical Complex
until the value of O2 was 21.5%, with
The catalyst caught fire several times when laboratory checks in different points.
the catalyst was in contact with ambient air
because air was leaking into the system. After checking the inspection activity takes
place.
There was absence of the catalyst at the top
of the bed number 3 People of Contractor, Catalyst Services,
Vendor and Pemex checked the inside parts
Worked around the clock divided in two of the reactor.
shifts.
Catalyst Services people took a video, bed
by bed of the reactor finding in bed number
3 a hole in the base of the central tube center
screen.
Vendor’s people presented a complete
instruction of the repair activities, and the
repair procedure.
Photo 7. Catalyst on fire
2010 167 AMMONIA TECHNICAL MANUAL
Photo 9. Reparation kit.
Photo 8. Hole in center screen.
Hole in the base of the central tube center screen
Two sections: the first of 34” approximately and
the second of 9” length.
Repair Process
Contractor’s people informed that a failure of
the central tube was found and they presented
procedure for repair.
Reparation kit of the central tube (center
screen), was constituted by:
3 sections of perforate plate with a thickness
of ½”
Photo 10 Center Screen repaired.
One ring of two pieces for the union of the
perforate plates with the central tube.
The basic objective of the reparation kit is to
One mesh of inconel with its three give a better structural resistance and a bigger
subjection belts. flow area to an area that has a low profile
normally and is relative inactive.
The double mesh minimizes the possibility of
catalyst carryover.
AMMONIA TECHNICAL MANUAL 168 2010
Results
The reactor is successfully back on stream after
the repair
A complete removal of catalyst was performed
under inert atmosphere with no incidents, which
was a great achievement since Pemex have
never done this kind of job before.
Conclusions
A thoroughly planned process insured the safe
success of the task, avoiding further damage to
the catalyst reactor and further losses of
operation time.
The entire catalyst for the synthesis reactor was
replaced because the remaining catalyst was
oxidized.
The differential pressure inside the synthesis
reactor decreased from 4.7 kg/cm2 (before the
problem) to 2.4 kg/cm2 (after problem).
The learned lesson with regard to the 2000
reactor modifications is the decrease of the
synthesis loop pressure from 180 kg/cm2 to 160
kg/cm2.
2010 169 AMMONIA TECHNICAL MANUAL
PI-71
150
152
154
156
158
160
162
164
166
168
170
12/10/2000
10/11/2000
08/12/2000
05/01/2001
22/06/2001
20/07/2001
AMMONIA TECHNICAL MANUAL
11/09/2001
15/10/2001
03/01/2002
31/01/2002
07/04/2002
06/05/2002
03/06/2002
01/07/2002
29/07/2002
02/09/2002
05/10/2002
02/11/2002
170
30/11/2002
28/12/2002
26/01/2003
23/02/2003
23/03/2003
synthesis reactor
19/07/2003
16/08/2003
13/09/2003
11/10/2003
Figure 2. Process diagram, synthesis loop.
08/11/2003
06/12/2003
03/01/2004
31/01/2004
27/10/2004
24/11/2004
22/12/2004
PI-71 Loop pressure PDI-162 Differential pressure of the synthesis reactor.
19/01/2005
16/02/2005
16/03/2005
Figure 3: Pressure and pressure drop profile for ammonia of the 3rd bed of the
0
1
2
3
4
5
6
PDI-162
2010
