Catalyst Catastrophes in Syngas Production - II

Gerard B. Hawkins, Managing Director

 This follows our earlier Catalyst Catastrophes
in Syngas Production - I that concentrated on
major plant disasters
 This paper covers more frequent but still
costly incidents related to catalysts with the
focus of pulling together recommendations to
avoid repetition of the same losses
www.gbhenterprises.com

 Review of incidents by reactor
◦ Primary reforming
◦ Secondary reforming
◦ HTS
◦ LTS
◦ Methanator
 Reactor loading
 Support media
 Some general comments on alternative actions
when a plant gets into abnormal operation

 Most common incidents:
◦ Poor furnace balancing from catalyst loading
◦ Reformer damage caused by burners lighting
procedure on start-up
◦ Carbon deposition
◦ Steam condensation/rapid drying

 UNIDENSETM is now established as key to an
even reformer loading
 However UNIDENSE requires some care to
achieve its full potential
 A reformer in South America was loaded by an
inexperienced team and ……….
 Lesson – check experience of UNIDENSE
loading supervisors
UNIDENSE is a trademark of Yara International ASA

 Lighting burners during start-up is a critical
activity
 The clear requirement is to increase the number
of lit burners as the plant rate is increased
◦ and ensure the pattern of burners always gives an
even heat input
 Obvious – but was one component leading to
this:

 Lesson – light only the number of burners you
need at each stage of start-up and keep the
pattern/heat generation even

 Carbon deposition will occur when excess
hydrocarbons are introduced
 There are several ways to do this:
◦ Inadequate purging during a plant trip can lead to feed
being stored in the desulfurization vessel/ pipe work
 Then introducing nitrogen purge pushes this
hydrocarbon into the reformer
◦ Naphtha fed plants have a high risk of feed
condensing and sitting in dead legs until some motive
force pushes this into the furnace
◦ Erroneous feed flow measurement – more critical in
low steam ratio plants

 Introduction of nitrogen during a start-up
increased the reformer pressure drop from 1.4
to 7 bar in 2 minutes
 The nitrogen feed line was 100mm diameter and
around 1km long, capable of holding up to 10te
of naphtha
 A spectacle plate was not swung during earlier
operation
 On previous occasions a drain valve was
opened on the nitrogen compressor – this time
the valve was not operable

Situation After Plant Trip
Steam to
Reformer
Flow
Feed CV
Feed ESDV
Steam to
Preheat
Coil
F
M
Final ZnO
Bed
Feed
To Collector
or Flare
PCV
S Pt
Trapped Feed After Plant Trip

 A couple of ammonia plants in South America had
problems before the natural gas condensate removal
plant was installed
 These plants took their feed off the bottom of the
supply line and hence took any liquids that were
present
 The liquid did not register in the flowmeters which
were orifice plate type – thereby reducing the actual
steam to feed ratio
 Non-alkalized catalysts lasted as little as 6 weeks

 Lessons
◦ Gas flowmeters largely ignore the presence of
condensed higher hydrocarbons
◦ Note also that during startup flowmeters may read in
error if not compensated for temperature and
pressure
◦ Commercial alkalized reforming catalysts give very
significant additional margin against carbon
formation in primary reformers

 A plant in North America was the sole user of
gas that came down a branch that went under
a river
 During a start-up after an extended shutdown -
when lighting burners – liquid was seen
flowing from a few burners onto tubes
 While the operator exited and radioed the
control room to shut off the fuel - a tube burst
leading to significant damage to the
furnace/tubes

 It was thought that hydrocarbons had
condensed in the cooler section of pipe under
the river
 Lessons:
◦ Consider potential for condensation of higher
hydrocarbons, especially
 If lines are cooled below normal
 If levels of higher hydrocarbons increase

 We have another example of catalyst breakage
from condensation on start-up
 A naphtha fed plant was not able to provide
nitrogen purge for the initial phase of start-up
and so heated the reformer using steam
 Around 20 start-ups from cold eventually led to
breakage of catalyst, poor flow distribution, hot
spots and required catalyst change

 Lesson:
◦ Reforming catalyst should be warmed up to 50°C
above the dew point before introduction of steam

 All incidents on secondary reformers are
related to the burners
 The problem of increasing plant rate to the
point that there is inadequate mixing zone is
well understood but requires detailed CFD
modelling to predict

Cost of Problems:
7-10 day turnaround
Short Catalyst life
$65K/yr less than 10yr
Mechanical repairs
Estimated $65K/year
Poor mixing Burner failure
Bed damage Refractory damage

 Small flame cores
from all nozzles
 No flame
attachment to rings
 Good mixing of the
process gas and air

 The main problem with HTS reactors is
upstream boiler leaks
 We have another case where dehydration of
the catalyst has lead to an exotherm on startup

 This is a potential problem on ammonia plants with
high pressure boilers upstream of the HTS
 Boiler leaks put stress onto the HTS catalyst by:
◦ rapid wetting/drying and
◦ pressure-drop build-up from accumulated boiler solids
 These leaks are inevitable with steam pressures of
100bar
◦ A serious leak will occur approximately every 12 years
 Selecting a catalyst with high in-service strength
significantly improves probability of survival

 A plant in Europe suffered a complete tube
failure that tripped the plant
◦ Catalyst was unaffected by this incident:
0
20
40
60
80
100
120
0.0 20.0 40.0 60.0 80.0 100.0 120.0
% Bed Depth
NormalizedTemperature
Dec-02
Jan-03

 A plant in Europe had kept a charge of HTS catalyst in a
spare reactor for 1 year – but had left this reactor open to
the air – so the catalyst had adsorbed water
 The start-up required nitrogen heating for 2 days to dry
the whole bed – and in doing so dehydrated the catalyst in
the top/bulk of the bed
 100% steam was switched into the reactor against our
advice of 5%
 An exotherm started and then (unrelated) the plant tripped
(site power trip) which held the reactor with 100% steam
 The exotherm reached 530°C, and look several hours to
cool down with N2
 The final activity when on-line looked good, with expected
low pressure-drop.

 Do not leave catalysts exposed to damp
atmospheres
 VULCAN HTS catalyst give the best survival of
boiler leak and over-reduction incidents
 Incorporate the GBH Enterprises procedures
when over-reduction is suspected

 A plant in North America had to top skim its
LTS bed due to high pressure drop
 The main cause was poor atomization of
quench water
 This was not helped by the (non VULCAN)
catalyst installed which developed very poor
strength when wetted

 Lessons:
◦ Ensure quench water nozzles are on the shutdown
inspection list
◦ Check for adequate pipe length for vaporization
◦ Use catalyst with good strength after wetting

 The main hazards when methanation reactors
are shutdown are nickel carbonyl (see earlier
presentation) and self heating when exposed
to air
 An example of self heating comes from a
methanator on an olefine cracker

 The plant was shut down and purged with
nitrogen
 The inlet and exit valves and thermocouples
were removed for repair
 Open ends were covered in plastic sheet
 Catalyst was in reduced state, with N2 purge

 A reading of 454°C/850°F was seen on re-connection
of the thermocouples
◦ The plastic sheeting was not adequate isolation
◦ Air entered the vessel and
 A downward purge of nitrogen then gave a reading
of 649°C/1200°F on the bottom thermocouple
 Decided to change catalyst as needed 5 yr run

 Reduced methanation catalyst becomes very
hot when exposed to air
 Secure isolation/inert purge is essential for
maintenance on vessels containing reduced
catalyst
 With little or no gas flow, thermocouples do
not show the peak temperature

 Don’t spoil a ship for a few cents worth of tar!
 Below Bed:
◦ Support media does a key job preventing catalyst
pass through the exit collector – and doing this with
low pressure-drop
 Above Bed:
◦ Support media placed on top of the bed protects
catalysts from high inlet gas velocities - which have
the potential to break catalysts through disturbance
and milling
◦ High voidage media can also be used to reduce the
effects of boiler solid build-up

 A plant decided to use some old support balls
that had been stored outside for some years
 This was a LTS duty so either alumina or silica-
alumina would be suitable
 Shortly after start-up the reactor pressure-drop
started to increase
 This eventually required a shutdown to address

 Investigation showed failure of the support
media
 The catalyst had to be replaced
 Cause is believed to have been rapid drying of
support that had got wet during storage

 Silica-alumina support is cheaper
 A plant decided to use silica-alumina balls in a
high temperature shift bed
 It was thought that this would be a low enough
temperature for silica migration not to be an
issue
 Not true – silica migrated downstream and
collected on the tubes of the exchanger before
the LTS – which required regular shutdowns to
clean
 A recent enquiry associated with HDS and HTS
catalysts simply specified ‘ceramic balls’

 For the most severe duties, including
secondary reformers GBH Enterprises
recommends fused alumina lumps
◦ High density
◦ High strength
◦ Inert (high purity alumina)
◦ Difficult to blow around!

 Lessons:
◦ Store support media to the same standard as
catalysts – the cost will be the same if they fail!
◦ Only use high purity alumina support above 300°C in
steam environments
◦ Use VULCAN ‘Shift Shield’ for protection against
accumulation of boiler solids from boiler leaks
◦ Use fused alumina lumps for the ultimate protection
against bed disturbance

 Don’t be tempted to put that last bit in!
 A methanol plant with a water cooled reactor
experienced an increasing pressure-drop on a new
charge of catalyst
 Eventually the plant had to be shut down
 Inspection showed that catalyst had been loaded on
top of the tube-sheet as well as in the tubes
 Removal of the catalyst on top of the reactor and
150mm down the tubes restored the pressure-drop to
normal

 A hydrogen plant in Europe implemented a plant up-
rate and as part of this increased the HTS volume (we
advised it could be lowered)
 In order to maximize the catalyst volume the hold-
down system was removed!
 Milling then increased the pressure-drop
 A reactor ‘inlet distributor’ is better described as ‘inlet
gas momentum destruction device’
 Lesson – gas distribution/bed protection requires
careful design along with the rest of a plant up-rate

 A plant with a HTS reactor with two beds (one
vessel) went with a short load and split the
short load equally between each of the two
beds.
 The net effect was a bed L/D of 0.2 – a long way
below the minimum recommendation of 1.0
 The charge had to be replaced after 2 years
 One can debate the merits of two beds with L/D
of 0.2 with gas mixing in-between or one bed
with an L/D of 0.4
 The key is neither – but to load the bed(s)
carefully:

 The ideal catalyst loading method is by sock
with the minimum or raking
 Any raking will introduce density differences
that will lead to early discharge of the catalyst
due to the uneven flow distribution produced
 Lesson: allowing your loading company to
rake catalyst is equivalent to throwing catalyst
away

 There is no universal advice – but some up-
front thinking can lead to faster more
confident decisions
 A number of incidents have involved
exotherms on catalysts which threatened the
integrity of their reactors

 Hydrogen was being removed from a process
stream using a copper oxide catalyst
 During commissioning a hydrogen stream was
mistakenly introduced and the catalyst
temperature rose to 1000°C
 Technical support staff on site advised
immediate depressurization
 Vessel damage was avoided
 There were problems later on downstream mol
sieve driers from water produced which
accumulated in a dead leg

 With the previous example in mind it is worth
reflecting on the merits of depressurization
and purging

 Several advantages:
◦ It decreases the partial pressure of reactants which
may help slow the temperature rise
◦ It reduces the stress on equipment enabling the
handling of higher temperatures
◦ No motive force is required – so it is reliable
◦ Lowering the pressure makes purging more effective

 Risks
◦ Depressurization can generate high gas velocities –
enough to fluidize catalyst beds
◦ Fluidized catalyst beds can lose their top protective
layer (into the bed) and suffer:
 flow distribution problems or
 pressure drop increase if loss of the top layer allows
milling

 Advantages
◦ Can maintain plant pressure (but is better if pressure
reduced)
◦ Fluidization risks to catalyst beds much lower

 Disadvantages
◦ Difficult to achieve high flow-rates – steam is often
the purge gas with highest availability
◦ Steam can deactivate catalysts through oxidation
and in some cases sintering
◦ Nitrogen is a good inert material – but often the
available flow is limited
◦ Need to consider trace oxygen in nitrogen
 Ideal is nitrogen with enough hydrogen to ensure
reducing conditions

 A broad conclusion from the first set of
catalyst catastrophes was the value of having
a system to force a ‘stop and think’ when the
plant gets away from familiar operating regime
 The incidents here suggest:
◦ Selecting the right catalyst has a significant impact
on the ability of a plant to continue operation through
an unplanned event
◦ Operator training/procedures are key to avoiding
incidents

Catalyst Catastrophes in Syngas Production - II

Catalyst Catastrophes in Syngas Production - II

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Catalyst Catastrophes in Syngas Production - II