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

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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

FM

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:

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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

Technology

Transcript of Catalyst Catastrophes in Syngas Production - II