Avoiding Syngas Catalyst Mal-Operation

By Gerard B. Hawkins

Managing Director, C.E.O.

Objective

This presentation covers frequent and costly incidents related to catalysts mal-operation with the focus of providing the plant operator with recommendations to avoid plant outages and catalyst losses.

Process Information Disclaimer

Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the Product for its own particular purpose. GBHE gives no warranty as to the fitness of the Product for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability for loss or damage resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.

Content

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

Reformer Catalyst Loading

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 had to be unloaded and reloaded with 20 % catalyst losses.

Lesson – check experience of UNIDENSE loading supervisors

UNIDENSE is a trademark of Yara International ASA

Reforming – Burners Lighting

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:

Reforming – Burners Lighting

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

Reformer - Carbon

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

Reformer – Carbon from Naphtha

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

Reformer – Naphtha in Dead Legs 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

Reforming - Carbon from Liquid HCs

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 flow meters which were orifice plate type – thereby reducing the actual steam to feed ratio

Non-alkalized catalysts lasted as little as 6 weeks – and when replaced by alkalized products lasted a 2 year run

Reforming - Carbon from Liquid HCs

Lessons • Gas flow meters 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

• Alkalized reforming catalysts give very significant additional margin against carbon formation in primary reformers

Reforming – Tube Failure from Higher Hydrocarbons

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

Reforming – Tube Failure from Higher Hydrocarbons

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

Reforming – Failure from Condensation

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

Reforming – Failure from Condensation

Lesson: • Reforming catalyst should be warmed up to

50°C above the dew point before introduction of steam

Secondary Reforming

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

Secondary Burner Problems

Cost of Problems:

7-10 day turnaround

Short Catalyst life $52K/yr less than 10yr

Mechanical repairs

Estimated $65K/year

Poor mixing Burner failure

Bed damage Refractory damage

Secondary Burner Solution

Small flame cores from all nozzles

No flame attachment to rings

Good mixing of the process gas and air

HTS

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

HTS - Boiler Leaks

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

HTS VSG-F101 Resists Boiler Leaks

A plant in China suffered a complete tube failure that tripped the plant • F101 was unaffected by this incident:

HTS - Dehydration

A plant in China 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.

HTS Lessons

Do not leave catalysts exposed to damp atmospheres

VSG-F101 give the best survival of boiler leak and over-reduction incidents

Incorporate the GBHE VULCAN Series procedures when over-reduction is suspected

LTS

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 competitive catalyst installed which developed very poor strength when wetted

LTS

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

Methanator

The main hazards when methanation reactors are shutdown are nickel carbonyl (see plant safety presentation on nickel Carbonyl) and self heating when exposed to air

An example of self heating comes from a methanator on an olefin cracker

Methanator self heating

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

Methanator Self-heating

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 GBHE had product on site within 4 days

(including a weekend)

Methanator Learning

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

Support Media

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

Support Media - problems

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

Support Media - problems

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

Support Media – use of Si/Al

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’

Support Media – Catalyst Protection

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

Support Media - advice

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 GBHE ‘A2ST’ for protection against accumulation of boiler solids from boiler leaks

• Use fused alumina lumps for the ultimate protection against bed disturbance

‘A2ST’ Advanced Alumina Support Technology

Reactor loading 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

Reactor overloading

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

Reactor loading

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:

Reactor Loading

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

Priorities When Things go Wrong

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

Example exotherm and action 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

GBHE 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

Depressurization vs Purging

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

Depressurization

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

Depressurization

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

Purging

Advantages • Can maintain plant pressure (but is

better if pressure reduced) • Fluidization risks to catalyst beds much

lower

Purging

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

Conclusions

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