The document provides guidelines for safely starting up and reducing steam reforming catalyst. It discusses warm-up procedures to avoid condensation, reducing the catalyst with hydrogen or hydrocarbons, and gradually introducing feedstock. It also summarizes a case study where overfiring during start-up led to tube failures due to much higher than normal temperatures as a result of deviations from proper procedures.
3. Introduction
Steam reformer is complex
• heat exchanger
• chemical reaction over catalyst
• combustion, leading to steam generation
Common symptoms of poor performance
• high exit methane slip
• high approach to equilibrium
• high tube wall temperature
• high pressure drop
Need properly active catalyst
4. As supplied - NiO on support
Active species - Ni crystallites
Reduction process needed:
NiO + H2 Ni + H2O
Steam Reforming Catalyst
6. NiO Reduction
Faster at high temperature
Slower in presence of steam
Thermodynamically, very little hydrogen
needed
Support can affect ease of reduction
7. Catalyst Reduction
Requires high temperature
• fire steam reformer
Requires hydrogen
• supply H2 or reduce gas
• re-circulation or once-through
Since little or no steam reforming is taking place,
• less heat is required to warm up gas:
50% steam rate, with 5:1 steam: H2 ratio
requires 1/7 fuel of normal operation
Extreme danger of local
overheating!
9. Warm-Up
1. Purge plant of air with N2
- must be free of hydrocarbons and carbon oxides
2. Heat reformer above condensation temperature
2. Add steam when exit header temperature 50oC
(90oF) above condensation temperature
- low pressure favours good distribution and
lowers this temperature
4. Increase steam rate to 40 - 50 % of design rate
- min 30 %
5. Stop N2 circulation
Air warm-up possible, but not for previously reduced
catalyst (possible carbon)
10. Warm-Up - Feedstock Isolation
• Before a flow of steam is established in the
steam reformer, hydrocarbons must not be
present
– Carbon formation!
• Ensure that hydrocarbon feed lines are fully
isolated
– Double-block and bleed
– Do not rely on block or control valves
• Or keep the pressure of the hydrocarbon feed
supply below hydrogen plant start-up
pressure
11. Traditionally 50oC (90oF) /hr
Modern materials 100oC (180oF) /hr
Catalyst 150 - 170oC (270 - 350oF) /hr
Warm-Up rates
Rapid warm-up minimises energy usage/time
Limited by mechanical considerations of steam
reformer
Assess effect on plant equipment
• thermal expansion of inlet/exit pipes
• steam reforming tensioners
• steam reformer tubes
• refractory linings
12. • Water can damage the steam reforming
catalyst
• Temperature “shock”
• Rapid drying of “wet” catalyst
•The expansion of water to steam in the
catalyst pores causes catalyst break-up
• Pre-reforming catalyst much more sensitive
to water
• Essential to avoid condensation
Warm-up - Avoiding Condensation
13. Steam
Reformer
Cold Pipework
Steam
If upstream pipe work is cold, then it is good
practice to warm up by steam flow with vent
to prevent carry-over of water
Warm-up - Avoiding Condensation
To Vent
15. Temperatures
Temperatures referred to are true catalyst
temperatures at exit of tube
Measured temperatures during normal operation
are 10 - 100oC (18 - 180oF) cooler due to heat losses
Most catastrophic failures of tubes in top-fired
furnaces occur during start-up
Cannot rely on plant instrumentation during start-
up
• lower flows than normal
• higher heat losses than normal
• fewer burners can give severe local effects
Frequent visual inspection of reformer tubes and
refractory is essential during start-up
16. Effect of Pressure and Temperature
800 900 1000 1,100 1,200
1
10
100
1,000
10,000
100,000
1,000,000
10,000,000
Tube Wall Temperature oC (oF)
( 1500 ) ( 1650 ) ( 1830 ) ( 2010 ) ( 2200 )
30 bar
5 bar
Steam Reformer Tube Life
18. Reduction Procedures
Reduction with hydrogen
Reduction with natural gas
Reduction with other sources of hydrogen
• higher hydrocarbons
• ammonia (not discussed)
• methanol (not discussed)
19. Reduction with Hydrogen
H2 or H2-rich gas can be added at any time
to the steam when plant is free of O2
Steam: hydrogen ratio normally 6:1 - 8:1
Get tube inlet temperature as high as
possible
Increase exit temperature to design value
(>700oC/1292oF)
Hold for 2-3 hours
20. Hydrogen Source
Hydrogen must be
• free of poisons (S, Cl)
Special consideration must be given to the
presence in impure hydrogen sources of
• carbon oxides
• hydrocarbons
Also applies to nitrogen (or inert) source
used for purge/warm-up
21. Carbon Oxides
Re-circulation loop may include HDS unit
(at temperature)
Carbon oxides above 250oC (480oF)
methanate over unsulfided CoMo catalyst
• temperature rise 74oC (133oF) per 1% CO
converted
• temperature rise 60oC (108oF) per 1%
CO2 converted
If H2 contains >3% CO or >13 %CO2
or a mixture corresponding to this,
then by-pass the HDS system
22. Hydrocarbons
May be converted to carbon oxides in
reformer
May crack thermally to give carbon
23. Reduction with Natural Gas
1. Warm-up as before
2. Introduce natural gas at 5% of design rate
3. Slowly increase gas rate to give 7:1 steam: carbon
over 2-3 hours
4. Simultaneously increase reformer exit temperature to
design level (>700oC/1292oF)
5. Increase inlet temperature as much as possible (to
crack natural gas to hive H2)
6. Monitor exit methane hourly: reduction complete
when methane reaches low, steady value (4 to 8
hours)
24. Reduction with Higher Hydrocarbons
Increased possibility of carbon formation
Great care needed
Longer time periods needed
More precision in all measurements
needed
Hydrogen addition recommended
Purification issues
Only use if absolutely necessary
26. Feedstock Introduction
1. Introduce feedstock at high steam: carbon
ratio
(5:1 for natural gas; 10:1 for higher
hydrocarbons)
2. Steam reforming will give small increase in
inlet pressure, cooling of tubes, and lower
exit temperature
3. Need to increase firing to maintain exit
temperature
4. Then increase feedstock flow
5. Increase pressure to operating pressure
6. Adjust steam: carbon ratio to design
27. Feedstock Introduction
Increase flow of natural gas to design steam:
carbon ratio (2 hours)
Maintain exit temperature
Check that exit methane stays low
• (reducing steam: carbon ratio will increase
methane slip and heat load)
if not, hold at 7:1 steam : carbon for 2 hours
Increase throughput to design level
Increase pressure to design level
Always increase steam rate before feed rate
28. Steam Reformer Re-starts
Shorter re-reduction recommended
• typically 4-6 hours for heavy feeds
Not essential to carry-out reduction with
Natural gas or light off-gas feedstock
• start-up at 50% design rate, high steam:
carbon ratio
30. Case Study: Overfiring
Large modern top-fired steam reformer
Significant tube failures during start-up
Caused by over-firing at start-up due to a
number of coincident factors
31. Case Study: Background
Site steam shortages requiring conservation of
steam
Pressure to avoid a shut-down (due to low
product stocks)
Burner fuel usually from two sources, mixed:
• one low-calorific value
• one high-calorific value
At time of incident, all high-calorific value
(unexpectedly) fuel received
Operators had seen many shut-down/start-ups
during past two years
32. Case Study: Events
Plant trip (loss of feedstock to steam reformer)
due to valve failure
Feedstock to steam reformer not isolated
adequately by valve
Setpoint on reformed gas pressure not reduced
Steam introduced for plant restart at reduced
rate
All burners lit (deviation from procedure)
Reformer tubes remained at normal
operating pressure of 16 barg (232 psig)
33. Case Study: Events (Contd.)
Steam reformer tubes “looked normal”
Nearly 3x as much fuel going to burners than there should
have been
High calorific value fuel added an extra 15% heat release
First tubes rupture
High furnace pressure (trip bypassed)
Oxygen in flue gas dropped to zero
Flames seen from peep holes
Normal furnace pressure
Visual inspection revealed “white hot furnace” and tubes
peeling open
30minutes
Emergency Shutdown Activated!
34. Case Study: Conclusions
Reformer exit gas temperature on panel never
exceeded 700oC (1290oF)
• cannot use this instrument as a guide to tube
temperature
Reformer start-up at normal operating pressure
• tube failure temperature 250oC (450oF) lower
than normal for start-up
All burners lit
• far too much heat input resulted in excessive
temperatures
35. Summary
Start-up procedures
• Warm-up
Feedstock isolation
Rate
Condensation
True temperatures/Tube temperatures
• Catalyst reduction
Using hydrogen
Using hydrocarbon
Feed introduction
• Case Study