Reduction and Start-Up of Steam Reforming Catalyst

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Introduction
Start-up procedures
Warm-up
Catalyst reduction
Feed introduction
Case study

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Reduction and Start-Up of Steam Reforming Catalyst

  1. 1. Reduction and Start-up of Steam Reforming Catalyst By: Gerard B. Hawkins Managing Director, CEO
  2. 2. Contents  Introduction  Start-up procedures • Warm-up • Catalyst reduction • Feed introduction  Case study
  3. 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. 4. As supplied - NiO on support Active species - Ni crystallites Reduction process needed: NiO + H2 Ni + H2O Steam Reforming Catalyst
  5. 5. 400 500 600 700 800 100 200 300 500 700 Temperature oC (oF) Partial Pressure of H2O / Partial Pressure H2 EquilibriumConstant Reducing Conditions Oxidizing Conditions (752) (932) (1112) (1292) (1472) Reduction of Bulk Nickel Oxide
  6. 6. NiO Reduction  Faster at high temperature  Slower in presence of steam  Thermodynamically, very little hydrogen needed  Support can affect ease of reduction
  7. 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!
  8. 8. Contents  Introduction  Start-up Procedure • Warm-up • Catalyst Reduction • Feed Introduction  Case Study
  9. 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. 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. 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. 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. 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
  14. 14. Warm-up - Condensation Ensure that all drain points are operational To steam reformer Steam Condensate to drain
  15. 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. 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
  17. 17. Contents  Introduction  Start-up procedure • Warm-up • Catalyst reduction • Feed introduction  Case study
  18. 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. 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. 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. 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. 22. Hydrocarbons  May be converted to carbon oxides in reformer  May crack thermally to give carbon
  23. 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. 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
  25. 25. Contents  Introduction  Start-up procedure • Warm-up • Catalyst reduction • Feed introduction  Case study
  26. 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. 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. 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
  29. 29. Contents  Introduction  Start-up procedure • Warm-up • Catalyst reduction • Feed introduction  Case study
  30. 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. 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. 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. 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. 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. 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

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