Steam Reforming - Practical Operations

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Reaction Chemistry
Typical Reformer Configurations
Catalyst Design Criteria
Carbon Formation and Prevention
Catalyst Deactivation
Steaming Reforming Catalysts
Monitoring Reforming Catalysts
Catalyst Loading
Reduction & Start-Up
Tube Wall Temperature Measurement

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Steam Reforming - Practical Operations

  1. 1. Steam Reforming Practical Operation C2PT Catalyst Process Technology By Gerard B Hawkins Managing Director, CEO
  2. 2. Contents Reaction Chemistry Typical Reformer Configurations Catalyst Design Criteria Carbon Formation and Prevention Catalyst Deactivation Steaming Reforming Catalysts Monitoring Reforming Catalysts Catalyst Loading Reduction & Start-Up Tube Wall Temperature Measurement
  3. 3. Steam Reforming Reactions The conversion of hydrocarbons to a mixture of CO, CO and H2 Two reactions: Reforming and Shift Water gas shift (slightly exothermic) CO + H2O CO2 + H2 Steam Reforming (very endothermic) CH4 + H2O CO + 3H2 CnH(2n+2) + nH2O nCO + (2n+1)H2 Overall the reaction is highly endothermic
  4. 4. Equilibrium Considerations  Both reforming and shift reactions are reversible  Rate of shift is fast compared to reforming  Methane conversion favoured by: low pressure high temperature high steam to carbon ratio  CO conversion to CO2 favoured by: low temperature high steam to carbon ratio  GBHE kinetics developed using full size reformer tube with whole pellets under industrial conditions
  5. 5. Reformer is a Heat Exchanger
  6. 6. Primary Reformer  The Primary Reformer is a heat exchanger  Its function is to heat up process gas  Catalyst and reaction in the tubes  Combustion on the shell side  Dominant heat transfer by radiation  Multiple fuel supply points
  7. 7. Reformer Furnace  3 major types of reformer  Each tackles the duty in different ways  No clear best choice  Choice dictated by Contractor history  Terrace wall - Foster Wheeler  Side Fired - Topsoe, Selas, Chiyoda  Top Fired - H & G, Davy, Toyo, Howe Baker, Kellogg, KTI etc
  8. 8.  Many reformers now heat transfer limited  Catalyst not limiting reformer size or operation  Especially the case for hydrogen & methanol plants  Important for design and uprating  Fluegas exit flow and temperature often limits  Heat exchange must not be ignored  Claims of +30% capacity treated with caution Reformer Furnace
  9. 9. Top Fired Reformer
  10. 10. Side Fired Reformer
  11. 11. Terrace Walled Reformer
  12. 12. Heat Transfer - Top Fired  Nearly all heat transfer is by radiation  Radiation from the fluegas to the tubes  Little direct radiation from refractory to tube  Refractory acts as a reflector  Radiation from flame to tube at tube top
  13. 13. Heat Transfer - Top Fired Radiative heat flows
  14. 14. Heat Transfer - Side Fired / Terraced  Same for side fired and terrace walled  Nearly all heat transfer is by radiation  Radiation from the fluegas to the tubes  Major direct radiation from refractory to tube  Significant heat transferred from flame to wall  Carried out by convection  Radiation from flame to tube all down
  15. 15. Typical Reforming Configuration Steam Steam Steam + Gas Steam Reformer 500°C 870°C 1200°C 3% CH4
  16. 16. Reformer Firing Balancing  Must have an even heat input to the furnace  Same reasons as for an even process gas flow  Exit temperature variations give high average approach  Need to keep exit temps the same  Trim air and fuel flows to individual burners  Monitor tube wall temps and exit temps  Must be done as air ducting, tunnels etc may have a systematic effect upon heat input
  17. 17. Reformer Firing Balancing  Usual problems for uneven heat input:-  Burner problems  Burner fouling with liquid fuels or offgas  Air leaks around burners or tube tops  Tunnel problems (mods or collapses)  Air ducting problems (internal refractory)
  18. 18. Typical Primary Reformer Catalyst Loading LoadedLength-12.98m HalfLoadDip-7.38m Tubes 352 Tubes id : 95 mm Loaded Length : 12.98 m Catalyst Types : 50% VSG-Z101 50% VSG-Z102 Loaded density : VSG-Z101 0.857 kg/l VSG-Z102 0.828 kg/l Total volume loaded 32.38m3 Full Tube Dip 0.89m Catalyst Support Grid VSG-Z102 6.49m VSG-Z101 6.49m Ammonia Plant
  19. 19. Primary Reformer  Reforming involves heating the process gas  The position of equilibrium is constantly changing  The catalyst tries to react the gas to equilibrium  The catalyst is essentially chasing the heat input  Top of tube: slow reaction rate, high heat flux  Bottom of tube: high reaction rate, low heat flux  High approach to equilibrium at top of tube, low approach at bottom of tube  Can never achieve a zero approach to equilibrium
  20. 20. 0 100 200 300 400 500 600 700 800 900 0 0.2 0.4 0.6 0.8 1 Fraction down tube Temperature(°C) Gas T Eqm. T ATE Approach to Equilibrium
  21. 21. Primary Reformer Catalyst Requirements  High and Stable Activity  Low Pressure Drop  Good Heat Transfer  High resistance to Carbon Formation  High Strength  Robust Formulation / Simple Operation
  22. 22. High and Stable Activity  Low methane slip  Lower tube wall temperature  Reduced Fuel usage
  23. 23. Low Pressure Drop  Savings in Compression Power / Fuel  Possible Throughput increase
  24. 24. Improved Heat Transfer  Reduced tube wall temperatures  Increase firing /higher throughput  Smaller catalyst particles improve heat transfer from wall to bulk gas  Smaller particles increase pressure drop  Catalyst shape should be optimised for high heat transfer with low PD
  25. 25. Steam Reforming Catalysts  Nickel on a ceramic support  Three key factors in catalyst design: i) geometric surface area ii) heat transfer from tube to gas iii) pressure drop  Also of concern: i) packing in the tube ii) breakage characteristics
  26. 26. Diffusion Limitation  The reforming reaction is very fast on the Ni sites  Reaction limited to catalyst surface (<0.1mm)  Reaction rate controlled by film diffusion  High geometric surface area gives high activity
  27. 27. Diffusion into and out of Catalyst Bulk Gas Gas Fil m Ni Sites CO H O2 H2 CH4 Catalyst Support
  28. 28. Key Reaction Steps 1. Fast Diffusion of the molecules in the bulk gas phase 2. Slow Diffusion of the molecules through the gas film 3. Slow Diffusion through catalyst pores 4. Fast Absorption of the molecules onto the Ni sites 5. Fast Chemical reaction to produce CO2 and H2
  29. 29. Reaction Rate Reaction rate controlled by film diffusion - Most of the reaction takes place on the catalyst surface (<0.1mm) - Pore diffusion not limiting as film diffusion controls the overall rate Catalysts with higher geometric surface area (GSA) per unit volume of catalyst will have a higher activity. Pore size/distribution is not significant for most commercial grades of reforming catalyst
  30. 30. Un-sintered Catalyst 0.001 mm (1/25 thou)
  31. 31. Sintered Catalyst 0.001 mm (1/25 thou)
  32. 32. Outside Tube Wall T 830°C Fluegas T 1200°C Inside Tube Wall T 775°C Gas film Tube Wall Heat Transfer Bulk Process Gas T 715°C
  33. 33. Catalyst Heat Transfer  Reforming involves large heat flows into tubes  Absolute requirement to keep tubes cool  Major limitation is at the tube wall  Need to minimize thickness of stationary gas film at tube wall  The catalyst acts as a heat transfer enhancer to improve heat transfer from tube wall to gas  Promotes turbulence at the wall  Promotes gas mixing from walls to tube centre  Smaller catalyst particles improve heat transfer from wall to bulk gas and hence reduce tube wall temperatures
  34. 34. Catalyst Heat Transfer  Heat transfer to catalyst normally very good (high GSA)  Minor limitation is radially in the catalyst  Catalyst also improves radial heat transfer  Smaller pellets improve wall transfer  Larger pellets improve radial transfer  Smaller usually better overall  BUT smaller particles increase pressure drop  Catalyst shape needs to be optimized for high heat transfer with low PD
  35. 35. Catalyst Shape  The traditional catalyst shape is a ring  Smaller rings give higher activity and heat transfer but higher pressure drop  Shape optimised catalysts offer high GSA and heat transfer with low PD  Important that shape also provides good packing and breakage characteristics
  36. 36. Tube Wall Temperature Profile Top Fired Reformer 660 680 700 720 740 760 780 800 820 840 860 0 0.2 0.4 0.6 0.8 1 Fraction Down Tube TubeWallTemperature(°C) Base case with twice GSA Base case with twice heat transfer Base case
  37. 37. Heat Transfer and Pressure Drop 1 2 3 4 1 2 3 4 Voidage 0.49 0.6 0.58 0.59 Relative PD 1 0.9 0.9 0.8 Relative HTC 1 1.3 1.1 1
  38. 38. Catalyst Design Criteria Conclusions Design of catalyst shape is a complex optimization of: - Higher GSA (Needed for activity - diffusion control) - Higher HTC (Needed for cooler reformer tubes) - Lower Pressure drop (Plant Efficiency / Capacity) Need also to consider breakage characteristics and loading pattern inside the reformer tube
  39. 39. VULCAN VSG-Z101 VULCAN VSG-Z102
  40. 40. Catalyst Breakage  Catalyst breaks up in service  Main mechanism due to startup / shutdown  The tube when cooling exerts massive forces (several tonnes)  Forces exerted by carbon formation immeasurable  Pressure drop rises about 10% per year  Minimum Catalyst strength for handling & charging approx. 10 kgf  The key is to ensure the catalyst does not fragment into small bits/dust.  Careful charging essential
  41. 41. Breakage Characteristics Contraction of tube - some readjustment - some breakage Cold ColdHot Initial catalyst level Expansion of tube - some settling All catalysts show breakage with time No support can withstand tube forces
  42. 42. Pressure Drop due to Catalyst Breakage Relativepd(%) % Breakage 100 200 0 5 10 15 20 pd limit Shape with good breakage characteristics Shape with poor breakage characteristics Conventional rings Breakage Characteristics is an Important Consideration
  43. 43. Breakage Characteristics
  44. 44. Breakage Characteristics
  45. 45. Packing Characteristics Uniform loading of catalyst - Uniform tube pressure drops - Uniform tube temperatures (no hot spots) Long cylinders with hole(s) through the centre give good uniform packing Short cylinders (tablets) with hole(s) through the centre can stack resulting in poor gas distribution down the reformer tube
  46. 46. Packing Characteristics Extended External Surface Area - "Cogs" Void
  47. 47. Catalyst Support Three types commercially available – Alpha Alumina – Calcium Aluminate – Magnesium Aluminate Spinel
  48. 48. Catalyst Support - Bulk Chemistry Alpha Alumina Calcium Aluminate Magnesium Aluminate Spinel Structure Corundum Spinel-like Spinel Stability to Sintering Extremely Stable Relatively Stable Relatively Stable Chemical Stability (Hydrolysis) Inert Stable ‘Free’ MgO Hydration under Steaming Conditions
  49. 49. Catalyst Support - Surface Chemistry Alpha Alumina Calcium Aluminate Magnesium Aluminate Spinel Surface Area Low Higher Higher Basicity Inert Basic Sites Most Basic Support Surface Interaction with Ni / NiO No Chemical Interaction Moderately Reactive Surface Some bonding of Ni 2+ ions Most Reactive Surface Strongest bonding of Ni 2+ ions
  50. 50. Catalyst Support - Solid Solutions Magnesium Aluminate Spinel NiO / MgO Solid solution NiAl2O4 formed NiO / Ni Fresh catalyst High surface area Heat In use - Low surface area Difficult to reduce NiO Important to consider in-service activity and ease of catalyst reduction
  51. 51. Catalyst Support - Reduction Temperatures AlphaAlumina CalciumAluminate Temperature (°F) Temperature (°C) 800 1000 1200 1400 1600 400 500 600 700 900 Magnesium aluminate spinel material usually supplied pre-reduced MagnesiumAluminateSpinel
  52. 52. Tube Wall Temperature Stability 0 200 400 820 840 860 880 900 920 940 DAYS ON LINE ICI RINGS COMP A SHAPE COMP B SHAPE COMP B SHAPE 2 ICI SHAPE POWEROUTAGE CATALYSTCHANGE AT584DAYS REDUCED CATALYSTCHANGE AT280DAYS PDLIMITRATE REDUCED CATALYSTCHANGE AT421DAYS 0 0 0 0200 400200 200 200 TWT LIMIT RATE25% CATALYSTCHANGE AT258DAYS 600 700 800 900 MAXIMUMTUBEWALLTEMPERATURE(°C)
  53. 53. Carbon Formation and Prevention  Carbon formation is totally unwanted  Causes catalyst breakage and deactivation  Leads to overheating of the tubes  In extreme cases carbon formation causes a pressure drop increase
  54. 54.  Cracking CH4 ⇔ C + 2H2 C2H6 ⇔ 2C + 3H2 etc  Boudouard 2CO ⇔ C + CO2  CO Reduction CO + H2 ⇔ C + H2O If carbon formation rate is faster than removal rate then carbon will be deposited
  55. 55. Carbon Formation - Heavy Feeds Hydrocarbon Feed Intermediates (Olefins, Paraffins, CH4, & H2) Carbon CH4, H2, CO2 & CO Catalyzed Partial Decomposition Thermal Cracking Polymerization Steam Reforming Carbon Gasification H2O H2O
  56. 56. Effect of Carbon Formation 1. Physical poisoning -Carbon covers the catalyst surface 2. Pressure drop increase - Usually only in severe situations - Carbon fills catalyst bed voids -Carbon formed in catalyst pores will weaken or break catalyst 3. Hot tubes - Carbon laydown on the inside of the tube wall - Lower catalyst activity
  57. 57. Carbon Formation and Prevention Giraffe Necking Hot TubeHot Band Reformer tube appearance - Carbon laydown
  58. 58. Carbon Formation and Prevention  Under normal conditions carbon gasification by steam and CO2 is favored i.e. gasification rate > C formation rate)  Problems of carbon formation may occur when: i) steam to carbon ratio is too low ii) catalyst is not active enough iii) higher hydrocarbons are present iv) tube walls are too hot (high flux) v) catalyst has poor heat transfer characteristics
  59. 59. Carbon Formation and Prevention  Methods of preventing carbon formation: – Use more active catalyst – Use better heat transfer catalyst – Reduce levels of higher hydrocarbons – Increase the steam ratio – Use a potash doped catalyst (VULCAN- series) which reduces probability of carbon formation
  60. 60.  Alkali greatly accelerates carbon removal  Addition of potash to the catalyst support reduces carbon formation in two ways: a) increases the basicity of the support b) promotes carbon gasification (aids adsorption of water) C + H2O ⇔ CO + H2  Potash is mobile on the catalyst surface  Level of potash required depends on feed and heat flux  Potash doped catalyst is only needed in the top half of the reformer tube Carbon Formation and Prevention OH -
  61. 61.  Increasing the content of alkali (potash) allows: Higher heat flux for light feeds Heavier hydrocarbons in feed Lower steam to carbon ratios Faster carbon removal during steaming Carbon Formation and Prevention
  62. 62. Carbon Formation and Prevention Increasing potash addition Methane feed/Low heat flux Methane feed/High heat flux Propane, Butane feeds (S/C >4) Propane, Butane feed (S/C >2.5) Light naphtha feed (FBP < 120 °C) Heavy naphtha feed (FBP < 180 °C) K2O wt% 0 2-3 4-5 6-7
  63. 63. Methane Cracking 100 10 1.0 0.1 Temperature (°C ) (pH2) 2 pCH4 Carbon Formation Zone No Carbon Formation CH4 2H2 + C 550 600 650 700 750 800
  64. 64. Methane Cracking - Kinetic Limitation Carbon Formation Zone No Carbon Formation Deposition rate < removal rate Promoted by alkali Deposition rate > removal rate Promoted by acid 550 600 650 700 750 800 Temperature (°C ) 100 10 1.0 0.1 (pH2) 2 pCH4
  65. 65. Methane Cracking - Kinetic Limitation 0.6 0.5 0.4 0.3 Fraction of tube length from top 550 600 750 800 Temperature C 100 10 1.0 0.1 Carbon Formation Zone No Carbon Formation Deposition rate < removal rate Deposition rate > removal rate 650 700650 700 O (pH2) 2 pCH4
  66. 66. Methane Cracking - Basic Catalyst Support 0.6 0.5 0.4 0.3 0.25 More basic support 550 600 800 Temperature C 100 1.0 0.1 No Carbon Formation 650 700 750 10 Carbon Formation Zone O (pH2) 2 pCH4
  67. 67. Methane Cracking - Increased Potash Content Carbon Formation Zone Increasing Potash Content 550 600 800 Temperature C 100 1.0 0.1 No Carbon Formation 650 700 750 10 0.6 0.5 0.4 0.3 0.25 O (pH2) 2 pCH4
  68. 68. Carbon Formation and Prevention Fraction Down TubeTop Bottom Non-Alkalised Catalyst Ring Catalyst Optimised Shape (4-hole Catalyst) Inside Tube Wall Temperature 920°C 820°C 720°C 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Alkalised Catalyst Carbon Forming Region
  69. 69. Carbon Formation and Prevention For light feeds and LPG etc. using lightly alkalized catalyst (VSG-Z102) - Potash is chemically locked into catalyst support - Potash required only in the top 40-50% of the reformer tube - Catalyst life influenced by Poisoning Ni sintering Process upsets etc. VSG-Z101 VSG-Z102
  70. 70. Carbon Formation and Prevention For heavy feeds, potash needs to be mobile Utilize VSG-Z101 series catalyst Removes carbon on the catalyst surface and inside of the tube wall Potash slowly released by a complex reaction VSG-Z101 VSG-Z102
  71. 71. Carbon Formation and Prevention Potash release mechanism (VSG-Z102 series) K2O-Al2O3-SiO2 CaO-Al2O3 MgO-Al2O3 CO2 + H2 CaO-Al2O3-SiO2 CaO-MgO-SiO2 K2CO3 H2O 2KOH + CO2 Note:- MgO is chemically locked into catalyst support material Catalyst Support Material
  72. 72. Carbon Formation and Prevention VSG-Z102 Series catalysts  Catalyst life determined by residual potash remaining in the catalyst  Minimum amount typically 2-3 %wt at bottom of potash promoted catalyst bed - but minimum level depends on feedstock type and operational severity
  73. 73.  Carbon formation by polymerization – Suppressed by having NiO / MgO solid solutions as the active catalyst component – Need to increase total NiO content to overcome loss of steam reforming activity – Zirconia addition also further enhances catalyst activity Carbon Formation and Prevention
  74. 74. Potash promotion  Lowers hydrocarbon carbon cracking rate  Increases carbon removal rate  VSG-Z102 series can remove carbon from tube wall ("mobile" potash)  VSG-Z102 series contain some NiO/MgO solid solutions to lower polymerisation activity  Greatly facilitates carbon removal during steaming operation (after severe carbon formation) Summary
  75. 75. Natural Gas Reforming Catalysts Associated Gas Reforming Catalysts Dual Feedstock Reforming Catalysts Naphtha Reforming Catalysts Un-alkalised Lightly Alkalised Moderately Alkalised Heavily Alkalised VSG-Z101 VSG-Z102 VSG-Z102 VSG-Z102 Naphtha 3.0 – 3.5 Light Naphtha 6.0 – 8.0 3.0 – 4.0 2.5 – 3.0 Butane 4.0 – 5.0 2.5 – 3.5 2.0 – 3.0 Propane / LPG 3.0 –4.0 2.5 – 3.0 2.0 – 2.5 Refinery Gas 6.0 – 10.0 3.0 – 4.0 2.0 – 3.0 2.0 – 2.5 Associated Gas 5.0 – 7.0 2.0 – 3.0 2.0 – 2.5 Natural Gas 2.5 – 4.0 1.5 – 2.0 1.0 – 2.0 Pre-reformed Gas 2.0 – 3.0 1.0 – 2.0 1.0 – 2.0
  76. 76. Catalyst Activity Die Off 2 major factors:  Poisoning by sulfur Affects upper tube and tube temperatures  Thermal sintering Affects lower tube and approach Some effect on upper tubes and tube temperatures
  77. 77. Poisons  Many poisons affect reforming catalysts  Halides, phosphates, sulfur  Heavy metals, alkali metals etc  Major poisons are: Sulfur ex feedstock Phosphate ex BFW Metals ex BFW or liquid feed
  78. 78. Sulfur Poisoning  Nickel is a very good sulfur adsorbent  Sulfur sticks to the nickel surface  Do not need a lot of sulfur to give problems  Can totally deactivate a reforming catalyst
  79. 79. Sulfur Poisoning Pellet S SS S S S S S S S Nickel CH4 H2O
  80. 80. Sulfur Poisoning  Depends upon the catalyst temperature  Occurs in the cooler regions  Upper section of tubes vulnerable  Also depends upon sulfur exit the HDS
  81. 81. Effect of sulfur coverage on activity 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Coverage Activity
  82. 82. Sulfur Poisoning  Sulfur poisoning is reversible  Steam the catalyst for 24 hours  Steam slowly strips off the sulfur  Re-oxidizes the catalyst in addition  May lose some activity permanently
  83. 83. Steaming Reforming Catalysts  Steaming of the catalyst is required when there is:  Severe carbon formation Loss of steam Incorrect steam to carbon ratio operation sulfur poisoning  sulfur poisoning Poor performance of the desulfurization system
  84. 84. Steaming Reforming Catalysts  Isolate hydrocarbon feed  Maintain steam flow at highest possible level  Adjust reformer firing to achieve 750°C reformer exit temperature or higher if possible  Monitor carbon oxides or H2S in the exit gases  Addition of a small amount of nitrogen into the steam facilitates reliable sample analysis
  85. 85. Potash promoted catalyst  Accelerates carbon gasification during steaming  Required since carbon laydown often occurs in the top section of the reformer tubes where high temperatures needed for steaming are not easily achieve  Design of catalyst needs to ensure potash release is controlled during steaming  Release rate for VSG-Z102 series is approximately double the normal rate during steaming (24 hours of steaming ages the catalyst by 48 hours) Steaming Reforming Catalysts
  86. 86. Effect of Steaming - Alkalized Catalyst Steaming Temperature °C (equivalent to 1 year operation) Potash Retention - Steaming Test 500 550 600 650 700 750 0 0.5 1 1.5 2 2.5 3 Residualwt%ofpotash VSG-Z102 Comp. A Comp. B
  87. 87. Steaming Reforming Catalysts Carbon Removal By Steaming 50 0 100 150 200 1.2 0 0.2 0.4 0.6 0.8 1 1.4 1.6 Time (hours) 0 5 10 15 20 ResidualCarbon(%)
  88. 88. 5 10 15 20 25 30 35 2.5 0 3 3.5 4 4.5 Time on line (months) HDS Problem Catalyst Steamed Methaneslip(mol%dry) Design Steaming Reforming Catalysts
  89. 89. Sulfur removal  Catalyst performance can be restored  High reformer inlet temperature during steaming is important for successful sulfur removal  Need to monitor the H2S slip during steaming Steaming Reforming Catalysts
  90. 90. Sulfur Levels in Discharged Catalyst 0 100 200 300 0 10 20 Distance Down Reformer Tube (m) sulfur(ppmwt) Before Steaming After Steaming
  91. 91. Steaming Reformer Catalyst  MgO in catalyst support must not be "free" otherwise during steaming, the MgO will hydrolyse MgO + H2O Mg(OH)2  Hydration of the MgO causes rapid loss of catalyst strength and severe catalyst break-up and high reformer pressure drop  VSG-Z102 series catalyst contain MgO that is chemically locked into the the catalyst support - No hydration
  92. 92. Reformer Catalyst Monitoring
  93. 93. Reformer Catalyst Monitoring Monitor frequently (daily)  Exit Methane  Tube Wall Temperature (TWT)  Tube Appearance Monitor Less Frequently  Pressure Drop  Approach to Methane Steam Equilibrium
  94. 94.  Methane Slip – Dependant on throughput, heat load & catalyst activity – If these conditions vary then exit CH4 will vary  Approach to Equilibrium – ATE defined as Difference between Actual Temperature & Equilibrium Temperature – Better guide to catalyst activity – ATE increases as catalyst activity decreases Reformer Catalyst Monitoring
  95. 95. Reformer Catalyst Monitoring  Tube Wall Temperature – Dependant on catalyst loading, catalyst activity & physical catalyst condition – As maximum TWT is approached, rate must be reduced. In worst case catalyst will need to be changed  Appearance – A good indication of how reformer is operating – Tubes should look cool. Poor catalyst performance will mean tubes looking hot. .
  96. 96. Pressure Drop –PD will increase with time due to physical blockage/breakage of catalyst –Too high PD will result in throughput limitation –Should back-calculate PD at design conditions (independent of throughput) Since PD α (velocity) 2 Normalised PD = Measured PD 100 % design( (2
  97. 97. Approach to Equilibrium (ATE)  The approach to equilibrium (ATE) at any point along the catalyst bed is the difference between the actual gas temperature and the equilibrium temperature corresponding to the gas composition.  The ATE can be used as a good measure of the performance of the catalyst when the operating temperature of the reactor is held constant, and when the reaction is equilibrium limited, such as with primary reforming.
  98. 98. Calculation of ATE Steam Reforming Reactions CH4 + H2O ⇔ CO + 3H2 Methane Steam (MS) CO + H2O ⇔ CO2 + H2 Water Gas Shift (WGS) Since the WGS reaction is so fast it can be assumed to be at equilibrium under reformer exit conditions This means then the equilibrium temperature for this reaction (TWGS) can be used as a reliable estimate of the actual reformer exit temperature (Measurements are unreliable)
  99. 99. For the WGS reaction the equilibrium constant (KWGS) can be calculated Then equilibrium tables can be used to determine the equilibrium temperature for this reaction (TWGS) For the MS reaction the equilibrium constant (KMS) can also be calculated and equilibrium tables then used to determine the equilibrium temperature for this reaction (TMS)
  100. 100. The ATE can then be calculated as the difference between TWGS (equal to the actual reformer exit temperature) and TMS GBHE uses a computer program to calculate ATE
  101. 101. Example Calculation of ATE Reformer Exit Composition % v/v H2 68.0 N2 1.6 CH4 9.6 CO 10.2 CO2 10.6 Total 100.0 Dry H2O 76.6 Total 176.6 Wet Reformer Exit Pressure = 31.6 ata Reformer Exit Temperature (TWGS) = 796°C
  102. 102. pCH4 = 9.6 x 31.6 = 1.7178 ata 176.6 pH2 = 68.0 x 31.6 = 12.1676 ata 176.6 pH2O = 76.6 x 31.6 = 13.7065 ata 176.6 pCO = 10.2 x 31.6 = 1.8251 ata 176.6
  103. 103. Example Calculation of ATE KMS = pCH4 . pH2O pCO . (pH2)3 = 1.7178 x 13.7065 1.8251 x (12.1676) 3 = 7.161 x 10-3 From Tables TMS = 792°C ATE = 796 - 792 = 4°C
  104. 104. Catalyst Handling  Catalysts are expensive & should be treated with care at all stages of: – Handling on arrival – Storage – Charging – Storage in vessel before start-up  Careful & detailed supervision at all stages is essential  Safety: proper equipment is essential both for the safety of workers & to prevent damage to catalyst
  105. 105. Handling & Storage  On Arrival – Use suitable fork lift truck or crane to transfer to storage – Don't drop drums off tail board of lorry – Don't roll drums – Inspect drums for damage & repair broken lids  Storage – Store under cover (long term storage) – Avoid damp / wet conditions – Store drums in upright position – Stack no higher than 4 drums – Catalyst not affected by extremes of temperature – (-50°C to +50°C) provided kept dry.
  106. 106. Catalyst Loading  If loading is poor, variety of flows in tubes  Each tube has different exit temperature  Each tube has a close approach  Methane slip not linear with temperature  Mixture of all tubes far from equilibrium  Made worse by the flow imbalance
  107. 107. Base Case Reformer Exit: 20 ata 870°C design 10 °C approach to equilibrium Maldistribution 10 °C approach to equilibrium Tube 1: 105% flow 850°C Exit T Tube 2: 95% flow 890°C Exit T
  108. 108. Base Case Maldistribution Case Tube 1 Tube 2 Exit Temperature (°C) 870 850 890 Relative Flow (%) 100 105 95 Approach to Equilibrium (°C) 10 10 10 Methane Slip (% dry) 3.583 4.698 2.687 Average Methane Slip (% dry) 3.583 3.743 Average Approach to Equilibrium (°C) 10 13.1
  109. 109. Catalyst Charging - Tubes  Inspect empty tubes  Check pressure drop on tubes both empty and full  'Sock' or 'Unidense' method recommended  Avoid excessive hammering and vibration  Final PDs should be within 5% of mean  Better to discharge tubes with high PDs rather than over-vibrate tubes with low PDs  Weighing is a useful check on charged bulk density, but not essential
  110. 110.  Catalyst supplied in pre-weighed socks
  111. 111.  Sock slightly narrower than tube bore  End of sock folded over
  112. 112.  Lowered down tube on rope  ‘Tugged’ to release fold  Free fall <0.5m allowable
  113. 113.  Vibrated with hammer after each sock
  114. 114.  PD measured empty / ½ full / full  Adjusted to ± 3 - 5%
  115. 115.  Outage Adjusted
  116. 116.  Norsk Hydro technology - available through Hydro Agri Europe  Simple & fast loading technique  No pre-socking and no tube vibration required  Applicable to a range of catalyst types & reformer designs  Offers high uniform catalyst density Catalyst Charging - Unidense Method
  117. 117. Charging Technique  Weighed amount of catalyst is poured into the tube & the loading rope is gradually pulled out of the tube as the catalyst layer builds up.  The brushes with flexible springs reduce the speed of the catalyst particles so that breakage is avoided.  This results in a loading without bridges & voids, hence there is no need for tube vibration / hammering. Catalyst Charging Unidense Method
  118. 118. Catalyst Charging - Unidense Method Support grid Charging chute Loading rope with flexible springs
  119. 119. Benefits  Reduced loading time  Reduced possibility of bridging / less hot spots  Contributes to lower tube wall temperatures and prolonged tube life  Narrow pressure drop variation in tubes  Slightly higher PD than sock method  Minimal further settling / PD increase Catalyst Charging - Unidense Method
  120. 120. Precommisioning / Periods of Shutdown  Completely close reactor after charging  Box up under N2 if necessary  After commissioning leave temperature points connected and check regularly during shutdown periods  Check drains regularly  After shutdown keep under positive N2 pressure (natural gas OK for sulfur removal catalysts)  On decommissioning Nickel containing catalysts must be purged free from carbon oxides before temperature falls below 250°C
  121. 121. Normally  Process feed on flow control  Process steam on ratio control from feed rate  Purge fuel / flash gas to fuel header  Fuel header on pressure control  Fuel to reformer on flow control ◦ Adjusted to maintain reformer exit temperature
  122. 122. Reduction & Start-Up
  123. 123.  Introduction  Start-up Procedures  Warm-up  Catalyst Reduction  Feed Introduction  Shut-down  Case Studies Contents
  124. 124.  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 Introduction
  125. 125. As supplied - NiO on support Active species - Ni Crystallites Reduction process needed:- NiO + H2 ⇔ Ni + H2O Introduction - Catalyst Reduction
  126. 126. 400 500 600 700 800 100 200 300 500 700 Temperature °C (°F) Partial Pressure of H2O / Partial Pressure H2 EquilibriumConstant Reducing Conditions Oxidising Conditions (752) (932) (1112) (1292) (1472) Introduction - Catalyst Reduction
  127. 127.  Faster at high temperature  Slower in presence of steam  Thermodynamically, very little hydrogen needed  Support also affects ease of reduction Introduction - Catalyst Reduction
  128. 128. Extreme danger of local overheating!  Requires high temperature - fire steam reformer  Requires reducing conditions - supply H2 or reducing 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 Introduction - Catalyst Reduction
  129. 129.  Introduction  Start-up Procedure  Warm-up  Catalyst Reduction  Feed Introduction  Shut-down  Case studies Start-Up Procedure
  130. 130. Air warm-up possible, but not for previously reduced catalyst (possible carbon)  Purge plant of air with N2 (Care: must be free of hydrocarbons and carbon oxides)  Heat reformer above condensation temperature  Add steam when exit header temperature 50°C above condensation temperature (low pressure favours good distribution and lowers this temperature)  Increase steam rate to 40 - 50 % of design rate (min 30%)  Stop N2 circulation Start-Up Procedure - Warm Up
  131. 131.  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  reformer tube tensioners  reformer tubes  refractory linings Traditionally: 50°C per hour Modern material: 100°C per hour Catalyst: 150 - 170°C per hour Start-Up Procedure - Warm Up
  132. 132.  If upstream pipe-work cold, good practice to warm up by steam flow to vent to prevent carry-over of water. Steam Steam Reformer Cold Pipe-work Start-Up Procedure - Warm Up
  133. 133.  Temperatures referred to are true catalyst temperatures at exit of tube  Measured temperatures during normal operation are 10 -100°C 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 essential during start-up Start-Up Procedure - Warm Up
  134. 134. Effect of Pressure and Temperature 1 10 100 1,000 10,000 100,000 1,000,000 10,000,000 Tube Wall Temperature °C (°F) TubeLife(hours) 800 900 1000 1100 1200 (1500) (1650) (1830) (2010) (2200) 5 bar30 bar Start-Up Procedure - Tube Life
  135. 135.  Introduction  Start-up Procedure  Warm-up  Catalyst Reduction  Feed Introduction  Shut-down  Case studies Start-Up Procedure - Catalyst Reduction
  136. 136.  Reduction with Hydrogen  Reduction with Natural Gas  Reduction with other sources of hydrogen  Higher hydrocarbons  Ammonia  Methanol Start-Up Procedure - Catalyst Reduction
  137. 137.  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 >700°C  Hold for 2-3 hours Catalyst Reduction with Hydrogen
  138. 138. Hydrogen must be free of poisons (S, CI) 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 Catalyst Reduction with Hydrogen
  139. 139.  Recirculation loop may include HDS unit (at temperature)  Carbon oxides above 250°C (480°F) methanate over unsulphided CoMo catalyst: temperature rise 74°C per 1% CO converted temperature rise 60°C per 1% CO2 converted If H2 contains > 3 % CO or > 13 % CO2 or a mixture corresponding to this then by-pass the HDS system Catalyst Reduction with Hydrogen
  140. 140. Natural Gas  Will be converted to carbon oxides + hydrogen in reformer  May crack thermally to give carbon Catalyst Reduction with Natural Gas
  141. 141.  Warm-up as before (N2 then steam)  Introduce natural gas at 5% of design rate  Slowly increase gas rate to give 7:1 steam:carbon over 2-3 hours  Simultaneously increase reformer exit temperature to design level i.e. >700°C  Increase inlet temperature as much as possible (to crack natural gas to give H2)  Monitor exit methane hourly  Reduction complete when methane reaches low, steady value (4 to 8 hours) Catalyst Reduction with Natural Gas
  142. 142. E.g. propane  Increased possibility of carbon formation  Much greater care needed  Longer time periods needed  More precision in all measurements needed  Hydrogen addition recommended if possible  Purification issues - Desulfurization - Methanation of carbon oxides Catalyst Reduction with Higher Hydrocarbons Not normally recommended
  143. 143.  Crack ammonia in ammonia cracker  Crack ammonia in steam reformer  inject liquid ammonia upstream of steam reformer  bypass HDS  Procedure as for hydrogen reduction  Exit temperature 800°C (1470°F) to maximise ammonia cracking Catalyst Reduction with Ammonia
  144. 144.  Uncommon Procedure  Methanol decomposes to give H2 and CO  Regulate flow of liquid methanol to give 6:1 - 8:1 steam:hydrogen ratio exit steam reformer  Do not recycle exit gas (potential methanation of carbon oxides) Catalyst Reduction with Methanol
  145. 145.  Introduction  Start-up Procedure  Warm-up  Catalyst Reduction  Feed Introduction  Shut-down  Case studies Start-Up Procedure - Feed Introduction
  146. 146.  Introduce feedstock at high steam:carbon ratio (5:1 for natural gas; 10:1 for higher hydrocarbons)  Steam reforming will give small increase in inlet pressure, cooling of tubes, and lower exit temperature  Need to increase firing to maintain exit temperature  Then increase feedstock flow  Increase pressure to operating pressure  Adjust steam:carbon ratio to design Start-Up Procedure - Feed Introduction
  147. 147.  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 Start-Up Procedure - Feed Introduction
  148. 148.  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 Start-Up Procedure - Restart
  149. 149.  Introduction  Start-up Procedure  Warm-up  Catalyst Reduction  Feed Introduction  Shut-down  Case Studies Case Studies
  150. 150.  Reduce tube exit temperature to 750°C  Decrease feed and steam flows in stages to 40% design - always decrease hydrocarbon flow first - adjust firing to keep exit temperature steady  Keep steam flow constant, shut off hydrocarbon feed - adjust firing to maintain exit temperature - purge system of hydrocarbons  Decrease exit temperature to 550°C at 100°C per hour Shut-down
  151. 151. Add flow of N2 and continue cooling Shut off steam 50°C above condensation temperature Continue cooling with N2 flow When catalyst below 50°C tubes may be emptied Shut-down
  152. 152.  Introduction  Start-up Procedure  Warm-up  Catalyst Reduction  Feed Introduction  Shut-down  Case Studies Case Studies
  153. 153.  Large modern top-fired steam reformer  Significant tube failures during start-up  Caused by overfiring at start-up due to a number of coincident factors Case Studies - No 1
  154. 154.  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 shutdown/start-ups during past two years Case Studies - No. 1
  155. 155.  Plant trip (loss of feedstock to reformer) due to valve failure  Feedstock to 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 Case Studies - No. 1
  156. 156.  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" Emergency Shutdown Activated! 30minutes Case Studies - No. 1
  157. 157.  Reformer exit gas temperature on panel never exceeded 700°C Cannot use this instrumentation as a guide to tube temperature  Reformer start-up at normal operating pressure Tube failure temperature 250°C lower than normal for start-up  All burners lit Far too much heat input resulted in excessive temperatures Tubes Fail Rapidly! Case Studies - No. 1
  158. 158. Ammonia Plant  LTS reduction loop included steam reformer  CO2 released from LTS reduction  Carbon formed in steam reformer Case Studies - No. 2
  159. 159.  LTS reduction with closed loop circulation  Normally condenser, compressor and pre-heat coil  This time included steam reformer, pre-heater and waste heat boiler  Steam reformer fired to TWT of 900°C  LTS reduction liberates CO2  By 2am, LTS reduction almost complete - 50 % CO2 in recirculation gas - also some H2 present Case Studies - No. 2
  160. 160.  Steam reformer pushed to give apparent LTS temperature of 200°C  Due to instrument error, in fact 380°C  Between 5am - 7 am, steam reformer PD increased  Tubes looked hot  Reformer steamed for 18 hours  No reduction in PD  Plant shutdown Case Studies - No. 2 Reformer catalyst black and badly broken up - due to severe carbon formation
  161. 161. Check to ensure that recirculation loops do not contain high levels of carbon oxides Case Studies - No. 2  CO2 can shift in LTS to CO CO2 + H2 CO + H2O  CO in presence of H2 gives carbon CO + H2 C + H2O  CO can methanate in steam reformer (if some catalyst reduction due to presence of H2 is seen) forming CH4 CO + 3H2 CH4 + H2O  This cracks to form carbon CH4 C + 2H2
  162. 162. Importance of Tube Wall Temperature Measurement  Need accurate information  Tube life  Artificial limitation on plant rate
  163. 163. Effect of Tube Wall Temperature on Tube Life 850 900 950 1000 (1560) (1650) (1740) (1830) Temperature °C (°F) TubeLife(Years) Design + 20°C 20 10 2 5 1 0.5 0.2
  164. 164. Tube Wall Temperature Measurement  Contact - Surface Thermocouple  "Pseudo-contact“ - Gold Cup Pyrometer  Non-contact Disappearing Filament Infra Red Optical Pyrometer Laser Pyrometer
  165. 165. Surface Thermocouples  Continuous measurement, by conduction  "slotting" can weaken tube wall  Spray-welding leads to high readings  Short, unpredictable lives (6 -12 months) Not commonly used for steam reformer tubes
  166. 166. Disappearing Filament  Hand held instrument  Tungsten filament superimposed on image of target  Current through filament altered until it "disappears“  Current calibrated to temperature  Range 800-3000°C Very operator sensitive Largely displaced by IR
  167. 167. Infra-red Pyrometer  Easy to use  Need to correct for emissivity and reflected radiation  Inexpensive
  168. 168. Laser Pyrometer  Laser pulse fired at target and return signal detected  Can determine target emissivity  Must correct for background radiation  High spacial selectivity  Very accurate for flat surfaces
  169. 169. Gold Cup Pyrometer  Excludes all reflected radiation  Approximates to black body conditions  High accuracy / reproducibility  BUT - Limited access - Awkward to use
  170. 170. Gold Cup Pyrometer Tube Furnace Wall Water Cooling To Recorder Gold Cup Lance *
  171. 171. Accurate Temperature Measurement  Combination of IR pyrometer and Gold cup pyrometer  Gold cup pyrometer allows calculation of emissivity  Full accurate survey of reformer possible with IR pyrometer
  172. 172. Temperature Measurement Corrections epyrometer (Tm)4 = etube (Tt )4 + rtube (Tw)4 Measured True Averaged target target background temperature temperature temperature e = emissivity r = reflectance = (1-e)
  173. 173. Accurate Temperature Measurement (Tm)4 = etube (Tt )4 + (1 - etube) (Tw)4 - Set IR Pyrometer emissivity at 1 - Measure Tm and Tw with Pyrometer - Measure Tt with Gold Cup - Calculate etube
  174. 174. Background Temperature Measurement NORTH A a 2 a 1
  175. 175. Comparison of Infra-red Pyrometer and Calculated Tube Wall Temperature MeasurementsTemperature(°C) Temperature(°F) Fraction Down Tube 0 0.2 0.4 0.6 0.8 1 950 900 850 800 750 1742 1652 1562 1472 1382 Uncorrected Pyrometer Corrected Pyrometer Calculated = Gold Cup Measurements
  176. 176. Tube Wall Temperature Measurement - Conclusions  IR Pyrometer typically reads high  Top-fired reformer 32°C  Side-fired reformer 50°C  IR Pyrometer with Gold Cup "calibration“  Top-fired reformer 2°C  Side-fired reformer 16°C
  177. 177. Classroom Exercise 2 - PROBLEM Reformer exit gas composition (dry %)  H2 73.19  N2 + Ar 1.11  CH4 3.04  CO 15.55  CO2 7.11  Total (dry) 100.00  H2O 41.34  Total (wet) 141.34 Reformer exit pressure 18.11 barg Reformer exit temperature 875°C Calculate the approach to equilibrium
  178. 178. Classroom Exercise 2 - ANSWER Exit Pressure (ata) = (18.11 / 1.013) + 1 = 18.88 ata pCH4 = 3.04 x 18.88 = 0.4061 ata 141.34 pH2 = 73.19 x 18.88 = 9.7766 ata 141.34 pH2O = 41.34 x 18.88 = 5.5221 ata 141.34 pCO = 15.55 x 18.88 = 2.0771 ata 141.34 pCO2 = 7.11 x 18.88 = 0.9497 ata 141.34
  179. 179. KWGS = pH2 . pCO2 pH2O . pCO = 9.7766 x 0.9497 5.5221 x 2.0771 = 8.09 x 10-1 From Tables TWGS = 875°C Reformer exit temperature = 875°C
  180. 180. KMS = pCH4 . pH2O pCO . (pH2)3 = 0.4061 x 5.5221 2.0771 x (9.7766) 3 = 1.15 x 10-3 From Tables TMS = 874°C ATE = 875 - 874 = 1°C

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