Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Theory and Practice of Steam Reforming

9,845 views

Published on

Steam reforming reactions
Steam reforming catalyst
Equilibrium considerations
Carbon formation
Poisoning
Steam reformer modeling
Pre-reforming
Post-reforming

Published in: Technology, Business

Theory and Practice of Steam Reforming

  1. 1. The Theory and Practice of Steam Reforming By: Gerard B. Hawkins Managing Director, CEO
  2. 2. Contents  Steam reforming reactions  Steam reforming catalyst  Equilibrium considerations  Carbon formation  Poisoning  Steam reformer modelling  Pre-and post-reforming
  3. 3. Steam Reforming of Methane CH4 + H2O CO + 3H2 (Steam Reforming)) CO + H2O CO2 + H2 (water Gas Shift) • Overall strongly endothermic • Need to get large amounts of heat in – narrow-bore steam reformer tubes
  4. 4. Steam Reforming of Heavier Hydrocarbons CnHm + nH2O nCO + (n+m/2)H2  Still endothermic  Easier than methane  More prone to carbon formation
  5. 5. Contents  Steam reforming reactions  Steam reforming catalysts • catalyst activity • catalyst development and testing • importance of gas and htc  Equilibrium considerations  Carbon formation  Poisoning  Steam reformer modelling  Pre - and post reforming
  6. 6. Steam Reforming Catalyst  Steam reforming can be done without catalyst, but needs very high temperatures • partial oxidation  Modern steam reforming catalyst use nickel on a ceramic support • with or without promoters and stabilisers • precious metals offer alternatives to Ni  Supports must be strong; inert; thermally and chemically stable  Catalysts lower the temperature at which steam reforming occurs at a high rate
  7. 7. Steam Reforming Catalyst Activity  Reaction highly endothermic • may be limited by process of getting heat in to reactant sites  Process may also be limited by diffusion
  8. 8. Activity Testing  Define some measure of reaction • exit methane  Measure for a range of catalysts under fixed conditions • flow, temperature pressure, catalyst
  9. 9. Reactants Products Reaction Gas Film • Two types: - molecular diffusion - Knudsen diffusion Diffusion Effects Bulk Gas Bulk Gas
  10. 10. Diffusion Processes  Molecular diffusion, Dm • determined by rate at which molecules collide with each other • depends on pressure • independent of pore radius  Knudsen diffusion, Dk • determined by the rate at which molecules collide with pore walls • depends on pore radius
  11. 11. Check for Knudsen Diffusion  Mean free path of molecules must be greater than pore radius for Knudsen diffusion to dominate • at 700oC (1290oF), mean free path is 100 Angstrom  Typical pore radius for steam reforming catalyst is 150 - 1000 Angstrom • Not Knudsen regime
  12. 12. Steam Reforming Catalyst Activity  Intrinsic activity (chemical reaction only)  Extrinsic activity (includes heat and mass transfer effects)  Steam reforming dominated by extrinsic effects  Influence of pressure significant
  13. 13. Pressure bar (psi) Catalyst B Catalyst A 1 (14.5) 10 (145) 20 (290) Pressure Dependence
  14. 14. Adsorption Desorption Adsorption Dehydrogenation Surface Reaction ** OH2 C + 2H2 CH4 * H2O CO + H2 CH4 Surface Science
  15. 15. Photo of XPS
  16. 16. Activity Testing  Techniques exist to measure intrinsic activity • plug-flow reactors and CSTR systems • tests for mass/heat transfer limitations  Quantify other effects explicitly • measure htc • measure diffusional effects
  17. 17. Activity Testing  Intrinsic activity measurements  Bench-scale for screening  Scale-up to include heat/mass transfer effects
  18. 18. Activity Testing Microreactor Semi-tech
  19. 19. Steam Reforming Catalysts  Require • high geometric surface area (gsa) • high heat transfer coefficient (htc) • low pressure drop (pd)  Balance of properties  Cubes; rings; optimised shapes
  20. 20. Nickel crystallites No further reaction Reaction zone Catalyst Pellet Pore Reactants Products Effect of gsa
  21. 21. Steam Reformer Tubes  Need to get a lot of heat in • narrow bore tubes  High temperatures and pressures • tubes in creep region • tubes will fail by rupture • tube life very sensitive to temperature
  22. 22. 850 (1560) 900 (1650) 950 (1740) 1000 (1830) Temperature oC (oF) 0.1 0.2 0.5 1 2 5 10 20 Design Effect of Tube Wall Temperature on Tube Life + 20oC (+ 36oF)
  23. 23. Top Fired Reformer Distance Down Tube m (ft) TubeWallTemperature DegC(DegF) 0 1 2 3 4 5 6 7 8 9 10 11 12 BASE CASE BASE CASE WITH TWICE SURFACE AREA BASE CASE WITH TWICE HEAT TRANSFER 840 800 760 720 (1544) (1472) (1400) (6) (12) (18) (24) (30) (36) Effect of Catalyst Design Variables on Tube Wall Temperature
  24. 24. Tube Wall Bulk Process Gas Temp. 715oC (1319oF) 1200oC (2192oF) 830oC (1526oF) 775oC (1427oF) Fluegas Outside tube wall temperature Inside tube wall temperature Gas film Temperature Profile Top-fired reformer, 40% down
  25. 25. TemperatureDegC(DegF) Tube Wall Temperature Limit Poor stability Good stability Days on Line 0 1,000500100 200 300 400 600 700 800 900 925 (1697) 900 (1652) 875 (1607) 850 (1562) Effect of Catalyst Stability on Tube wall Temperature
  26. 26. Contents  Steam reforming reactions  Steam reforming catalysts  Equilibrium considerations • equilibrium curves • effect of process variables  Carbon formation  Poisoning  Steam reformer modelling  Pre-and post-reforming
  27. 27. Methane Steam Equilibrium CH4 + H2O CO + 3H2 P [CH4] P [H2O] Kms = P [CO] P [H2] 3 – equilibrium tables – equilibrium curves
  28. 28. Equilibrium curves (methane) 508 203 102 Equilibrium%CH4(drybasis) Pressure(psig) Pressure(barg) Steam Ratio 2.0 3.0 4.0 5.0 (Illustration only - limited accuracy) 35 14 7
  29. 29. Equilibrium curves (methane) Pressure : 30 bar (435 psi) Temperature : 850°C (1562°F) Steam:Carbon Ratio : 3.5 What is exit CH4 at these conditions? Equilibrium value 5.6% CH4 (Illustration only - limited accuracy) Steam Ratio 2.0 3.0 4.0 5.0 100 50 20 10 5.0 2.0 1.0 35 14 7 508 203 102 Equilibrium%CH4(drybasis)
  30. 30. F[CH4 ] F[H2O] 1 Kms = F[CO ] F[H2]3 Pt2 F[CO ] F[H2]3 Kms Pt2 F[CH4] = F[H2O] Equilibrium Considerations CH4 + H2O CO + 3H2
  31. 31. Effect of Pressure • Exit methane proportional to pressure squared • lower exit methane at lower pressures • overall plant economics dictate higher pressures, typically 20 bar (300 psi) CH4 + H2O CO + 3H2 F[CO ] F[H2]3 Kms Pt2 F[CH4] = F[H2O]
  32. 32. Effect of Steam- to- Carbon Ratio • Exit methane inversely proportional to steam • lower methane requires more steam • actual value depends on overall plant design • s/c ratio typically 5-6 on older plants • s/c ratio typically 3 on newer plants CH4 + H2O CO + 3H2 F[CO ] F[H2]3 Kms Pt2 F[CH4] = F[H2O]
  33. 33. • Exit methane proportional to Kms • Kms approx inversely proportional to temperature • lower methane requires higher temperatures • limited by tube metallurgy Effect of Temperature CH4 + H2O CO + 3H2 F[CO ] F[H2]3 Kms Pt2 F[CH4] = F[H2O]
  34. 34. Temperature Pressure Steam/Carbon Ratio Exit Temperature Exit Pressure Steam/Carbon Ratio Exit Gas Composition (% dry) 850 800 850 850 850 850 1562 1472 1562 1562 1562 1562 30 30 20 35 30 30 435 435 290 508 435 435 3.5 3.5 3.5 3.5 3.0 4.0 73.35 70.68 74.76 72.67 72.15 74.26 (°C) (°F) (atas) (psi) 5.35 9.31 3.35 6.30 6.70 4.36 12.18 9.73 13.09 11.78 12.79 11.59CO CH4 CO 2 H2 9.12 10.28 8.80 9.25 8.36 9.78 Effect of Temperature, Pressure, S/C Ratio
  35. 35. Feedstock Refinery Off Gas Methane Butane Naphtha C/H Ratio CH6 CH4 CH2.5 CH2.2 Exit Gas CH4 CO CO2 H2 6.67 8.14 4.45 80.74 5.35 12.18 9.12 73.35 4.29 14.17 12.36 69.16 4.01 14.73 13.77 67.49 All at exit temperature 850 Deg C (1562 Deg F) Exit pressure 30 atas (435 psi) Steam/carbon ratio 3.5 Effect of Feedstock
  36. 36. 70 60 50 40 30 20 10 0 Methane Feedstock Exit Temperature 850 C (1472 F) Exit Pressure 30 atas (435 psi) Steam/Carbon Ratio 3.5 New Old CH4 CO CO2 H2 Catalyst activity Composition(%dry) Effect of Catalyst Activity
  37. 37. Approach to equilibrium  The system is not actually at equilibrium, but close to it  A measure of catalyst performance is the Approach to Equilibrium, ATEms • ATEms = 0 when at equilibrium • the bigger ATEms, the further from equilibrium
  38. 38. Temperature oC (oF) 770 780 790 800 810 820 2 4 6 8 10 12 Methaneslip(%) (1418) (1454)(1436) (1472) (1490) Exit CH4 Approach to Equilibrium (1508) ATE Equilibrium Temp Gas Temp
  39. 39. 0 0.2 0.4 0.6 0.8 1 200 (392) 400 (752) 600 (1112) 800 (1472) Fraction down tube TemperatureoC(oF) Gas Temp Eq'm Temp Approach to equilibrium
  40. 40. Contents  Steam reforming reactions  Steam reforming catalysts  Equilibrium considerations  Carbon formation • formation and removal reactions • role of alkali • range of catalysts  Poisoning  Steam reformer modelling  Pre-and post-reforming
  41. 41. Carbon Formation Depends on: - feedstock - operating conditions - catalyst
  42. 42. Carbon Deposition Carbon Catalyst surface 1 mm (40 thou)
  43. 43. Carbon Formation CH4 C + 2H2 (Thermal Cracking) CO + H2 C + H2O (CO Reduction) 2CO C + CO2 (CO disproportionation “Boudouard”)
  44. 44. Carbon Formation  Direction of reaction determined by process gas conditions  Generally, CO reduction and Boudouard are carbon removing  Generally, cracking restricted to top half of reformer
  45. 45. pH2 2 pCH4 10 1.0 0.1 550 600 650 700 750 800 Carbon Formation Zone No Carbon Formation Deposition rate < removal rate Deposition rate > removal rate 1100 1200 1300 1400 (°F) 100 Carbon Formation Removal Reactions Temperature (°C)
  46. 46. 10 0 10 1.0 0.1 550 600 650 700 750 800 0.6 0.5 0.4 0.3 Carbon Formation Zone Temperature (°C) Proportion of tube length from inlet 1100 1200 1300 1400 (°F) Carbon Formation - Inside Reformer Tube pH2 2 pCH4 No Carbon Formation
  47. 47. 100 10 1.0 0.1 550 600 650 700 750 800 0.6 0.5 0.4 0.3 Carbon Laydown Zone 1100 1200 1300 1400 (°F) Carbon Formation - Hot Band Carbon Formation Zone Temperature (°C) pH2 2 pCH4 No Carbon Formation
  48. 48. Carbon Formation C + H2O CO + H2 (CO Reduction - in reverse!) Catalyzed by OH-
  49. 49. 800 100 10 1.0 0.1 0.6 0.5 0.4 0.3 550 600 650 700 750 Increasing Potash Content 1100 1200 1300 1400 (°F) Carbon Formation - Effect of Alkali Carbon Formation Zone Temperature (°C) pH2 2 pCH4 No Carbon Formation
  50. 50. Role of Alkali  Reduces likelihood that carbon will be formed  Enables carbon to be removed readily  Incorporation into support must be done correctly • Release rate not too fast/slow • Effect on activity
  51. 51. Fraction Along Tube Inlet Outlet Non-Alkalised Catalyst Rings Optimised Shape Inside Tube Wall Temperature 920 (1688) 820 (1508) 720 (1328) 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 TemperatureoC(oF) Carbon Formation
  52. 52. Methane feed/Low heat flux Increasing Alkali Addition 0 Methane feed/High heat flux Propane, Butane feeds (S/C >4) 2-3 Propane, Butane feed (S/C >2.5) Light naphtha feed (FBP < 120oC, 248oF) 4-5 Heavy naphtha feed (FBP < 180oC, 356oF) 6-7 Role of Alkali K2O wt%
  53. 53. Feedstock Natural Gas Reforming Non- alkalised Associated Gas Ref Lightly alkalised Dual Feedstock Reforming Moderately alkalised Naphtha Reforming Heavily alkalised Non-alkalised Low alkali Moderate alkali High alkali 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 Typical Steam Ratios for Catalyst/ Feedstock Combinations
  54. 54. Alternatives to Alkali • Precious metals can also be used instead of Ni as the catalyst – Significant higher activity and hydrogenation activity yields lower carbon formation rates – Platinum, Ruthenium …etc – Effective “ultra”-purification essential • Lanthanum used in addition to Ni – Helps also with the removal of carbon • Magnesium/Ni – Also suppresses carbon formation rates – However, magnesium not stable with steam
  55. 55. Contents  Steam reforming reactions  Steam reforming catalysts  Equilibrium considerations  Carbon formation  Poisoning • sulphur • sintering  Steam reformer modelling  Pre-and post-reforming
  56. 56. Sulfur Poisoning  Most common poison  Severe levels (.5ppm) can lead to rapid catalyst deactivation  “Normal” levels (20-30ppbv) leads to very slow deactivation  Sulfur equilibrium depends on temperature
  57. 57. (752) 400 500 600 700 800 900 0 0.2 0.4 0.6 0.8 1 RelativeCatalystDeactivation (932) (1112) (1292) Temperature oC (oF) (1472) (1652) Sulfur Poisoning
  58. 58. Sulfur Poisoning  Complex; some disagreement in literature, particularly at low levels  Low level Sulfur will lead to increased twt with time  Other deactivation mechanisms also operate
  59. 59. Sulfur Poisoning - Precious Metals Reforming • Precious metals require ultra-low poison levels – Typically <5 ppbv – Use specialised purifcation absorbent downstream of ZnO • Typical S slip 1-2 ppbv
  60. 60. Catalyst Sintering  Initial rapid sintering  Slower subsequent sintering  Temperature dependent  Both Ni crystallites and support sinter
  61. 61. Photos of Catalyst Sintering Fresh Catalyst Sintered Catalyst
  62. 62. Contents  Steam reforming reactions  Steam reforming catalysts  Equilibrium considerations  Carbon formation  Poisoning  Steam reformer modelling  Pre-and post-reforming
  63. 63. Steam Reforming Modelling  Detailed simulation models can be developed for • reformer design • evaluation of performance • prediction of changes
  64. 64. Steam Reformer Types  Cylindrical (limited to small plants)  Top-fired  Side-fired  Terraced wall  Bottom-fired (relatively rare)  Heat exchange type (relatively new)
  65. 65. Top-Fired Steam Reformer
  66. 66. Terrace Wall Steam Reformer - Schematic
  67. 67. Model Results  Input reformer details  Model output: gas temperatures and compositions down tube  Radial effects considered also
  68. 68. Temperature Deg C 0.0 0.5 1.0 FractionDownTube Process Gas Tube Wall Furnace Gas 400 600 800 1000 1200 1400 1600 Temperature Deg F 750 1500 2250 3000 Temperature Profiles
  69. 69. Fraction Down Tube Composition Wet mol% Composition Wet mol% 0.0 0.2 0.4 0.6 0.8 1.0 1.5 1.0 0.5 10 20 30 40 50 60 70 80 C2 CH4 H2O C4+ C3 CO2 CO H2 Composition Profiles
  70. 70. Contents  Steam reforming reactions  Steam reforming catalyst  Equilibrium considerations  Carbon formation  Poisoning  Steam reformer modelling  Pre-and post-reforming • pre-reforming concept • retrofitting and new plants • post-reforming concept • retrofitting
  71. 71. Pre-reforming  Low temperature adiabatic steam reforming  Wide range of feedstocks  Requires highly active, high nickel catalyst  Exo/endothermic, depending on feedstock  Converts all heavy hydrocarbons to methane
  72. 72. Temperature 475 deg C (890 deg F) 410 deg C (770 deg F) 0 10050 NG Pre-reformer Temperature Profile Percentage Down Bed
  73. 73. 450 Deg C (842 Deg F) 500 Deg C (932 Deg F) Percentage Down Bed Temperature Naphtha Pre-reforming temperature Profile
  74. 74. Reformed Gas Steam Pre-reformer Desulphurised Feed Incorporation of a Pre-reformer
  75. 75. Post-reforming  Heat exchange type of steam reformer  Uses steam reformer exit gas as heating medium for fresh feed  Compact design • small footprint  Uses conventional catalyst  No extra fuel firing needed • no increase in Nox emissions  Typically allows 25 % increase in rate
  76. 76. Gas Heated Reactor Shell Shift Internals
  77. 77. Steam Reformer Heat Exchange Reformer Reformed Gas Desulphurised Feed Steam Incorporation of a Post-reformer
  78. 78. Summary  Steam reforming reactions  Steam reforming catalyst  Equilibrium considerations  Carbon formation  Poisoning  Steam reformer modelling  Pre- and post-reforming

×