Steam Reforming - A Comprehensive Review
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Equilibrium, Approach & Kinetics Carbon Laydown Potash Doping Catalyst Loading Pigtail Nippers GHR & AGHR Secondary Reforming Metal Dusting
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Steam Reforming - A Comprehensive Review
- 2. Equilibrium, Approach & Kinetics Carbon Laydown ◦ Potash Doping Catalyst Loading Pigtail Nippers GHR & AGHR Secondary Reforming Metal Dusting
- 3. Equilibrium, Approach and Kinetics
- 4. Steam Reforming limited by Heat transfer Catalyst activity Kinetic rate Equilibrium
- 5. Approach to Equilibrium 770 780 790 800 810 820 2 4 6 8 10 12Methaneslip(%) Gas Exit TEq'm T ATE (1418) (1454)(1436) (1472) (1490) Temperature Deg C (Deg F) Exit CH4
- 6. Approach to Equilibrium CH4 + H2O <=> CO + 3H2 Approach Tms = Actual T gas - EquilibriumT gas (A.T.E.) measured calculated •Measure of catalyst activity –If ATE = O, system at equilibrium –As catalyst activity decreases, ATE increases
- 7. Kinetics •Reversible action, zero rate at equilibrium •Reforming reaction very fast •Limited to pellet surface only-diffusion limit •Depends upon catalyst GSA •Depends upon gas composition •Exponential with temperature •C2, C3 & C4 considered as not reversible
- 8. Diffusion Limitation inactive active Pore Catalyst pellet CH H O H CO CO 4 2 2 2 CO H2 CO2 nickel crystal
- 9. Steam Reforming Catalysis 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 CO and H2
- 10. Kinetics - Natural Gas •Kinetic model used for natural gas feeds •Methane kinetics well validated •C2 ,C3, & C4 kinetics ok for natural gas tail only •NOT to be used for LPG feeds •Kinetics for steam reforming reactions •Shift reaction taken to be at equilibrium •Shift very fast compared to reforming
- 11. Kinetics - Natural Gas •Form of equation as below: R[CH4] = K.GSA.Ract.exp(T).P[CH4].(Kp’-Kp) P[H2O] R[CH ] = Rate of methane reaction T = Temperature K = Constant P[X] = Partial pressure Ract = Catalyst relative activity Kp' = Equilibrium constant of gas GSA = GSA Kp = Equilibrium constant at T
- 12. Geometric Surface Area • GSA for short –Area per unit volume –Typically 200-500 m2/m3 • Important as apparent activity is a very strong function • Function of –shape –size –number
- 13. Kinetics •Major points of interest –Steam is a poison –Kp’- Kp can be changed to approach –GSA term is included –Relative activity term is included
- 14. Kinetics •For higher hydrocarbons •R[C2H6] = K.GSA.Ract.exp(T).P[C2H6] •R[C3H8] = K.GSA.Ract.exp(T).P[C3H8] •R[C4H10]= K.GSA.Ract.exp(T).P[C4H10] •Simple first order kinetics-non reversible •Hence limit to natural gas tail only
- 15. Catalyst Types
- 16. VSG-Z101: Gas reforming
- 17. Catalyst Applications • VSG-Z101 –Other light feeds - refinery offgas –Light duties e.g. side fired reformers –Use up to butane at 4.0 S:C ratio –Use in top fired reformers at tube top
- 18. Steam Ratios for Catalyst/Feedstock Combinations 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
- 19. Pre - reduction • This will maximize the activity of a catalyst • The start up will be easier and quicker • Catalyst should remain more active at tube top • Useful for low inlet temperature reformers
- 20. Catalyst Support - Reduction Temperatures AlphaAlumina CalciumAluminate MagnesiumAluminateSpinel Temperature (Deg F) Temperature (Deg C) 800 1000 1200 1400 1600 400 500 600 700 800 900 Magnesium Aluminate spinel material usually supplied pre-reduced
- 21. Carbon Laydown
- 22. Carbon Formation •Carbon formation formed by side reactions •Totally unwanted due to damage caused •Catalyst break up and deactivation •Catalyst tubes overheated - hot bands –Premature tube failure –Catalyst activity reduction –Pressure drop increases
- 23. Carbon Formation •Carbon forms when –Steam ratio is too low –Catalyst has too little activity –Higher hydrocarbons are present –Tube walls are hot - high flux duties –Catalyst has poor heat transfer coefficient
- 24. Carbon Formation - Types •Carbon cracking •Boudouard •CO Reduction
- 25. Carbon Formation •Cracking –CH4 <=> C + 2H2 –C2H6 => 2C + 3H2 etc •Boudouard –2CO <=> C + CO2 •CO Reduction –CO + H2 <=> C + H2O
- 26. Heavier Feedstocks •Steam reforming –Not practical to increase steam to carbon ratio using gas reforming catalysts –Carbon formation more problematic Need promoters to limit carbon and/or increase its removal once formed Carbon formed not just by cracking but also by polymerisation of intermediates
- 29. Heavier Feedstocks •Carbon removal (heavy feed reforming) –Potash (alkali) incorporated into catalyst support Inhibits cracking rate Accelerates carbon gasification Needs to be “mobile” to remove carbon on the inside tube wall surface - Potash released by complex chemical Release reaction controlled by temperature Required only in the top section of the reformer tube - Lower section of catalyst absorbs liberated potash
- 30. Heavier Feedstocks •Potash Addition (Heavy feed reforming) –Reduces catalyst activity Need extra Ni –MgO/NiO solid solution Low Polymerization activity - reduces carbon formation MgO must be “fixed” so as to avoid hydration of the “free” MgO to magnesium hydroxide - weakens catalyst pellet severely (cannot steam catalyst!)
- 31. Carbon Formation •Use of potash to prevent carbon formation •Increases the rate of carbon removal •Does not stop the formation from cracking •Potash catalyzes the rate of steam gasification •Balance of kinetics altered to favour removal •Can steam with care
- 32. 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
- 33. Role of Alkali - Lightly Alkalized For light feeds and LPG etc using lightly alkalized catalyst –Potash is chemically locked into catalyst support –Potash required only in top 30-50% of the reformer tube Catalyst life influenced by –Poisoning –Ni sintering – Process upsets etc Lightly alkalized Non-alkalized
- 34. Carbon Formation and Prevention Top Fraction Down Tube Bottom Non-Alkalised Catalyst Ring Catalyst Optimised Shape (4-hole Catalyst) 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 Optimized Shape VSG-Z101
- 35. Potash and Activity •However, potash is a catalyst poison –Potash does reduce activity •At low levels 2%, the effect is minimal •Use a gas type catalyst in bottom of tube •Use naphtha catalyst in top of tube for carbon •Therefore no carbon and low exit approach
- 36. Potash Levels in Heavy Feed Steam Reforming Catalyst 0 20 40 60 80 100 0 1 2 3 4 5 6 7 % Down Reformer Tube wt% Potash 2 year 1 year 0 years Potash Promoted Non-akalised
- 37. Must watch the interface temperature At >650°C Potash leaching too high Leads to ◦ Fouling of WHB etc ◦ Loss of carbon resistance ◦ Hot banding etc
- 38. Naphtha Loading 0 2 4 6 8 10 12 14 16 180 220 Final boiling point Deg C %Reductioninnaphtha loading % reduction 200 210190
- 39. Naphtha Loading 0 5 10 15 20 25 30 10 20 Aromatic content (%) Reductioninnaphthaloading 12 14 16 18
- 40. When to Use and Why • Used when major feedstock variations – NG to LPG • Feedstock flexibility – When 650oC limit is reached • Problem of Potash Leaching
- 41. Catalyst Loading
- 42. Sock Loading - Measurements • Key part of charging procedure • Aim to pack catalyst to uniform voidage • Measure pd – Not outage in tube at any one time – Not weight per tube – Not catalyst density – After 50% – After full loading • Use defined and consistent procedure throughout
- 43. DP Measurement • Use VSC Pressure Drop Rig • If too high then – suck out catalyst and recharge • If too low then – Vibrate tube – Top up if outage too great
- 44. Pressure Drop Measurement • Fixed flow of air (choked flow through orifice) • Mass flow rate through orifice function of – Upstream pressure – Orifice diameter (known) – Temperature (known) • Downstream pressure is measure of pd
- 45. Measurement of Pressure Drops PD rig Inlet pigtail Exit pigtail 4a. Exit pigtail Empty tube PD rig 4b. Catalyst catalyst PD rig 4c. Inlet pigtail catalyst
- 46. Digital Pressure Drop Instrument
- 47. Sock Loading - Vibration • Electric or pneumatic vibrators –rotating cams are noisy • Soft-faced shot-filled hammer –need consistent blows
- 48. “UNIDENSE” Method • Developed by Norsk Hydro – licensed to a number of organisations • Tried in a number of plants – ammonia, hydrogen and DRI • Leads to “denser” packing – less pd variation • more uniform gas flows – easier procedure • shorter loading time (70%) – slightly higher pd • effect on throughput?
- 49. Pigtail Nippers
- 50. Pigtail Nipper This hydraulic device designed by ICI is operated at a safe distance from the leaking tube and squeezes the pigtail flat with the plant still operating. Allows furnace to stay on line. No thermal cycle.
- 51. Pigtail Nipper
- 52. Pinched Pigtail with Clamp in Place
- 53. Pinched Tube in Steam Reformer Row of steam reformer tubes Pinched tube
- 54. Gas Heated Reformer (GHR) Advanced Gas Heated Reformer (AGHR)
- 55. GHR Based Reforming Secondary Reformer Steam + Gas Air / Oxygen GHR
- 56. LCM Flowsheet Purifier Saturator GHR Secondar y Converte r Preheater Purge to fuel Topping Column Refining Column Process condensate water Fusel oil Natural gas OxygenSteam Refined methanol Purge Crude methanol
- 57. GHR History • Developed for ammonia process - LCA • Early 1980's - Paper exercise • Mid 1980's - Sidestream unit at Billingham • Mid 1980's - LCA design developed • Late 1980's - ICI Severnside plants start up • 1991 - BHPP LCM plant designed • 1994 - BHPP plant start up • 1998 - AGHR Start Up • 1998 - MCC Start Up
- 58. GHR Shellside Design •Shellside heat transfer usually poor •Minimize tube count with expensive alloys •Tubes are externally finned •Designed as double tubes - Sheath tube •Produces much smaller tube bundle •Allows scale up to higher capacities
- 60. GHR and Secondary Arrangement
- 61. Normal Operating Conditions Secondary Reformer GHR Syngas Gas/steam 425`C 701`C 975`C 515`C 742`C 21,000 Nm3/Hr Oxygen30`C 1200`C 2,590 Nm3/Hr 43.7 Barg 39.2 Barg 38.6 Barg 37.9 Barg 22.0% Methane 16.6% Methane 0.4% Methane 40.6 Barg
- 62. LCA GHR
- 64. Advanced GHR -Shellside heat transfer enhancement -Non bayonet design -Hot end tubesheet -Sliding seal system
- 65. Uprate Capabilities •GHRs can be used in parallel to existing primary reformer •Potential to uprate capacity by 40% •Severely impacts steam system •Most applicable to hydrogen plants •No changes to radiant or convection sections of reformer / fans burner etc. •New WHB may be required •Rest of plant must be uprated
- 66. Fluegas Heated Reformers FHR Combustion chamber Natural gas & Steam Fuel Air Fluegas to stack Combustion air compressor Fluegas recycle compressor Fluegas power recovery turbine Syngas GHR
- 68. Keys to Good Performance •Burner Design •Mixing Space •Catalyst
- 69. Poor Mixing Performance • Good mixing is absolutely essential • Poor mixing in combustion zone gives high approach and high methane slip • Problem is poor mixing can not be differentiated from poor catalyst performance UNLESS thermocouples are in the bed. •Bed temps will show divergence •Bed temps will show odd behaviour
- 70. Catalyst Bed Sizing •Based on a space velocity technique •Wet Gas Space Velocity (WGSV) •Uses exit flow with steam included •See attached graph •Modify space velocity by catalyst GSA •See table for relative catalyst GSA
- 71. WGSV Chart 0 5 10 15 20 25 0 2000 4000 6000 8000 10000 Wet Gas Space Velocity (Nm³/m³) Approach(°C)
- 72. Two types LCM style ◦ Single ‘pipe’ Ring burner
- 74. Metal Dusting
- 75. Metal Dusting Key features •Catastrophic carburization •Occurs at "low" temperatures 700 - 450°C •Induction period sometimes required •Often local pitting - pits coalesce •Can have general corrosion •Can be very rapid 3mm/year •Carbon formation occurs
- 76. Metal Dusting
- 77. Mechanism of Metal Dusting Initiation •Gas has propensity to deposit carbon 2 CO=> CO2 + C (Boudouard) CO + H2 => H2O + C (CO Reduction) •Oxide film breakdown exposes active Fe, Ni, Co sites •Carbon deposits at active sites
- 78. Metal Dusting and GBHE •GBHE have great experience •GBHE have a solution •Proven to work in operation