Steam reforming - The Basics of Reforming
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This document discusses catalyst process technology for steam reforming of hydrocarbons. It covers the chemical reactions involved, catalyst design considerations like shape and chemistry, and carbon formation and removal. Key points discussed include the conversion of hydrocarbons to syngas, reforming and shift reactions, factors that influence methane conversion, reformer design, optimizing catalyst shape for heat transfer and pressure drop, using alkali-doped catalysts to prevent carbon formation, and tailored catalyst requirements.
Steam reforming - The Basics of Reforming
- 1. C2PT Catalyst Process Technology By Gerard B Hawkins Managing Director
- 2. Steam Reforming Catalysis : ◦ Chemical reactions ◦ Catalyst shape design ◦ Catalyst chemistry ◦ Carbon formation and removal
- 3. The conversion of hydrocarbons to a mixture of CO, CO2 and H2 Two reactions: Reforming and Shift Steam Reforming (very endothermic) CH4 + H2O CO + 3H2 CnH2n+2 + nH2O nCO + (2n + 1)H2 Water gas shift (slightly exothermic) CO + H2O CO2 + H2 Overall the reaction is highly endothermic
- 4. Both reforming and shift reactions are reversible Rate of shift is fast compared to reforming Methane conversion favored by: – low pressure – high temperature – high steam to carbon ratio
- 5. Steam Secondary Reformer Steam Steam + Gas Steam Reformer Air / Oxygen500°C 780°C 450°C 1200°C 950°C 10% CH4 0.5% CH4
- 6. 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
- 8. 0 0.2 0.4 0.6 0.8 1 200 300 400 500 600 700 800 900 fraction down tube temperature(°C) gas temp Eq temp ATE
- 9. Nickel on a ceramic support Three key factors in catalyst design: – geometric surface area – heat transfer from tube to gas – pressure drop Also of concern: – packing in the tube – breakage characteristics
- 10. Top Fired Reformer 0 0.2 0.4 0.6 0.8 1 660 680 700 720 740 760 780 800 820 840 860 fraction down tube tubewalltemperature(°C) base case base case with twice GSA base case with twice heat transfer
- 11. Outside tube wall temperature 830°C Bulk Process Gas Temp. 715°C 1200°C Fluegas Inside tube wall temperature 775°C Gas film Tube Wall
- 12. Need to minimize thickness of gas film at tube wall Smaller catalyst particles improve heat transfer from wall to bulk gas and reduce tube temperatures Smaller particles increase pressure drop Catalyst shape should be optimized for high heat transfer with low pressure drop
- 13. The traditional catalyst shape is a ring Smaller rings give high activity and heat transfer but higher pressure drop Optimized catalysts offer high surface area and heat transfer with low PD Important that shape also provides good packing and breakage characteristics
- 14. Relative Pressure Drop Relative HTC Voidage 1 0.9 0.9 0.8 1 2 3 4 1 1.3 1.1 1.0 0.49 0.6 0.58 0.59 1 2 3 4
- 15. Design of catalyst shape is a complex optimization of: – Higher surface area (needed for activity - diffusion control) – Higher heat transfer (needed for cooler reformer tubes) – Lower pressure drop (efficiency consideration) Need also to consider breakage characteristics and loading pattern inside the reformer tube
- 16. Catalyst loading can be improved using various dense loading techniques
- 18. 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
- 19. Carbon Formation and Prevention Giraffe Necking Hot TubeHot Band Reformer tube appearance - Carbon laydown
- 20. Cracking – CH4 C + 2H2 – C2H6 2C + 3H2 etc Boudouard – C + CO2 2CO Gasification – C + H2O CO + H2
- 21. Under normal conditions carbon gasification by steam and CO2 is favored (gasification rate > C formation rate) Problems of carbon formation occur when: – steam to carbon ratio is too low – catalyst is not active enough – higher hydrocarbons are present – tube walls are too hot – catalyst has poor heat transfer characteristics Use of a potash doped catalyst reduces probability of carbon formation
- 22. Methods of preventing carbon formation: – Use more active catalyst – Use better heat transfer catalyst – Reduce level of higher hydrocarbons – Increase the steam ratio – Use VSG-Z102 (3-7) -hole tailored catalysts catalyst (potash-promoted)
- 23. 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 Potash is mobile on the catalyst surface Potash doped catalyst is only needed in the top half of the reformer tube C + H2O CO + H2 OH -
- 24. Increasing the content of alkali (potash) – Higher heat flux possible for light feeds – Heavier hydrocarbons can be steam reformed – Lower steam to carbon ratios – Faster carbon removal during steaming
- 25. Fraction Down TubeTop Bottom Non-Alkalised Catalyst Ring Catalyst Optimised Shape (4-hole Catalyst) Inside Tube Wall Temperature 920 C (1688 F) 820 C (1508 F) 720 C (1328 F) 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 O O O O O O
- 26. For light feeds and LPG etc using lightly alkalised catalyst VSG-Z101 – Potash is chemically locked into catalyst support – Potash required only in the top 30-50% of the reformer tube – Catalyst life influenced by Poisoning Ni Sintering Process upsets etc VSG-Z101 VSG-Z102
- 27. 0 0.5 1 1.5 2 2.5 3 1.2m 3m 5m 6m 9m Catalyst samples at various depths down reformer tube Fresh 1 year 2 years 4 years 6 years wt% of potash VSG-Z102 VSG-Z102
- 28. Requirements : ◦ High and stable activity ◦ Low pressure drop ◦ Good heat transfer ◦ High resistance to carbon ◦ High strength ◦ Robust formulation/simple operation Best achieved with VSG-Z101 (3-7) -hole tailored catalysts