Theory and Operation of Methanation Catalyst
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The document discusses the theory and operation of methanation catalysts, detailing aspects such as catalyst reduction, normal operation, troubleshooting, and shutdown procedures. It highlights the importance of monitoring temperature, carbon oxide levels, and catalyst poisoning during operation. Additionally, it addresses the hazards associated with nickel carbonyl and offers recommendations for safe handling and operation.
Theory and Operation of Methanation Catalyst
- 1. Theory and Operation of Methanation Catalyst By: Gerard B. Hawkins Managing Director, CEO
- 2. Contents Introduction and Theoretical Aspects Catalyst Reduction and Start-up Normal Operation and Troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard
- 3. Introduction Carbon oxides are poisons for many hydrogenation reactions Used on older plants (without PSA) CO2 removal followed by methanation Uses nickel-based catalyst
- 4. Theoretical Aspects Strongly exothermic reactions: CO + 3H2 CH4 + H2O CO2 + 4H2 CH4 + 2H2O H = 206 kJ/mol -89 BTU/lbmol H = -165 kJ/mol -71 BTU/lbmol (Reverse of steam reforming) Temperature rise: 74OC (133OF) for each 1% of CO converted 60OC (108OF) for each 1% of CO2 converted
- 5. 270°C 518°FInlet Composition (%) CO 0.2 CO2 0.1 H2 93.9 CH4 3.3 H2O 2.5 Outlet Composition (%) CO CO2 H2 93.5 CH4 3.6 H2O 2.9 291°C 556°F Typical Process Conditions }<5ppmv
- 7. Mechanism of Reaction Equilibrium concentrations of carbon oxides 10-4 ppmv Governed by Kinetics CO inhibits methanation of CO2 Two stage reaction: i) CO2 reverse - shifts to CO CO2 + H2 CO + H2O ii) CO methanates CO + 3H2 CH4 + H2O Intrinsic reaction rates very high (diffusion limited at higher temperature
- 8. Catalyst Composition Iron originally studied Ruthenium good at low temperature (“ultra-methanation”) Nickel conventionally used Support matrix with 20-40% (wt) nickel Promotors to reduce sintering Small pellets (5 mm x 3 mm)
- 9. Contents Introduction and Theoretical Aspects Catalyst reduction and start-up Normal operation and troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard
- 10. NiO + H2 Ni + H2O NiO + CO Ni + CO2 ∆H = +3 kJ/mol +1 BTU/lbmol ∆ H = -30 kJ/mol -13 BTU/lbmol Catalyst Reduction little temperature rise from reduction itself metallic nickel will lead to methanation during reduction reduction gas should not contain carbon oxides (<15) need to heat catalyst to 400-450oC (750-840oF) for maximum activity BUT THEREFORE
- 11. Pre-Reduced Catalyst Now available Simplifies start-up Maximises activity at low temperatures
- 12. Contents Introduction and Theoretical Aspects Catalyst Reduction and Start-up Normal Operational and troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard
- 13. Methanation Catalyst Temperature Profile Over designed originally, high catalyst activity Most reaction in top of bed Catalyst lives 10-15 years
- 14. 320 310 300 290 280 (2) (4) (6) (8) (10) (12) Temperature°C(°F) 0 1 2 3(536) 4 (554) (572) (590) (608) Bed depth m (ft) Methanation Reaction Profile
- 15. Normal Operation Conversion of carbon oxides depends on outlet temperature If CO inlet increases, exit temperature also increases, reaction rate increases and exit carbon oxide level decreases • this may allow a reduction in inlet temperature
- 16. Top Bottom Temperature Bed Depth - ageing mechanism is gradual poisoning - profile moves down the bed Methanation Catalyst Ageing
- 17. 1. Gradual steady rise across whole bed • inadequate reduction? • poisoning 2. Sudden movement of reaction zone with no change in slope • poisoning of top? • Poor reduction of top? 3. Normal temperature profile, high outlet carbon oxides • channelling through bed? • mechanical problems? (by pass valve; heat exchanger) • analytical problems? Abnormal Conditions
- 18. Unusual Operating Conditions 1. High CO levels • LTS by-passed • total concentration of carbon oxides <3% • inlet temperature 210-250oC (410-480oF) • if necessary, lower rate through HTS and increase S/C ratio 2. High Water Levels • normal level 2-3% H2O in inlet gas • if >3%, can lead to high CO2 in exit gas • may need to increase bed inlet temperature • operating experience up to 7% H2O
- 19. Plant Mal-Operation Normal maximum exit temperature is 450OC (480OF) Excursions to 600OC (1100OF) for several hours can be tolerated In this event of a temperature runaway, the vessel must be protected: • isolate on inlet side • blow down to atmospheric • purge with nitrogen to aid cooling • exclude air to avoid exothermic oxidation
- 20. Catalyst Poisons S is a poison but normally present unless LTS by- passed Most poisons originate from CO2 removal system Carry-over of a small amount of liquid not generally serious Large volumes will have a serious effect Common Poisons Effect Blocks pores; removable Serious, irreversible poisoning K2CO3 As2O3 Sulpholane Decomposes to S; poison
- 21. Process Chemical Effect Benfield Vetrocoke Benfield DEA Sulphinol MEA, DEA MDEA Rectisol Catacarb Selexol Aqueous potassium carbonate Aqueous potassium carbonate plus arsenious oxide Aqueous potassium carbonate With 3% di-ethanolamine Aqueous potassium carbonate with borate additive Sulpholane, water di-2-propanolamine Mono- or di-ethanolamine in aqueous solution Aqueous solution of methyl di-ethanolamine and activators Methanol Dimethyl ether of polyethylene glycol Blocks pores of catalyst by evaporation of K2CO3 Blocks pores of catalyst by evaporation of K2CO3 . (DEA is harmless) Blocks pores of catalyst by evaporation of K2CO3 . As2O3 is also a poison; 0.5% of As on the catalyst will reduce its activity by 50% Blocks pores of catalyst by evaporation of K2CO3 Sulpholane will decompose and cause sulphur poisoning None None None None CO2 Removal Systems
- 22. Contents Introduction and Theoretical Aspects Catalyst Reduction and Start-up Normal Operation and Troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard
- 23. Shutdown If process gas temperature > 200OC (390OF), can be left in atmosphere of process gas for short periods Below 200OC (390OF), must be purged with an inert to prevent carbonyl formation Reduced catalyst pyrophoric; oxidation very exothermic • spread catalyst thinly on ground • have water hoses available • transport in metal skips/metal/sided trucks
- 24. Catalyst Back-washing for K2CO3 Removal Considerations • catalyst strength • water quality and temperature • reactor cooling and purging • plant isolations
- 25. Methanator Back-washing - Effect on Performance Catalyst performance fully regained • CO + CO2 slip < 6 ppm Catalyst strength unaffected by repeated washings No effect on catalyst pressure drop
- 26. Contents Introduction and Theoretical Aspects Catalyst Reduction and Start-up Normal Operation and troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard
- 27. • Colorless, mobile liquid flammable in air, insoluble in water • Boiling point 43°C (109°F) • Vapor pressure: (°C) -12 18 24 43 (°F) 10 64 75 109 v p (bar) 0.10 0.25 0.51 1.01 v p (psi) 1.4 3.6 7.4 14.6 Nickel Carbonyl Ni(CO)4 EXTREMELY TOXIC!
- 28. Toxicity of Ni(CO)4 4 ppm v/v for 1 minute gives severe toxic effects 2 ppm v/v short time leads to illness target value (daily average concentration) 0.001 ppm v/v Ni + 4 CO Ni(CO)4
- 29. Guidelines 1. Under normal operating conditions, concentrations are too low to be a problem • steam reformer has high CO, high Ni, but high temperatures • after LTS, temperatures low, but low Co, low Ni 2. Under abnormal operating conditions (eg start-up or shut-down) it is possible to get conditions favourable for the formation of Ni(CO)4 Keep temperatures above 200°C (390°F) to avoid formation of Ni(CO)4
- 30. 0 100 200 300 400 0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 Temperature °C (°F ) Favorable Not Favorable (32) (212) (392) (572) (752) 30 bar 1 bar Conditions for the formation of 0.001 ppmv Nickel Carbonyl FormationPartialPressureofCO(bar)
- 31. Conclusions Reviewed methanation reactions and catalyst Described normal operation Described abnormal conditions Poisoning Mentioned catalyst back-washing Reviewed nickel carbonyl hazard