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Ammonia CO2 Removal Systems


CO2 Removal - Process Technology for Ammonia Plants

CO2 Removal - Process Technology for Ammonia Plants

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  • 1. CO2 Removal - Process Technology for Ammonia Plants By: Gerard B. Hawkins Managing Director, CEO
  • 2. Why Remove CO2 ?  Poisons Synthesis catalyst  Can be used downstream in Urea production  If allowed into the loop would  Also act as inert in loop  Increase the compression costs  Used as a product in its own right  Carbonated drinks  Refrigeration units  Can be exported to methanol plant
  • 3. Systems for Removing CO2  Started with Monoethanolamine  Chemical absorption  Highly corrosive  High energy consumption (14% of total plant)  Modern plants use MDEA  Other options include physical solvents  Selexol (UOP)  Benefield (hot potassium carbonate)  Vetrocoke
  • 4. MDEA  Uses diethanolamine with an amine activator  No corrosion inhibitors required  No solvent degradation is observed  Acts as a combination of physical and chemical solvent  CO2 can be recovered by flashing only
  • 5. Typical System for MDEA Treated Gas Feed Gas CO2
  • 6. Advantages of MDEA  Some stated above  Can use MDEA to retrofit plants using MEA  Can tailor balance between physical and chemical  Allows for versatility especially in retrofit cases
  • 7. Benfield System Treated Gas Feed Gas CO2 and H2O Semi Lean 120°C Lean 75°C Reboilers Absorber Regenerator
  • 8. CO2 Removal issues  Benfield Problems • Dirty solution • Foaming due to contaminants or degradation products • DEA degradation products • Difficulty of oxidizing V4+ to V5+  Results • Low DEA concentration • Poor corrosion protection • Reduced CO2 removal capability
  • 9. Vanadation  Method of protecting stable magnetite layer from corrosion  Must reduce V5+ and deposition on surface  Steady state is reached when V5+ : V2O5 is 0.4  If ratio less than 0.3 - rapid break down and corrosion  High levels can lead to degradation of DEA  Most plants have a slow reduction of V5+ to V4+  Can counter by addition of KNO2
  • 10. Benefield - Water Balance  Large volumes of solution being circulated  Must keep water balance right to ensure strength of solution is between 27 and 30%  >30% - crystallisation and vessel corrosion  <27% - poor scrubbing
  • 11. Benfield Water Balance Benfield System Process Gas Inlet Absorber Pump Gland Seals Purges to levels 34 te/hr 2 te/hr 21 te/hr 1 te/hr 52 te/hr 6 te/hr Absorber Inlet Catchpot PG Exit Absorber CO2 Catchpot Drain CO2 Export Main
  • 12. Common Problems - Benfield  High DP on Absorber • High level in base - flooding inlet gas line • Foaming due to contaminants • Fouled packing causing restriction to flow • High gas flow causing solution to be held up • High circulation rates leading to flooding • Temperature variations
  • 13. Common Problems - Benfield  Consequences • Solution help up creating low levels in regen tower • Make gas supply to syn gas machine reduced • Vibrations/Surging of machine • Benfield solution carried forward from absorber and deposited on methanator catalyst
  • 14. Common Problems - Benfield  High PD in Regenerator • High vapour rates leading to hold up of solution from low level in base of regen column • High make gas temperatures or excessive steam usage in reboilers • Foaming • Fouled/Choked packing • Mechanical failure of trays or distributors
  • 15. Common Problems - Benfield  Consequences • Levels fall in base of regen column • Reduction in suction head available to pumps - cavitation • Reduced flows to absorber - high CO2 in exit gas • Trips methanator on high temperature • Carry over of benfield solution into CO2 condensate recovery system • Excess boiling of solution - changes in concentration and potential for corrosion
  • 16. Amine Promotion Mechanism Slow reaction of CO2 and hydroxylation Fast reaction of CO2 and amine Hydrolysis of carbamate to amine Free amine diffusion Carbamate diffusion HCO3 - Promotion mechanism of organic Amine on CO2 Absorption into Potassium Carbonate solution interfaceGas phase Liquid phase CO2 CO2 HCO3 -
  • 17. Benfield CO2 Removal Improvements  LRS 10 Benfield Promoter • British Gas Technology • New amine mixture • Completely compatible with DEA • Replaces DEA in hot potash solutions • Improves absorption and desorption of CO2 • Reduces CO2 removal energy
  • 18. Effect of 3% LRS 10 to Benfield Time (hours) %CO Conc CO2 concentration in absorber overheads
  • 19. Benefits of LRS 10 Promotion  Retrofit easy, just add 3% LRS solution  CO2 slip reduces immediately, giving scope to:- • Increase plant throughput, or • reduce steam/carbon ratio, or • reduce methane to synloop, and • plant becomes more controllable
  • 20. UOP Lo-Heat Process Schematic