Ammonia CO2 Removal Systems


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CO2 Removal - Process Technology for Ammonia Plants

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Ammonia CO2 Removal Systems

  1. 1. CO2 Removal - Process Technology for Ammonia Plants By: Gerard B. Hawkins Managing Director, CEO
  2. 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. 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. 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. 5. Typical System for MDEA Treated Gas Feed Gas CO2
  6. 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. 7. Benfield System Treated Gas Feed Gas CO2 and H2O Semi Lean 120°C Lean 75°C Reboilers Absorber Regenerator
  8. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 18. Effect of 3% LRS 10 to Benfield Time (hours) %CO Conc CO2 concentration in absorber overheads
  19. 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. 20. UOP Lo-Heat Process Schematic