The Benefits and Disadvantages of Potash in Steam Reforming

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Why do we include potash ?
What are the benefits ?
What are the disadvantages ?
Catalyst Deactivation
Carbon Deposition : Thermodynamics & Kinetics
Carbon formation margin
Reaction chemistry (Tube inlet)
Hydrocarbons undergo cracking reactions on hot surfaces at the tube inlet
Products of catalytic cracking reactions can form polymeric carbon

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The Benefits and Disadvantages of Potash in Steam Reforming

  1. 1. The Benefits and Disadvantages of Potash In Steam Reforming Gerard B. Hawkins Managing Director www.GBHEnterprises.com
  2. 2. Introduction  Why do we include potash ?  What are the benefits ?  What are the disadvantages ? www.GBHEnterprises.com
  3. 3. Why do we include Potash ?  Two reasons • Prevents carbon formation within  The catalyst  On the inside wall of the tube • If carbon is laid down, helps gasification of carbon  Potash has to be mobile so that it gets to the hottest point that the process gas sees • The inside tube wall www.GBHEnterprises.com
  4. 4. Why do we include Potash ?  Catalyst Deactivation  Carbon Deposition : Thermodynamics & Kinetics pH2 2 pCH4 10 1.0 0.1 550 600 650 700 750 800 1100 1200 1300 1400 (°F) 100 Temperature (°C) High Methane Concentrations Increasing Potential for Carbon Deposition www.GBHEnterprises.com
  5. 5. Why do we include Potash ?  Catalyst Deactivation  Carbon Deposition : Thermodynamics & Kinetics pH2 2 pCH4 10 1.0 0.1 550 600 650 700 750 800 1100 1200 1300 1400 (°F) 100 Temperature (°C) Increasing Rate of Carbon Deposition www.GBHEnterprises.com
  6. 6. Why do we include Potash ?  Catalyst Deactivation  Carbon Deposition : Thermodynamics & Kinetics pH2 2 pCH4 10 1.0 0.1 550 600 650 700 750 800 1100 100 Temperature (°C) Carbon Deposition Zone 1200 1300 1400 (°F) Deposition possible but rate low Deposition not favored www.GBHEnterprises.com
  7. 7. Why do we include Potash ?  Catalyst Deactivation  Carbon Deposition : Thermodynamics & Kinetics pH2 2 pCH4 10 1.0 0.1 550 600 650 700 750 800 1100 100 Temperature (°C) CDZ 1200 1300 1400 (°F) Composition - temperature profile along reformer tube No carbon deposition www.GBHEnterprises.com
  8. 8. Why do we include Potash ?  Catalyst Deactivation  Carbon Deposition : Thermodynamics & Kinetics pH2 2 pCH4 10 1.0 0.1 550 600 650 700 750 800 1100 100 Temperature (°C) CDZ 1200 1300 1400 (°F) Zone of carbon deposition 30% of tube length www.GBHEnterprises.com
  9. 9. Why do we include Potash ?  Catalyst Deactivation  Carbon Deposition : Prevention • If carbon deposition occurs by : CH4 C + 2 H2 • Then carbon deposition rate > carbon removal rate • Deposition rate is difficult to modify • Faster carbon removal is possible by leveraging an additional removal reaction : C + H2O CO + H2 • Potash acts to increase the rate of this reaction www.GBHEnterprises.com
  10. 10. Why do we include Potash ?  Catalyst Deactivation  Carbon Deposition : Impact of Potash pH2 2 pCH4 10 1.0 0.1 550 600 650 700 750 800 1100 100 Temperature (°C) 1200 Faster rate of carbon removal shrinks CDZ No carbon deposition CDZ 1300 1400 (°F) www.GBHEnterprises.com
  11. 11. Why do we include Potash ?  In terms of modelling we consider the margin to carbon formation  This is defined as the difference between the • Equilibrium temperature • Process gas temperature  Assumes GOM natural gas  Care with gases that are heavier than this www.GBHEnterprises.com
  12. 12. Why do we include Potash ?  Definition of carbon formation margin pH2 2 pCH4 10 1.0 0.1 550 600 650 700 750 800 1100 100 Temperature (°C) CDZ 1200 1300 1400 (°F) No carbon deposition Margin to Carbon Formation www.GBHEnterprises.com
  13. 13. Why do we include Potash ? 600 650 700 750 800 850 0 2 4 6 Distance Down Tube (m) Temperature(°C) Inside TWT Carbon Equilibrium Margin to Carbon Formation www.GBHEnterprises.com
  14. 14. Why do we include Potash ? 600 650 700 750 800 850 0 2 4 6 Distance Down Tube (m) Temperature(°C) Base Case Inside TWT Base Case Carbon Forming Equilibrium www.GBHEnterprises.com
  15. 15. Why do we include Potash ? 600 650 700 750 800 850 0 2 4 6 Distance Down Tube (m) Temperature(°C) Base Case Inside TWT Base Case Carbon Forming Equilibrium Potash Carbon Formation www.GBHEnterprises.com
  16. 16. Carbon Formation Margin -50 0 50 100 150 200 250 0 2 4 6 Distance Down Tube (m) Temperature(°C) Base Case Margin www.GBHEnterprises.com
  17. 17. Naphtha Steam Reforming  Reaction chemistry (Tube inlet) • Hydrocarbons undergo cracking reactions on hot surfaces at the tube inlet • Products of catalytic cracking reactions can form polymeric carbon • High strength catalysts required • Carbon resistant catalysts required CxHy Cx + y/2H2 CxHy CH4 + H2 + Cx-1H2x-2 Thermal Catalytic Polymers www.GBHEnterprises.com
  18. 18. Steam Reforming - Reaction Chemistry Thermal cracking & carbon formation Catalytic cracking and olefin polymerisation Steam reforming reactions Water gas shift reaction Heavy Naphtha Light Naphtha www.GBHEnterprises.com
  19. 19. Steam Reforming - Catalyst Design High strength catalysts to tolerate carbon deposition Carbon resistant catalysts High activity catalysts High activity catalysts Heavy Naphtha Light Naphtha or Mixed Feeds www.GBHEnterprises.com
  20. 20. So What are the Benefits ?  Prevents carbon formation in applications were carbon formation is an issue  For example • Highly stressed reformers  High heat fluxes  High throughput • High levels of C2+  Even then can be insufficient • Low steam to carbon ratios www.GBHEnterprises.com
  21. 21. What are the Disadvantages ?  It is assumed that the potash will be captured on the catalyst in the bottom of the tubes  This is not always the case  In a number of situations there have been problems  Usually linked to potash loss • In some cases fouls the WHB • Sometimes reaches the HTS • Can cause SCC of downstream heat exchangers • Loss of catalyst strength www.GBHEnterprises.com
  22. 22. What are the Disadvantages ?  Waste Heat Boiler Fouling  For NG feeds main issue has been; • H2 plants are more vulnerable as process gas temperatures are higher • Rate of evolution of potash from catalyst is higher  Customer X WHB exit temperature increased at 2°F per day • Fouling in WHB was 23% potash www.GBHEnterprises.com
  23. 23. What are the Disadvantages ?  Stress corrosion cracking  On train at South American Methanol Plant • 2nd BFW heater (3 in total) suffered from SCC • Process gas on shell side  Design flaw – should have been on tube side as it is the dirty duty • Material specified as SS due to supply issue  Design flaw – should have been CS • Huge amount of cracking – due to SCC  Exchanger irreparable www.GBHEnterprises.com
  24. 24. What are the Disadvantages ? Effect on WHB  For Naphtha feeds – a variety of problems on plants • Again with fouling of WHB • Older plants have large fouling margin • Modern plants have had this cut and so have insufficient surface area • Requires regular WHB cleaning • Sometimes bypass valve becomes coated and does not operate correctly • Sometimes potash deposited in cold end of WHB and needs digging out www.GBHEnterprises.com
  25. 25. What are the Disadvantages ? Effect on HTS  Most WHB have internal bypass for exit temperature control – automatic control  If tubes are fouled, tube exit temperature rises  Bypass therefore closes to keep WHB exit temperature constant  Once bypass fully closes, exit WHB T rises  Therefore inlet HTS temperature rises  So CO slip rises  If HTS catalyst or vessel temperature limit is reached then plant rate has to be reduced www.GBHEnterprises.com
  26. 26. What are the Disadvantages ? Effect on HTS  Potash can also coat the surface of the HTS bed  Deactivates leading edge of the bed  Increases CO slip  Increases DP www.GBHEnterprises.com
  27. 27. What are the Disadvantages ? Effect on Catalyst  As potash is removed then strength of catalyst is reduced  Causes excessive breakage www.GBHEnterprises.com
  28. 28. What are the Disadvantages ? Effect on Catalyst 0 20 40 60 80 100 120 140 160 Z203 Z202 Z201 Z101 Catalyst type kg Min Avg Max www.GBHEnterprises.com
  29. 29. What are the Disadvantages ? Effect on Catalyst Carbon Laydown Test: Feed cyclohexane + steam at 500 oC and S/C = 3.5 isolate steam form carbon remove/inspect sample after set times 2 9 15 Timewithoutsteam(mins) 46-3 46-3Q VSG-Z101 survives where Comp fails www.GBHEnterprises.com
  30. 30. Conclusions  Potash has many more advantages than disadvantages  We have options for the majority of plants to address carbon formation  Even the very worst cases  On some plants there are problems were the WHB is very sensitive to fouling A Business Unit of GBH Enterprises, Ltd.

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