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The Benefits and Disadvantages of Potash in Steam Reforming
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The Benefits and Disadvantages of Potash in Steam Reforming

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Why do we include potash ? …

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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  • 1. The Benefits and Disadvantages of Potash In Steam Reforming Gerard B. Hawkins Managing Director www.GBHEnterprises.com
  • 2. Introduction  Why do we include potash ?  What are the benefits ?  What are the disadvantages ? www.GBHEnterprises.com
  • 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. What are the Disadvantages ? Effect on Catalyst  As potash is removed then strength of catalyst is reduced  Causes excessive breakage www.GBHEnterprises.com
  • 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. 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. 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.