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FCC Off Gas Treatment


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Back Ground
Typical Impurities & Their Effect
Typical Impurities & Their Removal
Typical Off Gas Processing Scheme
GBH Enterprises / Haiso Technology
H2S Absorption
COS Removal
RSH Removal
Acetylene / Oxygen / Arsine / Phosphine / Nitriles / Cyanides Removal
Chlorides Removal
Nitrogen Oxides Removal
Mercury Removal
FCC Offgas Oxygen Converter
Process Objectives, Process Requirements, Performance Guarantees, Catalyst: Classical Definition,
Vulcan VGP Series Catalyst and Absorbents
Chemical & Physical Properties
Reaction Chemistry
NOx Removal
Control During Normal Operation
Catalyst Poisons
Catalyst Foulants
Catalyst Regeneration

Published in: Technology, Business
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FCC Off Gas Treatment

  1. 1. C2PT Catalyst Process Technology By Gerard B Hawkins Managing Director FCC Off Gas Treatment
  2. 2.  Refinery off gas sourced from a number of units  FCC / Coker / HDT / etc  FCC off gas contains valuable olefins which can be recovered by stand-alone cryogenic unit  Gas stream typically contains many contaminants which affect either final product specs or processing options
  3. 3.  H2S Catalyst poison  COS Impacts on C3= product spec  RSH Impacts on C2= / C3= product spec  Acetylene Impacts on C2= / C3= product spec  Oxygen Impacts on C2= / C3= product spec  Chlorides Corrosive to aluminium  Ammonia Potential reactant to form NH4NO3  Nitric oxides Can react to form explosive nitroso gums  Mercury Attacks aluminium in cold section  Arsine Impacts C3= product spec  HCN Impacts C2= / C3= product spec  H2O Freezes in cold section
  4. 4.  H2S Amine/caustic wash + absorbent guard bed  COS Hydrolysis or solid bed absorption  RSH Caustic and/or solid bed absorption  Acetylene Hydrogenation to ethylene across catalyst  Oxygen Hydrogenation to water across catalyst  Chlorides Solid bed absorbent  Nitric oxides Hydrogenation to NH3 across catalyst  Mercury Solid bed absorbent  Arsine Solid bed absorbent  HCN Solid bed absorbent or hydrogenation across catalyst  H2O Regenerable mol sieve
  5. 5. Products for sale as polymer feeds and LPG Feed Gas Compression Acetylene conversion Acid Gas Removal Contaminant Absorption Steps Drying Cryogenic Recovery Unit Product Fractionation Off gas to fuel or hydrogen recovery Refinery off gas streams Typical Off Gas Processing Scheme
  6. 6.  Supply of the full range of catalysts / absorbents  for off gas processing  and / or treatment of fractionated product streams  Commercial references for both duties under the product brand name VULCAN VGP Series  Input to the flow sheet and reactor design  All spent catalysts / absorbents can be reprocessed
  7. 7.  Amine and caustic systems most cost effective route for bulk H2S removal  For specifications of < 3 ppmv H2S then a fixed bed absorbent bed on polishing duty most cost effective  Polishing guard bed also provides insurance for amine or caustic unit upsets  VULCAN VGP Series H2S absorbents market leader in gas processing industry and wide-spread use in refineries to protect catalysts from sulfur poisoning
  8. 8.  Amine & caustic systems not effective at COS removal  COS can be hydrolysed across a catalyst COS + H2O -----> H2S + CO2 or absorbed directly by reaction with a non-regenerable absorbent bed  Performance of the catalyst dependant on component partial pressures and temperature  VULCAN Absorbent references on reducing off-gas streams and polymer grade propylene streams
  9. 9.  Amine systems not effective at RSH removal  Caustic will remove RSH but struggles to meet tight exit specifications  VULCAN Series absorbents remove RSH onto a non- regenerable absorbent at ambient temperature  VULCAN Series RSH absorbents in use on refineries to protect catalysts from sulfur poisoning and meet product specifications
  10. 10.  Sulfided metal catalyst removes all  acetylene hydrogenated to ethylene  oxygen hydrogenated to water  arsine, phosphine and cyanides absorbed  nitriles converted to amines  cyanides converted to ammonia  Key to activity is proprietary catalyst sulfiding procedure  VULCAN Series catalyst proven for selective hydrogenation duties on both refineries & petrochemical plants
  11. 11.  HCl removed by chemical reaction onto a non- regenerable absorbent bed  Care needed if Aluminas specified because of danger of synthesis of organic chlorides which are difficult to remove R= + HCl ------> RCl This does not occur with VULCAN Series absorbents  VULCAN Series absorbents market leader in refineries for catalytic reformer off gas dechlorination
  12. 12.  NO and NO2 hydrogenated across a catalyst NOx + H2 -----> NH3 + H2O  Key is to ensure no co-hydrogenation of olefins  Proven on side-stream reactor unit on commercial FCC unit
  13. 13.  Mercury reacts with absorbent to form mercuric sulfide which remains as part of absorbent structure  Exit mercury specification determined by equilibrium  Typical exit specs < 0.001 ppb  VULCAN VGP Series mercury absorbents proven in many gas processing plants to meet transmission specifications and protect aluminium equipment
  14. 14. The primary objective of the oxygen converter is to reduce the Oxygen content of the treated gas to less than 0.1 ppm (V) Acetylene < 1 ppm (wt) Arsine < 5 ppb (wt) Phosphines < 5 ppb (wt)
  15. 15. Cost effectiveness: VULCAN VGP Series catalyst must meet several criteria. • Meet Oxygen specification of less than 0.1 vppm • Meet Acetylene specification of less than 0.1 wppm • Meet Arsine specification of less than 5 vppb • Meet Phosphine specification of less than 5 vppb Provide a 18 month cycle length.
  16. 16. Client minimum guarantee levels for the following performance metrics at SOR: • Oxygen content of treated gas to be less than 0.1 ppmv at maximum treat gas rate. • Acetylene specification of less than 0.1 wppm at maximum treat gas rate. • Arsine specification of less than 5 vppb at maximum treat gas rate. • Phosphine specification of less than 5 vppb at maximum treat gas rate.
  17. 17. Catalysts promote chemical reactions and accelerate the rate at which a chemical reaction approaches equilibrium. The catalyst provides a suitable surface for reactants to adsorb and for products to desorbed. Primary Function - to lower the activation energy of the reaction by providing a suitable reaction pathway.
  18. 18. GBHE offers VULCAN VGP Series catalyst for the treatment of fluidized catalytic cracker unit (FCCU) off gas to remove a variety of impurities, which are unacceptable for downstream processing in which useful olefins are recovered in a cold train.  Acetylene and higher acetylenes are hydrogenated selectively by VULCAN VGP Series catalyst using the large excess of hydrogen in the process stream without significant onward hydrogenation of the olefins, which are to be recovered.  Oxygen is removed by hydrogenation to H2O.  Traces of Phosphine and arsine are removed by chemisorption.
  19. 19. CATALYST DESCRIPTION Form Spheres  Size nominal 8 mm TYPICAL CHEMICAL COMPOSITION Component Wt. % dry Ni 0.5% Co 0.15% Cr 0.05% Al2O3 84 – 89% PHYSICAL DESCRIPTION Crush Strength > 50 Kgs Bulk Density 1 – 1.1 Kg/L
  20. 20.  VULCAN VGP Series: Catalyst of the sulfided Ni type.  The catalyst is principally designed to hydrogenate acetylene to ethylene by the reaction: C2H2 + H2  C2H4
  21. 21. Typical Feed Impurity Levels ◦ Inlet acetylene 1000 - 3000 ppm mol ◦ Outlet acetylene < 1 ppm mol The VULCAN VGP hydrogenates other acetylene and diene compounds in FCCU off gas. (MA/PD) Removal efficiency 60 - 80 % (BD) Removal efficiency 20 - 30 %
  22. 22. O2 Oxygen is removed by the reaction: O2 + 2 H2  2 H2O Typical oxygen Levels: ◦ Inlet oxygen 300 - 1000 ppm mol ◦ Outlet oxygen < 1 ppm mol can be achieved Optimum Inlet temperature range: 190 – 200 oC.
  23. 23. Operating temperature range: 150 – 260 oC (300 – 500 oF). ◦ Delta T: The temperature rise is typically 30 – 50 oC  Dependent on the amount of acetylenes, dienes and oxygen in the feed  The catalyst selectivity ◦ Activity for the desired hydrogenation reactions and selectivity for olefin hydrogenation is controlled by;  Continuous sulfur doping with H2S  Operating temperature Both of which must be varied as the catalyst ages.
  24. 24. ◦ A new charge of catalyst requires sulfiding prior to being brought on line, and after periodic steam/air regenerations. ◦ The reactions take place in the presence of a large excess of hydrogen over acetylene  Typically 5 - 10 mol % and 2000 ppm mol (respectively) ◦ High selectivity and minimizes undesirable hydrogenation  ethylene and propylene products.
  25. 25. The catalyst performance slowly deteriorates over time online due to the build up of foulants on the catalyst surface. Once the selectivity and activity of VULCAN VGP become unacceptable, the catalyst requires regeneration. ◦ The regeneration frequency depends on the operating conditions, the catalyst age, vessel sizing and the level of acetylene slip or ethylene loss that the operator can tolerate. ◦ Regenerations are usually required every few months but may be more or less frequent.
  26. 26. ◦ Regeneration conditions require heating of the catalyst in air/steam at approximately 500 oC for approximately one week until the fouling species are removed. ◦ Re-sulfiding before the catalyst is ready for re-use ◦ VULCAN VGP operates in a sulfided form, which has activity for the absorption of various species  Phosphine  Arsine.
  27. 27. NiO is a known catalyst for the reduction of NOx with both CO and H2, at temperatures below 392 0F (200 0C), with the principal product of reaction being N20. NO and NO2 hydrogenated across a catalyst NOx + H2 -----> NH3 + H2O References: E Echigoya et al, Bull. Japan. Petr. Inst, 1975, 17, 232 G V Glazneva et al., Dokl. Akad. Nauk. SSR, 1973, 213, 971 T P Kobylinski et al., J. Catal, 1973, 31, 450 F Nozaki et al, Bull. Chem. Soc. Japan. 1975, 48, 2764
  28. 28. Control parameters: A combination of rate of sulfur injection and inlet temperature optimization Higher temperature increases activity but diminishes selectivity so that the acetylene conversion is increased but also the level of ethylene hydrogenation increases Oxygen conversion can be affected also Conversely, higher sulphur decreases activity but augments selectivity
  29. 29. There are numerous factors which affect the catalyst performance: - Hydrogen partial pressure: Increases hydrogenation activity and therefore lower selectivity Sulfur (ppm): Any sulfur injection must be adjusted to compensate for varying levels of feed sulfur - Moisture: Increasing H2O diminishes activity
  30. 30. • - CO level: Increasing CO decreases activity and helps selectivity but large changes are needed for the effect to be significant; • - Space velocity: Faster gas flow results in less apparent activity but more selectivity; • - Olefin partial pressure: In theory the selectivity will decrease with increasing olefin levels but the effect is small.
  31. 31. • Typically inlet temperature 200 oC (392 oF) Reaction may be substantial at 190 oC (374 oF) or may require an increase in temperature to 220 oC (428 oF) Typical Sulfur levels: 2 - 50 ppmv (total sulphur) Over the lifecycle, temperatures should be increased and sulfur injection rates reduced, in order to maintain catalyst activity. - Decreased selectivity Under certain conditions, the catalyst will desulfide to give Ni rather than NiS and this is an effective ethylene hydrogenation catalyst so selectivity may collapse.
  32. 32. Permanent Poisons Arsenic, lead, mercury, cadmium  Silica, Iron Oxide…. Temporary Poisons Sulfur, carbon
  33. 33. - Heavy metals in FCC feedstocks • Nickel • Vanadium - Tramp iron contamination, formed as corrosion products within the pipe work - Scale and carbon deposits from heaters and exchangers - Polymerization caused by reactive molecules within the feedstock - Particulates contained within the feedstock or caused by upstream attrition of catalysts.
  34. 34. 0 0.01 0.02 0.03 0.04 0.05 0.06 0.001 0.01 0.1 1 10 100 1000 Pore size (microns) Cumulativevol(mls/g) Catalyst A Catalyst B The contaminants may vary in size from sub micron to several hundred microns and be deposited in the interstitial voids between the catalyst spheres. Result: Flow restriction, channeling, and a reduction in catalyst activity by; - Deposition and encapsulation of the catalyst surface - Pore mouth narrowing mechanism leading to eventual total pore plugging
  35. 35. During operation, carbonaceous polymer builds up on the catalyst surface for side reactions of unsaturated hydrocarbons in the process gas. As a result, catalyst selectivity and activity declines. VULCAN VGP can be regenerated using a steam/air oxidation at high temperature to remove the carbonaceous polymer deposits. The regeneration frequency depends on the operating conditions, the catalyst age, vessel sizing and the level of acetylene slip or ethylene loss that the operator can tolerate.
  36. 36. V5 V2 V4 FeedGAS V1 BA V6 V3 PRODUCT GAS Regeneration conditions require heating of the catalyst in air/steam at approximately 500 oC for until the fouling species are removed.
  37. 37.  VULCAN VGP is formulated to hydrogenate acetylene to ethylene. Other diene compounds can also be hydrogenated with relatively high efficiency, and impurities removed by chemisorption  The amount of temperature rise will vary depending on the inlet impurity content and catalyst selectivity  Continuous sulfur injection is required to maintain catalyst selectivity  VULCAN VGP is regenerable