Feedstock Purificatiion in Hydrogen Plants
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Feedstock Purificatiion in Hydrogen Plants

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Introduction ...

Introduction
reasons for purification, types of poisons, and typical systems
2. Hydrogenation
3. Dechlorination
4. Sulfur Removal
5. Purification system start-up and shut-down

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Feedstock Purificatiion in Hydrogen Plants Presentation Transcript

  • 1. Feedstock Purification in Hydrogen Plants G B Hawkins Managing Director GBH Enterprises Ltd.
  • 2. Feedstock Purification in Hydrogen Plants  1. Introduction  reasons for purification, types of poisons, and typical systems  2. Hydrogenation  3. Dechlorination  4. Sulfur Removal  5. Purification system start-up and shut-down GBH Enterprises Ltd.
  • 3. 1. Reasons for Feedstock Purification  Steam reforming catalyst requirements • process gas feed to reformer (dry basis)  sulfur <0.1 ppmv: poison  chlorides <0.1 ppmv: poison  As/V/Pb/Hg <5ppbv: poison  olefins <1-2 vol %: carbon formation  LTS catalyst requirement • process gas feed to LTS (dry basis)  chlorides <5 ppb: severe poison  sulfur <0.1 ppmv: poison
  • 4. 1. Reasons for Feedstock Purification  Steam reformer catalyst poisoning • Increased methane slip  low plant efficiency • Hot tubes  tube life reduction or failure • Carbons formation  increased pressure drop  increased methane slip and hot tubes • Sulfur poisoning can be recovered by steaming the steam reforming catalyst GBH Enterprises Ltd.
  • 5. 1. Reasons for Feedstock Purification  LTS catalyst poisoning • Reduced life  premature plant S/D due to high Co slip and high pressure drop • Chloride deactivates catalyst at concentrations of only 0.05 wt% • Cu poisoning is not reversible GBH Enterprises Ltd.
  • 6. Natural Gas Feeds Mercury may be present in some NG supplies *H2S & reactive organic S compounds (odoring agents often added) Component NG (mol %) CH4 93.2 C2H6 4.8 C3H8 1.2 C4H10 0.4 C5+ 0.4 Total Sulfur* 2-20 ppmv 1. Sources of Poisons GBH Enterprises Ltd.
  • 7. Component "Typical" Composition (mol %) Ranges C3H8 0.5 0.1 - 90 iC4H10 30 10 - 99 nC4H10 69 10 - 99 C5H12 0.5 0.1 - 10 Total Sulfur (Organic) * 20 ppm (wt) 0 - 100 ppm (wt) * Reactive types Olefins may be present! LPG Feeds 1. Sources of Poisons GBH Enterprises Ltd.
  • 8. Component Offgas #1 (mol %) Offgas #2 (mol%) H2 27.6 35.2 CH4 35.6 26.5 C2H6 19.2 15.2 C3H8 9.9 12 C4H10 6.7 8.8 C5H12 0.8 2 C6H14 0.2 0.3 Total Sulfur* 2 ppmv 10 ppmv Total Chloride 1-2 ppmv - Refinery Offgas Feeds 1. Sources of Poisons *H2S & reactive organic S compounds GBH Enterprises Ltd.
  • 9. Refinery Offgas Feeds (Contd.) 1. Sources of Poisons  COS may be present • particularly if CO2 is present  Cl may be present as NH4Cl  Significant variation in poison content may occur • hydrogenation duty designed for peaks • poisons absorption capacity designed for average concentrations GBH Enterprises Ltd.
  • 10. Type of Sulfur Typical Split of S (%) H2S Trace RSH 36 R2S2 3 R2S 51 *Unreactive S 10 Naphtha Feeds - Sulfur Species * Stable > 400 Deg C (752 Deg F) - e.g. Thiophene 1. Sources of Poisons GBH Enterprises Ltd.
  • 11. Naphtha Feeds (Contd.) 1. Sources of Poisons  Large variation in S level • 0.1 - 500 ppm wt  Chloride level typically 0.1 - 2 ppm wt  Pb/As/Va may be present GBH Enterprises Ltd.
  • 12. Hydrocarbon Feed Hydrogenation Chloride Removal Sulfur Removal Hydrocarbon Feed Hydrogenation Sulfur Removal Chloride Removal Hydrocarbon Feed Sulfur Removal Hydrogenation 1. Typical Purification Flowsheets GBH Enterprises Ltd.
  • 13. Feedstock Purification in Hydrogen Plants  1. Introduction  Reasons for purification, types of poisons, and typical systems  2. Hydrogenation  3. Dechlorination  4. Sulfur Removal  5. Purification system start-up and shutdown GBH Enterprises Ltd.
  • 14. Hydrogenation Reactions CoMo or NiMo type catalysts Exothermic reactions, but little temperature rise due to low concentrations C2H5Cl + H2 C2H6 + HCl C2H5SH + H2 C2H6 + H2S C4H4S + 4H2 n-C4H10 + H2S NH4Cl NH3 + HCl Hydrogen requirement fixed by feed type 2. Hydrogenation GBH Enterprises Ltd.
  • 15. Feed Type Min H2 Requirement (mol %) Typical H2 Levels (mol %) NG 0 2-5 LPG 10 12 Light Naphtha 20 25 H. Naphtha <20% Aromatics 25 25 H. Naphtha >20% Aromatics 30 30 ROG feeds usually have sufficient hydrogen content Hydrogenation Hydrogen Requirements 2. Hydrogenation GBH Enterprises Ltd.
  • 16. Feedstock Temperature SOR Temperature EOR ROG 370°C (698°F) 390°C (734°F) *LPG 360°C (680°F) 380°C (716°F) Naphtha 375°C (707°F) 400°C (752°F) Hydrogenation Inlet Temperatures - Lower inlet temperatures needed C4s can crack more readily 2. Hydrogenation GBH Enterprises Ltd.
  • 17. 2. Typical Hydrogenation Catalyst Characteristics - CoMo Typical composition (wt %):- CoO 4.0 % MoO3 12.0 % Cement Balance Form:- Usually extruded thin cylinders with high porosity A true catalyst! GBH Enterprises Ltd.
  • 18. 2. Hydrogenation - CoMo  Most common hydrogenation catalyst  Active in the sulfided state  Side reactions • methanation  CO + 3H2 → CH4 + H2O  CO2 + H2 → CH4 + H2O  use NiMo if CO>3 vol% or CO2 >13 vol% • hydrocracking  very low activity - carbon slowly formed • can achieve very long lives  6-20 years GBH Enterprises Ltd.
  • 19. 2. Typical Hydrogenation Catalyst Characteristics - NiMo Typical composition (wt, loss free):- NiO 4.0% MoO3 14.0% Cement Balance Form:- Usually extruded thin cylinders with high porosity A true catalyst! GBH Enterprises Ltd.
  • 20. 2. Hydrogenation - NiMO  Active in the sulfided state  Side reactions • methanation suppressed when catalyst is sulfided • hydrocracking  low activity - carbon slowly formed (activity marginally higher than CoMo)  Can achieve long lives (6-20 years)  Olefin hydrogenation activity slightly higher than CoMo so NiMo usually chosen when olefin concentration >1 vol% GBH Enterprises Ltd.
  • 21. 2. Hydrogenation  Typical operating conditions (CoMo & NiMo): • Operating temperature range  290-430OC (550-750OF) • Operating pressure range  1 - 50 atm (15 psig - 750 psig) • Space velocity  300 - 8000 hour-1  more typically 1000 - 4000 hour-1 GBH Enterprises Ltd.
  • 22. 2. Hydrogenation  Organometallic compounds absorbed by CoMo/NiMo • approx. 1wt% of catalyst can be absorbed • special catalyst grades exist that can increase metals pick-up to approx. 2 wt%  useful for high Pb content naphthas • extra catalyst design volume required  catalyst volume for metals absorption plus catalyst volume for hydrogenation GBH Enterprises Ltd.
  • 23. 2. Hydrogenation  Low sulfur feeds • CoMo/NiMo can over-reduce if S level <1-2ppmv  permanent partial deactivation • Hydrocracking  carbon formation • Need to sulfur-inject if alternate S- containing feeds are expected • Equilibrium charts GBH Enterprises Ltd.
  • 24. 826 F 665 F1040 F 540 F 1/Temperature Co Mo Ni Sulfided Phase Reduced Phase 2. Co, Mo & Ni Sulfur Equilibrium Phase Diagram GBH Enterprises Ltd.
  • 25. 2. Hydrogenation  Aromatics Hydrogenation • Naphtha feeds contains aromatics • Hydrogenation rate very slow over CoMo/NiMo  in reality - negligible  Olefin hydrogenation • Maximum olefins to steam reformer = 1-2 vol% • Hydrogen “consumption” needs to be taken into account (increase hydrogen R/C” • Temperature rise implications  re-circulation system can be used to limit impact of temperature rise GBH Enterprises Ltd.
  • 26. Hydrogenation 320°C (608°F) 388°C (730°F) 2. Hydrogenation - Olefin Conversion Using a Recirculation System H2 29.1% C3’ 0.1% C4’ 48.9% C4” 21.3% C5’ 0.6% H2 10.0% C3’ 0.2% C4’ 89.1% C4” 0.001% C5’ 0.7% Recirculator Cooler GBH Enterprises Ltd.
  • 27. 2. Hydrogenation  Reaction of COS over CoMo/NiMo • COS is not absorbed by amine systems • Low temperature operation • At temperatures <290 OC (550 OF), then hydrogenation activity is very low • Catalysts containing higher active metal contents May be used for temperatures down to 240 OC (464 OF) COS + H2O H2S + CO2 GBH Enterprises Ltd.
  • 28. 2.Hydrogenation - Typical Problems  Pressure drop increase  carbon formation • formed from hydrocarbon cracking  carry-over of solids  Sulfur slippage  low temperature of operation • e.g. small plants with high heat loss  rate Increase  sulfur level increase • very significant if sulfur is unreactive type GBH Enterprises Ltd.
  • 29. Feedstock Purification in Hydrogen Plants  1. Introduction  2. Hydrogenation  3. Dechlorination  sources of chloride  effects of chloride  removal of chloride  4. Sulfur Removal  5. Purification system start-up & shut- down GBH Enterprises Ltd.
  • 30. 3. Chloride Removal  Possible sources of chlorides • offgas from certain catalytic reformer plants  HCI & NH4Cl • LPG and naphtha feeds  organic chlorides Some chlorides might originate from the process steam due to incorrect boiler feed water quality control GBH Enterprises Ltd.
  • 31. Hydrocarbon Feed Hydrocarbon Feed Hydrogenation Chloride Removal Sulfur Removal Hydrogenation Sulfur Removal Chloride Removal Hydrocarbon Feed Sulfur Removal Hydrogenation Typical Purification Flowsheets GBH Enterprises Ltd.
  • 32. 3. Chloride Removal  ZnO catalyst • Some of the chlorides will react with the ZnO to form ZnCl2  this significantly reduces the ZnO capacity to absorb sulfur  weakens the catalyst  ZnCl2 sublimes at purification section normal operating temperatures and can deposit Zn and Cl on downstream reforming catalyst Why remove the chlorides before ZnO? GBH Enterprises Ltd.
  • 33. HClZnO Crystallites Catalyst Pore s Effect of Chloride on ZnO Sulfur Removal Catalyst 1. Fresh ZnO 2. Poisoned ZnCl2 blocks catalyst surface and pores to prevent sulfur absorption 3. Chloride Removal GBH Enterprises Ltd.
  • 34. HCl + NaAlO2 AlOOH + NaCl 2HCl + 2NaAlO2 Al2O3 + 2NaCl + H2O Removing chlorides at elevated temperatures requires a chemical absorbent Physical absorbents like activated aluminas can not operate at normal purification system temperatures as absorbent must operate downstream of the hydrogenation catalyst Need to use a promoted alumina - e.g. Na2O/Al2O3 3. Chloride Removal GBH Enterprises Ltd.
  • 35. 3. Chloride Removal - Operational Aspects  Operation very straightforward  Temperature range • 0 - 400OC (32 - 752OF)  Pressure range • 0 - 50 atm (14 - 750 psig)  Space velocity • experience of up to 10000/hr • typically 1000-4000/hr  Absorbent sensitive to condensation • pressure drop increase could be due to condensation GBH Enterprises Ltd.
  • 36. • Design Cl slip = <0.1ppmv • (Typically 0.05 ppmv or less) • Monitor HCl slip on a regular basis • If inlet chloride known, then life of catalyst can be calculated approximately • 12-14 weight % of chloride in catalyst • High space velocities are possible • Catalyst can be installed as a "ZnO" top-up • Other Halogens • Fluoride and bromide can also be removed 3. Chloride Removal GBH Enterprises Ltd.
  • 37. Comparative Performance of Promoted Alumina and Alumina 3. Chloride Removal 0 2 4 6 8 10 12 14 %wtChloridein Absorbent 0 20 40 60 80 100 Bed Depth Sodium Promoted Alumina Alumina GBH Enterprises Ltd.
  • 38. Feedstock Purification in Hydrogen Plants  1 Introduction  2 Hydrogenation  3 Dechlorination  4 Sulfur Removal • catalysts/absorbents • sulfur pick-up • operational aspects  Purification system start-up & shut- down GBH Enterprises Ltd.
  • 39. • Fe3O4 (reduced Fe2O3) not ideally suitable due to high S slip • ZnO used almost universally “black” ZnO - Lower S capacity H2S + ZnO H2O + ZnS Mercaptans can also crack C2H5SH + ZnO H2O + ZnS + C + CH4 4. Sulfur Removal Chemical Reaction of H2S with absorbent GBH Enterprises Ltd.
  • 40. Typical compositions:- 1. ZnO 90-94.0 wt% Cement Balance 2. ZnO 99 wt% Forms:- - Large variation •Pelleted cylinders •Extrudates •Granulated spheres Typical Sulfur Removal Catalyst Characteristics Target is to achieve maximum accessible ZnO GBH Enterprises Ltd.
  • 41. 4. Sulfur Removal - Total Pick-up  Catalyst requirements (high S pick-up) • High porosity  allows access of H2S to centre of catalyst pellet  porosity maintained as ZnO is converted to ZnS  upstream chloride slip has lower effect on catalyst S capacity • Highly accessible surface area  sharp S absorption profile at high space velocities GBH Enterprises Ltd.
  • 42. 4. Sulfur Removal - Operational Aspects  Temperature range • 300 - 400OC (572 - 752OF)  Pressure range • 1 - 50 atm (14 - 750 psig)  Space velocity • experience of up to 8000hr-1 • typically 500 - 4000hr-1  Sulfur slip • usually designed for 0.1 ppmv sulfur • achieved S slip <0.05 ppmv for fresh beds GBH Enterprises Ltd.
  • 43. 4. Sulfur removal - Monitoring and Life Assessment  Monitor for H2S regularly • daily for “stressed” beds (6 month lives) • or daily/weekly  Also monitor other organic S compounds • weekly Note:- If average inlet S is known, life of ZnO can be predicted using expected S pick-up value (eg 20-35 wt%) - NOT theoretical pick-up based on ZnO quantity! Monitoring still important GBH Enterprises Ltd.
  • 44. Temperature Affect on Total Sulfur Absorption 100 200 300 400 0 20 40 60 80 100 Temperature (°C) Total amount of S absorbed prior to breakthrough. % theoretical 4. Sulfur removal - ZnO Absorbent Capacity Low pressures (<12 bar, 17 psig) also decreases total amount of S absorbed GBH Enterprises Ltd.
  • 45. 4. Sulfur Removal - Typical Problems  Premature sulfur slip • check for organic S  CoMo/NiMo problems • check for chlorides  an operating plant achieved only 2-5 wt% S pickup with 1-2 ppmv Cl • check for changes in feed sulfur specification and operating conditions  higher space velocities will decrease original predicted sulfur pick-up  Hot reformer tubes (hot bands etc) • cross-check S analysis results! GBH Enterprises Ltd.
  • 46. Lead-Lag • Series arrangement • Configuration can be reversed • Upstream reactor can be operated with H2S slip to maximise S pick-up • Catalyst bed changed on-line 4. Sulfur removal - Series Beds GBH Enterprises Ltd.
  • 47. 4. Sulfur removal - Carbon Beds  Beds of activated carbon promoted with copper  Carbon removes organic sulfur and copper removes H2S  Regenerable • Steam generation removes organic sulfur • H2S can not be easily removed from Cu unless steam/air regeneration used • Effluent problems  H2S removal capabilities decrease with time GBH Enterprises Ltd.
  • 48. Feedstock Purification in Hydrogen Plants 1. Introduction 2. Hydrogenation 3. Dechloration 4. Sulfur Removal 5. Purification system start-up and shut down GBH Enterprises Ltd.
  • 49. 5. Purification System Start-up  Usually heated-up with an inert gas or NG • Heat up rate typically 50OC/hr (90OF/hr) • If sour NG is used, avoid passing to the steam reformer until conditions are reached for H2S conversion and adsorption  For re -start of naphtha/LPG based plants, ensure that the catalyst beds have been fully purged of hydrocarbons before reformer is brought on line GBH Enterprises Ltd.
  • 50. 5. Purification System - Start-up  CoMo/NiMo usually sulfided as hydrocarbon feed is introduced • In some cases, in situ pre-sulfiding may be required  Feeds with high CO2/CO content  Sulfur-free C4 stream  Involves injection of carbon disulfide or dimethyl disulfide etc in a flow of N2 or NG at 200OC (390OF)  Purification system usually effective at reduced rates once 300OC (572OF) is achieved • monitoring of S slip still important however GBH Enterprises Ltd.
  • 51. 5. Purification System - Shut-down  Beds should be purged with inert gas cooling to < 38OC (100OF) before depressurization • For naphtha/LPG type feeds, if steam is already isolated, purging should be done to flare and not through the reformer  Discharged catalyst should be considered pyrophoric • Fine carbon, residual hydrocarbons & iron carry-over • During discharge, have water hoses ready GBH Enterprises Ltd.
  • 52. Purification Catalyst for Hydrogen Plants - Summary  Types of poisons, required poison limits, and typical purification systems  Hydrogenation • CoMo/NiMo • Aromatics and Olefin hydrogenation • Sulfur equilibrium • Dechlorination • Sulfur removal • Start-up and shut-down GBH Enterprises Ltd.
  • 53. GBH Enterprises Ltd.