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Ammonia Plant - Methanation
Operations
By:
Gerard B. Hawkins
Managing Director, CEO
Methanation
• Introduction and Theoretical Aspects
• Catalyst Reduction and Start-up
• Normal Operation and
Troubleshootin...
Introduction
 Carbon oxides are poisons for
ammonia synthesis catalyst
 Methanation is the final stage
of purification o...
Methanation Reactions & Operating
Conditions
 Reactions are highly exothermic
• +74oC (133oF) for every 1% CO converted
•...
Mechanism of Reaction
 Equilibrium concentrations of carbon
oxides 10ppm
 Governed by kinetics
 CO inhibits methanation...
Typical Flowsheet
Gas from
CO2 Removal
290oC
(554oF)
Process gas to
Ammonia Loop
318oC
(604oF)
Inlet Composition
(vol % dr...
Methanator
METHANATOR
CATALYST
VSG-N101
Medium bauxalite
(40mm x 60mm)
Flat
Mesh
75 mm layer small bauxalite
150 mm layer ...
Catalyst Composition
 Iron originally studied
 Ruthenium good at low temperature (“ultra -
methanation”)
 Nickel conven...
Methanation Catalyst SV & Inlet
Temperature
SV (Hr )-1
220 240 260 280 300 320 340 360
2,000
4,000
6,000
8,000
10,000
Inle...
Methanation Temperature Profile
240
245
250
255
260
265
2 2.5 3 3.5 4 4.5 5 5.5 6
Bed depth (m)
Temperature(DegC)1000 te/d...
Catalyst Reduction
 If catalyst supplied in the oxidised form must be
reduced in the reactor to the active nickel form
 ...
Reduction Procedure
 Purge methanator free of air with N2
 Heat catalyst to 200oC (390oF) in process gas, N2 or
NG
– do ...
Reduction Procedure (con’t)
 Increase inlet temperature to 325-350oC (620-
660oF)
– 25oC (45oF) per hour
– maintain until...
Methanation Catalyst Reduction
400
300
200
100
0 10 20 0 10 20
10
20
30
40
50
60
70
Begin End
Exit
Inlet
CO
CO
Time (hr)
T...
Methanation Catalyst Reduction
Depth m (ft)
Inlet Exit
Flow 70% Design
Inlet CO 0.09%
Inlet CO2 0.11%
Partial LTS Slip
(In...
Pre-reduced Methanation Catalyst
 Catalyst first reduced, then partially stabilised
– Faster plant start-up
– Higher cata...
Inlet Temperature
 Usually controlled by inlet/outlet gas
heat exchanger and pre-heater
 With no pre-heat, heat of react...
Normal Operation
 Conversion of carbon oxides depends
on outlet temperature
 If CO inlet increases, exit temperature
als...
Methanation Catalyst AgeingTemperature
Top Bed Depth Bottom
- ageing mechanism is gradual poisoning
- profile moves down t...
Methanation Catalyst Monitoring
 Monitor Frequently:
– Inlet/Exit Temperatures
– Exit Carbon Oxides (CO + CO2)
 Monitor ...
Prediction of Remaining Catalyst Life
0 1 2 3 4 5 6 7 8 9 10 11 12
290
300
310
320
330
Bed Depth ft
TemperatureC
Plot end ...
Prediction of Remaining Catalyst Life
0 2 4 6 8 10 12 14
0
2
4
6
8
10
12
14
Time on-line (years)
Effectiveendofbed(ftfromi...
Catalyst Poisons
 S is a poison but not normally present
unless LTS by-passed
 Most poisons originate from CO2 removal
s...
CO2 Removal Systems
Process Chemical Effect
Benfield Aqueous potassium
carbonate
Blocks pores of catalyst by evaporation o...
Unusual Operating Conditions
 High CO levels
– LTS by-passed
– total concentration of carbon oxides
<3%
– inlet temperatu...
Plant Mal-operation
 Normal maximum exit temperatures is
450oC (840oF)
 excursion to 600oC (1100oF) for several
hours ca...
Abnormal Conditions
 Gradual steady rise across whole bed
• inadequate reduction?
• Poisoning?
 Sudden movement of react...
Nickel Carbonyl Ni(CO)4
 colorless, mobile liquid, flammable in air,
insoluble in water
 boiling point 43oC (190oF)
 va...
Toxicity of Ni (CO)4
 4 ppm v/v for 2 minute gives
severe toxic effects
 2 ppm v/v short time leads to
illness
 target ...
Guidelines
 Under normal operating conditions,
concentrations are too low to be a
problem
• steam reformer has a high CO,...
Nickel Carbonyl Formation
Temperature oC (oF)
PartialpressureofCO(bar)
0 100 200 300 400
0.001
0.002
0.005
0.01
0.02
0.05
...
Catalyst Back-washing for
K2CO3 Removal
 Considerations
– catalyst strength
– water quality and temperature
– reactor coo...
Ammonia Plant - Methanation Operations
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Ammonia Plant - Methanation Operations

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Introduction and Theoretical Aspects
Catalyst Reduction and Start-up
Normal Operation and Troubleshooting
Shutdown and Catalyst Discharge
Nickel Carbonyl Hazard
Modern Methanation Catalyst Requirements

Published in: Technology, Business

Ammonia Plant - Methanation Operations

  1. 1. Ammonia Plant - Methanation Operations By: Gerard B. Hawkins Managing Director, CEO
  2. 2. Methanation • Introduction and Theoretical Aspects • Catalyst Reduction and Start-up • Normal Operation and Troubleshooting • Shutdown and Catalyst Discharge • Nickel Carbonyl Hazard • Modern Methanation Catalyst Requirements
  3. 3. Introduction  Carbon oxides are poisons for ammonia synthesis catalyst  Methanation is the final stage of purification of synthesis gas after CO2 removal to reduce carbon oxides to trace levels  Uses nickel-based catalyst
  4. 4. Methanation Reactions & Operating Conditions  Reactions are highly exothermic • +74oC (133oF) for every 1% CO converted • +60oC (108oF) for every 1% CO2 converted  Typical inlet temperatures • 270-290oC (520-555oF)  Typical inlet carbon dioxide • 0.1 - 1.0 vol%  Typical carbon oxide slip • <5 ppmv CO + 3H2 CH4 + H2O ∆H= -206 kJ/mol CO2 + 4H2 CH4 + 2H2O ∆H = -165 kJ/mol
  5. 5. Mechanism of Reaction  Equilibrium concentrations of carbon oxides 10ppm  Governed by kinetics  CO inhibits methanation of CO2  Two stage reaction: CO2 reverse -shifts to CO CO2 + H2 CO + H2O CO methanates CO + 3H2 CH4 + H2O  Intrinsic reaction rates very high (diffusion limited at higher temperature)
  6. 6. Typical Flowsheet Gas from CO2 Removal 290oC (554oF) Process gas to Ammonia Loop 318oC (604oF) Inlet Composition (vol % dry) CO2 CO H2 CH4 N2+A 0.3 0.1 74.7 0.3 24.6 Outlet Composition (vol % dry) CO2 CO H2 CH4 N2+A <5ppm 74.2 0.8 25.0
  7. 7. Methanator METHANATOR CATALYST VSG-N101 Medium bauxalite (40mm x 60mm) Flat Mesh 75 mm layer small bauxalite 150 mm layer large bauxalite (60mm x 90mm) large bauxalite (60mm x 90mm) 25 mm layer small bauxalite (25mm x 40mm)
  8. 8. Catalyst Composition  Iron originally studied  Ruthenium good at low temperature (“ultra - methanation”)  Nickel conventionally used  Support matrix with 20-40% nickel  Promoters to reduce sintering  Small pellets (5mm x 3mm)  Low temperature operation  210-230oC (410-445oF)  Therefore low COx slip, < 5 ppm  Long lifetimes proven in service  VSG-N101 and VSG-N102 (Both available as Pre-reduced)
  9. 9. Methanation Catalyst SV & Inlet Temperature SV (Hr )-1 220 240 260 280 300 320 340 360 2,000 4,000 6,000 8,000 10,000 Inlet Temperature ( C) VSG-N101 VSG-N102 Competitor o
  10. 10. Methanation Temperature Profile 240 245 250 255 260 265 2 2.5 3 3.5 4 4.5 5 5.5 6 Bed depth (m) Temperature(DegC)1000 te/day Ammonia Plant in PR China VULCAN VSG-N101-Series Catalyst
  11. 11. Catalyst Reduction  If catalyst supplied in the oxidised form must be reduced in the reactor to the active nickel form  NiO + H2 Ni + H2O ∆H = + 3 KJ/mol  NiO + CO Ni + CO2 ∆H = - 30KJ/mol  Reduction process gives little temperature rise  BUT - metallic nickel will lead to methanation during reduction  THEREFORE - reduction gas should not contain carbon dioxide (<1%)  Need to heat catalyst to 400-450oC (750-840oF) for maximum activity
  12. 12. Reduction Procedure  Purge methanator free of air with N2  Heat catalyst to 200oC (390oF) in process gas, N2 or NG – do not use NG at temperatures above 200oC (390oF) due to possible methane cracking occurring  Reduction starts around 200-250oC (390-480oF) – temperature will rise – limit carbon oxides in feed to <1% – control inlet temperature
  13. 13. Reduction Procedure (con’t)  Increase inlet temperature to 325-350oC (620- 660oF) – 25oC (45oF) per hour – maintain until bed exit temperature equals maximum in bed – takes about 6 hours  Increase catalyst temperature to 400-450oC (750-840oF) for a few hours – max temperature 450oC (840oF) – may need controlled bypass of LTS to increase CO content of inlet  Decrease bed inlet temperature to design, increase plant rates
  14. 14. Methanation Catalyst Reduction 400 300 200 100 0 10 20 0 10 20 10 20 30 40 50 60 70 Begin End Exit Inlet CO CO Time (hr) TemperatureC(F) Time (hr) ExitCOandCO2(ppm) (212) (392) (572) (752) Partial by-pass of LTS oo 2
  15. 15. Methanation Catalyst Reduction Depth m (ft) Inlet Exit Flow 70% Design Inlet CO 0.09% Inlet CO2 0.11% Partial LTS Slip (Inlet CO 0.35%) * On line TemperatureC(F) 1 2 3 (3) (6) (9) 250 (482) 300 (572) 350 (662) 2 3.5 5 7 * 24 Time (hr) oo
  16. 16. Pre-reduced Methanation Catalyst  Catalyst first reduced, then partially stabilised – Faster plant start-up – Higher catalyst activity  Reduction taken place under controlled conditions – Higher Ni surface area  Lower inlet temperatures possible. – Minimum inlet temperature 170oC (338oF)  Catalyst is ready to go  Do not need to load full charge - only top portion needs to be pre reduced
  17. 17. Inlet Temperature  Usually controlled by inlet/outlet gas heat exchanger and pre-heater  With no pre-heat, heat of reaction may not achieve required inlet temperature  Increase carbon oxides in inlet gas by partial by-passing of LTS or CO2 removal
  18. 18. Normal Operation  Conversion of carbon oxides depends on outlet temperature  If CO inlet increases, exit temperature also increases, reaction rate increases and exit carbon oxide level decreases  This may allow a reduction in inlet temperature
  19. 19. Methanation Catalyst AgeingTemperature Top Bed Depth Bottom - ageing mechanism is gradual poisoning - profile moves down the bed
  20. 20. Methanation Catalyst Monitoring  Monitor Frequently: – Inlet/Exit Temperatures – Exit Carbon Oxides (CO + CO2)  Monitor less Frequently: – Pressure Drop – Temperature profile movement with time (poisoning rate). This is the major parameter in assessing Methanator Catalyst life expectancy
  21. 21. Prediction of Remaining Catalyst Life 0 1 2 3 4 5 6 7 8 9 10 11 12 290 300 310 320 330 Bed Depth ft TemperatureC Plot end of active catalyst bed as function of time 2.8 Co 16°C 2.8 C ~ 465ppm CO2 where tangent intercepts line, 16 C above 2.8 C point exit CO2 = 2ppm o o o o
  22. 22. Prediction of Remaining Catalyst Life 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Time on-line (years) Effectiveendofbed(ftfrominlet) Actual Predicted End of Bed
  23. 23. Catalyst Poisons  S is a poison but not normally present unless LTS by-passed  Most poisons originate from CO2 removal system  Carry-over a small amount of liquid not generally serious  large volumes will have a serious effect Common Poisons Effect K2CO3 Blocks Pores; is removable AS2O3 Serious irreversible poisoning Sulfolane Decomposes to S; poison
  24. 24. CO2 Removal Systems Process Chemical Effect Benfield Aqueous potassium carbonate Blocks pores of catalyst by evaporation of K2CO3 Vetrocoke Aqueous potassium carbonate plus arsenious oxide Blocks pores of catalyst by evaporation of K2CO3. As203 is also a poison; 0.5% of As on the catalyst will reduce its activity by 50%. Benfield DEA Aqueous Potassium carbonate with 3% di-ethanolamine Blocks pores of catalyst by evaporation of K2C03. DEA is harmless Sulphinol Sulpholane, water di-2-propanolamine Sulpholane will decompose and cause sulphur poisoning MEA, DEA di-ethanolamine in aqueous solution None MDEA Aqueous solution of methyl di-ethanolamine and activators None Rectisol Methanol None Catacarb Aqueous potassium carbonate with borate additive Blocks pores of catalyst by evaporation of K2C03 Selexol Dimethyl ether of polyethylene glycol None
  25. 25. Unusual Operating Conditions  High CO levels – LTS by-passed – total concentration of carbon oxides <3% – inlet temperature 210-250oC (410-480oF) – if necessary, lower rate through HTS and increase S/C ratio  High water levels – normal level 2-3% H2O in inlet gas – if >3%, can lead to high CO2 in exit gas – may need to increase bed inlet temperatures – operating experience up to 7% H2O
  26. 26. Plant Mal-operation  Normal maximum exit temperatures is 450oC (840oF)  excursion to 600oC (1100oF) for several hours can be tolerated  in the event of a temperature runaway, the vessel must be protected: – isolate on inlet side – blow down to atmospheric – purge with nitrogen to aid cooling – exclude air to avoid exothermic oxidation
  27. 27. Abnormal Conditions  Gradual steady rise across whole bed • inadequate reduction? • Poisoning?  Sudden movement of reaction zone with no change in slope • poisoning of top? • poor reduction of top?  Normal temperature profile, high outlet, carbon oxides • channelling through bed? • Mechanical problems? (bypass valve, heat exchanger) • analytical problems?
  28. 28. Nickel Carbonyl Ni(CO)4  colorless, mobile liquid, flammable in air, insoluble in water  boiling point 43oC (190oF)  vapor pressure (oC) -12 18 24 43 (oF) 10 64 75 109 Vp (bar) 0.10 0.25 0.51 1.01 Vp (psi) 1.4 3.6 7.4 14.6 EXTREMELY TOXIC
  29. 29. Toxicity of Ni (CO)4  4 ppm v/v for 2 minute gives severe toxic effects  2 ppm v/v short time leads to illness  target value (daily average concentrations) 0.001ppm v/v Ni(s) + 4CO(g) Ni(CO)4(g)
  30. 30. Guidelines  Under normal operating conditions, concentrations are too low to be a problem • steam reformer has a high CO, high Ni, but high temperatures • after LTS, temperatures low, but low CO, low Ni  Under abnormal operating conditions (eg. start-up or shut-down) it is possible to get conditions favourable for the formation of Ni(CO)4 Keep temperatures above 200oC (390oF) to avoid formation of Ni(CO)4
  31. 31. Nickel Carbonyl Formation Temperature oC (oF) PartialpressureofCO(bar) 0 100 200 300 400 0.001 0.002 0.005 0.01 0.02 0.05 0.1 0.2 0.5 1 Favorable Not Favorable (32) (212) (392) (572) (752) 30 bar 1 bar Conditions for the formation of 0.001 ppm
  32. 32. Catalyst Back-washing for K2CO3 Removal  Considerations – catalyst strength – water quality and temperature – reactor cooling and purging – plant isolations  Catalyst performance fully regained – CO + CO2 slip < 6ppm – catalyst strength unaffected by repeated washings – no effect on catalyst pressure drop

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