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

1. Theory and Operation
of Methanation Catalyst
By:
Gerard B. Hawkins
Managing Director, CEO

2. Contents
 Introduction and Theoretical
Aspects
 Catalyst Reduction and Start-up
 Normal Operation and
Troubleshooting
 Shutdown and Catalyst Discharge
 Nickel Carbonyl Hazard

3. Introduction
 Carbon oxides are poisons for many
hydrogenation reactions
 Used on older plants (without PSA)
 CO2 removal followed by methanation
 Uses nickel-based catalyst

4. Theoretical Aspects
Strongly exothermic reactions:
CO + 3H2 CH4 + H2O
CO2 + 4H2 CH4 + 2H2O
H = 206 kJ/mol
-89 BTU/lbmol
H = -165 kJ/mol
-71 BTU/lbmol
(Reverse of steam reforming)
Temperature rise:
74OC (133OF) for each 1% of CO converted
60OC (108OF) for each 1% of CO2 converted

5. 270°C
518°FInlet
Composition (%)
CO 0.2
CO2 0.1
H2 93.9
CH4 3.3
H2O 2.5
Outlet
Composition (%)
CO
CO2
H2 93.5
CH4 3.6
H2O 2.9
291°C
556°F
Typical Process Conditions
}<5ppmv

6. Methanator Vessel

7. Mechanism of Reaction
 Equilibrium concentrations of carbon oxides
10-4 ppmv
 Governed by Kinetics
 CO inhibits methanation of CO2
 Two stage reaction:
i) CO2 reverse - shifts to CO
CO2 + H2 CO + H2O
ii) CO methanates
CO + 3H2 CH4 + H2O
Intrinsic reaction rates very high (diffusion limited
at higher temperature

8. Catalyst Composition
 Iron originally studied
 Ruthenium good at low temperature
(“ultra-methanation”)
 Nickel conventionally used
 Support matrix with 20-40% (wt) nickel
 Promotors to reduce sintering
 Small pellets (5 mm x 3 mm)

9. Contents
 Introduction and Theoretical Aspects
 Catalyst reduction and start-up
 Normal operation and troubleshooting
 Shutdown and Catalyst Discharge
 Nickel Carbonyl Hazard

10. NiO + H2 Ni + H2O
NiO + CO Ni + CO2
∆H = +3 kJ/mol
+1 BTU/lbmol
∆ H = -30 kJ/mol
-13 BTU/lbmol
Catalyst Reduction
 little temperature rise from reduction itself
 metallic nickel will lead to methanation during
reduction
 reduction gas should not contain carbon oxides
(<15)
 need to heat catalyst to 400-450oC (750-840oF)
for maximum activity
BUT
THEREFORE

11. Pre-Reduced Catalyst
 Now available
 Simplifies start-up
 Maximises activity at low temperatures

12. Contents
 Introduction and Theoretical Aspects
 Catalyst Reduction and Start-up
 Normal Operational and
troubleshooting
 Shutdown and Catalyst Discharge
 Nickel Carbonyl Hazard

13. Methanation Catalyst Temperature
Profile
 Over designed originally, high catalyst
activity
 Most reaction in top of bed
 Catalyst lives 10-15 years

14. 320
310
300
290
280
(2) (4) (6) (8) (10) (12)
Temperature°C(°F)
0 1 2 3(536) 4
(554)
(572)
(590)
(608)
Bed depth m (ft)
Methanation Reaction Profile

15. 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

16. Top Bottom
Temperature
Bed Depth
- ageing mechanism is gradual poisoning
- profile moves down the bed
Methanation Catalyst Ageing

17. 1. Gradual steady rise across whole bed
• inadequate reduction?
• poisoning
2. Sudden movement of reaction zone with no change
in slope
• poisoning of top?
• Poor reduction of top?
3. Normal temperature profile, high outlet carbon
oxides
• channelling through bed?
• mechanical problems? (by pass valve; heat
exchanger)
• analytical problems?
Abnormal Conditions

18. Unusual Operating Conditions
1. 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
2. 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 temperature
• operating experience up to 7% H2O

19. Plant Mal-Operation
 Normal maximum exit temperature is 450OC
(480OF)
 Excursions to 600OC (1100OF) for several hours
can be tolerated
 In this 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

20. Catalyst Poisons
 S is a poison but normally present unless LTS by-
passed
 Most poisons originate from CO2 removal system
 Carry-over of a small amount of liquid not
generally serious
 Large volumes will have a serious effect
Common Poisons Effect
Blocks pores; removable
Serious, irreversible poisoning
K2CO3
As2O3
Sulpholane Decomposes to S; poison

21. Process Chemical Effect
Benfield
Vetrocoke
Benfield DEA
Sulphinol
MEA, DEA
MDEA
Rectisol
Catacarb
Selexol
Aqueous potassium carbonate
Aqueous potassium carbonate
plus arsenious oxide
Aqueous potassium carbonate
With 3% di-ethanolamine
Aqueous potassium carbonate
with borate additive
Sulpholane, water
di-2-propanolamine
Mono- or di-ethanolamine
in aqueous solution
Aqueous solution of methyl
di-ethanolamine and activators
Methanol
Dimethyl ether of polyethylene
glycol
Blocks pores of catalyst by evaporation of K2CO3
Blocks pores of catalyst by evaporation of K2CO3 .
(DEA is harmless)
Blocks pores of catalyst by evaporation of K2CO3 .
As2O3 is also a poison; 0.5% of As on the catalyst
will reduce its activity by 50%
Blocks pores of catalyst by evaporation of K2CO3
Sulpholane will decompose and cause sulphur
poisoning
None
None
None
None
CO2 Removal Systems

22. Contents
 Introduction and Theoretical Aspects
 Catalyst Reduction and Start-up
 Normal Operation and Troubleshooting
 Shutdown and Catalyst Discharge
 Nickel Carbonyl Hazard

23. Shutdown
 If process gas temperature > 200OC
(390OF), can be left in atmosphere of
process gas for short periods
 Below 200OC (390OF), must be purged with
an inert to prevent carbonyl formation
 Reduced catalyst pyrophoric; oxidation
very exothermic
• spread catalyst thinly on ground
• have water hoses available
• transport in metal skips/metal/sided
trucks

24. Catalyst Back-washing for K2CO3
Removal
 Considerations
• catalyst strength
• water quality and temperature
• reactor cooling and purging
• plant isolations

25. Methanator Back-washing - Effect
on Performance
 Catalyst performance fully regained
• CO + CO2 slip < 6 ppm
 Catalyst strength unaffected by repeated
washings
 No effect on catalyst pressure drop

26. Contents
 Introduction and Theoretical
Aspects
 Catalyst Reduction and Start-up
 Normal Operation and
troubleshooting
 Shutdown and Catalyst Discharge
 Nickel Carbonyl Hazard

27. • Colorless, mobile liquid flammable in air,
insoluble in water
• Boiling point 43°C (109°F)
• Vapor pressure:
(°C)
-12
18
24
43
(°F)
10
64
75
109
v p (bar)
0.10
0.25
0.51
1.01
v p (psi)
1.4
3.6
7.4
14.6
Nickel Carbonyl Ni(CO)4
EXTREMELY TOXIC!

28. Toxicity of Ni(CO)4
 4 ppm v/v for 1 minute gives severe toxic
effects
 2 ppm v/v short time leads to illness
 target value (daily average concentration)
0.001 ppm v/v
Ni + 4 CO Ni(CO)4

29. Guidelines
1. Under normal operating conditions, concentrations
are too low to be a problem
• steam reformer has high CO, high Ni, but high
temperatures
• after LTS, temperatures low, but low Co, low Ni
2. 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 200°C
(390°F) to avoid formation of Ni(CO)4

30. 0 100 200 300 400
0.001
0.002
0.005
0.01
0.02
0.05
0.1
0.2
0.5
1
Temperature °C (°F )
Favorable
Not Favorable
(32) (212) (392) (572) (752)
30 bar
1 bar
Conditions for the formation of 0.001 ppmv
Nickel Carbonyl FormationPartialPressureofCO(bar)

31. Conclusions
 Reviewed methanation reactions and catalyst
 Described normal operation
 Described abnormal conditions
 Poisoning
 Mentioned catalyst back-washing
 Reviewed nickel carbonyl hazard