1) Methanation is the final stage of synthesis gas purification to reduce carbon oxides like CO and CO2 to trace levels using a nickel-based catalyst. 2) Typical methanation reactions are highly exothermic and occur between 270-290°C with carbon oxide inlet levels of 0.1-1.0% and carbon oxide slip less than 5 ppm. 3) Normal methanator operation involves monitoring inlet/outlet temperatures and carbon oxide levels to detect any issues like catalyst aging or poisoning that could impact the removal of carbon oxides to the required levels.

1. Ammonia Plant - Methanation
Operations
By:
Gerard B. Hawkins
Managing Director, CEO

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

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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