theory and operation of methanation catalyst
DESCRIPTION
Introduction and Theoretical Aspects Catalyst Reduction and Start-up Normal Operation and Troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl HazardTRANSCRIPT
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Theory and Operation of Methanation Catalyst
By:
Gerard B. Hawkins Managing Director, CEO
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Contents
Introduction and Theoretical Aspects
Catalyst Reduction and Start-up Normal Operation and
Troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard
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Introduction
Carbon oxides are poisons for many hydrogenation reactions
Used on older plants (without PSA) CO2 removal followed by methanation Uses nickel-based catalyst
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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
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270°C 518°F Inlet
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
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Methanator Vessel
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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
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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)
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Contents
Introduction and Theoretical Aspects Catalyst reduction and start-up Normal operation and troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard
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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
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Pre-Reduced Catalyst Now available
Simplifies start-up
Maximises activity at low temperatures
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Contents
Introduction and Theoretical Aspects Catalyst Reduction and Start-up Normal Operational and
troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard
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Methanation Catalyst Temperature Profile
Over designed originally, high catalyst activity
Most reaction in top of bed Catalyst lives 10-15 years
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320
310
300
290
280
(2) (4) (6) (8) (10) (12)
Tem
pera
ture
°C (°
F)
0 1 2 3 (536) 4
(554)
(572)
(590)
(608)
Bed depth m (ft)
Methanation Reaction Profile
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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
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Top Bottom
Tem
pera
ture
Bed Depth
- ageing mechanism is gradual poisoning - profile moves down the bed
Methanation Catalyst Ageing
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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
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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
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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
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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
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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
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Contents
Introduction and Theoretical Aspects Catalyst Reduction and Start-up Normal Operation and Troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard
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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
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Catalyst Back-washing for K2CO3 Removal
Considerations • catalyst strength • water quality and temperature • reactor cooling and purging • plant isolations
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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
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Contents
Introduction and Theoretical Aspects
Catalyst Reduction and Start-up Normal Operation and
troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard
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• 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!
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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
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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
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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 Formation Pa
rtia
l Pre
ssur
e of
CO
(bar
)
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Conclusions
Reviewed methanation reactions and catalyst Described normal operation Described abnormal conditions Poisoning Mentioned catalyst back-washing Reviewed nickel carbonyl hazard
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