Theory and Operation of Methanation Catalyst

By:

Gerard B. Hawkins Managing Director, CEO

Contents

Introduction and Theoretical Aspects

Catalyst Reduction and Start-up Normal Operation and

Troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard

Introduction

Carbon oxides are poisons for many hydrogenation reactions

Used on older plants (without PSA) CO2 removal followed by methanation Uses nickel-based catalyst

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

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

Methanator Vessel

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

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)

Contents

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

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

Pre-Reduced Catalyst Now available

Simplifies start-up

Maximises activity at low temperatures

Contents

Introduction and Theoretical Aspects Catalyst Reduction and Start-up Normal Operational and

troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard

Methanation Catalyst Temperature Profile

Over designed originally, high catalyst activity

Most reaction in top of bed Catalyst lives 10-15 years

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

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

Top Bottom

Tem

pera

ture

Bed Depth

- ageing mechanism is gradual poisoning - profile moves down the bed

Methanation Catalyst Ageing

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

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

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

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

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

Contents

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

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

Catalyst Back-washing for K2CO3 Removal

Considerations • catalyst strength • water quality and temperature • reactor cooling and purging • plant isolations

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

Contents

Introduction and Theoretical Aspects

Catalyst Reduction and Start-up Normal Operation and

troubleshooting Shutdown and Catalyst Discharge Nickel Carbonyl Hazard

• 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!

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

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

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

)

Conclusions

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