a kinetic model of methanol formation over lts catalysts
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A Kinetic Model for Methanol Formation over LTS Catalysts
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Gerard B. Hawkins Managing Director
Contents
Impact of by-product methanol Catalyst chemistry and methanol formation Factors affecting by-product methanol formation Development process for the kinetic model Conclusions
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Contents
Impact of by-product methanol Catalyst chemistry and methanol formation Factors affecting by-product methanol
formation Development process for the kinetic model Conclusions
www.gbhenterprises.com
Impact of By-product Methanol (1)
Environmental • Licence to operate under tighter regulations
and/or legislation Control of VOC emissions
• Deaerator vents; condensate strippers • Odor from by-product amines
BOD of process condensate • Cost of mitigation strategies
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Impact of By-product Methanol (2)
Operational (1) • MeOH in recycle condensate
Complicates stripping and recycle Trace acid formation lowers pH
• Leads to increased operating costs
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Impact of By-product Methanol (3)
Operational (2) • CO2 removal systems
MeOH break down to HCOOH degrades solvent
• CO2 product for sale/urea production MeOH and break down products may need
scrubbing Selectivity and corrosion issues in urea
plants
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Impact of By-product Methanol (4)
Plant efficiency • Formation of MeOH consumes H2
CO2 + 3 H2 => CH3OH + H2O No longer available to make NH3
• Low selectivity LTS catalysts cost money Up to 4 – 5 tonne/day MeOH (2000 mtpd
plant) 1.1 tonne NH3/tonne MeOH + US$
350/tonne NH3 Lost NH3 value may be US$ 500,000/year
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Contents
Impact of by-product methanol Catalyst chemistry and methanol formation Factors affecting by-product methanol formation Development process for the kinetic model Conclusions
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Reactions over HTS and LTS Catalysts
Water Gas Shift reaction • CO + H2O CO2 + H2 -41.16 kJ/mol
Unwanted reactions: by-product methanol
• CO + 2 H2 CH3OH -90.73 kJ/mol • CO2 + 3 H2 CH3OH + H2O -49.57 kJ/mol
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Methanol Formation: Effect of Catalysts
Catalysts accelerate reaction rate • Influence kinetics • Reaction moves towards, and maybe reaches,
equilibrium Temperature also influences reaction rate
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Methanol Formation in HTS Converters
MeOH formation reaches equilibrium • Equilibrium limited reaction • Higher temperature (than LTS) BUT
equilibrium position disfavors MeOH Level depends on HTS exit conditions
• E.g. temperature • Thus – higher activity HTS catalysts operate at
lower temperatures => higher MeOH make
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Methanol Formation in LTS Converters
MeOH formation does not reach equilibrium • Kinetically limited reaction • Lower temperature and catalyst activity for
MeOH formation Level depends on LTS exit conditions and MeOH
activity of LTS catalyst
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Methanol Formation in HTS & LTS Converters
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Contents
Impact of by-product methanol Catalyst chemistry and methanol formation Factors affecting by-product methanol formation Development process for the kinetic model Conclusions
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Process Factors Affecting LTS Methanol Formation
Higher steam ratio • Increasing steam ratio reduces methanol
make Lowering LTS inlet temperature
• MeOH formation is kinetically limited • Lower inlet temperature reduces MeOH make
Higher space velocity • MeOH formation is kinetically limited • Lower residence time reduces MeOH make
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Process Factors Affecting LTS Methanol Formation
Lower operating pressure • Higher pressure favors forward reaction • Lower pressure reduces MeOH make
BUT • Window to change operating conditions is
limited • Changes may compromise rate and/or
efficiency
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Catalyst Factors Affecting LTS Methanol Formation
Catalyst age • As the catalyst ages its activity declines • Older catalyst forms less MeOH • BUT shift activity has also declined
Catalyst selectivity • More selective catalyst reduces MeOH make
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Methanol Formation: Effect of LTS Catalysts
To make less MeOH • Modify LTS catalyst formulation • Reduce its influence on MeOH formation
kinetics (slower reaction rate) BUT Without reducing effect on shift reaction
• Influence on shift kinetics maintained
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Methanol Formation: Effect of LTS Catalysts
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Methanol Formation: Effect of LTS Catalysts
Low MeOH LTS catalysts • MeOH formation can be suppressed • Add controlled levels of alkali metal oxides • Combination of ~2 wt% of K2O and Cs2O
MeOH levels • reduced to ~15% of that made by a standard
(non-alkali) catalysts
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Contents
Impact of by-product methanol Catalyst chemistry and methanol formation Factors affecting by-product methanol formation Development process for the kinetic model Conclusions
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Development Process – 3 Stages
Calculate limiting conditions for study • Dew points (avoid condensation)
Normal margin then applied (15 – 20°C) • Equilibrium MeOH concentrations
Also other possible C1 by-products
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Development Process – 3 Stages
Scoping experiments • Initial period necessary to stabilize catalyst
activity • Confirm lack of diffusion limitations • Define envelope of experimental conditions for
the detailed kinetic study
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Development Process – 3 Stages
Kinetic study experiments • Focussed on T range 200 – 230°C (392 –
446°F) • Fixed CO2 and H2 levels in dry gas • Variables include
CO and N2 in dry gas Steam to dry gas ratio Pressure GHSV
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Concept for Experimental Program
CO + H 2 O CO 2 + H 2 - 41.16 kJ/mol
Unwanted reactions:
CO + 2H 2 CH 3 OH - 90.73 kJ/mol
CO 2 + 3H 2 CH 3 OH+H 2 O - 49.57 kJ/mol
LT - WGS Main Bed
Working Bed
Extended Bed
X CO
X CO =X CO, eq (WGS)
X CO =X CO, eq (WGS)
WGS reaches equilibrium
CO + H 2 O CO 2 + H 2 - 41.16 kJ/mol
Unwanted reactions:
CO + 2H 2 CH 3 OH - 90.73 kJ/mol
CO 2 + 3H 2 CH 3 OH+H 2 O - 49.57 kJ/mol
LT - WGS Main Bed
Working Bed
Extended Bed
X CO
X CO =X CO, eq (WGS)
X CO =X CO, eq (WGS)
Flow
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VULCAN TECHNOLOGY Test Rig
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VULCAN TECHNOLOGY Test Rig
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General Conclusions from Experimental Work
Initial rapid activity die-off observed • Very active sites “burn out” to attain stable
active state Synthesis through CO2 implied
• CO concentration has minimal effect on by-product MeOH
• H2O has strong inhibiting effect on by-product MeOH formation
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Methanol Formation Model: Form of Kinetic Model
Empirical model derived by non-linear least squares data regression
Power law based model of the form
• Where
rs = reaction rate kr = rate constant Px = partial pressure of component x nx = order of reaction of component x
O H CO H CO n O H
n CO
n H
n CO
RT Ea r s P P P P e k r 2
2
2
2
2
2 ) ( ) ( ) ( ) ( / − =
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Methanol Formation Model: Results Fit
experimental result *10^10
0 50 100 150 200 250 300 350 400
regr
esse
d re
sult
*10^
10
0
100
200
300
400
exp. point vs rs3 y=x
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Methanol Formation Model: Validation
GBH Enterprises LTS predictive model • VULCAN Technology MeOH kinetic model
incorporated Updates GBHE MeOH kinetics
• Includes activity die off factors for MeOH • Includes condensate catch pot conditions
LTS predictive model tested • Data from a number of NH3 plants
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Methanol Formation Model: Validation
1500 MTPD Europe am Ammonia plant
0
20
40
60
80
100
120
140
160
0.0 0.5 1.0 1.5 2.0 2.5 3 .0 3 .5 4 .0
time yea rs
met
hnao
l pp
m
m easure d pre dic ted
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Methanol Formation Model: Validation
1200MTPD European NH3 plant
0
40
80
120
160
200
0 0.5 1 1.5 2time, years
met
hano
l, m
g/l
predicted measured
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Methanol Formation Model: Validation
GBH Enterprises LTS predictive model • Good agreement with measured MeOH levels • Realistic activity die off factor to ensure
predictions do not over-promise
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Contents
Impact of by-product methanol Catalyst chemistry and methanol formation Factors affecting by-product methanol formation Development process for the kinetic model Conclusions
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Conclusion
MeOH formation • Raises environmental, operational and
efficiency issues • Occurs over HTS and LTS catalysts • Control over LTS by operating conditions
and catalyst choice Accurate MeOH prediction provides assurance
of environmental compliance (licence to operate)
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Conclusion
GBH Enterprises Improved MeOH formation kinetic model • Validated against plant data
Enhanced MeOH prediction capability
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