A Kinetic Model of Methanol Formation Over LTS Catalysts

A Kinetic Model for Methanol
Formation over LTS Catalysts
www.gbhenterprises.com
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

Contents
 Impact of by-product methanol
 Catalyst chemistry and methanol formation
 Factors affecting by-product methanol
formation
 Development process for the kinetic model
 Conclusions

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

 Operational (1)
• MeOH in recycle condensate
 Complicates stripping and recycle
 Trace acid formation lowers pH
• Leads to increased operating costs

 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

 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

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

Methanol Formation: Effect of Catalysts
 Catalysts accelerate reaction rate
• Influence kinetics
• Reaction moves towards, and maybe reaches,
equilibrium
 Temperature also influences reaction rate

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

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

Methanol Formation in HTS & LTS
Converters

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

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

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

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

Catalysts

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

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

 Scoping experiments
• Initial period necessary to stabilize catalyst
activity
• Confirm lack of diffusion limitations
• Define envelope of experimental conditions for
the detailed kinetic study

 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

Concept for Experimental Program
CO + H2O CO2+ H2 - 41.16 kJ/mol
Unwanted reactions:
CO + 2H2 CH3OH - 90.73 kJ/mol
CO2 + 3H2 CH3OH+H2O - 49.57 kJ/mol
LT-WGS Main Bed
Working Bed
Extended Bed
XCO
XCO=XCO,eq(WGS)
XCO=XCO,eq(WGS)
WGS reaches equilibrium
CO + H2O CO2+ H2 - 41.16 kJ/mol
Unwanted reactions:
CO + 2H2 CH3OH - 90.73 kJ/mol
CO2 + 3H2 CH3OH+H2O - 49.57 kJ/mol
LT-WGS Main Bed
Working Bed
Extended Bed
XCO
XCO=XCO,eq(WGS)
XCO=XCO,eq(WGS)
Flow

VULCAN TECHNOLOGY Test Rig

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

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
OHCOHCO n
OH
n
CO
n
H
n
CO
RTEa
rs PPPPekr 2
2
2
2
2
2
)()()()(/−
=

Methanol Formation Model: Results Fit
experimental result *10^10
0 50 100 150 200 250 300 350 400
regressedresult*10^10
0
100
200
300
400
exp. point vs rs3
y=x

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

1500MTPD Europeam 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
methnaolppm
m easure d pre dicted

1200MTPD European NH3 plant
0
40
80
120
160
200
0 0.5 1 1.5 2
time, years
methanol,mg/l
predicted measured

 GBH Enterprises LTS predictive model
• Good agreement with measured MeOH levels
• Realistic activity die off factor to ensure
predictions do not over-promise

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)

Conclusion
 GBH Enterprises
Improved MeOH formation kinetic model
• Validated against plant data
 Enhanced MeOH prediction capability

A Kinetic Model of Methanol Formation Over LTS Catalysts

A Kinetic Model of Methanol Formation Over LTS Catalysts

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