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


Contents



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


Contents



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





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


Contents



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 + 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)


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



Flow


VULCAN TECHNOLOGY Test Rig


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

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 ) ( ) ( ) ( ) ( / − =


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


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



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



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



GBH Enterprises LTS predictive model • Good agreement with measured MeOH levels • Realistic activity die off factor to ensure

predictions do not over-promise


Contents



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
