130
parameters on methanol production, while the adsorption constant of water had very
low relevance to methanol formation. Finally, the microkinetic model previously
developed by Ovesen and co-workers (1996) in which the water-gas shift reaction
occurs via a redox mechanism and then methanol is formed by the successive
hydrogenation of carbon dioxide could also be used to evaluate morphology changes
during the reaction, such as changes in the Cu surface area. All three models were
found to be valid in the range of experimental data used. However, macrokinetic
models showed to be more accurate in predicting kinetics in the evaluated experimental parameters in space useful for reactor modeling, while the microkinetic
model could include catalyst morphology changes, which are important during
methanol production.
The microkinetic model developed by Van den Bussche and Froment (1996) has
been frequently used to describe experimental data obtained not only at the laboratory scale but also at the industrial scale (Stanislaw et al. 2013). The model was validated with experiments performed over a Cu–ZnO–Al 2 O 3 commercial catalyst at
temperatures ranging from 180 to 280 °C and pressures ranging from 15 to 51 bar.
The reactions (1) to (3) were considered to occur only on the copper surface, while
zinc oxide was considered to act only as structural promoter. Moreover, carbon
dioxide was assumed to be the main source of carbon for methanol production. In
this model, hydrogen and carbon dioxide adsorb dissociatively on the copper surface. Carbon dioxide adsorption generates carbonates that are first hydrogenated
into bicarbonate structures and then subsequently hydrogenated to formate, formaldehyde, methoxy species, and methanol. Also, carbon dioxide hydrogenation of
methanol releases surface oxygen that is also hydrogenated first into hydroxyl
groups and then into water. The kinetic equations described correctly the influence
of temperature, pressure, as well as feed composition.
Most of the kinetic models for methanol synthesis take into consideration the
reaction on different copper surfaces, such as Cu
+
and Cu
0
. However, very few
kinetic models are based on three-site adsorption (Cu
+
, Cu
0
, ZnO). Park et al. (2014)
addressed this issue by proposing a mechanism based on a Langmuir–Hinshelwood–
Hougen–Watson mechanism over a commercial Cu–ZnO–Al 2 O 3 catalyst and taking
into consideration reactions (1) to (3). The kinetic data was collected using a continuous tubular fixed-bed microreactor at a temperature range of 250–340 °C and at
a pressure range from 50 to 90 bar. A total of 16 elementary steps were considered,
and the model was validated by comparing the experimental and simulation data. As
expected, the model showed that the reaction was favored by low space–time velocity around 800 mLg cat
−1
h
−1
, due to high residence times, and by high pressures
around 50 bar since there is a decrease in the total number of moles during the reaction. Simulations also showed that carbon monoxide conversion rates were mainly
influenced by the thermodynamic equilibrium when the particle size was smaller
than 0.5 mm. On the contrary, for particle sizes larger than 1.5 mm, the conversion
rate was mainly influenced by the reaction rates. However, contrary to other kinetic
models reported on the literature, this model showed similar contributions of carbon
monoxide and carbon dioxide hydrogenations.
D. P. Minh et al.
parameters on methanol production, while the adsorption constant of water had very
low relevance to methanol formation. Finally, the microkinetic model previously
developed by Ovesen and co-workers (1996) in which the water-gas shift reaction
occurs via a redox mechanism and then methanol is formed by the successive
hydrogenation of carbon dioxide could also be used to evaluate morphology changes
during the reaction, such as changes in the Cu surface area. All three models were
found to be valid in the range of experimental data used. However, macrokinetic
models showed to be more accurate in predicting kinetics in the evaluated experimental parameters in space useful for reactor modeling, while the microkinetic
model could include catalyst morphology changes, which are important during
methanol production.
The microkinetic model developed by Van den Bussche and Froment (1996) has
been frequently used to describe experimental data obtained not only at the laboratory scale but also at the industrial scale (Stanislaw et al. 2013). The model was validated with experiments performed over a Cu–ZnO–Al 2 O 3 commercial catalyst at
temperatures ranging from 180 to 280 °C and pressures ranging from 15 to 51 bar.
The reactions (1) to (3) were considered to occur only on the copper surface, while
zinc oxide was considered to act only as structural promoter. Moreover, carbon
dioxide was assumed to be the main source of carbon for methanol production. In
this model, hydrogen and carbon dioxide adsorb dissociatively on the copper surface. Carbon dioxide adsorption generates carbonates that are first hydrogenated
into bicarbonate structures and then subsequently hydrogenated to formate, formaldehyde, methoxy species, and methanol. Also, carbon dioxide hydrogenation of
methanol releases surface oxygen that is also hydrogenated first into hydroxyl
groups and then into water. The kinetic equations described correctly the influence
of temperature, pressure, as well as feed composition.
Most of the kinetic models for methanol synthesis take into consideration the
reaction on different copper surfaces, such as Cu
+
and Cu
0
. However, very few
kinetic models are based on three-site adsorption (Cu
+
, Cu
0
, ZnO). Park et al. (2014)
addressed this issue by proposing a mechanism based on a Langmuir–Hinshelwood–
Hougen–Watson mechanism over a commercial Cu–ZnO–Al 2 O 3 catalyst and taking
into consideration reactions (1) to (3). The kinetic data was collected using a continuous tubular fixed-bed microreactor at a temperature range of 250–340 °C and at
a pressure range from 50 to 90 bar. A total of 16 elementary steps were considered,
and the model was validated by comparing the experimental and simulation data. As
expected, the model showed that the reaction was favored by low space–time velocity around 800 mLg cat
−1
h
−1
, due to high residence times, and by high pressures
around 50 bar since there is a decrease in the total number of moles during the reaction. Simulations also showed that carbon monoxide conversion rates were mainly
influenced by the thermodynamic equilibrium when the particle size was smaller
than 0.5 mm. On the contrary, for particle sizes larger than 1.5 mm, the conversion
rate was mainly influenced by the reaction rates. However, contrary to other kinetic
models reported on the literature, this model showed similar contributions of carbon
monoxide and carbon dioxide hydrogenations.
D. P. Minh et al.