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Propylene Partial Oxidation
Objective: To Build and Compare Microkinetic and Macrokinetic Models
In this example, we build kinetic models for the partial oxidation of propylene to acrolein. This
example is adapted from the work presented in [1] on propylene oxidation with bismuth molybdenum
catalyst. As noted in [1], the Bismuth/Molybdenum ratio strongly affects reactivity and selectivity, and
the Alpha, Beta and Gamma phases have vastly differing by-product profiles. For such systems, a
microkinetic model can be very useful to see the relationship between the catalyst composition and
the surface reactions. Here, we show how REX can be used to build both microkinetic and
macrokinetic models. You may download the REX files for this model here. The model is built with
fictitious experimental data.
Features Illustrated
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Microkinetics Model for Catalytic Oxidation of propylene
Macrokinetic Model based on Langmuir Hinshelwood kinetics
Use of Parity Plots to evaluate Model Predictions
Main Reactions of Propylene Partial Oxidation
The product of interest is Acrolein (C3H4O), which is formed by the partial oxidation of Propylene
(C3H6) on a solid catalyst:
C3H6 + O2 → C3H4O + H2O
Acetaldehyde (CH3CHO) is a byproduct which could be created from oxidation of either Propylene or
Acrolein:
C3H6 + 2O2
→ CH3CHO + H2O + CO2
C3H4O + 1.5O2 → CH3CHO + H2O + CO2
However, experimental data taken with Acrolein in the feed does not show any increase in
Acetaldehyde formation. Thus, we discard the path of Acrolein to Acetaldehyde.
Another byproduct is CO2. Aside from being a co-product of Acetaldehyde reactions, it can also be
produced from total oxidation of Propylene and Acrolein:
C3H6 + 4.5O2 → 3CO2 + 3H2O
C3H4O + 3.5O2 → 3CO2 + 2H2O
In this example, we limit the reactions to those shown above. In reality, there are additional
byproducts such as acrylic acid and acetic acid.
Setting up the Microkinetic Model in REX
First, we consider the surface adsorption/desorption of species from the bulk gas to the free catalyst
sites, that are represented by “s”:
+ 2s ⇄ 2 O-s
O2
+ s ⇄ H2O-s
H2O
+ s ⇄ C3H6-s
C3H6
C3H4O + s ⇄ C3H4O-s
+ s ⇄ CO2-s
CO2
The suffix “-s” indicates the active sites occupied by the adsorbed species. All the
adsorption/desorption reactions are considered to be at equilibrium.
The other surface reactions are:
C3H6-s
+ O-s ⇄ C3H5-s + OH-s
+ O-s ⇄ C3H5O-s + s
C3H5-s
C3H5O-s + O-s ⇄ C3H4O-s + OH-s
2OH-s
⇄ H2O-s + O-s
+ 9O-s ⇄ 3CO2-s + 3H2O-s + 4s
C3H6-s
C3H4O-s + 7O-s ⇄ 3CO2-s + 2H2O-s + 3s
+ 4O-s ⇄ CO2-s + H2O-s + CH3CHO + 3s
C3H6-s
These reactions are implemented in the PPYtoACR-Micro.rex file contained in the zip. In the
Chemistry, the Enable Special Models → Detailed Catalyst Model is enabled by selecting that option
(available on right click on the node). Then im Catalyst node we indicate the free active site “s” as the
Catalyst, while the adsorbed species on the catalyst surface are marked as Catalyst Complexes:
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Experimental Data
All experimental data are from a fixed bed reactor, with Flow automatically adjusted to keep Pressure
constant.
The experimental design includes:
→ Catalyst Mass Variation
→ Temperature Variation
→ Feed Concentration Variation
→ To check the effects of adsorption
→ To investigate compound orders on reaction
→ Product Concentrations
→ To check the effects of competitive adsorption
→ Product degradation pathways
There is also substantial amount of low conversion data to ensure that we capture the kinetics
accurately.
The experiment design is summarized in the Experiments node as shown below. Detailed data are
entered in the Measurements node.
Results from Microkinetics Model
We intend to match the outlet composition of Propylene, Acrolein, Acetaldehyde and CO2. Thus, they
are selected in the Weights node to be reconciled. Hybrid weights are then generated for these
compounds. The estimation project is run and we inspect the parity plots shown below. In general, we
see good predictions from the microkinetics model:
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We can also visualize the profiles of catalyst surface coverage. If we right-click on the Single Set
option in the Model-Data Comparison node, we can see the profile of several compounds for a single
set. Surface fractions are not reconciled since there are no surface measurements, but they provide
valuable information on the competitive adsorption on the surface:
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In the charts above, it shows that free catalyst fraction increases as reaction proceeds indicating that
product adsorption is not yet a significant barrier to propylene conversion. This should be considered
with caution since these experiments are run with high Nitrogen dilution. Another aspect that merits
caution is parameter uniqueness when dealing with microkinetic models. Since surface coverages
are rarely measured in experiments, there may be multiple sets of parameters that may match the
bulk species well, although the surface coverage may be quite different for each parameter set.
Setting up the alternate macrokinetic model in REX
While the microkinetic model gives us a fundamental understanding of the chemistry, they are often
hard to implement for process design in commercial process simulators. Furthermore, they are harder
to solve and suffer from parameter uniqueness issues which we discussed earlier. An alternate
approach is to implement the competitive adsorption phenomena by using the Langmuir HInshelwood
models. Langmuir Hinshelwood (LHHW) models may be derived from the microkinetics, but this is not
always possible. In such cases, we use an approximate LHHW model that mathematically
incorporates the reduction in rates due to active site coverage by competitive adsorption. These
models consider only the bulk species. The catalyst surface is not explicitly modeled, so they are
simpler and faster to solve. They can also be implemented in commercial process simulators. An
approximate LHHW model is used in the PPYtoACR-Macro.rex file contained in the zip, and is
described in detail below.
The reactions studied are those observed at macroscopic level:
C3H6
C3H6
C3H6
C3H4O
+
+
+
+
O2
2O2
4.5O2
3.5O2
→
→
→
→
C3H4O + H2O
CH3CHO + H2O + CO2
3CO2 + 3H2O
3CO2 + 2H2O
Reaction rates are defined using LHHW Sites that represent the denominator of the rate expression
below and account for the adsorption in the catalyst. For example, rate for Propylene oxidation to
Acrolein can be written as:
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The site adsorption inhibition terms are defined for the three most important species on the catalyst
surface, according to what was seen for the microkinetics model: Propylene occupies most of the
catalyst surface, together with Acrolein and followed by O2 in a smaller amount.
The proposed formulation with Site inhibition is not a mathematically exact equivalent of the
microkinetics model. Still, with the proper definition of site terms, the LHHW model can provide a
simpler representation without compromising on the predictive ability.
The use of LHHW kinetics is enabled by defining the Sites in Parameters node:
Then we assign the site to all reactions in the Kinetics Sites node, with exponent of one:
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After running the LHHW model, estimating the rate constants and the LHHW Sites parameters, we
observe that the weighted least square errors (LSQ) is similar to the microkinetics model. In the
results node, you may compare the predictions between the microkinetic and the LHHW models and
confirm that the LHHW model is adequate to describe the overall performance of this reaction.
Further studies
You may move the LHHW model project to optimization mode to evaluate the effect of temperature
profile on the selectivity to acrolein. A sample rex file (PPYtoACR-opt.rex) that studies three different
temperature profiles and its effect of acrolein selectivity is available in the zip.
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References
1. Fansuri, Hamzah, 2005, Catalytic Partial Oxidation of Propylene to Acrolein: The Catalyst
Structure, Reaction Mechanisms and Kinetics. PhD Thesis, Curtin University of Technology.
Weblink:
http://espace.library.curtin.edu.au/R?func=dbin-jump-full&local_base=gen01era02&object_id=16386
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