Chemical Pathway Validation, Kinetics, and Reactor Scale-Up

This assignment addresses the validation of a chemical pathway and kinetic parameters, reactor design and scale-up, and the effects of temperature on kinetics and equilibrium limitations. The student validates the pathway by comparing selectivity and yield curves from Aspen simulations with reference data. The assignment explores the use of the Damkohler Number for reactor scale-up and the impact of temperature on reaction rates, using the Arrhenius equation to estimate activation energies. It also investigates the need to consider reversibility in reactions and calculate equilibrium constants using the REQUIL model in Aspen. Supporting calculations and rationale are provided throughout the analysis, including the estimation of reaction rate constants for both forward and reverse reactions. The assignment covers topics like plug flow reactor design, sensitivity analysis, and the importance of operating temperature on selectivity.

[Part a] Validation of Chemical Pathway and Kinetic Parameters:
Question:
How would you validate the pathway and kinetic parameters?
Answer:
The validation can be done by the comparison of selectivity and yield curves of the reference
paper (Anunziata et al., 1999) and the curves of selectivity and yield change with respect to
Temperature obtained by sensitivity analysis from the Aspen simulation of the plant.
Sensitivity analysis across the reactor will give the selectivity curve. If the curves obtained
from Aspen and the curves in the reference paper corroborate, then the validation is
confirmed.
Can you please, do the ASPEN part for this?

Question:
Based on the data presented in the reference paper (Anunziata et al., 1999) is the kinetic
information provided to you sufficient to scale-up the reactor experiments (a Plant Design
scale)?
Answer:
(YES / NO)?
The design and scale up of the reactor using the dimensionless Damkohler Number and the
assumption that L/D ratio of the Plug Flow Reactor is 1/5.(Supporting calculations ?
Therefore, along with the data given in the reference paper, which could give the volume of
the reactor, an assumption for the length and diameter ratio was required to dimensionalize
the reactor. This was demonstrated to be working in the scale up in the Aspen file for the
whole plant.
Please re-answer this question with supporting calculations.
[Part b] Temperature Effects on Equilibrium Limitations:
Question:
If you were to consider the possibility of reversibility, which reactions will warrant a correction
in their rate of reaction expressions?
Answer:
which reactions will warrant a correction in their rate of reaction expressions?
All the forward reactions were modelled with Requil to estimate the equilibrium constant in
Aspen. Then, their reverse reaction constants were estimated. The model for a PFR was
then used and the reverse reactions along with the forward reactions were used in the
model.
and Explain your rationale and indicate the modifications needed, along with supporting
(evidence) calculations.

Question:
Would there be any merit in considering operating the reactor at a different (lower, or
higher?) temperature than the reference value?
Answer:
The high temperature of 700 oC shows the greatest selectivity for BTX, so there is great merit
to operate it on that temperature as selectivity is very crucial. (This is a partial answer) ..
Explain your rationale and provide supporting calculations.
The following is also a partial answer …
In a preliminary phase all the reactions will be considered irreversible, but there is a large
amount of hydrogen generated by this mechanism. Thus, depending on the operating
conditions, some of the reactions may be limited by equilibrium. The kinetics therefore must
be completed by adding the reversal reaction term. The equilibrium constants, Kc for each
individual reaction have been calculated using REQUIL in Aspen and then the reaction rate
constants for the backward reactions was determined. All the forwards reactions are
considered first-order with respect to the reactants (ethane, C2H6, ethylene, C2H4, or
propylene, C3H6 while hydrogen will be in excess), with the exception of the methane
formation, which is considered elementary when written as
Rxn 3: Methane formation 1/3 C3H6 + H2  CH4 with r3 = k3 (CPropylene)1/3 CHydrogen
Table 1: The estimated reaction rate constants for concentrations in M (or moles/liter = kmoles/m3)
Reaction
No.
Forward reaction
rate constant, k at
Tref (1/sec)
Equilibrium constant,
Kc
1 12.1 0.227059043
2 14.5 0.688829581
3 16.5 187.521333
4 59.2 214920.65
5 47.2 3.32E+16
6 3.9 5.65498E+13
7 4.5 7505.94797
Tref = 700 C (or 973.15 K) and R = 0.008314 kJ/mol K.
Any numbers for activation energies and frequency factors as it is isothermal, can be used.
The reactor operates at 700 C and is catalyzed by Mo-ZSM-11 zeolite [10]. The equilibrium
constants were calculated and are shown in the table above. As mentioned earlier, Requil model
was used in Aspen to calculate these constants.

Using the equilibrium constants, the rates of reversal reactions were calculated and are shown
in Table 1.b.
Table1.b: the rates of reversal reactions
[Part c,] Estimation of Temperature Effects on Kinetics:
Question:
How would you use the data provided in the reference paper (Anunziata et al., 1999) to
estimate the effect of Temperature on the Kinetics (on the reaction rate constants)?
Answer:
The Temperature effect on the reaction rate constant can be estimated by Arrhenious
Equation which can be represented as:
k = A . e−(Ea / RT)
Where,
A is the pre-exponential factor
k is the reaction rate constant
Ea is the Activation Energy
R is the universal gas constant
T is the Temperature
To come up with the correct correlation for k and T, first we need to estimate the Activation
Energy.
Estimate the Activation Energies for the main reactions in the Mechanism, justify your
choices/estimates, and provide supporting calculations.
Reaction
No.
Keq result from
Aspen Given k+ k-
1 0.227059043 12.1 53.2901
2 0.688829581 14.5 21.0502
3 187.521333 16.5 0.08799
4 214920.65 59.2 0.000275
5 3.32E+16 47.2 1.42E-15
6 5.65498E+13 3.9 6.9E-14
7 7505.94797 4.5 0.0006