A discussion of how your simulation might differ from reality and the top three things
you would do to improve the fidelity of the work [2 pages].
Reactor modelling
The method of simulation chosen for the reactor leads to a deviation from how the process
would operate in reality. It was decided to model the reactor based on a plug flow reactor,
with the assumption that an ideal plug flow is achieved and that there are no changes with
radial position.
As the type of reactor for this process is described as a catalytic fixed bed reactor, the flow
through the reactor can be approximated to a plug flow. While this is an reasonable
assumption for the modelling of a catalytic fixed bed reactor with gaseous reactions,
inevitably, this results in a deviation from reality as the real flow will not be a perfect plug
flow, therefore resulting in changes with the radial position, for example conversion of
propylene in the reactor. (Levenspiel, 1999)
Ideal plug flow however, considers that there are no changes at different radial positions,
and so this difference between real flow and ideal plug flow will result in deviations for
conversions of reactants achieved.
The simulation results obtained for the reactor also only consider the reactions involving
propylene as a reactant, with the additional reactions for the formation of carbon dioxide,
water and acrylic acid from acrolein not being simulated in the reactor. The effect of these
additional reactions being neglected results in the mass flow of acrolein exiting the reactor to
be calculated higher than what the mass flow would be in reality, as it has not been
considered that some of the acrolein formed reacts with oxygen to produce more products.
This also results in less of the oxygen in the reactor being used compared with in reality.
Another difference from the simulation compared to the reality of the process is that the
simulation only considers the energetics of individual reactions, neglecting the presence of a
catalyst, however, in the brief the reactor specified is a catalytic fixed bed reactor. The
reaction kinetics used do not consider the presence of a catalyst in this process, and in
reality a catalyst is used to increase the selectivity and rate of reaction of the oxidation
process. In industry, a range of bismuth molybdate catalysts, typically mixed with additional
metal oxides are used in order to increase the activity of the reaction. The use of these
multicomponent catalysts typically achieves a conversion of 90-95%, compared to the
results of the simulation which show a conversion of 77.2% of propylene. This can be
attributed to the lowering of activation energy by providing an alternate reaction pathway.
The selectivity with respect to acrolein in the process is improved firstly by the ability of
lattice oxygen to readily transfer at the reaction temperature between the phases comprising
the catalyst and to the propylene reacting, and secondly that the point of activation on the
propylene is one of the C-H bonds on the methyl group, allowing the production of a surface
allyl intermediate. By this action, the catalyst allows propylene to be readily and more easily
oxidised to acrolein. (Santen, 2005)
The simulation also differs from reality in that the recycled propylene distillate stream has not
been investigated, and so this has not been shown in the simulation. In reality, this distillate
stream containing propylene would be recycled back to the reactor, and so a greater
production of acrolein should be achieved in reality.
Improving the fidelity of the work
As there is a discrepancy between the achieved conversions using the kinetic reaction
modelling compared with the expected conversion for catalytic oxidation of propylene, the
first improvement to the fidelity of the work would be to model the reactions to include the
presence of the bismuth molybdate catalysts used in the process. Instead of inputting the
reactions as kinetics, the reactions can be inputted as heterogeneous catalytic reactions,
using activation energies that include the effect of the catalyst. The results from this can then
be compared to the expected conversions of propylene given by literature when using this
particular catalyst, and can be compared in order to verify the validity of the results obtained.
Reliability testing can also be conducted by repeating the simulation under different
conditions. This will allow irregularities or anomalies to be identified in the process, and they
can be removed when identified. By repeating the simulation, its reliability and robustness
can also be demonstrated, and by elimination of any irregularities, the simulation will
become more repeatable and closer to reality in terms of its accuracy.
To further examine if the simulation is reliable, the inputted data can be kept the same with
the exception of one parameter or dimension, for example the overall effect of changing the
length of the reactor on the results of the simulation. By conducting this method of testing,
the understanding of both the simulation and the effect of different parameters on the results
obtained can be improved, and through this the accuracy of the simulation can be improved
by identifying sources of uncertainty.
The last thing that could be done to improve the realism of the work is to investigate the
effect of recycling the distillate stream containing a high concentration of propylene back to
the reactor to investigate the effect this would have on the final mass flow of acrolein that the
process would achieve. This will improve the realism of the work as in reality, the unreacted
propylene in the distillate would be recycled to the reactor in order to better utilise the
reactant, producing a greater amount of acrolein.
References
Levenspiel, O. (1999). Chemical reaction engineering. 3rd ed. Hoboken, NJ: Wiley, p.477.
Santen, R. (2005). Catalysis. 2nd ed. Amsterdam: Elsevier, pp.249-250.