Why Density Change Cannot Be Ignored in a Plug Flow Reactor (PFR)

This Demonstration shows the effect of ignoring density change due to mole change in a gas-phase reaction when calculating conversion in an isothermal plug flow reactor (PFR). A second-order, gas-phase reaction () takes place in an isothermal, isobaric PFR. Because of the mole change in the reaction, the volumetric flow rate increases down the reactor. This decreases the residence time compared to a reaction with the same kinetics but no change in the number of moles, and thus conversion decreases. This decrease in conversion can instead be viewed as a decrease in the reactant concentration due to the increased number of moles.
This Demonstration can be used to compare reactor models that account for density change by calculating the changing volumetric flow rate (the blue line, which is the correct solution) and models that assume constant volumetric flow rate (the dashed line). Two types of comparisons can be made: (1) at the same reactor volumes and observing the difference in conversion, or (2) at the same conversion and observing the difference in reactor volumes. This comparison is done by adjusting the reactor volumes.


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Material balances:
= molar flow of A
= molar flow of B
= molar flow of C
= volume
= rate constant dependent on temperature
= pre-exponential factor
= activation energy
= ideal gas constant
= isothermal temperature
= concentration of A
= inlet molar flow rate of A
= inlet concentration of A (pure A fed)
Density change ignored:
= constant volumetric flow rate
Density change taken into account:
= volumetric flow rate, changes with distance down the reactor
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