Residence Time Distribution (RTD) in Stirred Tank Reactor: OpenFOAM Simulation

You see an OpenFOAM simulation of a single phase stirred tank reactor using AMI technique. This simulation is performed to find the residence time distribution (RTD) in a stirred tank reactor.

In the design and scale-up of chemical reactors, the residence time distribution of fluid particles inside the reactor should be known. This information can help us to better design a reacting system (find the operating problem of a reactor) or predict the performance of a reactor under a given operating conditions.

The Residence Time Distribution (RTD)is obtained by injecting an inert substance or molecule, as tracer, into the reactor (reactor inlets) at t = 0 and measuring the tracer concentration in the outlet stream of the reactor. The tracer should be nonreactive. Its physical properties should be similar to the fluid and it should not affect the flow behavior of the fluid in the reactor. It should not be adsorbed on the reactor walls so that it completely reflects the flow behavior of the reactor.

In experiments, colored or radioactive materials are commonly used as tracer. The injection method can be a pulse input or a step input. The same procedure can be followed in flow simulations based on CFD. A number of tracer particles are injected from the reactor inlet and the inventory content of tracer particles or the exit concentration of tracer particles is tracked over time to find Residence Time Distribution (RTD).

In the simulation below, tracers are small , artificial particles with density 1000 kg/m3 (similar to fluid density). These particles with small time constant follow the flow path and since program performs one-way coupling, tracer presence does not affect the flow behavior. Since the perfect pulse injection is not possible, tracer particles are injected for 1 second.

OpenFOAM solvers do not implicitly provide such information to us. However, there are some useful tools for particle/tracer tracking in flow system that can be used in simulations. Here is a tutorial on how to use these tools.

In the figure below, the results of the above CFD simulation were post-processed to obtain E(t) function (RTD function) in the reactor. The results are also compared with the ideal CSTR model.

RTD in an stirred tank reactor
The RTD of tracers in the reactor obtained from CFD simulation

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