Comparing CSTR (fReactors) and pipe Reactors
Lets now compare the RTD of a cascade of CSTRs with that of a laminar flow pipe reactor. Within the pipe, fluid in the centre, where it is moving fastest, spends least time in the reactor, whilst fluid near the edge spends a much longer time. Here the model has been set up to allow you to vary the number of modules (each of 2ml) in the cascade of flow reactors; as you change these the length of the pipe is changed to match the total volume.
Set the number of fReactors in your cascade along with the mean residence time.
As before, the graphs show the distribution of residence times of fluid elements within the reactors. The behaviour of the laminar tubular reactor is very different to that of a stirred tank cascade (and the fReactor-Classic). The shortest time an element of fluid can spend is in the fastest moving part of the flow in the centre of the pipe - so there is a minimum residence time. But you will also see a very broad distribution of residence times including a long tail which represents the very slow fluid near the wall.
However, the residence time distribution isn't the end of the story. A reactor can have a narrow residence time distribution but still have poor mixing; neighbouring packets of fluid could spend the same time travelling through the reactor, but not necessarily mix with each other. Mixing in a laminar flow tubular reactor relies entirely on diffusion, which is relatively slow. For multiphasic flows within a tube (i) for a gas-liquid system there is a plug of liquid followed by a plug of gas; (ii) with a solid-liquid system, particles can drop to the bottom of the pipe.
There are some enhancements to the mixing in laminar tubular pipes that include coiling the pipe which can promote secondary mixing, or adding internals into the pipe such as static mixers that can improve the overall characteristics of the reactor.
Active mixing with CSTRs, where energy is continually added to the fluid through a stirrer, can better support good mixing and for multiphasic flows good contact between the phases, since energy is being added to the system to create this interaction.
Notes: The laminar flow tubular reactor model assumes the fluid is Newtonian, which leads to a parabolic flow profile. It also assumes laminar flow (you can check out conditions for this here). The effect of diffusion is neglected in this analysis - it is a purely convective model.The concept of residence time distribution is well covered in Chemical Reaction Engineering by Levenspiel and from which the models for RTD for the pipe flow reactor is taken.