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Laboratory experiment: Residence Time Distribution (Cascade)

Behaviour of Non-Ideal Reactors

In non-ideal reactors, different flow effects can occur. These effects can influence the ideal behaviour strongly or weakly. They can affect the reactor design, the yield or temperature development of a chemical process.

Dead Zones and Short Circuits

So called dead zones or stagnant region can appear in technical reactors. Areas with a low or no mixing are meant by this. This reduces the available effective reaction volume. A further effect in non-ideal reactors is a short circuit current. An unfavourable location for input and output causes a portion of the reactants to be transported directly to the output before that portion has been able to be part of the reaction.


The figure shows the typical "Dead Zone" (1), "Short Circuit" (2) and ideal mixing (3) and ideal plug flow (4) behaviour in corresponding reactors. A combination of dead zone and short circuit effects can occur both in continuously stirred tank reactors and flow reactors.

Flow Profiles and Flow Types


A further criteria of non-ideal flow reactors is a flow profile that deviates from the ideal plug flow (1). Radial concentration and temperature differences occur, depending on the particular flow conditions (e.g. (3) laminar or (2) turbulent).


Further non-ideal conditions can occur depending on the state of the mixing of the reaction fluids. A complete mixing of the reactants only occurs with low-viscosity fluids and gases - resulting in a micro fluid. In an incomplete mixture, local concentration differences will exist. This effect is called segregation. Segregation effects always occur in heterogeneous systems (e.g. suspensions and emulsions). Segregated elements in a reactor may cause different residence time distribution differing from ideal behaviour and can influence the performance of the reacting system. This special type of non-ideal conditions can also be determined by the experimental determination of the residence time distribution.

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