Mixing

Mixing is another important classifying feature for continuous reactors. Good mixing improves the efficiency of heat and mass transfer.

In terms of trajectory through the reactor, the ideal flow condition for a continuous reactor is plug flow (since this delivers uniform residence time within the reactor). There is however a measure of conflict between good mixing and plug flow since mixing generates axial as well as radial movement of the fluid. In tube type reactors (with or without static mixing), adequate mixing can be achieved without seriously compromising plug flow. For this reason, these types of reactor are sometimes referred to as plug flow reactors.

Continuous reactors can be classified in terms of the mixing mechanism as follows:

Mixing by diffusion

Diffusion mixing relies on concentration or temperature gradients within the product. This approach is common with micro reactors where the channel thicknesses are very small and heat can be transmitted to and from the heat transfer surface by conduction. In larger channels and for some types of reaction mixture (especially immiscible fluids), mixing by diffusion is not practical.

A simple tube can be used as a reactor. Small bore systems usually rely on mixing by diffusion

Mixing with the product transfer pump

In a continuous reactor, the product is continuously pumped through the reactor. This pump can also be used to promote mixing. If the fluid velocity is sufficiently high, turbulent flow conditions exist (which promotes mixing). The disadvantage with this approach is that it leads to long reactors with high pressure drops and high minimum flow rates. This is particularly true where the reaction is slow or the product has high viscosity. This problem can be reduced with the use of static mixers. Static mixers are baffles in the flow channel which are used to promote mixing. They are able to work with or without turbulent conditions. Static mixers can be effective but still require relatively long flow channels and generate relatively high pressure drops. The oscillatory baffled reactor is specialised form of static mixer where the direction of process flow is cycled. This permits static mixing with low net flow through the reactor. This has the benefit of allowing the reactor to be kept comparatively short.

The static mixer permits mixing with or without turbulent conditions

The oscillatory baffled reactor uses a combination of static mixing and cycling of flow direction.

Mixing with a mechanical agitator

Some continuous reactors use mechanical agitation for mixing (rather than the product transfer pump). Whilst this adds complexity to the reactor design, it offers significant advantages in terms of versatility and performance. With independant agitation, efficient mixing can be maintained irrespective of product throughput or viscosity. It also eliminates the need for long flow channels and high pressure drops.

One less desirable feature associated with mechanical agitators is the strong axial mixing they generate. This problem can be managed by breaking up the reactor into a series of mixed stages separated by small plug flow channels.

The most familiar form of continuous reactor of this type is the continuously stirred tank reactor (CSTR). This is essentially a batch reactor used in a continuous flow. The disadvantage with a single stage CSTR is that it can be relatively wasteful on product during start up and shutdown. The reactants are also added to a mixture which is rich in product. For some types of process, this can have an impact on quality and yield. These problems are managed by using multi stage CSTRs. At the large scale, conventional batch reactors can be used for the CSTR stages. At the small scale, this is less practical and Agitated Cell Reactors provide a simpler alternative.

The Agitated Cell Reactor is a multi stage CSTR