Plug Flow Reactors

What is Plug Flow?

Flow reactors are sometimes referred to as plug flow reactors or PFRs. Plug flow is an important characteristic of flow reactors, whereby any two molecules entering the reactor at time zero, exit at a similar time. This provides an effective means of controlling the reaction time while optimising the separation of reactants and products. Good plug flow is essential for good performance in all but a few applications and means that fluid travels through the reactor in a time-orderly way and without back-mixing.

Advantages of Plug Flow

Efficient reaction time control: as products are removed from the reactor on formation
Efficient utilisation of reactor volume: since reacted material is not retained within the reactor
Optimum reaction yield and purity: due to efficient heat and mass transfer properties and removal of products on formation, reducing by-products
Low start-up and shutdown losses: due to the smaller volumes involved

Changing Concentration in Plug Flow

Fluid going through a Plug Flow Reactor may be modelled as flowing through the reactor as a series of infinitely thin coherent 'plugs', each with a uniform composition, travelling in the axial direction of the reactor, with each plug having a different composition from the ones before and after it.

The key assumption is that; as a plug flows through a plug flow reactor, the fluid is perfectly mixed in the radial direction but not in the axial direction (forwards or backward). Each plug of differential volume is considered as a separate entity, effectively an infinitesimally small continuous stirred tank reactor, limiting to zero volume.