When fluid moves through a large pipe or channel with sufficiently large Renolds number, it approximates plug flow, which means there is no variation of axial velocity over the cross section. It is assumed that the fluid movement through the Plug Flow Reactor can be described as plug flow, that is to say, there is no concentration or temperature gradients in the radial coordinate.
 
 

In contrast to a CSTR, sterile feed to a PFR automatically implies zero biomass concentration in the effluent: plug flow prevents a slice of fluid moving through the vessel from ever being inoculated. One way to circumvent this problem is by recycle, so that the incoming stream is inoculated before entering the vessel.

For a single reaction with ordinary kinetics, the PFR provides greater substrate conversion and higher product concentration than the CSTR of equal volume. The opposite is true if the kinetics are autocatalytic (higher rates with decreasing substrate concentration). For microbial processes, the PFR typically maximizes effluent product concentration. However, the requirement of continuous inoculation and practical difficulties with gas exchange for PFRs often result in use of their analog, the batch reactor, when high final-product concentration is important. For exponential microbial growth, the CSTR is more efficient than a PFR.

Tisseyre et. al.(Ref. 1) describes the design and use of a new type of Continuous Plug Flow Reactor (CPFR). The reactor comprises a continuous glass tube (diameter of 19 mm) arranged into a spiral (0.4 m external diameter) consisting of five loops, which rotate around an axle. The CPFR has advantages over other reactors in that continuous fermentation is possible without dilution of the culture and biomass or metabolite yield is increased. In addition temperature and water activity may be adjusted to a different optimum level for each physiological stage of the cultivated microorganism.
 

Reference

1. B. Tisseyre, J. C. Coquille, P. Gervais, Conception and characterization of a continuous plug flow bioreactor, Bioprocess Engineering, 13(3), 113-118, 1995
 
 

Last modify: 05/01/2000, by Xuezhen Kang