Enzyme Technology
Fluidised bed reactors
These reactors
generally behave in a manner intermediate between CSTRs and PBRs. They consist
of a bed of immobilised enzyme which is fluidised by the rapid upwards flow of
the substrate stream alone or in combination with a gas or secondary liquid
stream, either of which may be inert or contain material relevant to the
reaction. A gas stream is usually preferred as it does not dilute the product
stream. There is a minimum fluidisation velocity needed to achieve bed
expansion, which depends upon the size, shape, porosity and density of the
particles and the density and viscosity of the liquid. This minimum fluidisation
velocity is generally fairly low (about 0.2 -I.0 cm s−1) as most immobilised-enzyme particles have densities close to that of the bulk liquid. In this case
the relative bed expansion is proportional to the superficial gas velocity and
inversely proportional to the square root of the reactor diameter. Fluidising
the bed requires a large power input but, once fluidised, there is little
further energetic input needed to increase the flow rate of the substrate stream
through the reactor (Figure 5.3). At high flow rates and low reactor diameters
almost ideal plug -flow characteristics may be achieved. However, the kinetic
performance of the FBR normally lies between that of the PBR and the CSTR, as
the small fluid linear velocities allowed by most biocatalytic particles causes
a degree of back-mixing that is often substantial, although never total. The
actual design of the FBR will determine whether it behaves in a manner that is
closer to that of a PBR or CSTR (see Figures 5.5
-5.9). It can, for example, be
made to behave in a manner very similar to that of a PBR, if it is baffled in
such a way that substantial backmixing is avoided. FBRs are chosen when these
intermediate characteristics are required, e.g., where a high conversion is
needed but the substrate stream is colloidal or the reaction produces a
substantial pH change or heat output. They are particularly useful if the
reaction involves the utilisation or release of gaseous material.
The FBR is normally used with fairly small
immobilised enzyme particles (20-40 mm diameter) in order to achieve a high
catalytic surface area. These particles must be sufficiently dense, relative to
the substrate stream, that they are not swept out of the reactor. Less-dense
particles must be somewhat larger. For efficient operation the particles should
be of nearly uniform size otherwise a non-uniform biocatalytic concentration
gradient will be formed up the reactor. FBRs are usually tapered outwards at the
exit to allow for a wide range of flow rates. Very high flow rates are avoided
as they cause channelling and catalyst loss. The major disadvantage of
development of FBR process is the difficulty in scaling-up these reactors. PBRs
allow scale-up factors of greater than 50000 but, because of the markedly
different fluidisation characteristics of different sized reactors, FBRs can
only be scaled-up by a factor of 10 -100 each time. In addition, changes in the
flow rate of the substrate stream causes complex changes in the flow pattern
within these reactors that may have consequent unexpected effects upon the
conversion rate.
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This page was established in 2004 and last updated by Martin
Chaplin on
6 August, 2014
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