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