Packed bed reactors

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Packed beds can either be run in the submerged mode (with or without aeration) or in the trickle flow mode as shown in Figure 7.

Figure 7. Schematic presentation of packed bed reactors (a) packed bed; (b) packed bed with external aeration column (adopted by ref. 5).

A variety of packings are available for this purpose, both polymer, ceramic, glass and natural (wood or bark) in dumped and structured forms, porous or non-porous in a variety of shapes and sizes. Recently, monoliths have become available similar to those in catalytic exhausts from cars. These give very high surfaces for cell immobilization and thin films with minimal mass transfer limitations can be formed. The flow velocities in the channels can be high to eliminate external mass transfer limitation in the adjacent liquid film as well. Simultaneously, plugging can be avoided, albeit at the cost of high pressure drop. Fixed beds in anaerobic operations (e.g. digestion with methane production) can be run in recycle mode to avoid gas building up in the packing. With low gas production rates, recycling is not needed and a gradient of concentration may be established. In contrast to fluidised beds, no hydrodynamic requirements have to be fulfilled, except for the even distribution of the flow over the entire cross-section. It should be stressed that as with gas absorbers or other packed bed operations any maldistribution of the liquid flow will result in poor performance (e.g. channelling). Plugging of the beds may occur as rapidly as every 1 or 2 days and this makes periodic cleaning necessary. Injecting air and using a higher liquid flow rate for a short period will normally dislodge excess biomass. This sudden burst of biomass in the effluent has to be separated (e.g. by sedimentation) and collected for further treatment. Biosorption technology has been implemented traditionally in packed bed columns similar to the ion exchange systems. However, biosorption as a metal ion removal mechanism can be implemented in any appropriate reactor configuration.

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