Fluidised bed techniques

Fluidised-bed techniques, pioneered with low-density polyethylene, have been applied to PVC powders. These powders can be produced by grinding of conventional granules, either at ambient or sub-zero temperatures or by the use of dry blends (plasticised powders). The fluidised bed process is somewhat competitive with some well-established paste techniques, and has the advantage of a considerable flexibility in compound design. 

The utilisation of the fluidised bed technique has become more common in combustion because it allows the combustion of low heat capacity fuels. The environmental benefits associated with biomass fuel (e.g. forest residue, bark, sawdust and wood chip) and waste material (e.g, RDF, industrial based waste) has increased their use in energy production during the last decade. These fuels are often problematic and cause problems like fouling and agglomeration during combustion. The tendency towards agglomeration also depends on bed material, additives, and on the boiler conditions. " ... 

Breaking points are the availability of the raw materials and primary containers of reliable suppliers, the feasibility of analysis of the drug substance and the preparation and the availability of equipment. As an example, preparation processes such as tableting, freeze-drying or aseptic production are accessible in a few pharmacies. The preparation of oral solids with controlled release is not possible in pharmacies mainly to lacking equipment (fluidised-bed techniques and instrumental analysis, etc). Working with radiopharmaceuticals also requires very specific facilities, as is the case with preparation of solid dosage forms with hazardous substances. 

Fig. 2. An automatic, in-line powder coating system, the Autocoat, based on the fluidised bed technique. It is capable of repetitively coating 15,000 articles per 8 hr. shift in polyethylene, PVC or nylon powder. (Courtesy of Plastic Coatings Ltd.)...

In the case of the electrostatic fluidised bed, a combination of the electrostatic spray technique and the fluidised bed technique is used. The apparatus consists of an array of electrostatic elements on the bottom of a shallow bed. A small quantity of powder is placed on the elements and fluidised by the normal method. The elements impart a negative charge to the powder with the result that a cloud of charged powder is obtained. An earthed object held in the cloud collects a layer of powder by electrostatic rather than fluidised bed principles. Unfortunately, the density of the cloud varies considerably from top to bottom and is sensibly uniform over only 4 in. This is offset somewhat by the fact that an electrostatic fluidised bed requires only one-tenth of the raw material compared with an equal sized conventional fluidised bed and this helps considerably as far as stock holding of colours is concerned. 

It was claimed that the maximum continuous operating temperature in most chemical environments was 120°C and even 140-150°C in some instances. The major chemical applications were in the form of pipe and tank linings and injection moulded valve and pump parts. Coatings could be applied to metals by means of fluidised bed, water suspension and organic dispersion techniques. 

Thermoplastic resins, such as vinyl chlorides, vinyl acetates and polyamides are employed, particularly in the water industry, on buried pipes and fittings. To provide both internal and external coating, application may be by one of these principle techniques dipping in a plastisol, fluidised beds or electrostatic spray. 

The fluidised bed coating technique is used extensively for wirework items such as dish drainer racks, vegetable racks, office trays etc. The technique is also widely used for street furniture e.g. metal lampposts, signposts and balustrading, and for metal office furniture and domestic garden furniture. It also provides chemical corrosion resistance on valves, pipes, couplings etc. 

The efficacy of polymers when used to protect metals from corrosive environments is influenced by their efficiency as barrier materials. When applied to metals by some techniques, such as fluidised bed coating, there is always the danger of macro-diffusion through pinholes which are gross imperfections in the surface and which do not have to be visible to be very much greater than the dimension of penetrating molecules. 

There is much current interest aimed at the implementation of processes that integrate the upstream and downstream operation for protein recovery.131419 Although adsorption in fluidised beds provides a considerable saving in cost and time over conventional purification techniques, it still deploys a discrete operation with which the desired protein is captured at termination of fermentation or once a cell suspension has been disrupted. The main... 

Although the application of fluidisation techniques to sublimation-desublimation processes was first proposed by Matz" 11, the technique has not yet been widely adopted for large-scale commercial use, despite its obvious advantage of improving both heat and mass transfer rates. G aiko 112 1 has, however, reported on a fluidised-bed de-sublimation unit operating in the United States for the production of aluminum chloride at the rate of 3 kg/s (11 tonne/h). 

As costs of precombustion hydrodesulphurisation and post combustion flue gas clean-up have escalated and as environmental regulations have further limited the sulphur dioxide emission rates, there has been a growing interest in technology designed to effect fuel desulphurisation during the combustion process. Desulphurisation during fluidised bed combustion of coal has been a leading technique in these developments. 

This is included as an example of the techniques used in pilot scale studies of industrial crystallisers. A schematic of a fluidised bed system is shown in Fig.7. The apparatus could be operated in free-drift or constant composition conditions as described above. 

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