Fluidisation

of density 900 kg/m3 and viscosity 3 mN s/m2, is passed vertically upwards through a bed of catalyst consisting of approximately spherical particles of diameter 0.1 mm and density 2600 kg/m3. At approximately what mass rate of flow per unit area of bed will (a) fluidisation, and (b) transport of particles occur  

Calculate the minimum velocity at which spherical particles of density 1600 kg/m3 and of diameter 1.5 mm will be fluidised by water in a tube of diameter 10 mm on the assumption that the Carman-Kozeny equation is applicable. Discuss the uncertainties in this calculation. Repeat the calculation using the Ergun equation and explain the differences in the results obtained. 

As a wall effect applies in this problem, use is made of equation 4.23 to determine the correction factor, f where  

As noted in Section 6.1.3 of Volume 2, the Carman-Kozeny equation applies only to conditions of laminar flow and hence to low values of the Reynolds number for flow in the bed. In practice, this restricts its application to fine particles. Approaches based on both the Carman-Kozeny and the Ergun equations are very sensitive to the value of the voidage and it seems likely that both equations overpredict the pressure drop for fluidised systems. 

In a fluidised bed, Ao-octane vapour is adsorbed from an air stream onto the surface of alumina microspheres. The mole fraction of Ao-octane in the inlet gas is 1.442 x 10 2 and the mole fraction in the outlet gas is found to vary with time as follows  

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J. F. Davidson and D. Harrison, Fluidised Particles, Cambridge University Press, Cambridge, U.K., 1963. 

Formation of emissions from fluidised-bed combustion is considerably different from that associated with grate-fired systems. Flyash generation is a design parameter, and typically >90% of all soHds are removed from the system as flyash. SO2 and HCl are controlled by reactions with calcium in the bed, where the lime-stone fed to the bed first calcines to CaO and CO2, and then the lime reacts with sulfur dioxide and oxygen, or with hydrogen chloride, to form calcium sulfate and calcium chloride, respectively. SO2 and HCl capture rates of 70—90% are readily achieved with fluidi2ed beds. The limestone in the bed plus the very low combustion temperatures inhibit conversion of fuel N to NO. ... 

The technologies suitable for LLDPE manufacture include gas-phase fluidised-bed polymerisation, polymerisation in solution, polymerisation in a polymer melt under high ethylene pressure, and slurry polymerisation. Most catalysts are fine-tuned for each particular process. 

The hot ammonia used as carrier gas serves simultaneously as fluidising agent and inhibits deammonisation. 

Various methods are used for evaluatiag the quaflty, ie, physical strength and ensyme dust formation, of the granulate. In the elutriation process, a sample of product is fluidised ia a glass tube with a perforated bottom plate for 40 miautes. Dust from the sample is collected oa a filter and the ensyme activity measured. An acceptable dust level is when less than 5—10 ppm of the activity of the sample has been collected. In the so-called Heubach method, 20 g of granulate is elutriated. During the elutriation, four steel balls are rotated ia the bed ia order to evaluate the impact of attritioa oa the dust release of the ensyme. The dust is collected oa a filter and measured. The acceptable dust level is very low. 

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. 

Nylon 11 is also used in powder form in spraying and fluidised bed dipping to produce chemical-resistant coatings. Although more expensive than the polyolefin and PVC powders, it is of interest because of its hardness, abrasion resistance and petrol resistance. 

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. 

Provided due care is taken with respect to predrying and to crazing tendencies, polycarbonates may also be thermoformed, used for fluidised bed coating and machined and cemented. Like metals, but unlike most thermoplastics, polycarbonates may be cold formed by punching and cold rolling. Cold rolling can in fact improve the impact resistance of the resin. 

Bujac, P.B. and Mullin, J.W., 1969. A rapid method for the measurement of crystal growth rates in a fluidised bed crystallizer. Symposium on Industrial Crystallization. London, 1969. Rugby Institution of Chemical Engineers, pp. 121-129. 

Di Felice, R., 1995. Hydrodynamics of liquid fluidisation. Chemical Engineering Science, 50, 1213-1245. 

Foscolo, P.U., Gibilaro, L.G. and Waldram, S.P., 1983. A unified model for particulate expansion of fluidised beds and flow in porous media. Chemical Engineering Science, 38, 1251-1260. 

Gibilaro, L.G., 2001. Fluidisation Dynamics. Oxford Butterworth-Heinemann. 

Jones, A.G., Wachi, S. and Delannoy, C.C., 1992b. Precipitation of calcium carbonate in a fluidised bed reactor. In Fluidization VII. Eds. O.E. Potter and D.J. Nicklin, New York Engineering Foundation, pp. 407M14. 

Khan, A.R and Richardson, J.F., 1990. Pressure gradient and friction factor for sedimentation and fluidisation of uniform spheres in liquids. Chemical Engineering Science, 45, 255-265. 

Mulliii, J.W. and Garside, J., 1967. Crystallization of aluminium potassium sulphate a study in the assessment of crystallizer design data I Single crystal growth rates, II Growth in a fluidised bed. Transactions of the Institution of Chemical Engineers, 45, 285-295. 

Patwardhan, V.S. and Chi Tien, 1985. Sedimentation and liquid fluidisation of solid particles of different sizes and densities. Chemical Engineering Science, 40, 1051-1060. 

Combustion of coal may take place in conventional fixed beds using lump coal and in which temperatures up to 1 300°C may be reached by entrained flow in which pulverised coal is injected into the combustion zone with the air, reaching temperatures up to 1 500°C or in the more recently developed systems of fluidised-bed combustion, again using pulverised coal but with... 

Fluidised-bed systems produce higher combustion intensities at lower temperatures than combustion of pulverised fuel in conventional fossil-fuel-fired boilers. The mineral matter for corrosion does not form fused salts and is not expected to release corrosive species. Fluidised bed combustors can, therefore, burn lower grade, cheaper fuel in smaller plant with better pollution control than traditional boilers... 

Minchener el al. report that the bubble phase of atmospheric fluidised bed combustion has a pOi in the range 2 x 10 to 2 x 10 Combustion in the dense phase is sub-stoichiometric, with the /Oj as low as 10 and SO2 and SO3 present in the range 500-5 000ppm. Low Cr-Mo steels show heavy scaling in these conditions, whereas 9-12% Cr steels show good resistance to sulphidation up to 650°C. Roberts et however, report that for pressurised fluidised-bed combustion, ferritic steels at or below 9% Cr show heavy general corrosion above 540-560°C. 

In Fig. 7.68 the oxidising and sulphiding potentials of four different atmospheric environments, i.e. conventional coal combustion (A), fluidised bed combustion (B), conventional coal gasification (C) and coal gasification using nuclear heat (D), are shown on the thermochemical phase stability... 

Fluidised bed This process is used for powder coating Basically, the equipment consists of a dip tank with a perforated shelf near the bottom. The powder is placed on this shelf and low pressure air is fed under the perforated shelf, resulting in a cloud of fine powder in the body of the dip tank. The article is heated to a little above the melting point of the powder and is then dipped into the fluidised bed for a short period. It is then withdrawn... 

Fluidised bed Most suitable for small articles Produces thick films. Varying metal gauge could produce uneven films and weak spots... 

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. 

Fluidised bed The fluidised bed consists of two boxes on top of one another. The top and larger one contains the powder, and the lower one is separated from it by metal mesh and a semipermeable membrane. Air is pumped under pressure into the lower compartment and then diffuses through the membrane and through the powder. The powder particles are lifted and separated by the air. This results in a considerable reduction in the bulk density so that the item to be coated can easily be submerged in the powder. 

The pretreated metalwork to be coated is heated in an oven to a temperature of between 260 and 360°C, depending on the metal thicknesses and the coating to be applied. It is then withdrawn from the oven and dipped into the fluidised powder. Here the fine powder particles are blown onto the hot metal where they melt. After a few seconds (5-lOs is normal), the item is removed from the powder and the unfused outer particles are allowed to fuse. Then either the item is allowed to air cool or it is water quenched. The cooling method can affect crystal structure and hence surface finish and... 

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. 

Plastics used for fluidised bed powder coatings include polyethylene, p.v.c., nylon, p.v.f.2, p.e.c.t.f.e. and a variety of polyolefins and their copolymers. 

Polyolefin copolymers Although there is a wide variety of these available, the only one currently commercially available as a compounded powder is saponified EVA. This is reported to have good weatherability and will not suffer from ESC. One major advantage this coating has is that it can be applied by the fluidised bed process at low temperatures and this offers the possibility of coating temperature-sensitive metals such as galvanised steel. 

Nylon 11 is applied using a fluidised bed process to a wide variety of substrates including metal chair frames, door furniture and wire dishwasher baskets. It can also be applied by electrostatic spraying, but generally only where the application is decorative and where the metal work is thin, i.e. less than 0.2 mm. 

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. 

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