Fluidised bed systems

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... 

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. 

A more detailed overview of the development of continuous fluidised bed systems is covered in a review by Teunou and Poncelet (2002). Figure 16.7 shows a schematic diagram of a continuous top-spray fluid bed coater. 

Entrainment may be defined as the carryover of ejected particles, while selective entrainment of finer or less dense particles is often referred to as elutriation. In most industrial processes, neither entrainment nor elutriation are desirable, which is in sharp contrast to this particular application. Consequently, there is very little research aimed specifically at enhancing the selective removal of less dense material from fluidised beds. Most research on entrainment is based on dimensional analysis applied to experimental data either with no or very limited consideration of the underlying physics Predictions made from these correlations are limited to very simple geometries. They may vary widely even for reactor airangements close to the experimental conditions they are based on, and are often completely unreliable when conditions are markedly different. In several intemal studies they have been found inadequate for entrainment and elutriation predictions in the fluidised bed system under investigation. The problem is too complex to be adequately represented by a small number of ordinary equations that would simply require substitution of a few parameters to obtain the rales of entrainment of the different particle size ftactions. 

Sands bonded with cold-setting resins may be regenerated using simple treatment techniques, due to the fiagility of the binder layer. Mechanical regeneration systems (e.g. fluidised bed systems) are based on interparticle friction or impact. 

The fixed bed approach suffers from complexity over the fluidised bed system as valve switching is required to change process streams from one vessel to another. It can also be higher capital because multiple vessels are required and each additional one adds to the total cost in a linear fashion. There is also a time dependency to the compositions and flows from fixed bed processes due to the semi-batch nature of the process and this may require the addition of mixing tanks to be added to reduce the fluctuations for downstream equipment. 

In a study of the kinetics of the thermal decomposition of gibbsite and boehmite, Stacey [8] also employed constant reaction rate conditions but using a fluidised bed system operating... 

Fouling phenomena diminish as concentration polarisation decreases. Concentration polarisation can be reduced by increasing the mass transfer coefficient (high flow-velocities) and using low(er) flux membranes. Also the use of various kinds of turbulence promoters w ill reduce fouling, although fluidised bed systems and rotary module systems seem not veiy feasible from an economical point of view for large scale applications but they may attractive for small scale applications. 

In gas phase processes, gaseous propylene comes into contact with the solid catalyst which is intimately dispersed in dry pol5aner powder. Industry uses two different methods of carrying out this reaction depending on the chosen method of heat removal. The Unipol PP process uses a modification of the Unipol polyethylene fluidised bed system. The Novolen PP process and Innovene PP process use mechanically agitated dry powder beds with evaporative cooling in vertical and horizontal reactors, respectively. Unipol PP was originally developed by Union Carbide and Shell, the Novolen PP process by BASF and the Innovene PP process by Amoco. 

If the viscosity of the liquid is not excessive, lime and gypsum coatings can be avoided with a fluidised-bed system. For this purpose, the forced-circulation -type evaporator with vertically arranged heating grid is used (Figure 11.9). 

Figure 5. A figure illustrating the impact of exit effects in circulating fluidised bed systems. The lines show density profiles for identical conditions of gas velocity (7.1 m/s) and circulation rate (73 kg/mh) for an abrupt exit (circles) and a smooth exit (triangles). The solids are returned 1.98 m above the gas distributor causing the "nose" in the profile, (Brereton, 1987).

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. ... 

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... 

Fig. 7.68 Thermochemical stability diagram for the system Fe-S-O at I 000 K showing the relative corrosion potentials of the atmospheres in conventional coal combustion (A), fluidised bed combustion (B), conventional coal gasification (C) and coal gasificiation using nuclear heat (D) (after Gray and Starr )...

Equation (17.7.2.9) was originally used to correlate the minimum fluidisation velocity for gas-solid fluidisation beds but has been successfully employed by Lan and his co-workers42 for adsorbents in the field of direct recovery using liquid-solid systems (Figure 17.4). 

Interest extends from transfer to single particles to systems in which the particles are in the form of fixed or fluidised beds. The only case for which there is a rigorous analytical solution is that for heat by conduction and mass transfer by diffusion to a sphere. 

The movement of individual particles in a liquid-solid fluidised bed has been measured by Handley et a/.(40) Carlos(41,42), and Latif(43). In all cases, the method involved fluidising transparent particles in a liquid of the same refractive index so that the whole system became transparent. The movement of coloured tracer particles, whose other physical properties were identical to those of the bed particles, could then be followed photographically. 

In general, the behaviour of gas-fluidised systems is considerably more complex than that of liquid-fluidised systems which exhibit a gradual transition from fixed bed to fluidised bed followed by particle transport, without a series of transition regions, and with bed expansion and pressure drop conforming reasonably closely to values calculated for ideal... 

Part of the complication with gas-solid systems arises from the fact that the purely hydrodynamic forces acting on the particles are relatively small compared with frictional forces between particles, electrostatic forces and surface forces which play a much more dominant role when the particles are very fine. As the gas velocity in a fluidised bed is... 

The differences between liquid and gas fluidised systems have also been studied theoretically by Jackson121 who showed that small discontinuities tend to grow in a fluidised bed, although the rate of growth is greater in a gas-solids system. 

Because of the complex flow pattern, it is not generally possible to determine visually the velocity at which the system ceases, in effect, to be a fluidised bed. From measurements of pressure fluctuations, the following approximate relation may be used, although with considerable care ... 

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