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The 'fluidised bed is the name given to a bed of solid particles, which through the impact energy of a fluid, is caused to undergo fluidisation. The term fluidisation in this context refers to the phenomenon whereby the solid components of a particulate bed exhibit fluid like properties, owing to the upwelling of a fluid.

Fluidisation bed processes are important process engineering systems, finding application in areas such as spray drying, drying of granular media, combustion, or in applications such as catalytic cracking or coffee roasting

Oldest power station utilising circular fluidised bed technology in Lünen, Germany

Properties of fluidised beds

A fluidised-bed consists of fluid-solid mixture that exhibits fluid-like properties. As such, the upper surface of the bed is relatively horizontal, which is analogous to hydrostatic behaviour. The bed can be considered to be an inhomogenous mixture of fluid and solid that can be represented by a single bulk density.

Furthermore, an object with a higher density than the bed will sink, whereas an object with a lower density than the bed will float, thus the bed can be considered to exhibit the fluid behaviour expected of Archemidies' principle. As the "density" of the (actually the solid volume fraction of the suspension) of the bed can be altered by changing the fluid fraction, objects with different densities comparative to the bed can, by altering either the fluid or solid fraction, be caused to sink or float.

In fluidised beds, the contact of the solid particles with the fluidisation medium (a gas or a liquid) is greatly enhanced when compared to packed beds. This behaviour in fluidised combustion beds enables good thermal transport inside the system and good heat transfer between the the bed and its container Similarly to the good heat transfer, which enables thermal uniformity analgous to that of a well mixed gas, the bed cna have a significant heat-capacity whilst maintaining a homongenous temperature field which is advantageous for the proper combustion of potentially harmful substances

Characteristic of the fluidisations mechanisms of the fluidised bed is the solid's volume profile

Fluidised bed types

One decides between *Stationary or bubbling beds, where the fluidisation of the solids is relatively stationary, with some fine particles being entrained. *Circulating beds, where the fluidisation suspends the particle bed, due to a larger kinetic energy of the fluid. As such the surface of the bed is less smooth and the larger particles can be entrained from the bed These can then form clusters whereby in the fluidised bed either internally or externally can be classified by a cyclone sperator and separated from or returned to the bed.

Bed conditions

Bubbling bed condition

In order to approximately go from a packed bed, whereby a fluid stream is passed through and the gas velocity is continually raised, there will be a point, known as a minium fluidisation point, whereby the bed is suspended (condition A) The corresponding fluid velocity, known as the "mimum fluidisation velocity" is generally referred to by $u_{mf}$. Raising the fluid energy further will cause the bed to exhibit the bubbling bed condition (condition b) , whree the bubbles are seen to arise from a relatively stationary bed.

In a relatively large range of gas velocities, this condition does not change considerably. Depending upon the particle form, the size, density distribution etc, the fluidised-bed, for $u$ 5-6 greater than $u_{mf}$, can exhibit bubbling bed characteristics. With further increases to the gas velocity, the suspension's volume fraction will decrease In this region, one finds that the fluidised bed solids fraction is from 20 to 40 %.

The gas velocity in this condition is very dependant upon the single-grain sedimentation velocity of the particles. The particles in this region exhibit a swarm-like behaviour. Particles tend to align in "strings" such that the drag coefficient ($c_{w}$) is higher than that for than the single-particle drag-coefficient. Similarly the ..., so that these ..... The result is a compact suspension with exactly defined surfaces, which at the surface appear to bubble.

The bubblng behaviour of the bed is charachteristic of the intensive intermixing in a vertical direction. Depending upon the cross-sectional area, picture yourself spatious circulation energy In general a rise in solids in the central region of the bed results in the bed edges sinking Due to the very large cross-sectional area there is more ascent and descention zones. This effect can through special arrangement of the gas distributor plate be enhanced, for example in order to increase the combustion mixing in the distributor zone to enhance the fluidisation behaviour.

By estimating the bedd material and vessel cross-sectional area can the "percussive" fluidisation be generated, that which in such beds the bubble size is in the same cross-section enlarged By increassing the gas velocity further, the process begins to discharge fine material, that can, for example be sent into a gas cyclone and returned to the bed. Circumstance D

A fither increase in fluid energy causes the futher discharge of finer particles in the fluidised bed The return of the particles, whether suspended by gas or fluid, can be controlled by the installation of a gas cyclone to return separated solid particles. The gas cyclone separates the gas from the solides, whereby the surfoce of the fluidised bed can be utilised to inprove circulation in the bed In the case of condition D, the ? gas and solid components are so large that the cyclone cannot any more in the combustion chamber a ? can be integrated

The advantages of this technical method are the circulation of the bed reinforces the bubbling bed behaviour, due to the higher fluid velocities, which are generated by combutionprocesses due to extra combustion. For catalyic processes, such as fluid-catalytic cracking, the catalyst can be injected without the need for a mechanical installation of the catalsyst in the regenerator.

The disadvantages of the circualting bubbling bed are that the bubbling bed has higher operating costs, due to the higher energy requirements for the required blowers.

The choice of which bed to utliise is based upon the size of the something-energy. Small plants can frequently use bubbling beds, where as larger installations may utilise the circulating variety.

Through the higher fluid-energies will relatively many particles be caught in the free-board zone (or combustion zone) and partly will be directed to the gas cyclone A known quantitiy of the solidparticles will fall back in the bed after being entrained in the surface-layer In the so-called "Freeboard" region, sometimes a denser zone can result in a flow pattern which is konown as the a "core-annulus" pattern. In a relatively broad particle for the solid flake suspension, while the direct to the brim of the bed (in industrial scales, the vessel can typically be from 30 to 350 cm in diameter), the solids can form clusters with higher and lower motion.

At the upper part of the vessel, there is a solids volume concentration profile, which reaches a maximum is reached at the surface layer of the bed, and the minium is reached at the head of the vessel. The solids volume fraction in the freeboard section, has typically, in modern combustion beds, a fraction of less than 1%, Catalytic crackers can have an higher solids volume-fraction.

Geldart Groupings

Dr. Geldart Schüttgüter in proposed the grouping of powerders in to for so-called "Geldart Groups". The groups are defined by there locations on a diagram of solid-fluid desnity difference and particle size.

Group A the particle size is between 20 and 100 ?m, and the particle density is typically 1400kg/m³. Prioer to the initiation of a bubbling bed phase, beds from these particles will expand by a factor of 2 to 3 at incipient fluidisation, due to a decreased bulk density. Most powder-catalysed beds utilise this group.

Group B The particle size lies between 40 and 500 ?m and the particle density between 1400 and 4500 kg./m³ Bubbling typically forms directly at incipient fluidisation.

Group C' This group is the smallsed and therfore the most cohesive particles with a size of 20 to 30 ?m Due to the strong cohesive foces, these particles fluidise with very difficult to achieve conditions, for example with the aid of mechanical agitation.

Group D the particles in this region are above 600?m and typically have high particle densities. Fluidisation requires very high fluid energies, and this typically causes high levels of abrasion.

Mathematical modelling

When the packed bed has a fluid passed over it, the pressuredrop is approximately poporitional to the fluids superficial velocity. At the minimum fluidisation velocity ($u\geq u_{mf}$), the bed material will be suspended by the gas-stream The pressure drop will therefore remain relatively constant.

At the base of the vessel is the apparent pressuredrop multiplied by the cross-sectiona area of the bed can be equated to the force of the weight of the solid particles (less the boyancy of the solid in the fluid).

$\Delta p_{w}=H_{w}(1-\epsilon _{w})(\rho _{s}-\rho _{f})g$

History

In 1922 von Winkler designed a reactor that for the firstt tume utilised the coal gasifcation process. Through the application of the fluidised bed in for the catylitic cracking of mineral oils in the 1940s. During this time theoretical and experiemental research improved the design of the fluidised bed In the 1960s VAW-Lippewerk in Lönen implemented the first industrial bed for the combustion of coal and later for the calcination of aluminium hydroxide Inzwischen werden Wirbelschichtanlagen f\374r viele verschiedene Zwecke benutzt.

Application

Fluidised beds are used as a technical process which has the ability to promote high levels of contact between gasses and solids. In a fluidised bed can a charachteristic set of basic properties be utilised in rocess engineering and in chemical rection engineer, these properties include:

• high relative velocities between the fluid and the dispersed solid phase.
• Frequent particle-particle and particle-wall collisions.
• High levels of intermixing of the particulate phase.

Due to these properties fluidised beds are an important technologyin in process engineering.

Fluidised beds have been used a s a drying process, in chemical reactions and in the conversion of energy for gasification and combustion processes.