Turbulent fluidisation

The onset of turbulent fluidisation appears to be almost independent of bed height, or height at the minimum fluidisation velocity, if this condition is sufficiently well defined. It is, however, strongly influenced by the bed diameter which clearly imposes a maximum on the size of the bubble which can form. The critical fluidising velocity tends to become smaller as the column diameter and gas density, and hence pressure, increase. Particle size distribution appears to assert a strong influence on the transition velocity. With particles of wide size distributions, pressure fluctuations in the bed are smaller and the transition velocity tends to be lower. 

It may be seen by comparison with equations 6.49 and 6.50 that the turbulent fluidised bed region covers an approximately 2-4 fold range of fluidising velocities. 

However, on die basis of the relation between pressure drop and die minimum fluidisation velocity of particles, the point of transition between a packed bed and a fluidised bed has been correlated by Ergun41 using (17.7.2.3). This is obtained by summing the pressure drop terms for laminar and turbulent flow regions. 

They suggest that particle interaction effects are underestimated by equation 5.71, particularly at high voidages (e > 0.9) and for the turbulent region, and that for a given value of voidage (e) the equation predicts too high a value for the sedimentation (or fluidisation)... 

The heat transfer characteristics of liquid-solid fluidised systems, in which the heat capacity per unit volume of the solids is of the same order as that of the fluid are of considerable interest. The first investigation into such a system was carried out by Lemlich and Caldas193, although most of their results were obtained in the transitional region between streamline and turbulent flow and are therefore difficult to assess. Mitson194 and Smith(20) measured heat transfer coefficients for systems in which a number of different solids were fluidised by water in a 50 mm diameter brass tube, fitted with an annular heating jacket. 

The presence of adjacent spheres caused an increase in the coefficient because the turbulence was thereby increased. The effect became progressively greater as the concentration increased, although the results were not influenced by whether or not the surrounding particles were free to move. This suggests that the transfer coefficient was the same in a fixed or a fluidised bed. 

Fluidised beds may be divided into two classes. In the first, there is a uniform dispersion of the particles within the fluid and the bed expands in a regular manner as the fluid velocity is increased. This behaviour, termed particulate fluidisation, is exhibited by most liquid-solids systems, the only important exceptions being those composed of fine particles of high density. This behaviour is also exhibited by certain gas-solids systems over a very small range of velocities just in excess of the minimum fluidising velocity—particularly where the particles are approximately spherical and have very low free-falling velocities. In particulate fluidisation the rate of movement of the particles is comparatively low, and the fluid is predominantly in piston-type flow with some back-mixing, particularly at low flowrates. Overall turbulence normally exists in the system. 

As the gas or vapour production rate increases, the flow regime may change from churn-turbulent to droplet flow, in which a fluidised bed of liquid droplets is present in the reactor (see Figure A3.1). This is of less practical interest for relief system sizing because if the gas or vapour rate is so high as to give droplet flow, the relief system size is likely to be impractically large. 

Industrial reactors generally operate at very high velocities (of order 1 m/s) much in excess of terminal falling velocity for at least the finest powder fractions. Powder is continually elutriated and returned to the bed via cyclones. Under these conditions there is disagreement as to whether or not bubbles retain their identity and such beds have been described as "turbulent" or "fast fluidised". What little evidence there is supports the continued existence of bubbles but now in a much disturbed or heterogeneous dense phase and with a less definite shape. Until more is known about this physical situation it is not easy to see how the bubbling bed reactor models should be modified correctly to describe this flow regime. 

At present the direct synthesis of alkyl- and arylchlorosilanes is often carried out in apparatuses which operate using the phenomenon of fluidising. Turbulent movement of components in such a reactor guarantees good contact of reactants with contact mass, as well as steady temperature. Reactors with the fluidised layer are cylindrical apparatuses of various diameter with heat exchange elements. Fig.9 features a reactor with a heat exchange element in the form of a Field tube, and Fig. 10 shows a reactor with a heat exchange element in the form of a small-diameter tube bundle. 

Noordman TR, De Jonge A, Wesselingh JA, Bel W, Dekke M, Ter Vorde E, and Grijpma SD, Application of fluidised particles as turbulence promoters in ultrafiltration improvement of flux and rejection, J. Membr. Sci. 2002 208 157-169. van der Waal MJ, van der Velden PM, Koning J, Smolders CA, and van Swaay WPM, Use of fluidized beds as turbulence promoters in tubular membrane systems, Desalination. 1977 22 465 83. 

Abed R, The characterisation of turbulent fluid bed hydrodynamics, in Fluidisation IV, editiors Kunii D, Toei R, 137 (1984)... 

If it is not possible to use a fixed-bed reactor for the OXCO process, some form of fluidised-beo reactor will be necessary. Heat removal and temperature control are greatly facilitated in fluidised-beds because of the excellent backmixing of the solid phase. Considerable opportunity also exists to vary the mode of gas/solids contact by operation in either the bubbling, turbulent or fast (circulating) fluidised-bed regimes. 

Sulfur dioxide emissions from fluidised bed boilers are routinely reduced by adding limestone to the bed (e.g., [12.11]). The favoured top size of limestone for the bubbling and turbulent bed systems is less than 3 mm, with a bottom size of about 250 pm. More finely divided limestone is injected into re-circulating bed systems, as the fine fraction is recycled to the bed. 

Patel M.K and Cross M., The Modelling of Fluidised Beds for Ore Reduction, Numerical Methods in Laminar and Turbulent Flow, Pineridge Press Ltd, Swansea, U.K., p. 2051, 1989. 

During cleaning the flow is reversed to wa off the deposited solids, this is called back-flushing . This often fluidises the media, and frequently air scour is also used. The flow conditions are, therefore, veiy aggressive and turbulent, and particles attached to... 

Two-dimensional electrodes. Two-dimensional electrodes appropriate for metal recycling are typically based on the tank electrolyser to enable ready removal of metal plated electrodes. The simplest cells are the vertical, plate or mesh, electrode in tank units where turbulence is provided by using either inert fluidised beds [8] (Chemelec Cell, BEWT Water Engineers Ltd.) or air agitation (Reconwin cell), in conjunction with electrolyte pumping (see Figure 11.3). 

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. 

Fig. 4.2 — Convective diffusion regimes commonly found in industrial cells, (a) flow through channel formed by two parallel electrode (or by one electrode and a membrane), (b) flow through such a channel but containing a turbulence promoter (e.g. a set of non-conducting bars or a net), (c) fluidised bed electrode, (d) packed bed electrode, (e) rotating cylinder electrode within a concentric tube.

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