Liquid solid fluidisation

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. 

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. 

This equation predicts that the heat transfer coefficient should pass through a maximum as the velocity of the liquid increases. It may be noted that for liquid-solids fluidised beds, (1 — e) falls as Re increases, and a maximum value of the heat transfer coefficient is usually obtained at a voidage e of about 0.6 to 0.8. 

Liquid-solid fluidised systems are generally characterised by the regular expansion of the bed which takes place as the liquid velocity increases from the minimum fluidisation velocity to a value approaching the terminal falling velocity of the particles. The general form of relation between velocity and bed voidage is found to be similar for both Newtonian and inelastic power-law liquids. For fluidisation of uniform spheres by Newtonian liquids, equation (5.21), introduced earlier to represent hindered settling data, is equally applicable ... 

Juma AKA, Richardson JF. Particle segregation in liquid-solid fluidised beds. Chem Eng Sci 34 137-143, 1979. 

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). 

If a gas is introduced at the bottom of a bed of solids fluidised by a liquid, the expansion of the bed may either decrease or increase, depending on the nature of the solids and particularly their inertia. It is generally found that the minimum fluidising velocity of the liquid is usually reduced by the presence of the gas stream. Measurements are difficult to make accurately, however, because of the fluctuating flow pattern which develops. 

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. 

At the present time, three-phase fluidised-beds are not often chosen for gas-liquid-solid reactions despite their advantages of good heat and mass transfer and, in principle, freedom from the blockages that can occur with fixed-beds(30). The reason may be that, because of the pronounced hydrodynamic interactions between the phases as indicated in Fig. 4.16, development of a three-phase fluidised-bed... 

In liquid-solid systems, and for gas-solid systems of small particles, increasing the fluidising velocity beyond wmf gives rise to a uniform expansion of the bed. The homogeneous expansion of a gas-solid system is generally described using the Richardson-Zaki equation 13... 

The velocity and hold-up distribution of the solid phase in a liquid-solid riser has been studied with radioactive particle tracking and computed tomography (CT).[ 1 The goal of this research was the development of an understanding of the variables affecting the performance of liquid-solid risers, and of fundamentally-based scale-up rules. An improved PEPT system has recently been developed, capable of continuously following the 3D trajectory of a radiotracer particle (as small as 500/um) moving at 0.1 ms with a resolution of 5 mm. The system has been used to measure in situ flow patterns of solids in a gas-solids Interconnected Fluidised Bed reactor. 

Liquid-solid mass transfer in fluidised beds... 

Figure 5.15 Correlations for liquid-solid mass transfer in beds fluidised by power-law liquids...

Prediction of gas solid flow fields, in processes such as pneumatic transport lines, risers, fluidised-bed reactors, hoppers and precipitators are crucial to the operation of most process plants. Up to now, the inability to accurately model these interactions has limited the role that simulation could play in improving operations. In recent years, CFD software developers have focused on this area to develop new modelling methods that can simulate gas-liquid-solid flows to a much higher level of reliability. As a result, process industry... 

Bascoul A, Delmas H, Couderc JP. Caracteristiques hydro-dynamiques de la fluidisation liquide-solide influence du distributeur. Chem Eng J 37 11-24, 1988. 

Bordet J, Coeuret F, Le Goff P, Vergues F. Etude par conductance electrique des fluctuations de porosite locale dans les lits fluidises liquide-solide. Powder Technology 6 253-261, 1972. 

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. 

Adsorption in expanded or fluidised beds is now widely adopted for the direct recovery of protein products from particulate feedstocks. As an integrative protein recovery operation it circumvents process bottlenecks encountered with the solid liquid separation required upstream of fixed bed adsorption, while achieving considerable concentration and primary... 

The technique of passing a gas into a solid which is in the form of tiny granules and thus making the solid behave as if it were a dense, viscous liquid. Fluidised materials may be easily piped from place to place, but a particular application in the rubber industry is the vulcanisation of extruded sections. See Fluid Bed Vulcanisation. Fluorinated Elastomers... 

Flexible tubing for the conveyance of fluids (fluidised solids, liquids or gases) the constmction, which includes textile and/or wire reinforcement, varies according to the pressure and the type of fluid. 

The need to remove suspended dust and mist from a gas arises not only in the treatment of effluent gas from a plant before it is discharged into the atmosphere, but also in processes where solids or liquids are carried over in the vapour or gas stream. For example, in an evaporator it is frequently necessary to eliminate droplets which become entrained in the vapour, and in a plant involving a fluidised solid the removal of fine particles is necessary, first to prevent loss of material, and secondly to prevent contamination of the gaseous product. Further, in all pneumatic conveying plants, some form of separator must be provided at the downstream end. 

As already indicated, when a liquid is the fluidising agent, substantially uniform conditions pervade in the bed, although with a gas, bubble formation tends to occur except at very low fluidising velocities. In an attempt to improve the reproducibility of conditions within a bed, much of the earlier research work with gas fluidised systems was carried out at gas velocities sufficiently low for bubble formation to be absent. In recent years, however, it has been recognised that bubbles normally tend to form in such systems, that they exert an important influence on the flow pattern of both gas and solids, and that the behaviour of individual bubbles can often be predicted with reasonable accuracy. 

Regular and even expansion of the bed does not always occur when particles are fluidised by a liquid. This is particularly so for solids of high densities, and non-uniformities are most marked with deep beds of small particles. In such cases, there are significant deviations from the relation between bed voidage and velocity predicted by equation 6.31. 

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. 

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