Suspension of solid particles

There are a large number of processes in the chemical industries that handle a variety of suspensions of solid particles in liquids. The application of filtration techniques for the separation of these heterogeneous systems is sometimes very costly. If, however, the discrete phase of the suspension largely contains settleable particles, the separation can be effected by the operation of sedimentation. The process of sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling. This unit operation is divided into thickening,... 

Zweiteriiig, T.N., 1958. Suspension of solid particles in liquids by agitators. Chemical Engineering Science, 8, 244. 

A vertical centrifuge, operating at 100 rpm, contains an aqueous suspension of solid particles with SG = 1.3 and radius of 1 mm. When the particles are 10 cm from the axis of rotation, determine the direction in which they are moving relative to a horizontal plane. 

The first phase in the process is the formation of the sol . A sol is a colloidal suspension of solid particles in a liquid. Colloids are solid particles with diameters of 1-100 nm. After a certain period, the colloidal particles and condensed silica species link to form a gel - an interconnected, rigid network with pores of submicrometer dimensions and polymeric chains whose average length is greater than one micrometer. After the sol-gel transition, the solvent phase is removed from the interconnected pore network. If removed by conventional drying such as evaporation, so-called xerogels are obtained, if removed via supercritical evacuation, the product is an aerogel . 

With some concentrated suspensions of solid particles, particularly those in which the liquid has a relatively low viscosity, the suspension appears to slip at the pipe wall or at the solid surfaces of a viscometer. Slip occurs because the suspension is depleted of particles in the vicinity of the solid surface. In the case of concentrated suspensions, the main reason is probably that of physical exclusion if the suspension at the solid surface were to have the same spatial distribution of particles as that in the bulk, some particles would have to overlap the wall. As a result of the lower concentration of particles in the immediate vicinity of the wall, the effective viscosity of the suspension near the wall may be significantly lower than that of the bulk and consequently this wall layer may have an extremely high shear rate. If this happens, the bulk material appears to slip on this lubricating layer of low viscosity material. 

Water may be evaporated from a solution or a suspension of solid particles by spraying the mixture into a vessel through which a current of hot gases is passed. In this way, a... 

Nienow, A.W. (1992) The Suspension of Solid Particles. Mixing in the Process Industries, 2nd edn, Butterworth-Heinemann, Oxford, pp. 364-411. 

Aerosol is defined herein as an atmospheric suspension of solid particles including adsorbed water and absorbed volatile organic components. 

Fluidized beds have also been used for generating suspensions of solid particles with diameters in the range of 0.5-40 gm. Air flows through the fluidized bed, which contains beads kept suspended by the motion of the air dust injected into the bed is broken up into small particles and carried out with the air flow (Raabe, 1976). 

Criteria for suspension of solid particles in the slurry phase... 

Bubble columns in which gas is bubbled through suspensions of solid particles in liquids are known as slurry bubble columns . These are widely used as reactors for a variety of chemical reactions, and also as bioreactors with suspensions of microbial cells or particles of immobilized enzymes. 

Inorganic Materials. Sol-gel chemistry involves first the formation of a sol, which is a suspension of solid particles in a liquid, then of a gel, which is a diphasic material with a solid encapsulating a solvent A detailed description of the fundamental chemistry is available in the literature. The chemistry involving die must commonly used precursors, the alkoxides (M(OR) ,). can be described in terms of two classes of reactions ... 

The sol-gel process involves first the formation of a sol followed by that of a gel. A sol, which is a liquid suspension of solid particles ranging in size from 1 nm to 1 micron, can be obtained by the hydrolysis and partial condensation of a precursor such as an inorganic salt or a metal alkoxide. The further condensation of sol particles into a three-dimensional network produces a... 

Joosten et al. (1977) and Kolar (1967) also studied suspension of solids in stirred vessels. The correlations of Baldi et al. (1978) and Zwietering (1958) are based on data over a wide range of conditions and are also in good agreement with each other. Baldi et al. (1978) also proposed a new model to explain the mechanism of complete suspension of solid particles in cylindrical flat-bottomed stirred vessels. According to this model the suspension of particles at rest on certain zones of the tank bottom is mainly due to turbulent eddies of a scale of the order of the particle size. The model leads to an expression... 

In order to achieve simultaneous suspension of solid particles and dispersion of gas, it is necessary to define the state when the gas phase is well dispersed. Nienow (1975) defined this to be coincident with the minimum in Power number, Ne, against the aeration number, 1VA, relationship (see Fig. 12 [Sicardi et al., 1981]). While Chapman et al. (1981) accept this definition, their study also showed that there is some critical particle density (relative to the liquid density) above which particle suspension governs the power necessary to achieve a well-mixed system and below which gas dispersion governs the power requirements. Thus, aeration at the critical stirrer speed for complete suspension of solid particles in nonaerated systems causes partial sedimentation of relatively heavy particles and aids suspension of relatively light particles. Furthermore, there may be a similar (but weaker) effect with particle size. Wiedmann et al. (1980), on the other hand, define the complete state of suspension to be the one where the maximum in the Ne-Ren diagram occurs for a constant gas Reynolds number. 

Consider first the effect of a dispersed phase, of volume fraction continuous phase of viscosity D0 and dispersed particles (droplets) which do not attract. At low volume fractions the Einstein equation should apply to a suspension of solid particles at constant temperature,... 

Fig. 9.9 depicts some of the results from such trials. Despite some scatter in the angle range 95°-105°, it is readily seen that at 9< 95° none of the films ruptures, while at 0> 105° all the films rupture. Experiments with suspensions of solid particles (Teflon, hydrophobed glass beads and ground glass) showed that in the case of smooth particles, the defoaming... 

Additional work on the suspension of solid particles in agitated systems has been reported by Oyama and Endoh (013), on suspension of sands and resin particles, in baffled vessels 5.5, 6.7, and 10.8 in. in diameter. These authors used a 3.6-in. vaned disk, and 2.6- and 3.6-in. flat-blade turbines. A light beam passed through the vessel onto a photoelectric tube was used to monitor the particle concentration in a horizontal plane at a height about of the total liquid height. At low speeds the particles tended to congregate near the vessel bottom. As the... 

We first consider brielly why a polymer solution would be expected to have a higher viscosity than the liquid in which it is dissolved. We think initially of a suspension of solid particles in a liquid. The particles are wetted by the fluid, and the suspension is so dilute that the disturbance of the flow pattern of the suspending medium by one particle docs not overlap with that caused by another. Consider now the flow of the fluid alone through a tube which is very large compared to the dimensions of a suspended particle. If the fluid wets the tube wall its velocity profile will be that shown in Fig. 3-5a. Since the walls are wetted, liquid on the walls is stationary while the flow rate is greatest at the center of the tube. The flow velocity v increases from the wall to the center of the tube. The difference in velocities of adjacent layers of liquid (velocity gradient = civ/dr) is greatest at the wall and zero in the center of the tube. 

Jackson, R., Progress toward a mechanics of dense suspensions of solid particles. AIChE. Symp. Ser. 301 90, 1 (1994). 

Solid PVC resin is combined with water in a stirred tank to make a slurry (a suspension of solid particles in a liquid) containing about 10 wt% PVC. The slurry is pumped to a continuous chlorine absorber, a vessel equipped with an impeller that keeps the contents agitated. A stream of chlorine vapor at 25°C also enters the absorber. The absorber operates at 25°C. 

Emulsion Viscosity Equations. Only a few viscosity-concentration equations have been developed purely for emulsion systems. Most equations have been adapted by analogy from studies with suspensions of solid particles (20). 

Difficulties are encountered in some extractions with immiscible solvents because of the formation of emulsions which prevent a sharp separation of the layers. Suspension of solid particles tends to stabilize emulsions, and it is advisable, if possible, to filter the solution before extraction and to avoid violent shaking. Emulsions are often demulsified by the addition of more solvent, an electrolyte like salt, a few drops of alcohol (in the case of ether extracts), and application of suction to the vessel. 

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