Suspensions of coarse particles

The flow behaviour of suspensions of coarse particles is completely different in horizontal and vertical pipes. In horizontal flow, the concentration of particles increases towards the bottom of the pipe, the degree of non-uniformity increasing as the velocity of flow is decreased. In vertical transport, however, axial symmetry is maintained with the solids evenly distributed over the cross-section. The two cases are therefore considered separately. 

For suspensions of coarse particles The value calculated for the carrier fluid flowing alone at the mixture velocity is used. 

Richardson, 1. F. and Smith, 1. W. Trans. Inst. Chem. Eng. 40 (1962) 13. Heat transfer to liquid fluidised systems and to suspensions of coarse particles in vertical transport. 

Coagulation by inorganic electrolytes has been found to be effective only for fine colloids and not for suspensions of coarse particles, for example, of size over 0.074 mm. 

However, it is very difficult to suspend solid particles effectively and still maintain plug flow. An agitated or pulsed column reactor may be applicable (see section 43,1,5), but then the effects of the residence time distribution have to be taken into account. For large scale operations a series of CSTR s is often more practical, even if the residence time distribution is greater, since it offers better possibilities for effective suspension of coarse particles, and also for heat transfer to the vessel wall. However, one should be aware of the fact that in a continuous reactor with backmixing the conversion of solid particles can never be complete. Because of the residence time distribution, a fraction of the solid particles, those with an individual residence time shorter than the dissolution time, will leave the reactor. For obtaining a complete conversion of the solid, a tubular reactor, that guarantees a certain minimum residence time, will have to be installed after the last CSTR. An alternative is to separate the unconverted solid and return it to the first reactor. 

Pirie, R. L. Ph D. Thesis, University of Wales (1990). Transport of coarse particles in water and shearthinning suspensions in horizontal pipes. 

Richardson and Zaki (1954) showed that in the Reynolds number range Rep<0.2, the velocity uc of a suspension of coarse spherical particles in water relative to a fixed horizontal plane is given by the equation... 

It is important to note that suspensions of fine particles tend to behave rather differently from coarse suspensions in that a high degree of flocculation may occur as a result of the very high specific surface of the particles. For this reason, fine and coarse suspensions are considered separately, and the factors giving rise to flocculation are discussed in Section 5.2.2. 

If a < 1, some of the gas in the suspension phase undergoes short-circuit flow through the bubble, while only part of the bubble gas recirculates through the suspension. This type of flow is typical for fluidized beds of coarse particles (Geldart Group D). 

A criterion for the prediction of minimum stirrer rotation speeds for the suspension of coarse-grain particles (Archimedes number > 40) is derived by Molerus and Latzel (1987). They showed that the minimum stirrer rotation speeds can be predicted by the evaluation of two diagrams the drag of fluidized particles as a function of concentration, and the pressure-head volumetric flow-rate characteristics of the agitated vessel. The latter can be obtained using the similarity of fluid-kinetic machines and can be expressed as lAav = /([Pg.48]

An example of how slurry rheology is affected by combining a coarse size distribution (208-279 pm) with a fine size distribution (smaller than 45 pm) can be seen in Figs. 5 and 6. Adding increasing amounts of coarse particles made the suspensions examined less viscous until approximately 40% of the solids were coarse particles. Increasing the proportion of coarse particles beyond that level produced no further viscosity decrease and tended to destabilize the slurry so that the particles would rapidly settle out. ... 

Fig. 5 Changes in the rheology of 40wt% coal suspensions as the proportion of coarse particles (208-279 pm) to fine particles (<45pm) is increased. As the fraction of coarse particles increases, the viscosity drops and the yield stress is decreased. (From Ref.. )...

A comparison can be made between Table 7 and Table 6 (mineral suspensions). In both cases, the presence of coarse particles is characterised by a diffusion spectrum with a relatively flat shape due to the very light dependence of absorbance versus wavelength (diffraction). On the contrary, the presence of colloids is responsible for a high variation of absorbance with wavelength (Mie diffusion). The main difference between the two sets of spectra is the presence of structured elements (i.e. shoulders) on the spectra of wastewater, related to the chemical absorption of organic compounds bound to the particles (i.e. surfactants). 

One of the earlier studies was carried out in 1971 by Charles and Charleswho investigated the feasibility of transporting coarse materials in heavy media (sand in flocculated suspensions of clay in water). They concluded that the power requirement for the transport of one million tonnes/year of solids in a 200 mm diameter pipe could be reduced by a factor of about 6 by using heavy media in place of water. KenchingtoN who also studied the transport of coarse particles in china clay suspensions, concluded that a significant proportion of the coarse particles may be suspended in the dense medium without a noticeable increase in pressure drop. Duckworth et showed that it is... 

Clearly, colloidal particles lie on the border-line between coarse particles, which obey the ordinary sedimentation laws, and atoms and molecules, which do not it is therefore not surprising tnat colloidal particles behave as they do. Although we shall be concerned mainly with suspensions of solid particles in water, it should be realized that colloidal suspensions of solid particles in liquids other than water can be produced. 

In applying the theory of the attractive forces as developed in the preceding section to colloidal systems, we met with some difficulties, especially in the treatment of coarse suspensions, because the London theory as such is not relativ-istically invariant, and by working out this idea we found that precisely in the case of coarse particles a relativistic correction may well become important. 

Benah, M. and Shakourzadeh-Bolouri, K., The gas-sohd-solid packed conctactor hydrodinamic behavior of counter-current trickle of coarse and dense particles with a suspension of fine particles, Int. J. Multiphase Flow, 20, 161, 1994. 

Filter media efficiency tests are made using dilute suspensions of standard particles. A wide variety of test powders is available, in fine and coarse grades. Industrial users will tend to use a test mixture typical of the products of interest, e.g. yeast in the browing industry. In the gaseous field, both solid (sub-micron sodium chlorid crystals) and liquid particles (di-octyl phthalate dispersion) are used. 

From Figure 11.1, it may be inferred that suspensions of fine particles will require pressure filters whilst a coarse suspension will be suitably processed in pan or table filters. Where the size range is wide, the actual plant choice may be affected by the concentration of fines a slurry with 10% greater than 10 pm may require pressure filtration at the same average size an increase in proportion greater than 10 pm may make a rotary filter possible. 

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