Dispersion Elutriation

Fluidized combustion of coal entails the burning of coal particles in a hot fluidized bed of noncombustible particles, usually a mixture of ash and limestone. Once the coal is fed into the bed it is rapidly dispersed throughout the bed as it bums. The bed temperature is controUed by means of heat exchanger tubes. Elutriation is responsible for the removal of the smallest soHd particles and the larger soHd particles are removed through bed drain pipes. To increase combustion efficiency the particles elutriated from the bed are coUected in a cyclone and are either re-injected into the main bed or burned in a separate bed operated at lower fluidizing velocity and higher temperature. 

The elutriation method is really a reverse sedimentation process in which the particles are dispersed in an upward flowing stream of fluid. All particles with terminal falling velocities less than the upward velocity of the fluid will be carried away. A complete size analysis can be obtained by using successively higher fluid velocities. Figure 1.4 shows the standard elutriator (BS 893)(6i for particles with settling velocities between 7 and 70 mm/s. 

To obtain a suitable concentrate, the flocculated particles must be separated from the suspension. The usual method is sedimentation of the floes combined with elutriation of the dispersed particles. Flotation of the flocculated particles is a possible method to achieve that separation. The effect of polymers used as flocculants on the flotation of a few minerals has received... 

BANCO CENTRIFUGAL CLASSIFIER. This is a proprietary instrument that is essentially a centrifugal air elutriator. Air and dispersed powder samples are drawn thru the cavity of a rotating hollow disc in a radially inward direction against centrifugal forces. The powder particles are thus divided into under- and oversize fractions, collected, and weighed. Separation into different size-fractions is made by altering the air velocity. About 20g of sample are required for analysis, and 8 size determinations can be made in 2 hours (Ref. 10)... 

The USP 28 describes kaolin as a native hydrated aluminum silicate, powdered and freed from gritty particles by elutriation. The BP 2004 similarly describes light kaolin but additionally states that it contains a suitable dispersing agent. Light kaolin (natural) BP contains no dispersing agent. Heavy kaolin is described in the BP 2004 and PhEur 2005 as a purified, natural hydrated aluminum silicate of variable composition. The JP 2001 describes kaolin as a native hydrous aluminum silicate. 

Elutriation. In fluid classification, various forces act to control the separation of dispersed particles. As discussed earlier, the kinetic behavior of particles is influenced by gravitation (as with elutriators) or centrifugal or Coriolis force (in classifiers). The medium is usually water or air. In general, all fluid classifiers can be divided into two classes counterflow equilibrium (elutriation) and inverse flow separation. Elutriation is discussed briefly below (8). 

The efficiency of separation (Ep) in dense-medium baths was found to depend on the plastic viscosity of the media, and a high yield stress (to) of the medium is claimed to cause elutriation of the finer particles into the float as they (and near-density particles) are unable to overcome the threshold shear stress required before the movement takes place. When a particle is held in a suspension, the yield stress is responsible for it but when it is moving, its velocity is a function of plastic viscosity. It is understandable then that dispersing agents used to decrease medium viscosity may improve separation efficiency quite significantly. They may also decrease mag-nitite loses. 

The process of selective flocculation is used for solid-solid separations (i.e. to separate a desired component from a mixture of particles). The process involves the selective adsorption of a polymeric flocculant on the desired component, resulting in the formation of agglomerates known as floes, and the separation of such aggregates from particles of other component(s) in the dispersed phase. The competition between different surfaces for the flocculant has to be controlled in order to achieve adsorption on the targeted compo-nent(s). The floes are separated from the suspension either by sedimentation/elutriation or floc-flotation. In order to improve the concentrate grade, if possible, the floes may be subjected to further washing. 

In addition to scale-up difficulties, there are a number of problems related to the stable operation of a bubble column associated with hydrodynamics. For example, consider the important commercial application of bubble columns in hydroprocessing of petroleum resids, heavy oils and synthetic crudes. Hydrodynamic cold flow and hot flow studies on the Exxon Donor Solvent coal liquefaction process (Tarmy et al., 1984) showed that much of the literature correlations for the hydrodynamic parameters (holdup, interfacial area and dispersion coefficients) obtained with cold flow units, at ambient conditions, are not applicable for commercial units operating at relatively higher pressures. In addition, the flow pattern in commercial units was considerably different. In the hydroprocessing of petroleum residues by the H-Oil and LC-Fining processes, refinery operations have experienced problems with nonuniform distribution of gas and liquid reactants across the distributor, maintaining stable fluidization and preventing temperature excursions (Beaton et al., 1986, Fan, 1989 and Embaby, 1990). Catalyst addition, withdrawal and elutriation have also been identified as problems in these hydrotreaters. 

As is normal in such cases, as soon as a result appears promising, the demands increase Thus the 650 kg/h achieved were not acceptably high, and since the model we had built was a 1 6 scale model and the capacity required in full scale was 24 t/h, we were asked to prove that the unit would perform as well with 4 t/h. The scale factor of 1 6 was based on the cross section of the upper elutriator tube, in which velocities similar to those in the full scale version were maintained. Since we had not really thought of how the dispersing device was to be scaled up (loading ratio , residence time , mean transport distance ), all we could do was again set the system up and run it ... 

Fig. 5. Effect of enhanced dispersion on the performance of a standard elutriator.

HT- and AT-CNTs were dispersed in 2-propanol solution with a small amount of polyvinylbutyral as a dispersant by ultrasonic agitation. Only the 2-propanol solution containing HT-CNTs were jet-milled. After the obtained suspensions were kept quietly for 3 days, the upper layers of them were elutriated to obtain well dispersed CNTs suspensions. The elutriated CNTs suspensions were mixed with high purity alumina powder (TM-DAR, Taimei Chemicals, Japan) by ball-milling for 24 h. The dried powder mixtures containing 1.6 wt%... 

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