Particulate fluidisation

The relation between bed voidage e and fluid velocity uc for particulate fluidisation of uniform particles which are small compared with the diameter of the containing vessel is given by ... 

At values of a Froude group of less than unity, particulate fluidisation normally occurs and, at higher values, aggregative fluidisation takes place. Much lower values of the Froude number are encountered with liquids because the minimum velocity required to produce fluidisation is less. A theoretical justification for using the Froude group as a means of distinguishing between particulate and aggregative fluidisation has been provided by Jackson and Murray . 

An alternative way of calculating the index n in equation 6.31 for the expansion of particulately fluidised systems is now considered. Neglecting effects due to the container wall then ... 

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. 

Godard, K. E. and RICHARDSON, J. F. Chem. Eng. Sci. 24 (1969) 363. Correlation of data for minimum fluidising velocity and bed expansion in particulately fluidised beds. 

Consider a fluidised bed catalytic reactor in which particulate fluidisation of catalyst particles takes place (Figure 4.27) and no bubble is formed. All the terms and notations defined in Section 4.2.2.1.1 for a packed bed catalytic reactor are applicable here. 

Harrison D, Davidson JF, De Kock JW. On the nature of aggregative and particulate fluidisation. Trans Inst Chem Engrs 39 202-211, 1961. 

Damronglerd S, Couderc JP, Angelino H. Mass transfer in particulate fluidisation. Trans Instn Chem Engrs 53 175-180, 1975. 

Foscolo, P.U., Gibilaro, L.G. and Waldram, S.P., 1983. A unified model for particulate expansion of fluidised beds and flow in porous media. Chemical Engineering Science, 38, 1251-1260. 

Fluidised beds have been used previously for the industrial-scale recovery of the antibiotics streptomycin and novobiocin.30 However, more recently, considerable interest has been shown in the use of fluidised beds for the direct extraction of proteins from whole fermentation broths.31 In a packed bed, the adsorbent particles are packed within the contactor. The voidage, that is, the inter-particle space, is minimal and thus feedstock clarification is mandatory to avoid clogging of the bed. In a fluidised/expanded bed, the adsorbent bed is allowed to expand by irrigation with feedstock. Bed voidage is increased, allowing the passage of particulates in the feed. The diameters of the adsorbent beads are exaggerated for illustrative clarity. 

Gilchrist, G.R., Direct fluidised bed adsorption of protein products from complex particulate feedstocks . PhD thesis, University of Birmingham, 1996. 

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... 

In an early attempt to differentiate between the conditions leading to particulate or aggregative fluidisation, Wilhelm and Kwauk suggested using the value of the Froude number (u f/gd) as a criterion, where ... 

Nienow, A.W. and Naimer, N.S., Continuous mixing of two particulate species of different density in a gas fluidised bed, Trans. Inst. Chem. Engrs., 58 (1980) 181-186. 

It would be desirable to have simple tests capable of characterising the fluidisation behaviour or flowability of particulate materials on the basis of their bulk properties. To this end, Carr19 developed a system to characterise bulk solids with respect to flowability. Table 6 summarises the properties which are determined. In Carr s method a numerical value is assigned to the results of each of these tests, and is summed to produce a relative flowability index for that particular bulk material. Given the extensive use of these empirical techniques in academia and industry, a brief review on the subject is reported here. Nevertheless, it should be emphasised that these techniques allow measurements of the flow-ability or cohesion of materials solely in their stationary or compressed status and at ambient conditions. A direct relationship between these... 

The fluidised bed is only one of the many reactors employed in industry for gas-solid reactions, as reported by Kunii and Levenspiel.25 Whenever a chemical reaction employing a particulate solid as a reactant or as a catalyst requires reliable temperature control, a fluidised bed reactor is often the choice for ensuring nearly isothermal conditions by a suitable selection of the operating conditions. The use of gas-solid fluidised beds... 

From the data obtained, it could be concluded that, at coal fluidised bed combustion, not only higher PAH amounts are emitted but these PAH undergo different distribution between solid and gas phase. While, with coal-limestone feed, the PAH emitted are supported on the particulate matter, in limestone absence when only coal is fed, the PAH are emitted on the phase that goes through a filter with a 1 micrometer pore size. 

It is possible to use solid catalysts in particulate forms in tubular reactors through the use of fluidised or fluid bed reactors, where the upward flow of the feed is sufficient to suspend the particulate catalyst in such a way that it seems to behave like a liquid (Figure 1.3). It is however preferable to use more structured catalysts, since better flow characteristics can be achieved, thus minimising hydrodynamic uncertainties and maximising volumetric reaction rates. 

Y. Doganoglu, V. Jog, K.V. Thambimuthu, and R. Clift, Removal of fine particulates from gases in fluidised beds , Trans IChemE 56 (1978), 239-248. 

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