Fluidized Beds Of Large Particles Research Articles

Caractéristiques au minimum de fluidisation et expansion des couches fluidisées de particules de la catégorie D de Geldart

This paper presents results of an experimental study concerning the hydrodynamic behaviour at incipient fluidization and expansion of fluidized beds of large particles of category D of Geldart's classification. The effect of the physical properties of the solids and of the column geometry was analysed particularly with air at room temperature and atmospheric pressure. Relationships have been derived to predict the gas velocity and the voidage at incipient fluidization and expansion of the beds. These correlations take into account all the literature data relative to this kind of particle.

Slugging behaviour of fluidized beds of large particles

An experimental study has been carried out to investigate the slugging behaviour of fluidized beds of large particles (Group D according to the Geldart classification). The influence of the bed diameter on the transition from bubbling to slugging flow conditions has been quantified by means of a simple relationship, which allows faithful predictions. Slug length and frequency, and average bed expansion have been measured as functions of the excess gas flow rate, u— u mf, with beds of different diameters and overall heights. An attempt is presented to correlate the experimental data in terms of the main physical and operating variables.

Modelling heat transfer in fluidized beds of large particles and its applications in the freezing of large food items

Bed-to-surface heat transfer coefficients in fluidized beds of large particles were studied. It was found that heat transfer coefficients in fluidized beds of large particles are two to three times larger than those in forced air convection and that particle diameter had a reverse effect on heat transfer coefficient (e.g. larger particle has smaller bed-to-surface heat transfer coefficient). Freezing curves were compared for two food items in (a) fluidized beds and (b) forced air convection. Freezing time and moisture loss in fluidized beds were both less than those in forced air convection. Computer simulation showed close agreement between the predicted and experimental results.

The gas contribution to heat transfer between fluidized beds of large particles and immersed surfaces

The gas contribution to heat transfer between large particle fluidized beds and immersed surfaces is analysed through a heterogeneous model for the dense phase. An approximate solution of the governing balances provides a conceptually appropriate and precise enough expression for the gas heat transfer coefficient, h g . For most practical conditions h g amounts to 75—85% of the limiting heat transfer coefficient at the surface wall, h wg . The results are compared with previous correlations for experimental conditions at which they were formulated. The analogous mass transfer process is included in this analysis as a particular case of the proposed theory.

Gas-to-particle and particle-to-particle heat transfer in fluidized beds of large particles

Gas-to-particle and interparticle transfer rates were measured, at room temperature, for three different materials, in the range 1 ≤ U/ U mf ≤ 3.5 and 0.900 ≤ d p ≤ 2.250 mm by means of batch runs in which wet capillary-porous particles were dried in fluidized beds of similar dry ones. It was observed that gas-to-particle convective transfer coefficients vary only slightly with the air velocity. A “slow-bubble model” convenient for large particle fluidization, was developed to explain this finding and provided interesting data concerning the gas flow distribution in beds of large particles. The particle-to-particle heat transfer coefficients, nearly independent on the particle thermal properties, were correlated by the following equation: h pp = 206 d p −1.22 ( U/ U mf ) −0.56. A theoretical analysis of the interparticle heat transfer confirmed the observed trends and its application to our results allowed to estimate the dense phase expansion.

Heat transfer between fluidized beds of large particles and horizontal tube bundles at high pressures

The experimental heat transfer data for horizontal square inline tube bundles ( D T = 13.0 mm and pitch values are 19.5, 29.3 and 39.0 mm) immersed in fluidized beds of glass beads ( d ̄ p = 1.25 and 3.1 mm) and sands( d ̄ p = 0.794 and 1.225 mm) are measured as a function of fluidizing velocity and system pressure (1.1, 2.6, 4.1 and 8.1 MPa). The heat transfer coefficient values are found to increase with particle diameter, system pressure but are almost independent of tube pitch in the range investigated here. The h w values are compared with the predictions of five different theories available in the literature. The two correlations for Nu max are also considered and evaluated. Significant conclusions are drawn on the basis of reported h w data for large particles at high pressures.

High pressure heat transfer investigations for fluidized beds of large particles and immersed vertical tube bundles

The overall heat transfer coefficient, h w, is measured for vertical tube bundles in fluidized beds of glass beads ( ƌ p = 1.25 and 3.10 mm) and sands ( ƌ p = 0.794 and 1.225 mm) at pressures of 1.1, 2.6,4.1 and 8.1 MPa and ambient temperature. Tube bundles of three different pitches (19.5, 29.3 and 39.0 mm) are employed and h w is reported as a function of velocity, G. It is found that h w increases with pressure, to a lesser extent with tube pitch, and with an increase in particle diameter. The experimental data are compared with the predictions of four theories for h w and two theories of h w,max. It appears that the theory of Ganzha et al. is most successful in reproducing the experimental data and it is recognized that the knowledge of reliable bed voidage at the heat transfer surface and in the bulk is crucial for its applicability.

A mechanistic theory for heat transfer between fluidized beds of large particles and immersed surfaces

A mechanistic theory for the heat transfer between an immersed surface and a fluidized bed of large particles is developed by adopting the well accepted concept that in the absence of radiation the total heat transfer coefficient is the sum of conductive ( h cond ) and convective ( h conv ) components. The solids are assumed to be distributed around the heat transfer surface in an arrangement of unit orthorhombic cells. h cond is then computed by considering a composite infinite layer of gas and solid and by solving the unsteady state heat conduction equations under well defined boundary and initial conditions. h conv is evaluated by assuming that the turbulent boundary layer on the heat transfer surface is disrupted at the front half of the particle and is reformed in its wake. The theoretical model predictions for the total heat transfer coefficient are in good agreement with the available experimental data on large particle systems. The proposed theory is considered a good predictive and design tool.

Mass transfer between the particles and the fluid in fluidized beds of large particles

Generalized empirical correlations available in the literature for particle to fluid mass transfer in fluidized beds are tested for their effectiveness in predicting the experimental mass transfer data for fluidized beds of large particles. Rowe's modification of Nelson and Galloway's theory of particle to fluid mass transfer in dense systems of fine particles is also re-examined and found to agree fairly well with large particle mass transfer data for liquid fluidized beds. A simplified correlation, which is shown to be an approximation of the Nelson-Galloway-Rowe asymptotic expression, is proposed for predicting the mass transfer rate.

Mass transfer to power law fluids in fluidized beds of large particles

Mass transfer in liquid fluidized beds has been investigated by measuring the rate of dissolution of solid particles in demineralized water and 1.0% aq. carboxyl methyl cellulose (CMC) solution. Results for aq. CMC solution are interpreted using the capillary tube bundle approach and are found to be in reasonable agreement with the present as well as previously published Newtonian mass transfer data.