Fluidized beds of large particles

Sheen, S. and Whitney, L.F., Modelling heat transfer in fluidized beds of large particles and its applications in the freezing of large food items, J. Food Eng., 12 (1990) 249-265. 

Fluidized beds of large particles are usually operated at only 2-10 times the minimum fluidization velocity. 

There have been a great many studies of heat transfer in fluidized beds of large particles, which are important for the design of fluid-bed boilers. The maximum coefficients are lower than with type A solids, and some experiments show a gradual decrease in h with increasing dp and a minimum hat dp = 2 3 mm [28]. Extensive data are presented in books [29,30] and in the proceedings of the Engineering Foundation Conferences on Fluidization. 

Bradshaw, R.D. and Myers, J.E., Heat and mass transfer in fixed and fluidized beds of large particles, AIChE J., 9(5) 590-595, 1963. 

Borodulya VA, Ganzha VL, Podberezsky AI, Upadhyay SN, Saxena SE. High pressure heat transfer investigations for fluidized beds of large particles and immersed vertical tube bundles, Int J of Heat and Mass Transfer 26 1577-584, 1983. 

Botterill JSM, Denloye AOO. Bed to surface heat transfer in a fluidized bed of large particles. Powder Tech 19 197-203, 1978a. 

Sittiphong N, George AH, Bushnell D. Bubble eruption diameter in a fluidized bed of large particles at elevated temperatures. Chem Eng Sci 36 1259-1260, 1981. 

Jovanovic G, Catipovic N, Fitzgerald T, Levenspiel O. The mixing of tracer gas in fluidized beds of large particles in fluidization. In Grace J, Matsen J, eds. Fluidization III. New York Plenum Press, 1980, pp 325-332. 

Devolasalle, C., and J. Vanderchuren, Gas-to-particle and particle-to-particle heat transfer in fluidized beds of large particles, Chem. Eng. Sci., 49, 769-779,1985. 

Measurements in large fluidized beds of fine particles indicate that bubble coalescence often ceases within a short distance above the gas distributor plate. Indications from density measurements or single bubble velocities are that bubble velocity Ug and diameter often reach maximum stable values, which are invariant with height or fluidizing gas velocity. 

Because of greater mechanical damage catalyst particles at high fluid velocities, and because of the poor retentivity of fines produced by attrition in beds of large particles, the use of small-diameter catalyst particles in fluidized beds is more usual (Gunn, 1968). In many applications of fluidization, the particles are in the range 30 -300 pm (Smith, 1981 ... 

Several tracers have been used in experiments describing axial mixing in fluidized beds of porous particles, e.g. acetone [37,57], Tryptophane [47], NaCl [49,56], radioactive tracers [58] and dextrane blue [59], It should be noted at this point, that measurement of RTD is not only important for determining possible domination of the chromatographic result by liquid mixing, Bo may as well be taken as a measure for the existence of a stable classified fluidized bed which is ready for sample application. Measurement of RTD in this case will provide a rational basis for the decision to start a large scale protein purification using a fluidized bed or to take measures for improvement of bed stability before application of valuable material. 

Satija, S. and Fan, L.-S. (1985). Characteristics of the Slugging Regime and Transition to the Turbulent Regime for Fluidized Beds of Large Coarse Particles. AIChE J., 31, 1554. Stewart, P. S. B. (1968). Isolated Bubbles in Fluidized Beds Theory and Experiment. Trans. Instn. Chem. Engrs., 46, T60. 

Satija, S. and Fan, L.-S. (1985). Characteristics of Slugging Regime and Transition to Turbulent Regime for Fluidized Beds of Large Coarse Particles. AIChE J., 31,1554. 

The chief assumption made in the development presented previously was that the gas enviromnent is known and essentially constant throughout the reactor. This assumption simplified the treatment considerably, since an analysis based on the solid phase alone could be used to describe the bed behavior. Many practical systems using large-particle beds fluidized with a large excess of gas conform to this situation. If vigorously fluidized beds of fine particles are involved, however, the composition of the gas seen by the solid would vary. The previous simple analysis would then be inapplicable. In these instances, the conservation equations for the gas-phase species... 

In this case, the fluidizing fluid moves upward relative to the bubble motion. This case is usual for beds of large particles and small bubbles. Since R is less than zero in this case, the majority of fluidizing fluid enters the void at the base and leaves from the roof. The fluidizing fluid, in essence, uses the bubble void as the shortcut. The fluidizing fluid penetrates the particulate phase freely from the bubble except for a small fraction of fluid in the shaded area, as shown in Fig. 12 in a circle of radius a, expressed in the following equation. 

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