Mixing in fluidized beds

In some cases, it is impractical to use a plenum chamber under the constriction plate. This condition arises when a flammable or explosive mixture of gases is being introduced to the reactor. One solution is to pipe the gases to a multitude of individual gas inlets in the floor of the reactor. In this way it may be possible to maintain the gas velocities in the pipes above the flame velocity or to reduce the volume of gas in each pipe to the point at which an explosion can be safely contained. Another solution is to provide separate inlets for the different gases and depend on mixing in the fluidized bed. The inlets should be fairly close to one another, as lateral gas mixing in fluidized beds is poor. 

Litka, T., and Glicksman, L. R., The Influence of Particle Mechanical Properties on Bubble Characteristics and Solid Mixing in Fluidized Beds, Powder Technol., 42 231 (1985)... 

Geldart, D., Mixing in fluidized beds, in Hamby, N., Edwards, M.E and Nienow, A.W., (eds.). Mixing in the process industries, Butterworth-Heinemann, Oxford, 1992. 

The mixing in fluidized beds is somewhat more complicated, as might be expected. The over-all results found by the previously mentioned authors are summarized by Cairns and Prausnitz (C4) ... 

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. 

Up to now, there have been no data available in the literature describing solids mixing in fluidized beds of particles suited for protein adsorption. Therefore we will be restricted to a general discussion on particle movement and have to extrapolate some findings to fluidized bed adsorption. In fluidized beds of... 

Borlai.O., Hodany, L., and Blickle, T., Investigation into the Mixing in Fluidized Beds, Proc. 

Rowe, P. N and Sutherland, K. S., Solids Mixing Studies in Gas Fluidized Beds, Part II The Behavior of Deep Beds of Dense Materials, Trans, lnstn. Chem. Engrs. 42, T55 (1964). Rowe, P. N., Partridge, B. A., Cheney, A. G., Henwood, G. A., and Lyall, Z., The Mechanisms of Solids Mixing in Fluidized Beds, Trans, lnstn. Chem. Engrs. 43,271 (1965). 

Squires, A. M. Role of solid mixing in fluidized-bed reaction kinetics, AlChE Symp. Ser. 69(127), 8-10 (1973). 

Particle Feeding. Many fluidized bed reactors require that particles be fed continuously or intermittently. Pneumatic feeding and screw feeders are most common. Because lateral mixing is much less than axial mixing in fluidized beds, it may be essential to introduce solids at a number of different positions... 

SECTION 1 3-9 HEAT TRANSFER AND MIXING IN FLUIDIZED-BED REACTORS... 

Frequency response methods have been found useful in both theoretical and experimental analysis of gas mixing in fluidized beds. Experiments in a fluidized-bed reactor related to mixing theory were made by Bamstone and Harriott 24). Testin and Stuart have measured diffusion coefficients in gas-solid adsorption studies 25). 

In this chapter we shall examine the mechanisms of mixing in fluidized beds and learn how to choose the right operating conditions so as to minimize segregation and ensure good mixing, where that is possible. First, however, it is necessary to outline some fundamental aspects of fluidization. 

Because it is the bubbles which are largely responsible for mixing in fluidized beds, it is important to know how much of the gas flowing into the bed forms bubbles. A first estimate of the bubble flow rate, (2b> is provided by the two phase theory. This pictures the bed as consisting of a dense phase in which the gas velocity is equal to that at minimum fluidization, U f, and a bubble phase which carries all the additional gas. Thus, at a superficial velocity U ... 

Solids are sphericaL isotropic, uniform size, and homogeneous. They are perfectly well mixed in fluidized bed. 

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