Fluidized beds bubbles

Bubbles and Fluidized Beds. Bubbles, or gas voids, exist in most fluidized beds and their role can be important because of the impact on the rate of exchange of mass or energy between the gas and soflds in the bed. Bubbles are formed in fluidized beds from the inherent instabiUty of two-phase systems. They are formed for Group A powders when the gas velocity is sufficient to start breaking iaterparticle forces at For Group B powders, where iaterparticle forces are usually negligible, and bubbles form immediately upon fluidization. Bubbles, which are inherently... 

Eor turbulent and fast-fluidized beds, bubbles are not present as distinct entities. The following expression for bed voidage, bed occupied by gas, where U is in m/s, has been suggested (17) ... 

This equation has been experimentally verified in liquids, and Figure 2 shows that it applies equally well for fluidized solids, provided that G is taken as the flow rate in excess of minimum fluidization requirements. In most practical fluidized beds, bubbles coalesce or break up after formation, but this equation nevertheless gives a useful starting point estimate of bubble size. 

The selection and design of a reactor for bench-scale kinetic experiments should be considered case by case. It is important to stress, however, that one should not try to build a bench-scale replica of what is believed to be or is the industrial reactor. Industrial reactors are designed to operate a process in a profitable way, which is not the case for experimental reactors. In industrial reactors heat, mass and momentum transport has to occur in an economically justifiable way, leading in general to temperature, concentration and/or pressure gradients inside the reactor. Also, the hydrodynamics can be rather complicated. Fluidized beds, bubble columns and trickle-flow reactors require model equations that involve several physical parameters, besides the intrinsic kinetic parameters. Empirical... 

In fluidized beds, bubbles form at the distribution plate or grid ports where fluidizing gas enters the bed (Figure 169).They form because the gas velocity at the interface to the bed represents an input rate that is larger than what can pass through the interstices with less frictional resistance than the bed weight. Therefore, holes are formed through whose porous surface the gas can enter the bed at the incipient fluidization velocity. This means that bubble formation is a way to increase the active interface between the gas and the particle bed. 

Catal3dic processes are carried out in several types of reactors like fixed bed, moving bed, trickle bed, two- and three phase fluidized beds, bubble columns, and stirred tanks. Examples of important fixed bed catal3dic processes with only one fluid phase are given in Table 11.1. Other fixed bed processes with particular catalyst designs exist as well. 

Contacting patterns between phases (e.g., packed bed, fluidized bed, bubble column)... 

It is reasonable to present the flow in disperse systems in a general way to avoid repetitions of this topic. Such disperse systems are gas or liquid fluidized beds, bubble or drop columns, and spray columns. In all cases the solid or fluid particles are suspended or moving due to the density difference A/ = p - p ) and the acceleration of gravity. In Fig. 3.5-1 a fixed bed on the left side and several fluidized beds with different flow patterns are depicted. The fluid flow density V , in the fluidized beds is greater than the minimum flow density V(,f necessary to achieve fluidization. The volumetric holdup of the continuous phase increases for Vj > Vjf with the fluid throughput. The relative velocity between the fluid and the suspended particles is inversely proportional to the volnmetric holdup Sg. With... 

The heat transfer coefficient between vertical heating and cooling surfaces and a dispersed system (gas and liquid fluidized beds, bubble and drop columns) has been determined experimentally. Often, a maximum heat transfer coefficient is found at a certain volume flux v, or of the continuous respectively the dispersed phase (Mersmann and Wunder 1978). Here, only information about this maximum value is provided. 

Two dimensional fluidized bed bubble fluidized bed Pressure fluidized bed Cylindrical reactor (butanol and ammonia)... 

As indicated above, it is detrimental to have bubbles in standpipes. For fluidized bed underflow standpipes with the standpipe entrance in the fluidized bed, bubbles can be sucked down the standpipe at its entrance if nothing is done to prevent this from occurring. This is especially true when the bed consists of Geldart group... 

Rowe PN and Everett DJ 1972 Fluidized bed bubbles viewed by X-rays, Trans. Instn Chem. Engrs,. 50,42-60... 

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