Bubbling dense beds

It is seen that for Geldart types A and B particles, fast fluidization requires superficial gas velocities approximately an order of magnitude greater than that for bubbling dense beds. In many applications of fast fluidization, the particles exiting top of the bed are captured by cyclones and recirculated for makeup injection at the bottom of the bed, hence this regime is also denoted as circulating fluidization, CFB. 

Although the individual bubble models are included in many texts and research papers, they are not reliable for predicting reactor performance under practical conditions. One problem is that the bubble size must be assumed to use the model, and it is hard to tell what size to choose. A further problem is that the interchange rate does not show the predicted dependence on diffusivity. Fontaine and Harriott [11] used frequency response tests to compare bubble-dense bed interchange rates for different tracers. At 0.11 and 0.18 m/sec, there was no difference between the results for He and CO2, in spite of the fourfold difference in diffusivity, and there was only a slight difference at 0.03 m/sec. DeVries and coworkers [12]... 

There is little information on the process of heat transfer between particles and gas in fast fluidized beds. One reason for this situation is the expectation that the heat transfer rates are high and therefore of little concern. The smaller concentration of particles (in comparison to bubbling dense beds) implies less intense heat source concentration in the cases of exothermic reactions, and this also alleviates potential concern. At the time of this writing, there is no established model or correlation for estimation of the heat transfer coefficient between particles and gas during fast fluidization. 

Optical probes were used to measure the bubble size, frequency and velocity within the dense bed. The bubble velocity for an actively bubbling bed was found to closely agree with the drift flux form proposed by Davidson and Harrison (1963). In contrast, the volumetric flow rate of the bubbles was found to be far less than that predicted by the two-phase hypothesis (Fig. 40). 

Bokkers, G. A., Laverman, J. A., Van Sint Annaland, M., and Kuipers, J. A. M., Modelling of large-scale dense gas-solid bubbling fluidised beds using a novel discrete bubble model, Chem. Eng. Sci. 61, 5590-5602 (2006). 

At higher gas velocities the BFB transforms into a TB—no distinct bubbles, much churning, and violent solid movement. The surface of the dense bed fades and solids are found increasingly in the lean region above the dense bed. 

The simplest model of a bubbling fluidized bed, with uniform bubbles exchanging matter with a dense phase of catalytic particles which promote a continuum of parallel first order reactions is considered. It is shown that the system behaves like a stirred tank with two feeds the one, direct at the inlet the other, distributed from the bubble train. The basic results can be extended to cases of catalyst replacement for a single reactant and to Astarita s uniform kinetics for the continuous mixture. 

On this view of the reactor it is not surprising to find Astarita and Occone s methods for uniform kinetics work for the bubbling fluid bed. Replacing kcp by kcp x, t) F[/ K(y)cp(y)dy]. Thus the balance over the dense phase is... 

Using the usual assumptions, we obtain the following dimensionless unsteady-state material and energy balance equations for the dense (emulsion) phase of the bubbling fluidized bed ... 

Dense-phase fluidized beds with bubbles represent the majority of the operating interests although the beds may also be operated without bubbles. The bubbling dense-phase fluidized bed behavior is fluidlike. The analogy between the bubble behavior in gas-solid fluidized beds and that in gas-liquid bubble columns is often applied. Dense-phase fluidized beds generally possess the following characteristics, which promote their use in reactor applications ... 

As more small bubbles/voids with low rise velocities and long residence times are present in the turbulent bed, they give rise to more significant dense bed expansion in the turbulent... 

Figure 36 is an illustration of a CFBC boiler retrofitted from a stoker-fired boiler. Also, at the bottom is a bubbling bed operated at a higher velocity of 3.5 m/s. A tube bundle is installed at an inclination of 15° in the dense bed, which extends to a height of 1.5 m from the air nozzles. A channel separator is also used, and the separated solid particles are returned through an air seal to the combustor. 

Adris et al. [1991] also determined that the reactor performance is weakly sensitive to the bubble size, bed porosity at minimum fluidization and flow distribution between bubble and dense phases. Furthermore, the bubbles which remove the products from the reaction mixture in the dense phase enhance the forward reaction and consequently breaks the barrier of the reaction equilibrium. 

The two-phase flow theory is adopted in the model it consists in two phases, a dense phase and a bubble phase separated by a film through which the mass transfer occurs. Gases move upward in both bubble and dense phase with plug flow, which proves to be adequate to describe the flow in a bubbling fluidized bed gasifier [17],... 

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