Contacting turbulent fluidization

Turbulent Fluidized Bed B Counter-cnrrent contacting is beneficial... 

Haider et al. [2] presented a theoretical model of mass transfer rate in a turbulent fluidized bed. The model considers a turbulent fluidized bed to consist of clusters of bed particles or solid agglomerates and voids [4]. Bubbles are absent, and gas forms the continuous phase. An active particle within the bed will come into contact with both the clusters of bed particles and voids. Therefore, mass transfer to or from an active particle is made up of two components ... 

Fig. 10.7 Types of fluidized bed reactors [43], Reprinted with permission from the authors. The primary fluidized bed reactor types 1 bubbling fluidized bed, 2 turbulent fluidized bed, 3 circulating fluidized bed, 4 riser, 5 drowner, 6 cross-current fluid bed, 7 counter-current fluid, 8 spouted fluidized bed, 9 floating fluidized bed, 10 twin fluidized bed. The key issues leading to re-design of the conventional bubbling fluid bed A higher gas velocity, B counter-current contacting is beneficial, C incompatible differences in desired environment, D dusty environment, E large paiticles/low gas load...

Farag et al. [65] adapted a pair of the reflective fibre optic sensors, originally developed by Sobocinski et al. [185], for measuring fluid dynamic properties (e.g., bubble fraction and bubble frequency) in a pilot scale turbulent fluidized bed. Each of the probes consisted of one emitter and two receivers. A pair of reflective fibre optic sensors, 3 mm apart, were used to measure fluid dynamic properties. The bubble fractions, (3, was defined as the cumulative contact times Xm divided by the total sampling time T ... 

The term three-phase fluidization requires some explanation, as it can be used to describe a variety of rather different operations. The three phases are gas, liquid and particulate solids, although other variations such as two immiscible liquids and particulate solids may exist in special applications. As in the case of a fixed-bed operation, both co-current and counter- current gas-liquid flow are permissible and, for each of these, both bubble flow, in which the liquid is the continuous phase and the gas dispersed, and trickle flow, in which the gas forms a continuous phase and the liquid is more or less dispersed, takes place. A well established device for countercurrent trickle flow, in which low-density solid spheres are fluidized by an upward current of gas and irrigated by a downward flow of liquid, is variously known as the turbulent bed, mobile bed and fluidized packing contactor, or the turbulent contact absorber when it is specifically used for gas absorption and/or dust removal. Still another variation is a three-phase spouted bed contactor. 

Technology Description Fluidized bed incinerators utilize a very turbulent bed of inert granular material (usually sand) to improve the transfer of heat to the waste streams to be incinerated. Air is blown through the granular bed materials until they are "suspended" and able to move and mix in a manner similar to a fluid, i.e., they are "fluidized".In this manner, the heated bed particles come in intimate contact with the wastes being burned. The process requires that the waste be fed into multiple injection ports for successful treatment. Advantages... 

At gas velocities higher than those used for BFBs we successively enter the turbulent (TB), fast fluidized (FF), and the pneumatic conveying (PC) regimes. In these contacting regimes solids are entrained out of the bed and must be replaced. Thus in continuous operations we have the CFB, shown in Fig. 20.1. Flow models are very sketchy for these flow regimes. Let us see what is known. 

Fluidization is the operation by which fine solids are transformed into a fiuidlike state through contact with a gas. At certain gas velocities, the fluid will support the particles, giving them freedom of mobility without entrainment. Such a fluidized bed resembles a vigorously boiling fluid with solid particles undergoing extremely turbulent motion, which increases with gas velocity. 

A fluidized bed is made up of a mass of particles buoyed up out of permanent contact with each other by a flowing fluid. Turbulent activity in such a bed promotes high rates of heat and mass transfer and uniformity of temperature and composition throughout. The basic system includes a solids feeding device, the fluidizing chamber with a perforated distributing plate for the gas, an overflow duct for removal of the dry product, a cyclone and other equipment for... 

The effectiveness of the gas-solid mass transfer in a circulating fluidized bed (see Chapter 10) can be reflected by the contact efficiency, which is a measure of the extent to which the particles are exposed to the gas stream. As noted in Chapter 10, fine particles tend to form clusters, which yield contact resistance of the main gas stream with inner particles in the cluster. The contact efficiency was evaluated by using hot gas as a tracer [Dry et al., 1987] and using the ozone decomposition reaction with iron oxide catalyst as particles [Jiang etal., 1991], It was found that the contact efficiency decreases as the particle concentration in the bed increases. At lower gas velocities, the contact efficiency is lower as a result of lower turbulence levels, allowing a greater extent of aggregate formation. The contact efficiency increases with the gas velocity, but the rate of increase falls with the gas velocity. 

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