Spout fluidized

Vahlas C, Juarez F, et al (2002) Fluidization, spouting, and metal-organic CVD of platinum group metals on powders. Chemical Vapor Deposition 8(4), 127-144... 

For solids discharge of the fluidized spout, the interconnecting aperture represents the main resistance to solids flow. By treating the solids emulsion as an inviscid fluid, as proposed by Jones and Davidson (1965) and recommended by Leung (1980), the solids discharge is likened to the efflux... 

Determination of the dimensions of the fluidized spout is based on operation experience. According to Li et al. (1982), the cone angle, 6, for smooth fluidized flow in the conical section may be taken as 6 = 10°. The minimum thickness of the spout is set by the requirement that the spout base area should not be less than the cross-sectional area of the interconnecting aperture. The diameter of the dipleg should be greater than four times the equivalent diameter of the aperture. These will ensure an aperture restriction capable of controlling solids flow through the valve. 

By comparison with the fluidized spout, the nonfluidized spout has a number of advantages such as continual and stable operation, reduced gas bypassing, improved sealing ability, and high discharge rate. 

Porous or particulate fixed bed electrodes have been recognized as the more efficient three-dimensional cathodes used for metal electrodeposition from dilute solutions, mainly due to their uniform effective conductivity of the solid phase. However, electrode clogging due to metal electrodeposition restricts their use as only for very dilute solutimis in which the long operational time would justUy their use [11, 12]. In order to overcome this Umitatimi and make the electrochemical process continuous, mobile electrodes, such as fluidized, spouted, and inclined bed electrodes, were proposed [13-16]. In these electrodes, the conductivity of the... 

Grace and Lim (1987) proposed a simple criterion based on stable spouting correlation by Chandnani and Epstein (1986), as shown in Eq. (154), as a neees-sary but not sufficient condition for permanent flame jet formation in fluidized, spout-fluid, and spouted beds. They also observed that inereasing temperature has a significant destabilizing influence on jet formation, and that a lower ratio of orifiee and particle diameter is required for permanent jet formation at high temperatures. Larger auxiliary gas flow can also lead to unstable jets and absence of permanent flame jets. For jets other than in the vertical direction, the formation of permanent flame jets is less likely. 

Group D particles are large, on the order of 1 or more millimeters (1000 fim) in average particle size. In a fluidized bed, they behave similarly to Group B particles. Because of the high gas velocities required to fluidize Group D particles, it is often more economical to process these particles in spouted or in moving beds, where lower gas rates suffice. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

Chisso-Asahi uses a spouted bed process for the production of their coated materials (12). A 12,000 t/yr faciHty is located in Japan. The semicontinuous process consists of two batch fluid-bed coaters. A dilute polymer solution is prepared by dissolving 5% polymer and release controlling agent into a chlorinated hydrocarbon solvent such as trichloroethylene. The solution is metered into the spouted bed where it is appHed to the fertilizer core. Hot air, used to fluidize the granules, evaporates the solvent which is recovered and reintroduced into the process. Mineral talc, when used, is either slurried into the polymer solution or introduced into the fluidizing air. 

Spouted beds are used for coarse particles that do not fluidize well. A single, high velocity gas jet is introduced under the center of a static particulate bed. This jet entrains and conveys a stream of particles up through the bed into the vessel freeboard where the jet expands, loses velocity, and allows the particles to be disentrained. The particles fall back into the bed and gradually move downward with the peripheral mass until reentrained. Particle-gas mixing is less uniform than in a fluid bed. 

Spouted Beds The spoutedT)ed technique was developed primarily for sohds which are too coarse to be handled in fluidized beds. Although their applications overlap, the methods of gas-sohds mixing... 

Employing wood chips, Cowan s drying studies indicated that the volumetric heat-transfer coefficient obtainable in a spouted bed is at least twice that in a direct-heat rotaiy diyer. By using 20- to 30-mesh Ottawa sand, fluidized and spouted beds were compared. The volumetric coefficients in the fluid bed were 4 times those obtained in a spouted bed. Mathur dried wheat continuously in a 12-in-diameter spouted bed, followed by a 9-in-diameter spouted-bed cooler. A diy-ing rate of roughly 100 Ib/h of water was obtained by using 450 K inlet air. Six hundred pounds per hour of wheat was reduced from 16 to 26 percent to 4 percent moisture. Evaporation occurred also in the cooler by using sensible heat present in the wheat. The maximum diy-ing-bed temperature was 118°F, and the overall thermal efficiency of the system was roughly 65 percent. Some aspec ts of the spouted-bed technique are covered by patent (U.S. Patent 2,786,280). 

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. 

Figure 11.10(b) can be modeled as a piston flow reactor with recycle. The fluid mechanics of spouting have been examined in detail so that model variables such as pressure drop, gas recycle rate, and solids circulation rate can be estimated. Spouted-bed reactors use relatively large particles. Particles of 1 mm (1000 pm) are typical, compared with 40-100 pm for most fluidizable catalysts. 

The spouting regime of fluidization is used for the fluid-solid noncatalytic reactions, especially drying and combustion. 

Like enzymes, whole cells are sometime immobilized by attachment to a surface or by entrapment within a carrier material. One motivation for this is similar to the motivation for using biomass recycle in a continuous process. The cells are grown under optimal conditions for cell growth but are used at conditions optimized for transformation of substrate. A great variety of reactor types have been proposed including packed beds, fluidized and spouted beds, and air-lift reactors. A semicommercial process for beer used an air-lift reactor to achieve reaction times of 1 day compared with 5-7 days for the normal batch process. Unfortunately, the beer suffered from a mismatched flavour profile that was attributed to mass transfer limitations. 

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