Fluidized beds conduction heat transfer

Fundamental models correctly predict that for Group A particles, the conductive heat transfer is much greater than the convective heat transfer. For Group B and D particles, the gas convective heat transfer predominates as the particle surface area decreases. Figure 11 demonstrates how heat transfer varies with pressure and velocity for the different types of particles (23). As superficial velocity increases, there is a sudden jump in the heat-transfer coefficient as gas velocity exceeds and the bed becomes fluidized. 

Chen, J. C., Chen, K. L., Analysis of Simultaneous Radiative and Conductive Heat Transfer in Fluidized Beds, Chem. Eng. Commun., 9 255-271 (1981)... 

It is seen that the Nusselt numbers for BFBs fall below those for convection from a single sphere, for Reynolds numbers less than 20. In fact, the magnitude of Nup for fluidized beds drops below the value of 2.0, which represents the lower limit of conduction heat transfer. The cause of this is the bubbling phenomenon. Low Reynolds numbers correspond to beds of fine particles (small flip and C/g), wherein bubbles tend to be clouded with entrained particles. This diminishes the efficiency of particle-gas contact below that represented by idealized plug flow, resulting in reduced values of Nup. As particle diameter increases (coarse particle beds), bubbles are relatively cloudless and gas particle contact improves. This is shown in Fig. 2 where the Nusselt numbers of fluidized beds are seen to increase with... 

Chen JC, Chen KL. Analysis of simultaneous radiative and conductive heat transfer in fluidized beds. Chem Eng Commun 9 255 271, 1981. 

The fluidized bed reactor is about 60 years old, but only in recent years has its apphcation to coal combustion taken on commercial significance. The fluidized bed is the dispersion of a solid, usually in powder form, by a gas, under flow conditions such that the solid takes on the properties of a gas. Such reactors can be designed to operate continuously. Thermal conduction (heat transfer) in such systems can be high, and, as a result, in the case of coal and air, the combustion can occur at much lower temperatures than in the fixed bed system. Thus, the addition of limestone (CaCOs) or dolomite (CaCOs MgCOs) to the fluidized bed system can result in the reduction of oxides of sulfur and oxides of nitrogen. 

Conduction heat transfer always occurs through the contacted area between particles or between particles and wall. Generally, such conduction heat transfer due to elastic deformation includes two mechanisms conduction due to particle—particle static contact (particularly common in a packed bed), and conduction due to particle—particle collision (often occurs in a moving or fluidized bed). 

The heat transfer between an immersed tube and a fluidized bed depends on many factors, such as the contacts of particles with the tube, porosity, and gas flow around the tube. These factors are affected by many variables related to operational conditions. Gas velocity is one of the most important parameters in affecting the heat transfer, which can be seen in Fig. 23. With the increase of from 0.08 to 0.50 m/s, the overall FITC increases. However, when the exc is further increased from 0.50 to 0.80 m/s, the HTC decreases. The effect of particle thermal conductivity on the local HTC was also examined and shown in Fig. 25A (Hou et al., 2012b). The local HTC increases with the increase of from 1.10 to 100 W/(mK). However, such an increase is not significant for from 100 to 300 W/(m K). The variation of percentages of different heat transfer modes with ks is further shown in Fig. 25B. When is lower than 100 W/ (m K), the conductive heat transfer increases with the increase offej, while the convective heat transfer decreases. Further increase of fej has no significant effects. 

The combined CFD-DEM approach, incorporated with heat transfer models of convection, conduction, and radiation, has been developed and can be used in the study of heat transfer in packed and fluidized beds. It has various advantages over the conventional experimental techniques and continuum simulation approaches. For example, the detailed conductive heat transfer between particles can be examined and the factors such as local particle-fluid structure and materials properties in determining heat transfer can be investigated. 

Measurements of heat-transfer coefficients and effective thermal conductivity for gas-liquid fluidized beds have also been carried out by Manchanda (M2). 

We studied the polyamidation of nylon 4,6, and varied the reaction time, reaction temperature, partical size, starting molecular weight, and type of reactor gas. At the same time we looked at the molecular weight broadening and the degradation with colour formation. In order to have good heat and mass transfer the reactions were mainly conducted on fine powder in a fluidized bed reactor and with dry nitrogen as carrier gas. 

The concept of a fluidized bed consisting of electrically conducting particles as a statistically continuous electrode was first discussed by Le Goff et al. (Lie). Interesting similarities with heat-transfer studies in fluidized beds may be exploited to advantage by use of the limiting current method. 

In contrast to the strong effect of gas properties, it has been found that the thermal properties of the solid particles have relatively small effect on the heat transfer coefficient in bubbling fluidized beds. This appears to be counter-intuitive since much of the thermal transport process at the submerged heat transfer surface is presumed to be associated with contact between solid particles and the heat transfer surface. Nevertheless, experimental measurements such as those of Ziegler et al. (1964) indicate that the heat transfer coefficient was essentially independent of particle thermal conductivity and varied only mildly with particle heat capacity. These investigators measured heat transfer coefficients in bubbling fluidized beds of different metallic particles which had essentially the same solid density but varied in thermal conductivity by a factor of nine and in heat capacity by a factor of two. 

These authors numerically solved the system of equations with appropriate boundary conditions to derive the time-averaged radiant and conductive heat fluxes between the fluidized bed and the heat transfer surface. Using... 

Steinfeld et al. [133] demonstrated the technical feasibility of solar decomposition of methane using a reactor with a fluidized bed of catalyst particulates. Experimentation was conducted at the Paul Scherrer Institute (PSI, Switzerland) solar furnace delivering up to 15 kW with a peak concentration ratio of 3500 sun. A quartz reactor (diameter 2 cm) with a fluidized bed of Ni (90%)/Al2O3 catalyst and alumina grains was positioned in the focus of the solar furnace. The direct irradiation of the catalyst provided effective heat transfer to the reaction zone. The temperature was maintained below 577°C to prevent rapid deactivation of the catalyst. The outlet gas composition corresponded to 40% conversion of methane to H2 in a single pass. Concentrated solar radiation was used as a source of high-temperature process heat for the production of hydrogen and filamentous... 

SPHER [Shell Pellet Heat Exchange Retorting] A process for extracting oil from shale. The process is conducted in a fluidized bed in which heat is transferred by inert pellets of two... 

Temperature in a fluidized bed is uniform unless particle circulation is impeded. Gas to particle heat flow is so rapid that it is a minor consideration. Heat transfer at points of contact of particles is negligible and radiative transfer also is small below 600°C. The mechanisms of heat transfer and thermal conductivity have been widely studied the results and literature are reviewed, for example, by Zabrodsky (1966) and by Grace (1982, pp. 8.65-8.83). 

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