Title: HEAT AND MASS TRANSFER TO A GAS-FLUIDIZED BED OF SOLID PARTICLES

Volume II describes the details of heat-transfer studies in a dry fluidized-bed system (called ''heat tray''), which has been proposed for heat recovery from hot gases and for heat management in exothermic reactions. In particular, this report presents the results of bench-scale and pilot-scale experimental studies which quantify heat transfer between a hot supernatant gas (S-gas) and a flowing shallow fluidized bed of solids. A fractional-factorial design of experiments has been performed on two heat-tray systems using three different solids. The results show that fine fluid cracking catalyst (FCC) particles out-perform larger alumina spheres as a fluidized solid. Heat transfermore » coefficients between the supernatant gas and the shallow fluidized bed approaches 440 W/m/sup 2/-K using FCC with a heat-exchange area of 0.124 m/sup 2/. Various S-gas inlet nozzle configurations have been studied, with a nozzle height equal to one-half of the static bed height (0.051 m) giving the best results. The study shows that short heat-tray lengths (< 0.8 m) are desirable and that S-gas redistributors are needed to compartmentalize the unit. An economic analysis shows that the proposed heat tray would be economically feasible for adaption as a boiler feedwater preheater in a small steam-generation facility, using boiler combustion gases as the S-gas. The payback time for the system would be as short as 1.9 years when used continuously. The heat transfer results from a supernatant gas to a flowing shallow fluidized bed represent the only data reported thus far, and have led to a better understanding of the heat management in the proposed ''heat-tray'' reactor for Fischer-Tropsch synthesis. 20 refs., 46 figs., 15 tabs.« less

BS>The problem of heat or mass transfer between the fluidizing agent and the solid particles is examined, both in the case of homogeneous fluidization and in that of nonhomogeneous fluidization. In the first case the experimental results are correlated, by using,the following equations Nu =0.426 Re/sup 0.30/ Ar/sup 0.17/ Pr/sup 1/3/ for Re Ar/sup -0.40/ <2.15 and Nu = 0.943 Re/sup -1/ Ar/ sup 0.69/ Pr/sup 1/3/ for Re Ar/sup -0.40/> 2.15, where Nu, Re, Ar, and Pr are Nusselt's, Reynolds', Archimedes', and Prandtl' s numbers. Nu, Re, and Ar numbers are computed with the diameter of the particle. Inmore » the second case, of non-homogeneous fluidization, the problem of the adequate definition of the transfer coefficient is discussed and the reason why small values are obtained for the transfer coefficients, such as they are defined in the literature, is explained. (auth)« less

The lack of reliable data on the fluidization and heat-transfer characteristics of low density particles in a fluidized bed has prompted an experimental and analytical investigation into this subject. Seven groups of particles ranging in diameter from 0.25 to 2.0 mm and density from 2.5 to 32 pcf have been successfully fluidized and shown to be generally well predicted by classical fluidization and fluidized-bed heat-transfer theory. Two other groups of particles, also in this approximate range of particle diameter and density, are, however, unable to be fluidized due to significant inter-particle and static electric attractions. Using the experimental data andmore » results as a basis of analysis, two application of low-density particle fluidization in a building efficient energy-management program are discussed. A fluidized bed can be incorporated into the wall cavity of a building for use as either a collector of solar energy or as a heat exchange medium in a building space heating/cooling program. As a solar collector, it is shown that the low density particle fluidized bed would thermally perform between comparable conventional liquid and air-cooled flat-plate solar collectors. It would require less water pumping power and plumbing than the liquid collector and less air pumping power than the air collector.« less

One of the largest challenges for 21st century is to fulfill global energy demand while also reducing detrimental impacts of energy generation and use on the environment. Gasification is a promising technology to meet the requirement of reduced emissions without compromising performance. Coal gasification is not an incinerating process; rather than burning coal completely a partial combustion takes place in the presence of steam and limited amounts of oxygen. In this controlled environment, a chemical reaction takes place to produce a mixture of clean synthetic gas. Gas-solid fluidized bed is one such type of gasification technology. During gasification, the mixingmore » behavior of solid (coal) and gas and their flow patterns can be very complicated to understand. Many attempts have taken place in laboratory scale to understand bed hydrodynamics with spherical particles though in actual applications with coal, the particles are non-spherical. This issue drove the documented attempt presented here to investigate fluidized bed behavior using different ranges of non-spherical particles, as well as spherical. For this investigation, various parameters are controlled that included particle size, bed height, bed diameter and particle shape. Particles ranged from 355 µm to 1180 µm, bed diameter varied from 2 cm to 7 cm, two fluidized beds with diameters of 3.4 cm and 12.4 cm, for the spherical and non-spherical shaped particles that were taken into consideration. Pressure drop was measured with increasing superficial gas velocity. The velocity required in order to start to fluidize the particle is called the minimum fluidization velocity, which is one of the most important parameters to design and optimize within a gas-solid fluidized bed. This minimum fluidization velocity was monitored during investigation while observing variables factors and their effect on this velocity. From our investigation, it has been found that minimum fluidization velocity is independent of bed height for both spherical and non-spherical particles. Further, it decrease with decreasing particle size and decreases with decreasing bed diameter. Shadow sizing, a non-intrusive imaging and diagnostic technology, was also used to visualize flow fields inside fluidized beds for both spherical and non- spherical particles and to detect the particle sizes.« less

Experimental data were obtained for the heat transfer coefficient between electrically-heated horizontal tube bundles and square fluidised beds of silica sand and alumina as a function of air fluidising velocity. Heat transfer data for a bundle of tubes are compared with a single tube under otherwise identical conditions, and are compared with existing correlations and theoretical models in the literature for maximum heat transfer coefficient. A correlation for the maximum heat transfer coefficient between horizontal tube bundles and a gas-solid fluidised bed of small particles is proposed which includes the influence of tube pitch in the bundle. This correlation willmore » be particularly useful in the design of a low-rank coal fluidised-bed combustor where crushed coal is burnt in an inert bed of silica sand or alumina of particle size less than 1 mm.« less