Circulating fluidized beds mass transfer

Subbarao, D. and Gambhir, S., Gas to particle mass transfer in risers, in "Proceedings of 7th International Circulating Fluidized Beds Conference", pp. 97-104, Canadian Society for Chemical Engineering, Niagara Falls. 

Zhang, N., EMMS-based Meso-Scale Mass Transfer Model and Its Application to Circulating Fluidized Bed Combustion Simulation, Ph.D. thesis (in Chinese), Institute of Process Engineering, Chinese Academy of Sciences, Beijing (2010). Zhang, J., Ge, W. and Li, J., Chem. Eng. Sci. 60(11), 3091-3099 (2005). 

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. 

Basu, P. and Nag, P. K. (1987). An Investigation into Heat Transfer in Circulating Fluidized Beds. Int. J. Heat Mass Transfer, 30,2399. 

The book is arranged in two parts Part I deals with basic relationships and phenomena, including particle size and properties, collision mechanics of solids, momentum transfer and charge transfer, heat and mass transfer, basic equations, and intrinsic phenomena in gas-solid flows. Part II discusses the characteristics of selected gas-solid flow systems such as gas-solid separators, hopper and standpipe flows, dense-phase fluidized beds, circulating fluidized beds, pneumatic conveying systems, and heat and mass transfer in fluidization systems. 

By means of a model reaction it has recently been proved that in many cases circulating fluidized-bed reactors cannot be characterized by solely considering mixing phenomena [111]. Instead, the presence of mass transfer limitations and bypassing was found to have a significant influence. In analogy to low-velocity fluidized beds a detailed description of the local flow structure within the reaction volume must serve as a basis for appropriate reactor modeling. 

A common feature of all models for the upper part of circulating fluidized beds is the description of the mass exchange between dense phase and dilute phase. Analogously to low-velocity fluidized beds, the product of the local specific mass-transfer area a and the mass-transfer coefficient k may be used for this purpose. Many different methods for determining values for these important variables have been reported, such as tracer gas backmixing experiments [112], non-steady-state tracer gas experiments [117], model reactions [115], and theoretical calculations [114],... 

Andersson B.-A. Leckner B. (1992) Experimental methods of estimating heat transfer in circulating fluidized beds. Int. J. Heat/Mass Transfer, 35, 3353-3362. 

Vollert, J. and Werther, J Mass Transfer and Reaction Behaviour of a Circulating Fluidized Bed Reactor , Chem. Eng. TechnoL, 17, 201(1994). 

Breault RW A review of gas-sohd dispersion and mass transfer coefficient correlations in circulating fluidized beds. Powder Technol 163 9-17, 2006. 

Lints M. Particle to wall heat transfer in circulating fluidized beds. PhD dissertation. Mass Inst Tech Cambridge, MA, 1992. 

Bolland O, Nicolai R. Describing mass transfer in circulating fluidized beds by ozone decomposition, AIChE Symp Ser 95(321) 52-60, 1999. 

Li J, Zhang X, Zhu J, Li J. Effects of cluster behavior on gas solid mass transfer in circulating fluidized beds. In Fan LS, Knowlton TM, eds. Fluidization IV. New York Engineering Foundation, 1998, pp 405-412. 

Lockhart C, Zhu J, Brereton CMH, Lim CJ, Grace JR. Local heat transfer, solid concentration and erosion around membrane tubes in a cold model circulating fluidized bed. Int J Heat Mass Transfer, 38 2403 2410,1995. 

Schlichthaerle P, Hartge EU, Werther J. Interphase mass transfer and gas mixing in the bottom zone of a circulating fluidized bed. In Kwauk M, Li J, Yang WC, eds. Fluidization X. New York Engineering Foundation, 2001, pp 549-556. 

Vollert J, Werther J. Mass transfer and reaction behavior of a circulating fluidized bed reactor. Chem Eng Technol 17 201-209, 1994. 

Werther J, Hartge EU, Kruse M. Gas mixing and interphase mass transfer in the circulating fluidized bed. In Potter OE, Nicklin DJ, eds. Fluidization VII. New York Engineering Foundation, 1992, pp 257-264. 

Xie D, Bowen BD, Grace JR, Lim CJ. Two-dimensional model of heat transfer in circulating fluidized beds. Intern J Heat Mass Transf. In press, 2003. 

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