Reactor circulating fluidized bed

A circulating fluidized-bed (CFB) reactor is very similar to a bubbling fluidized-bed reactor. The major difference is the gas velocity. If the gas velocity of a bubbling fluidized bed increases, at some point the force exerted mi the particle will be enough to move it upwards, carrying it outside of the bed. This situation is called elutriation. 

In a CFB, elutriation carries both sand and char particles outside of the bed. Once char and sand are separated from the main gas stream in the cyclone, the char is combusted to provide additional energy for the sand. The hot sand is then recirculated 

As with bubbling fluidized beds, CFBs enjoy good temperature control and uniformity of conditions, as well as easy scale-up. The integrated system provides a way to save energy, but the system is also more complex and requires careful control in order to function properly. 

Because of the control requirements, CFBs are typically only used in commercial application. Lab-scale fluidized beds are almost always of the bubbling type. 

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A new process for the partial oxidation of n-butane to maleic anhydride was developed by DuPont. The important feature of this process is the use of a circulating fluidized bed-reactor. Solids flux in the rizer-reactor is high and the superficial gas velocities are also high, which encounters short residence times usually in seconds. The developed catalyst for this process is based on vanadium phosphorous oxides... 

Dessalces, G., Kolenda, F., andReymond, J. P., Attrition Evaluation for Catalysts used in Fluidized or Circulating Fluidized Bed Reactors, AIChE Preprints of the First Int. Particle Technol. Forum, II 190, Denver, Colorado (1994)... 

Patience, G. S., and Mills, P. L., Modelling of Propylene Oxidation in a Circulating Fluidized-Bed Reactor, New Developments in Selective Oxidation II, p. 1 (1994)... 

Marschall KJ, Mleczko L. CFD modeling of an internally circulating fluidized-bed reactor. Chem Eng Sci 1999 54 2085-2093. 

A New Process for n-Butane Oxidation to Maleic Anhydride Using a Circulating Fluidized Bed Reactor. In Circulating Fluidized Bed Technology TV. Ed. A. A. Avidan. New York AlChE Publications. 

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 typical example of a circulating fluidized-bed reactor model has been presented by Schocnfeldcr et al. [112, 116]. Its structure shown in Fig. 21 is based on the definition of four axial zones. Above the bottom zone the splash zone is located. It yields a mixing condition to link the bottom zone with the upper part of the reactor. Due to the internal backmixing this upper section is referred to as the recirculation zone. At the riser outlet, an exit zone on top of the recirculation zone yields a second boundary condition. Here, complete mixing is assumed. 

Figure 21. The four-stage model of the circulating fluidized-bed reactor (adapted from Ref. 116).

Shingles, T., Jones, D. H. The development of Synthol circulating fluidized bed reactors, ChemSA 12, 179-182 (1986). 

Engstrom, Folke. Apparatus for separating solid material from flue gases in a circulating fluidized bed reactor, CA 1261122. 

Engstrom, Folke. Method and apparatus for separating solid particles from flue gases in a circulating fluidized bed reactor, PATENT PCT International WO 8603986 A1 DATE 860717, APPLICATION WO 86FI2 (860109) FI 85134 (850111), 11 pp. (1986). 

Gianetto, A., Pagliolico, S., Rover, G., and Ruggeri, B. Theoretical and Practical Aspects of Circulating Fluidized Bed Reactor (CFERs) for Complex Chemical Systems, Chem. Eng. Sci., Vol. 45, pp. 2219-2225 (1990). 

Jiang, Peijun, Bi, Hsiaotao, Jean, Rong Her, and Fan, Liang Shih. Baffle effects on performance of catalytic circulating fluidized bed reactor, AIChE J., vol. 37, No. 9, pp. 1392-1400(1991). 

Kaczmarzyk, Grzegorz and Sciazko, Marek. Coal pyrolysis in a circulating fluidized-bed reactor for hybrid gas-steam power plants, Koks, Smola, Gas, vol. 35, No. 8, pp. 175-84 (1990). 

Shingles, T., and Silverman, R. W. Determination of Standpipe Pressure Profiles and Slide Valve Office Discharge Coefficients on Synthol Circulating Fluidized-Bed Reactors. Powder Technology, vol. 47, No. 2, p. 129 (1986). 

Weiss, V., and Fett, F. N. Modelling the decomposition of solidum bicarbonate in a circulating fluidized bed reactor, in Circulating Fluidized Bed Technology (Basu, P., ed.), pp. 167-172. Pergamon Press, Canada (1986). 

Weiss, V., Fett, F. N., Helmrich, H., and Janssen, K. Mathematical modelling of circulating fluidized bed reactors by reference to a solids decomposition reaction and coal combustion, Chem. Eng. Process, vol. 22, No. 2, pp. 79-90 (1987). 

Yamaoka, Hideyuki. Flow characteristics of gas and powders at circulating fluidized bed reactor," Tetsu to Hagane, vol. 75, No. 3, pp. 424-431 (1989). 

Ntouros, Z.. Karas, E., and Papageorgiou, N. Experimental investigation of coal combustion in the circulating fluidized bed reactor of NTUA, Proceedings of International Symposium on Coal Combustion Science and Technology, China Machine Press, Beijing, pp. 381-388 (1991). 

Akzo Nobel is an important producer of chlorine, vinyl chloride and PVC, strongly interested in a process for feedstock recycling of MPW containing PVC. Since 1994 they have chosen a fast pyrolysis process in a circulating fluidized-bed reactor, based on the technique developed by the Battelle Memorial Institute (CO, USA) for biomass gasification. 

The data given in this problem are based on kinetics data given in H. Schoe-nfelder, I. Hinderer, J. Weither, F.J. Keil, Methanol to olefins—prediction of the performance of a circulating fluidized-bed reactor on the basis of kinetic experiments in a fixed-b reactor, Chem. Eng. Set. 49, 5377 (1994).]... 

This demonstration plant has been installed at the Zeltweg power plant operated by DRAUKRAFT (15). The BIOCOMB process is designed for preparation of biofuels for co-combustion by partial gasification and attrition due to mechanical and thermal stress in a circulating fluidized bed reactor (CFB) (Fig. 2). 

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