Cocurrent downflow circulating fluidized bed (downer) reactors — A state of the art review

Abstract

In the last decade, cocurrent downflow circulating fluidized bed reactors, called "downer" reactors, have been proposed as an alternative to upflow circulating fluidized bed, or "riser", reactors. In this paper, published results on downer studies are siimmarized and future directions of research are recommended. Downer reactors are shown to have several distinct advantages over upflow circulating fluidized bed reactors and can potentially be used in many industrial processes, mainly due to a more uniform gas and solids flow structure compared with risers.Depuis une dizaine d'annks, les rtacteurs B lit fluidist circulant B cwourant descendant ont ktk propos6s pour faire concurrence aux rhcteurs B lit fluidisk circulant ascendant. Dans cet article, on rtsume les rksultats publits des ttudes sur les rtacteurs de type descendant, et des directions sont recomrnand&s pour la recherche. On a trouvt que les rhcteurs descendants offraierit des avantages particuliers par rapport aux rhcteurs B lit fluidid circulant ascendant et qu'ils Ctaient potentiellement inttressants dans de nombreux praddts industriels, en raison principalement d'une structure d'kcoulement du liquide et des solides plus uniforme que dans le cas des rtacteurs ascendants.Keywords. downer reactor, co-current downflow, fluidized bed, short contact reactor. n the last two decades, considerable progress has been Nomenclature = particle diameter, m = axial gas dispersion coefficient, m2/s = axial solids dispersion coefficient, m2/s = radial gas dispersion coefficient, m2/s = radial solids dispersion coefficient, m2/s = functions of radial ratio, defined by Equations (10) = Froude number, [ =(Us-t~,)/(gd,,)~.~l = gravitational acceleration constant, m/s2 and (13), respectively

Fast drying of cut tobacco in drop tube reactor and its effect on petroleum ether tobacco extracts

“…When tobacco solids were transported in DTR, particles clustering were less obvious and the clusters are more unstable than those in the riser. [7,9] This means the solid particles could be dispersed more uniformly, and the uniform gas-solid flow structure of DTR is beneficial to heat and mass transfer. On the other hand, the superheated steam has high heat transfer coefficient when it is used as a drying medium.…”

“…When compared with the riser dryer with gas-solid concurrent upward flow, due to the uniform gas-solid flow structure of DTR as reported in many fluidization literatures, [6][7][8][9] the new type of dryer would provide the better product homogeneity. On the other hand, in the present work, the superheated steam as the drying medium showed a better effect on the retention of petroleum ether extract in tobacco than the conventional hot air drying.…”

“…[8] Over the last two decades, the gas-solid concurrent drop tube reactor (DTR) has been developed as a new type of fluidized reactors, which takes gas-solid concurrent with gravity field movement. [9] Compared with the riser dryer, the DTR has several distinct advantages, such as narrow residence time distribution for particulate materials, low energy consumption, better product homogeneity, and high particles loads. [7] In view of its advantages, the DTR has received wide attention in many industrial fields.…”

“…[10] For petroleum catalytic cracking and biomass pyrolysis, the DTR was used as pyrolysis reactors to improve product yields. [9,11] In addition, the DTR has also been proposed as a fluidized dryer. [8] However, the drying characteristics of particulate materials in a DTR, including drying kinetics and dried product quality, still requires further investigation and optimization.…”

Desarrollo de un micro-reactor de lecho transportado y su aplicación al estudio craqueo catalítico.

“…Uno de los problemas de los reactores de lecho ascendente es la importante retromezcla que occure con el sólido 48 , lo cual implica una amplia distribución del tiempo de residencia del catalizador dentro del reactor, lo cual disminuye la selectividad a los productos de mayor interés (gasolina y olefinas C 3 y C 4 ) en favor de gas seco y coque 49 .…”