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
