Chemical Engineering Science 57 (2002) 3851–3859 www.elsevier.com/locate/ces Masstransferinrollingrotarykilns:anovelapproach M.D.Heydenrych a ; * ,P.Gree a ,A.B.M.Heesink b ,G.F.Versteeg b a Department of Chemical Engineering, University of Pretoria, Pretoria, South Africa b Department of Chemical Technology, University of Twente, Enschede, The Netherlands Received 29 November 2001; received in revised form 3 June 2002; accepted 23 June 2002 Abstract Anovelapproachtomodelingmasstransferinrotarykilnsorrotatingcylindersisexplored.Themovementofgasintheinterparticle voidsinthebedofthekilnisconsidered,whereparticlesmoveconcentricallywiththegeometryofthekilnandgasisentrainedbythese particles. The approach considers a dierential section along the length of a rotary kiln where the gas concentration in the freeboard is assumedtobeuniforminthatsection.Areactormodellingapproachhasbeenusedtoderiveeectivenessfactorsforthebedasafunction ofbedll,reactionkineticsandrotationspeed.Inmanycases,theentrainedgasbecomesdepletedwithinthebed,leadingtoasimplied modelforthebedeectivenessfactor.Experimentaldataconrmsthevalidityofthismodelforslowerrates.Atfasterrates,masstransfer canbemuchhigherthanthemodelpredicts,indicatingthatothermechanisms,suchasdispersionordiusionarealsoimportantinthese conditions. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: Rotating drum; Reaction engineering; Drying; Convective transport; Granular materials; Passive layer 1. Introduction Rotary kilns are used industrially in many applications suchasdrying,incineration,mixing,pre-heating,humidi- cation,calcining,reducing,sinteringandgas–solidreactions (Barr, Brimacombe, & Watkinson, 1989; Jauhari, Gray & Masliya,1998). Formanyrotarykilns,heattransferisthelimitingfactor, both in the heating section of the rotary kiln, and in the reaction zone (Barr et al., 1989, Tscheng and Watkinson, 1979).Consequently,mostfocusintheliteraturehasbeen directedatunderstandingtheheattransferprocessesinrotary kilns.Itisimportantthoughtounderstandalloftheprocesses thatoccurinrotarykilnsonafundamentallevelbeforerotary kilnscanbedesignedandoperatedoptimally. With physical processes like drying and humidication for example, mass transfer is also important, as well as in gas–solidreactingsystemswithhighspecicreactionrates suchasincineration.Inthiswork,wewillexaminethemass transferinrollingrotarykilnsandproposeanovelapproach ∗ Corresponding author. Tel.: +27-12-420-2199; fax: +27-12-660-1518. E-mail address: mike@heydenrych.info (M. D. Heydenrych). todescribethephenomenathatdeterminetherateofmass transfer. 2. Modelling studies The rst published experimental studies on rotary kilns recorded the relationship of rotation speed and kiln incli- nation on bed depth and solids residence time (Sullivan, Maier,Ralston,1927).Amodelwaslaterdevelopedbased ontheassumptionthatparticlesinarollingbedmoveina circular motion with the rotation of the kiln, and then fall downthesurfaceofthebedinathinlayer(Saeman,1951). Thetimetakentofalldownthesurfacewasassumedtobe smallcomparedtothetimeforaparticletomovewiththe kilnfromthebottomhalftothetophalfofthebed.Using the geometry of an inclined rotary kiln, the angle of incli- nationnecessarytomaintainaconstantbedheightoverthe length of the rotary kiln could be determined for a given rotationspeed.Thisbasicmodelpredictedtheoriginaldata (Sullivan et al., 1927)well,andthemodelwasfurtherre- ned to predict axial movement of particles with dierent bed lls, taking into account the time for particles to fall downthesurfaceofthebed(Kramers&Crookewit,1952). In later work that specically measured the movement of 0009-2509/02/$-see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII:S0009-2509(02)00312-3