Chemical Engineering Science 55 (2000) 3515}3527 Gas #ow maldistribution in moving beds of monosized particles W. R. Paterson, E. L. Berresford, D. L. Moppett, D. M. Scott*, V. K. Simmons, R. B. Thorpe Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA, UK Received 19 March 1999; received in revised form 6 December 1999; accepted 13 December 1999 Abstract This work is concerned with the structure of moving beds, in particular the steady-state dense phase downwards #ow of monosized particles in a vertical cylindrical vessel. The structure of a moving bed is probed using gas #ows through a &frozen' bed, which is a moving bed which has been carefully stopped to preserve its structure. The gas #ows are exploited in two ways, (i) by injecting a pulse of CO  tracer into a steady inlet #ow of air, and measuring its concentration in the outlet air, thus measuring the gas residence time distribution (RTD), and (ii) by injecting a pulse of CO  at a particular position in the bed, and measuring its concentration at a position vertically above the injection point, thus measuring the gas velocity at a particular radial position in the bed; a radial traverse then yields a radial pro"le of tracer gas velocity. The concentration of the CO  tracer is measured using a mass spectrometer. Separate experiments using glass spheres of diameter 1, 3 and 5 mm in a bed of diameter 0.14 m show signi"cant gas #ow maldistribution, in both RTD and radial pro"le of tracer gas velocity. The level of maldistribution depends crucially on the frictional properties of the wall of the vessel. When the wall of the vessel is smooth, the RTD has the Gaussian shape characteristic of axially dispersed plug #ow, and the radial pro"le of tracer gas velocity is #at; when the wall of the vessel is rough, the RTD is wide and asymmetric, and the gas #ow maldistribution is shown directly in the radial pro"le of tracer gas velocity. Limited results are also presented for polypropylene spheres and cylinders, and steel spheres. The measured RTD, with maldistribution, is successfully described by modelling the gas #ow using axially dispersed plug #ow with no radial mixing but taking into account the measured gas velocity variation over a bed cross section, together with end zones each consisting of a well-mixed volume and a dead vol- ume.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Moving bed; Packed bed; Granular materials; Particulate processes; Residence time distribution; Maldistribution 1. Introduction A moving bed is a device in which granular solids, moving vertically downwards relative to the walls of the containing vessel under the in#uence of gravity, are con- tacted by a #uid stream #owing co- or counter-currently. Moving beds are widely used in industry in applications such as continuous catalyst regeneration reactors, blast furnaces and gasi"ers. The transport properties in mov- ing beds are commonly modelled by applying conven- tional "xed-bed correlations, because the relative velocity of the #uid to the particles is well below the #uidizing velocity. In a well-compacted "xed bed, the solids are * Corresponding author. Tel.: #1223-334782; fax: #1223-334796. E-mail address: dms1@cheng.cam.ac.uk (D. M. Scott)  On leave of absence July '98}July '99 at Department of Chemical Engineering, Monash University, Clayton, Victoria 3168, Australia. stationary and the #ow of #uid is close to plug #ow. However, previous workers (Norton, 1946; Bower & Reintjes, 1961; Young & Barnard, 1962; Norgate, Batterham, Thurlby & Povey, 1982) have found that the gas-to-solid heat transfer coe$cients (htcs) in industrial moving beds are an order of magnitude smaller than those predicted from "xed-bed correlations. Paterson, Hart and Scott (1991) performed laboratory scale heat transfer experiments on moving and "xed beds to determine gas-to-solid htcs, and found a dis- parity similar to that measured on the industrial scale. The gas-to-solid htcs for moving beds were found to be a factor 10}15 times lower than those found for the equivalent "xed beds, that is those of the same mean voidages and operating at the same particle- based Reynolds number. The htcs are controlled by the development of the #uid boundary layer around the solids, and it is inconceivable that the micro-scale #uid dynamics is sensitive to the particle velocities of 1 or 0009-2509/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 5 9 8 - 9