MICROSTRUCTURAL ASPECTS OF THE BEHAVIOUR OF CIRCULATING FLUIDIZED BEDS C. M. H. BRERETON and J. FL GRACE Department olChetttic*ll Enginwring, University of Btitish Columbia, Vancouver, Canada V6T 124 (First zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA r eceived 18 Sept ember 1991; bx ept eif w publication in rroiwd zyxwvutsrqponmlkjihgfedcbaZYXWVU form 16 June 1992) Ahsmct--Local instantaneous and tiime-averaging suspension densities were determined in a K2mm &meter by 9.3 m tall circulating fluidiaed-bed riser using anedlc cawtitanat pmbt ihstrtcd fiorn the side. Radial profile wete obtainad zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA at three different heighls and for thw di&rent solids circulation fluxes for sand particles 01 mean diameter 148 pn. The results confinn that &Cal densities a~ vtec near the wall than in the interior of the column The tit-fluidization regime is shown to consist of a developing flow with tren&r or pclrticlrxto and from the wall n@on. An intermittmy index, which would be Z.CD for perfect care-annulus flow and one for perfect cluster flow, is wed to characterize the plow; it shows that the behaviour is always between these two limits but tending towards the former with increasing height up the column. Circulating fluid&d beds have a number of attractive F&L~UES which lend themselves te processes where gas-solids Eontacting is important. Their ability to accommodate widely differing panicdate materials with high gas throughputs per unit cross-section, temperature uniformity, effective gas-solids contact- in& and controllability by varying the external recycle rate of partic)= are advantages which have been expIoited in catalytic presses like fluid cataiytic cracking and Fischer-Tropsch synthesis, and in gas-solid reactions like calcination and combustion. Despite their advantages and a history of several decades, there is surprisingly little understanding of the dynamics of circulating fiuidjzed beds. Although they must incorporate gas separation devices (e.g. cyclone, inertial separator) and solids return systems (e.g. loop seal, L-valve, &&valve system). it is the riser which provides most of the gas-solid contacting and which is subj~t to the greatest uncctiainty. At superficial gas velocities of interest (roughly 3-l S m/s) and at the solids fluxes encountered (typi- cally 20-300 kg/m’s for gas-solid reactions such as combustion and an order of magnitude or more higher for finer particle catalytic processes operatd with slide valves), the riser ope.ra& in the fast-fluid- ization hydrodynamic regime. En this regime, there is no distinct “bed” surface; instead, one finds a relat- ively dense suspension of solid particles where thm is considerable local dowaflow of the particles, as well as the predominant upflow. Both particle clusters and downflow of solids in an annular zone along the inside wall of the reactor have been identified as being important in maintaining the high concentration of particles within the riser, but there is limited under- standing of which of these is more important or how the cluster or annular sheets form and interact with other parts of the flow. In this paper we report the measurements of local time-varying voidage in a cold-model circulating fluidized-bed riser. The measurements, obtained using a capacitance probe, lead to an improved understand- ing of the relative importance of clusters and core-annular segregation, thereby improving the un- derstanding of the structure and the performance of circulating ffuidized beds. EXPERIMENTAL EQUIPMENT AND PROCEDUBE All experiments were carried out using a circulating fluid&c-IM apparatus constructed mostly of poly- acrylic tubing and consisting of a riser. 152mm diameter x 9.3 m fall, primary and secondary cyclones. a low-velocity storage he.& and an L-v&e f or returning particles to the base oi the riser in a controlled manner. The riser was equipped with a multi-orifice distributor plate of free area 19% and, for all the tests described in this paper, with an abrupt exit at the top. The solids re-enter4 the riser at a height of 76 mm above the distributor. No secondary air was used for any of the data presented in this paper. Further details are given by Bremton (1987) and in papers where we have presented the results. obtained in the same apparatus, on such aspects as the measurement of solids circulation rats (Burkell et aI_, lSSSj, heat transfer (Wu et aZ., 1989) and end effects (Brereton and Grace, 1992). The riser was equipped with 7.9 mm diameter ports at 457 mm intervals over its entire height to allow insertion of probes or connection to manometers or pressure .transducers for measuring the apparent- density profiles or pressure Ructuations. Test sections of the c&~mn were interchangeable, allowing various configurations to be explored. A capacitance probe was developed for application in this work. Beginning with Morse and Ballou (195i), capacitance probes have often been employed in fluid- ized beds as a means of distinguishing voids from dense ph&e and of measuring lo-1 voidages. They have proved to be the most popular and versatile of probes for fluidizatiotr systems (Grace and Baeyena. 2565