8458
Ind. Eng. Chem. Res. 2008, 47, 8458–8470
Analysis of Particle Segregation and Intermixing in Solid-Liquid Fluidized Beds Prakash V. Chavan and Jyeshtharaj B. Joshi* Department of Chemical Engineering, Institute of Chemical Technology, UniVersity of Mumbai, Matunga, Mumbai, India 400 019
Particle segregation and intermixing have been studied in 50 mm i.d. and 1.2 m long solid-liquid fluidized bed (SLFB). An ion-exchange resin was used as a solid phase in five size ranges with average particle sizes (dry basis) of 427, 500, 605, 725, and 855 µm. Expansion characteristics of beds were investigated separately for all the particle sizes. The expansion characteristics were also investigated using 2, 3, 4, and 5 sizes having all the combinations among 427, 500, 605, 725, and 855 µm. In all cases, the concentration profiles of the individual sizes were measured along the bed height. The segregation velocity of dense and light particle was predicted and compared with experimental results for binary mixtures. Criteria have been developed for segregation/intermixing in laminar (Re∞ < 0.1), transition (0.1 < Re∞ < 500), and turbulent (Re∞ > 500) regimes for binary particle systems. The solid dispersion coefficient has also been evaluated for each particle size present in the mixture of particles of different sizes at given operating conditions. 1. Introduction Solid-liquid fluidized beds (SLFB) are used in industry for hydrometallurgical operations, catalytic cracking, ion exchange, adsorption, crystallization, sedimentation, etc. These practical applications of liquid fluidization have stimulated theoretical and experimental investigations over the past three decades. A significant amount of work related to solid-liquid fluidization has been reported in the literature. Theoretical and empirical models have been used, and several correlations are now available for estimation of design parameters. Di Felice1 and Joshi2 presented excellent state-of-the-art reviews on the transport phenomena in solid-liquid fluidization. Fluidization of a multiparticle system provides several challenges because of the complexity of solid-fluid and solid-solid interactions. For example, when a binary mixture of solids (differing in size only) is fluidized, the degree of segregation and intermixing mainly depends on the size ratio and superficial liquid velocity. The system becomes complex when the size ratio is accompanied by density differences. Such multiparticle systems have received wide attention. However, there is a need to analyze all the published information with a coherent theme. Further, it is also desired to collect new data which are useful to fill the existing knowledge gaps. In view of this, it was thought desirable to undertake a systematic investigation in terms of bed expansion, segregation/intermixing, and solid-phase dispersion.
used for estimating the settling rate of a single particle (or the fluid velocity necessary to suspend a single particle) needs to be modified to account for the presence of other particles. There have been many empirical, semiempirical, and theoretical correlations available in the literature for the velocity-voidage relationship for monosize particle systems. Some of those which are widely used in the literature are due to Joshi,2 Richardson and Zaki,3 Hanratty and Bandukwala,4 Happel,5 Loeffler and Ruth,6 Barnea and Mizrahi,7 Garside and Al-Dibouni,8 and Foscolo et al.9 Table 1 briefly summarizes the velocity-voidage relationship widely used for the monosize particle systems. It is clear that the velocity-voidage relationship for the monosize particle systems can be described with a sufficient degree of confidence. However, these relationships are not compared on a common platform. Therefore, it was thought desirable to compare these relationships with the experimental data to validate their applicability. 2.2. Binary Particle Systems. Extensive literature is available on the SLFB consisting of binary particles. Pruden and Epstein10 considered that the degree of segregation depends on the difference between the bulk densities of the two particle species when each is fluidized separately, the one having the higher bulk density being at the bottom. The following equation has been proposed for constant density particles
[( )
FBL - FBS ) (FSS - FL)∈LL
2. Previous Work Previous work has been divided into three parts: (i) monosize particle systems, (ii) binary size particle systems, and (iii) multisize particle systems. 2.1. Monosize Particle Systems. The terminal settling velocity is defined by the following force balance π 1 π 3 d (F -F )g ) CD∞ dP2 FLVS∞2 (1) 6 P S L 4 2 However, in the presence of other particles, the flow field around a given particle is affected and hence the drag coefficient * To whom correspondence should be addressed. Tel.: +91-2224145616. Fax: +91-22-24145614. E-mail:
[email protected].
dPL dPS
(3-m⁄mn)
]
-1
(2)
where m varies from 1 in the Stokes region (Re∞ < 0.1) to 2 in the turbulent (Re∞ > 500) regime and n is a Richardson-Zaki index. Equation 2 is applicable when the Richardson-Zaki indices of both particles are comparable. Further, eq 2 can be applied to extreme cases only, i.e., they cannot be applied in the transition regime where it is difficult to find an appropriate value of m. Lockett and Al-Habbooby11 developed a physical model for binary suspensions, and Mirza and Richardson12 extended their model to multisize suspensions. Both the groups assumed that within any sedimenting zone the settling velocities of larger particles were unaffected by the sizes and velocities of surrounding particles. This assumption is physically unacceptable
10.1021/ie800504z CCC: $40.75 2008 American Chemical Society Published on Web 10/04/2008
entire range
