Stability of Spouting Regimes in Conical Spouted Beds with Inert Particle Mixtures M. S. Bacelos and J. T. Freire* UniVersidade Federal de Sa˜ o Carlos, Centro de Cieˆ ncias Exatas e de Tecnologia, Programa de Po´ s-Graduac¸ a˜ o em Engenharia Quı ´mica, Centro de Secagem de Pastas, Suspenso˜ es e Sementes, Rod. Washington Luiz, Km 235, 13565-905, CP 676 Sa˜ o Carlos, SP, Brasil Because of their reduced level of segregation, conical spouted beds (CSBs) have become a promising technique for many chemical engineering processes involving particle mixtures. However, for possible applications, the identification of particle segregation and flow-regime stabilities in CSBs is needed. Therefore, this paper presents an investigation to quantify the stability of spouting regimes (by pressure fluctuations) and particle segregation levels in CSBs operated at 60 °C with a variety of particle combinations with uniform sizes or a size distribution. The data show that, at a standard deviation from pressure signals of 50 Pa, beds with inert particle mixtures can achieve stable spouting regimes as the diameter ratio of larger particles to smaller particles (d pL /d pS ) is reduced from 2.85 to 1.98 and the bed height is increased from 0.105 to 0.195 m. Moreover, analysis of the segregation indexes revealed that our CBS showed itself to be more restrictive in operating with particle mixtures than those reported by San Jose´ et al. (Ind. Eng. Chem. Res. 1994, 33, 1838). 1. Introduction Conical spouted beds (CSBs) with coarse uniform size particles have commonly been used for a great variety of unit operations such as the drying of grains 1,2 and of pasty materials 3-11 and pneumatic conveyance, 12 among others. For applications in beds with wide particle size distributions, CSBs have been playing important roles as a promising chemical reactor, mainly in coal gasification; 13 in catalytic polymeriza- tion; 14 and recently, in the pyrolysis of sawdust and plastics 15-17 as well. With regard to cost effectiveness, the choice of CSBs as possible contactors is competitive in small-scale production with conventional spouted beds and fluidized and vibrofluidized beds, 18 as CSBs can be operated in the presence of uniform sizes or particle size distributions for good solids mixing coupled with satisfactory gas-particle contact and high rates of heat and mass transfer to the system when compared to the other contactors. On the other hand, depending on the application, CSBs can be operated in parallel modules. Such an alternative not only can favor an increase in spouted bed capacity, but also can be advantageous for production-line flexibility in industries such as pharmaceutical or juice manufacturing, which operate with both batch and intermittent processes. In practice, to process solid materials with uniform sizes or size distributions in CSBs, stable spouting regimes must be attained, and these regimes can be achieved within specific limits of operation. Therefore, many research efforts have concentrated on establishing such conditions by using a great variety of design parameters (i.e., R, d 0 , d i , d c ), properties of solids (F p , d p , d s ), and the gas velocity (V) to obtain the overall scope of these limits for application in the design of new CSBs. 19-21 Thus, to reach feasible stable operation in CSBs with a uniform or wide size distribution, the same authors obtained experimental data to visually verify the bed stability for binary mixtures alone and quantify segregation for both binary and ternary mixtures in the range of particle diameter ratios of 2 < d pL /d pS < 8, by using several arrangements of design parameters. Furthermore, the identification of spouted bed stability has been predominantly based on visual observation through trans- parent walls of full or half-columns, i.e., in cylindrical spouted beds of conical base 2,22,23 and in conical spouted beds. 19,20,24 The gas injected at the nozzle crosses the bed, and three flow regimes can be observed: packed bed, stable spouting, and unstable spouting. Unstable spouting regimes attained in cylindrical spouted beds of conical base have been visually characterized, not only for noncyclic and nonuniform particle movement, but also for fluctuations of pressure signals due to either swirling or pulsation of the spout over time for a given airflow injected into the bed. 25 As noted, the practical operation of conical spouted beds of inert particles depends on the identification of the scope limits for stable spouting regimes. Therefore, the aim of this research study is to apply a methodology proposed by Xu et al. 25 (i.e., for cylindrical spouted beds of conical base with uniformly sized particles) to identify stable or unstable spouting regimes in conical spouted beds composed of inert particles of uniform size or a size distribution. Furthermore, implications that CSBs can be operated with minor segregation and with stable spouting regimes were also analyzed. 2. Background In the late 1980s, research concerning the influence of fines on hydrodynamic behavior in binary mixtures was analyzed by describing the quality of mixtures and of the minimum spouting regime for cylindrical spouted beds of conical base. 26-28 A few years later, for conical spouted beds, Olazar et al. 20 studied the effect of wide diameter ratios between larger and smaller particles (d pL /d pS > 4) on hydrodynamic variables of conical spouted beds with binary mixtures of coarse particles in a range of particle diameter from 1 to 8 mm. They found that including only the diameter ratio between larger and smaller particles in mixing index variables is not sufficient for characterizing the influence of each mixture in the bed; thus, the size distribution must be considered as well. With regard to the spouting stability of binary or ternary mixtures, many studies have been performed to describe the * To whom correspondence should be addressed. Tel.: +55-16- 3351-8264. Fax: +55-16-3351-8266. E-mail: freire@power.ufscar.br. 808 Ind. Eng. Chem. Res. 2006, 45, 808-817 10.1021/ie050633s CCC: $33.50 © 2006 American Chemical Society Published on Web 11/10/2005