Original Paper Fluidization behavior of nano-particles by adding coarse particles Lianying Song, Tao Zhou * , Jingsi Yang College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, China article info Article history: Received 6 November 2008 Received in revised form 21 January 2009 Accepted 5 February 2009 Keywords: Nano-particles Fluidization Additive particles Richardson–Zaki equation abstract Fluidization behaviors of SiO 2 and TiO 2 nano-particles by adding coarse particles of FCC, ordinary Al 2 O 3 and heat-resistant Al 2 O 3 , has been investigated experimentally. The effect of size and inventory of coarse components on the fluidization behavior of nano-particles has been studied. The results reveal that the fluidization quality of the mixture systems can be improved with increasing the amount of the coarse particles. For SiO 2 nano-particles, the optimum of FCC, ordinary Al 2 O 3 and heat-resistant Al 2 O 3 are 30%, 40% and 50%, respectively. Similarly, the optimal amount of FCC, ordinary Al 2 O 3 and heat-resistant Al 2 O 3 is about 30% for TiO 2 nano-particles. Fluidization quality is almost the best for all these three coarse additives of 65–80 lm. The analysis performed from the standpoint of the Richardson–Zaki (R–Z) equa- tion reveals that the apparent terminal velocity derived through scaling experimental data to R–Z equa- tion affects the expansion index of the mixture systems. Ó 2009 The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder Technology Japan. All rights reserved. 1. Introduction The fluidization characteristics of nano-particles were investi- gated by many scholars [1–5] from various aspects. Due to strong interparticle forces, plug formation, channeling and particle agglomeration have been observed with increasing superficial gas velocity in conventional fluidized bed of nano-particles. These mac- roscopic phenomena generally reduce the heat and mass transfer efficiency of the fluid–solids system. Hence, there is a call for meth- ods to improve the fluidization quality of nano-particle beds. The methods that have been applied so far for promoting the fluidiza- tion of nano-particles are the same as those methods used for the cohesive fine particles in the Geldart group C category [6–9]. These methods include vibration agitation [10,11], applying acoustic fields [12,13], applying magnetic fields [14–16], mixing with coarse particles [17–18] and surface modification of the primary particles. The method of mixing with coarse particles is to add some par- ticles in the Geldart group A or B category into the nano-particle beds, working as agglomerate breakers to reduce the interparticle forces among nano-particles. The addition of coarse particles might not be as efficient as other methods such as acoustic, vibration, and magnetic, to promote nano-particle fluidization, since it does not have an external agitation control system. However, it can be easily implemented and does not require any additional equipment. This article has investigated the fluidization behavior of nano- particle beds of both SiO 2 and TiO 2 , as well as how it could be adjusted by the addition of three commonly used particles: FCC, ordinary Al 2 O 3 and heat-resistant Al 2 O 3 . The effects of particle size and amount of these three additives on the fluidization behavior have also been studied here. Finally, the Richardson–Zaki equation is applied to describe the bed fluidization behavior. 2. Experimental study The fluidization experiments were performed in a transparent polymethylmethacrylate (PMMA) column with 5 cm in diameter and 1 m in height. The pressure drop across the fluidized bed was measured by a manometer connected to a pressure tap that was placed just above the gas distributor. The powder bed height was measured by ruler placed at the column wall. Compressed air supplied by a compressor was used as a fluidizing gas, which was dried out by a silica gel column before it entered into the flu- idization column, as shown in Fig. 1. Gas flow rate was measured by a series of rotameters. The physical properties of SiO 2 and TiO 2 nano-particles (Lujiali nano-particle Ltd., China) are listed in Table 1, while the properties of the three coarse additives are de- scribed in Table 2. Note that each of the coarse additives has three different particle size ranges. The fluidization behavior of mixture systems is judged by pressure drop and bed expansion which is measured by the Bed Collapsing Device developed by Institute of Process Engineering, Chinese Academy of Science. 3. Results and discussion 3.1. Fluidization behavior of single component powder When trying to fluidize beds of SiO 2 nano-particles, it shows crack formation at low superficial gas velocities. With the increase 0921-8831/$ - see front matter Ó 2009 The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder Technology Japan. All rights reserved. doi:10.1016/j.apt.2009.02.010 * Corresponding author. E-mail address: zhoutao@mail.csu.edu.cn (T. Zhou). Advanced Powder Technology 20 (2009) 366–370 Contents lists available at ScienceDirect Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt