Ž . Powder Technology 109 2000 27–40 www.elsevier.comrlocaterpowtec Numerical simulation of metallic solid bridging particles in a fluidized bed at high temperature Kenya Kuwagi ) , Takafumi Mikami, Masayuki Horio Department of Chemical Engineering, Tokyo UniÕersity of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan Accepted 20 September 1999 Abstract Ž . High temperature fluidization of iron particles was investigated by numerical simulation based on the discrete element method DEM as a case study for metallic bridging. A model was developed for metallic solid bridging by surface diffusion mechanism including the effect of surface roughness. The simulated fluidization behavior was highly time dependent, which is completely different from our w Ž . x previous results for liquid bridging particles T. Mikami, H. Kamiya, M. Horio, Chem. Eng. Sci. 53 1998 1927. . Both the amplitude of pressure fluctuation and the absolute value of bed pressure drop decreased with time. These tendencies agreed well with the experimental w Ž . x data of Mikami et al. T. Mikami, H. Kamiya, M. Horio, Powder Techonol. 89 1996 231. although temperature and bed size were different. Size and shape of agglomerates were much different for different surface roughness models. Hypha-shaped agglomerates were Ž . more dominant in the case of the lowest cohesiveness three-microcontact-point model . The size of agglomerates grown on the wall was largest for the largest cohesiveness and smallest for the smallest cohesiveness. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Numerical simulation; Discrete element method; Surface diffusion; Iron particles; Agglomerate; Defluidization 1. Introduction Metallic solid bridging is a well-recognized factor dom- inant in metallic fluidized bed processes such as iron oxide reduction and silicon CVD if they are conducted in a wx fluidized bed. Mikami et al. 1 carried out an experimental investigation for a typical case of iron particles at high temperatures and concluded that the neck growth of solid bridging and the cohesion force between iron particles can be predicted by the surface diffusion model of Kuczynski’s wx 2 sintering models. Although investigations into metallic solid bridging fluidized beds have been conducted for w x process developments 3–5 , and defluidization models wx were recently proposed by Iwadate and Horio 6 and wx Knight et al. 7 , no attempt has ever been made to formulate the whole mechanism of agglomerating fluidiza- tion behavior of bridging metallic particles. Recently, wx Ž . Mikami et al. 8 developed a simulation code SAFIRE Ž . based on the discrete element method DEM , considering cohesion force by liquid bridging, and they successfully simulated agglomerating fluidized bed behavior of wet ) Corresponding author. Tel.: q 81-42-388-7067; fax: q 81-42-386- 3303; e-mail: quwagi@cc.tuat.ac.jp wx particles. Iwadate and Horio 9 extended such simulation into fluidized particles with van der Waals force interac- tion. However, the sintering phenomenon is quite time dependent because sintering necks between contacting par- ticles grow rather rapidly with time. Accordingly, the fluidization behavior of solid bridging particles is com- pletely different from that of liquid bridging particles or w x group C particles of Geldart 10 classification. In this paper, a solid bridging model is developed and applied for numerical simulation of high temperature fluidization of iron particles. Since agglomerating fluidization behavior depends on the properties of agglomerates, attention is focused also on the size and shape of agglomerates formed in the bed. 2. Theoretical analysis 2.1. Definition of time Three kinds of time are used in the present model. The first is the absolute time, t , starting from the beginning of computation. The second is the time of neck growth, t , neck corresponding to each neck. When a neck is broken in a collision, t for the neck is reset to zero. The third is the neck 0032-5910r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. Ž . PII: S0032-5910 99 00224-7