Chemical Engineering Science 59 (2004) 4637 – 4651 www.elsevier.com/locate/ces Pneumatic transport of granular materials through a 90 ◦ bend Lai Yeng Lee a , Tai Yong Quek a , Rensheng Deng b , Madhumita B. Ray a , Chi-Hwa Wang a, b, ∗ a Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore b Singapore-MIT Alliance, E4-04-10, 4 Engineering Drive 3, Singapore 117576, Singapore Received 12 January 2004; received in revised form 26 June 2004; accepted 7 July 2004 Abstract In the present study, a pneumatic conveying system incorporating a 90 ◦ bend is investigated. This study employs the use of three non-invasive instruments to measure solids concentration and velocity distribution determination in the pneumatic conveying system. They are namely the electrical capacitance tomography (ECT), particle image velocimetry and phase doppler particle analyzer. Pressure transducers were also used to monitor the pressure drop characteristics along the post-bend vertical pipe region. Two different classes of granular materials, polypropylene beads (2600 m, Geldart class D) and glass beads (500 m, Geldart class B), were used to investigate the differences in the flow characteristics for granular particles of various Geldart classes. The experimental results show a constant frequency pulsating flow for polypropylene beads in the dense-phase flow regime. This is illustrated by the visualization, ECT and pressure drop data. For dilute-phase flow regime, both polypropylene and glass beads show a continuous annulus flow structure. Numerical simulation using the Euler–Euler method was also conducted using computational fluid dynamics and the fluid and particle flow characteristics were compared with the experimental data obtained in the present study.  2004 Elsevier Ltd. All rights reserved. Keywords: Pneumatic conveying; Multiphase flow; Bend; Granular materials; Simulation 1. Introduction Pneumatic conveying is an important process in the food and pharmaceutical industry for transportation of granular particles. The transport phenomenon of the conveying pro- cess in gas–solid system is not fully understood despite numerous studies, both experimental and numerical, have been conducted on different pneumatic conveying systems to characterize the flow profiles of the solids in the pipes of different sizes and for different pipe bends. These stud- ies helped to optimize the pneumatic conveying process and to assess the different methods of monitoring the conveying systems. The gas–solid two-phase flow in a vertical pipe is heterogeneous by nature and locally unsteady. As the solids mass loading increases, particles may come together to form groups such as sheets, streamers or clusters. Some of the particle groups may even experience back-flow or slipping ∗ Corresponding author. Tel.: +1-65-6874-5079; fax: +1-65-6779-1936. E-mail address: chewch@nus.edu.sg (C.H. Wang). 0009-2509/$ - see front matter  2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.07.007 near the pipe wall (Rautiainen et al., 1999). Several studies are reported on different kinds of vertical pneumatic con- veying systems (Rautiainen et al., 1999; Yilmaz and Levy, 1998, 2001; Dyakowski et al., 2000; Van de Wall and Soo, 1994, 1998; Van den Moortel et al., 1997; Plumpe et al., 1993; Zhu et al., 2003). These studies report various fluctu- ations and clustering of particles in the vertical pipe. Table 1 summarizes the parameters investigated in some of the re- search. Most of these researches were conducted for smaller sized particles falling under class A of Geldart classification. The Geldart classification of particles provides a guideline describing the ease of fluidization of particles and its ease of handling in pneumatic conveying (Klinzing et al., 1997). Yilmaz and Levy (1998, 2001) measured solids velocity and mass concentration using a fiber optic probe in two dif- ferent 90 ◦ pipe bends with bend radius to pipe diameter ratio of 1.5 and 3.0. Their investigations suggested a continuous rope-like structure that formed within the elbow and disin- tegrated further downstream into large and discontinuous clusters. A continuous stream of particles was observed near