Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs Investigating the influence of the configuration of the bundle of heat exchanging tubes and column size on the gas holdup distributions in bubble columns via gamma-ray computed tomography Abbas J. Sultan a,b , Laith S. Sabri a,b , Muthanna H. Al-Dahhan a,c, ⁎ a Multiphase Reactors Engineering and Applications Laboratory (mReal), Department of Chemical and Biochemical Engineering, Missouri University of Science and Technology, Rolla, MO 65409-1230, USA b Department of Chemical Engineering, University of Technology, Baghdad, Iraq c Cihan University-Erbil, Iraq ARTICLE INFO Keywords: Pilot-scale bubble column Vertical internal tubes Configurations of vertical internals Gas holdup distribution Computed tomography (CT) ABSTRACT The impact of dense vertical internal tubes and their configurations on the gas holdup distributions and their diametrical profiles in pilot-scale bubble column is visualized and quantified for the first time ever using an advanced gamma-ray computed tomography (CT) technique. Two arrangements of vertical internals (circular and hexagonal configurations) occupying the same cross-sectional area (CSA) of the column (about 25% of the total cross-sectional area to represent the heat exchanging tubes that are used in the Fischer-Tropsch synthesis), were examined in addition to the measurement in the bubble column without vertical internals. Moreover, the gas holdup distribution results of the 18-inch (0.46 m in outer diameter, O.D.) bubble column are compared with an available data of 6-inch (0.15 m in O.D.) bubble columns with and without vertical internals. CT scans have been conducted for 18-inch bubble columns with and without vertical internals for the air-water system under a wide range of superficial gas velocity (0.05–0.45 m/s). The experimental results indicate that an improvement in the gas holdup distribution over the column's cross-sectional area is obtained when the vertical internal tubes (arranged in either a circular or a hexagonal configuration) were used. However, better cross-sectional gas holdup distribution was achieved in the bubble column with vertical internals arranged in a hexagonal con- figuration as compared to the bubble column without and with vertical internals arranged in a circular ar- rangement. Additionally, the averages of the cross-sectional gas holdup and their profiles for bubble column with and without vertical internals are close to each other when the bubble column with vertical internals is operating at a high superficial gas velocity, which is calculated based on the free cross-sectional area for the flow. Furthermore, the gas holdup distributions are further improved when the larger bubble column with vertical internals was used as compared to the 6-inch bubble columns with and without vertical internals. 1. Introduction Bubble/slurry bubble columns equipped with a bundle of heat-ex- changing tubes are well-fitted reactors for conducting highly exo- thermic reactions, such as Fischer-Tropsch (FT) synthesis, acetic acid production, cyclohexanol manufacturing, and many others [1–5]. The reason these reactors were selected for wide applications in industry is that they possess superior advantages in facilitating sufficient heat re- moval and temperature control (close to isothermal condition), which allow for a secure and high reactor performance [6–12]. Despite the wide variety of applications of bubble/slurry bubble columns (e.g., in industry), the design and scale-up of these reactors is a difficult engineering task due to the complex behavior of multiphase flow patterns and the absence of a phenomenological model that can reliably predict the flow patterns for these columns [13–16]. Ad- ditionally, the presence of the dense geometry of vertical tubes inside these reactors further alters the flow structure and the intensity of the mixing [17–21]. As a result, these vertical internal tubes make the design and scale-up even more challenging and complicated. Therefore, a comprehensive understanding of the impacts of vertical tubes on the hydrodynamics of these reactors is much needed to the successful de- sign, scale-up, and optimize the performance of a bubble/slurry bubble column equipped with a bundle of the intense heat exchanging tubes. One of the most critical hydrodynamic parameters for the design, scale-up, and modeling of bubble/slurry bubble columns is the gas holdup because of its impacts on the momentum, heat, and mass https://doi.org/10.1016/j.expthermflusci.2018.05.005 Received 14 February 2018; Received in revised form 10 May 2018; Accepted 10 May 2018 ⁎ Corresponding author at: Chemical & Biochemical Engineering Department, Missouri University of Science and Technology, Rolla, MO, 65409, USA. E-mail address: aldahhanm@mst.edu (M.H. Al-Dahhan). Experimental Thermal and Fluid Science 98 (2018) 68–85 Available online 11 May 2018 0894-1777/ Published by Elsevier Inc. T