Investigation of dense slurry suspensions with coaxial mixers: Influences of design variables through tomography and mathematical modelling

Highlights

• Dispersion of solids with coaxial mixers was examined via ERT and CFD.

• Loss of energy was minimal for the system with an impeller spacing of 13.3 cm.

• Axial homogeneity enhanced with a decrease in inner impeller clearance.

• Inner stirrer diameter of 18 cm was proper to achieve better suspension quality.

• Use of inner A320 stirrer improved both axial and radial dispersion of solids.

Abstract

The coaxial mixers enhance the suspension of concentrated slurries in an agitated reactor. In this research work, the complex slurry suspension and dissemination behavior in a coaxial slurry mixing system (comprised of a close clearance anchor rotating with a low speed and an inner axial impeller rotating with a high speed) was analyzed employing ERT (electrical resistance tomography, a non-intrusive flow visualization technique), and computational fluid dynamics (CFD). The numerical models were validated by comparing the axial solid concentration profiles generated using the ERT data and the CFD simulation results. The influences of various important parameters such as the diameter of the inner axial impeller, the inner impeller type, and the inner impeller spacing on the hydrodynamic characteristics of the slurry suspensions in a coaxial mixing vessel were thoroughly analyzed. The radial and axial velocity profiles of solid particles were generated using the validated mathematical models. The assessment of energy loss due to the solid–solid collisions, the particle–fluid frictions, and the particle–vessel wall collisions was conducted. The evaluation of optimum inner impeller clearance and inner impeller diameter is essential to attain a high degree of solids suspension and dissemination in a coaxial slurry mixing system.

Introduction

The significance of two-phase liquid–solid mixing in various industrial utilizations namely cosmetics production, water purification, pulp–paper suspensions, pharmaceutical formulations, sludge disposal, food processing, consumer products manufacturing, minerals slurry processing, and petrochemicals refinement is paramount (Kresta, Etchells, Dickey, & Atiemo-Obeng, 2016; Paul, Atiemo-Obeng, & Kresta, 2004). Most of the research activities reported are limited to examine the impacts of various variables on hydrodynamic characteristics of slurry suspensions in dilute liquid–solid systems (Mishra & Ein-Mozaffari, 2016, 2017; Raghava Rao, Rewatkar, & Joshi, 1988; Zwietering, 1958; Grenville, Mak, & Brown, 2015; Grenville, Giacomelli, & Brown, 2016; Hosseini, Patel, Ein-Mozaffari, & Mehrvar, 2010; Kazemzadeh, Ein-Mozaffari, & Lohi, 2020a; Tahvildarian, Ng, D’Amato, Drappel, & Upreti, 2011; Harrison, Stevenson, & Cilliers, 2012; Liu, Xu, Fan, & Huang, 2018; Williams, Jia, & McKee, 1996). Very few researchers investigated the influences of various factors on hydrodynamic behavior of dense slurry suspensions in an agitated reactor (Ayranci & Kresta, 2011; Drewer, Ahmed, & Jameson, 1994; Jafari, Tanguy, & Chaouki, 2012; Kazemzadeh et al., 2020a, 2020b; Wang, Parthasarathy, Bong, Wu, & Slatter, 2012; Wu, Zhu, & Pullum, 2002; Mishra & Ein-Mozaffari, 2021). The effects of solid–liquid frictions, solid–vessel wall collisions, and solid–solid interactions on hydrodynamic characteristics of slurry suspensions need to be examined to get a better understanding of liquid–solid suspensions and dispersions in dense slurry systems (Bubbico, Di Cave, & Mazzarotta, 1998; Kazemzadeh et al., 2020a, 2020b).

Several approaches namely dispersion method (Barressi & Baldi, 1987; Buurman, Resoort, & Plaschkes, 1985), homogeneity approach (Harrison et al., 2012; Hosseini et al., 2010a, b, Williams et al., 1996), solids cloud height technique (Bittorf & Kresta, 2003; Bujalski et al., 1999), and suspension method (Baldi, Conti, & Alaria, 1978; Davies, 1986; Jafari et al., 2012; Yoshida, Kimura, Yoneyama, & Tezura, 2012; Zwietering, 1958) have been adopted over the years to characterize the solids suspension and dissemination quality in slurry agitated tanks.

Several researchers employed experimental methods to analyse the influences of various key variables such as density of solid and liquid (Ayranci & Kresta, 2014; Buurman et al., 1985; Yoshida et al., 2012; Zwietering, 1958), fluid viscosity (Ibrahim & Nienow, 2009; Wu, Zhu, & Pullum, 2001), liquid depth (Davarajulu & Loganathan, 2016), impeller-pumping mode (Tahvildarian et al., 2011), solids loadings (Godfrey & Zhu, 1994; Hosseini et al., 2010a; Mishra & Ein-Mozaffari, 2016), stirrer clearance (Armenante & Nagamine, 1998; Hicks, Myers, & Bakker, 1997; Micheletti, Nikiforaki, Lee, & Yianneskis, 2003), Stirrer types (Hosseini et al., 2010a; Mishra & Ein-Mozaffari, 2016; Williams et al., 1996), degree of turbulence intensity (Baldi et al., 1978), baffles adjustment (Brucato, Cipollina, Micale, Scargiali, & Tamburini, 2010), shape and roughness of tank bottom (Ghionzoli et al., 2007; Kondo, Motoda, Takahashi, & Horiguchi, 2007), particle diameter (Carletti, Montante, Westerlund, & Paglianti, 2014; Harrison et al., 2012; Tahvildarian et al., 2011; Tamburini et al., 2014), stirrer speed and power (Angst & Kraume, 2006; Hosseini et al., 2010a; Mishra & Ein-Mozaffari, 2016; Wang et al., 2012), and stirrer and tank diameter (Bittorf & Kresta, 2003; Raghava Rao et al., 1988) on hydrodynamic characteristics of slurry suspensions in single stirrer slurry vessels. It has been reported that the axial-flow stirrers are more effective than the radial-flow stirrers in suspending and disseminating the solids in slurry mixing vessels (Kazemzadeh et al., 2020a; Mishra & Ein-Mozaffari, 2020; Paul et al., 2004). Liu et al. (2018) studied the slurry suspensions behavior in a coaxial slurry mixing system operating at very low solids concentration (up to 7.7 v/v).

The effectiveness of electrical resistance tomography (ERT) in visualizing the complex fluid-flow patterns generated by impellers in different mixing systems has been widely reported (Carletti et al., 2014; Hashemi, Ein-Mozaffari, Upreti, & Hwang, 2016; Hosseini et al., 2010a; Kazemzadeh et al., 2020a, 2020b; Kazemzadeh, Ein-Mozaffari, Lohi, & Pakzad, 2016; Mishra & Ein-Mozaffari, 2016; Pakzad, Ein-Mozaffari, Upreti, & Lohi, 2013). The ERT technique provided consistent and reliable results in analyzing complex slurry suspension behavior (Carletti et al., 2014; Harrison et al., 2012; Hosseini et al., 2010a; Mishra & Ein-Mozaffari, 2016).

In recent years, several researchers examined the complex hydrodynamic behavior of slurry suspensions using the computational fluid dynamics (CFD) approach (2020b, Hosseini, Patel, Ein-Mozaffari, & Mehrvar, 2010; Kazemzadeh et al., 2020a; Mishra & Ein-Mozaffari, 2017; Wadnerkar, Utikar, Tade, & Pareek, 2012; Wadnerkar, Tade, Pareek, & Utikar, 2016; Xie & Luo, 2018). Most researchers employed Eulerian and Eulerian (E–E) multi-phase numerical modeling approach to obtain reliable and accurate results in the case of dense slurry suspension systems (Kazemzadeh et al., 2020a, 2020b; Wadnerkar et al., 2012). Several researchers highlighted the dominant impact of the drag force on the system hydrodynamics in an agitated slurry vessel (Fletcher & Brown, 2009; Khopkar, Kasat, Pandit, & Ranade, 2006; Ljungqvist & Rasmuson, 2001). The impacts of the added mass force, the lift force, and the buoyancy force on slurry suspension behavior depend on the liquid to solid density ratio (Khopkar et al., 2006). Various drag models have been proposed over the years to incorporate the influence of drag force on the system hydrodynamics in slurry vessels (Brucato, Grisafi, & Montante, 1998; Gidaspow, 1994; Khopkar et al., 2006; Wen & Yu, 1966). It has been reported that the standard k–ε turbulent modeling approach provided precise and accurate information regarding the impact of turbulence on hydrodynamic behavior of slurry suspensions (Fradette et al., 2007; Hosseini et al., 2010b; Kazemzadeh et al., 2020a, 2020b; Mishra & Ein-Mozaffari, 2017; Wadnerkar et al., 2016).

Several researchers analyzed the impacts of various key parameters namely solids content (Hosseini et al., 2010b; Xie & Luo, 2018; Wadnerkar et al., 2016; Mishra & Ein-Mozaffari, 2016; Kazemzadeh et al., 2020b), Impeller speed and power (Fradette et al., 2007; Hosseini et al., 2010b; Mishra & Ein-Mozaffari, 2017), particle diameter (Eng & Rasmuson, 2015; Kazemzadeh et al., 2020b; Khopkar et al., 2006; Mishra & Ein-Mozaffari, 2017), impeller types (Gu et al., 2017; Hosseini et al., 2010b; Kazemzadeh et al., 2020a), drag force (Derksen, 2003; Khopkar et al., 2006); turbulence intensity (Kazemzadeh et al., 2020a, 2020b); stirrer off-bottom clearance (Kazemzadeh et al., 2020b; Hosseini et al., 2010b), the specific gravity of solid (Hosseini et al., 2010b; Mishra & Ein-Mozaffari, 2017), and baffles adjustment (Mishra & Ein-Mozaffari, 2017) on hydrodynamic characteristics of the slurry suspension in single impeller slurry mixing systems using the CFD numerical modeling technique.

No information is available in the literature regarding the mixing efficacy of coaxial mixers to suspend and disseminate solids in dense slurry suspension systems. In this research study, the complex slurry suspension and dissemination behavior in a coaxial slurry mixing system (comprised of an outer anchor and an inner axial impeller) was analyzed employing the ERT and CFD methodologies. The impacts of different key parameters namely the impeller spacing, the inner stirrer diameter, and the inner stirrer types on the hydrodynamic characteristics of dense slurry suspension systems were analyzed based on the ERT conductivity data and the CFD simulation results. The solids suspension and dissemination quality in coaxial slurry mixing systems was assessed using three different mixing indexes. The radial and axial velocity profiles of solid particles were generated using the CFD simulations. The influence of turbulent kinetic energy on the system hydrodynamics for a coaxial slurry mixing system was examined using the CFD numerical modeling.