Magnetic intensification of mass transfer between fluidizing gas and Geldart-B nonmagnetizable particles: Property effects of added magnetizable particles
Introduction
Magnetic fields have long been utilized to intensify the contact performance between the fluidizing gas and Geldart-B magnetizable particles in the bubbling fluidized bed (BFB) by suppressing the bubble formation or reducing the bubble size, creating the well-known magnetized fluidized bed (MFB) (Espin et al., 2013, 2017; Hristov, 2002; Liu et al., 1991; Rosensweig, 1979a; Sajc et al., 1994; Siegell, 1991; Zong et al., 2013, 2017). Compared with other techniques, the magnetic field has the merit of being unaffected by the harsh reaction condition inside the bed since it works from the outside (Zhu et al., 2019, 2021a,b). However, most particles in nature are magnetically non-susceptible and cannot respond to the magnetic field, severely limiting the application scope of the magnetized fluidization technique. How to extend this versatile technique to the numerous nonmagnetizable particles is an urgent problem to solve. In other words, it is practically important to intensify the interphase mass transfer between the fluidizing gas and nonmagnetizable particles by using the magnetic field.
To the best of our knowledge, Arnaldos and Casal (1987) took the lead in addressing such an issue. They found that the mass transfer (quantified by the single-pass conversion of the fluidizing gas, η) between the fluidizing gas and the binary admixture of magnetizable and nonmagnetizable particles could be significantly enhanced by the magnetic field. However, it could not be concluded from this study whether the mass transfer between the fluidizing gas and nonmagnetizable particles was intensified as compared with that in the BFB with purely nonmagnetizable particles. They took the binary admixture as a whole and focused merely on the effects of the magnetic field. No comparison with the gas–solid contact performance in the BFB with purely nonmagnetizable particles was conducted. For the ease of narration, the MFB with the binary admixture of magnetizable and nonmagnetizable particles was simply termed the admixture MFB (Ganzha and Saxena, 1998, 1999; Thivel et al., 2004; Wu et al., 1997a,b) in this paper.
Khristov et al. (2000) also explored the gas–solid contact performance (quantified by the volumetric mass transfer coefficient, KGa) in the admixture MFB. Nevertheless, they compared the mass transfer performance therein to that in the fixed bed with the binary admixture and that with purely nonmagnetizable particles. They focused on the advantage of low pressure drop in the admixture MFB as compared with that in the fixed bed when the gas velocity (Ug) exceeded the minimum fluidization velocity (UmfN) of nonmagnetizable particles. Consequently, we could not derive from this study as well whether the mass transfer between the fluidizing gas and nonmagnetizable particles was intensified as compared with that in the BFB with purely nonmagnetizable particles.
For this reason, our previous study (Zhou et al., 2021) further explored the mass transfer intensification between the fluidizing gas and nonmagnetizable particles by simultaneously introducing magnetizable particles and applying the magnetic field. As shown in Fig. A1a, the resulting admixture MFB exhibited three distinct flow regimes for Ug beyond UmfN (Zhu et al., 2019, 2021b). The hydrodynamic characteristics of each flow regime could be easily found in our previous works (Zhu et al., 2016, 2019, 2021b). It was found that when the resulting admixture MFB was located in the magnetic stabilization flow regime, the contact performance between the fluidizing gas and nonmagnetizable particles could be significantly intensified as compared with that in the BFB with purely nonmagnetizable particles (Fig. A1b). In fact, it approached that in the fixed bed with purely nonmagnetizable particles. To be specific, η of SO2 could be maintained approximately at 1.0 for a long time from the beginning of adsorption, which was, however, impossible to achieve in the conventional BFB with purely nonmagnetizable particles. In short, the complete adsorption, which could not be reached in the conventional BFB, was successfully achieved in the admixture MFB. Note that the short time period (tc) corresponding to complete adsorption (defined as η > 0.98 in this study) could be improved by forming a circulating system. The saturated nonmagnetizable particles were continuously removed from the bottom of the admixture MFB and returned from the top after regeneration.
Apparently, the added magnetizable particles successfully transmitted the stabilizing effect of the magnetic field to the nonmagnetizable particles (Gros et al., 2008a,b; Yu et al., 2005). The mechanism was as follows. First, the magnetizable particles aggregated into the so-called magnetic chains (Zhu and Li, 1996) under the influence of the magnetic field. Then the magnetic field aligned these chains in the direction of field lines, thus forming a network inside the bed (Arnaldos et al., 1985; Hristov, 2002; Thivel et al., 2004; Wang et al., 2013a, b; Wu et al., 1997a, b). Finally, the network entrapped the nonmagnetizable particles (forming a Bacon-like structure), stabilized their motion, suppressed the bubble formation, and eliminated any gas-bypassing in the form of gas bubbles (Cohen and Chi, 1991; Liu et al., 1991; Rodríguez et al., 1999; Rosensweig, 1979a, b; Tao et al., 2021; Wang et al., 2008; Xu et al., 2019; Yu et al., 2005). Results from the numerical simulation (Wang et al., 2013a, b) vividly illustrated how the magnetic field worked on the nonmagnetizable particles through the added magnetizable particles (Fig. A2).
Given that the stabilizing effect of the magnetic field on the nonmagnetizable particles has to be transmitted by the magnetizable particles, it can be anticipated that the magnetic intensification of mass transfer between the fluidizing gas and nonmagnetizable particles will be strongly affected by the amount, size, and density of the added magnetizable particles. However, to the best of our knowledge, no such investigation has been reported in the literature. The purpose of this work was to address these issues. The work done here could provide people with useful guidance on how to match suitable magnetizable particles to the given nonmagnetizable particles so that the magnetic stabilization could be fully transmitted and the interphase mass transfer between the fluidizing gas and nonmagnetizable particles could be intensified as much as possible by the magnetic field.
Section snippets
Materials
The gas–solid contact performances under various operating conditions were evaluated via the adsorption of SO2 from the influent gas by the alumina particles. The physical properties of alumina particles and various magnetizable particles are summarized in Table 1. When the amount effect of magnetizable particles on the magnetic intensification of mass transfer was explored, the iron-III particles were used. The amount of iron-III particles in the resulting binary admixture was quantified by
Amount effect of added magnetizable particles on magnetic intensification of mass transfer
Given that the adsorption of SO2 was always not a steady process, it was difficult to choose one single parameter to reflect the mass transfer performance for a given operating condition. Hence, the breakthrough curves were compared directly in this study. The breakthrough curves at various values of xvM are illustrated in Fig. 4a. Keep in mind that 6 g of alumina particles were used in all these cases and only the added amount of iron particles was varied to change xvM in the resulting binary
Conclusions
This work focused on the magnetic intensification of mass transfer between the fluidizing gas and Geldart-B nonmagnetizable particles. In particular, the property effects of added magnetizable particles on the performance of intensification were explored. A compromise value of xvM in the resulting binary admixture was found to be 0.40 to transmit the magnetic stabilization and save the bed volume. When xvM was kept constant (0.40), there existed an optimal size/density for the added
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (No. 21808232).
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