Elsevier

Powder Technology

Volume 380, March 2021, Pages 462-474
Powder Technology

Flow behavior and heat transfer in bubbling fluidized-bed with immersed heat exchange tubes for CO2 methanation

https://doi.org/10.1016/j.powtec.2020.11.027Get rights and content

Highlights

  • Heat removal is needed for bubbling fluidized bed (BFB) reactor for CO2 methanation.

  • A 2D CFD model was validated with experimental data at various gas velocities.

  • Hydrodynamics, CO2 conversion, and heat transfer were examined for the BFB reactor.

  • Overall heat transfer coefficient from the bed to tubes was estimated at 114 W/m2/K.

Abstract

This study aims to investigate hydrodynamic and heat transfer characteristics of a BFB reactor with immersed heat exchange tubes for CO2 methanation using a two-dimensional (2D) gas-solid Eulerian computational fluid dynamics (CFD) model. A reaction kinetics model for Ni-based catalyst was coupled with the CFD model. The 2D-CFD model with the Huilin and Gidaspow drag was validated with experimental data for the bed expansion of Geldart B particles according to gas velocity. It was demonstrated that the heat of reaction was effectively removed in the BFB reactor with a 25% heat exchange area and that the reactor maintained isotherm conditions. The CO2 conversion was 92% in the BFB reactor at 400 °C and 5 bar. The overall heat transfer coefficient from the bed to the heat exchange tubes was estimated at 114 W/m2/K for an inlet gas velocity of 0.13 m/s.

Introduction

The total world-installed wind energy production increased noticeably from 198 GW in 2010 to 656 GW in 2018, while solar photovoltaic power has shown a steeper increase from 40 GW in 2010 to 647 GW in 2018 [1,2]. A storage system is required to integrate those energy sources into the power grid, because natural sources (wind and solar energy) are strictly time-independent [3,4]. Among the various alternatives for storing this excess energy, synthetic natural gas conversion through Power-to-Gas (PtG) technology [4] has been recognized for its high storage capacity, long charge/discharge period [3], and good overall efficiency of energy storage and reuse [5]. PtG typically involves a two-step process: H2 production from water electrolysis and H2 conversion to CH4 via a methanation reaction using external carbon sources such as CO or CO2 [4].

The CO2 methanation process, which is also called the Sabatier process (CO2+4H2cat.CH4+2H2O ΔHr =  − 165 kJ/mol), has an important role in reducing CO2 greenhouse gas emissions by recycling CO2 captured from industrial activities [6] as well as has great potential for process efficiency improvement through reactor design [7,8]. Due to the high amount of exothermic reaction heat in CO2 methanation, reactor design has focused on effective heat removal from the catalyst bed to minimize catalyst deactivation caused by thermal stress [[9], [10], [11]] and for the management of the removed heat [8,12]. Numerous studies on reactor design for heat removal have reported the use of jacket cooled fixed-bed reactors [7], fixed-bed reactors with internal heat exchange tubes [12,13], plate-type heat exchanger reactors [8], fluidized-bed reactors [13,14], and integrated reactors with fixed and fluidized beds [15,16].

The bubbling fluidized-bed (BFB) allows for isothermal operation due to its excellent mixing and heat transfer [11,15,17]. Kopyscinski et al. (2011) examined the effects of catalyst loading, gas velocity, and dilution rate on the gas composition and bed temperature in a bench-scale BFB without a heat exchanger [18]. For syngas methanation with a Ni-based catalyst, Liu and Ji (2013) reported that a bench-scale fluidized-bed reactor had a higher CH4 yield, lower coke content, and lower bed temperature than a fixed-bed. [16]. Sun et al. (2018) investigated the effect of operating parameters on syngas methanation in a full loop circulating fluidized-bed reactor using a two-dimensional (2D) Eulerian computational fluid dynamics (CFD) [9]. Using a one-dimensional (1D) two-phase fluidized-bed (TPFB) model, Jia et al. (2020) [14] investigated reaction characteristics according to the operating conditions in a bench-scale fluidized-bed without an internal heat exchanger. Ngo et al. (2020) [13] examined the flow behavior and reaction kinetics of a BFB CO2 methanation reactor using a 1D TPFB model. In the large-scale industrial applications of BFB, immersed heat exchangers such as coils, plates, and tubes are frequently used inside reactors to remove thermal energy, which maintains the desired temperature in the gas-solid system [19]. However, few researchers have addressed BFB reactors with heat exchangers as a means to manage the reaction heat of CO2 methanation.

Gas-solid multiphase CFD models have been presented to investigate hydrodynamics, reaction kinetics and heat transfer in fluidized-beds [9,[20], [21], [22]]. CFD approaches can be classified into two main categories including the Eulerian–Eulerian (EE) and Eulerian–Lagrangian (EL) methods. Both the gas and particle phases are treated as a continuum in the EE method, whereas in the EL method, each solid particle is calculated individually using Newton's equation of motion [[20], [21], [22]]. For the BFB system, the EE method has a computational cost advantage over the EL method [22] because the EL method consumes considerable time during the calculation of particle trajectories in a dense solid phase. Liu and Hinrichsen (2014) [21] applied the EE method to CO and CO2 methanation without the energy conservation equation under isothermal conditions. Sun et al. (2018) presented the EE method including the energy equation for syngas methanation in both a BFB [23] and a circulating fluidized bed [9], wherein the Gidaspow drag model [24] for momentum transfer and the empirical equation for heat transfer between gas and solid phases by Gunn [25] were used.

Various drag models suitable for gas-solid fluidized beds have been developed: Syamlal-Obrien (1989) [26], Gidaspow (1994) [24], Huilin and Gidaspow (2003) [27], Yang et al. (2003) [28], and Gao et al. (2009) [29]. The first three drag models were developed based on pressure drop and bed expansion, whereas the last two drag models [28,29] included the formation of particle clusters based on an energy minimization multi-scale (EMMS) approach. Several tuning parameters in the Syamlal-Obrien drag model were proposed for compensating particle properties [26]. The Huilin and Gidaspow drag model improved the Gidaspow drag model by introducing a smoothing function at a local solid volume fraction of 0.8 [27]. Liu and Hinrichsen (2014) [21] compared the Syamlal-Obrien, Gidaspow, and EMMS drag models in a BFB for syngas methanation, where the Gidaspow drag model [24], which predicted bed expansion well, was chosen for the CFD simulation. Kshetrimayum et al. (2020) [30] compared the Gidaspow drag model and EMMS drag models in terms of bed expansion and pressure drop at different flow regimes for cold-rigs with poly-dispersed particles. Li and Yang (2019) [17] used a local-structure-dependent drag model in the Eulerian CFD simulation for a lab-scale BFB reactor. Nevertheless, the CFD simulation of BFB reactors with an internal heat exchanger has not been widely researched to investigate heat transfer between the catalyst particles and the tubes.

This study aims to examine the hydrodynamics, CO2 conversion, and heat transfer characteristics of a BFB reactor with immersed heat exchange tubes for CO2 methanation using an EE CFD model. Three drag models proposed by Gidaspow [24], Huilin and Gidaspow [27], and Gao et al. [29] are compared with existing experimental data, and the best drag model is chosen. Three BFB reactor types are considered to compare hydrodynamic features: (1) no heat exchange tube and no reaction (nnBFB), (2) reactions but no tube (nrBFB), and (3) immersed heat exchange tubes and reactions (trBFB). In nnBFB, only the hydrodynamics are considered, whereas the reaction kinetics are added to nrBFB. In trBFB, the hydrodynamics, reaction kinetics, and heat transfer are examined. Finally, the performance of trBFB is highlighted in terms of temperature uniformity, CO2 conversion, and heat transfer coefficient from the bed to the heat exchange tubes.

Section snippets

Process description

Three BFB reactors with a diameter (D) of 0.08 m and a height (H) of 0.45 m are shown in Fig. 1. The initial bed height (Hinit) filled with catalyst particles was 0.2 m. The first BFB reactor with no heat exchange tube and no chemical reaction (nnBFB) shown in Fig. 1a was used to investigate only the hydrodynamic characteristics such as bed voidage, bed expansion, and pressure drop. The hydrodynamics coupled with the chemical reactions were examined in the BFB reactor with no heat exchange tube

Model description

In the reaction rate model for CO2 methanation [35] presented in Appendix A, the temporal concentration was converted to the spatial concentration with a constant gas velocity. The gas-solid EE CFD model included solid particle motion in the continuous phase via the kinetic theory of granular flow (KTGF) [36] and the CO2 methanation reaction on catalyst particles. In the dense gas-solid flows, the inertial energy loss was mainly caused by local flow contraction, expansion, and change in the

Results and discussion

The mesh independency on the three mesh structures was first evaluated in terms of pressure, velocity, bed voidage, and CH4 mole fraction. The most suitable drag model was then determined by comparison with experimental data. The hydrodynamic behavior for the three BFB reactors (nnBFB, nrBFB, and trBFB) was examined using the validated CFD model. Finally, the hydrodynamic and heat transfer characteristics were investigated along the reactor height using the 2D CFD model for trBFB.

Conclusion

Reactor design for CO2 methanation has focused on effective heat removal from the catalyst bed because of the exothermic heat of reaction. The bubbling fluidized-bed (BFB) is suitable for exothermic reactions due to excellent mixing and heat tranfer. This study investigated the hydrodynamics, CO2 conversion, and heat transfer characterisitcs of a BFB reactor with immersed heat exchange tubes for CO2 methanation using a two-dimensional (2D) gas-solid Eulerian computational fluid dynamics (CFD)

CRediT authorship contribution statement

Son Ich Ngo: Conceptualization, Data curation, Formal analysis, Methodology, Software, Validation, Visualization, Writing - original draft. Young-Il Lim: Resources, Supervision, Writing - review & editing, Funding acquisition, Project administration. Doyeon Lee: Investigation, Resources. Myung Won Seo: Funding acquisition, Project administration.

Declaration of Competing Interest

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.

Acknowledgment

This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) in the Korean Government (grant number: NRF-2020R1F1A1066097). This work was also was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 2019281010007B).

References (50)

  • J. Witte et al.

    Direct catalytic methanation of biogas – part I: new insights into biomethane production using rate-based modelling and detailed process analysis

    Energy Convers. Manag.

    (2018)
  • B. Liu et al.

    Comparative study of fluidized-bed and fixed-bed reactor for syngas methanation over Ni-W/TiO2-SiO2 catalyst

    J. Energy Chem.

    (2013)
  • J. Li et al.

    Multi-scale CFD simulations of bubbling fluidized bed methanation process

    Chem. Eng. J.

    (2019)
  • H. Martin

    Heat transfer between gas fluidized beds of solid particles and the surfaces of immersed heat exchanger elements, part I

    Chem. Eng. Process.

    (1984)
  • S.I. Ngo et al.

    Effects of fluidization velocity on solid stack volume in a bubbling fluidized-bed with nozzle-type distributor

    Powder Technol.

    (2015)
  • M. Chiesa et al.

    Numerical simulation of particulate flow by the Eulerian–Lagrangian and the Eulerian–Eulerian approach with application to a fluidized bed

    Comput. Chem. Eng.

    (2005)
  • L. Sun et al.

    Production of synthetic natural gas by CO methanation over Ni/Al2O3 catalyst in fluidized bed reactor

    Catal. Commun.

    (2018)
  • D.J. Gunn

    Transfer of heat or mass to particles in fixed and fluidised beds

    Int. J. Heat Mass Transf.

    (1978)
  • L. Huilin et al.

    Hydrodynamics of binary fluidization in a riser: CFD simulation using two granular temperatures

    Chem. Eng. Sci.

    (2003)
  • N. Yang et al.

    CFD simulation of concurrent-up gas–solid flow in circulating fluidized beds with structure-dependent drag coefficient

    Chem. Eng. J.

    (2003)
  • K.S. Kshetrimayum et al.

    EMMS drag model for simulating a gas–solid fluidized bed of geldart B particles: effect of bed model parameters and polydisperity

    Particuology

    (2020)
  • B. Formisani et al.

    Analysis of the fluidization process of particle beds at high temperature

    Chem. Eng. Sci.

    (1998)
  • W. Roetzel et al.

    Chapter 8 - experimental methods for thermal performance of heat exchangers

  • R.K. Niven

    Physical insight into the Ergun and Wen and Yu equations for fluid flow in packed and fluidised beds

    Chem. Eng. Sci.

    (2002)
  • S.I. Ngo et al.

    Hydrodynamics of cold-rig biomass gasifier using semi-dual fluidized-bed

    Powder Technol.

    (2013)
  • Cited by (17)

    • Experimental investigation on the impact of tube bundle designs on heat transfer coefficient in gas-solid fluidized bed reactor for Fischer-Tropsch synthesis

      2022, International Communications in Heat and Mass Transfer
      Citation Excerpt :

      However, a fluidized bed reactor is preferred for performing this reaction for the Fischer-Tropsch process. This preference was based on the fact that this reactor provides several advantages, including offering high rates of heat and mass transfer, achieving a uniform particle mixing by this reactor, providing a uniform temperature gradient, handling high pressure and temperature operating conditions, and allowing to process the large volume of fluid [8,16,18–28]. One of these reactor's essential characteristics is the uniformity of the temperature distribution inside the reactor.

    • Two phase modelling of Geldart B particles in a novel indirectly heated bubbling fluidized bed biomass steam reformer

      2022, Chemical Engineering Journal
      Citation Excerpt :

      Furthermore, Córcoles et al. [41] presented the simulation of a bubbling fluidized bed with immersed surfaces, using a Computational Particle Fluid Dynamics (CPFD) model, based on the multiphase particle-in-cell (MP − PIC) method. Eulerian – Eulerian two-fluid models (TFM) have also been employed for the simulation of similar geometries, however to a lesser extent [42–50]. In a conceptually different approach then for the cases described above, Jašo et al. [51], investigated a fluidized bed membrane reactor for oxidative methane coupling via CFD simulations.

    View all citing articles on Scopus
    View full text