Effect of Solids Viscosity on Unary and Binary Gas-Solid Fluidization: ECT Measurements and CFD Simulations

Industrially relevant applications like coal gasification (involving fluidization of coal and ash), catalytic polymerization, waste incineration, powder granulation, etc. involve fluidization of mixture of solids with different properties (size, shape, density, etc.). Gas-solids flow in fluidized bed reactors is inherently dynamic and performance of such reactors largely depends on the temporal and spatial distribution of the different solids phases in the bed. Further, depending on the operating conditions and solids properties, segregation of solids phases can also occur, which in turn influences the local heat and mass transport and consequently the overall reactor performance. Prediction of time-averaged flow behavior in multiphase reactors (e.g. gas-solid fluidized beds) using Eulerian multi-fluid models is possible to a reasonable accuracy, but most of the models seem less promising in their ability to predict the time-evolution (dynamics) of phase distribution in the reactors. The ability to predict the dynamics is imperative since most transport and kinetic effects in multiphase reactors occur on time scales which are significantly smaller than the time-scale of averaging. While long recognized, relatively fewer research efforts have focused on establishing the prediction of the dynamics using Eulerian multi-fluid models owing to the paucity of reliable time- and spatially- resolved experimental data.

The gas-solid interphase momentum exchange models (drag force) are considered to be very crucial in Eulerian multi-fluid simulations, and most often modified by researchers to reduce the discrepancies between the results of simulation and their corresponding measurements. In addition to the drag force, the force due to solid shear stresses also play an important role in the bubbling behavior [1,2]. It was seen that the previous study on the effect of solid viscosities are carried out on pseudo-3D beds for unary gas-solid fluidization, and the effect of solid viscosities on the segregation and mixing phenomena of binary gas-solid fluidization was not reported in the previous study.

In the present work, a cylindrical fluidized bed of internal diameter of 9 cm and height of 150 cm was used for measurements and the same geometry was used for the Eulerian multi-fluid simulations. Air and glass beads of different sizes (96 µm, 430 µm, and 922 µm; density = 2500 kg/m³) were used for the gas and solid phases. Also, different compositions (30%, 50%, and 70% by weight) of 96 µm and 922 µm particles were used to make different mixtures of solids phases for binary fluidization. A commercial measurement instrument Electrical Capacitance Tomography (ECT) system was used to measure temporally and spatially resolved solid volume fraction. In case of Eulerian multi-fluid simulations, the conservation equations (mass and momentum) of gas and solid phases, and the Kinetic Theory of Granular Flow (used to model solid stresses and energy losses due to particle-particle collisions) are solved in commercial software Ansys Fluent 18.0. The details of the measurements (e.g. sensor calibration, image reconstruction algorithms, etc.) and simulations (e.g. grid size, boundary conditions, solution techniques, discretization schemes, etc.) will be given in the full-length manuscript.

The dynamics of segregation and fluidization of unary particles and binary mixtures (with varying compositions of unary particles) with different size ratio (d_pr = ~2- 9) in a cylindrical fluidized bed were investigated using the temporally- and spatially-resolved measurements of solid volume fraction performed using the ECT. In the segregating bed conditions, the segregation-time measured using the ECT was compared with that measured using the high-speed imaging and it was shown that ECT data can be analyzed to serve as a quantitative measure of the segregation behavior. The ECT measurements were used further to quantify dynamics of unary and binary fluidization behavior using the time-evolution of spatial solid volume fraction fluctuations, corresponding frequency distribution, and bubble size distribution. Further, a relation between the measured variance of solid volume fraction fluctuations with the spatial solid volume fraction distribution and corresponding flow structures under different fluidization conditions was established. These measurements helped to provide a database to test the Eulerian multi-fluid CFD simulations of segregation and fluidization of binary mixtures.

The Eulerian multi-fluid simulations were performed for unary and binary gas-solids fluidization, and the time-averaged, as well as instantaneous solid volume fraction distribution, were compared with their corresponding measurements. The drag force and force due to solid shear stresses in a cell were compared. It was found that besides the drag force, the force due to solid shear stresses strongly influence the bubbling behavior by influencing the compactness in the solid phases. Higher the compactness in the solids phase, the less gas is transported through the boundary of the bubbles and therefore bigger gas bubbles were observed. It was also seen that among all the solids viscosities (kinematic, collisional, bulk, and frictional), the effect of frictional viscosity on the bubbling behavior was found more dominating than the other viscosities. Generally, three types of frictional pressure models have been used widely in the literature i.e. KTGF-based, Johnson and Jackson [3], and Syamlal et al. [4]. However, the frictional viscosity values obtained using the KTGF-based, Johnson and Jackson [3], and Syamlal et al. [4] were significantly different and therefore choosing the right model for frictional pressure became a challenge in the Eulerian multi-fluid simulations especially in the case of binary gas-solid fluidization.

It was found that the use of KTGF-based frictional pressure leads to produce relatively small-sized but large number of the bubbles per unit time. With an increase in the solid viscosity caused by the use of Johnson and Jackson [3] or Syamlal et al. [4] frictional pressure models at different critical solid volume fractions, the bubbles size increased in comparison to the size of the bubbles observed using the KTGF-based frictional pressure model. Also, the number of bubbles per unit time decreased in comparison to the KTGF-based model. Due to the change in the size, and number of bubbles per unit time in the binary gas-solid fluidization, there was a significant change observed in the segregation rate. It is worthwhile to mention that the agreement between the measured and simulated bubbling characteristics (size and frequency) and segregation rate can be improved by changing the frictional viscosity of the solids in the bed.

In the full length manuscript, the detailed comparison of time-evolution of solid volume fraction for unary and binary gas-solid fluidization measured using the ECT will be reported. Also, through the Eulerian multi-fluid simulations, the effect of solid viscosities on the time-evolution of solid volume fraction will be discussed. The bubble chord length distribution and power spectra of bubble chord length fluctuations will be reported with their corresponding experimental validation. In addition, the change in segregation rate due to the change in the bubbling characteristics for different mixture compositions have been analyzed and it will be reported in the full length manuscript. The present work will be helpful in understanding the role of solid viscosities on the segregation and bubbling behavior of binary gas-solid fluidization in a cylindrical fluidized bed.

References:

[1] D.J. Patil, M. Van Sint Annaland, J.A.M. Kuipers, Critical comparison of hydrodynamic models for gas-solid fluidized beds - Part I : Bubbling gas-solid fluidized beds operated with a jet, Chem. Eng. Sci. 60 (2005) 57–72. doi:10.1016/j.ces.2004.07.059.

[2] S. Cloete, S.T. Johansen, A. Zaabout, M. van Sint Annaland, F. Gallucci, S. Amini, The effect of frictional pressure, geometry and wall friction on the modelling of a pseudo-2D bubbling fluidised bed reactor, Powder Technol. 283 (2015) 85–102. doi:10.1016/j.powtec.2015.04.060.

[3] P.C. Johnson, R. Jackson, Friction collisional constitutive relations for granular materials, with application to plane shearing, J. Fluid Mech. 176 (1987) 67–93. doi:10.1017/S0022112087000570.

[4] T.J. Syamlal, M., Rogers, W., O’Brien, Mfix documentation: volume I, theory guide. Technical Report DOE/METC-9411004, NTIS/DE9400087, 1993.