Axial Solid Concentration Distribution Research Articles

Impacts of solid physical properties on the performances of a slurry external airlift loop reactor integrating mixing and separation

Solid physical properties are vital for the design, optimization, and scale-up of gas–liquid–solid multiphase reactors. The complex and interactional effects of the solid physical properties, including particle diameter, density, wettability, and sphericity, on the hydrodynamic behaviors in a new external airlift loop reactor (EALR) integrating mixing and separation are decoupled in this work. Two semi-empirical equations are proposed and validated to predict the overall gas holdup and liquid circulating velocity satisfactorily, and then the individual influence of such solid physical properties is further investigated. The results demonstrate that both the overall gas holdup in the riser and the liquid circulating velocity in the downcomer increase with the contact angle, but decrease with particle size, density, and sphericity. Additionally, the impact of the particle size on the liquid circulating velocity is also profoundly revealed on a micro-level considering the particle size distribution. Moreover, the axial solid concentration distribution is discussed, and the uniformity of the slurry is described by the mixing index of the solid particles. The results show that a more homogeneous mixture can be achieved by adding finer particles other than attaining violent turbulence. Therefore, this work lays a foundation for the design, scale-up, and industrialization of the EALRs.

Experimental investigation of axial Solid concentration distribution in fluidized bed

The axial solid concentration distributions were measured by using pressure inspection instrument in a glass bead fluidized bed. Set 1-3 belong to single size distribution and Set 4-5 have wide size distribution, respectively. The change trend of axial solid concentration distributions were investigated under different superficial gas velocities, particle size and particle density. The results showed that, with increasing superficial gas velocity, the solid concentrations in dense region decreased, but increased in dilute region. The glass bead particles whose particle size and particle density were smaller had good fluidization quality. It was also found that the joining of fine particles can significantly change the flow characteristics of the fluidized solid particles.

Solid concentration and velocity distributions in an annulus turbulent fluidized bed

Solid concentration and particle velocity distributions in the transition section of a ϕ 200mm turbulent fluidized bed (TFB) and a ϕ 200mm annulus turbulent fluidized bed (A-TFB) with a ϕ 50mm central standpipe were measured using a PV6D optical probe. It is concluded that in turbulent regime, the axial distribution of solid concentration in A-TFB was similar to that in TFB, but the former had a shorter transition section. The axial solid concentration distribution, probability density, and power spectral distributions revealed that the standpipe hindered the turbulence of gas–solid two-phase flow at a low superficial gas velocity. Consequently, the bottom flow of A-TFB approached the bubbling fluidization pattern. By contrast, the standpipe facilitated the turbulence at a high superficial gas velocity, thus making the bottom flow of A-TFB approach the fast fluidization pattern. Both the particle velocity and solid concentration distribution presented a unimodal distribution in A-TFB and TFB. However, the standpipe at a high gas velocity and in the transition or dilute phase section significantly affected the radial distribution of flow parameters, presenting a bimodal distribution with particle concentration higher near the internal and external walls and in downward flow. Conversely, particle concentration in the middle annulus area was lower, and particles flowed upward. This result indicated that the standpipe destroyed the core-annular structure of TFB in the transition and dilute phase sections at a high gas velocity and also improved the particle distribution of TFB. In conclusion, the standpipe improved the fluidization quality and flow homogeneity at high gas velocity and in the transition or dilute phase section, but caused opposite phenomena at low gas velocity and in the dense-phase section.

Gas–solids flow structure and prediction of solids concentration distribution inside a novel multi-regime riser

In this study, a novel multi-regime riser featured by combination of a dense-phase bottom region operated in turbulent fluidization and a dilute upper region was presented. Systematic research was conducted to investigate both axial and radial solids concentration distributions and the flow development in this riser. Experimental results indicated that high solids concentration and uniform flow structure were achieved in the diameter-enlarged bottom section, and the flow development was comprehensively analyzed in terms of local solids concentration, radial nonuniformity index and one-dimensional slip velocity. An empirical correlation of solids concentration in the turbulent section of various multi-regime risers was also developed. This correlation was further extended to predict axial solids concentration distribution in the diameter-enlarged section of this novel riser. Moreover, Boltzman function was adopted to correlate local solids concentration with cross-sectional averaged solids concentration and radial position in bottom dense region and upper flow developing region of the diameter-enlarged section, respectively. In general, a satisfactory agreement between predictions of these correlations and experimental results was obtained.

Experimental Study in a Short Circulating Fluidized Bed Riser

Experiments were performed with gas and solids flow in a 2.42 m high circulating fluidized bed (CFB). This equipment has both solids and gas fed into the downer section. Local solids holdup was measured using an optical fiber probe. By the axial solids concentration distribution, it was verified that (1) the curve that precedes the entrance into the riser provides further acceleration to the flow and (2) the abrupt exit causes an increase in solids concentration in the top zone. Results of radial distributions in the bottom zone show that the flow is more concentrated near the wall. For the exit zone, the distributions show high values of solids holdup both near the wall and on the axis.

The simulation and experimental validation on gas-solid two phase flow in the riser of a dense fluidized bed

Gas-solid flow in dense CFB (circulating fluidized bed)) riser under the operating condition, superficial gas 15.5 m/s and solid flux 140 kg/m2s using Geldart B particles (sand) was investigated by experiments and CFD (computational fluid dynamics) simulation. The overall and local flow characteristics are determined using the axial pressure profiles and solid concentration profiles. The cold experimental results indicate that the axial solid concentration distribution contains a dilute region towards the up-middle zone and a dense region near the bottom and the top exit zones. The typical core-annulus structure and the back-mixing phenomenon near the wall of the riser can be observed. In addition, owing to the key role of the drag force of gas-solid phase, a revised drag force coefficient, based on the EMMS (energy-minimization multi-scale) model which can depict the heterogeneous character of gas-solid two phase flow, was proposed and coupled into the CFD control equations. In order to find an appropriate drag force model for the simulation of dense CFB riser, not only the revised drag force model but some other kinds of drag force model were used in the CFD. The flow structure, solid concentration, clusters phenomenon, fluctuation of two phases and axial pressure drop were analyzed. By comparing the experiment with the simulation, the results predicted by the EMMS drag model showed a better agreement with the experimental axial average pressure drop and apparent solid volume fraction, which proves that the revised drag force based on the EMMS model is an appropriate model for the dense CFB simulation.

Nickel solids concentration distribution in a stirred tank

Nickel solids concentration distribution in a stirred tank was investigated using computational fluid dynamics (CFD) and experimental methods. The laser Doppler velocimetry (LDV) method was used to measure the velocity field for the liquid-only system and an optical technique was employed to determine the axial solids concentration distribution. The poly-disperse multiphase simulation approach was used to investigate the influence of the particle size on the solids concentration distribution. The effect of the solids loading on the liquid flow field was investigated and a good agreement was obtained between the experimental and simulation results. It was shown that the turbulent dispersion force is important for solids suspension, especially for high solids loading. Regions of inhomogeneity in the tank were identified. It was found that, for a given solids loading, the solids concentration distribution depended on both particle size and particle size distribution. Solids suspension theories are discussed and a correlation between these theories and scale-up rules is proposed.

Axial solid concentration distribution in tapered and cylindrical bubble columns

The distribution of axial solid concentration was investigated experimentally and theoretically in tapered and cylindrical slurry bubble columns using air as the gas phase, tap water as the liquid phase and quartz sands as the solid phase. Based on the sedimentation–dispersion model commonly used in cylindrical columns, a mathematical model was presented to predict the solid concentration distribution in the tapered column. Superficial gas velocities up to 0.28 m/s, slurry concentrations up to 159 kg(solid)/m 3(slurry), and static slurry height from 1.6 to 2.7 m were measured. The axial solid concentration distribution becomes uniform with an increase in superficial gas velocity or average solid concentration and with a decrease in particle diameter or static slurry height. The experiments were carried out in the cylindrical column under similar operating conditions to compare differences in the axial solid distribution measured in cylindrical and tapered columns. The results showed that the tapered column provides a more uniform profile of axial solid concentration than the cylindrical column. An empirical correlation for Peclet number was developed using dimensional analysis, which can predict the axial solid concentration distribution in tapered columns.

Axial solids distribution in slurry bubble columns

The axial sold concentration distribution in a slurry bubbly column was studied in both batch and continuous operation. Air and water were used as the gas and liquid phases, respectively. The gas velocity ranged from 0.016 to 0.173 m/s, and the slurry velocity ranged from 0.0 to 0.031 m/s. Glass beads of diameters 163,97, and 49 {mu}m were used as the solid phase. The holdup distribution of each solid in binary mixtures was also studied. The effects of gas velocity, slurry velocity, and particle size on the axial solid concentration distribution were examined. A mechanistic model is developed to describe the solid distribution in the slurry bubble columns. The model accounts for the solid distribution for both batch and continuous operation involving monodispersed and binary mixtures of solid particles.

The sedimentation‐dispersion model for slurry bubble columns

Slurry bubble columns have been widely used in petrochemical and coal conversion industries, and increasingly applied in fermentation and wastewater treatment fields. A slurry bubble column operated under the expanded bed regime is characterized by an axial solids concentration distribution in the column. Quantitative description of this distribution has been based mainly on the sedimentation-dispersion (S-D) model. In this paper, the physical nature of the S-D model in the literature is discussed in light of a rigorous analysis of the axial solids concentration distribution based on a macroscopic balance of the solid phase. General validity of the model for various modes of operation of slurry bubble column systems is also discussed.