Axial Solid Distribution Research Articles

Steady-state multiscale CFD simulation of a circulating fluidized bed riser

Compared to transient simulation, steady-state simulation of circulating fluidized bed risers is more efficient, but is also harder to perform due to the complex scale-dependency of dense gas-solid flows. In this work, steady-state computational fluid dynamics (CFD) simulation of a riser is performed using the steady energy-minimization multi-scale (EMMS) drag. It is found that the steady state corresponds to an extremely large scale of length and time, thus the grid size required in steady-state simulation is larger than that in transient one. The time-averaged two-fluid model (TFM) coupled with the steady-state EMMS/1M drag model enables a good prediction of the S-shaped, axial solids distribution and the choking transition, whereas the two-phase turbulence and solids stress models are important in predicting the radially core-annular distribution of solids. So far as we know, this is the first time that one can predict the choking transition in a steady-state CFD simulation. Further improvement may need an EMMS modeling of the time-averaged solid stresses.

UV-photodegradation of R6G dye in three-phase fluidized bed reactor: Modeling and optimization using adaptive neuro-fuzzy inference system and artificial neural network

The development of an efficient photoreactor is still the most challenging task in the photocatalytic process. The main challenges include the determination of the optimal hydrodynamic conditions required for the efficient photodegradation of biorecalcitrant pollutants. In this study, a TiO2-ZnO/BAC composite catalyst was applied to investigate the hydrodynamic characteristics of a three-phase fluidized bed reactor in the UV-photodegradation of rhodamine 6G dye. Artificial neural network (ANN) and adaptive neuro-fuzzy inference systems (ANFIS) were used for modeling and optimization of the photodegradation process. From the preliminary study, 70° column inclination angle and 0.028 ms−1 superficial gas velocity, showed good axial solid distribution and average gas holdup. Subsequently, 0.019 ms−1 gas flow rate at 70° column inclination angle was found to be the optimum condition for the removal of rhodamine 6G (97.0 %). Moreover, solid distribution was found to be a dominant limiting factor in the photodegradation process as compared to gas holdup. Meanwhile, sensitivity analysis showed that all the input parameters (lamp position, inclination angle, and superficial gas velocity) were above 10%, confirming a strong influence on the process. The ANN-trainlm and ANFIS-hybrid with R-values of 0.9911 and 0.9546, respectively confirmed that the predicted model fits well with the experimental data. Furthermore, the inclination angle of 70° can be important in solar photoreactors to attain a relatively efficient tilt angle when using solar energy.

Numerical investigation on the intensification of coaxial slurry mixing systems

The dense solid concentration brought difficulties to solid suspension, and the mixing intensification of the coaxial mixer, which comprised a close-clearance anchor and a center propeller, was researched numerically in the challenging system. The results indicated that by employing the propeller in an up-pumping direction rather than a down-pumping direction, the state of solid accumulation at the center bottom could be significantly improved, whereas the uniformity of axial solid distribution was also relatively poor. The quality of the suspension deteriorated at the dense solid concentrations, and energy loss would be more significant, particularly near the location of the center propeller. In addition, a larger outer impeller speed was necessary to prevent solid deposition, but it also led to a decrease in cloud height. A larger off-bottom spacing was advised for the up-pumping propeller, and the structure could achieve superior intensification performance for dense solid-liquid mixing.

Fluidization behaviors of nanoparticle agglomerates with high initial bed heights

The fluidization behaviors of SiO2 nanoparticles with high initial bed heights of 8–24 cm are investigated experimentally. The effects of the initial bed height and the superficial gas velocity on the minimum fluidization velocity, bed heights, axial solid distributions and the agglomerate diameter are discussed. The results show that the minimum fluidization velocity decreases significantly with the increase of the initial bed height. Over 80% of nanoparticles are fluidized at the bottom dense region of the bed under all fluidization conditions. The high bed expansion arises from a significant increase of the upper dilute region of the bed. The mean diameter of agglomerates measured in the dilute region increases with the initial bed height and the superficial gas velocity. There exists a low-frequency mass transfer between the dense and dilute regions of the bed.

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.

Hydrodynamics of a bubble-driven liquid-solid fluidized bed

A new fluidization system, the bubble-driven liquid-solid fluidized bed, adopting particles with a slightly higher density than the fluid, has been proposed. During the experiment process, air was introduced into the bottom of the column as driving force, and superficial gas velocity ranged from 0 to 28 mm/s and liquid was continuous phase but maintained at the stationary state. The variation of pressure drops in the bottom section near the gas distributor was measured to identify the demarcation of flow regimes and determine the loose velocity (Uloose) and initial fluidization velocity (Uin). It was found that Uloose and Uin both increase with the initial hydrostatic height but are independent of the initial liquid height. Moreover, solids holdup was analyzed using a photographic technique while gas holdup was calculated from the pressure drop variation and solids holdup. A non-uniform axial solids distribution exists for all gas velocities. Additionally, two fluidization mechanisms were proposed based on the experimental observations.

A hydrodynamic model for circulating fluidized beds with low riser and tall riser

Based on different flow regimes classified by the saturation carrying capacity in the riser, a comprehensive hydrodynamic model for circulating fluidized beds was proposed and validated with experimental data. The model can describe the different axial solids distributions or regimes in previous literatures, especially the boundaries among them. The calculation of model shows good agreement with experiments not just for gas solids behavior in riser, but also for that in standpipe.The influences of the riser height on the axial solids distribution in riser were discussed. With the increase of the distance between the upper end of dense phase and the bottom of riser, the axial solids distribution in low riser appears as the exponential profile, the quasi exponential profile and the quasi S-shaped profile continuously. For the tall riser, the quasi exponential profile is replaced by the S-shaped profile due to the dilute phase in the upper. In addition, the saturation carrying capacity prevails in the tall riser, and with greater inventory solids circulation rate would exceed the saturation carrying capacity to form the quasi S-shape profile.

Experimental validation of the gas–solid flow in the CFB riser

In this paper, an experimental study is performed to investigate the flow structure in a circulating fluidized bed (CFB). The typical core-annulus structure and small amount of back-mixing of particles near the wall of the riser were observed. The axial solid concentration distributions contain a dilute region towards the up-middle zone and a dense region near the bottom and the top exit zones. Furthermore, the solid concentration decreases with the increase of the superficial gas velocity, and increases with the increment of the circulation rate at the same height position. The total pressure drop of the main bed represents a linear relationship with the solid flux rate. In the dense phase zone, the solid concentration increases linearly with the augmentation of the solid flux, however, the change of the solid concentration is slight, even unchangeable at the up zones. In addition, based on the Energy-Minimization Multi-Scale (EMMS) method, a revised drag force model is proposed, which is coupled in the Eulerian two-fluid model for simulating the flow structure in the riser. Numerical results are consistent with the experimental data, which indicate the revised drag force model is very successful in simulating flow structure of the dense gas–solid two-phase flow.

Experimental investigation on flow asymmetry in solid entrance region of a square circulating fluidized bed

To study the influence of back feeding particles on gas–solid flow in the riser, this paper investigated the flow asymmetry in the solid entrance region of a fluidized bed by particle concentration/velocity measurements in a cold square circulating fluidized beds (CFB). The pressure drop distribution along the riser and the saturation carrying capacity of gas for Geldart-B type particles were first analyzed. Under the condition of u 0 = 4 m/s and G s = 21 kg/(m 2 s), the back feeding particles were found to penetrate the lean gas–solid flow near the entrance (rear) wall before reaching the opposite (front) wall, thus leading to a relatively denser region near the front wall in the bottom bed. Higher solid circulation rate ( u 0 = 4 m/s, G s = 33 kg/(m 2 s)) resulted in a higher particle concentration in the riser. However the back feeding particles with higher momentum increased the asymmetry of the particle concentration/velocity profile in the solid entrance region. Lower air velocity ( u 0 = 3.2 m/s) and G s = 21 kg/(m 2 s), beyond the saturation carrying capacity of gas, induced an S-shaped axial solid distribution with a denser bottom zone. This limited the penetration of the back feeding particles and forced the fluidizing air to flow in the central region, thus leading to a higher solid holdup near the rear wall. Under the conditions of u 0 = 4 m/s and G s = 21 kg/(m 2 s), addition of coarse particles ( d p = 1145 μm) into the bed made the radial distribution of solids more symmetrical.

Recent Patent and Modelling of Phosphate Rotary Dryer

We review recent patents on a phosphate rotary dryer, and we present in this paper two-fold. The first aim is the modelling of the phosphate rotary dryer and presentation of the dynamic model which we elaborated for the simulation. So far, no studies to investigate this have been reported. The second aim is to realize tests on the rotary dryer existing in Oued-Zem Complex of drying Khouribga-Morocco. In the present study, an improved numerical heat and mass transfer model has been developed for the rotary dryer used for drying the phosphate. The developed model is obtained on the basis of the equations of heat and mass transfer between the gaseous and solid phases. These equations are simplified by using suitable assumptions. The latter were validated by the tests carried out on the location. In order to better understand the behaviour of the product during drying process and to validate the mathematical model developed, we conducted tests on a rotary dryer installed in Oued-Zem Complex of drying. The purpose of these tests was the determination of phosphate humidity and temperature gas profiles along the dryer. The computer results show that the present model can predict the length of the dryer as well as axial solid and gas temperature distributions with reasonably good accuracy. The model developed is in a dynamic regime and it makes possible to take into account the thermal convective process, the porosity of the product as well as the generation of humidity. The results of the simulation of the model are compared with the experimental data.

Evaluation of solids suspension in a pilot-scale mechanical flotation cell: The critical impeller speed

This paper investigates solids suspension in a pilot-scale mechanical flotation cell in terms of the critical impeller speed, N js . Understanding solids suspension has become increasingly important in recent years due to dramatic increases in flotation cell sizes but appears to be relatively poorly researched. The critical impeller speed is commonly used to indicate the effectiveness of solids suspension in stirred tanks, but has seldom been investigated in flotation cells. In this study, critical impeller speeds were visually determined and concentration profiles were measured through sample withdrawal in a 125 l Batequip (Bateman) pilot flotation cell. Two solids size fractions (75–106 and 150–250 μm) were tested, in ungassed and gassed conditions ( J G = 0, 1 cm/s) and at various impeller speeds (300–900 rpm). The effectiveness of solids suspension was quantified in three ways; (i) the extent of off-bottom solids suspension, (ii) the extent of axial solids distribution (suspension height) and, (iii) the variability of axial solids distribution. Consistent trends were found when these effectiveness criteria were considered against relative impeller speed, in terms of percentage of critical impeller speed, N/ N js . As per definition, off-bottom solids suspension was found to be complete at or above 100% of N js , with significant sedimentation occurring as the impeller speed dropped below 60% of N js . Suspension heights consistently reached a level equivalent to 90% of the tank diameter at the critical impeller speed, and dropped off significantly as N/ N js dropped below 60%. The relative standard deviation of the vertical solids distribution consistently reached a value of around 30% at N js . The paper concludes that the critical impeller speed, as per stirred tanks, is an appropriate measure for describing and benchmarking the effectiveness of solids suspension in a mechanical flotation cell. The authors speculate that, when solids suspension is considered as a precondition to flotation, flotation cells should not be operated at specific ‘impeller speeds’ but rather at specific ‘percentages of critical impeller speed’ analogous to the ‘critical speed’ used in the operation of grinding mills.

Hydrodynamics of an annulus airlift reactor

The hydrodynamics of an annulus airlift reactor (AALR) was studied and compared with that of a slurry bubble column reactor (SBCR) with silica sands of 75–125 μm in size as solids, city tapping water as liquid phase, and air as gas phase in the present investigation. The effects of superficial gas velocity and solids concentration on gas holdup and solids distributions were investigated. The results showed that the local average gas holdup decreased along the column height, and the average gas holdup decreased with the increasing solids concentration, but this tendency became less at higher solids concentrations. It was found that the effect of superficial gas velocity on axial solids distribution was negligible over the gas velocity range investigated, as long as the solids in the column could be suspended. Increasing solids concentration led to flatter axial solids holdup profiles. The axial distributions of solids concentration and gas holdup in the AALR were much more uniform than those in the SBCR, and slurry circulation in the AALR damped the effects of increasing solids concentration on the hydrodynamics. These advantages of an AALR over a SBCR are especially important for some catalytic reaction processes in three-phase systems such as Fischer–Tropsch synthesis and methanol synthesis.

Axial Solids Distribution and Flow Development in the Risers of Circulating Fluidized Beds with Different Heights

The axial apparent solids holdup and flow development is studied in two riser circulating fluidized bed reactors of different heights (10 m and 15.1 m risers), but similar diameter (76 mm and 100 mm). Pressure transducers were used for the measurement of axial solids distribution and flow development. For a higher riser, the axial distribution of solids holdup becomes more uniform along the riser. The apparent solids holdup increases with the increase of solids circulating rate or decrease of superficial gas velocity. However, the influence of these operating conditions on solids holdups at different axial regions have different tendency of change, which is quite different at particle acceleration region and fully developed region. A more accurate correlation is proposed to predict average solids holdup at a fully developed region of CFB riser.

Non-Newtonian annular vertical flow of sand suspensions in aqueous solutions of guar gum

In this work we present an experimental study of non-Newtonian solid–liquid suspension flow in the annular space of a concentric-pipe system that simulates the geometry of wells during drilling for petroleum exploration and production, excluding effects of drill rotation. The liquids used were solutions of biodegradable guar gum at various concentrations and the solid phase was siliceous sand. We studied how the mean annular velocity, polymer concentration and degree of polymer cross-linking affect axial solids distribution. The results show that the solids suspension capacity of the solutions is enhanced by the presence of polymer: the addition of guar gum decreases the minimum annular velocity needed to achieve a homogeneous axial solids distribution in the annular region. However, the amount of solids that is carried by the solutions is not always increased in the presence of polymer. The chemical cross-linking of guar gum with borax promotes a substantial increase of transported solids in the annular space. Overall, the results presented show that relatively high viscosities, high polymer relaxation times under shear flow and non-zero residual shear stresses are desired rheological characteristics for a drilling fluid to obtain a uniform solids distribution in the annulus, as well as high solids carrying capacity.

Non-Newtonian annular vertical flow of sand suspensions in aqueous solutions of guar gum

In this work we present an experimental study of non-Newtonian solid–liquid suspension flow in the annular space of a concentric-pipe system that simulates the geometry of wells during drilling for petroleum exploration and production, excluding effects of drill rotation. The liquids used were solutions of biodegradable guar gum at various concentrations and the solid phase was siliceous sand. We studied how the mean annular velocity, polymer concentration and degree of polymer cross-linking affect axial solids distribution. The results show that the solids suspension capacity of the solutions is enhanced by the presence of polymer: the addition of guar gum decreases the minimum annular velocity needed to achieve a homogeneous axial solids distribution in the annular region. However, the amount of solids that is carried by the solutions is not always increased in the presence of polymer. The chemical cross-linking of guar gum with borax promotes a substantial increase of transported solids in the annular space. Overall, the results presented show that relatively high viscosities, high polymer relaxation times under shear flow and non-zero residual shear stresses are desired rheological characteristics for a drilling fluid to obtain a uniform solids distribution in the annulus, as well as high solids carrying capacity.

The Influence of the Distributor Plate on the Bottom Zone of a Fluidized Bed Approaching the Transition from Bubbling to Turbulent Fluidization

The dynamics of the bottom zone of a narrow fluidized bed, from bubbling to turbulent regimes, was studied in a cold model of 0.1 m i.d. and 1.3 m high. Tested distributor types were perforated perspex plates, with six different perforation grids, metallic mesh and porous ceramic, with pressures drops ranging from 0.05 to 350 kPa, corresponding to superficial air velocities from 0.1 to 2.3m s−1. Group B silica ballotini, within the range 0.355–0.425 mm, were used as bed material. The experimental data consisted of pressure drop and absolute pressure fluctuating signals, together with visual observations. The bottom zone presented a dynamic condition that produced higher pressure drop values than those expected with the incipient fluidization condition, particularly for the distributor plates with a higher open area. A simple model is used to describe the axial solids distribution and the dynamics characteristics of the voids created in that zone, and an attempt is made to differentiate the bubble voidage from that of the dense phase, with a value of through flow estimated in a systematic way.

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.

Characterization of downflowing high velocity fluidized beds

AbstractA downer‐riser circulating high velocity fluidization apparatus was developed to study the fundamentals of downflowing gas‐solid particle mixtures. The acceleration and deceleration of solids due to the influences of the entrance and exit sections result in a relatively uniform axial solids distribution. Radial solid density profiles detected with an X‐ray imaging system in the downer show the existence of a core‐annulus flow with a dilute core surrounded by a denser wall region. Local solids flux profiles were obtained with an aspirating probe device and the solid velocity profile obtained from the two measured quantities. These confirm that the majority of solids segregates in a wall region that flows faster than the dilute core region. Thus, the shorter residence time in the high‐speed downer wall region is coupled with faster reaction rates due to the accompanying high concentration of catalyst, while the dilute core has slower reaction rates with longer residence time due to the lower catalyst concentration and flow velocity. This results in much more uniform reaction extent over the cross‐sectional area of the downer and, therefore, should improve the product selectivity.

Classification and characterization of hydrodynamic and transport behaviors of three-phase reactors

According to axial profile of solid concentration, the cocurrent upward threephase reactors with liquid as continuous phase can be classified into three types (a) gassparged slurry reactors, (b) threephase bubble columns, and (c) threephase fluidized beds Comparative study shows that the gas hold up, bubble characteristics and mass transfer are significantly dependent on the type of threephase reactors Three types of reactors exhibit the different hydrodynamic and transport behaviors with particle size, solid concentration and gas holdup The structural analysis of the axial solid distribution indicates the bubble and bubble wake dynamics are the key factors to the hydrodynamic and transport behaviors of three-phase reactors.

Hydrodynamic behavior of slurry bubble column at high solids concentrations

Hydrodynamics of a slurry bubble column have been investigated up to slurry concentrations of 40 vol.% and gas velocities of 0.26 m/s. High slurry concentrations represent high catalyst loading to increase reactor productivity, while high gas velocities would be required to increase reactor throughputs. The solid particles used are 35 μm glass beads representing typical particle size for catalytic slurry reactors. The two important hydrodynamic parameters investigated are gas holdup and solids concentration profiles. The average gas holdups decreased with increasing slurry concentration but the rate of decrease slowed down for higher slurry concentrations. The analysis of axial gas holdup profiles indicated that decrease in gas holdup due to solids addition could be attributed to a decrease in bubble breakup rates. The experimental gas holdups are compared with literature correlations and models. The axial distribution of slurry concentration followed the classical sedimentation–dispersion model. Effects of gas velocity on axial solids distribution are found to be minimal over the range of gas velocities investigated.