Circulation Time Of Particles Research Articles

Numerical investigation of the particle circulation characteristics in a tapered fluidized bed

PurposeThe objective of this study is to investigate several issues related to particle circulation within the TFB, including exploring an appropriate method to quantify particle circulation time, the effects of different operational parameters on particle circulation time, and the relationship between particle mixing and particle circulation.Design/methodology/approachThe computational fluid dynamics coupled with the discrete element method (CFD-DEM) is applied to investigate the particle circulation characteristics of a tapered fluidized bed (TFB). An approach for defining particle circulation, which accounts for the horizontal motion of each particle, is proposed to estimate particle circulation time.FindingsIt is found that the overall particle circulation in a TFB could be accelerated by increasing air velocity and wall inclination angle, while an increase in particle size and an increase in inter-particle cohesive forces decelerate particle circulation; the increase in the open area ratio of the central region of the air distributor would decelerate the particle circulation. Moreover, the particle circulation time and mixing rate are independent variables that describe the flow dynamics of particles from different perspectives.Practical implicationsA large part of fluidized beds in industrial applications can be classified as TFB. This study presents a numerical method to obtain detailed knowledge about particle circulation in a TFB, which is essential for the design, optimization, and control of related processes.Originality/valueThe particle circulation in a TFB is important but rarely investigated, and it is hard to be quantified using existing experimental approaches. The proposed numerical workflow reveals the characteristics of particle circulation from a particle-scale perspective.

PEPT validated CFD-DEM model of aspherical particle motion in a spouted bed

Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) has been used to model the dynamics of aspherical particles in a spouted bed. CFD-DEM results were rigorously validated against high-resolution, three-dimensional experimental data acquired using Positron Emission Particle Tracking (PEPT). A workflow facilitating the direct, detailed, quantitative comparison of experimental and numerical data, and thus validation of CFD-DEM simulations, was developed. Sensitivity analysis considering a wide range of particle properties and drag correlations was carried out. It was observed that the Young’s modulus, Poisson’s ratio, and restitution coefficient do not have a significant effect on the system’s dynamics across the range of parameters explored, and that Gidaspow’s correlation using equivalent intrinsic particle density is an appropriate choice for modelling drag forces. Analysis shows PEPT-validated CFD-DEM models quantitatively capture key features including particle circulation time, particle velocity and occupancy distributions, and the proportion of residence time in the bulk-bed and freeboard regions, all of which are crucial to understanding mass and heat transfer within the system. Our results also demonstrated that models using simple, spherical particles were – with suitably modified friction and drag models – capable of quantitatively reproducing the dynamics of a range of highly aspherical materials.

Eulerian-Lagrangian Modelling of Turbulent Two-Phase Particle-Liquid Flow in a Stirred Vessel: CFD and Experiments Compared

A 3D Eulerian-Lagrangian CFD model is set up to simulate the turbulent flow and mixing of coarse dense particles in a standard batch vessel, mechanically agitated by a down-pumping pitched-blade turbine. Flow is investigated in the just-suspended regime and regimes much above. All simulations are fully and successfully validated by experimental Lagrangian measurements provided by positron emission particle tracking. Predicted distributions of the local 3D phase-velocity components, local particle slip velocities and local particle concentration show very good agreement with experiment. Lagrangian particle trajectories are exploited to infer detailed accurate information on particle circulation time and local residence time. The predicted two-phase flow number is in excellent agreement with experimental values under all flow regimes. Being accurate and robust, the numerical model can be extended to study even more complex particle-liquid suspensions of industrial relevance, and to provide much needed Lagrangian data to other data-driven modelling techniques.

Homologous tumor cell membrane vesicles active preferential self-recognition of tumor cells in vitro

Cell membrane vesicles, as delivery carriers of drugs or biological agents in vivo, are an important therapeutic mode in the study of disease treatment. Tumor membrane-derived vesicles have been widely used in tumor therapy because of their good tumor enrichment effect. The most common method is the surface of nanoparticles coated with tumor cell membrane, which can effectively prolong the circulation time of particles in the blood and the enrichment of tumors. In this study, we prepared vesicles of different tumor cell membrane derivate and studied their targeting to tumors detailly. The results showed that homologous vesicles have high targeting to homologous tumor cells. The fluorescence of vesicles in homologous tumor cells was significantly higher than that in other tumor cells. This study will provide a new strategy and guidance for the clinical treatment of cancer based on the tumor cell membrane system.Graphical

Solids flow characteristics and circulation rate in an internally circulating fluidized bed

Internally circulating fluidized beds (ICFBs) enable effective control of the reactions and heat distribution in reactors. The ICFB contains two or more connected fluidized regions with different gas velocities to promote controlled solid circulation. The control of solid circulation rate (G0) is a critical factor. We recorded single particle trajectories by tracing a fluorescent particle, based on which particle flow behaviors were analyzed in different regions. G0 was obtained for a wide range of operating parameters. An increase in gas velocity in the down- and upflow beds shortened the particle circulation time in both beds and G0 increased significantly. As the static bed height increased, the differential pressure on both sides of the circulation port increased, which resulted in an increase in the solid circulation rate. As the orifice area increased, the flow resistance through the orifice decreased and thus the solid circulation rate increased. G0 increased with the decrease in particle size. The gas velocity in the upflowing bed and orifice area was the most important parameter to control the solid circulation rate. G0 was compared with the experimental measurements in literature and predictions using the correlation based on Bernoulli’s equation, and they agreed well.

Insights into the granular flow in rotating drums

Scaling relations for the velocity profile and circulation time, which can be used in design and scale-up, are of great importance in rotating drums. We conducted simulations by discrete element method at various operating conditions (which covers both rolling and cascading regimes) for spherical and non-spherical particles. New scaling relations were proposed for evaluating and fully characterize velocity profile and circulation time in both rolling and cascading regimes. Using dynamic angle of repose, effect of shape was included in these correlations to extend them to spherical and non-spherical particles. Simulation results can satisfactorily reproduce experimental measurements. Visual results show that transition from rolling to cascading regime depends not only on Froude number, fill level and particle size, but also on the particle shape. The surface velocity is scaled with peak velocity and half chord length and its profile is asymmetric with the maximum occurring after the mid-chord position for all simulations. We obtained a correlation for the peak velocity based on our simulations as well as available experimental data in literature. We also found the velocity profiles along the bed depth in active and passive layers. Our results show that the circulation time of particles follow a log-normal distribution. The mean circulation time decreases with rotation speed and drum diameter and increases with fill ratio. This value is greater for spherical particles compared to non-spherical particles. Correlations are also proposed for mean and standard deviation of the circulation time.

Dynamic and cellular interactions of nanoparticles in vascular-targeted drug delivery.

Vascular-targeted drug delivery systems could provide more efficient and effective pharmaceutical interventions for treating a variety of diseases including cardiovascular, pulmonary, inflammatory, and malignant disorders. However, several factors must be taken into account when designing these systems. The diverse blood hemodynamics and rheology, and the natural clearance process that tend to decrease the circulation time of foreign particles all lessen the probability of successful carrier interaction with the vascular wall. An effective vascular-targeted drug delivery system must be able to navigate through the bloodstream while avoiding immune clearance, attach to the vascular wall, and release its therapeutic cargo at the intended location. This review will summarize and analyze current literature reporting on (1) nanocarrier fabrication methods and materials that allow for optimum therapeutic encapsulation, protection, and release; (2) localization and binding dynamics of nanocarriers as influenced by hemodynamics and blood rheology in medium-to-large vessels; (3) blood cells' responses to various types of nanocarrier compositions and its effects on particle circulation time; and (4) properties that affect nanocarrier internalization at the target site.

Dynamic and cellular interactions of nanoparticles in vascular-targeted drug delivery (review).

Vascular-targeted drug delivery systems could provide more efficient and effective pharmaceutical interventions for treating a variety of diseases including cardiovascular, pulmonary, inflammatory, and malignant disorders. However, several factors must be taken into account when designing these systems. The diverse blood hemodynamics and rheology, and the natural clearance process that tend to decrease the circulation time of foreign particles all lessen the probability of successful carrier interaction with the vascular wall. An effective vascular-targeted drug delivery system must be able to navigate through the bloodstream while avoiding immune clearance, attach to the vascular wall, and release its therapeutic cargo at the intended location. This review will summarize and analyze current literature reporting on (1) nanocarrier fabrication methods and materials that allow for optimum therapeutic encapsulation, protection, and release; (2) localization and binding dynamics of nanocarriers as influenced by hemodynamics and blood rheology in medium-to-large vessels; (3) blood cells' responses to various types of nanocarrier compositions and its effects on particle circulation time; and (4) properties that affect nanocarrier internalization at the target site.

PEPT visualisation of particle motion in a tapered fluidised bed coater

Detailed knowledge of particle motion is essential in better understanding fluidised bed coating processes. This paper reports on the use of Positron Emission Particle Tracking (PEPT) in order to visualise and quantify the features of the powder movement inside a laboratory-scale GPCG-1 fluidised bed coater (Glatt GmbH, Germany) with a tapered (conical) chamber. In all cases, a clear circulating motion, upwards in the centre and downwards along the walls, was revealed. This circulating pattern lends itself to operations such as particle coating, where controlled application of the coating onto the particles is essential. In contrast to the fluidisation patterns observed in cylindrical fluidised beds or spouted beds without a draft tube, radial movement was stronger and occurred at all heights in the powder bed. By introducing a binary nozzle and increasing the nozzle atomisation air pressure, the circulating motion of the fluidised particles in a tapered vessel was found to be accelerated. When spraying a coating solution downwards onto the fluidised powder particles, effects on the particle motion, depending on coating solution properties and process parameters, were observed. Besides visualisation, PEPT was also used to derive quantitative information such as bed height and particle circulation time, as well as to investigate the size of the coating zone. Generally, it was demonstrated that PEPT can be successfully used to characterise the particle motion in a tapered fluidised bed under both drying and coating conditions.

Amphiphilic polyether branched molecules to increase the circulation time of cationic particles

The preparation, physicochemical and biological properties of amphiphilic polyether branched molecules is described. These ‘bunch shaped’ molecules when inserted into cationic liposomes/DNA complexes have shown efficient surface charge shielding. As a consequence they efficiently inhibited the non specific interactions with blood components and significantly enhanced circulation time of the particles in the blood track. Formulations containing these molecules compared positively with those containing PEG lipids, providing a 5-fold increase in circulation time.

Numerical analysis of particle mixing characteristics in a single helical ribbon agitator using DEM simulation

Numerical analysis of three-dimensional motion of particles in a single helical ribbon agitator was carried out by means of the Discrete Element Method (DEM). To validate the computed results experiments were carried out with a cold scale model of 0.3 m inside diameter. Circulation time of particles in the agitator and the horizontal particle velocity distribution in the core region predicted by the present simulation agreed well with those obtained by experiments. Based on DEM simulation, the particle circulation and mixing characteristics in the agitator vessel were investigated in detail. Vertical mixing of particles was found rather poor during upward and downward flows through the blade and core regions, respectively. However, in the core region, particles flow in the manner of funnel flow. Additionally, in the blade region the vertical velocity decreases toward the wall and reduces to zero near the boundary between the blade and core regions. Particles were carried up to the bed surface by the helical ribbon flow down into the center of the core, and those not carried completely to the surface flow into the outer periphery of the core. The effect of bed height on circulation and mixing was analyzed numerically in terms of tracer particle concentration response as well as particle velocity and angular velocity distributions, which are quite difficult to obtain by experiment. The bed height was found to be the most important factor for mixing and circulation. When the bed height is appropriate, particles circulate rather regularly. If the bed height is lower than the blade top, particles mix rapidly because of the high fall of particles from the blade top. On the other hand, if the bed height is higher than the blade top, the velocity of particles in the bed surface region is much lower than that of the other region and accordingly a semi-stagnant eddy is formed.

Fluidized bed granulation: gas flow, particle motion and moisture distribution

In fluidized bed granulation the liquid is often distributed by means of a two-fluid-nozzle that is positioned inside the bed. Then the wetting process takes place in the jet that is formed by the atomizing air. A model has been derived for a free jet to calculate the particle and gas velocities, the void fraction and the cross-sectional area of the jet. The results are compared with experimental data for the particle circulation time in the fluidized bed. Based on the particle and gas motion the liquid deposition is calculated, resulting in particle moisture distributions. The influence of different parameters on the moisture distribution and possible consequences on the granulation process are discussed.

On the Behavior of Particles in a Horizontally Rotating Cylinder

An experimental study was carried out on the flow pattern of particles at the cross section of a horizontally rotating cylinder with twenty-three kinds of particulate systems of different characteristics. The following relations were obtained as the experimental results.(1) The width of the static zone was expressed with the volumetric charge ratio of particles F/V(-) and the mixing Froude number N2R/g(-) in the following equation.(h/R)=[a-b(N2R/g)](F/V)where, h is the width of the static zone (cm), R is the inner radius of the cylinder (cm), N is the rotating speed of the cylinder (1/min), F is the total volume of particles charged in the cylinder (cm3), V is the inner volume of the cylinder (cm3) and g is the acceleration of gravity (cm/min2) respectively, and a and b are the experimental constants.(2) The following relation was obtained between the mean circulation time of particles T(sec) and the rotating speed N'(1/sec) for the range of rotating speed less than one revolution per second or so.N'T=0.77(F/V)1/2

Studies on the Circulation Time of Particles and the Thickness of Static Mass in the Holizontal Cylindrical Mixer

In a holizontal cylindrical mixer, the circulation flow of particles by the rotation of the mixer consists of the mixing zone and the static mass.In this report, the thickness of the static mass of particles was analysed with the factors of the radious of mixer, R, the rotation speed of mixer, N, and the total volume of feed particles in the mixer vessel, F.As a result of this analysis, an experimental equation was derived as a function of Froude number of mixing, N2R/g, and the charge ratio of the particles in the mixer vessel, F/V, as follows:(h/R)=(1.55-30N2R/g)(F/V)By using this relation, the circulation time of the particles in the cylindrical mixer can be obtained by:T=R2(F/V)/(N/60)h(2R-h)Thus, the mixing time required to reach the desired uniformity of mixture can be calculated when the particles is easily flowable.

Particle circulation time in propeller stirred tanks

Particle circulation time in propeller stirred tanks