Powder Hold-up Research Articles

Exergy efficiency and thermocline degradation of a packed bed thermal energy storage in partial cycle operation: An experimental study

To enable the exploitation of the industry sectors’ huge waste heat potential this study investigates the utilization of packed bed thermal energy storage systems for the waste heat recovery in the iron and steel industry. The main goal is to assess the partial cycle operation of a packed bed thermal energy storage with industrial exhaust gas that is contaminated with high amounts of metal powder. A continuously rising accumulation of powder particles inside the packed bed requires the storage to be charged from the bottom in order to facilitate the removal of powder hold-up. Therefore, investigations focus on the dependency of the thermal performance of the storage on the flow direction of heat transfer fluid during charging/discharging. Exergy efficiency and thermocline degradation are evaluated as key performance indicators for different flow directions of the heat transfer fluid using a lab-scale test rig. Results show that, compared to charging from the top, a slightly faster thermocline degradation occurs for a storage that is charged from the bottom. Still, energy and exergy efficiencies in partial cycle operation are well above 85% and 80% respectively regardless of the heat transfer fluid flow direction. The thermal power rate during discharging is stable between 74% and 93% with respect to the maximum input rate for both charging the storage from the top and from the bottom. All in all, packed bed thermal energy storage systems are found to be suitable for waste heat recovery in industrial processes. Only a marginal deterioration of the thermal performance of the storage has to be expected if the storage is required to be charged from the bottom.

Packed bed thermal energy storage for waste heat recovery in the iron and steel industry: A cold model study on powder hold-up and pressure drop

Waste heat recovery in the energy intensive industry is one of the most important measures for the mitigation of climate change. The present study examines the integration of a packed bed thermal energy storage for waste heat recovery in the iron and steel industry. Along with the highly fluctuating availability of excess heat the main difficulty of waste heat recovery in industrial processes is the high amount of powder that is transported by the hot exhaust gases. Therefore, the experimental investigations in this study focus on the powder hold-up and pressure drop in a packed bed thermal energy storage that is operated with a gas-powder two phase exhaust gas as heat transfer fluid. The ultimate goal is, to assess its suitability and robustness under such challenging operational conditions. The results indicate, that 98% of the powder that is introduced into the system with the heat transfer fluid during charging accumulates in the packed bed. Remarkably, most of the powder hold-up in the packed bed is concentrated near the surface at which the heat transfer fluid enters the packed bed. When reversing the flow direction of the heat transfer fluid to discharge the storage with a clean single phase gas, this gas is not contaminated with the powder that has been accumulated in previous charging periods. The entirety of these findings reinforces the suitability of packed bed thermal energy storage systems for waste heat recovery in the energy intensive industry.

Investigating the effects of material properties on the mixing dynamics of cohesive particles in a twin screw mixer using a discrete element method approach

A discrete element method (DEM) model was developed for a co-rotating twin-screw mixer (TSM) consisting of conveying and kneading elements. This model was used to simulate the mixing of a cohesive binary system of particles (P1 and P2). The particles were calibrated using angle of repose and dynamic yield strength simulations. Properties of P1 were kept constant throughout the study while properties of P2 were varied. The TSM model was used to study the influence of particle size, density, and flowability of P2 on process outputs such as powder holdup, mean residence time (MRT), and degree of mixing along the length of the TSM. All the three particle properties were varied at three-factor levels with flowability ranging from good flow (GF), poor flow (PF), and very very poor flow (VVPF) based on Carr’s classification. The model provides fundamental insights into the differences in mixing in the conveying sections of the mixer for materials with different flowability. Due to the co-rotating nature of the screws, the majority of the material in the conveying sections was carried on one side of the TSM for GF and PF particles, while both the screws carried VVPF particles. The total steady-state holdup and the mean residence time (MRT) of particles inside the TSM were heavily influenced by particle flowability compared to the other two tested particle parameters. The holdup in the conveying compartments ranged from 11%–15% of the total holdup, while the kneading compartment comprised 40% of the total holdup in the mixer. The simulations found that VVPF particles mixed the fastest and that the GF particles had the highest tendency to demix. The model was also used to suggest and test other screw design configurations to improve mixing. A screw design with kneading elements closer to the mixer outlet resulted in better mixing of GF particles. A screw design consisting only of conveying elements achieved the same degree of mixing as the original screw design for VVPF particles. • A model for a co-rotating twin-screw mixer was developed using DEM. • Holdup and mean residence time was influenced by the particle flowability. • Particles were transported mainly on one of the conveying screws. • The model was also used to suggest changes to screw design configurations.

Quantification of powder holdups in the presence of a cavity with lateral gas–powder injection in a packed bed

AbstractThe lateral flow of gas–powder through a packed bed in a cold model is studied to understand the flow and holdup behaviour of powder in the presence of a cavity, nozzle (tuyere) protrusion, and decreasing gas condition, a system used in the ironmaking blast furnace. Experiments conducted in the current study included a two‐dimensional (2D) slot‐type packed bed. A previously published mass balance and elutriation velocity concept formed the basis for accurately quantifying the static and dynamic powder holdups. Experiments conducted under different conditions such as powder size and flux, gas flow rate, and packed particle density and size resulted in quantifying the powder holdups. The pressure drop in both horizontal and vertical directions is studied in all two‐phase flow experiments. The formation of the static holdup with time in the packed bed is studied. The reproducibility of the experiments was confirmed. The static holdup inside the packed bed at various locations along the vertical direction (i.e., height) is also quantified. The static holdup correlation developed based on experimental data resulted in a 95% confidence interval. Static powder holdup increases with a decrease in the superficial gas velocity, an increase in the size of the powder particle, and powder flux. Dynamic holdup also showed a similar trend.

CFD-DEM modelling of the migration of fines in suspension flow through a solid packed bed

In this work, based on the unresolved CFD-DEM framework, a smoothed volume distribution model (SVDM) is developed to simulate the migration of fines in suspension flow through a solid packed bed. In this model, the porosity of coarse solids is calculated by a smoothed particle volume distribution. The model is validated using two solid-liquid systems. Compared with divided particle volume method (DPVM) with a low-resolution mesh, the SVDM method allows a high-resolution mesh and can achieve a high efficiency and applicability. Moreover, the effects of inlet fine concentration and solid/fine size ratio on fines migration behaviour are studied. The results indicate that more holdup of fine powders especially in the upper zone can be captured, when a higher concentration fine phase or a larger size ratio is used. Based on the local holdup information, four powder migration mechanisms are proposed, i.e., size exclusion, powder bridging, powder deposition and powder locking.

Modelling Research of Gas Flow Resistance and the Powder Holdup during Two-Phase Flow (Gas + Powder) through the Descending Packed Bed

In the shaft metallurgical units (blast furnace, Corex, Finex and Midrex installations) the powder is generated within the process with charge materials as its source or it is introduced into the unit within the process procedure. Accumulation of small particles inside the furnace is diversified and depends on the applied process procedure. Noticeable disruptions of the flow might occur at large degree of dustiness. The article presents the results of model investigation of gas flow resistance and the powder holdup during two-phase flow (gas + powder) through the descending packed bed. The aim of the research was to determine the conditions of the descending bed suspension in the following systems: sinter + coal powder and blast furnace pellets + coal powder. Air was used as a gas. The study results were the basis for defining the conditions in which bed suspension of the model systems occurs. It was found that in case of sinter + coal powder system the most intensive powder holdup in the bed occurs together with the optimal conditions for bed movement distortions.

3d Model Study of Radial Distribution of Powder Holdup in the Descending Packed Bed of Metallurgical Shaft Furnaces

3d Model Study of Radial Distribution of Powder Holdup in the Descending Packed Bed of Metallurgical Shaft Furnaces

Effects of processing parameters and blade patterns on continuous pharmaceutical powder mixing

The present study summarizes the experimental characterization of a new continuous powder mixer (GCG-70 by Glatt®) using common pharmaceutical ingredients. The powder hold-up and residence time distribution were used to characterize the bulk behavior of the mixer as a function of impeller rotational speed, total throughput (mass flow rate) and blade configuration. The relative standard deviation (RSD), calculated from samples taken at the outlet of the blender, was used to characterize its mixing performance. The hold-up and the mean residence time decreased with increasing impeller rotational speed. The mean centered variance and the number of blade passes increased with increasing impeller rotational speed. The effect of the blade configuration on the mixing dynamics diminished as the rotation rate increased. The hold-up and mean residence time were sensitive enough to demonstrate the effects of blade configurations. The mixing performance, depending on the processing parameters, was found to be between 5% and 10% RSD for 5% w/w active pharmaceutical ingredient (API), and <3% for 30% w/w API. These results showed improvements in the mixing performance when compared to studies of other continuous mixers using similar materials and analytical techniques for quantification.

Mathematical Model of Gas, Powder and Bed Flow in Metallurgical Shaft Furnaces

This paper presents the second phase of model investigations. In the first phase research into flow for the system “gas transporting powder - moving packed bed” was conducted in the physical modeling. The influence of bed, powder and gas parameters on values of interaction forces and phenomena occurring in investigated system was defined.The article discusses the successive stage of investigations into gas flow carrying the powder through the descending packed bed. The research was performed with the application of mathematical modeling after tests with a physical model in use had been accomplished. The elaborated mathematical model was used to calculate resistance values of gas flow carrying the powder through the descending packed bed, masses of ‘static’ and ‘dynamic’ powders as well as total mass of powder holdup in the bed. Then the verification of the model was done comparing the obtained results with those from the physical model.

Influence of the Bed Type on the Flow Resistance Change During the Two-Phase (Gas + Powder) Flow through the Descending Packed Bed

Abstract The flow of gases with powder in the countercurrent to the charge materials occurs in many chemical processes. In the shaft metallurgical devices, the physical and chemical processes take place also in the countercurrent system. An important issue is that there are no disruptions of the flow in this multiphase system. Under real operating conditions of the device, the powder is generated within the process and its source is the charge or it is inserted to the device within the process procedure. In this system, a problem of bed particle suspension appears. That is why the author undertook investigations on the gas - powder flow in the descending bed. A physical model of this system was constructed. The experiments were performed and the influence of gas velocity, a type and size of the bed and powder particles as well as the powder concentration in the gas was established. Conditions when the descending bed suspension occurs were defined. In the case of physical model with glass materials, the suspension of bed did not occur. Therefore, investigations using beds of high alumina materials, blast furnace pellets and iron powder were performed. The results are presented below. When the bed of glass spheres was replaced with the bed of alumina spheres, a considerable increase in the volume of powder held up in the bed the gas flow resistance were observed. The surface properties of bed particles changed and better conditions for powder holdup were created. The actual gas velocity in the bed increased due to void fraction reduction. Replacement of the glass powder with the iron powder caused a change in the powder density, its surface properties and the shape factor. Greater amounts of the iron powder were held up in the bed and the gas flow resistance increased. Comparing the alumina particle bed - iron powder system to the blast furnace pellet bed - iron powder system, changes in the surface properties of bed particles and the void fraction of bed changed. The study results were the basis for defining conditions of the descending bed suspension.

Effect of operating conditions and design parameters in a continuous powder mixer

An experimental investigation was carried out to study the mixing performance and flow behavior in a continuous powder mixer for a typical pharmaceutical mixture. Blender performance, characterized by the relative standard deviation (RSD) of composition of blend samples taken at the blender discharge and by the variance reduction ratio (VRR) of the blender, was measured as a function of impeller rotation rate, flow rate and blade configuration. The flow behavior in the continuous mixer was characterized using the residence time distribution (RTD) and powder hold-up measurements. To quantify the strain applied to the powder in the blender, the number of blade passes experienced by the powder in the blender was calculated using the residence time measurements. The relationship between different experimental parameters and mean residence time and mean centered variance was examined. The mixing performance was largely dominated by the material properties of the mixture, which had a larger effect than the ingredient flow rate variability contributed by the feeders. Holdup was strongly dependent on impeller rotation rate; as impeller rotation rate increased, holdup (and therefore, residence time) decreased sharply. As a result, intermediate rotation rates showed the best mixing performance. Blade configuration affected performance as well; blade patterns where some of the blades push the powder backwards improved the mixing performance.

Modelling of Multiphase Flow in Ironmaking Blast Furnace

A mathematical model for the four-phase (gas, powder, liquid, and solids) flow in a two-dimensional ironmaking blast furnace is presented by extending the existing two-fluid flow models. The model describes the motion of gas, solid, and powder phases, based on the continuum approach, and implements the so-called force balance model for the flow of liquids, such as metal and slag in a blast furnace. The model results demonstrate a solid stagnant zone and dense powder hold-up region, as well as a dense liquid flow region that exists in the lower part of a blast furnace, which are consistent with the experimental observations reported in the literature. The simulation is extended to investigate the effects of packing properties and operational conditions on the flow and the volume fraction distribution of each phase in a blast furnace. It is found that solid movement has a significant effect on powder holdup distribution. Small solid particles and low porosity distribution are predicted to affect the fluid f...

Penetration of fine cohesive powders through a powder-particle fluidized bed

AbstractThe penetration of fine cohesive powders through a powder-particle fluidized bed (PPFB) was studied and correlated with the hold-up of fine powders in the bed. Silica sand, limestone, glass beads, bentonite, composite clay and silica gel of 550 μm mean size and silica sand of 780 μm mean size were used as coarse particles. Aluminum hydroxide powders of 0.5, 1.0 and 3.0 μm mean size were used as fine powders. The effects of aerosol concentration and size of fine powders, type of coarse particles and superficial gas velocity on the penetrating fraction of fine powders were investigated under various hold-ups of fine powders in the bed at the non-equilibrium state. In the early stage of penetration, the penetrating fraction of fine powders was hardly affected by the inlet aerosol concentration of fine powders, superficial gas velocity and size of coarse particles, but strongly affected by the size of fine powders and type of coarse particles. A fluidized bed of angular coarse particles having a rough surface or a highly rough surface with a granular structure demonstrated very low penetration of aerosol fine powders until relatively high hold-up of fine powders in the bed. Beds of coarse particles having a higher angle of repose gave higher equilibrium hold-up of fine powders in the bed.

Solid circulation rate in a circulating fluidized bed in the presence of fine powders

Fine powders (Geldart's group C) are added to a circulating fluidized bed (CFB) of coarse particles (Geldart's group A) and the solid circulation rate (SCR) is investigated with addition of fine powders of different sizes and different fractions (different hold-ups) to the bed. Experiments were carried out in a CFB of 2 m in height and 0.052 m in diameter, using FCC catalyst particles of 66 μ m as the coarse particles and cohesive aluminum hydroxide powders of 0.5– 15 μ m as the fine powders. The effects of hold-up of fine powders in the bed, fine powders size, and superficial gas velocity on the SCR were investigated. The SCR strongly depended on the hold-up of fine powders of 0.5 – 1.0 μ m in size and noticeably decreased with increasing the hold-up of fine powders under constant gas velocity. This dependency disappeared when the size of fine powders was larger than 3.0 μ m . Thus, depending on the size of fine powders added to the CFB, two distinct regions for the changes of SCR could be clearly identified.

Evaluation of the turnover times of the bed particles and of the fine powders in a circulating powder-particle fluidized bed (CPPFB)

Circulating fluidizing system of binary Geldart C powders and Geldart A particles was formed, and was called a circulating powder-particle fluidized bed (CPPFB). Solid residence in the CPPFB was concerned in two aspects, namely, bed turnover time and average turnover time of fine powders. The former represented the average time needed for all the bed particles to be circulated, while the latter represented the time needed for all the fine powders in the bed to be discharged out of the bed. Both parameters were investigated under different operating conditions as to the superficial gas velocity, size and hold-up of fine powders. FCC particles of 66 μm were used as coarse particles and 1–5 wt.% Al(OH) 3 powders of different sizes ranging from 0.5 to 15 μm were used as fine powders. The bed turnover time decreased with increasing the size of fine powders to a certain level and then became almost constant with further increase of the size of fine powders. When the size of fine powders was larger than this critical size, neither the size nor the hold-up of fine powders affected the bed turnover time. The bed turnover time drastically increased with increasing the hold-up of fine powders for the cases of using very fine powders of 1.0 μm or smaller. On the other hand, the average turnover time of fine powders decreased with increasing the size of fine powders to a minimum at around 3.5 μm and then increased with increasing the size of fine powders. It also decreased with increasing the gas velocity and decreasing the hold-up of fine powders in the bed. The average turnover time of fine powders was several times larger than the bed turnover time at the same operating conditions.

Separation rate of fine powders from a circulating powder-particle fluidized bed (CPPFB)

In this study, group C powder in Geldart's classification was added to a circulating fluidized bed or a fluid bed of group A particles. We define this system as a circulating powder-particle fluidized bed (CPPFB). The separation rate of fine powders from a CPPFB was investigated using two different types of solid collectors. A cyclone or a settling chamber was used as a solid collector. The effects of the superficial gas velocity (0.65–1.0 m/s), the holdup of fine powders in the bed (0–5 wt.%), and the size of fine powders (0.5–15 μm) on the separation rate of fine powders from the system were investigated. The separation rate increased with the increases in the holdup of fine powders in the bed and the superficial gas velocity. Irrespective of the type of the solid collector, the separation rate of fines was strongly influenced by the fine powder size and reached the maximum at a fine powder size around 3.5 μm. However, operation with a settling chamber causes a threshold of holdup of fine powders in the bed, below which the separation of fine powders did not occur. An empirical equation for the separation rates of fine powders of 0.5–3.0 μm in size from the CPPFB was obtained.

Numerical Analysis of Static Holdup of Fine Particles in Blast Furnace

With increased in pulverized coal injection rate into blast furnace, the importance of understanding flow characteristics of powders within packed bed increases because more unburned char and coke fragments would be generated, it would be deteriorate permeability in the blast furnace. Although flow characteristics of dynamic or total holdup of powders in packed beds is found in several reports, 3-6) behavior of powders in blast furnace using separate treatment of dynamic and static holdup has yet to be reported. In this study the behavior of static powders was examined through numerical simulation using the “four fluid model”, 3) which included the formulation of static powders behavior. The model results were compared with two dimensional temperature distributions measured in the furnace for three different pulverized coal injections (PCI) rates (100, 200 and 250 kg/thm) for validated. The model was applied to quantitative analysis of static powders holdup in the blast furnace operation with high-rate pulverized coal injection. The higher amount of static powders holdup was found mostly in the center lower deadman, above and below the tuyere level, and in upper shaft, for PCI rate 100 kg/thm, 150 kg/thm, 200 kg/thm and 225 kg/thm respectively. The lower amount of static powders holdup was found in the raceway region throughout the surface of deadman.

Numerical Analysis Of Powder Deposition InBlast Furnace By Using Multi-fluid ProcessSimulator

Blast furnace is a principal metallurgical reactor in ironmaking process, and usually treated as a counter current moving bed reactor. In the lower part of this reactor, flows of gas, solid, liquid and powder exist at the same time. With recent increase in pulverized coal injection, operation problems due to deposition of fine particles in this zone have been arising. The authors developed a process simulator of blast furnace treating more than four different phases that had individual flow mechanisms. In this study, mathematical formulations describing powder deposition rate by difference between adhering and departing rates has been introduced, and the simulator is able to handle flowing and deposited powders separately. It is assumed that the deposited powder phase has flow pattern identical to the solid one because it is trapped in the interstice among packed particles or on the particle surface. The simulator successfully reproduces temperature field and operation status of the blast furnace at different pulverized coal injection rates. The simulation results revealed the behavior of powder in the blast furnace, such as distributions of static powder hold-up, concentration of flowing powder, major zones of powder deposition, and so on.

Studies on gas–solid–solid circulating fluidized-bed reactors

In the gas–solid–solid circulating fluidized-bed reactor (GSSCFR), a packed bed and a fluidized bed are interconnected. Large catalyst particles are fixed in a packed bed, in which the catalytic reaction takes place. Powder, often used as adsorbent or heat carrier, is circulated between the two beds to enhance the efficiency of mass and heat transfer. The hydrodynamic behaviors of gas-powder flow through the packed bed, such as the mass flow rates, the powder hold-up and the pressure gradient along the packed bed are studied under different operating conditions. The catalytic oxidation–dehydrogenation of butene (C 4H 8) to butadiene (C 4H 6) in the GSSCFR (as an example) is described. The experimental results show that the C 4H 8 conversion and the C 4H 6 yield are increased by 10–20% and 10–15%, respectively, and the H 2O/C 4H 8 molar flux ratio in the feed is reduced from 16 : 1 to 8 : 1 when compared with those existing in industrial adiabatic packed-bed reactors. Finally, a hydrodynamic model is established. It further testifies that the GSSCFR can effectively combine the chemical reaction with heat transfer by using the powder as a heat carrier.

Gas-solids circulating fluidization in a packed bed

Gas-solid-solid circulating fluidized bed is a novel gas-solid contactor which offers interesting heat and mass transfer properties. In this type of equipment, large particles (often used as the catalyst) are fixed in the bed forming a packing section while fine powders (often used as the absorbent or heat carrier) are carried by the gas and flow through the packing. In this paper, the pressure gradient and powder hold-up in the packing have been measured in a rectangular fluidized bed. A theoretical analysis for the prediction of pressure drop, which is caused by the powder hold-up and the friction between gas and packing and between powders and packing, has been proposed. The predicted results agree well with the experimental results.