Modeling Solids Friction for Dense-Phase Pneumatic Conveying of Powders

This article results from an ongoing investigation aimed at developing a new validated test-design procedure for the accurate prediction of pressure drop for dense-phase pneumatic conveying of powders. Models for combined pressure drop coefficient (“K”) for solids-gas mixture were derived using the concept of “suspension density” by using the steady-state “straight pipe” pressure drop data between two different tapping locations of the same pipe and also for two different diameter pipes. It was observed that the derived models were different depending on the location of tapping points (for the same pipe) and selected pipe diameters. The derived models were then evaluated by predicting the pressure drop for pipelines with various diameters or lengths (69 mm I.D. × 168 m, 105 mm I.D. × 168 m, 69 mm I.D. × 554 m) for the conveying of power station fly ash. A comparison between the predicted pneumatic conveying characteristics (PCC) and the experimental plots showed that the models resulted in significant over-predictions. In the second part of the article, the “system” approach of scaleup was evaluated. “Total” pipeline pressure drop characteristics for test-rig pipelines were scaled up to predict the PCC for larger/longer pipes. It was found that the “system” approach generally resulted in grossly inaccurate predictions. It was concluded that further studies are needed for a better understanding of the solids-gas flow mechanism under dense-phase conditions.     

Investigating Straight-Pipe Pneumatic Conveying Characteristics for Fluidized Dense-Phase Pneumatic Conveying

This article presents results from an investigation into the pneumatic conveying characteristics (PCC) for horizontal straight-pipe sections for fluidized dense-phase pneumatic conveying of powders. Two fine powders (median particle diameter: 30 and 55 µm; particle density: 2300 and 1600 kg m^?3; loose-poured bulk density: 700 and 620 kg m^?3) were conveyed through 69 mm I.D. × 168 m, 69 mm I.D. × 148 m, 105 mm I.D. × 168 m and 69 mm I.D. × 554 m pipelines for a wide range of air and solids flow rates. Straight-pipe pneumatic conveying characteristics obtained from two sets of pressure tappings installed at two different locations in each pipeline have shown that the trends and relatively magnitudes of the pressure drops can be significantly different depending on product, pipeline diameter and length and location of tapping point in the pipeline (indicating a possible change in transport mechanism along the flow direction). The corresponding models for solids friction factor were also found to be different. There was no distinct pressure minimum curve (PMC) in any of the straight-pipe PCC, indicating a gradual change in flow transition (change in flow mechanism from dense to dilute phase). For total pipeline conveying characteristics, the shapes of the PCC curves and the location of the PMC were found to be significantly influenced by pipeline layout (e.g., location and number of bends) and not entirely by the dense-to-dilute-phase transition of flow mechanism. Seven existing models and a new empirically developed model for PMC for straight pipes have been evaluated against experimental data.     

An Investigation into Pressure Fluctuations and Improved Modeling of Solids Friction for Dense-Phase Pneumatic Conveying of Powders

The aim of this paper is to investigate into flow mechanism with the help of pressure signal fluctuations analysis and modeling solids friction in case of solids–gas flows for fluidized-dense-phase pneumatic conveying of fine powders. Materials conveyed include fly ash (median particle diameter 30 µm; particle density 2300 kg m^?3; loose-poured bulk density 700 kg m^?3) and white powder (median particle diameter 55 µm; particle density 1600 kg m^?3; loose-poured bulk density 620 kg m^?3). These were conveyed in different flow regimes varying from fluidized-dense-to-dilute phase. To obtain information on the nature of flow inside pipeline, static pressure signals were studied using technique of Shannon entropy. Increase in the values of Shannon entropy along the flow direction through the straight-pipe sections were found for both the powders. However, drop occurred in the Shannon entropy values after the flow through bend(s). Change in slope of straight-pipe pneumatic conveying characteristics along the flow direction is another factor which provided indication regarding change in flow mechanisms along the flow. A new technique for modeling solids friction factor has been developed using a solids volumetric concentration and ratio of particle terminal settling velocity to superficial air velocity by replacing the conventional use of solids loading ratio and Froude number, respectively. The new model format has shown promise for predictions under diameter scale-up conditions.     

The effect of oscillating flow on a horizontal dilute-phase pneumatic conveying

ABSTRACT

A horizontal dilute-phase pneumatic conveying system using vertically oscillating soft fins at the inlet of the gas–particle mixture was studied to reduce the power consumption and conveying velocity in the conveying process. The effect of different fin lengths on horizontal pneumatic conveying was studied in terms of the pressure drop, conveying velocity, power consumption, particle velocity, and intensity of particle fluctuation velocity for the case of a low solid mass flow rate. The conveying pipeline consisted of a horizontal smooth acrylic tube with an inner diameter of 80 mm and a length of approximately 5 m. Two types of polyethylene particles with diameters of 2.3 and 3.3 mm were used as conveying materials. The superficial air velocity was varied from 10 to 17 m/s, and the solid mass flow rates were 0.25 and 0.20 kg/s. Compared with conventional pneumatic conveying, the pressure drop, MPD (minimum pressure drop), critical velocities, and power consumption can be reduced by using soft fins in a lower air velocity range, and the efficiency of fins becomes more evident when increasing the length of fins or touching particles stream by the long fins. The maximum reduction rates of the MPD velocity and power consumption when using soft fins are approximately 15% and 26%, respectively. The magnitude of the vertical particle velocity for different lengths of fins is clearly lower than that of the vertical particle velocity for a non-fin conveying system near the bottom of the pipeline, indicating that the particles are easily suspended. The intensities of particle fluctuation velocity of using fins are larger than that of non-fin. The high particle fluctuation energy implies that particles are easily suspended and are easily conveyed and accelerated.     

Effect of Close-Coupled Bends in Pneumatic Conveying

Pressure drop in a close-coupled double bend in pneumatic conveying of fly ash is studied. Tests are carried out with a 6.35 cm (2.5 in) diameter 169.8 m (557 ft) long pipeline with various combinations of airflow, ash flow, phase density, and conveying velocity. The pressure drop across two close-coupled 90-degree bends is compared to the pressure drop in an isolated single 90-degree bend. Six ash samples of different physical and chemical compositions are used in the tests. Resulting bend pressure drops are correlated to the corresponding phase density and superficial air velocity at the bend inlet. The correlation pattern represented by the relationship {\Delta P_{solids} \over {SLR}} = Y1 \cdot V^{Y2} is established and found to vary with ash properties. For both single and close-coupled double bends and operating test conditions with \Delta {\rm P}_{\rm solids} / {\rm SLR} 0.15 at the bend entry, 86% of the measured test points fall within the range of - 20% of the \Delta {\rm P}_{\rm solids} / SLR calculated point. Below this threshold, the test results show that the pressure drops due to solids flow through a close-coupled double bend and single bends are often indistinguishable. Consequently, the loss through a close-coupled double bend cannot be considered as the cumulative effect of two isolated single bends.     

CHARACTERISTICS OF A HORIZONTAL SWIRLING FLOW PNEUMATIC CONVEYING WITH A CURVED PIPE

ABSTRACT

In order to prevent flow blockage phenomenon and to reduce the impact of particles on the wall of the bend, an experimental study of the swirling flow pneumatic conveying system with a horizontal curved pipe was carried out in this work. The experiment was performed in a 90-deg pipe bend with pipe diameter 75 mm and centerline curvature ratio 12. The straight pipes with 75 mm inside diameter at the upstream and downstream of the bend were 1.3 m and 4.0 m in lengths, respectively. The initial swirl number was varied from 0.22 to 0.60, the mean air velocity from 10 to 20 m/s, and the solid mass flow rate from 0.07 to 0.68 kg/s. It is found that in the lower air velocity range, the overall pressure drop of the swirling flow pneumatic conveying shows a lower tendency than that of axial flow pneumatic conveying. The minimum air velocities can be decreased by using the swirling flow pneumatic conveying. From the visualization of particle flow patterns, the impact of particles on the wall of the bend can be reduced using the swirling flow.     

Development and investigation of cellular light weight bio-briquette ash bricks

The present paper deals with the development of cellular light weight bricks using bio-briquette ash. The necessary physical and chemical tests were conducted on a bio-briquette ash sample to investigate its suitability for the development of bricks. Physico-mechanical, durability and thermal conductivity tests were conducted on cellular light weight bio-briquette ash bricks that fulfilled the requirements of Indian standard. The test results of cellular light weight bio-briquette ash bricks were compared with commercially available fly ash bricks. With reference to fly ash bricks, the cellular light weight bio-briquette ash bricks were found 43 % light in weight, having 13 % higher compressive strength and resulted in 66 % lesser thermal conductivity. A small scale model room (1 m × 1 m × 1 m) made up of fly ash bricks was designed. A similar built form for the cellular light weight bio-briquette ash bricks was also modelled. Both the models were analysed for indoor temperature control and cost. When compared with fly ash model room, cellular light weight bio-briquette ash brick model resulted in a 6 % better indoor temperature control and 29 % cost savings. Thus, the developed cellular light weight bio-briquette ash bricks were found suitable as an alternate construction material for non-load bearing walls.     

Effect of Particle Properties on Slug Flow Conveying in a Horizontal Pneumatic Pipeline

The influence of particle properties on slug flow conveying was experimentally examined by using polyethylene particles of different densities from 825 kg/m³ to 945 kg/m³ in a horizontal pipeline 5.5 m in length, inside diameter of 32 mm, for air speeds below 8 m/s. It was found that hardness affects the slug flow conveying in such a way that for soft particles lower limiting velocity as well as boundary air velocities for suspension flow and slug flow increases. Additionally, it was found that the frictional characteristics of a particle influence its flow pattern. Also, there are two types of slug flow, that is, a solitary slug flow and a consecutive slug flow. In a solitary slug flow, there is at most only one plug in the pipeline. In a consecutive slug flow, the particles are conveyed continuously as slugs. There is always at least one slug in the pipeline.     

Numerical Study of Air Compressibility During Horizontal Pneumatic Transport

Using numerical simulations, the effect of the compressibility of air on the flow pattern of particles and pressure drop in the presence of particles during horizontal pneumatic transport operating under negative pressure was examined. The length and inside diameter of the pipeline were 30 m and 40 mm, respectively, and the chosen particles (4 mm in diameter) had densities of ρ_p = 1000 and 2000 kg/m³. The mean air velocities at pipe the inlet were U_inlet = 19, 22, and 28 m/s, and the range of the mass flow rate ratios of particle to air, μ, was varied up to 2.0. For a given inlet air velocity, the difference in the flow pattern between compressible and incompressible flow calculation is generally small. For ρ_p = 1000 kg/m³ particles the additional pressure drop in compressible flow increases when μ is above 0.5 and U_inlet is 28 m/s, μ is above 1.3 and U_inlet is 22 m/s, and μ is above 1.5 and U_inlet is 19 m/s. In these cases, the particle flow pattern is homogeneous. For ρ_p = 2000 kg/m³ particles, the pressure drop increases only when μ is above 1.5 and U_inlet is 28 m/s. The difference is not noticeable when the particle flow pattern is heterogeneous. Also, the difference in the additional pressure drop is much larger during homogeneous flow than heterogeneous flow.     

Simulation of Gas-Solid Flow in Vertical Pipe by Hard-Sphere Model

ABSTRACT

This article presents a two-dimensional study of the gas-solid flow in a vertical pneumatic conveying pipe by means of a hard-sphere model where the motion of individual particles can be traced. Simulations were performed for a pipe of height 0.9 m and width 0.06 m, with air as gas phase and particles of density 900 kg/m³ and diameter 0.003 m as solid phase. Periodic boundary conditions were applied to the solid phase in the axial direction. Different cases were simulated to examine the effects of the number of particles used, superficial gas velocity, and restitution coefficient. The results show that the main features of plug flow can be reasonably captured by the proposed simulation technique. That is, increasing the number of particles in a simulation will increase the length of plugs but does not change the velocity of plugs; the solid fraction of a plug is relatively low if the number of particles is small. In particular, it is shown that increasing superficial gas velocity will increase the velocity of plugs and the frequency of plugs, and the pressure drop through a rising plug increases linearly with the plug length, suggesting that the total pressure of a conveying system with a given length can be quantified from the information of plug length and plug frequency. Increasing the restitution coefficient can promote the momentum transfer between particles and hence the raining down of particles from the back of a plug in vertical pneumatic conveying. The simulation offers a useful technique to understand the fundamentals governing the gas-solid flow under pneumatic conveying conditions.     

On Improving Scale-Up Procedures for Dense-Phase Pneumatic Conveying of Powders

This article presents results of an ongoing effort toward improving the modeling and scale-up procedures for the dense-phase pneumatic conveying of fine powders through pipes. Two new approaches are employed in this study. One approach, derived by modifying an existing reliable dilute-phase model to make it suitable for the dense-phase, has resulted in relatively stable predictions for diameter and length scale-up for two types of fly ash, ESP dust, pulverized brown coal and fly ash/cement mixture. Although some over-predictions still remain for the cases of diameter scale-up, there seems to be a substantial relative improvement in the overall accuracy of predictions (compared to the existing design methods). Another method has been derived using the concept of “two-layer” slurry flow modeling (suspension flow occurring on top of a non-suspension moving layer), and this has also resulted in similar improvements. Although the “two-layer” technique is believed to be more representative of the actual flow conditions under dense-phase conveying, the simpler “modified” method appears to be adequate for practical design purposes.     

Mineralogical Studies of Coal Fly Ash for Soil Application in Agriculture

Coal fly ash procured from Guru Gobind Singh Super Thermal Power Plant, Ropar, Punjab, India, was analyzed for its mineralogical content and thermal stability by x-ray diffraction (XRD), thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and physicochemical properties. XRD studies showed that major crystalline phases observed were quartz (SiO₂) and aluminum silicon oxide (Al_4.52Si₁.₄₈) with macro- and microelement (N, P, K, Mg, Zn, S, and Fe). Fly ash showed thermal stability up to 500°C and reduction in weight was up to 200°C, primarily due to loss of water and decarboxylation as revealed by TGA plots. FTIR of fly ash showed that the most prominent peaks in the spectra corresponded to Si–O and Al–O stretch vibrations. Coarse-grain accumulation of fly ash indicated the presence of 70% of fine-grained particles of 0.075 mm. Coal fly ash was alkaline in nature (pH 7.85 ± 0.03) with an electrical conductivity of 0.14 ± 0.02 µS m^?1, water holding capacity of 62%, low bulk density of 0.99 g cm^?3, and a surface area of 0.96 m² g^?1. With properties similar to that of soil coal, fly ash represents a suitable material for use in specific quantities as a soil amending agent in agriculture.     

Power Spectral Density Analysis of Pressure Fluctuation in Pneumatic Conveying of Powders

In order to reveal the unsteady features of gas–solid flow, the pressure fluctuations were measured at different locations along the length of the pipeline while conveying powders through the pipeline. Power spectral density (PSD) functions were obtained for the analysis of the pressure fluctuation. Two types of powders (fly ash and alumina) were used in this analysis. The PSD analysis was conducted by taking into account different aspects such as flow conditions (dilute or dense), location of transmitter (top and bottom transmitters), location of transmitter along the length of the pipeline (three different locations), material property (fly ash or alumina), etc. Analysis of signals from top and bottom transmitters shows that it is not possible to identify the flow mode at upper and lower portions of pipeline. The magnitude of power is found to be higher for alumina as compared to fly ash. PSD parametric analysis reveals that frequency bandwidth and average power decreases exponentially with increase in solid loading ratio.     

Experimental Study on Particle Deposition and Blockage in Small-Size Straight Fracture Channels

Based on the technology of using particles to block air-leakage fractures around drainage boreholes, pneumatic conveying experiments were conducted to explore the mechanism of particle deposition and blockage in fractures. Two small-size straight fractures (1000 mm × 40 mm × 2 mm and 1000 mm × 40 mm × 4 mm) were performed. The experimental results show that for the conditions under which fractures could not be blocked, average deposition height fluctuates around a certain value. This value is significantly influenced by air pressure but little impacted by mass flow rate. The average deposition height fluctuates sharply when solid flow rate increases. The effective blockage length curve slowly increases followed by a fast decrease. According to the effective blockage length, the optimum blockage effect is obtained at mass flow rate of 0.042 kg/s and air pressure of 0.25 MPa for a 2 mm-wide fracture, while for a 4 mm-wide fracture, the optimum blockage effect is obtained at mass flow rate of 0.042 kg/s and air pressure of 0.15 MPa. Both air pressure and fracture width have approximately equivalent effects on the blockage time, whereas mass flow rate does not contribute a noticeable effect.     

Variation of physical and rheological properties of fly ash slurries with particle size and its effect on hydraulic transportation at high concentrations

In thermal power plants, fly ash is collected at the bottom of electrostatic precipitator (ESP) hoppers and transported to common sump for further disposal to the ash pond by slurry pipelines. The fly ash from different fields of ESP hoppers vary widely in particle size as well as quantity. Depending on the sequence of evacuation, the overall particle size distribution (PSD) would vary with time which in turn would affect the head requirement in the high concentration slurry disposal (HCSD) system. Fly ash samples from different fields of ESP hoppers of a thermal power plant have been analyzed for their physical properties namely the PSD, specific gravity, settling characteristics, pH of the slurry, etc. and for rheological properties in the concentration range of 60–70% (by weight). The particle size (d_wm) of the fly ash samples decreases with the increase in ESP field, whereas the static settled concentration and specific density increase. The pH values of all samples are almost constant and nonreactive in nature. The rheological properties namely yield stress and Bingham viscosity of the fly ash slurries from different fields of ESP hoppers increase with increase in concentration. Further at any given concentration, these parameters exhibit a strong dependence on particle size. Using these properties and treating the distribution of particles across the pipe cross section as homogeneous in the concentration range of 60–70% (by weight), CFD computations are made to evaluate the head requirement in a HCSD pipeline. The head loss increases with increase in concentration for all fields of ESP hoppers. The present study also shows that head requirement varies significantly by mixing different proportion of fly ash from different ESP fields.     

An application of blended fly ash and residual rice husk ash for producing green building bricks

The present study investigates the possibility of using a blended class-F fly ash (FA) and residual rice husk ash (RHA) in the production of green building bricks through the application of densified mixture design algorithm (DMDA) in order to provide a new use for solid waste materials. This study uses unground rice husk ash (URHA) as a partial fine aggregate substitution (10–40%) in the studied cementitious mixtures. Solid bricks of 220 × 105 × 60 mm in size were prepared under forming pressure of 25–35 MPa, a curing temperature of 90 °C, and a relative humidity of 50%, for tests that assessed: compressive strength, flexural strength, bulk density, void volume, and water absorption. The test results showed that all brick samples demonstrated excellent properties. Compressive strength and flexural strength ranged, respectively, between 20.2–33 MPa and 5.4–6.9 MPa. Additionally, up to 30% of URHA content, the values of water absorption and void volume ranged, respectively, between 8.8–15.7% and 1.5–2.1%. All of these values not only conformed well to the requirements of the Vietnamese codes but also demonstrated great potential for using a blended FA–RHA in producing green building bricks.     

A NUMERICAL SIMULATION OF SWIRLING FLOW PNEUMATIC CONVEYING IN A HORIZONTAL PIPELINE

ABSTRACT

A numerical simulation for swirling and axial flow pneumatic conveying in a horizontal pipe was carried out with a Eulerian approach for the gas phase and a stochastic Lagrangian approach for particle phase, where particle-particle and particle-wall collisions were taken into consideration. The k-? turbulence model is used to characterize the time and length scales of the gas-phase turbulence. Models are proposed for predicting the particle source and additional pressure loss. The numerical results are presented for polyethylene pellets of 3.1 mm diameter conveyed through a pipeline of 13 m in length with an inner diameter of 80 mm, solid mass flow rate was 0.084 kg/s, and gas velocity was varied from 10 m/s to 18 m/s. The particle flow patterns, the particle concentration and the particle velocity, and additional pressure loss were obtained. It is found that the particle velocity and concentration has almost same value along flow direction in swirling flow pneumatic conveying. The profile of particle concentration for swirling flow pneumatic conveying exhibits symmetric distribution towards the centerline and the higher particle concentration appears in neighbor of wall in the acceleration region. At downstream, the uniform profile of particle concentration is observed. The particle velocity profile, on the other hand, is uniform for both swirling and axial flow pneumatic conveying. A comparison of the calculations with the measured data shows a good agreement within an average error of less than 15 percent.     

Structural Characterization of Four South African Fly Ashes and Their Structural Changes with β-Cyclodextrin

This article examines the structural characteristics of four South African fly ashes and their structural changes with β-cyclodextrin so as to compare their structural responses to fly ash-β-cyclodextrin (FA-βCD) composite. The four different fly ashes, obtained from different power stations in South Africa were subjected to x-ray fluorescence (XRF), particle size distribution, x-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) analysis. FA-βCD composites were subjected to XRD and FTIR analyses. The XRF analysis showed that all the fly ash samples used are in class F with SiO₂ + Al₂O₃ + Fe₂O₃ greater than 70%. The average particle sizes of all fly ash samples were less than 0.075 mm; the major mineral phase in all fly ash samples was quartz (SiO₂). The FTIR analysis showed Si-O-Si asymmetric and Al-O symmetric stretching vibrations in all fly ash samples. FA-βCD composites for all the fly ashes revealed additional upcoming peaks between diffraction angles (2θ) 10° and 25°, which was not in the raw fly ashes. Shift in FTIR spectra frequencies and an additional peak at approximately 1155 cm^?1 attributed to O-Si-O bending vibration were observed in all the composite samples.     

Formation of Carbon Nanotubes from Carbon-Rich Fly Ash: Growth Parameters and Mechanism

Fly ash is a by-product, generated due to the use of heavy oil or coal as a fuel in power plants. Recently, carbon-rich fly ash is found to have ideal compositions as a catalyst and precursor for carbon nanotube (CNTs) growth by the chemical vapor deposition method. The existing methods have a limitation on producing CNTs in a large scale. In this work, the parameters effecting the growth of CNTs of fly ash were investigated. These include temperature, gas pressure, growth time, and gas flow rate. The produced samples were characterized by different techniques. Consistent results are demonstrated in terms of CNT uniformity, conversion and lengths as produced under optimized values. The optimum values provide multiwall CNTs with diameters in the 20–40 nm range. Raman spectrum shows G and D bands (the G/D intensity ratio ～1.4). The as-grown CNTs displayed peaks of sp² and sp³ carbons in the X-ray photoelectron spectroscopy spectrum with an intensity ratio 7.5 eV. Two small bands are also observed at 288.2 and 290.5 eV, which could be assigned to –COO group and π–π* transition carbon. A proposed mechanism for CNT growth is also presented. The goal of this method is to develop a large-scale production of CNTs using fly ash for various applications.     

Influence of Process Parameters on Cu–Fly Ash Composite by Powder Metallurgy Technique

In this study influence of compaction pressure, sintering temperature, and sintering time on mechanical and wear behavior of the fly ash reinforced copper-based composites are analyzed. The composites were prepared by powder metallurgy (P/M) technique with copper as matrix, 5 and 10 wt% of fly ash as reinforcement. The green compacts were prepared at three different pressures such as 350, 400, and 450 MPa. The prepared green composites were sintered at 700, 800, and 900 °C for the time period of 30, 60, and 90 min, respectively. From the results it is observed that when the process parameter increases the density, hardness, compression strength, and wear resistance increases.