Pneumatic Conveying of Granular Materials
J.S. Hua and C.H. Wang, “Electrical Capacitance Tomography Measurements of Gravity-driven Granular Flows”, Ind. Eng. Chem. Res., 38 621-630 (1999).
An Electrical Capacitance Tomography (ECT) system is used in this study to investigate the density waves generated in the gravity-driven granular flow through a vertical pipe. The experiments are conducted in two stages. Firstly, the time-averaged quantities of particle concentration and velocity are measured and correlated with the mass flow rate of particles. Secondly, the power spectra of particle concentration fluctuations are examined to determine the condition for the formation of density waves. The present work finds that the time-averaged particle concentration is higher towards the centerline of the pipe and the particle velocity is relatively uniform over the cross section of the pipe. The conducted load cell and fiber optic probe measurements are in reasonable agreement with the ECT measurements. The density wave formation has a close correlation with the local particle concentration. The experimental results also indicate that the dominant frequency might vary while the particles pass through the vertical pipe.
Temporal variation of particle concentration at the cross section of the pipe. The flow rate of glass beads is 0.31 kg/s. The number at the upper left corner of each frame (at the time internal of 0.02 s) refers to the corresponding time sequence.
S.M. Rao, K. Zhu, C.H. Wang, and S. Sundaresan, “Electrical Capacitance Tomography Measurements on the Pneumatic Conveying of Solids”, Ind. Eng. Chem. Res. 40(20) 4216-4226 (2001).
Pneumatic conveying of solids has applications in several industries such as food, pharmaceutical, chemical process, mines and thermal power plants. An efficient conveying system should be able to transport large amounts of solids steadily and continuously with minimum energy input. Hence, the optimum design of any pneumatic conveying system requires a complete knowledge about the flow patterns, pressure drop and the various variables affecting them. Based on a parameter called mass load ratio, defined as the ratio between the solid mass flow rate and the air mass flow rate, pneumatic conveying is broadly classified as the dilute phase and the dense phase. The dilute phase can be observed at a relatively high gas velocity with low mass load ratio in connection with low pressure drop, while the dense phase conveying is observed at low gas velocities and high mass load ratios associated with high pressure drops. In the dilute phase the solids are homogeneously dispersed in the pipe. The dense phase conveying occurs when the gas velocity is below the saltation velocity, defined as the gas velocity in a horizontal pipe at which the solid particles start to drop out of the gas-solid suspension and settle on the bottom surface of the pipe. The minimum pressure drop occurs at the saltation point. Compared to the dilute phase the dense phase regime offers lower energy consumption and lower wear and attrition to the pipe. The low gas flow rates beyond a certain value could hamper the solids flow in the pipe thus ultimately blocking the pipe. An experimental facility has been built to investigate the pneumatic conveying of solids. The present study focuses on the dense phase conveying. The Electrical Capacitance Tomography(ECT) technique is used to observe particles distribution in the pipe cross section at various locations along the pipe length for various operating conditions. The corresponding pressure fluctuation data are also being measured using pressure transducers. This work could serve as a guiding tool for the development of a control system to the pneumatic transport of particles.
Experimental Pneumatic Conveying Setup
S. M. Rao, K. W. Zhu, C. H. Wang, S. Sundaresan, “Electrical Capacitance Tomography Measurements on the Pneumatic Conveying of Solids”, Ind. Eng. Chem. Res. 40, 4216-4226 (2001).
We have evaluated the usefulness of electrical capacitance tomography (ECT) as a tool for monitoring pneumatic conveying in horizontal ducts. Power spectra of solids concentration flucturations obtained from single-plane ECT data were used to identify the various flow regimes, and these were confirmed through visual observation. From single-plane ECT data, the instantaneous and time-averaged distributions of particle concentration over the cross section of the conveying pipe have been determined in various flow regimes. Propagation velocities of patterns were evaluated from cross correlation of twin-plane ECT data. The solids mass flow rate, determined independently by load cell measurements, was found to be roughly proportional to the product of the pattern velocity, the particle density, and the average solids holdup in the pipe, and the proportionality factor depended on the material being transported through the pipe. In our experiments involving flows past a 90 smooth bend. ECT was able to detect significant temporal and spatial nonuniformity in particle concentration in the postbend region.
Flow past a 90º smooth elbow U =14.3 m/s, Gs =20.5 kg/m2 ∙ s : (a) Snapshot of a traveling solid layer in the riser pipe; (c) Four images from ECT data obtained in the vertical pipe shortly after the bend, showing the ringlike structure.
K.W. Zhu, S.M. Rao, Q. H. Huang, C. H. Wang, S. Matsusaka, H. Masuda, “On the Electrostatics of Pneumatic Conveying of Granular Materials using Electrical Capacitance Tomography”, Chemical Engineering Science 59, 3201-3213, (2004).
In this work the electrostatics of the pneumatic conveying of granular materials in a non-conducting (PVC) vertical pipe is studied using Electrical Capacitance Tomography (ECT) system. The non-conducting wall in general attains static charges arising from particle-wall collisions in the initial periods of conveying process and then reaches equilibrium with the surroundings. The polarity of particles and conveying pipe inner wall agree reasonably well with the contact potential difference measurements. The perturbations in the capacitance signal due to charge accumulation are larger with smaller air superficial velocity. The denser flow regimes give larger wall residual charge. Wall charging process shows similar trend described by surface potential taken from electrostatic voltmeter to that revealed by ECT measurements. Also the addition of small amount (0.5% by weight) of anti-static agent (Larostat-519) in the powder form decreases the electrostatic charge generation by altering the patterns for particle-particle and the particle-wall collisions.
Radial distribution of time averaged effective solids concentration values (data averaged over 10 sec) from an ECT sensor located at z = 2.05 m away from the bottom elbow: (a) wall charge contribution not subtracted; and (b) wall charge contribution subtracted. ECT data were taken after transporting the polypropylene granules (ds = 2.8mm) for “t, sec” into a vertical PVC pipe (i.d, D = 40.4mm). Air superficial velocity U = 17.3 m/s and solids flow rate Gs = 0.08 kg/s.
K. W. Zhu, S. M. Rao, C. H. Wang, S. Sundaresan, “Electrical Capacitance Tomography Measurements on Vertical and Inclined Pneumatic Conveying of Granular Solids”, Chemical Engineering Science, 28, 4225-4245 (2003).
Pneumatic conveying of granular solids in vertical and inclined risers was studied using electrical capacitance tomography (ECT). The focus of the study was on flow development past a smooth bend connecting the riser to a horizontal duct which brought the gas-particle mixture to the riser. In the vertical riser, dispersed flow manifested a core-annular structure, whose development is discussed. Three different time-dependent flow patterns were imaged. Slugging flow, which appeared to be intrinsic to riser flow, took the form of alternating bands of core-annular disperse flow and a slug with a particle-rich core. Averaging over these two structures yield a composite distribution with high particle concentration both at the axis and the wall region. Pulsing flow, whose ECT fingerprint was similar to that of slugging flow, was largely an entrance effect. Stationary and moving annular capsules with a dilute core were also observed, and such flow patterns do not appear to have been reported previously. Our ECT measurements probing the development of disperse flow in an inclined riser past a bend revealed that the particle loading initially decreased, subsequently increased and then leveled off. Regimes such as eroding dune flow and flow over a settled layer could be easily imaged using ECT. The surface of the settled layer had a concave shape, suggesting that the particles were picked up from the settled layer by airflow at the centre and deposited on the sides of the tube.
Flow over a settled layer in a 45º inclined pipe. Transport of polypropylene particles by air. Ug=11.7 m/s. Negligible net transport of particles. (a) snapshot of the flow pattern. (b) ECT images sequence with number on the top left showing the frame number. Rotary valve vent was closed in these experiments. The ECT measurements were made at distance of 2.16m downstream of the bend.
K. W. Zhu, C. K, Wong, S. M. Rao, C. H. Wang, “Pneumatic Conveying of Granular Solids in Horizontal and Inclined Pipes”, AIChE Journal, 50 (8), 1729-1745 (2004).
Computational fluid dynamics simulations are used to investigate the pneumatic conveying of granular solids through an inclined pipe at different inclinations. Model predictions agreed reasonably well with the measurements reported by Tsuji and Morikawa in 1982 for mean gas and solids velocities. The results of influence of model parameters, inclination angle, and feeding conditions on the flow patterns were also reported. Particle-wall collisions were found to have a very significant effect on the solids distribution over the cross section of the conveying tube for large particles. Upon introducing finit-amplitude sinusoidal fluctuations in the gas velocity at the inlet of the pipe, solids density waves were observed to move along the conveying line with significant axial dispersions.
(a) Simulation geometry: The inclination angle of the conveying pipe varies from 0 to 90º. The diameters of the pipe used in the current studies are 3.05, 4.0, and 8.0 cm. The flow quantities predicted normally taken at the fully developed region along the center line L at the cut section A, ZL meters away from the feeding end. The length of the conveying pipe is ZP. (b) Influence of inclination angle on the flow quantities along line L of pneumatic conveying of 3.0-mm particles in a pipe with a diameter of 8.0 cm, ug/U.
L. Y. Lee, T. Y. Quek, R. S. Deng, M. B. Ray, C. H. Wang, “Pneumatic Transport of Granular Materials through a 90º Bend”, Chemical Engineering Science, 59, 4637-4651 (2004).
In the present study, a pneumatic conveying system imcorportating a 90º bend is investigated. This study employs the use of three non-invasive instruments to measure solids concentration and velocity distribution determination in the pneumatic conveying system. They are namely the electrical capacitance tomography (ECT), particle image velocimetry and phase doppler particle analyzer. Pressure transducers were also used to monitor the pressure drop characteristics along the post-bend vertical pipe region. Two different classes of granular materials, polypropylene beads (2600 μm, geldart class D) and glass beads (500 μm, Geldart class B), were used to investigate the differences in the flow characteristics for granular particles of various Geldart classes. The experimental results show a constant frequency pulsating flow for polypropylene beads in the dense-phase flow regime. This is illustrated by the visualization, ECT and pressure drop data. For dilute-phase flow regime, both polypropylene and glass beads show a continuous annulus flow structure. Numerical simulation using the Euler-Euler method was also conducted using computational fluid dynamics and the fluid and particles flow characteristics were compared with the experimental data obtained in the present study.
(a) ECT images obtained for pneumatic conveying of polypropylene in the vertical pipe. Gs=31.1 kg∙m-2∙s-1, Ug=11.9 m∙s-1 and (b) Predicted polypropylene solids volume fraction (αS) in the symmetry plane of the entire geometry. High solid concentration in the sharp bend corner, with solid clusters above it and disperse rope at the top of the vertical section. Enlarged sections show the clustering and roping regions of the vertical pipe.
J. Yao, Y. Zhang, C. H. Wang, S. Matsusaka, H. Masuda, “Electrostatics of the Granular Flow in a Pneumatic Conveying System”, Ind. Eng. Chem. Res. 43, 7181-7199 (2004).
The phenomenon of electrostatic charge generation and its effects on granular flow behavior in a pneumatic conveying system was studied. The main parameters used for quantitative characterization of the phenomenon were the induced current, particle charge density and equivalent current of the charged granular flow. These were measured using a Digital Electrometer, Faraday Cage and Modular Parametric Current Transformer (MPCT) respectively. Three different flow patterns corresponding to different electrostatic effects within the pneumatic conveying system were observed and these were named the disperse flow, half-ring flow and ring flow patterns. It was found that the induced current, particle charge density and equivalent current increased with decreasing flow rates. Electrostatic effects generally become stronger with time and this may lead to clustering behavior occurring even in the disperse flow regime. The effects of several factors such as pipe wall material, particle composition, relative humidity of the conveying air used and the presence of an anti-static agent in the system were investigated and found to be important in determining the electrostatic charge generation characteristics and granular flow patterns observed.
Comparison three flows: disperse flow (air flow rate 1600 L/min, solids feed rate 35.3 ± 3.2 kg/m2 s); half-ring flow (air flow rate 1000 L/min, solids feed rate 13.8 ± 2.4 kg/m2 s); ring flow (air flow rate 860 L/min, solids feed rate 8.1 ± 1.6 kg/m2 s): (a) induced current value (using Advanced R8252 digital electrometer) acquired at the vertical pipe; (b) particle charge density (using Faraday Cage).
T. Y. Quek, C. H. Wang, M. B. Ray, “Dilute Gas-Solid Flows in Horizontal and Vertical Bends”, Ind. Eng. Chem. Res. 44 (2005).
Simulations of dilute-phase gas-solid flow in pipe bends of different radii of curvature were conducted using computational fluid dynamics (CFD). The renormalization group k-e turbulence model was used for the flow calculations of the continuous phase while the Lagrangian approach was used for calculating the discrete-phase trajectory. The model predictions were first validated with particle concentration and velocity measurements adopted from the literature. Subsequently, the deposition pattern of fine particles in horizontal bends was compared with the model predictions. Other flow quantities such as secondary flow intensity and residence time distribution of the particles in the bend were also investigated. Simulation indicates that the rope formation in vertical bend is stronger for the longer bend radius than that in the shorter radius and inlet turbulence intensity (<10%) does not bring about significant effects on particle concentration through the bend whereas particle size is found to have the highest impact on the extent of flow dispersions observed in the bend with different radii of curvature.
Concentration profiles for 2.3 mm diameter particles. Positions are at the inlet of the bend (q = 0o), in the middle of the bend (q = 45o), at the exit of the bend (q = 90o), at z/D=1, z/D=3, z/D=5, conveying velocity =10 m/s.
Lim, E. W. C., C. H. Wang and A. B. Yu, “Discrete Element Simulation for Pneumatic Conveying of Granular Material”, AIChE Journal, 52(2), 496–509. 2006.
The pneumatic transport of granular material is a common operation frequently employed to transport solid particles from one location to another. It is well established in the literature that different flow regimes can arise in such transportation processes depending on the system geometry and operating conditions used. In this study, the pneumatic transports of solid particles in both vertical and horizontal conveying lines were studied numerically using the Discrete Element Method coupled with Computational Fluid Dynamics. The simulation outputs corresponded well with reported experimental observations in terms of the different flow regimes obtained at different operating conditions. In the vertical pneumatic conveying simulations, two different flow patterns corresponding to the experimentally observed dispersed flow and plug flow regimes were obtained at different gas velocities and solid concentrations. Similarly, the homogeneous flow, stratified flow, moving dunes and slug flow regimes previously reported to occur in horizontal pneumatic conveying were also reproduced computationally in this study. Solid concentration profiles obtained by spatial averaging along the length of the pipe showed a symmetrical but non-uniform distribution for dispersed flow and an almost flat distribution for plug flow in vertical pneumatic conveying. The profile for stratified flow in horizontal pneumatic conveying showed higher solid concentration near the bottom wall due to the effects of gravitational settling while that for slug flow was flat. Hysteresis in solid flow rates was observed in vertical pneumatic conveying near the point where transition between the dispersed and plug flow regimes was expected to occur. Solid flow rates were also found to be more sensitive towards the coefficient of friction than the coefficient of restitution of particles and the pipe walls in a sensitivity analysis study of these parameters.
Vertical pneumatic conveying in the dispersed flow regime with 1000 particles and gas velocity 24 m s-1
Horizontal pneumatic conveying in the slug flow regime with 1500 particles and gas velocity 10 m s-1
J. Yao, C. H. Wang, E. W. C. Lim, J. Bridgewater, “Granular Material in a Rotary Valve: Attrition Product Size and Shape”, Chemical Engineering Science, 61 (2006), 3435-3451.
The rotary valve is a widely used mechanical device in solids-handling industrial processes, but it is responsible for much attrition. Here, the attrition occurring in a rotary valve operating both as a stand-alone device and as part of a pneumatic conveying system was, for the first time, investigated. In the former case, attrition at the three rotary valve speeds was consistent with the Gwyn correlation. For polypropylene, attrition was dependent on the number of rotations of the valve, but for PVC there was also a further rate dependent effect. In the latter case the Gwyn parameter was higher than those for higher air flow rates. This may be ascribed to a greater amount of material accumulating at the exit of the rotary valve.
The attrition product tended to assume a narrow, elongated morphology. Attrition products was found to be in a five basic shapes, these being dependent on the shearing action of the vanes in the rotary valve. The new concepts of shear frequency and attrition frequency are proposed. Over a wide range of attrition product sizes and shapes, the principles of attrition in the rotary valve were unified in a remarkably simple manner. As such, these concepts described well all findings and offered a characterization of attrition in complex systems.
(a) Microscope image of the attrition product (sampled about 1g):2.00-2.36 mm; (b) Attrition product shape distribution displayed via the plots of length- length ratio, original: particle size 3.35-4.1mm.
Lim, E. W. C. and C. H. Wang, “Diffusion Modeling of Bulk Granular Attrition”, Industrial and Engineering Chemistry Research, 45(6), 2077–2083. 2006.
A simple empirical model for bulk granular attrition is proposed and investigated in this paper. We attempt to model the attrition process occurring in various types of experimental systems reported in the literature with a diffusion type equation. The proposed empirical model is found to reproduce much of the experimentally observed behavior and our own numerical simulation results. This may suggest similarities in statistical characteristics or other fundamental properties between the process of bulk granular attrition and diffusion of material. A comparison of the model with the well-established Gwyn correlation may also provide insights on why such a power-law type correlation has been generally successful in describing granular attrition behavior.
Comparisons of model with attrition data reported for experiments conducted using annular shear cells. The granular material used were (a) 1.7 – 2.0 mm sodium chloride granules12 (D = 6.82 ´ 10-5 s-1) and molecular sieve beads1 (D = 2.36 ´ 10-5 s-1) with a constant applied normal stress of 41 kPa and (b) 2.0 – 2.36 mm porous silica catalyst carrier beads8 with varying applied normal stresses of 25 (○), 50 (◊), 100 (D) and 200 (□) kPa (D = 6.39 ´ 10-5, 1.77 ´ 10-4, 9.20 ´ 10-4, 3.67 ´ 10-3 s-1 respectively).
Comparisons of model with attrition data reported for experiments conducted using fluidized beds. The granular material used were (a) 2 mm agglomerate particles made up of 63 – 90 mm soda glass beads2 with varying superficial gas velocities of 1.1 (○), 1.2 (D) and 1.3 (□) times the minimum fluidization velocity (D = 7.16 ´ 10-7, 1.28 ´ 10-6, 3.46 ´ 10-6 s-1), (b) 1764 mm lime sorbents in a circulating fluidized bed3 with fluidizing velocities of 2 (○) and 4 (D) m s-1 (D = 2.33 ´ 10-6, 1.11 ´ 10-5 s-1), (c) 2.0 – 2.36 mm foamed glass particles13 with gas velocities 0.412 (○), 0.463 (D) and 0.512 (□) m s-1 (D = 1.06 ´ 10-9, 3.15 ´ 10-9, 5.49 ´ 10-9 s-1) and (d) 351 – 417 mm granular slug particles14 with jet velocities of 47.2 (○) and 70.7 (D) m s-1 (D = 1.40 ´ 10-6, 6.55 ´ 10-6 s-1).