- Email: [email protected]
Solids deposition in low-velocity slug flow pneumatic conveying
Chemical Engineering and Processing 44 (2005) 167–173
Solids deposition in low-velocity slug flow pneumatic conveying J. Li a,∗ , C. Webb a , S.S. Pandiella a , G.M. Campbell a , T. Dyakowski a , A. Cowell b , D. McGlinchey b b
a Department of Chemical Engineering, Satake Centre for Grain Process Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, UK Centre for Industrial Bulk Solids Handling, School of Engineering, Science and Design, Glasgow Caledonian University, Glasgow G4 0BA, UK
Received 26 August 2003; received in revised form 4 December 2003; accepted 11 February 2004 Available online 25 September 2004
Abstract Solids deposition in the horizontal pipeline of a pneumatic conveying system was studied both mathematically and experimentally. Mathematically modelled results using the coupled discrete element method (DEM) and computational fluid dynamics (CFD) approach have demonstrated an intensive exchange of particles between the stationary layer (deposited particles) and the moving slug and a variation of solids concentration and pressure and velocity distributions across the slug. Slug flows were also visualised experimentally through a glass section and analysed by a high-speed video camera. The amount of particle deposition in the pipeline after a conveying was calculated by controlling the solids feeding rate using a rotary valve and by monitoring the solids flow out of the system using dynamic load cells. Experimentally generated data have quantitatively shown a tendency of more solids deposition with lower gas mass flow rate in slug flows except that, below a certain amount of solids mass flow rate, the deposition becomes independent of gas flow rate. © 2004 Published by Elsevier B.V. Keywords: Pneumatic conveying; Slug flow; Solids deposition; CFD model; DEM simulation
1. Introduction Low-velocity slug flow pneumatic conveying has become more and more common in many industrial sectors ranging across chemical, pharmaceutical and food industries, due to its advantages over conventional high-velocity suspension flow in preventing product degradation and plant wear and in delivering high throughput and efficient power utilisation [1]. One of the distinguishing characteristics in such an operation is the gravitational deposition of solid particles in pipelines, which is due to the low gas velocity used, usually below the solids saltation velocity. A layer of particles at the bottom of the horizontal conveying line is commonly seen in these systems and this solids deposition significantly affects the transition and flow of slugs along the pipeline. This paper presents mathematical and experimental studies of solids deposition characteristics in the horizontal pipeline of a dense phase slug flow pneumatic conveying
∗ Corresponding author. Tel.: +44 1236 878448; fax: +44 1236 872837. E-mail address: [email protected] (J. Li).
0255-2701/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.cep.2004.02.011
system. To obtain an insight into the slug flow and solids deposition characteristics, a coupled discrete element method (DEM) and computational fluid dynamics (CFD) approach was used to simulate the solids and gas flows. Solids deposition in the horizontal section of the conveying system was visualised through a glass section and recorded by a high-speed video camera. The amount of particle deposition was calculated by closely monitoring the solids flow into and out of the system. Mathematically and experimentally generated data were analysed in order to develop a useful tool for the future prediction of solids deposition and slug flow behaviours in low-velocity pneumatic conveying.
2. Characteristics of slug flow - DEM modelling Although low-velocity slug flow pneumatic conveying has been studied intensively in the past two decades [1–8], due to the practical restrictions of applying modern measuring tools to gas and solids multiphase flows there is still a lack of understanding of how gases and particles interact inside a moving slug and how these interactions affect the formation
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J. Li et al. / Chemical Engineering and Processing 44 (2005) 167–173
Fig. 1. Initial conditions used in the simulation of slug flow in pipes.
and flow of slugs. A recent study by Klinzing [8] demonstrated that the slugging behaviours of solids in a pipeline are complex, involving intensive interchanges of particles between the stationary layer and the moving slug, i.e. picking up particles at the front of a slug and dropping off at the tail. He also stated that a thin layer of particles in the pipeline was essential for the development of stable slugs. To obtain this realistic experimental condition several pre-runs were conducted in his experiments to let particles lay down in the pipeline. Obviously, a quantitative description of this particle layer, which is equivalent to the amount of particle deposition after a conveying, would bridge the understanding of solids slugging behaviour and the performance of the transport system. To understand the mechanisms of solids slug flows, a two-dimensional coupled DEM/CFD numerical model was built to simulate the motion of a pre-formed slug (ca. 0.3 m long) in a 1 m long horizontal 50 mm bore pipe as shown in Fig. 1. The pipe was initially filled with a layer of particles, approximately 15 mm thick at the bottom. (The thickness of this stationary layer was determined based on experience from previous experiments and computer test runs). In the model, the solids phase was modelled as discrete particles using DEM and the gas phase was modelled as a continuum by CFD. The influence of solid particles on the gas flow was considered as a source of mass, momentum and energy, via the change to its volume fraction, the aerodynamic forces exerting on the gas and the heat transfer with the gas. The effect of the gas flow on particle motion was calculated via the fluid drag forces. (Detailed description of the model is found in a previous publication of the authors, Li and Mason [9]). This method allows individual particle trajectories and interstitial gas flows to be traced, and local gas-particle, particle-particle and particle-wall interactions to be computed with the evolution of the slug flows. The basic advantage of this coupled approach over conventional continuum techniques is that DEM simulates effects at the particle level. There is less need for global assumptions and the assembly response is a direct output from the simulation [10]. DEM simulations have been used to model different granular systems by a number of researchers and have reported positive results: e.g. Tsuji et al. [11] on a fluidised bed, Langston et al. [12] on hopper flow, Frank et al. [13] on channel flow, Cleary [14] on ball mills and Stewart et al. [15] and Kuo et al. [16] on solids mixing. The main disadvantage of DEM modelling is that it requires a large amount of resource to compute increased number of parti-
cles, which makes it difficult to model fine particles or in three-dimensional geometries. Therefore, discrepancies are expected when comparing the current two-dimensional simulations with experiments. Particles used in this simulation were 5 mm spherical polyethylene pellets with a particle density of 880 kg/m3 . The gas was introduced into the pipe inlet with a constant superficial gas velocity of 2 m/s and all particles in the solids slug and the stationary layer were assigned a low initial velocity at 0.01 m/s to represent a dynamic starting condition. The simulation started with an ambient pressure in the whole pipe and this ambient pressure was kept constant at the pipe outlet while the pressure rose in the pipeline when gas was introduced from the inlet. Fig. 2 shows six snapshots of slug flows along the pipe from the DEM/CFD modelling at a time interval of 0.2 s for a total time length of 1.0 s. This simulation demonstrates a similar picture as described by Klinzing [8], in terms of particle motion in the slug. That is, particles in the stationary layer (dark coloured particles at the bottom of the pipeline) ahead of the slug wave are drawn into the slug (light coloured particles) and form part of it when being moved along the pipe by the pressurised gases. The slug wave actually compresses the particle layer just ahead of it, pushing some of the particles up from the layer into the wave. It is also interesting to see that at the tail of the slug particles fall into the bottom part of the pipeline and form the stationary layer. The exchange of particles between the stationary layer and the slug has shown a unique slugging mechanism in slug flows, which explains why particles move slower than slugs [8]. A statistical analysis shows that, after the slug flow, the number of particles deposited in the pipe is about 90% of the number of particles originally placed in the stationary layer. Taking into account the entrance effect (much less particle deposition in the front section from 0 to 0.3 m), this shows a reasonable comparison between the assumed layer thickness (15 mm) and the modelled particle deposition, though an uneven particle layer has been produced after the slug flow. This also suggests that the uneven particle layer from the current model should be adopted as initial input for further modelling. It can be clearly seen from Fig. 2 that there is a variation of solids concentration across the slug with a looser front and a denser back. Some particles are even pushed ahead of the slug wave at the front and this group of particles behave in a very dynamic and suspended fashion. This phenomenon has been confirmed by analysing video footages from experiments in slug flows using similar particulate materials (3 mm spherical polymer pellets), as shown in Fig. 3. Although the general slugging behaviour is determined by the aeration, permeability and de-aeration characteristics of the particulate material, the local variation of solids concentration in the slug is very much dependent upon the actual interstitial gas flow through the slug and the pressure distribution along the pipe. Fig. 4 shows a plot of pressure drops along the pipeline at both top and bottom of the pipe cross-section at
J. Li et al. / Chemical Engineering and Processing 44 (2005) 167–173
Fig. 2. Solids slug flow and deposition – DEM/CFD simulation (time interval between each picture 0.2 s and total time 1.0 s).
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Fig. 3. Snapshots of slug flow from video footages recorded in experiments.
an instantaneous time of 0.4 s. A clear response of pressure build-up at the back of and across the slug is shown and a pressure difference between top and bottom of the pipe cross-section at the back of the slug is observed, both of which have been confirmed by experiments using accurate pressure measurement [17]. Obviously the pressure build-up
is due to the blockage of gas passage by particles in the pipe cross-section and the gas retention characteristics in front of the slug plays an important role in determining solids slugging behaviours. The presence of particles has severely altered the gas flow field in the vicinity of the slug. Figs. 5 and 6 show the simulated gas velocity profiles horizontally along the pipeline and vertically in the pipe cross-section at different locations, respectively. In general the axial gas velocity is much higher at the top of the pipe cross section than at the bottom and the highest occurs at the back of the slug (around 0.4 m), which follows the profile of the solids assembly at the back of the slug. However, gas velocity profiles become much flatter in the middle of the slug as shown in Fig. 5 at 0.5 m. It is also interesting to see the change of gas flow direction in the pipe cross-section as shown in Fig. 6 (positive value indicating an up-flow and negative a down-flow). An up-flow of gas is observed at the front of the slug (0.6 and 0.7 m) and the far back of the slug (0.3 m) and a down-flow at the back of the slug (0.4 m) but a complex picture in the middle of the slug (at 0.5 m). In general, these cross section velocity profiles comply with the axial velocity profiles in terms of mass balance in the gas phase.
Fig. 4. Pressure drop along the pipeline – DEM/CFD simulation (at 0.4 s).
Fig. 5. Gas velocity profile along the pipe – DEM/CFD simulation (at 0.4 s).
J. Li et al. / Chemical Engineering and Processing 44 (2005) 167–173
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Fig. 6. Gas velocity profile in pipe cross section – DEM/CFD simulation (at 0.4 s).
The gas velocity profiles may explain why a loose slug front is generated. That is particles in the front of the slug wave are pushed by the slug body and dragged ahead of the slug by the high local gas velocity at the top pipe cross section while being agitated through severe interactions incurred by the rising particles from the stationary layer beneath the slug front and floating over them. The agitated particle layer underneath the slug front is actually acting as multiple fluidised rolling wheels for the slug, which greatly reduces the frictional forces comparing with a direct interaction with the pipe bottom wall. In some cases, even an air pocket or a particle swirling flow is created between the suspended loose slug front and the bottom particle layer, of which the resulting mechanisms and its consequence on slug flow need further studies.
3. Experimental analysis Experiments were conducted to determine quantitatively the amount of particle deposition in the pipeline of a specially designed pneumatic conveying system [17,18]. A schematic layout of the rig is shown in Fig. 7. Briefly
it comprises of a feed vessel, a receiving vessel, a 13 m long conveying pipeline with interchangeable 5.1, 7.6 and 10.2 cm (i.e. 2, 3 and 4 in.) nominal bore pipes, a bank of air supply nozzles, a variable speed rotary valve with dropout box, an air filter unit, and connecting pipes and control valves. In the system, both gas and solids mass feeding rates are closely controlled through the choked nozzle bank and the variable speed rotary valve. A set of three load cells is mounted underneath the receiving vessel to monitor the weight of solids conveyed. This particular design allows the desired solids mass flow rate to be selected by adjusting the rotary valve speed while the actual conveying rate is calculated simultaneously from the load cell readings. Fig. 8 shows a typical diagram of a non-suspension wave-like flow. Each step in the mass collected history is caused by the arrival of a slug of solids. It is noticed that there is a difference between the mass of solids flowing into and out of the system and the difference in the final masses is the deposition of solids in the system, which usually forms a particle layer in the pipeline at the end of conveying. Different combinations of gas and solids flow rates were tested in a 7.6 cm bore pipe with a common industrial granular material - polyethylene pellets. The pellets were spher-
Table 1 Solids deposition in a pneumatic conveying system Ug (ca.) (m/s)
25 8 6 4 2.6 a
Mg (kg/s)
0.1534 0.0491 0.0368 0.0245 0.0160
Ms1
Ms2
Ms3
Ms4
Fixed (kg/s)
Deposition (kg)
Fixed (kg/s)
Deposition (kg)
Fixed SLR5 (kg/s)
Deposition (kg)
Fixed SLR10 (kg/s)
Deposition (kg)
1.2270 1.2270 1.2270 1.2270 1.2270
0.00 3.94 6.28 6.05 9.79
0.6135 0.6135 0.6135 0.6135 0.6135
0.00 9.35a 7.29 8.03 8.92
0.7669 0.2454 0.1840 0.1227 0.0798
0.00 11.35a 9.18a 8.33a 11.70
1.5337 0.4908 0.3681 0.2454 0.1595
0.00 9.51 5.43 8.05 12.88a
Large difference ( > 2 kg) of load cell readings occurred between repeated runs, which may be due to the unstable flow of slugs in these flow conditions.
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Fig. 7. Schematic layout of the pneumatic conveying rig.
ical in shape with an average diameter of 3 mm, a particle density of 880 kg/m3 and an approximate bulk density of 550 kg/m3 . The experiments were started initially with high superficial gas velocity at 25 m/s, in which very little particle deposition occurred (This was confirmed with visualisation via a 3 m glass section in the conveying line). When using
low gas velocity (< 10 m/s), a particle layer was built at the bottom of the pipeline after a slug passing and at the end of each test case. This solids deposition was assumed equivalent to the difference of the final load cell readings between the tested cases and that obtained using a high gas velocity (25 m/s).
Fig. 8. Mass trace and solids deposition in slug flows.
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Table 1 summarises solids depositions in the four groups of tests conducted: Ms1 - fixed mass flow rate of solids with a higher rate at 1.2270 kg/s, Ms2 - fixed mass flow rate with a lower rate at 0.6135 kg/s, Ms3 - fixed solids loading ratio (ratio between solids and gas flow rates) SLR = 5, and Ms4 - fixed SLR = 10, totalling 20 experimental cases. From Ms1 , a strong tendency for more solids deposition with a lower gas mass flow rate is shown. However, this effect does not appear to be true for either the lower solids mass flow rate Ms2 , or the two groups with fixed SLR, Ms3 and Ms4 (also with lower mass flow rate < 0.5 kg/s). It shows that below a certain amount of solids mass flow rate the deposition of solids becomes independent of gas flow rate, which can be explained by the fluidised motion of particles before a full slug moves and the gradual build-up of long slugs in the down stream pipeline as discussed previously in Li et al. [18]. Further experiments with a wider range of flow conditions using advanced measuring tools such as tomographic image analysis [19] are required to validate the model and to build up a quantitative description of solids deposition for different particulate materials.
4. Conclusions Gravitational solids deposition in a horizontal pipeline of a pneumatic conveying system was studied both mathematically and experimentally. Mathematically modelled results using the DEM/CFD simulation have demonstrated an intensive exchange of particles between the slug body and the stationary layer as slugs move along the pipeline, and a large variation of solids concentration and pressure and velocity distributions across the slug. A slug wave actually compresses the particle layer just ahead of it, pushing some of the particles up from the layer into the wave. On the other hand, particles at the tail of the slug fall into the bottom part of the pipeline and form a stationary particle layer behind the slug. These phenomena have been confirmed by analysing recorded video footages. Experimentally generated data have quantitatively shown a tendency of increased solids deposition at lower gas mass flow rate in slug flows except that, below a certain amount of solids mass flow rate, the deposition becomes independent of the gas flow rate, which is due to particle fluidised motion before a slug flow commences. Further work will focus on experiments with a wider range of flow conditions using sophisticated measuring tools to validate the model and to build up a quantitative protocol for the description of solids deposition in conveying pipes.
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Acknowledgements Funding for this project was provided in part by the Satake Corporation of Japan. References [1] K. Konrad, Dense-phase pneumatic conveying: a review, Powder Technol. 49 (1986) 1–35. [2] T. Ramakrishnan, K. Ramakoteswara Rao, M.A. Parameswaran, S. Sivakumar, Experimental investigation of a dense-phase pneumatic transport system, Chem. Eng. Process. 32 (1993) 141–147. [3] A.J. Jaworski, T. Dyakowski, Investigations of flow instabilities within the dense pneumatic conveying system, Powder Technol. 125 (2002) 279–291. [4] R. Pan, P.W. Wypych, Pressure drop and slug velocity in low-velocity pneumatic conveying of bulk solids, Powder Technol. 94 (1997) 123–132. [5] Y. Tomita, K. Tateishi, Pneumatic slug conveying in a horizontal pipeline, Powder Technol. 94 (1997) 229–233. [6] O. Molerus, A. Burschka, Pneumatic transport of coarse-grained materials, Chem. Eng. Process. 34 (1995) 173–184. [7] A. Levy, Two-fluid approach for plug flow simulations in horizontal pneumatic conveying, Powder Technol. 112 (2000) 263–272. [8] G.E. Klinzing, Dense phase (plug) conveying – observations and projections, in: A. Levy, H. Kalman (Eds.), Handbook of Conveying and Handling of Particulate Solids, vol. 10, Elsevier, 2002, pp. 291–301. [9] J. Li, D.J. Mason, A computational investigation of transient heat transfer in pneumatic transport of granular particles, Powder Technol. 112 (2000) 273–282. [10] J.P.K. Seville, U. Tuzun, R. Clift, Processing of Particulate Solids, Blackie Academic & Professional, London, 1997, p. 372. [11] Y. Tsuji, T. Kawaguchi, T. Tanaka, Discrete particle simulation of two-dimensional fluidized bed, Powder Technol. 77 (1993) 79–87. [12] P.A. Langston, U. Tuzun, D.M. Heyes, Distinct element simulation of granular flow in 2D and 3D hoppers: dependence of discharge rate and wall stress on particle interactions, Chem. Eng. Sci. 50 (1995) 967–987. [13] T.H. Frank, K.-P. Schade, D. Patrak, Numerical simulation and experimental investigation of a gas-solid two-phase flow in a horizontal channel, Int. J. Multiphase Flow 19 (1993) 187–198. [14] P.W. Cleary, Predicting charge motion, power draw, segregation and wear in ball mills using discrete element methods, Miner. Eng. 11 (1998) 1061–1080. [15] R.L. Stewart, J. Bridgwater, Y.C. Zhou, A.B. Yu, Simulated and measured flow of granules in a bladed mixer – a detailed comparison, Chem. Eng. Sci. 56 (2001) 5457–5471. [16] H.P. Kuo, P.C. Knight, D.J. Parker, Y. Tsuji, M.J. Adams, J.P.K. Seville, The influence of DEM simulation parameters on the particle behaviour in a V-mixer, Chem. Eng. Sci. 57 (2002) 3621–3638. [17] D.J. Mason, J. Li, A novel experimental technique for the investigation of gas-solids flow in pipes, Powder Technol. 112 (2000) 203–212. [18] J. Li, S.S. Pandiella, C. Webb, T. Dyakowski, M.G. Jones, Analysis of gas-solids feeding and slug formation in low-velocity pneumatic conveying, Part. Sci. Technol. 21 (2003) 57–73. [19] T. Dyakowski, J.F.C. Jeanmeure, A. Jaworski, Applications of electrical tomography for gas-solids and liquid-solids flows – a review, Powder Technol. 112 (2000) 174–192.
Solids deposition in low-velocity slug flow pneumatic conveying J. Li a,∗ , C. Webb a , S.S. Pandiella a , G.M. Campbell a , T. Dyakowski a , A. Cowell b , D. McGlinchey b b
a Department of Chemical Engineering, Satake Centre for Grain Process Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, UK Centre for Industrial Bulk Solids Handling, School of Engineering, Science and Design, Glasgow Caledonian University, Glasgow G4 0BA, UK
Received 26 August 2003; received in revised form 4 December 2003; accepted 11 February 2004 Available online 25 September 2004
Abstract Solids deposition in the horizontal pipeline of a pneumatic conveying system was studied both mathematically and experimentally. Mathematically modelled results using the coupled discrete element method (DEM) and computational fluid dynamics (CFD) approach have demonstrated an intensive exchange of particles between the stationary layer (deposited particles) and the moving slug and a variation of solids concentration and pressure and velocity distributions across the slug. Slug flows were also visualised experimentally through a glass section and analysed by a high-speed video camera. The amount of particle deposition in the pipeline after a conveying was calculated by controlling the solids feeding rate using a rotary valve and by monitoring the solids flow out of the system using dynamic load cells. Experimentally generated data have quantitatively shown a tendency of more solids deposition with lower gas mass flow rate in slug flows except that, below a certain amount of solids mass flow rate, the deposition becomes independent of gas flow rate. © 2004 Published by Elsevier B.V. Keywords: Pneumatic conveying; Slug flow; Solids deposition; CFD model; DEM simulation
1. Introduction Low-velocity slug flow pneumatic conveying has become more and more common in many industrial sectors ranging across chemical, pharmaceutical and food industries, due to its advantages over conventional high-velocity suspension flow in preventing product degradation and plant wear and in delivering high throughput and efficient power utilisation [1]. One of the distinguishing characteristics in such an operation is the gravitational deposition of solid particles in pipelines, which is due to the low gas velocity used, usually below the solids saltation velocity. A layer of particles at the bottom of the horizontal conveying line is commonly seen in these systems and this solids deposition significantly affects the transition and flow of slugs along the pipeline. This paper presents mathematical and experimental studies of solids deposition characteristics in the horizontal pipeline of a dense phase slug flow pneumatic conveying
∗ Corresponding author. Tel.: +44 1236 878448; fax: +44 1236 872837. E-mail address: [email protected] (J. Li).
0255-2701/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.cep.2004.02.011
system. To obtain an insight into the slug flow and solids deposition characteristics, a coupled discrete element method (DEM) and computational fluid dynamics (CFD) approach was used to simulate the solids and gas flows. Solids deposition in the horizontal section of the conveying system was visualised through a glass section and recorded by a high-speed video camera. The amount of particle deposition was calculated by closely monitoring the solids flow into and out of the system. Mathematically and experimentally generated data were analysed in order to develop a useful tool for the future prediction of solids deposition and slug flow behaviours in low-velocity pneumatic conveying.
2. Characteristics of slug flow - DEM modelling Although low-velocity slug flow pneumatic conveying has been studied intensively in the past two decades [1–8], due to the practical restrictions of applying modern measuring tools to gas and solids multiphase flows there is still a lack of understanding of how gases and particles interact inside a moving slug and how these interactions affect the formation
168
J. Li et al. / Chemical Engineering and Processing 44 (2005) 167–173
Fig. 1. Initial conditions used in the simulation of slug flow in pipes.
and flow of slugs. A recent study by Klinzing [8] demonstrated that the slugging behaviours of solids in a pipeline are complex, involving intensive interchanges of particles between the stationary layer and the moving slug, i.e. picking up particles at the front of a slug and dropping off at the tail. He also stated that a thin layer of particles in the pipeline was essential for the development of stable slugs. To obtain this realistic experimental condition several pre-runs were conducted in his experiments to let particles lay down in the pipeline. Obviously, a quantitative description of this particle layer, which is equivalent to the amount of particle deposition after a conveying, would bridge the understanding of solids slugging behaviour and the performance of the transport system. To understand the mechanisms of solids slug flows, a two-dimensional coupled DEM/CFD numerical model was built to simulate the motion of a pre-formed slug (ca. 0.3 m long) in a 1 m long horizontal 50 mm bore pipe as shown in Fig. 1. The pipe was initially filled with a layer of particles, approximately 15 mm thick at the bottom. (The thickness of this stationary layer was determined based on experience from previous experiments and computer test runs). In the model, the solids phase was modelled as discrete particles using DEM and the gas phase was modelled as a continuum by CFD. The influence of solid particles on the gas flow was considered as a source of mass, momentum and energy, via the change to its volume fraction, the aerodynamic forces exerting on the gas and the heat transfer with the gas. The effect of the gas flow on particle motion was calculated via the fluid drag forces. (Detailed description of the model is found in a previous publication of the authors, Li and Mason [9]). This method allows individual particle trajectories and interstitial gas flows to be traced, and local gas-particle, particle-particle and particle-wall interactions to be computed with the evolution of the slug flows. The basic advantage of this coupled approach over conventional continuum techniques is that DEM simulates effects at the particle level. There is less need for global assumptions and the assembly response is a direct output from the simulation [10]. DEM simulations have been used to model different granular systems by a number of researchers and have reported positive results: e.g. Tsuji et al. [11] on a fluidised bed, Langston et al. [12] on hopper flow, Frank et al. [13] on channel flow, Cleary [14] on ball mills and Stewart et al. [15] and Kuo et al. [16] on solids mixing. The main disadvantage of DEM modelling is that it requires a large amount of resource to compute increased number of parti-
cles, which makes it difficult to model fine particles or in three-dimensional geometries. Therefore, discrepancies are expected when comparing the current two-dimensional simulations with experiments. Particles used in this simulation were 5 mm spherical polyethylene pellets with a particle density of 880 kg/m3 . The gas was introduced into the pipe inlet with a constant superficial gas velocity of 2 m/s and all particles in the solids slug and the stationary layer were assigned a low initial velocity at 0.01 m/s to represent a dynamic starting condition. The simulation started with an ambient pressure in the whole pipe and this ambient pressure was kept constant at the pipe outlet while the pressure rose in the pipeline when gas was introduced from the inlet. Fig. 2 shows six snapshots of slug flows along the pipe from the DEM/CFD modelling at a time interval of 0.2 s for a total time length of 1.0 s. This simulation demonstrates a similar picture as described by Klinzing [8], in terms of particle motion in the slug. That is, particles in the stationary layer (dark coloured particles at the bottom of the pipeline) ahead of the slug wave are drawn into the slug (light coloured particles) and form part of it when being moved along the pipe by the pressurised gases. The slug wave actually compresses the particle layer just ahead of it, pushing some of the particles up from the layer into the wave. It is also interesting to see that at the tail of the slug particles fall into the bottom part of the pipeline and form the stationary layer. The exchange of particles between the stationary layer and the slug has shown a unique slugging mechanism in slug flows, which explains why particles move slower than slugs [8]. A statistical analysis shows that, after the slug flow, the number of particles deposited in the pipe is about 90% of the number of particles originally placed in the stationary layer. Taking into account the entrance effect (much less particle deposition in the front section from 0 to 0.3 m), this shows a reasonable comparison between the assumed layer thickness (15 mm) and the modelled particle deposition, though an uneven particle layer has been produced after the slug flow. This also suggests that the uneven particle layer from the current model should be adopted as initial input for further modelling. It can be clearly seen from Fig. 2 that there is a variation of solids concentration across the slug with a looser front and a denser back. Some particles are even pushed ahead of the slug wave at the front and this group of particles behave in a very dynamic and suspended fashion. This phenomenon has been confirmed by analysing video footages from experiments in slug flows using similar particulate materials (3 mm spherical polymer pellets), as shown in Fig. 3. Although the general slugging behaviour is determined by the aeration, permeability and de-aeration characteristics of the particulate material, the local variation of solids concentration in the slug is very much dependent upon the actual interstitial gas flow through the slug and the pressure distribution along the pipe. Fig. 4 shows a plot of pressure drops along the pipeline at both top and bottom of the pipe cross-section at
J. Li et al. / Chemical Engineering and Processing 44 (2005) 167–173
Fig. 2. Solids slug flow and deposition – DEM/CFD simulation (time interval between each picture 0.2 s and total time 1.0 s).
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Fig. 3. Snapshots of slug flow from video footages recorded in experiments.
an instantaneous time of 0.4 s. A clear response of pressure build-up at the back of and across the slug is shown and a pressure difference between top and bottom of the pipe cross-section at the back of the slug is observed, both of which have been confirmed by experiments using accurate pressure measurement [17]. Obviously the pressure build-up
is due to the blockage of gas passage by particles in the pipe cross-section and the gas retention characteristics in front of the slug plays an important role in determining solids slugging behaviours. The presence of particles has severely altered the gas flow field in the vicinity of the slug. Figs. 5 and 6 show the simulated gas velocity profiles horizontally along the pipeline and vertically in the pipe cross-section at different locations, respectively. In general the axial gas velocity is much higher at the top of the pipe cross section than at the bottom and the highest occurs at the back of the slug (around 0.4 m), which follows the profile of the solids assembly at the back of the slug. However, gas velocity profiles become much flatter in the middle of the slug as shown in Fig. 5 at 0.5 m. It is also interesting to see the change of gas flow direction in the pipe cross-section as shown in Fig. 6 (positive value indicating an up-flow and negative a down-flow). An up-flow of gas is observed at the front of the slug (0.6 and 0.7 m) and the far back of the slug (0.3 m) and a down-flow at the back of the slug (0.4 m) but a complex picture in the middle of the slug (at 0.5 m). In general, these cross section velocity profiles comply with the axial velocity profiles in terms of mass balance in the gas phase.
Fig. 4. Pressure drop along the pipeline – DEM/CFD simulation (at 0.4 s).
Fig. 5. Gas velocity profile along the pipe – DEM/CFD simulation (at 0.4 s).
J. Li et al. / Chemical Engineering and Processing 44 (2005) 167–173
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Fig. 6. Gas velocity profile in pipe cross section – DEM/CFD simulation (at 0.4 s).
The gas velocity profiles may explain why a loose slug front is generated. That is particles in the front of the slug wave are pushed by the slug body and dragged ahead of the slug by the high local gas velocity at the top pipe cross section while being agitated through severe interactions incurred by the rising particles from the stationary layer beneath the slug front and floating over them. The agitated particle layer underneath the slug front is actually acting as multiple fluidised rolling wheels for the slug, which greatly reduces the frictional forces comparing with a direct interaction with the pipe bottom wall. In some cases, even an air pocket or a particle swirling flow is created between the suspended loose slug front and the bottom particle layer, of which the resulting mechanisms and its consequence on slug flow need further studies.
3. Experimental analysis Experiments were conducted to determine quantitatively the amount of particle deposition in the pipeline of a specially designed pneumatic conveying system [17,18]. A schematic layout of the rig is shown in Fig. 7. Briefly
it comprises of a feed vessel, a receiving vessel, a 13 m long conveying pipeline with interchangeable 5.1, 7.6 and 10.2 cm (i.e. 2, 3 and 4 in.) nominal bore pipes, a bank of air supply nozzles, a variable speed rotary valve with dropout box, an air filter unit, and connecting pipes and control valves. In the system, both gas and solids mass feeding rates are closely controlled through the choked nozzle bank and the variable speed rotary valve. A set of three load cells is mounted underneath the receiving vessel to monitor the weight of solids conveyed. This particular design allows the desired solids mass flow rate to be selected by adjusting the rotary valve speed while the actual conveying rate is calculated simultaneously from the load cell readings. Fig. 8 shows a typical diagram of a non-suspension wave-like flow. Each step in the mass collected history is caused by the arrival of a slug of solids. It is noticed that there is a difference between the mass of solids flowing into and out of the system and the difference in the final masses is the deposition of solids in the system, which usually forms a particle layer in the pipeline at the end of conveying. Different combinations of gas and solids flow rates were tested in a 7.6 cm bore pipe with a common industrial granular material - polyethylene pellets. The pellets were spher-
Table 1 Solids deposition in a pneumatic conveying system Ug (ca.) (m/s)
25 8 6 4 2.6 a
Mg (kg/s)
0.1534 0.0491 0.0368 0.0245 0.0160
Ms1
Ms2
Ms3
Ms4
Fixed (kg/s)
Deposition (kg)
Fixed (kg/s)
Deposition (kg)
Fixed SLR5 (kg/s)
Deposition (kg)
Fixed SLR10 (kg/s)
Deposition (kg)
1.2270 1.2270 1.2270 1.2270 1.2270
0.00 3.94 6.28 6.05 9.79
0.6135 0.6135 0.6135 0.6135 0.6135
0.00 9.35a 7.29 8.03 8.92
0.7669 0.2454 0.1840 0.1227 0.0798
0.00 11.35a 9.18a 8.33a 11.70
1.5337 0.4908 0.3681 0.2454 0.1595
0.00 9.51 5.43 8.05 12.88a
Large difference ( > 2 kg) of load cell readings occurred between repeated runs, which may be due to the unstable flow of slugs in these flow conditions.
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Fig. 7. Schematic layout of the pneumatic conveying rig.
ical in shape with an average diameter of 3 mm, a particle density of 880 kg/m3 and an approximate bulk density of 550 kg/m3 . The experiments were started initially with high superficial gas velocity at 25 m/s, in which very little particle deposition occurred (This was confirmed with visualisation via a 3 m glass section in the conveying line). When using
low gas velocity (< 10 m/s), a particle layer was built at the bottom of the pipeline after a slug passing and at the end of each test case. This solids deposition was assumed equivalent to the difference of the final load cell readings between the tested cases and that obtained using a high gas velocity (25 m/s).
Fig. 8. Mass trace and solids deposition in slug flows.
J. Li et al. / Chemical Engineering and Processing 44 (2005) 167–173
Table 1 summarises solids depositions in the four groups of tests conducted: Ms1 - fixed mass flow rate of solids with a higher rate at 1.2270 kg/s, Ms2 - fixed mass flow rate with a lower rate at 0.6135 kg/s, Ms3 - fixed solids loading ratio (ratio between solids and gas flow rates) SLR = 5, and Ms4 - fixed SLR = 10, totalling 20 experimental cases. From Ms1 , a strong tendency for more solids deposition with a lower gas mass flow rate is shown. However, this effect does not appear to be true for either the lower solids mass flow rate Ms2 , or the two groups with fixed SLR, Ms3 and Ms4 (also with lower mass flow rate < 0.5 kg/s). It shows that below a certain amount of solids mass flow rate the deposition of solids becomes independent of gas flow rate, which can be explained by the fluidised motion of particles before a full slug moves and the gradual build-up of long slugs in the down stream pipeline as discussed previously in Li et al. [18]. Further experiments with a wider range of flow conditions using advanced measuring tools such as tomographic image analysis [19] are required to validate the model and to build up a quantitative description of solids deposition for different particulate materials.
4. Conclusions Gravitational solids deposition in a horizontal pipeline of a pneumatic conveying system was studied both mathematically and experimentally. Mathematically modelled results using the DEM/CFD simulation have demonstrated an intensive exchange of particles between the slug body and the stationary layer as slugs move along the pipeline, and a large variation of solids concentration and pressure and velocity distributions across the slug. A slug wave actually compresses the particle layer just ahead of it, pushing some of the particles up from the layer into the wave. On the other hand, particles at the tail of the slug fall into the bottom part of the pipeline and form a stationary particle layer behind the slug. These phenomena have been confirmed by analysing recorded video footages. Experimentally generated data have quantitatively shown a tendency of increased solids deposition at lower gas mass flow rate in slug flows except that, below a certain amount of solids mass flow rate, the deposition becomes independent of the gas flow rate, which is due to particle fluidised motion before a slug flow commences. Further work will focus on experiments with a wider range of flow conditions using sophisticated measuring tools to validate the model and to build up a quantitative protocol for the description of solids deposition in conveying pipes.
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