An experimental investigation on a fluidized motion conveying system

Powder Technology 167 (2006) 72 – 84 www.elsevier.com/locate/powtec

An experimental investigation on a fluidized motion conveying system S.K. Gupta a , V.K. Agrawal b , S.N. Singh c,⁎, V. Seshadri c , David Mills d a

Civil Engineering, North Eastern Regional Institute of Science and Technology, Nirjuli (Itanagar), Arunachal Pradesh, India b ITMMEC, IITDelhi, New Delhi, India c Applied Mechanics, IITDelhi, New Delhi, India d Glasgow Caledonian University, UK Received 4 February 2006; received in revised form 18 June 2006; accepted 20 June 2006 Available online 14 August 2006

Abstract The paper presents a description and the performance characteristics of a 3.7 m long fluidized motion conveying system, which transports the dry particulate materials through closed conveying channel section at different conveyor inclinations. The conveying capacity of the system has been investigated using coal ash of median particle size 108 μm as the solid material. Data on the effect of operating superficial air velocity, channel inclination, and the material supply valve opening on material mass flux, plenum chamber pressure and the material bed depth have been presented. It is found that the increase in the operating superficial air velocity increases the material mass flow rate, which finally reaches an asymptotic value at about 1.5–1.7 Umf. It is observed that the plenum chamber pressure is independent of the flow resistance offered by the moving material bed, and is only dependent on the supply of the airflow rate into the plenum. The solids mass flow rate decreases as the conveyor orientation changes from downward to upward direction. Further, the upward inclination of the conveyor requires a higher operating superficial air velocity to start the material flow, and also increases the material bed depth in the channel. The increase in the opening of material supply valve increases both the material mass flux and the bed depth in the channel for all the cases of the conveyor inclinations. The flow visualization in the horizontal and upward incline cases of the conveyor shows that the pulsatory flow mode plays a major role in the transport of the material, whereas, only sliding bed/non-pulsatory flow mode exist in the down-incline of the conveyor. The observed flow patterns have been categorized and classified in terms of flow regimes. © 2006 Elsevier B.V. All rights reserved. Keywords: Dense phase flow; Fluidization; Airslide; Fluidized motion conveying system; Experimental investigation

1. Introduction Numerous products and intermediates in mining and process industries exist in dry particulate form. These bulk particulate materials need to be stored and/or transported for their processing purpose. To transport these particulate materials, pneumatic conveying systems are widely used in industries due to their inherent advantages such as low cost, flexible routing, low maintenance, easy automation, enclosed nature, and general environmental hygiene, etc. [1]. These systems convey dry particulate materials broadly in two different flow modes, i.e., dilute suspension mode and dense phase mode [2]. Out of the

⁎ Corresponding author. Tel.: +91 11 26591180; fax: +91 11 26581119. E-mail address: [email protected] (S.N. Singh). 0032-5910/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2006.06.004

two available modes of the transportation, dense phase mode offers relative advantages of reduced specific energy consumption, pipe wear and particle attrition [3]. Further, in a relatively recent study, Jones and Williams [4] have proposed an empirical model for solids friction factor to determine the pressure drop contribution due to solids in fluidized-dense-phase conveying. They have demonstrated the solids friction factor to be relatively independent of the particle properties under different operating conditions, and validated their model by conveying polypellets in the same pipeline (50 m long with 53 mm NB) geometry. Pan [5] has suggested that the three flow modes, which occur in dense phase pneumatic conveying in a pipeline, can be classified on the basis of properties of the materials to be conveyed. In studies of Tsuji et al. [6], Lim et al. [7], and Fraige and Langston [8], the flow characteristics of dense phase pneumatic conveying through pipelines have been successfully simulated using DEM

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Nomenclature F L t Uo Umf Uo/Umf X θ

Percentage opening of the material supply valve (%) Length of the conveyor/conveying distance (m) Thickness of material bed in the permeameter (m) Operating superficial air velocity (cm/s) Minimum fluidization velocity (cm/s) Non-dimensional operating superficial air velocity Distance from the channel feed end in flow direction (m) Angle of inclination of the channel with horizontal (°)

(Discrete Element Method). Fraige and Langston [8] have also compared their simulated results with the data of Molerus [9,10] and found a good agreement relating to the prediction of pressure gradient, solids/gas flow rates and flow pattern as well. These detailed investigations have improved the understanding of dense phase conveying systems, which were earlier suffering from frequent pipe blockage [11]. However, these systems are sensitive to the feed conditions, which are to be set carefully or else they may lead to process instability [10]. Practically, a horizontal pipe is more critical than a vertically oriented one, since it has a greater uncertainty in terms of the flow parameter and characteristics, and hence, is more vulnerable to blockage [8]. This has led to the development of a Fluidized Motion Conveying System, which conveys the dry particulate materials in ultra-dense-phase mode without causing the problem of blockage of conveying duct. Fluidized motion conveying system uses an aeration technique to convey the dry form of bulk particulate materials. It has proved its large conveying capability in industries with its additional advantages of lower wear, particle attrition, air requirement and operating cost [12]. Powdered materials like cement, alumina, plastic metal powders, soda ash, coal dust, flour, resins, etc. have already been conveyed successfully in industries using these systems [13]. Muskett et al. [14] have conveyed fluidized sand down-incline in a gravity conveyor to study the effect of superficial air velocity on the mass flow rate of solids. They have also evaluated the performance of a vertical baffle wall to control the flow rate of solids. Botterill et al. [15] determined the rheological behaviour of fluidized bauxilite by using a modified Brookfield viscometer and a closed-circuit (shape similar to one obtained by joining two U-tubes together at their ends) open channel, which had 1.0 m test length in one of the limbs of the circuit. They found that the fluidized bauxilite exhibits nonNewtonian flow property and the results obtained by both the methods are comparable. Botterill and Bessant [16] used sand of mean particle size 200 μm and established that the fluidized particulate solids show non-Newtonian rheological behaviour. Ishida et al. [17] conveyed glass beads in a 954 mm long and 39 mm wide open channel. They measured the velocity distribution of solid particles using an optical probe in the downward inclined channel and attempted to categorize the flow pattern on the basis of non-dimensional superficial air velocity (Uo/Umf) and angle of inclination of the channel. Rao and Tharumarajan [18] carried out parametric investigation on a 2.5 m

long airslide using raw meal (crushed grain-bulk density 0.91 g/ cm3 and particle density 2.71 g/cm3) as the conveyed material. Kosa [19] fixed a specially formed air distributor plate having oblique slots in an open channel of 1 m length, and named it the Aerokinetic Canal. He conveyed polyethylene and fertilizer granules separately and proposed a physical model for the system. Latkovic and Levy [20] investigated the flow characteristics of fluidized magnetite powder in an open channel of length 1.3 m. They further extended the conveying distance to 2.3 m for minimizing the effect of entry and exit disturbances. Hanrot [21] has reported a case study where fluidized alumina was conveyed over a horizontal distance of 180 m. Woodcock and Mason [22] have systematically presented the variations of airslides used by different researchers to convey the dry particulate materials in all three types of the conveyor inclinations. Klinzing et al. [23] have given a systematic design procedure for Airslides/Gravity Conveyors for downward inclined channel. The available literature reveals that the airslides have been studied mostly in downward inclinations, even if the powdered materials were required to be conveyed at the same level. The reason behind the placement of the conveyors in down-incline position is either to increase the material haulage efficiency of the system, or the lack of the users' confidence in the design of the system working in horizontal position. This resulted in the necessity of a sufficient elevation difference between the feed and exit points of the conveyor, especially when the conveying distance was large. On the other hand, the material transportation in up-incline cases of the conveyors needed a specially formed air distributors to be fixed in the conveying channel, which ultimately affected the basic simplicity of the airslides. The change in the constructional features of an airslide resulted in the problems, which are associated to the intricacy involved in the manufacture of the airslide fabric/air distributor and the clogging of the permeable inclined slots of air distributor during the operation. This finally resulted in a complicated and inefficient material haulage system. The deterioration of the airslide fabric/ air distributor has also been, perhaps, another reason for the nonadoption of the airslides on a larger scale. However, the need of the regular replacement of an airslide fabric/air distributor at specified time intervals could be overlooked, if the advantages offered by these systems and the problems associated with belt conveyors and its multiple components are simultaneously kept in perspective for comparison along with the environmental issues. Therefore, in order to derive the advantages of ultra dense

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phase mode conveying and upward conveying potential, a minor change in the constructional features of an airslide has been incorporated in the form of enclosing the conveying channel. This made it possible to transport the material up-incline with additional advantages of environmental cleanliness and simplicity. Thus, the characteristics of such kind of airslides that retain their basic simplicity and convey the dry particulate materials in upward incline cases are yet to be fully explored, especially for long distances. The present study aims at the assessment of the horizontal and upward conveying potential of an airslide having a closed conveying duct. Although the downward conveying potential is quite obvious, the performance characteristics need to be investigated for the purpose of comparison with the other two cases of the conveyor inclinations. Therefore, the downward conveying potential has also been investigated. The present system can convey the particulate materials through a closed conveying duct for all the three inclinations (downward, horizontal and upward) of the conveyor and hence is referred to as the Fluidized Motion Conveying System as against the airslide that convey the materials through an open channel in down-incline case only. The effect of the airflow rate, channel inclination and material feed rate has been investigated in relation to the material mass flux, plenum chamber pressure, and the material bed depth. Subsequently, the system's conveying length has been extended to 5.5 m and 7.5 m and the results of the conveying potential for the longer distances in horizontal cases have been given elsewhere [24].

reaction show irregular but non-random fluctuations related to non-linear dynamics of the system. The chaotic behaviour of the fluidized bed gets further enhanced and a sequence of subsequent changes in the fluidization regimes as fixed bed, bubbling regime, slugging regime, turbulent regime, fast fluidization and finally the pneumatic conveying regime occurs with increasing airflow rate. The bubbling, slugging and turbulent regimes are together referred to as aggregative fluidization [30]. The transition of the flow regime with increasing airflow rate also causes an increase in the material bed voidage [30–32], and therefore, the fluidized density of the powdered material bed will also vary with the changes in the fluidization regimes. The fluidized motion conveying system makes use of this fluidized character of the particulate solids to transport the material mass, and therefore, its performance characteristics are expected to be governed by the hydrodynamics of the various fluidization regimes occurring in the channel with increasing airflow rate. However, Botterill and Abdul-Halim [33] have already shown that the flowing fluidized sand and the ash beds exhibit the pseudo-plastic characteristics at relatively low airflow rate and dilatant property at higher fluidizing air velocities. 3. Experimental program The description of various components of the experimental setup, material properties, test procedure and parameters covered during the test runs are given in the following subsections.

2. Regimes in fluidization process 3.1. Test rig If air, at a relatively low flow rate, is passed upward through a bed of dry particulate materials supported over a porous membrane, it merely filters through the interstitial voids of the bed of the discrete particles. As the airflow rate continues to increase, a stage comes at which solid particles tend to float in the airflow. The floating tendency of solid particles causes a rearrangement in the material bed configuration, and thus, the bed expands. At this stage, the pressure drop across the material bed becomes equal to the weight of materials per unit cross-section area of the bed; and the pressure drop hereafter remains almost same with increasing airflow rate [22]. In fact, the solid particles do not precisely float into the airflow; instead, they circulate within the material bed and cause the bed to become non-homogeneous and anisotropic [16]. Stein et al. [25] used a non-invasive positron emission particle tracking technique (PEPT), and observed that the solid particles move upward in central region and fall downward near the wall for a relatively deep material bed of particles in a cylindrical column. Despite this peculiar character of the solid particles in the material bed, the disposition of the powdered material resembles fluid-like characteristics and is said to be fluidized. Lim et al. [26] reported that the experimental measurements made by van der Stappen et al. [27,28], and Bouillard and Miller [29] show that the fluidized bed behaves as a chaotic system. Many properties of fluidized bed such as local pressure, local voidage and local concentration of chemical species undergoing

The schematic diagrams of the fluidized motion conveying system and the permeameter are given in Figs. 1 and 2, respectively. The different components of the fluidized motion conveying system have been described below. However, the design of the permeameter is in accordance with the usual practices being followed and, hence, is described only briefly. 3.1.1. Air supply system A root blower having a capacity of standard air delivery of 340 m3/h at a pressure of 8.27 × 104 Pa has been used for air supply. This root blower supplies air to a reservoir, which is subsequently connected to the plenum chamber of the fluidized motion conveying system. A pipeline of 50 mm NB has been used to connect the prime mover with the reservoir and the plenum chamber. A ball valve was fixed downstream of the reservoir in the air supply line to control the airflow rate into the plenum. Thus, the prime mover supplies the air to the reservoir, which finally delivers it to the plenum chamber at a desired flow rate. 3.1.2. Material feeding arrangement A supply hopper (Fig. 1) having geometry of a rectangular vertical bin with a wedge at the lower portion has been used to feed the material into the channel. It was fabricated from mild steel sheets and has a capacity of 0.5 m3 to hold approximately

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Fig. 1. Fluidized motion conveying system.

350 kg of the coal ash. A rectangular opening of 115 mm × 110 mm size was made below the truncated rectangular pyramid of the supply hopper. A slide valve was provided at the rectangular opening of the supply hopper to control the material feed rate into the channel. A projecting collar in the form of an open-ended rectangular box was provided just below the slide valve of the supply hopper to guide the flow of material down into a rubber tube which is slipped over the collar. The hopper was mounted on a 1.63 m high fabricated-steel-platform and a steel ladder was provided to access the top of the platform to refill the supply hopper with material.

3.1.3. Conveying duct Two U-shaped channel sections have been used to form a closed channel (Fig. 1). A 4.5 mm thick air-slide fabric was sandwiched between the two channel sections by using nuts and bolts. In this manner, there exist two compartments in the conveyor. The compartment formed above the air slide fabric is referred to as the conveying channel section, whereas the one below it is called the plenum chamber section and their depths are 150 mm and 75 mm, respectively. In comparison to the channel widths of 200 mm used by Rao and Tharumarajan [18] and 100 mm used by Latkovic and Levy [20], authors have adopted a

Fig. 2. Permeameter.

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channel of 150 mm width in line with the one used by McGuigan [34]. Two conveyor sections of 1.85 m length have been joined together to give the total conveying length of 3.7 m. The channel was fabricated using 12.5 mm thick Perspex sheets to facilitate the visual observation of the flow characteristics. The downstream end of the conveying section is open to allow the material to flow out, whereas the plenum chamber is closed at both ends. A rectangular opening at the upstream end of the conveyor section was made on the top face of the conveying channel section to receive the material falling from the supply hopper. A rectangular funnel-shaped collar was also attached to this opening to ensure free flow of the material. At the point where material drops inside the conveying channel section, a rectangular perspex sheet was fixed at 60° inclination to avoid the risk of damage of air-slide fabric, which can occur due to the direct impact of falling particulate materials. This inclined plate also imparts a slight forward motion to the falling particulate materials. Two hollow rectangular mild steel sections (25 mm × 75 mm) were provided beneath the perspex conveyor as supporting beams to give strength to it. The support structures, over which the conveyor assembly rests, were kept at a horizontal spacing of 1.54 m from each other. The conveyor assembly was hinged to the support structure placed near the feed end, whereas its other end was arranged to rest on another support platform provided with a screw-jack arrangement near the exit end. This arrangement allows the conveyor assembly to have the flexibility of rotation in the vertical plane, which is needed at the time of change in the conveyor inclination. There is 1.22 m vertical clearance between the floor level and the perspex channel bottom, which offers convenience to visualize the dynamic flow pattern of particulate solid materials effectively. 3.1.4. Collection vessels and interconnections The exit opening of the supply hopper and the feed opening of the conveying channel section were joined together with the help of a rubber tube similar to the one being used in the wheels of a heavy motor vehicle (Fig. 1). This tube was slipped over the collars attached to those two openings. During the change in the inclination of the channel, the flexible rubber tube adjusts itself according to the vertical clearance between the projected collar of the supply hopper opening and the funnel-shaped collar of the conveying channel section. An L-shaped channel section with a rectangular opening size of 178 mm × 150 mm has been attached to the exit end of the conveyor to divert the material flow down towards the collecting vessels of 150 l capacity. A total of six numbers of vessels were used to collect the particulate materials falling down at the conveyor exit. A cotton cloth in the form of tube was tied around the exit opening of the L-shaped section to avoid the dust dispersion into the environment and the spread of material out of the receiving buckets. 3.1.5. Instrumentation A pre-calibrated orifice plate assembly (Cd = 0.622) was provided in the air supply pipeline at the downstream of the control valve to measure the airflow rate. The desired airflow rate is fed into the plenum chamber with the help of a 65 mm NB

flexible hose which joins the air supply pipe line to the air inlet of the plenum chamber. The plenum chamber pressure was measured with a U-tube manometer attached at a distance of 2.8 m from the closed end of the channel. Two plastic scales of 150 mm length were glued on the outside surface of the conveying channel section at 0.91 m (3.0 ft) and 2.74 m (9.0 ft) distances from the upstream closed end of the conveyor. These scales are used to measure the material bed depth variation at these two locations along the channel length. A stopwatch having a resolution of 0.01 s was used to measure the time required to fill the desired number of buckets with the coal ash. The least count of the weighing machine that was used to measure the weight of collected material was 0.1 kg. The inclination of the conveyor was accurately measured by noting down the difference in the elevation of the upstream and downstream ends with respect to a common datum. 3.2. Permeameter This test rig (Fig. 2) has also been connected to the same roots blower through a bypass air supply line. The air supply line was connected to a 38 mm NB manifold having three rotameters in parallel lines. In order to cover the range of airflow rates, the flow capacities of three rotameters are 50– 550 lpm, 5–55 lpm and 1.5–15 lpm, respectively. The valves provided before each rotameter controlled the airflow rates through each rotameter. The delivery from each rotameter is supplied to a 38 mm NB common pipeline which is finally connected to the plenum of the permeameter. The outer diameter of the plenum is 0.30 m and depth is 0.15 m. A permeable fabric is placed above the opening in the top face of the plenum and a cylinder of Perspex sheet having a height 0.60 m and diameter 0.15 m was fixed above the fabric. The diameter of the opening in the plenum chamber was equal to the diameter of the cylinder. The two pressure taps, one at the bottom and another at a vertical spacing of 0.20 m from the first were used to measure the pressure drop across the material bed. One additional pressure tap was provided in the plenum chamber to monitor the plenum pressure. The pressure taps

Fig. 3. Particle size distribution for coal ash.

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on the cylinder were connected to a differential manometer and the plenum pressure was measured with the help of another manometer. 3.3. Material characteristics Coal ash has been used as the conveyed material. It was procured from a thermal power plant located at Panipat, Haryana, India. The particle size distribution of the coal ash is given in Fig. 3. Top particle size and the mean particle size of the coal ash have been determined as 300 μm and 108 μm, respectively, whereas 62% of the particles fall in the range of sizes 180–75 μm. Tests were conducted in the permeameter to determine the fluidization characteristic of the coal ash. The pressure drop was measured across 200 mm thick ash bed in the Permeameter. The minimum superficial air velocity corresponding to the constant pressure drop gives the minimum fluidization velocity, whereas the fluidized density of material is determined by using the volume occupied by the particulate materials at its incipient fluidization and the mass of the material in the permeameter. The fluidization characteristic of the coal ash is shown in Fig. 4 and the properties of the material are summarized in Table 1. The minimum fluidizing velocity has been determined as 2.51 × 10− 2 m/s, and the range of superficial air velocities covered in the test runs conducted on the fluidized motion conveying system have been non-dimensionalized using this value. The bulk density and the fluidized density of the coal ash have been determined as 773 kg/m3 and 665 kg/m3, respectively. 3.4. Experimental procedure and range of parameters covered The coal ash was manually poured into the supply hopper to fill it up fully. Before starting the measurements, material was conveyed for some time without taking any observations. This was done as a routine each time when tests were conducted. The objective was to ensure the proper fluidization of the material to be conveyed. After filling the material in the supply hopper, the slide valve of the hopper was adjusted to give the desired exit opening in the supply hopper. The roots blower was started and the flow control valve was opened to the desired extent to allow

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Table 1 Properties of the coal ash Sl. no.

Material property

Value

1 2 3 4 5

Specific gravity Bulk density (kg/m3) Mean particle size (μm) Minimum fluidizing velocity (cm/s) Fluidized density (kg/m3)

1.62 773 108 2.51 665.6

the airflow into the plenum chamber. The material from supply hopper falls into the conveying channel through the flexible rubber tube. The air that comes up through the porous fabric fluidizes the material to behave as liquid, and thus causes the material to flow towards the exit where it was collected in six large buckets. The time measurement was started only after the first two buckets were allowed to be filled up. The exclusion of the first two filled up buckets from the measurements is done to ensure the complete stabilization of the flow occurring in the channel. Pressure differential across the orifice plate assembly, plenum chamber pressure and the material bed depth at two locations were measured. The airflow control valve was closed immediately after the last bucket gets filled up, and simultaneously the stopwatch was stopped. Elapsed time to fill the last 4 buckets was noted down. Materials collected in the last 4 buckets were weighed. This completes one cycle of the test run. The effect of airflow rate, angle of the channel inclination and the material feed rate have been investigated in relation to the material mass flux, plenum chamber pressure and the material bed depth in the channel. The non-dimensional air velocity (Uo/ Umf) has been varied from the start-up value of approximately 0.85 to the upper limit of 4.28 as against to the upper limit values of 3 and 3.3 used by Botterill and Bessant [16], and Latkovic and Levy [20], respectively. The conveyor inclination was varied from 1.75° (downward) to − 1.68° (upward) at intervals of approximately 0.5°. Slide valve of the hopper was kept open for the two opening sizes, i.e., 50% open (63.25 cm2) and 65% open (82.23 cm2). However, the tests could not be conducted beyond 65% opening of the slide valve due to the excessive material flow rate and constraints on the quantity of material. In order to understand the flow mechanism, visual observations were made, and some still photographs and short movies of 30 s time interval using a digital camera were also captured during the test run. However, the data obtained by the direct visual observations were primarily used to draw the conclusions regarding the flow regimes. 4. Results and discussion The first part of this section gives the flow patterns observed during the test runs, whereas the other parts of the section deal with the effects of various parameters on performance characteristics of the fluidized motion conveying system. 4.1. Flow visualization

Fig. 4. Variation of pressure drop across the material bed in permeameter for fluidization velocity determination.

It has been observed that over the range of superficial velocities studied, the flow of material takes place in the form of a

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stratified bed, and a layer of air carrying insignificant solids concentration flows above the moving material bed. This way, two distinct layers of the flowing fluidized solids and the air could be clearly identified. Mason and Levy [35] have also mentioned that the similar two layers could be observed in the moving-bed flow of the particulate materials in horizontal pipes. The velocities of both the moving material bed and the air layer increase along the flow direction. It is observed that in all the three cases of the conveyor inclinations, i.e., downward, horizontal and upward, the initial increase in the operating superficial air velocity (Uo) causes no change in the material bed (Mode 1). This characterizes the fixed bed regime of the fluidized beds [30]. Further increase in the operating superficial air velocity builds up the material bed depth in all three cases of the conveyor inclination (Mode 2). Different flow patterns have been observed after this stage in three types of the conveyor inclinations. Hence, each case has been dealt separately in the following paragraphs.

Table 2 Description of flow patterns and classification Sl. Flow description no.

Mode Criteria

1 2

1 2

3

4

5 6

4.1.1. Downward conveyor inclination After Mode 2, the increase in the superficial air velocity causes a non-pulsatory movement of the partial material bed at the top layer of the bed (Mode 4). Further increase in the superficial air velocity causes the total material bed to slide down through the channel (Mode 6). However, an additional increase in the superficial air velocity causes the excess air to come out of the sliding material bed in the form of air bubbles (Mode 7) and it characterizes the bubbling regimes occurring in the fluidized beds [30]. This phenomenon is also accompanied with the vigorous particles agitation, and thereafter the flow remains in the sliding bed mode with increasing airflow rate (Mode 7). 4.1.2. Horizontal conveyor position In this case, the Mode 2 phenomenon is followed by a nonpulsatory flow of partial material bed at the top portion of material layer (Mode4) with increasing air velocity. Further increase in the superficial air velocity causes the pulsatory movement of the total material bed with less frequent pulses (Mode 5), and this flow mode (Mode 5) continues to exit over the range of superficial air velocities covered in the test runs. 4.1.3. Upward conveyor inclination As opposed to the above two cases, the increase in the operating superficial air velocity after Mode 2 pattern causes the pulsatory movement of the partial material bed (Mode 3)in the channel. Further increase in the superficial air velocity transforms this pulsatory movement of partial material bed into the non-pulsatory movement of the partial material bed (Mode 4). However, an additional increase in the airflow rate causes no change in the material flow mode at low feed rate condition, whereas for high feed rate condition it transforms it into a pulsatory movement of the total material bed (Mode 5). 4.1.4. Discussion on flow patterns and regime classification Table 2 presents different flow patterns observed during the test runs and these patterns have been classified as distinct flow regimes. The observed increase in the material bed depth with the initial increase in the Uo occurs due to two reasons; first, the

7

No movement of material Building up of material bed depth in channel followed by start of pulsatory movement of partial material bed from top layer and gives extremely low mass flux due to large time interval between two pulses Pulsatory movement of partial bed of material from top layer of bed and flow is significant Non-pulsatory movement of partial bed of material from top layer of bed Pulsatory movement of total bed of material Steady movement of total bed of material with occasional pulses Steady movement of total bed of material with vigorous bubbling and particles' agitation

Flow regime

Stationary bed Fixed bed Surface shear Wavy/ force-dominated pulsatory flow

3

4

5 6

Combined influence of gravity and shear force

Transition (pulsatory/ non-pulsatory)

Gravitydominated flow

Sliding bed

7

incoming material from the supply hopper and second, the expansion of the material bed due to fluidization. Therefore, the material bed depth in the channel builds up [20] before the flow starts; and as the bed acquires fluid-like character the material flow starts. Thus, the flow starts suddenly as the initial shear stress exceeds the minimum critical stress and the pulsatory movement of the material is seen. This characterizes the Bingham fluid property of fluidized beds [20]. It has also been observed that the bed depth decreases after the passage of a pulse of material. It shows that more material goes out of the channel than the material fed in, and it results in the continuous variation of the material bed depth at any location, as observed visually. At the same time in the upward cases of the channel inclination, the falling material causes flow disturbance in the fluidized material bed accumulated in the lower portion of the channel near feed end. These disturbances in the material bed grow further along the channel length due to the interaction of increasing shear stress with the moving material bed interface and thus help to make the flow pulsatory. Therefore, the pulsatory flow mode becomes prominent in the case of upward conveyor inclination. Further, it is anticipated that the magnitude of the shear force that acts on the surface of material bed decides the depth to which its drag effect permeates into the material bed, in a way similar to the growth of a boundary layer where viscous effect permeates into the flow region. Thus, depending upon the magnitude of the shear stress and consequent depth of the material bed penetrated, the flow pattern transforms from partial bed pulsatory flow (Mode 3) to partial bed non-pulsatory flow (Mode4) and finally to the total bed pulsatory flow in sequence (Mode 5). However, the inclusion of the gravity component arising due to the conveyor inclination affects the transition process in the flow patterns.

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Fig. 5. Variation of material mass flux with non-dimensional superficial air velocity at different conveyor inclinations.

4.2. Effect of superficial air velocity The superficial air velocity significantly affects the performance of the fluidized motion conveying system and the effect of this parameter is discussed in the following paragraphs.

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4.2.1. Material mass flux Fig. 5(a and b) shows the variation of the material mass flow rate with non-dimensional air velocity at seven different conveyor inclinations and for a given material supply valve opening. It is found that the increase in the non-dimensional air velocity increases the material mass flux initially and thereafter brings it to a saturation level at higher rates of airflow. Further, it is also very interesting to note from Table 3 that the material mass flux reaches the saturation level at Uo/Umf value falling in the range of 1.5 to 1.7 for all the cases of conveyor inclinations. Woodcock and Mason [36] have also suggested that the value of Uo/Umf should lie in the range of 1.5–2.0 for the optimum performance of the air gravity conveyor. The initial steep increase in the material mass flow rate is due to the two-way action of the airflow rate. Firstly, after the start of the material flow, the increase in the operating superficial air velocity causes the material to have an increased bed voidage [30] and thus decreased bed viscosity [33]. Secondly, the increased airflow rate increases the shear stress acting on the upper surface of moving material bed. However, the reason for the saturation level being attained by material mass flux lies in the fact that the solids feed rate remains the same at all the operating superficial air velocities due to the constraint posed by the slide valve opening of the supply hopper. At this stage, the carriage potential of the airflow rate is under-utilized due to unavailability of the sufficient material falling through the hopper. This phenomenon is confirmed by the observed decrease in the material bed depth at higher airflow rates. Thus, the carriage potential of the airflow rate is more effectively utilized at the initial stage than that at later stage. It seems obvious that the excessive increase in the airflow rate will transform flow modes in the channel to undergo a sequential change to finally reach the pneumatic conveying stage as it happens in the fluidized bed studies [30–32]. However, the studies could not be carried out at such high airflow rates due to the capacity limitation of the prime mover.

Table 3 Variation of material mass flux with non-dimensional superficial air velocity for two material supply valve openings at different conveyor inclinations Sl. no.

Conveyor inclination with horizontal (θ °) ↓

1

+1.75

2

+1.07

3

+0.62

4

0.0

5

−0.46

6

−1.17

7

−1.68

Uo/Umf →

0.9

Material supply valve opening (%) ↓

Material mass flux (kg/s) ↓

65 50 65 50 65 50 65 50 65 50 65 50 65 50

0.63 0.32 0.81 0.21 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

1.1

2.50 0.89 2.40 0.87 2.26 0.82 2.13 0.73 1.99 0.64 1.20 0.00 0.62 0.00

1.3

3.23 1.31 3.21 1.33 3.16 1.37 3.10 1.35 3.05 1.32 2.27 1.15 1.62 1.18

⁎Positive (+) sign stands for downward inclination and negative (−) sign stands for upward inclination.

1.5

1.7

1.9

2.1

2.3

2.5

3.27 1.54 3.25 1.56 3.20 1.59 3.15 1.56 3.10 1.51 2.81 1.47 2.19 1.43

3.27 1.65 3.25 1.64 3.20 1.63 3.15 1.59 3.10 1.53 3.00 1.49 2.43 1.43

3.27 1.68 3.25 1.66 3.20 1.64 3.15 1.59 3.10 1.53 3.04 1.49 2.52 1.43

3.27 1.69 3.25 1.66 3.20 1.64 3.15 1.59 3.10 1.53 3.05 1.49 2.54 1.43

3.27 1.69 3.25 1.66 3.20 1.64 3.15 1.59 3.10 1.53 3.05 1.49 2.54 1.43

3.27 1.69 3.25 1.66 3.20 1.64 3.15 1.59 3.10 1.53 3.05 1.49 2.54 1.43

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Fig. 6. Variation of non-dimensional superficial air velocity with conveyor inclination for start-up of material flow.

4.2.2. Onset of material flow The non-dimensional operating superficial air velocity at which the flow of material starts in the channel has been plotted against the conveyor inclination in Fig. 6. It is observed that a minimum critical operating superficial air velocity is to be exceeded to start the flow of material into the channel. This finding is in accordance to the findings of the many other researchers [14,20] and it indicates that the fluidized material shows the Bingham fluid characteristics [20]. The magnitude of start-up operating superficial air velocity increases as the conveyor inclination changes from downward to upward position. This can be directly attributed to the effect of gravity. More detailed discussion is given in a subsequent section.

along the channel length due to the incremental increase in the volumetric airflow rate. This results in an increasing shear force, which acts on the top surface of material bed along the flow direction and causes the material to accelerate. Therefore, there is a decrease in the material bed depth along the channel length. This acceleration effect is further promoted by the decrease in the bed viscosity, which occurs due to the decrease in the material bed depth [16,20]. Hence, material accelerates even further, which results in the low material bed depth at the exit end and high bed depth at the feed end. The difference in the material bed depth at the feed end and the exit end gives rise to a hydrostatic pressure gradient [37] within the material bed which tries to flatten the slope of the material surface profile, and probably this could also be one of the reasons of the pulsatory flow mechanism. However, this reasoning assumes that the fluidized bed behaves as the highdensity liquid. 4.3. Effect of the conveyor inclination The following subsections discuss the performance characteristics of the fluidized motion conveying system in relation to the conveyor inclination with respect to the horizontal. 4.3.1. Material mass flux It is clearly observed from Fig. 5(a and b) that the material mass flux decreases with the increase in the upward inclination of

4.2.3. Plenum chamber pressure The plenum chamber pressure has been plotted against the operating superficial air velocity in Fig. 7(a and b) for two different supply valve openings. It is observed that there exists a linear relation between the two. The linear relation between the two parameters reveals that the plenum pressure is directly dependent on the supply of the airflow rate and is independent of the thickness of the moving material bed in the channel. 4.2.4. Material bed depth In order to establish the variation of material bed depth in the conveying channel with operating superficial air velocity, the time-averaged bed depth was observed at two fixed locations along the channel length for a given material supply valve opening and conveyor inclination. This has been plotted against the operating superficial air velocity in Fig. 8(a–c). For the sake of brevity and conciseness, the plots have been given only for the three cases of conveyor inclinations, i.e., 1.75° downward, horizontal and 1.68° upward. It is observed that the increase in the airflow rate decreases the material bed depth at a given location. In fact, the higher airflow rate causes a higher shear stress on the top surface of the moving material bed which sweeps more material out of the channel, and hence results in the reduction of the material bed depth at any location. It is also clear from the figures that the bed depth at upstream location is always higher than that at down stream location. The reason lies in the fact that the magnitude of the shear stress caused due to the airflow increases

Fig. 7. Variation of plenum chamber pressure with non-dimensional superficial air velocity at all conveyor inclinations.

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causes a back-pressure on the inflow rate of material falling into the channel; thus resulting in the low saturation level in the upward inclined channel. The magnitude of the material mass flux, after the saturation level is achieved, is almost the same in all the three downward inclinations of the conveyor (Fig. 5(a and b)). This is due to the fact that in the downward movement of the material, gravity overrides the shear force to convey the material. This means that the gravity component corresponding to the 0.62° conveyor inclination can effectively sweep all the material falling into the channel. Further increase in the downward inclination of the channel up to 1.75° does not cause any increase in the material mass flux due to the constraint posed by the material supply valve opening, though the gravity force component increases with the downward inclining conveyor. Thus, almost the same magnitude of material mass flux at saturation is observed in all the three downward inclinations of the channel. Further, there is a steep rise at the beginning of the curves for all the downward inclinations of the conveyor (see Fig. 5(a and b)), which suggests the dominance of the gravity as compared to the shear stress caused by the airflow rate. This implies that the moment material bed attains the fluid character at extremely low airflow rate, gravity takes all the material out, and hence, there is a steep rise in the material mass flow rate. Fig. 9(a and b) reveals that, for a given airflow rate, the rate of change of material mass flow with conveyor inclination is low for +1.75° to 0° conveyor inclination, and thereafter, a sharp change occurs as the conveyor inclines upward. This happens in both the cases of the supply valve openings. In fact, the driving force for the material carriage has gravity and the shear force as

Fig. 8. Variation of material bed depth at a location with operating superficial air velocity for two feed rates and a given conveyor inclination.

the conveyor for a given airflow rate. This is more evident from Fig. 9(a and b), which shows the variation of material mass flux with channel inclination for different airflow rates. The reason is that the component of the gravity force acts backward, and the shear force acting on the material surface has to exceed this adverse gravity component to transport the material upward. Therefore, for a given airflow rate, material mass flux decreases with the increase in the upward channel inclination. Further, the asymptotic level of the material mass flux also reduces as the conveyor inclines upward. The reason could be the large holdup of fluidized material bed near the feed end (see Fig. 8(c)) which

Fig. 9. Variation of material mass flux with conveyor inclination.

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two primary components. When the conveyor is inclined upward, the gravity opposes the flow and it neutralises the large magnitude of the shear force, which tries to transport the material upward. Thus, the significant change in the reduction rate of material mass flux is observed, as the conveyor starts inclining upwards. 4.3.2. Onset of material flow It is observed in Fig. 6 that the increase in the upward conveyor inclination requires a higher operating superficial air velocity than that at downward inclinations to start the material flow. This phenomenon is obvious due to the adverse gravity effect that pulls the material backward. It is interesting to note that in all the cases of the downward conveyor inclinations, the flow of material starts at a lower operating superficial air velocity than the minimum fluidization velocity. In fact, the airflow that comes out of the porous membrane forms an air layer above the material bed through which it comes up, and this flowing air layer imparts a shear stress on the upper surface of the moving material bed, which keeps increasing at all the subsequent downstream sections of the channel. Therefore, the airflow fluidizes the material and exerts a shear stress on the upper surface of material bed. This way, in downward incline positions of the conveyor, the dominant gravity force drives all the material out as soon as the material bed attains quasi-fluid character caused due to the fluidization by air. The material flow in the horizontal case of the conveyor also starts at an operating superficial air velocity, which is lower than the minimum fluidization velocity. This is due to the increased material bed depth that increases the air velocity and consequently the shear stress on the top surface of the material bed. 4.3.3. Plenum chamber pressure It is also clear from Fig. 7(a and b) that the trend between the two parameters remains linear in all the cases of the conveyor inclination and they almost coincide with each other for both the supply valve openings. The variation of the plenum pressure for all the conveyor inclinations falls within ± 6% of the plenum pressure values obtained at horizontal. The large holdup of material in the lower stretch of the channel slightly increases the plenum pressure in the upward incline case of the channel but the variation is not very significant. Therefore, the plenum pressure can reasonably be treated unaffected by the conveyor inclinations and can be said to be independent of material bed thickness, as mentioned earlier. Moreover, the experimental error involved in the measurement of the plenum pressure could also be another reason for the variations observed. 4.3.4. Material bed depth As the channel is lifted up from the maximum downward inclination to the upward position, the bed depth for a given superficial air velocity at the two fixed locations increases (see Fig. 8(a–c)). This is due to the fact that the gravity force component starts diminishing from maximum to the zero at the horizontal position and finally plays an adverse role for the upward conveyor inclination. This way, the material velocity will reduce due to the decreased driving force, which results in the higher material bed depth in the conveyor at a given supply

valve opening and airflow rate. However, the reduced crosssection area for the layer of air increases the air velocity, which tries to impart higher driving force, but it is not sufficient to overcome the effect of dominating gravity force. At the same time, in upward incline cases the material accumulation inside the channel near feed end occurs, which will have permanent large fluidized material holdup. This large amount of material holdup can easily be visualized by inclining a partially filled water bottle in upward inclined position. This material holdup was obviously more significant at − 1.68° inclination with 65% material supply valve opening than that at − 0.46° and − 1.17° or with 50% supply valve opening (see Fig. 8(a–c)). This large holdup of fluidized material inside the channel creates a favorable condition for the generation of translatory waves that play a leading role in conveying the material in horizontal and up-incline cases of the channel. A translatory wave is defined as the temporal or spatial variation in the flow depth/flow rate, which conveys mass in the direction of wave propagation [38]. 4.4. Effect of material supply valve opening The feed rate of solids in the conveying channel also has a significant effect on the performance characteristics of fluidized motion conveying system and this is discussed in the following subsections. 4.4.1. Material mass flux A comparison of the Fig. 5(a) and (b) shows that, for a given airflow rate, the higher supply valve opening increases the material mass flux compared to the smaller opening of the supply valve. Table 3 clearly reveals the same. In fact, at higher material feed rate, the reduced cross-section area due to the thicker material bed raises the air velocity in the air layer. Thus, in addition to the higher pressure developed by the falling material at the feed end, the shear stress exerted by the airflow on the interface of the material bed also increases. This explains the reason of high mass flow rate at high feed rate. Therefore, at higher supply valve opening, the total conveying potential of the air is more efficiently utilized for material transport than that at smaller supply valve opening. 4.4.2. Onset of material flow Fig. 6 shows that at the 65% opening of the material supply valve, the start of the material flow occurs at an operating superficial air velocity lower than the minimum fluidization velocity in both horizontal and upward inclinations. This happens because of the decreased effective cross-section area available for the air layer. This increases the velocity of airflow, and hence, a higher shear stress on the upper surface of the material bed causes the start of material flow earlier than the occurrence of the minimum fluidization velocity. Another reason could also be the increased pressure developed by the falling material in the channel at the large supply valve opening. In the horizontal case of the conveyor at 50% opening of the material supply valve, the non-dimensional air velocity is less than unity for starting the material flow. This shows that the increased bed depth generates sufficient shear stress to start the flow. However, in upward-

S.K. Gupta et al. / Powder Technology 167 (2006) 72–84

incline cases, the increased bed depth could not result in the sufficient shear force to start the flow, and hence, the conveyor requires the higher operating superficial air velocity than the minimum fluidization velocity to start the material flow. Moreover, in downward cases of the conveyor inclination, the start-up air velocities, for two feed rate conditions, almost coincide because the primary role is played by the gravity whose magnitude is same for both the feed rates. The requirement of the air in these cases is only to inject quasi-fluid property to the material bed and the rest of the work is done by the gravity to transport the material. This is the reason for the same start-up air velocities obtained for both the feed rates. 4.4.3. Plenum chamber pressure A comparison of the Fig. 7(a) and (b) shows that the plenum chamber pressure is hardly affected by the material supply valve opening and this trend is quite clear. The reason is the insignificant change in the flow resistance offered to the airflow by the thicker material bed in the conveying channel through which it comes up. It is also found that the same linear relation holds for the two cases of the supply valve openings in downward and horizontal cases of the conveyor. However, in upincline cases of the conveyor, the two curves separate marginally for the two feed rates and the reason seems to lie in the large holdup of fluidized material in the lower portion of the conveyor. This material holdup, especially at higher material feed rate, chokes the cross-section area of the conveying channel near the feed end in the similar way to a large stationary slug, and thus offers higher resistance to the airflow coming out of the porous membrane. Therefore, a slightly higher pressure is observed in the plenum chamber for the up-inclinations of the conveyor in high feed rate conditions only. 4.4.4. Material bed depth The Fig. 8(a–c) shows that the larger material supply valve opening increases the material bed depth in the conveying channel for a given airflow rate and conveyor inclination. It indicates that the conveyor has to transport higher material mass flux at higher feed rate (see Table 3) to satisfy the continuity equation, and hence, higher material bed depth is observed in the channel. 5. Concluding remarks It is observed that the flow of material takes place in the stratified form, and the material transport starts even under the partial fluidized condition, i.e., at operating superficial air velocity lower than the minimum fluidizing velocity. It is clear from the present investigation that the solids mass flow rate increases with the increase in the non-dimensional superficial air velocity and it reaches a saturation level at Uo/Umf value in the range of 1.5–1.7 for all the three cases of the conveyor inclinations. The plenum chamber pressure has a linear relation with operating superficial air velocity, and hence, the mass flow rate of solids can be seen as independent of this parameter. The channel inclination strongly affects the startup of the material flow, material bed depth in the channel, and the solids mass flow rate as well. It is found that for

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the case of upward inclinations, the material mass flow rate of solids decreases and the material bed depth in the channel increases, for a given airflow rate. Further, both the material mass flow rate and material bed depth in the channel increase with the increase in the material supply valve opening. It is also concluded that the material bed depth varies with the material feed rate, operating superficial air velocity and the inclination of the channel. It has also been observed that the material bed depth varies along the channel length in flow direction. Thus, the material mass flow rate of any given solid can be viewed as the function of Uo/Umf, è and F. Additional studies with different material are needed to develop design methodology for the fluidized motion conveying systems. References [1] P.W. Wypych, J. Yi, Minimum transport boundary for horizontal dense phase pneumatic conveying of granular materials, Powder Technology 129 (2003) 111–121. [2] A.J. Jaworski, T. Dyakowski, Investigations of flow instabilities within the dense pneumatic conveying system, Powder Technology 125 (2002) 279–291. [3] J. Hong, Yi-shen Shen, Shu-lin Liu, A model for gas–solid stratified flow in horizontal dense-phase pneumatic conveying, Powder Technology 77 (1993) 107–114. [4] M.G. Jones, K.C. Williams, Solids friction factors for fluidized densephase conveying, Particulate Science and Technology 21 (2003) 45–56. [5] R. Pan, Material properties and flow modes in pneumatic conveying, Powder Technology 104 (1999) 157–163. [6] Y. Tsuji, T. Tanaka, T. Ishida, Lagrangian numerical simulation of plug flow of cohesionless particles in a horizontal pipe, Powder Technology 71 (1992) 239–250. [7] E.W.C. Lim, C.H. Wang, A.B. Yu, Discrete element simulation for pneumatic conveying of granular material, AIChE Journal 52 (2) (2006) 496–509. [8] F.Y. Fraige, P.A. Langston, Horizontal pneumatic conveying: a 3D distinct element method, Granular Matter 8 (2006) 67–80. [9] O. Molerus, Principles of Flow in Dispersed Systems, Chapman and Hall, London, 1993. [10] O. Molerus, Overview: pneumatic transport of solids, Powder Technology 88 (3) (1996) 309–321. [11] K. Konrad, Dense-phase pneumatic conveying: a review, Powder Technology 49 (1986) 1–35. [12] P. Butler, No-moving-parts conveyor shifts dry powdered solids, Process Engineering (Aug. 1974) 65. [13] D. Kunii, O. Levenspiel, Fluidization Engineering, Krieger, Huntington, N.Y., 1977. [14] W.J. Muskett, A.R. Leicester, J.S. Mason, The fluidized transport of powdered materials in an air-gravity conveyor, Proc. of Pneumotransport 2. Second Int. Conf. on Pneumatic Transport of Solids in Pipes, Paper F1, September 1973. [15] J.S.M. Botterill, M. Kolk, D.E. Elliott, S. McGuigan, The flow of fluidised solids, Powder Technology 6 (1972) 343–351. [16] J.S.M. Botterill, D.J. Bessant, The flow of properties of fluidized solids, Powder Technology 14 (1976) 131–137. [17] M. Ishida, H. Hatano, T. Shirai, The flow of solid particles in an aerated inclined channel, Powder Technology 27 (1980) 7–12. [18] M.M. Rao, S. Tharumarajan, Experimental investigations on fluidized gravity conveying, Bulk Solids Handling 6 (1) (Feb 1986) 99–103. [19] L. Kosa, Modelling the operation of aerokinetic canal, Powder Technology 54 (1988) 209–216. [20] D. Latkovic, E.K. Levy, The flow characteristics of fluidized magnetite powder in an inclined open channel, Powder Technology 67 (1991) 207–216. [21] J.P. Hanrot, Multipoint feeding of hoppers, mounted on aluminium smelter pots, by means of potential fluidization piping, Proc. of 115th Annual

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