Powder attrition in gas fluidized beds

Powder Technology 287 (2016) 1–11

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Powder attrition in gas fluidized beds Huili Zhang a,⁎, Jan Degrève a, Jan Baeyens b, Shu-Yii Wu c a b c

KU Leuven, Department of Chemical Engineering, Bio-&Chemical Systems Technology, Reactor Engineering and Safety Section, Heverlee 3001, Belgium Beijing University of Chemical Technology, Beijing 100029, China Department of Chemical Engineering, Feng Chia University, Taichung 407, Taiwan

a r t i c l e

i n f o

Article history: Received 1 May 2015 Received in revised form 2 August 2015 Accepted 3 August 2015 Available online 18 September 2015 Keywords: Fluidized bed Solids attrition Orifices Particle characteristics Correlation

a b s t r a c t New developments of fluidized beds are focusing on their use in powder circulation systems for thermal energy capture, storage and re-use. Although fast particle motion and associated high degree of mixing favor the high rate of heat transfer in fluidized beds, they however cause inter-particle collision and bed-to-wall impacts, both leading to particle attrition. Experimental work on attrition was carried out in batch gas fluidized beds of 6.62 cm and 10 cm I.D. The rate of attrition was determined both for bubbling fluidized bed conditions, and when adding jet-orifices above the distributor. The attrition was determined from the variation in composition of bed material and collected carryover. A literature survey determined the dominant fluidization parameters, including operating superficial gas velocity, orifice velocity and size, bed weight and diameter, and particle characteristics. Analysis of the experimental findings resulted in a correlation that encompasses all relevant operating characteristics. As a result, the attrition rate can be correlated as the sum of the bubble-induced and jet-induced 2 2 contribution: Rt ¼ K1 ½γðU−Um f Þ  W D  þ K2 ½nor  dor  Uor  with K1 and K2, the intrinsic attrition rate constants, being mainly a function of particle characteristics. K1 is ~10−5 for soft, ~10−6 for hard and ~10−7 for very hard particles, respectively. K2 is ~10−5 for silica sand. In general, attrition is negligible at low superficial gas velocities, but significantly increases through the jet-induced contribution if the orifice velocity exceeds ~30 m/s. © 2015 Elsevier B.V. All rights reserved.

Nomenclature A AI D dp dor dsv H Hmf HM K1 K2 Kf nor QB Rt t Uc Us Uj U Umf Uor

cross-sectional area of the bed abrasive index number (CEMA) diameter of the bed particle size the inside diameter of a single orifice surface-to-volume diameter height of the bed height of the bed at minimum fluidization Moh's hardness intrinsic constant for bubble-induced attrition intrinsic constant for jet-induced attrition fracture toughness the number of orifice (jets) visible bubble flow rate attrition rate at time t time velocity at cyclone inlet superficial velocity of fluidizing gas with distributor only superficial velocity of fluidizing gas with single nozzle system only the total superficial velocity of gas (=Us + Uj) minimum fluidization velocity velocity of gas at exit of each orifice

⁎ Corresponding author. E-mail address: [email protected] (H. Zhang).

http://dx.doi.org/10.1016/j.powtec.2015.08.052 0032-5910/© 2015 Elsevier B.V. All rights reserved.

m2 – m μm mm μm m m – – kg s/m4 N/m3/2 – m3/s kg/s s m/s m/s m/s m/s m/s m/s

W γ ρp, ρB σ

total bed weight bubble through flow factor absolute particle density and bulk density of the bed, respectively fracture energy (force/length)

kg – kg/m3 N/m

1. Introduction New developments of fluidized beds are focusing on their use in powder circulation systems for thermal energy capture, storage and re-use [1–4]. In these systems, mostly Geldart A and B type powders are used at low to moderate velocities, corresponding to a U/Umf-ratio below 3 to 5. Powders envisaged include sand, silicon carbide, limestone or alumina. Since such systems are supposed to operate on a continuous basis, without particle degradation or loss, the investigation of attrition is very important. A typical illustration of such a powder loop is provided in Fig. 1. The depicted powder circulation loop involves different gas–solid contacting modes. The upflow bubbling fluidized bed in the solar energy receiver is combined with moving beds in the hot and cold storage hoppers, a moving bed evaporator and superheater, a bubbling fluidized bed economizer, and different mechanical powder conveyors (screw conveyors, elevators) to connect the different sub-systems and to overcome the height difference.

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H. Zhang et al. / Powder Technology 287 (2016) 1–11

Fig. 1. Layout of a concentrated solar power plant (CSP) plant with powder circulation loop.

Although fast particle motion and associated high degree of mixing favor the use of fluidized beds, they however cause inter-particle collision and bed-to-wall impacts, both leading to particle attrition. Attrition generates fines that can be lost in the dust collection system, whereas the particle size distribution of the bed will alter during the operation. If fines are not recycled to the bed, bed material will gradually get coarser; if recycled, the bed particle size distribution may get too fine. Evaluating attrition is hence important in fluid-bed systems for a controlled process operation, i.e. (i) to limit the generation of fines; (ii) to minimize down-stream deposits or erosion, (iii) to possibly apply a controlled attrition to maintain material reactivity, as in sludge combustion; (iv) to respect environmental regulations (i.e. minimize fine particle collection cost) and (v) to reduce the loss of bed material. Attrition of fluid-bed solids can be caused by several mechanisms, including thermal stress, chemical stress, static mechanical stress and kinetic stress, as illustrated in Table 1 and Fig. 2. Erosion is linked to the effect of moving particles upon the construction materials of the equipment, and is not considered in the present paper. The paper deals with zones A and B only. The present work focuses on both the kinetic and the static mechanical stress, which combine the effects of (i) slow-moving particles subject to surface abrasion upon collision, (ii) fast-moving particles subject to surface abrasion upon collision, and/or (iii) fast-moving particles fracturing completely near high-velocity jets.

The findings hence differ entirely from results of Xiao et al. [6] and Chen et al. [7] (high velocity jet apparatus), Chen et al. [8] (circulating fluidized bed attrition), or Scala and Salatino [9] (attrition during calcination and sulfation). The underlying principles of attrition in these experiments are completely different from bubble-induced or nozzleinduced attrition and can therefore not be compared with our findings. Particle attrition in fluid-bed systems was first studied to characterize catalysts for fluid-cracking systems, and to rank different candidate catalysts towards their attrition behavior [10–12]. Vaux and Keairns [5] tested several sources of attrition, i.e. particle heating, calcination, sulfatation, low and high velocity impacts, and their importance to particle size reduction through wear, fracture, decrepitation, abrasion, splitting, shattering, chipping and disintegration. Their extensive work concluded that high and low velocity impacts needed to be distinguished. Low-velocity impact (bubbling bed attrition) has been shown to be less important than other sources. The rate of fines formation (kg/kg s) appears to be proportional to

Table 1 Sources of attrition (Vaux and Keairns [5]). Sources of powder attrition Mechanical stress Kinetic stress

Thermal stress Chemical stress

Screw feeder; rotary valves, etc. Impact of particles due to high velocity jets (orifices); in conveying lines; due to bubbling action; collision with tubes, baffles, wall; in cyclones, etc. Thermal shock of cold particles fed in a hot bed Evolving gases, water vapor, …

Fig. 2. Fluidized bed zones where attrition/erosion can occur. A: at the orifice plate. B: in the fluidized bed. C: in cyclones. D: at the entrance of rotating locks. E: in bends. F: at the walls of transport pipes. G: in any equipment where particles fall down.

H. Zhang et al. / Powder Technology 287 (2016) 1–11

(U − Umf)n with n ≥ 1, bed height and the solids' density. It is reduced by increased solid fracture energy. High velocity impacts (in jets, cyclones and transfer lines) are recognized as very important and the gas velocity at the distributor level seems the most important factor towards attrition. Vaux and Keairns [5] proposed that the rate of attrition is proportional to U2or, target or plate orientation, particle shape, particle size, target hardness and resiliency, gas properties and jet geometry. Some early qualitative work by Vaux and Fellers [13] studied the processes of three different calcium carbonates at high temperature through the change of particle specific surface area, particle size distribution and amounts of fine particles caused by attrition. Ray et al. [14] investigated calcium carbonate at room temperature and found that the attrition rate is not a function of particle size and weight of the bed, but proportional to the excess air velocity (U − Umf). Vaux and Schruben [15] and Lin et al. [16] found that the rate of attrition is a function of the excess gas velocity. According to Geldart and Baeyens [17], velocities at the distributor orifices in excess of 90 m/s are not to be recommended. Black and Petrie [18] also investigated attrition and demonstrated that attrition remains low for exit orifice velocities up to 50 m/s, whereas excessive breakage of the particles occurs at 100 m/s. Jeffrey et al. [19] studied the attrition of calcium carbonate absorbent in a circulating fluidized bed. They used the principle of activation energy to explain the potential of attrition and developed a relation of operating gas velocity and activation energy to predict the attrition rate. Stein et al. [20] reviewed previous work and concluded that the attrition of glass beads in a vertical jet increases with the weight of the bed material. For a single jet, the attrition rate increases exponentially with the product of Uor and dor. For multi-jet distributors, where dp / dor b 1, the attrition rate increases linearly with the excess velocity, with the smaller particles having higher attrition rates. When dp / dor N 2, attrition rates increase nonlinearly with the excess gas velocity, causing the larger particles to have a longer retention time in the jet area and a higher frequency of impacting. Blinichev et al. [21] suggested two attrition modes: either by particle shattering and elutriation of the fines produced whenever the freeboard velocity exceeds the particle terminal velocity, or by grinding small chips off large particles, again followed by carryover. According to Blinichev et al. [21] and other investigators [22–25], attrition is mostly the result of the second mode. Wu et al. [25] illustrated the mechanical and kinetic stress by a single jet effect, and developed empirical equations to predict the attrition rates. Werther and Reppenhagen [27] provided an extensive report on particle attrition. Through separate tests, 3 major contributions were

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individually characterized being bubble-induced, submerged jetinduced, and cyclone attrition. Literature presents a vast number of equations, as summarized in Table 2 often with empirical and powder-specific correlation coefficients. A distinction should be made between soft particles (e.g. coal, ash, and lime), hard particles (limestone, catalyst) and very hard particles (sand, SiC). Also the effect of chemical reactions in the bed, e.g. calcination or sulfatation, might create thermal and chemical stress within the particles, with enhanced attrition as a result. These thermochemical effects are not studied in the present paper. The present research aims to clarify the influence of particle size and nature, bed weight and diameter, fluidization gas velocity, action of orifices (jets) and orifice diameter. Experimental results will be compared with previous investigations, and will lead to the development of a new correlation to predict the rate of attrition in terms of the relevant parameters. 2. Experiments Experimental work on attrition was carried out in a batch gas fluidized bed of 6.62 cm inner diameter and 2.5 m height. The effect of bed diameter was investigated by also using a 10 cm I.D. set-up. The high velocity jets are installed 5 cm above the perforated plate. The operating velocity was kept between Umf and 5 Umf, the exit orifice velocity ranged from 12 m/s to 130 m/s with inner individual nozzle diameters of 3, 4 and 5 mm. The bed height at minimum fluidization was 0.5 m in all experiments. The materials used are silica sand, silicon carbide, fluid cracking catalyst (FCC), and limestone. Powder characteristics are given in Table 3. Particle sizes were determined by Malvern laser-diffraction. The experimental layout is given in Fig. 3-a and 3-b. A cyclone and bag filter collect all fines carried out. After each run, carried out for a given time period (per ~ 15 min between 0 and ~ 150 min), an air jet is used to clear the connecting pipe of all fines. The schematic diagram of the orifice system is shown in Fig. 3-b. A given weight of bed-material was charged into the bed. The bed was then fluidized at specific operating conditions (dor, Uor, U) and carryover samples were collected at regular intervals. After each run for a specific time, the experiment was stopped and both residual bed material and carryover particles were weighed and subject to a particle size analysis. The rate of attrition is defined from the respective weights and size distributions, as described below. The influences of particle size, bed diameter, given bed weight, fluidization velocities, nature of bed material, action of orifices (jets) and orifice diameter were investigated. In spite of the small orifice diameter used and the high orifice velocities, spouting was not observed. This

Table 2 Previous investigations on attrition rates in fluidized beds. Investigators

Experimental conditions

Major parameters in the attrition rate equation

Merrick and Highley [25] Vaux and Schruben [15] Lin et al. [16] Kono [28] Arena et al. [29] Vaux and Schruben [30] Jeffrey et al. [19] Scala et al. [31] Wu et al. [26] Werther and Reppenhagen [27]

91 × 91 cm2 (square bed); U = 0.61–2.44 m/s; ash and limestone Bed diameter 7.0 cm; U = 0.48 m/s; limestone 61 × 61 cm2 (square); U = 0.1–0.3 m/s; char and silica sand mixture Bed diameter 10.5–179.0 cm; U/Umf = 1.5–5; silica–alumina particles of 0.97–4.00 mm Bed diameter 14 cm; U = 0.78–1.6 m/s; carbon and sand; T = 850 °C 7.0 cm ID × 91.44 cm high; U = 0.46 m/s limestone, 350–495 μm Bed diameter 7.6 cm; U = 1.5–3.0 m/s; lime 40 mm ID × 1 m high; U − Umf = 0.4–1.3 m/s; sand and limestone mixture Bed diameter 6.6 cm; U = 0.22–0.72 m/s; Uor = 43–189 m/s (single-jet) silica sand Bed diameter 0.2 m; porous and perforated plate; fresh catalyst ~80 μm; U − Umf = 0.4–0.76 m/s

(U − Umf), W W, ρp, H (U − Umf) (U − Umf), W U3, Hmf/D, W (U − Umf), W/dp H, U − Umf, ρp, σ (U − Umf)2, W2 (U − Umf), W/dp Uor, W, QB/A 1) Bubble induced attrition: (U − Umf), W, particle nature 2) Cyclone attrition: dp, cyclone geometry, U2.5 c , feed rate to cyclone 3) Jet-induced attrition: Uor3, dor2, nor

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Table 3 Properties of bed materials.

Particle density, ρp (kg/m3) Average particle size, dsv (μm) Umf (cm/s) Geldart particle classification

Silica sand

Silica sand

Silica sand

FCC-

FCC

FCC

CaCO3

CaCO3

CaCO3

SiC

2581 195 3.65 B

2581 296 11.4 B

2600 421 25.2 B

1680 40 0.21 A

1680 26 0.14 A

1680 61 0.32 A

2560 255 9.8 B

2560 360 14.4 B

2560 470 19.2 B

3120 59 0.36 A

was expected since non-spouting criteria defined by Epstein and Grace [32] were met. Slugging was observed at higher values of the superficial gas velocity (in excess of ~0.2 m/s), as applied for coarse silica sand and limestone. Attrition of bed material produces extra fines, which are preferentially elutriated if their terminal velocity is smaller than the operating superficial velocity in bed and/or freeboard. Since elutriation however is not instantaneous, some finer material will remain in the bed. A full picture of attrition is therefore obtained from analyzing, after each run, the size distribution and quantities of both residual bed material and collected carryover. This is illustrated in Fig. 4-a and -b for bed and carryover samples respectively. It can be clearly seen that the amount of larger particles in the bed has decreased (area 2) in favor of both an increased quantity of fine material in the bed (area 1), as shown in Fig. 4-a, and in favor of enhanced elutriation (area 3), separately determines (Fig. 4-b).

The evolution of the particle size distribution of the bed material in function of the time is also illustrated for limestone particles in Fig. 4-c. The average particle size (at 50%) clearly shifts towards lower values as time progresses. The rate of attrition can therefore be determined from the variation in composition of bed material and collected carryover, and results in the following expression: 8 9 8 9 8 9 total loss of weight > > > extra weight of fines of > > entrained > > > > > < = > < = > < = of larger particles; size i in the bed; as weight R¼ ¼ þ : compared with initial > of fines of size i > > ðcorresponding > > > > > > > > > : ; : ; : ; to area 2Þ weight ðarea 1Þ ðarea 3Þ

From the figures, the attrition rate after 90 min, corresponds to R90 = 105 g ≈ 52 + 51 g, which is equal to 0.01944 g/s or 1.94 × 10−5 kg/s. 3. Results and discussion 3.1. Excess gas velocity and jet velocity Fig. 5 illustrates the attrition rates for the different particle systems by the sole action of fluidization, i.e. without adding excess air through the single or twin orifices. Operating at low air velocity does not produce significant attrition rates (b10−6 kg/s). At higher velocities, the attrition becomes important, with an increase by a factor of more than 10 for a twofold velocity increase. The effect of particle size within a group of similar particles (e.g. silica sand), is not represented solely by the gas velocity (or the excess gas velocity, when considering the increasing Umf), with coarser particles being less prone to attrition than their finer equivalents. Clearly, the air velocity cannot be considered as proportionality parameter. This will be discussed further in the text, when the visible bubble flow rate is introduced as relevant hydrodynamic parameter. For a given particle, as illustrated in Figs. 5 and 6, a higher superficial gas velocity results in an increased formation of fines, as a result of increasing attrition in the bed. Whereas for SiC (Fig. 6), a velocity ratio of U/Umf b 5 seems to have little effect (Umf = 0.21 cm/s), the attrition rate significantly increases at high velocities. Fig. 7 however illustrates that in addition to the excess gas velocity, the orifice velocity also plays an important role in the bed attrition, with higher orifice velocities considerably increasing the attrition rate. This is further illustrated in Fig. 8. The influence of the orifice diameter is pronounced in this case of high orifice gas velocity. Whereas attrition rates remain fairly low at low Uor-values, irrespective of dor, higher values of Uor in combination with an increasing dor, significantly increase the rate of attrition. The higher jet diameter produces a larger diameter of the jet area and particles are sucked into the orifice-jet impact area at a higher frequency. The attrition rate is hence expected to increase with dor2. 3.2. Multiple-jet velocity

Fig. 3. a: Experimental set-up 1. Fluidized bed; 2. Freeboard; 3. HE Stairmand Cyclone; 4. Fines; 5. Cyclone outlet to bag filter; 6. Windbox with sintered metal plate distributor; 7. Single/multiple orifice jet; 8. Compressed air; 9. Bed sampling; 10. Differential pressure to data logger. [D = 6.62 or 10 cm; total column height = 2.5 m; bed height, H, ~0.5 m]. dor = 3, 4 or 5 mm. Pitch = 2, 3, or 4 cm. b: Layout of orifice systems.

When multiple jets are present, the jet systems will have different regimes. Wu and Whiting [33] described the occurring phenomena as four regions, mostly as function of Uor and dor.

H. Zhang et al. / Powder Technology 287 (2016) 1–11

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Fig. 5. Attrition rate coefficient for the different particles (no jet action) in the 6.62 cm I.D. rig.

In region I, the nozzle velocity is small and the jets behave like noninteracting independent jets. In region II, the jet height increases with increasing nozzle velocity, the jets still do not interact. In this region, jets grow to a certain height, and pause before the bubble detaches, the jet height reaches a maximum value and the jets are less stable. This region is the transition zone from non-interacting jets to interacting jets. In region III, two jets begin to interact and form a large bubble, while the jet height is lower than that of region II. The jet area is separated at the initial height and merged at the end of the jets. In region IV, two jets first come together to form a large jet area and then a large bubble detaches from the jet top-end immediately: it is shown that the behavior of the multiple-jet system is similar to that of the two-jet system. Single jet and multiple-jet show the same trend for the attrition in fluidized beds. In line with previous investigations of Table 2, experimental results of single and twin jets of different orifice diameters were correlated in terms of Uor2 (for a single orifice, and representative of the kinetic energy) and 2Uor2 (for twin jets). The illustrations of Figs. 9 and 10 demonstrate that the combined number of jets and orifice velocity do not fully correlate results for a single particle species. Different jet diameters

Fig. 4. a: Bed particle size distribution after 90 min of fluidization (D = 6.62 cm, W = 1500 g, dsv = 195 μm, U = 37.5 cm/s, and dor = 5 mm) ■ initial size distribution size distribution after 90 min. b: The elutriated particle size distribution after 90 min of fluidization (D = 6.62 cm, W = 1500 g, dsv = 195 μm, U = 37.5 cm/s and dor = 5 mm). c: Illustration of the progressing particle size distribution of the bed material for 470 μm limestone particles (D = 10 cm; dsv = 470 μm; Umf = 19.2 cm/s; U − Umf = 8,6 cm/s; no orifices). Fig. 6. Attrition rate coefficient of SiC as mass of collected fines at different superficial bed velocities, without jet action, in the 6.62 cm I.D. rig.

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H. Zhang et al. / Powder Technology 287 (2016) 1–11

Fig. 7. Attrition rate coefficient for silica sand (296 μm) at a superficial gas velocity of 40 cm/s, without and with jet (80 m/s).

Fig. 9. Attrition rate coefficient versus Uor2, single jet (dp = 421 μm, D = 6.62 cm, U − Umf = 0.85 m/s, Umf = 25.2 cm/s, W = 1.5 kg, and t = 90 min).

have different slopes, especially after including the factor of 2 for twin jets. Clearly the larger diameter has a higher impact area and jet momentum. According to literature data by Werther and Reppenhagen [27] and Wu et al. [26], the dependency of attrition rate and orifice characteristics cannot be simply represented in terms of the total kinetic energy, since the orifice diameter has a considerable effect, especially when multiple jets operate at high velocities, as suggested by Wu and Whiting [33]. A better correlation to account for the effect of orifice diameter, orifice velocity, and number of orifices will be discussed in the sections below.

3.4. Effect of jet pitch

3.3. Effect of dor in the multiple-orifice experiments Werther and Xi [34], Sishtla et al. [35], Ghadiri et al. [36] and Stein et al. [20], have studied the dependency of the attrition rate on jet gas velocities and orifice diameter. Using the concept of Werther and Xi [34], the gas emerging from an orifice dissipates kinetic energy into the bed. In a different approach, Ray et al. [14] argue that the energy supplied to the bed equals the product of pressure drop and gas velocity, leading to a dependency of the attrition rate on the excess gas velocity. In this investigation and in accordance with Werther and Reppenhagen [27], we separated the contributions of the jet kinetic energy and the excess gas velocity.

Fig. 8. Attrition rate coefficient versus single nozzle velocities (W = 1500 g, dp = 421 μm, U = 37.5 cm/s and t = 90 min).

For a single jet system, the attrition rate is a function of the jet velocity and jet diameter. For two or more jets, the jet areas interact [33]. In this work, the attrition rate is changing with the jet pitch in a twin jet arrangement as illustrated in Fig. 11. For the twin jet arrangement, the higher attrition rate occurs at a pitch of 3 cm. According to the empirical approach of Wu and Whiting [33], the jets at each orifice will act individually at the investigated pitches and orifice diameters. The jets will however collapse into bubbles at about 0.1 m above the orifice. Within the orifice area, the jet areas are independent of each other. For a large pitch and considering the small diameter of the experimental rig used, the wall affects the jet areas, and the attrition rate decreases.

3.5. Comparison of single and multiple jets The number of jets induces a different behavior of jet interaction. For a single jet, no interaction is possible and the jet remains isolated. The attrition rate is dependent on the frequency of the particle entering into the jet area. Larger particles, with a higher inertia to movement,

Fig. 10. Attrition rate coefficient versus 2 Uor2, two jets (dp = 421 μm, D = 6.62 cm, U − Umf = 0.85 m/s, Umf = 25.2 cm/s, W = 1.5 kg, and t = 90 min).

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The data treatment in this work incorporates all experimental findings. Since the number of jets, their kinetic energy (~Uor2) and the excess gas velocity (U − Umf) are appropriate fitting parameters, we advocate a new equation to correlate the attrition rate in fluidized beds. The excess gas velocity should however be related to the visible bubble flow rate (Q B/A), representing the effective bubble action in the bed for different particle sizes. The two-phase theory of fluidization indeed postulated that the visible bubble flow rate was equal to the excess superficial gas velocity (Q B/A = U − Umf). Subsequent research, summarized by Baeyens and Wu [37], demonstrated that this equality is only valid for Geldart-A powders. For coarser powders, only a fraction of the excess gas flow rate is measured as visible bubble flow rate. This fractional relationship is represented as Q B/A = γ (U − Umf), with γ progressively decreasing from ~0.9 to 0.25 as the particle size increases from 100 to ~ 450 μm (Geldart-B powders), while assuming a nearly constant value of ~0.2 for Geldart-D powders (N450 μm). A full dependency of γ with particle characteristics is expressed by Baeyens and Wu [37] in terms of the powder/gas Archimedes number: Fig. 11. Attrition rate versus length of jet-pitch (D = 6.62 cm, W = 1500 g, dp = 421 μm, U = 37.5 cm/s, dor = 4 mm, Uor = 90 m/s and t = 90 min).

have less opportunity to get into the jet area, while smaller particles have a higher probability to be sucked into the impact zone; the attrition rate is expected to increase with decreasing particle size for a given excess gas velocity, as will be confirmed when comparing fine and coarse silica sand results as illustrated in Fig. 5. This was also confirmed for FCCresults operated at 3.8 cm/s with a single 3 mm I.D. orifice. The attrition rate increased from 0.29 × 10− 6 kg/s (61 μm), to 0.53 × 10−6 kg/s (40 μm), and 0.88 × 10−6 kg/s (26 μm). 3.6. Effect of bed diameter, bed height and the weight of bed materials Almost all of the previous investigations, reported in Table 2, have indicated that the attrition rate depends upon the bed geometry, with bed weight (W), generally accepted as fitting parameter. Kone [28] and Vaux and Schruben [30] moreover introduce Hmf/D, or H and ρp, respectively. The effect of the bed diameter as such, was not previously investigated, since a single bed diameter was used. The experiments of the present work considered 6.62 and 10 cm I.D. beds, for a constant bed weight. The results of the attrition rate for the 421 and 296 μm sands are shown in Table 4, illustrating that the attrition rate decreases with the bed diameter for both tested particle sizes. In view of the general acceptance of a proportionality of the attrition rate with bed weight, and the experimentally noticed effect of D, the factor W/D was introduced as a fitting factor in the present work.

 3 Ar ¼ dsv ρp −ρg ρg g = μ 2

ð1Þ

γ ¼ 2:27 Ar−0:21 for ArN100:

ð2Þ

Since the visible bubble flow rate (and its associated particle mixing) is suspected to be the driving force behind bubble-induced attrition, the visible bubble flow rate was introduced to measure the attrition rate Rt: Rt ∝

QB ¼ γðU−Um f Þ: A

ð3Þ

When high-velocity orifices are included, in line with the results of Werther and Reppenhagen [27], the experimental results support a proportionality with nor, dor2 and Uor2. The attrition rate can therefore be defined as a combination of bubble-induced and jet-induced effects: 0 0 11 0 1 weight       visible B orifice B to bed CC orifice orifice B B CC @ A attrition rate≈f @ ; ; þ ;@ bubble diameter AA number velocity diameter flow rate ratio

ð4Þ or   h i W 2 Rt ¼ K1 γðU−Um f Þ  þ K2 nor  dor  Uor 2 : D

ð5Þ

The attrition rate is linearly proportional to the number of jets provided no interaction occurs [33]. The effect of the particle nature is included in the intrinsic attrition constants, K1 and K2. Fitting all experimental results in terms of Eq. (5) allows assessing coefficients K1 and K2.

4. Data treatment and discussion 4.1. Overall correlation of the attrition rate The experimental results are in line with previous findings (Table 2) and stress the importance of particle nature, operating superficial gas velocity, jet velocity, orifice diameter, number of orifices, and bed weight. Table 4 Effect of bed diameter. Bed diameter (cm)

W (kg)

dsv (μm)

dor (mm)

Us (cm/s)

Uor (m/s)

Experimental Rt × 106 (kg/s)

10 6.62 10 6.62

1.5 1.5 1.5 1.5

421 421 296 296

4 4 4 4

37.5 37.5 23.7 23.7

116.7 116.7 116.7 116.7

5.62 6.91 6.88 8.04

4.1.1. Bubble-induced attrition Fig. 12 illustrates the results when no jets are used. The attrition rate is hence solely the result of bubble-induced particle movement. The operating excess gas velocities, U-Umf, are fairly low. The critical gas velocity to provoke fragmentation, as illustrated in Taylor [38], Ghadiri [39], Geldart and Baeyens [17] and Black and Petrie [18], is certainly not reached: attrition is therefore mostly due to grinding small chips of large particles. From fitting the experimental results presented in Fig. 12, values of K1 can be determined and are summarized in Table 5. Powder attrition appears to be a function of the nature of the particles and of the particle size, with larger particles leading to a reduced attrition at similar values of (U − Umf).

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H. Zhang et al. / Powder Technology 287 (2016) 1–11 Table 6 CEMA abrasive index of different bed materials.

Fig. 12. Attrition rate for different powders without jets.

The results clearly demonstrated that the attrition rate constant is inversely proportional with particle size, thus in line with previous correlation of Arena et al. [29] and Scala et al. [31], but contradicting the proportionality with dsv as proposed by Ghadiri [39] or Werther and Reppenhagen [27]. According to the research of Ghadiri [40], specific particle properties of hardness (HM) and fracture toughness (Kf) affect the attrition rate, with K1 being proportional to HM/K2f . Whereas the particle hardness is known in terms of e.g. the Moh's classification, fracture toughness is largely unknown [40]. To assess the differences between silica sand, FCC, limestone and SiC, the authors used powder-specific properties, as presented by the Conveying Equipment Manufacturers Association (CEMA), specifically towards the abrasive index (AI). Although the abrasive index (AI) is an index of abrasion of equipment components by powders, the AI encompasses major relevant particle characteristics, and can tentatively express the particle nature of attrition in this investigation too. The CEMA abrasion index [41] is composed of 4 contributions being the particle size, the particle bulk density, its shape and its Moh's hardness. The particle size is indicated by a letter and a number, where the number indicates the maximum sieve size of the powder: A200 meaning that all particles pass a 200 mesh sieve i.e. its maximum sieve size b 74 μm. Whereas A powders are given an impact factor of 1.0, B and C powders are given a factor of 1.15. The particle bulk density varies from 10 to 4900 kg/m3 and is given an increasing impact factor of 1.0 to 1.6. The Moh hardness varies from 1 to 10, and is characterized by an impact factor of 1 to 100. The particle shape is classified as rounded, Table 5 Values of bubble-induced attrition coefficient, K1. Powder

K1(−)

Silica sand, dp = 195 μm Silica sand, dp = 296 μm Silica sand, dp = 421 μm SiC, dp = 59 μm FCC, dp = 40 μm Limestone, dp = 255 μm Limestone, dp = 360 μm Limestone, dp = 470 μm

1.83 × 10−6 ± 0.27 × 10−6 1.49 × 10−6 ± 0.23 × 10−6 6.00 × 10−7 ± 0.98 × 10−6 1.54 × 10−7 ± 0.21 × 10−6 3.09 × 10−7 ± 0.31 × 10−6 8.93 × 10−5 ± 0.78 × 10−5 4.58 × 10−5 ± 0.38 × 10−5 3.18 × 10−5 ± 0.31 × 10−5

Physical characteristics

Value

CEMA impact factor

FCC (Moh) hardness, HM Particle shape Bulk material density Size Abrasive index number, AI 103

7.9 Sub-angular 752.9 kg/m3 A230 ~ A400

68.6 1.5 1.0 1.0

CaCO3 (Moh) hardness, HM Particle shape Bulk material density Size Abrasive index number, AI 22.6

3.7 Sub-angular 1329.5 kg/m3 A50 ~ A100

13.7 1.5 1.1 1.0

Silica sand (Moh) hardness, HM Particle shape Bulk material density Size Abrasive index number, AI 68.3

6.0 Sub-round 1489.7 kg/m3 B6 ~ C1/2

36 1.5 1.1 1.15

Silicon carbide (Moh) hardness, HM Particle shape Bulk material density Size Abrasive index number, AI 266

9 Sharp φ = 0.5 3120 kg/m3 A230 ~ A400

95.0 2.0 1.4 1.0

sub-angular and angular, and given an impact factor of 1.0, 1.5 and 2.0 respectively. The abrasion index, AI, is then calculated as the product of all impact factors. These impact factors and abrasive indexes of the different materials are shown in Table 6. With its lower hardness and lower abrasion index, limestone should produce the highest attrition rates. SiC largely exceeds both silica sand and FCC, and should provide the lowest attrition rates. These tentative AI-influences are confirmed by the experiments, with K1 increasing from SiC, to silica sand, FCC catalyst and limestone. A complete fitting of K1 in terms of AI could not yet be determined. Further research is required. If extrapolating to different powders for solar receivers, the AI-value is certainly a good tentative indication towards particle attrition, with very soft powders (low AI, such as limestone), more prone to attrition and hence less applicable in the fluidized bed system.

4.1.2. Addition of jet-induced attrition When specific jets are introduced into the bubbling fluidized bed, their effect is reflected in an increasing superficial velocity (U = Us + Uj), and in a separate factor that includes the jet characteristics nor, Uor, dor. Experimental results are presented in Fig. 13-a (single jet) and Fig. 13-b (double jets), where experimental attrition rates are plotted against Eq. (5). To eliminate the effect of bubble-induced attrition, these experimental data should be re-fitted in terms of jet-induced attrition only, as illustrated in the corresponding Fig. 14-a and -b for silica sand. K1 is taken as the experimentally determined value, whereas U is calculated as the sum of Us + Uj. In doing so, the proportionality constant K2 can be defined. As a result, the following values of K2 are obtained and presented in Table 7. It should be noticed that the contribution of jet-induced attrition is negligible (Rt b 0.1 kg/s) for Uor ≤ 30 m/s. This value corresponds with the critical velocity of fragmentation proposed by Taylor [34] and Ghadiri [35].

H. Zhang et al. / Powder Technology 287 (2016) 1–11

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Fig. 14. a: Rt-K1[γ (U-Umf)W/D] vs [nordor2Uor2], single jet. b: Rt-K1[γ (U − Umf)W/D] vs [nordor2Uor2], twin jets.

4.2. Comparison of experimental data with previous Literature

Fig. 13. a: Attrition rate for different powders with 1 jet. b: Attrition rate for different powders with 2 jets.

Only at Uor ≥ 30 m/s does particle fragmentation take over as the attrition mechanism from chipping.

4.1.3. Overall data fitting Knowing the values of K1 and K2, all experimental results can be compared with predictions by Eq. (5). Despite the scatter shown in Fig. 15, experimental and predicted attrition rates are in fair agreement, stressing the soundness of the approach developed.

Considering the experimental conditions of previous research, as presented in Table 2, the current research differs significantly, having been performed in a range of bed diameters (6.62 and 10 cm) typically to be used in solar receivers in concentrated solar power plants, and at rather low fluidization velocities (≲4 Umf). Previous research, as listed in Table 2, was performed in either fluidized beds of very large size and using beds of considerable height [25,27,

Table 7 Values of jet-induced attrition coefficient, K2. Powders Silica sand 296 μm Silica sand 421 μm Silica sand 421 μm, 3 mm Silica sand 421 μm, 4 mm

K2 (kg s/m4) 1 jet 1.36 × 10−5 ± 0.16 × 10−5 2.51 × 10−5 ± 0.22 × 10−5 2 jets 3.78 × 10−5 ± 0.47 × 10−5 3.05 × 10−5 ± 0.32 × 10−5

10

H. Zhang et al. / Powder Technology 287 (2016) 1–11

with K1 and K2, the intrinsic attrition rate constants. In general, attrition is negligible at low superficial gas velocities, but significantly increases if the orifice velocity exceeds 30 m/s. Funding sources “CSP2” project “Concentrated Solar Power in Particles”, EU Project No. 282932. Conflict of interest The authors declare no competing financial interest. Acknowledgment Authors acknowledge the European Commission for co-funding the “CSP2” project “Concentrated Solar Power in Particles” (EU Project No. 282932). The first author, Huili Zhang, would like to express sincere gratitude to China Scholarship Council for sponsoring her Ph.D. study at KU Leuven in Belgium (File No. 201206880024). References

Fig. 15. The predicted attrition rate versus the experimental attrition rate.

28], and/or generally at very high superficial gas velocities. Extrapolation of the current results, as expressed in Eq. (5), should hence be considered with great caution. In general, the equations by the different authors of Table 2 do not include all parameters that the present research found relevant, with only Werther and Reppenhagen [28] and Wu et al. [26] making an exception. The major differences in predictions between Eq. (5) and the literature correlations, some under- or overestimating by an order of magnitude, are due to this incomplete parameter consideration, with the use of visible bubble flow rate instead of the excess gas velocity being of major impact for Geldart-B or Geldart-D powders, and not accounted for in previous research, except by Wu et al. [26]. This is probably the main reason why predictions of Eq. (5) and results of Werther and Reppenhagen [28] (using Geldart-A powders, γ ~ 1), and Wu et al. [26] are in fair agreement (± 20%) with the predicted values. 5. Conclusions Fast particle motion and associated high degree of mixing favor the use of fluidized beds. They however cause inter-particle collision and bed-to-wall impacts, both leading to particle attrition. The rate of attrition was determined for bubbling fluidized bed conditions, and when adding jet-orifices above the distributor. Analysis of the experimental results resulted in a correlation to encompass all relevant operating characteristics:   h i W 2 þ K2 nor  dor  Uor 2 Rt ¼ K1 γðU−Um f Þ  D

ð5Þ

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