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Sand attrition in conical spouted beds
Particuology 10 (2012) 592–599
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Sand attrition in conical spouted beds Aranzazu R. Fernández-Akarregui a , Jon Makibar a , Isabel Alava a , Luis Diaz a , Fernando Cueva a , Roberto Aguado b , Gartzen Lopez b , Martin Olazar b,∗ a b
Ikerlan-IK4, Juan de 4 la Cierva 1, Arabako Parke Teknologikoa, E-01510 Minao, Araba, Spain Department of Chemical Engineering, University of the Basque Country, P.O. Box 644, E48080 Bilbao, Spain
a r t i c l e
i n f o
Article history: Received 19 October 2011 Received in revised form 4 January 2012 Accepted 20 February 2012 Keywords: Attrition Conical spouted bed Draft tube Sand attrition
a b s t r a c t A study was carried out on the attrition in conical spouted beds using two sands with different properties for several bed heights and gas flow rates. Furthermore, the influence of a draft tube was studied at ambient and high temperatures. The main objective was to acquire knowledge on the attrition of sand beds for biomass pyrolysis in a pilot plant provided with a conical spouted bed reactor. A first-order kinetic equation is proposed for sand attrition in a conical spouted bed at room temperature. The predicted attrition rate constant depends exponentially on excess air velocity over that for minimum spouting. Both the draft tube and temperature increase contribute to reduction of attrition. © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Spouted beds have been proposed as an alternative to fluidized beds for processes that call for efficient fluid–solid contact (Epstein & Grace, 1997), especially for particles larger than 1 mm (Freitas & Freire, 2001; Zhao, Li, Liu, Song, & Yao, 2008). An advantage of spouted beds over fluidized beds is the well-defined cyclic movement of the particles, which facilitates contact between gas and solid particles (Luo, Lim, Freitas, & Grace, 2004). In fact, both high solid circulation rate and efficient gas–solid contact favor the use of spouted beds in many chemical processes (Azizi, Hosseini, Moraveji, & Ahmadi, 2010; Vieira Neto, Duarte, Murata, & Barrozo, 2008). In spouted beds, the fluid is introduced through a central nozzle rather than uniformly through a distributor plate as in fluidized beds. The fluid enters the bed in the form of a jet and causes the particles to circulate in a uniform way creating a central spout zone. The fluid and particles are in counter-current flow in the annulus, which makes up the major portion of the spouted bed. Particle flow is co-current with the fluid in the spout, where velocities are high and residence time is short. Nowadays, spouted beds are widely used in physical processes, such as drying (Altzibar et al., 2008; Berghel, Nilsson, & Renstrom, 2008; Markowski, Sobieski, Konopka, Tanska, & Bialobrzewski, 2007), coating and granulation (da Rosa & dos Santos Rocha, 2010;
∗ Corresponding author. Tel.: +34 946392527; fax: +34 946393500. E-mail address: [email protected] (M. Olazar).
Oliveira, Peixoto, & Freitas, 2005) and in chemical processes involving combustion (Konduri, Altwicker, & Morgan, 1999), gasification (Belyaev, 2008; Spiegl, Sivena, Lorente, Paterson, & Milian, 2010), catalytic polymerization (Olazar, San Jose, Zabala, & Bilbao, 1994) and pyrolysis of different solid wastes (Aguado, Olazar, Barona, & Bilbao, 2000; Amutio et al., 2011; Elordi, Olazar, Lopez, Artetxe, & Bilbao, 2011; Lopez, Olazar, Aguado, & Bilbao, 2010). An overview of spouted bed reactors was carried out by Olazar, Alvarez, Aguado, and San Jose (2003). One of the challenges for scaling up the spouting regime is the understanding of the process controlling particle size reduction and the determination of attrition kinetics, which will help to design a process with high efficiency and reduced fine particle emission. Moreover, the fines formed must be removed from the out-going gas stream, thereby increasing the processing cost (Stein, Seville, & Parker, 1998). Although numerous papers address the attrition in fluidized beds, few provide information on spouted beds. Mathur and Epstein (1974) devoted a brief chapter in their book to this subject and, more recently, attrition in spouted beds was considered for the processing of carbonaceous materials (Buczek, 1983; Wongvicha & Bhattacharya, 1994), calcite (Flamant, Chraibi, Vallbona, & Bertrand, 1990) and selected polymers (Al-Senawi, Hadi, Briens, & Chabagno, 2008). To our knowledge, no paper has been published dealing with the attrition of spouted beds fitted with a draft tube. The attrition process is commonly divided into two different mechanisms causing particle breakage, namely, the abrasion of the particle surface and the disintegration or fragmentation of the particle matrix (Boerefijn, Ghadiri, & Salatino, 2007, chap. 25; Lin &
1674-2001/$ – see front matter © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.partic.2012.02.002
A.R. Fernández-Akarregui et al. / Particuology 10 (2012) 592–599
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Nomenclature DC Di DT D0 ds Ea HC Hf HT Ka Kp K0 k0 k1 LH m P Mw Rt t T umf u0 (ums )0 W W0 We Wmin
diameter of the cylindrical section of the contactor (m) diameter of the bed bottom (m) diameter of the draft tube (m) diameter of the bed inlet (m) Sauter diameter (m) attrition activation energy (m2 /s2 ) height of the conical section of the contactor (m) height of the fountain (m) total height of the contactor (m) attrition rate constant in Lee’s equation (s−1 ) dimensional constant in Gwyn’s equation (s−m ) frequency factor in an Arrhenius form (s−1 ) kinetic constant in Eq. (4) (s−1 ) kinetic constant in Eq. (4) (m2 /s2 ) height of the draft tube entrainment zone (m) exponent for time dependence on attrition in Gwyn’s equation, dimensionless pressure (kg/(m s2 )) molecular weight of air (kg/mol) attrition rate (kg/s) time (s) temperature (K) minimum fluidization velocity (m/s) inlet air velocity referred to the inlet diameter (m/s) minimum spouting velocity referred to the inlet diameter (m/s) weight of bed material (kg) initial mass of particles in the bed (kg) mass of particles in the bed at t time (kg) minimum weight of parent solids in the bed (kg)
Greek letters cone angle (rad) pressure drop (kg/(m s2 )) P
Wey, 2005; Welt, Lee, & Krambeck, 1977). The factors that affect attrition dynamics include: (i) the properties of the bed material, i.e., porosity, size, hardness, density, surface, cracks, shape and particle strength (Lee, Jiang, Keener, & Khang, 1993) and (ii) the reactor environment, i.e., exposure time, particle size and velocity (Lin & Wey, 2003, 2005; Shih, Chu, & Hwang, 2003) pressure system and type of enclosure (hard or soft surface), shear, temperature (Lin & Wey, 2003, 2005; Arena, D’Amore, & Massimilla, 1983) and viscosity and turbulence (Bemrose & Bridgwater, 1987). In fluidized beds, the jetting region is the most significant contributor to particle breakage (Boerefijn, Gudde, & Ghadiri, 2000; Boerefijn et al., 2007) and in the case of spouted beds the same role is probably played by the spout region. Many reports propose empirical correlations for attrition rate in bubbling and circulating fluidized bed systems, and interesting reviews have been reported by Lee et al. (1993) and Lin and Wey (2005), but no correlation for spouted beds has been found in the literature. This study addresses the kinetics of sand attrition in a conical spouted bed contactor under different hydrodynamic conditions. The kinetic equation corresponding to the experimental conditions studied has been obtained based on analogies with certain kinetic equations proposed for fluidized bed systems. The closest correlations are those by Gwyn (1969) and Lee et al. (1993), both for fluidized beds. Gwyn (1969) studied silica sand attrition
Fig. 1. Schematic of the experimental setup for attrition tests in a conical spouted bed contactor.
rate at room temperature and proposed a correlation of the following type: Rt = Kp mt m−1 W,
(1) = 4.47 × 10−6 –1.35 × 10−5 s−m .
Lee et al. with m = 0.46 and Kp (1993) investigated lime attrition in a circulating fluidized bed absorber at 20–180 ◦ C and found that the predominant mechanism was surface abrasion. According to these authors, lime attrition decreases as temperature is increased, due to changes in the material, whereas limestone attrition increases with temperature due to the decrepitation resulting from increased internal pressure. The total weight of particles in the bed at time t, We , follows the equation: We (t) = (W0 − Wmin )e−Ka t + Wmin ,
(2)
where W0 (kg) is the weight of the bed at t = 0, Wmin is the minimum weight for which the attrition is negligible and Ka is the attrition rate constant. These authors found that the constant Ka depends exponentially on activation energy, temperature, superficial air velocity and minimum fluidization velocity in the attrition process: Ka = Ko e−Ea RTCs /PMW u0 (u0 −umf ) .
(3)
For lime attrition, the authors related a frequency factor in an Arrhenius form, with Ko = 1.29 × 10−4 s−1 , and calculated an attrition activation energy, Ea = 3.38 × 10−3 kJ/kg. Other equations (Halder & Basu, 1992; Kono, 1981; Merrick & Highley, 1974; Wu, Baeyens, & Chu, 1999) are not suitable for spouted beds because they take into account phenomena that do not occur in these beds, such as the presence of bubbles, and those like the second-order model (Cook, Khang, Lee, & Keener, 1996) are of no direct use for the design of these types of beds. 2. Experimental system and test procedure The experimental set-up for sand attrition tests is shown in Fig. 1, including an air compressor, a mass flow meter and a controller, a conical spouted bed contactor, a cyclone, a pressure
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Fig. 2. Fractional size distribution of the two silica sands (A-GRS07, Sibelco Minerals), type I and type II.
sensor, a thermocouple and a data monitoring system. The attrition experiments were carried out in two different plants, i.e., a conical spouted bed contactor made of plexiglass and a non-porous draft tube conical spouted bed reactor made of stainless steel. The latter is the main component in a pilot plant unit designed and set up for continuously pyrolysing 25 kg/h of biomass. The experimental runs in this reactor were conducted at both ambient and high temperatures. The dimensions of both contactors are exactly the same (Fig. 1): the diameter of the upper section (DC ) is 242 mm, the angle of the contactor is = 32◦ , the height of the conical section (HC ) is 330 mm and the total height (HT ) is 1030 mm. The base diameter (Di ) is 52 mm and the inlet diameter (Do ) is 15 mm when no internal device is used, and 25 mm when a non-porous draft tube is used, given that the use of this internal device improves bed stability and, consequently, a greater inlet diameter may be used without instability problems (Altzibar et al., 2008, 2009; Cunha, Santos, Ataide, Epstein, & Barrozo, 2009; da Rosa & Freire, 2009; Olazar, Lopez, Altzibar, & Bilbao, 2011; Vieira Neto et al., 2008). The diameter of the draft tube (DT ) is 36 mm and the height of the entrainment zone (LH ) is 80 mm. These dimensions of the draft tube were determined in a previous paper (Makibar et al., 2011). A stainless steel mesh (55 mesh size) placed at the base of the contactor supports the sand. Air (25 ◦ C and 480 ◦ C, 1 atm) was used as the spouting medium. The outlet air from the contactor entered the cyclone where the elutriated fines are collected. The air flow rate was measured by a mass flow meter (Brooks, 5853S). Pressure was measured by a differential pressure transducer (RS 286-686). Pressure drop, temperature and air flow rate were continuously monitored and recorded by a Fluke NetDAQ Logger data acquisition system. Attrition study was carried out using two types of commercial silica sand (called sands I and II) provided by Sibelco Minerals. The density of both sands is 2600 kg/m3 and their loose bed porosity 0.46, as determined following the Brown and Richards (1970) method. The initial particle size distribution (before attrition tests) for both types of sand is shown in Fig. 2 (determined by sieving), with their Sauter average diameter being around 1.0 mm. Experiments were carried out using two amounts of sand, 4.5 and 6 kg, which account for bed heights (Ho ) of 24.5 and 27.8 cm, respectively. The height of the fountain (Hf ) was visually measured, whereas air velocities referred to the inlet diameter (u0 ), temperature and pressure drop (P) were continuously recorded throughout the whole test. In experiments, air flow rate is gradually increased to a value 1.5 times higher than the minimum spouting gas flow rate and is then
Fig. 3. Pressure drop evolution with air velocity (6 kg of silica sand I, Do = 15 mm, dS = 1.05 mm).
decreased to the set spouting velocity. This air rises through the spouting bed, with the sand particles moving upwards in the fountain and falling down again onto the annulus where they continue moving downwards until arriving at the spout, thus repeating the full cycle. The air supply is turned off at regular time intervals and sand is taken out, weighed and put back into the contactor. At the beginning of the experiments, the measuring intervals are shorter because the attrition process is more severe. When the weight reduction of the sand reaches a steady state, the test is concluded. The shape factor of bed particles is measured by optical microscopy at the beginning and end of each run, and the image analysis of final elutriated fines is also obtained by SEM microscopy. Following the methodology described, attrition tests without the draft tube are carried out at room temperature using air velocities 1.16 and 1.35 times higher than the minimum spouting velocity, (u0 )ms , corresponding to the original bed. Thus, air flow rates of 450 L/min and 574 L/min, corresponding to the spouting velocities (per unit of inlet cross-sectional area) of 48–54 m/s are used. Each experiment lasted 420 h. In the experiments carried out with a draft tube, the velocities are 1.35 and 1.65 times higher than those for minimum spouting at 25 and 480 ◦ C, respectively. Thus, the flow rates used (measured at ambient conditions) are 472 L/min for ambient temperature and 210 L/min for 480 ◦ C, and the runs are carried out using type II sand. The reduction in the gas flow rate required for the operation at high temperatures is explained by the increase in gas viscosity and, therefore, by the increase in the momentum transfer from the gas to the solid, which facilitates the spouting phenomenon (Olazar, Lopez, Altzibar, Aguado, & Bilbao, 2009). In experiments with a draft tube, each experiment lasted 248 h. 3. Experimental results The evolution of pressure drop with air velocity from the fixed to the spouted bed was analyzed in order to determine the minimum spouting velocity for the different systems studied. As shown in Fig. 3, at first, as air velocity is increased, pressure drop increases to a peak value, at which, with further increasing air velocity, a fountain is created and pressure drop decreases to a value that remains approximately constant for a wide range of air velocities. A highly pronounced hysteresis is noted upon reducing the air velocity, which is due to the fact that the peak pressure drop is much higher than the operating pressure drop and, furthermore, the velocity required to break up the bed and open the spout is higher than that corresponding to the peak pressure drop.
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Fig. 4. Particle size distribution of sand I at the beginning and end of the tests using air velocities of 1.2 and 1.35 times (u0 )ms .
It is widely accepted that the predominant attrition mechanism during fluidization is surface abrasion due to collisions between parent solids in the bed (Lee et al., 1993). Nevertheless, there are two different attrition mechanisms in a spouted bed: (i) particle impact attrition, which takes place in the fountain region and happens when particles collide against the vessel wall, and (ii) particle–particle surface abrasion, which takes place mainly in the spout region. Particles collide with each other and against the layer of annulus particles constituting the spout wall. The extent of particle-wall impact attrition depends on total impact surface area, number of impact cycles, particle velocity, and the particle and vessel material (Chen, Jim Lim, & Grace, 2007). Particle–particle surface abrasion depends on particle velocity and particle material. All these parameters change with time. This is due to the presence of fewer faults, flaws, or discontinuities affecting particles and to smaller particle size in the bed. Both mechanisms ensure that sand particles undergo attrition, increasing the fractional weight of fines and reducing the fractional weight of bigger particles.
3.1. Effect of gas velocity In order to study the effect of gas velocity on attrition, results are obtained at two operating velocities, both above the minimum, by using sand I. Fig. 4 shows the variation in the fractional size distribution at the beginning and end of the tests carried out at air velocities of 1.2 and 1.35 times higher than the minimum spouting velocity. As observed, a reduction in the particle size of the sand is noted at the end of the tests. In fact, the Sauter diameters measured at the end of the tests for these velocities are 0.90 mm and 0.94 mm, respectively. The higher Sauter diameter for the higher velocity (1.35 times the minimum one) is explained by the higher amount of elutriated fines for the higher gas flow rate. Knowledge on the attrition process of the bed material in conical spouted bed reactors is essential because changes in the fractional weight distribution involve changes in the hydrodynamic behavior of the spouted bed reactors. Thus, a decrease in the average particle diameter throughout the run due to attrition causes minimum spouting velocity to decrease and, consequently, the operating (u/ums )0 ratio to increase when a constant air flow rate is used in the run. Fig. 5 shows the evolution of (u0 )ms and (u/ums )0 with time using an initial inlet air velocity of 1.35 times the minimum one which accounts for a velocity of 54 m/s. This effect has already been observed by Lin and Wey (2005) in fluidized beds.
595
Fig. 5. Evolution of (u0 )ms and (u/ums )0 with time, for sand I and an initial inlet air velocity of 1.35 times (u0 )ms .
Furthermore, the fountain height increases with the increase in the small particle fraction by attrition. In fact, big particles tend to occupy radial positions near the annular–spout interface and so describe shorter trajectories than the smaller ones. The smaller particles circulate mainly along the outer periphery of the annular zone and describe wider cycles. The fountain therefore acts as a highefficiency distributor, which provides an interesting segregation of particles by diameter and/or density. Furthermore, as the fountain height increases the particle-wall impact surface area and velocity also increase. In addition, the number of particles contained in a given weight fraction is much higher when these particles are small and, moreover, their ratio of surface area to volume is higher, which increases the probability of collision. Fig. 6 shows the experimental data of bed mass evolution for two air inlet velocities. As observed, there is an exponential decrease in the weight of solids in the bed throughout the run. Particles initially undergo rapid attrition, which then decreases exponentially, leading to a minimum weight value (Wmin ) of steady state in which attrition could be considered negligible. This seems to be due to the fact that the irregular surface of the particles at the beginning of each experiment abrades easily, but the particles become harder to abrade when they are already rounded (Boerefijn et al., 2000; Lin & Wey, 2003). Wmin depends on air inlet velocity, so that the higher the air inlet velocity, the smaller the Wmin . The values of Wmin for the two sets carried out in this study are 5555 g and 5300 g for (u/ums )0 = 1.2 and 1.35, respectively. The higher air flow rate gave
Fig. 6. Experimental data for bed weight evolution (sand I) at two air velocities.
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Fig. 7. Microphotographs of (a) original sand I and (b) sand after 420 h spouting, (u/ums )0 = 1.2; (c) sand after 420 h spouting, (u/ums )0 = 1.35.
way to more violent collisions between the particles and with the contactor surfaces. In all the experiments carried out in this work, the time to reach the minimum value was around 400 h, and it is proven that the attrition rate increases rapidly by increasing air velocity. The rounding off and surface abrasion of silica sand were observed under a microscope, as illustrated in Fig. 7, showing the rough and irregular surface of the original sand I (Fig. 7(a)) and the abraded surface of the sand after 420 h of spouting with inlet air velocities of 48 and 54 m/s, respectively (Fig. 7(b) and (c)). The sharp edges gradually disappeared, and the surface became increasingly polished with time due to collisions. The sizes of the fines elutriated from the spouted bed contactor and retained in the cyclone range between 2 and 50 m, as illustrated by SEM microscopy in Fig. 8. 3.2. Effect of sand type The effects of sand types, I and II, and gas velocities on attrition are shown in Fig. 9 by observing the evolution of bed weight. The mass loss is observed for sand type I operating with an air velocity 1.2 times the minimum, and for sand type II operating with velocities 1.16–1.26 times the minimum. As observed, the mass loss is much higher with sand II than with sand I, even operating with lower gas velocities. In fact, when operation is carried out using the same (u/ums )0 ratio with both sands, the mass loss with sand II is twice that observed with sand I. The chemical compositions of both sands were analyzed by Xray diffraction, to show the main component in both cases to be quartz (>99.5%), with feldspar as the main impurity. The different behavior between the sands is due to their different textural properties, which were determined by means of a petrographic microscopy: sand II is characterized by its sharp edges and, consequently, when it is subjected to impacts in the spouting operation,
Fig. 8. SEM image of fines elutriated from the spouted bed.
breakage of these sharp edges causes faster and more significant mass loss. Other authors (Lee et al., 1993; Lin & Wey, 2003, 2005) studying attrition in fluidized beds, also observed the important effect of particle properties, such as porosity, size, shape, hardness and so on. 3.3. Attrition in a contactor fitted with a draft tube Operating with conical spouted beds has certain limitations for the treatment of fine materials and a crucial parameter that hinders bed stability in the scaling-up of the process is the ratio between the inlet diameter and particle diameter. Thus, the inlet diameter should be up to 20–30 times the average particle diameter in order to achieve spouting status (Olazar et al., 2011). The use of a draft tube is the usual solution to this problem (Swasdisevi et al., 2005). In fact, this internal device is the key for stable operation in largescale spouted beds and allows increasing the spoutable bed height and reducing bed pressure drop (Altzibar et al., 2009; Luo et al., 2004; Swasdisevi et al., 2004, 2005). Moreover, solid circulation rate, particle cycle time, gas distribution and so on are governed by the specifications of the draft tube (Altzibar et al., 2009; Cunha et al., 2009; Ishikura, Nagashima, & Ide, 2003; Zhao, Yao, & Li, 2006). The use of a draft tube allows increasing the versatility of the conical spouted bed and improving the flexibility of the gas–solid contact. Accordingly, a conical spouted bed provided with a draft tube has been developed for the biomass pyrolysis process. Attrition in the conical spouted beds provided with a draft tube was compared with that observed without it. Runs were carried out with a draft tube by using a higher velocity than the minimum for spouting in order to improve the less vigorous contact of these beds without a tube. Thus, the (u/ums )0 ratio used is 1.35 at 25 ◦ C and 1.65 at 480 ◦ C. In spite of operating with a higher (u/ums )0 ratio, the fountain height observed at 480 ◦ C (about 15 cm) is lower than that observed at 25 ◦ C (around 20 cm). It is noteworthy that this less vigorous contact at high temperatures reduces sand attrition.
Fig. 9. Comparison of sand mass loss for different types of sand and different gas velocities.
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Fig. 10. Effect of gas velocity and temperature on fountain height.
Thus, at the end of the run (248 h), the sand mass loss at 25 ◦ C is 180 g, whereas it is only 115 g at 480 ◦ C, even operating with a higher gas velocity. Certain authors observed the opposite operating with fluidized beds, i.e., more attrition at high temperatures (Bemrose & Bridgwater, 1987; Lin & Wey, 2003, 2005). This different behavior between spouted beds and fluidized beds at high temperatures is explained by the less vigorous gas–solid contact when temperature is increased in the spouted beds. In order to improve knowledge on the spouting regime at high temperatures, a study was conducted on the fountain height and gas–solid contact behavior at different (u/ums )0 ratios and temperatures from 25◦ to 500 ◦ C. Fig. 10 shows the evolution of fountain height as a function of temperature and (u/ums )0 ratio. As observed, the fountain height is reduced when operating at high temperatures and this effect seems to be violated if operating over 400 ◦ C. The reduction in the fountain height is related to a change in the gas flow pattern, i.e., a higher amount of gas crosses the bed through the annulus, which causes a reduction in the gas velocity in the spout and, consequently, in the acceleration of the particles in this region. In addition to the lower fountain height at high temperatures, it is visually observed that solid movement is less vigorous and, consequently, the solid circulation rate is lower. Both findings cause a reduction in the strength of particle collisions and therefore in the attrition rate at high temperatures. Likewise, Shih et al. (2003) related attrition in an internally circulating fluidized bed to the solid circulation rate. The lower attrition rate at high temperatures is related to the change in drag force from low to high temperatures. Increasing temperature leads to an increase in gas viscosity, whereas at the same time its density decreases. Since at low temperatures (i.e., well below 300 ◦ C) increasing gas viscosity is dominant in comparison with gas density, the drag force increases (Sanaei et al., 2010). Before comparing the results obtained at ambient temperature with and without a draft tube, it should be mentioned that the conical spouted bed without a draft tube is made of plexiglass, whereas the one with the draft tube is made of stainless steel. Consequently the collisions between the sand and the contactor wall are more severe in the latter case. Nevertheless, the mass loss with draft tube (and higher (u/ums )0 ratio) is less than a quarter of that observed without a draft tube. This difference is due to the solid circulation pattern when working with a draft tube. Thus, in the operation with a non-porous draft tube, the particles are incorporated into the spout at the bottom of the contactor, whereas in a plain conical spouted bed the incorporation of solids into the spout takes place at all heights throughout the spout. In the latter case, the particles that are incorporated into the spout above the bottom collide with
Fig. 11. Bed weight evolution for sand I and air velocities of (a) (u/ums )0 = 1.2 and (b) (u/ums )0 = 1.35. (Points: experimental data; lines: calculated.)
other particles arising at a high velocity, thus causing severe breakage due to the difference in their velocities. This situation is fully avoided when working with a draft tube because all the particles enter the spout at the bottom and their relative velocity is almost zero. Moreover, in a spouted bed without a draft tube, there are violent collisions between the particles in the spout and the layer of annulus particles constituting the spout wall. These collisions are avoided by operating with a draft tube. These findings reveal additional advantages for operating with a draft tube because the interest in reducing attrition is not only related to the avoidance of bed material loss but also to the cost of removing fine particles downstream of the fluidized or spouted bed (Stein et al., 1998).
4. Kinetics of sand attrition Experimental data were fitted to different first-order kinetic models reported in the literature, specifically those by Gwyn (1969) and Lee et al. (1993). Gwyn’s equation is the most versatile model describing granular attrition and states that the elutriated weight fraction of sand at room temperature is proportional to time under constant strain rate conditions, Eq. (1). To estimate the weight changes of the sand in the spouted bed using Lee’s formula, Eq. (2), the definition of parameter Ka must be modified by assuming that it depends exclusively on the excess air velocity over that corresponding to
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Fig. 13. Comparison of experimental data for bed weight with those calculated using Lee’s model.
Fig. 12. Bed weight evolution for sands I and II for different air velocities and initial bed weights of (a) 6 kg and (b) 4.5 kg. (Points: experimental data; lines: calculated.)
minimum spouting velocity at the beginning of the run (u − ums )0 . The empirical correlation proposed for Ka is: Ka = k0 e−k1 /u(u−ums )0 ,
(4)
with constants k0 and k1 depending on the contactor geometry and various parameters related to the bed (solid density, porosity, sphericity, particle mean diameter and bed height). Scilab algorithms have been developed to fit the experimental data to the above mentioned models. The objective function (OF) is defined as:
m
OF =
i=1
(Wcal − Wexp )2n +
m
n+m
j=1
(Wcal − Wexp )2m
,
(5)
where Wcal and Wexp are the bed weights calculated with the model and the experimental values, respectively, and n and m are the number of measurements in each one of the runs carried out. In order to determine the model of best fit for the attrition kinetics in a conical spouted bed reactor, the data obtained for sand I at different velocities were fitted to both models. Fig. 11 shows the results for two air velocities, 48 m/s (Fig. 11(a)) and 54 m/s (Fig. 11(b)). By comparing the experimentally measured bed weights with those calculated using the equations, it is clear that both models are acceptable for predicting attrition kinetics in spouted beds, but the one based on that proposed by Lee et al. (1993) fits better the trend for long time runs due to its exponential term. Consequently, the model developed by Lee et al. (1993) was fitted to the experimental results obtained for different sand types, gas velocities and bed heights to compare experimental and calculated evolutions of
bed weight as shown in Fig. 12(a) for initial bed weight of 6 kg, and in Fig. 12(b) for 4.5 kg, both showing how faithfully the model predicts the experimental evolution of bed weight, with the attrition parameters k0 = 6.77 × 10–3 h–1 and k1 = −0.068 (m2 /s2 ). Fig. 13 compares experimental data for bed weight with those calculated using Lee’s model to show the adequacy of the model proposed. Despite satisfactory prediction of the model it should be noted that, similar to other attrition models, it is highly dependent on the geometric factors determining the hydrodynamic performance in the contactor. Moreover, the physical properties of the sand also play a significant role on the attrition process. Consequently, the attrition parameters calculated in this paper should be extrapolated with care to other experimental conditions.
5. Conclusions Two attrition kinetic models proposed by Gwyn (1969) and Lee et al. (1993) were fitted to experimental data obtained with two types of silica sand in a spouted bed contactor at room temperature. Both provide acceptable results but the second one predicts better results for long time runs due to the exponential term involved in the equation. The attrition rate constant (Ka ) depends exponentially on the excess of air velocity over that corresponding to the minimum spouting velocity at zero time. The values of the constants k0 and k1 are 6.77 × 10–3 h–1 and −0.068 m2 /s2 , respectively, and they depend on contactor geometry and bed material properties. As inlet air velocity is raised attrition rate increases until bed weight reaches a minimum value (Wmin ) at steady state. Furthermore, the higher the inlet air velocity, the lower the final bed weight. Consequently, in order to maintain a given weight of bed material in the contactor, and therefore the height of the bed, a certain amount of material will have to be added. Particle properties play an important role in the attrition process. Especially noteworthy is the effect of textural properties, that is, sand particles (type II) with sharp edges undergo more severe particle breakage than the more rounded. A sharp reduction in attrition has been observed when operating with a draft tube. The insertion of an internal device avoids the incorporation of particles into the spout at different heights and, consequently, avoids violent impacts in this region. Furthermore, attrition decreases as temperature is increased, which is attributed to the less vigorous gas–solid contact and the resulting reduction in particle breakage.
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Fountain height is another essential parameter in the design of spouted beds, as it determines the total height of the reactor. Fountain height increases when small particles are handled and when attrition increases the fraction of these particles in the bed over time. Acknowledgments The authors would like to thank the Basque Government for the ETORTEK Strategic Programme, the Provincial Council of Gipuzkoa and the Spanish Ministry of Science and Education for the Project CTQ2010-16133. References Aguado, R., Olazar, M., Barona, A., & Bilbao, J. (2000). Char-formation kinetics in the pyrolysis of sawdust in a conical spouted bed reactor. Journal of Chemical Technology and Biotechnology, 75, 583–588. Al-Senawi, S., Hadi, B., Briens, C., & Chabagno, J. M. (2008). A comparison of the breakage mechanisms for attrition of selected polymers in pneumatic transport and spouted beds. International Journal of Chemical Reactor Engineering, 6. Altzibar, H., Lopez, G., Aguado, R., Alvarez, S., San Jose, M., & Olazar, M. (2009). Hydrodynamics of conical spouted beds using different types of internal devices. Chemical Engineering & Technology, 32, 463–469. Amutio, M., Lopez, G., Aguado, R., Artetxe, M., Bilbao, J., & Olazar, M. (2011). Effect of vacuum on lignocellulosic biomass flash pyrolysis in a conical spouted bed reactor. Energy and Fuels, 25, 3950–3960. Altzibar, H., Lopez, G., Alvarez, S., Jose, M., Barona, A., & Olazar, M. (2008). A draft-tube conical spouted bed for drying fine particles. Drying Technology, 26, 308–314. Arena, U., D’Amore, M., & Massimilla, L. (1983). Carbon attrition during the fluidized bed combustion of a coal. AIChE Journal, 29, 40–49. Azizi, S., Hosseini, S. H., Moraveji, M., & Ahmadi, G. (2010). CFD modeling of a spouted bed with a porous draft tube. Particuology, 8, 415–424. Belyaev, A. (2008). Gasification of low-grade fuels in a spouted bed for power generation. Solid Fuel Chemistry, 42, 335–341. Bemrose, C. R., & Bridgwater, J. (1987). A review of attrition and attrition test methods. Powder Technology, 49, 97–126. Berghel, J., Nilsson, L., & Renstrom, R. (2008). Particle mixing and residence time when drying sawdust in a continuous spouted bed. Chemical Engineering and Processing, 47, 1252–1257. Boerefijn, R., Ghadiri, M., & Salatino, P. (2007). Attrition in fluidised beds. In A. Salman, M. Ghadiri, & M. Hounslow (Eds.), Handbook of powder technology: Particle breakage (pp. 1019–1053). Amsterdam: Elsevier B.V. Boerefijn, R., Gudde, N. J., & Ghadiri, M. (2000). A review of attrition of fluid cracking catalyst particles. Advanced Powder Technology, 11, 145–174. Brown, R. L., & Richards, J. C. (1970). Principles of powder mechanics. Oxford: Pergamon. Buczek, B. (1983). Working of active-carbon by attrition in a spouted bed. Powder Technology, 35, 113–114. Chen, Z., Jim Lim, C., & Grace, J. R. (2007). Study of limestone particle impact attrition. Chemical Engineering Science, 62, 867–877. Cook, J. L., Khang, S. J., Lee, S. K., & Keener, T. C. (1996). Attrition and changes in particle size distribution of lime sorbents in a circulating fluidized bed absorber. Powder Technology, 89, 1–8. Cunha, F. G., Santos, K. G., Ataide, C. H., Epstein, N., & Barrozo, M. A. S. (2009). Annatto powder production in a spouted bed: An experimental and CFD study. Industrial & Engineering Chemistry Research, 48, 976–982. da Rosa, C. A., & Freire, J. T. (2009). Fluid dynamics analysis of a draft-tube continuous spouted bed with particles bottom feed using CFD. Industrial & Engineering Chemistry Research, 48, 7813–7820. da Rosa, G. S., & dos Santos Rocha, S. C. (2010). Effect of process conditions on particle growth for spouted bed coating of urea. Chemical Engineering and Processing: Process Intensification, 49, 836–842. Elordi, G., Olazar, M., Lopez, G., Artetxe, M., & Bilbao, J. (2011). Product yields and compositions in the continuous pyrolysis of high-density polyethylene in a conical spouted bed reactor. Industrial & Engineering Chemistry Research, 50, 6650–6659. Epstein, N., & Grace, J. R. (1997). Spouting of particulate solids. In M. E. Fayed, & L. Otten (Eds.), Handbook of powder science and technology. New York: Van Nostrand-Reinhold. Flamant, G., Chraibi, M. A., Vallbona, G., & Bertrand, C. (1990). Decarbonation and attrition of calcite in a plasma spouted bed reactor. Journal de Physique, 51, C527–C534. Freitas, L. A. P., & Freire, J. T. (2001). Heat transfer in a draft tube spouted bed with bottom solids feed. Powder Technology, 114, 152–162. Gwyn, J. E. (1969). On the particle size distribution function and the attrition of cracking catalysts. AIChE Journal, 15, 35–39.
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Halder, P. K., & Basu, P. (1992). Attrition of spherical electrode carbon particles during combustion in a turbulent fluidized bed. Chemical Engineering Science, 47, 527–532. Ishikura, T., Nagashima, H., & Ide, M. (2003). Hydrodynamics of a spouted bed with a porous draft tube containing a small amount of finer particles. Powder Technology, 131, 56–65. Konduri, R. K., Altwicker, E. R., & Morgan, M. H. I. (1999). Design and scale-up of a spouted-bed combustor. Chemical Engineering Science, 54, 185–204. Kono, H. (1981). Attrition rates of relatively coarse solid particles in various types of fluidised beds. AIChE Symposium Series, 77, 96–106. Lee, S. K., Jiang, X., Keener, T. C., & Khang, S. J. (1993). Attrition of lime sorbents during fluidization in a circulating fluidized bed absorber. Industrial and Engineering Chemistry Research, 32, 2758–2766. Lin, C. L, & Wey, M. Y. (2003). Effects of high temperature and combustion on fluidized material attrition in a fluidized bed. Korean Journal of Chemical Engineering, 20, 1123–1130. Lin, C. L., & Wey, M. Y. (2005). Influence of hydrodynamic parameters on particle attrition during fluidization at high temperature. Korean Journal of Chemical Engineering, 22, 154–160. Lopez, G., Olazar, M., Aguado, R., & Bilbao, J. (2010). Continuous pyrolysis of waste tyres in a conical spouted bed reactor. Fuel, 89, 1946–1952. Luo, B., Lim, C. J., Freitas, L. A. P., & Grace, J. R. (2004). Flow characteristics in a slot-rectangular spouted beds with draft plates. Canadian Journal of Chemical Engineering, 82, 83–88. Makibar, J., Fernandez-Akarregi, A. R., Alava, I., Cueva, F., Lopez, G., & Olazar, M. (2011). Investigations on heat transfer and hydrodynamics under pyrolysis conditions of a pilot-plant draft tube conical spouted bed reactor. Chemical Engineering and Processing: Process Intensification, 50, 790–798. Markowski, M., Sobieski, W., Konopka, I., Tanska, M., & Bialobrzewski, I. (2007). Drying characteristics of barley grain dried in a spouted-bed and combined IRconvection dryers. Drying Technology, 25, 1621–1632. Mathur, K. B., & Epstein, N. (1974). Spouted beds. New York: Academic Press. Merrick, D., & Highley, J. (1974). Particle size reduction and elutriation in a fluidized bed process. American Institution Chemical Engineering Symposium Series, 70, 366–378. Olazar, M., Alvarez, S., Aguado, R., & San Jose, M. (2003). Spouted bed reactors. Chemical Engineering & Technology, 26, 845–852. Olazar, M., Lopez, G., Altzibar, H., Aguado, R., & Bilbao, J. (2009). Minimum spouting velocity under vacuum and high temperature in conical spouted beds. The Canadian Journal of Chemical Engineering, 87, 541–546. Olazar, M., San Jose, M. J., Zabala, G., & Bilbao, J. (1994). A new reactor in jet spouted bed regime for catalytic polymerizations. Chemical Engineering Science, 49, 4579–4588. Olazar, M., Lopez, G., Altzibar, H., & Bilbao, J. (2011). Modelling batch drying of sand in a draft-tube conical spouted bed. Chemical Engineering Research and Design, 89, 2054–2062. Oliveira, H. V. A., Peixoto, M. P. G., & Freitas, L. A. P. (2005). Study on the efficiency of hard gelatin capsules coating in a spouted bed. Drying Technology, 23, 2039–2053. Sanaei, S., Mostoufi, N., Radmanesh, R., Sotudeh-Gharebagh, R., Guy, C., & Chaouki, J. (2010). Hydrodynamic characteristics of gas–solid fluidization at high temperature. Canadian Journal of Chemical Engineering, 88, 1–11. Shih, H. H., Chu, C. Y., & Hwang, S. J. (2003). Solids circulation and attrition rates and gas bypassing in an internally circulating fluidized bed. Industrial and Engineering Chemistry Research, 42, 5915–5923. Spiegl, N., Sivena, A., Lorente, E., Paterson, N., & Milian, M. (2010). Investigation of the oxy-fuel gasification of coal in a laboratory-scale spouted-bed reactor: Reactor modifications and initial results. Energy & Fuels, 24, 5281–5288. Stein, M., Seville, J. P. K., & Parker, D. J. (1998). Attrition of porous glass particles in a fluidised bed. Powder Technology, 100, 242–250. Swasdisevi, T., Tanthapanichakoon, W., Charinpanitkul, T., Kawaguchi, T., Tanaka, T., & Tsuji, Y. (2004). Investigation of fluid and coarse-particle dynamics in a two-dimensional spouted bed. Chemical Engineering & Technology, 27, 971–981. Swasdisevi, T., Tanthapanichakoon, W., Charinpanitkul, T., Kawaguchi, T., Tanaka, T., & Tsuji, Y. (2005). Prediction of gas-particle dynamics and heat transfer in a two-dimensional spouted bed. Advanced Powder Technology, 16, 275–293. Vieira Neto, J. L., Duarte, C. R., Murata, V. V., & Barrozo, M. A. S. (2008). Effect of a draft tube on the fluid dynamics of a spouted bed: Experimental and CFD studies. Drying Technology, 26, 299–307. Welt, J., Lee, W., & Krambeck, F. J. (1977). Catalyst attrition and deactivation in fluid catalytic cracking system. Chemical Engineering Science, 32, 1211–1218. Wongvicha, P., & Bhattacharya, S. C. 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Particuology journal homepage: www.elsevier.com/locate/partic
Sand attrition in conical spouted beds Aranzazu R. Fernández-Akarregui a , Jon Makibar a , Isabel Alava a , Luis Diaz a , Fernando Cueva a , Roberto Aguado b , Gartzen Lopez b , Martin Olazar b,∗ a b
Ikerlan-IK4, Juan de 4 la Cierva 1, Arabako Parke Teknologikoa, E-01510 Minao, Araba, Spain Department of Chemical Engineering, University of the Basque Country, P.O. Box 644, E48080 Bilbao, Spain
a r t i c l e
i n f o
Article history: Received 19 October 2011 Received in revised form 4 January 2012 Accepted 20 February 2012 Keywords: Attrition Conical spouted bed Draft tube Sand attrition
a b s t r a c t A study was carried out on the attrition in conical spouted beds using two sands with different properties for several bed heights and gas flow rates. Furthermore, the influence of a draft tube was studied at ambient and high temperatures. The main objective was to acquire knowledge on the attrition of sand beds for biomass pyrolysis in a pilot plant provided with a conical spouted bed reactor. A first-order kinetic equation is proposed for sand attrition in a conical spouted bed at room temperature. The predicted attrition rate constant depends exponentially on excess air velocity over that for minimum spouting. Both the draft tube and temperature increase contribute to reduction of attrition. © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Spouted beds have been proposed as an alternative to fluidized beds for processes that call for efficient fluid–solid contact (Epstein & Grace, 1997), especially for particles larger than 1 mm (Freitas & Freire, 2001; Zhao, Li, Liu, Song, & Yao, 2008). An advantage of spouted beds over fluidized beds is the well-defined cyclic movement of the particles, which facilitates contact between gas and solid particles (Luo, Lim, Freitas, & Grace, 2004). In fact, both high solid circulation rate and efficient gas–solid contact favor the use of spouted beds in many chemical processes (Azizi, Hosseini, Moraveji, & Ahmadi, 2010; Vieira Neto, Duarte, Murata, & Barrozo, 2008). In spouted beds, the fluid is introduced through a central nozzle rather than uniformly through a distributor plate as in fluidized beds. The fluid enters the bed in the form of a jet and causes the particles to circulate in a uniform way creating a central spout zone. The fluid and particles are in counter-current flow in the annulus, which makes up the major portion of the spouted bed. Particle flow is co-current with the fluid in the spout, where velocities are high and residence time is short. Nowadays, spouted beds are widely used in physical processes, such as drying (Altzibar et al., 2008; Berghel, Nilsson, & Renstrom, 2008; Markowski, Sobieski, Konopka, Tanska, & Bialobrzewski, 2007), coating and granulation (da Rosa & dos Santos Rocha, 2010;
∗ Corresponding author. Tel.: +34 946392527; fax: +34 946393500. E-mail address: [email protected] (M. Olazar).
Oliveira, Peixoto, & Freitas, 2005) and in chemical processes involving combustion (Konduri, Altwicker, & Morgan, 1999), gasification (Belyaev, 2008; Spiegl, Sivena, Lorente, Paterson, & Milian, 2010), catalytic polymerization (Olazar, San Jose, Zabala, & Bilbao, 1994) and pyrolysis of different solid wastes (Aguado, Olazar, Barona, & Bilbao, 2000; Amutio et al., 2011; Elordi, Olazar, Lopez, Artetxe, & Bilbao, 2011; Lopez, Olazar, Aguado, & Bilbao, 2010). An overview of spouted bed reactors was carried out by Olazar, Alvarez, Aguado, and San Jose (2003). One of the challenges for scaling up the spouting regime is the understanding of the process controlling particle size reduction and the determination of attrition kinetics, which will help to design a process with high efficiency and reduced fine particle emission. Moreover, the fines formed must be removed from the out-going gas stream, thereby increasing the processing cost (Stein, Seville, & Parker, 1998). Although numerous papers address the attrition in fluidized beds, few provide information on spouted beds. Mathur and Epstein (1974) devoted a brief chapter in their book to this subject and, more recently, attrition in spouted beds was considered for the processing of carbonaceous materials (Buczek, 1983; Wongvicha & Bhattacharya, 1994), calcite (Flamant, Chraibi, Vallbona, & Bertrand, 1990) and selected polymers (Al-Senawi, Hadi, Briens, & Chabagno, 2008). To our knowledge, no paper has been published dealing with the attrition of spouted beds fitted with a draft tube. The attrition process is commonly divided into two different mechanisms causing particle breakage, namely, the abrasion of the particle surface and the disintegration or fragmentation of the particle matrix (Boerefijn, Ghadiri, & Salatino, 2007, chap. 25; Lin &
1674-2001/$ – see front matter © 2012 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.partic.2012.02.002
A.R. Fernández-Akarregui et al. / Particuology 10 (2012) 592–599
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Nomenclature DC Di DT D0 ds Ea HC Hf HT Ka Kp K0 k0 k1 LH m P Mw Rt t T umf u0 (ums )0 W W0 We Wmin
diameter of the cylindrical section of the contactor (m) diameter of the bed bottom (m) diameter of the draft tube (m) diameter of the bed inlet (m) Sauter diameter (m) attrition activation energy (m2 /s2 ) height of the conical section of the contactor (m) height of the fountain (m) total height of the contactor (m) attrition rate constant in Lee’s equation (s−1 ) dimensional constant in Gwyn’s equation (s−m ) frequency factor in an Arrhenius form (s−1 ) kinetic constant in Eq. (4) (s−1 ) kinetic constant in Eq. (4) (m2 /s2 ) height of the draft tube entrainment zone (m) exponent for time dependence on attrition in Gwyn’s equation, dimensionless pressure (kg/(m s2 )) molecular weight of air (kg/mol) attrition rate (kg/s) time (s) temperature (K) minimum fluidization velocity (m/s) inlet air velocity referred to the inlet diameter (m/s) minimum spouting velocity referred to the inlet diameter (m/s) weight of bed material (kg) initial mass of particles in the bed (kg) mass of particles in the bed at t time (kg) minimum weight of parent solids in the bed (kg)
Greek letters cone angle (rad) pressure drop (kg/(m s2 )) P
Wey, 2005; Welt, Lee, & Krambeck, 1977). The factors that affect attrition dynamics include: (i) the properties of the bed material, i.e., porosity, size, hardness, density, surface, cracks, shape and particle strength (Lee, Jiang, Keener, & Khang, 1993) and (ii) the reactor environment, i.e., exposure time, particle size and velocity (Lin & Wey, 2003, 2005; Shih, Chu, & Hwang, 2003) pressure system and type of enclosure (hard or soft surface), shear, temperature (Lin & Wey, 2003, 2005; Arena, D’Amore, & Massimilla, 1983) and viscosity and turbulence (Bemrose & Bridgwater, 1987). In fluidized beds, the jetting region is the most significant contributor to particle breakage (Boerefijn, Gudde, & Ghadiri, 2000; Boerefijn et al., 2007) and in the case of spouted beds the same role is probably played by the spout region. Many reports propose empirical correlations for attrition rate in bubbling and circulating fluidized bed systems, and interesting reviews have been reported by Lee et al. (1993) and Lin and Wey (2005), but no correlation for spouted beds has been found in the literature. This study addresses the kinetics of sand attrition in a conical spouted bed contactor under different hydrodynamic conditions. The kinetic equation corresponding to the experimental conditions studied has been obtained based on analogies with certain kinetic equations proposed for fluidized bed systems. The closest correlations are those by Gwyn (1969) and Lee et al. (1993), both for fluidized beds. Gwyn (1969) studied silica sand attrition
Fig. 1. Schematic of the experimental setup for attrition tests in a conical spouted bed contactor.
rate at room temperature and proposed a correlation of the following type: Rt = Kp mt m−1 W,
(1) = 4.47 × 10−6 –1.35 × 10−5 s−m .
Lee et al. with m = 0.46 and Kp (1993) investigated lime attrition in a circulating fluidized bed absorber at 20–180 ◦ C and found that the predominant mechanism was surface abrasion. According to these authors, lime attrition decreases as temperature is increased, due to changes in the material, whereas limestone attrition increases with temperature due to the decrepitation resulting from increased internal pressure. The total weight of particles in the bed at time t, We , follows the equation: We (t) = (W0 − Wmin )e−Ka t + Wmin ,
(2)
where W0 (kg) is the weight of the bed at t = 0, Wmin is the minimum weight for which the attrition is negligible and Ka is the attrition rate constant. These authors found that the constant Ka depends exponentially on activation energy, temperature, superficial air velocity and minimum fluidization velocity in the attrition process: Ka = Ko e−Ea RTCs /PMW u0 (u0 −umf ) .
(3)
For lime attrition, the authors related a frequency factor in an Arrhenius form, with Ko = 1.29 × 10−4 s−1 , and calculated an attrition activation energy, Ea = 3.38 × 10−3 kJ/kg. Other equations (Halder & Basu, 1992; Kono, 1981; Merrick & Highley, 1974; Wu, Baeyens, & Chu, 1999) are not suitable for spouted beds because they take into account phenomena that do not occur in these beds, such as the presence of bubbles, and those like the second-order model (Cook, Khang, Lee, & Keener, 1996) are of no direct use for the design of these types of beds. 2. Experimental system and test procedure The experimental set-up for sand attrition tests is shown in Fig. 1, including an air compressor, a mass flow meter and a controller, a conical spouted bed contactor, a cyclone, a pressure
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Fig. 2. Fractional size distribution of the two silica sands (A-GRS07, Sibelco Minerals), type I and type II.
sensor, a thermocouple and a data monitoring system. The attrition experiments were carried out in two different plants, i.e., a conical spouted bed contactor made of plexiglass and a non-porous draft tube conical spouted bed reactor made of stainless steel. The latter is the main component in a pilot plant unit designed and set up for continuously pyrolysing 25 kg/h of biomass. The experimental runs in this reactor were conducted at both ambient and high temperatures. The dimensions of both contactors are exactly the same (Fig. 1): the diameter of the upper section (DC ) is 242 mm, the angle of the contactor is = 32◦ , the height of the conical section (HC ) is 330 mm and the total height (HT ) is 1030 mm. The base diameter (Di ) is 52 mm and the inlet diameter (Do ) is 15 mm when no internal device is used, and 25 mm when a non-porous draft tube is used, given that the use of this internal device improves bed stability and, consequently, a greater inlet diameter may be used without instability problems (Altzibar et al., 2008, 2009; Cunha, Santos, Ataide, Epstein, & Barrozo, 2009; da Rosa & Freire, 2009; Olazar, Lopez, Altzibar, & Bilbao, 2011; Vieira Neto et al., 2008). The diameter of the draft tube (DT ) is 36 mm and the height of the entrainment zone (LH ) is 80 mm. These dimensions of the draft tube were determined in a previous paper (Makibar et al., 2011). A stainless steel mesh (55 mesh size) placed at the base of the contactor supports the sand. Air (25 ◦ C and 480 ◦ C, 1 atm) was used as the spouting medium. The outlet air from the contactor entered the cyclone where the elutriated fines are collected. The air flow rate was measured by a mass flow meter (Brooks, 5853S). Pressure was measured by a differential pressure transducer (RS 286-686). Pressure drop, temperature and air flow rate were continuously monitored and recorded by a Fluke NetDAQ Logger data acquisition system. Attrition study was carried out using two types of commercial silica sand (called sands I and II) provided by Sibelco Minerals. The density of both sands is 2600 kg/m3 and their loose bed porosity 0.46, as determined following the Brown and Richards (1970) method. The initial particle size distribution (before attrition tests) for both types of sand is shown in Fig. 2 (determined by sieving), with their Sauter average diameter being around 1.0 mm. Experiments were carried out using two amounts of sand, 4.5 and 6 kg, which account for bed heights (Ho ) of 24.5 and 27.8 cm, respectively. The height of the fountain (Hf ) was visually measured, whereas air velocities referred to the inlet diameter (u0 ), temperature and pressure drop (P) were continuously recorded throughout the whole test. In experiments, air flow rate is gradually increased to a value 1.5 times higher than the minimum spouting gas flow rate and is then
Fig. 3. Pressure drop evolution with air velocity (6 kg of silica sand I, Do = 15 mm, dS = 1.05 mm).
decreased to the set spouting velocity. This air rises through the spouting bed, with the sand particles moving upwards in the fountain and falling down again onto the annulus where they continue moving downwards until arriving at the spout, thus repeating the full cycle. The air supply is turned off at regular time intervals and sand is taken out, weighed and put back into the contactor. At the beginning of the experiments, the measuring intervals are shorter because the attrition process is more severe. When the weight reduction of the sand reaches a steady state, the test is concluded. The shape factor of bed particles is measured by optical microscopy at the beginning and end of each run, and the image analysis of final elutriated fines is also obtained by SEM microscopy. Following the methodology described, attrition tests without the draft tube are carried out at room temperature using air velocities 1.16 and 1.35 times higher than the minimum spouting velocity, (u0 )ms , corresponding to the original bed. Thus, air flow rates of 450 L/min and 574 L/min, corresponding to the spouting velocities (per unit of inlet cross-sectional area) of 48–54 m/s are used. Each experiment lasted 420 h. In the experiments carried out with a draft tube, the velocities are 1.35 and 1.65 times higher than those for minimum spouting at 25 and 480 ◦ C, respectively. Thus, the flow rates used (measured at ambient conditions) are 472 L/min for ambient temperature and 210 L/min for 480 ◦ C, and the runs are carried out using type II sand. The reduction in the gas flow rate required for the operation at high temperatures is explained by the increase in gas viscosity and, therefore, by the increase in the momentum transfer from the gas to the solid, which facilitates the spouting phenomenon (Olazar, Lopez, Altzibar, Aguado, & Bilbao, 2009). In experiments with a draft tube, each experiment lasted 248 h. 3. Experimental results The evolution of pressure drop with air velocity from the fixed to the spouted bed was analyzed in order to determine the minimum spouting velocity for the different systems studied. As shown in Fig. 3, at first, as air velocity is increased, pressure drop increases to a peak value, at which, with further increasing air velocity, a fountain is created and pressure drop decreases to a value that remains approximately constant for a wide range of air velocities. A highly pronounced hysteresis is noted upon reducing the air velocity, which is due to the fact that the peak pressure drop is much higher than the operating pressure drop and, furthermore, the velocity required to break up the bed and open the spout is higher than that corresponding to the peak pressure drop.
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Fig. 4. Particle size distribution of sand I at the beginning and end of the tests using air velocities of 1.2 and 1.35 times (u0 )ms .
It is widely accepted that the predominant attrition mechanism during fluidization is surface abrasion due to collisions between parent solids in the bed (Lee et al., 1993). Nevertheless, there are two different attrition mechanisms in a spouted bed: (i) particle impact attrition, which takes place in the fountain region and happens when particles collide against the vessel wall, and (ii) particle–particle surface abrasion, which takes place mainly in the spout region. Particles collide with each other and against the layer of annulus particles constituting the spout wall. The extent of particle-wall impact attrition depends on total impact surface area, number of impact cycles, particle velocity, and the particle and vessel material (Chen, Jim Lim, & Grace, 2007). Particle–particle surface abrasion depends on particle velocity and particle material. All these parameters change with time. This is due to the presence of fewer faults, flaws, or discontinuities affecting particles and to smaller particle size in the bed. Both mechanisms ensure that sand particles undergo attrition, increasing the fractional weight of fines and reducing the fractional weight of bigger particles.
3.1. Effect of gas velocity In order to study the effect of gas velocity on attrition, results are obtained at two operating velocities, both above the minimum, by using sand I. Fig. 4 shows the variation in the fractional size distribution at the beginning and end of the tests carried out at air velocities of 1.2 and 1.35 times higher than the minimum spouting velocity. As observed, a reduction in the particle size of the sand is noted at the end of the tests. In fact, the Sauter diameters measured at the end of the tests for these velocities are 0.90 mm and 0.94 mm, respectively. The higher Sauter diameter for the higher velocity (1.35 times the minimum one) is explained by the higher amount of elutriated fines for the higher gas flow rate. Knowledge on the attrition process of the bed material in conical spouted bed reactors is essential because changes in the fractional weight distribution involve changes in the hydrodynamic behavior of the spouted bed reactors. Thus, a decrease in the average particle diameter throughout the run due to attrition causes minimum spouting velocity to decrease and, consequently, the operating (u/ums )0 ratio to increase when a constant air flow rate is used in the run. Fig. 5 shows the evolution of (u0 )ms and (u/ums )0 with time using an initial inlet air velocity of 1.35 times the minimum one which accounts for a velocity of 54 m/s. This effect has already been observed by Lin and Wey (2005) in fluidized beds.
595
Fig. 5. Evolution of (u0 )ms and (u/ums )0 with time, for sand I and an initial inlet air velocity of 1.35 times (u0 )ms .
Furthermore, the fountain height increases with the increase in the small particle fraction by attrition. In fact, big particles tend to occupy radial positions near the annular–spout interface and so describe shorter trajectories than the smaller ones. The smaller particles circulate mainly along the outer periphery of the annular zone and describe wider cycles. The fountain therefore acts as a highefficiency distributor, which provides an interesting segregation of particles by diameter and/or density. Furthermore, as the fountain height increases the particle-wall impact surface area and velocity also increase. In addition, the number of particles contained in a given weight fraction is much higher when these particles are small and, moreover, their ratio of surface area to volume is higher, which increases the probability of collision. Fig. 6 shows the experimental data of bed mass evolution for two air inlet velocities. As observed, there is an exponential decrease in the weight of solids in the bed throughout the run. Particles initially undergo rapid attrition, which then decreases exponentially, leading to a minimum weight value (Wmin ) of steady state in which attrition could be considered negligible. This seems to be due to the fact that the irregular surface of the particles at the beginning of each experiment abrades easily, but the particles become harder to abrade when they are already rounded (Boerefijn et al., 2000; Lin & Wey, 2003). Wmin depends on air inlet velocity, so that the higher the air inlet velocity, the smaller the Wmin . The values of Wmin for the two sets carried out in this study are 5555 g and 5300 g for (u/ums )0 = 1.2 and 1.35, respectively. The higher air flow rate gave
Fig. 6. Experimental data for bed weight evolution (sand I) at two air velocities.
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Fig. 7. Microphotographs of (a) original sand I and (b) sand after 420 h spouting, (u/ums )0 = 1.2; (c) sand after 420 h spouting, (u/ums )0 = 1.35.
way to more violent collisions between the particles and with the contactor surfaces. In all the experiments carried out in this work, the time to reach the minimum value was around 400 h, and it is proven that the attrition rate increases rapidly by increasing air velocity. The rounding off and surface abrasion of silica sand were observed under a microscope, as illustrated in Fig. 7, showing the rough and irregular surface of the original sand I (Fig. 7(a)) and the abraded surface of the sand after 420 h of spouting with inlet air velocities of 48 and 54 m/s, respectively (Fig. 7(b) and (c)). The sharp edges gradually disappeared, and the surface became increasingly polished with time due to collisions. The sizes of the fines elutriated from the spouted bed contactor and retained in the cyclone range between 2 and 50 m, as illustrated by SEM microscopy in Fig. 8. 3.2. Effect of sand type The effects of sand types, I and II, and gas velocities on attrition are shown in Fig. 9 by observing the evolution of bed weight. The mass loss is observed for sand type I operating with an air velocity 1.2 times the minimum, and for sand type II operating with velocities 1.16–1.26 times the minimum. As observed, the mass loss is much higher with sand II than with sand I, even operating with lower gas velocities. In fact, when operation is carried out using the same (u/ums )0 ratio with both sands, the mass loss with sand II is twice that observed with sand I. The chemical compositions of both sands were analyzed by Xray diffraction, to show the main component in both cases to be quartz (>99.5%), with feldspar as the main impurity. The different behavior between the sands is due to their different textural properties, which were determined by means of a petrographic microscopy: sand II is characterized by its sharp edges and, consequently, when it is subjected to impacts in the spouting operation,
Fig. 8. SEM image of fines elutriated from the spouted bed.
breakage of these sharp edges causes faster and more significant mass loss. Other authors (Lee et al., 1993; Lin & Wey, 2003, 2005) studying attrition in fluidized beds, also observed the important effect of particle properties, such as porosity, size, shape, hardness and so on. 3.3. Attrition in a contactor fitted with a draft tube Operating with conical spouted beds has certain limitations for the treatment of fine materials and a crucial parameter that hinders bed stability in the scaling-up of the process is the ratio between the inlet diameter and particle diameter. Thus, the inlet diameter should be up to 20–30 times the average particle diameter in order to achieve spouting status (Olazar et al., 2011). The use of a draft tube is the usual solution to this problem (Swasdisevi et al., 2005). In fact, this internal device is the key for stable operation in largescale spouted beds and allows increasing the spoutable bed height and reducing bed pressure drop (Altzibar et al., 2009; Luo et al., 2004; Swasdisevi et al., 2004, 2005). Moreover, solid circulation rate, particle cycle time, gas distribution and so on are governed by the specifications of the draft tube (Altzibar et al., 2009; Cunha et al., 2009; Ishikura, Nagashima, & Ide, 2003; Zhao, Yao, & Li, 2006). The use of a draft tube allows increasing the versatility of the conical spouted bed and improving the flexibility of the gas–solid contact. Accordingly, a conical spouted bed provided with a draft tube has been developed for the biomass pyrolysis process. Attrition in the conical spouted beds provided with a draft tube was compared with that observed without it. Runs were carried out with a draft tube by using a higher velocity than the minimum for spouting in order to improve the less vigorous contact of these beds without a tube. Thus, the (u/ums )0 ratio used is 1.35 at 25 ◦ C and 1.65 at 480 ◦ C. In spite of operating with a higher (u/ums )0 ratio, the fountain height observed at 480 ◦ C (about 15 cm) is lower than that observed at 25 ◦ C (around 20 cm). It is noteworthy that this less vigorous contact at high temperatures reduces sand attrition.
Fig. 9. Comparison of sand mass loss for different types of sand and different gas velocities.
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597
Fig. 10. Effect of gas velocity and temperature on fountain height.
Thus, at the end of the run (248 h), the sand mass loss at 25 ◦ C is 180 g, whereas it is only 115 g at 480 ◦ C, even operating with a higher gas velocity. Certain authors observed the opposite operating with fluidized beds, i.e., more attrition at high temperatures (Bemrose & Bridgwater, 1987; Lin & Wey, 2003, 2005). This different behavior between spouted beds and fluidized beds at high temperatures is explained by the less vigorous gas–solid contact when temperature is increased in the spouted beds. In order to improve knowledge on the spouting regime at high temperatures, a study was conducted on the fountain height and gas–solid contact behavior at different (u/ums )0 ratios and temperatures from 25◦ to 500 ◦ C. Fig. 10 shows the evolution of fountain height as a function of temperature and (u/ums )0 ratio. As observed, the fountain height is reduced when operating at high temperatures and this effect seems to be violated if operating over 400 ◦ C. The reduction in the fountain height is related to a change in the gas flow pattern, i.e., a higher amount of gas crosses the bed through the annulus, which causes a reduction in the gas velocity in the spout and, consequently, in the acceleration of the particles in this region. In addition to the lower fountain height at high temperatures, it is visually observed that solid movement is less vigorous and, consequently, the solid circulation rate is lower. Both findings cause a reduction in the strength of particle collisions and therefore in the attrition rate at high temperatures. Likewise, Shih et al. (2003) related attrition in an internally circulating fluidized bed to the solid circulation rate. The lower attrition rate at high temperatures is related to the change in drag force from low to high temperatures. Increasing temperature leads to an increase in gas viscosity, whereas at the same time its density decreases. Since at low temperatures (i.e., well below 300 ◦ C) increasing gas viscosity is dominant in comparison with gas density, the drag force increases (Sanaei et al., 2010). Before comparing the results obtained at ambient temperature with and without a draft tube, it should be mentioned that the conical spouted bed without a draft tube is made of plexiglass, whereas the one with the draft tube is made of stainless steel. Consequently the collisions between the sand and the contactor wall are more severe in the latter case. Nevertheless, the mass loss with draft tube (and higher (u/ums )0 ratio) is less than a quarter of that observed without a draft tube. This difference is due to the solid circulation pattern when working with a draft tube. Thus, in the operation with a non-porous draft tube, the particles are incorporated into the spout at the bottom of the contactor, whereas in a plain conical spouted bed the incorporation of solids into the spout takes place at all heights throughout the spout. In the latter case, the particles that are incorporated into the spout above the bottom collide with
Fig. 11. Bed weight evolution for sand I and air velocities of (a) (u/ums )0 = 1.2 and (b) (u/ums )0 = 1.35. (Points: experimental data; lines: calculated.)
other particles arising at a high velocity, thus causing severe breakage due to the difference in their velocities. This situation is fully avoided when working with a draft tube because all the particles enter the spout at the bottom and their relative velocity is almost zero. Moreover, in a spouted bed without a draft tube, there are violent collisions between the particles in the spout and the layer of annulus particles constituting the spout wall. These collisions are avoided by operating with a draft tube. These findings reveal additional advantages for operating with a draft tube because the interest in reducing attrition is not only related to the avoidance of bed material loss but also to the cost of removing fine particles downstream of the fluidized or spouted bed (Stein et al., 1998).
4. Kinetics of sand attrition Experimental data were fitted to different first-order kinetic models reported in the literature, specifically those by Gwyn (1969) and Lee et al. (1993). Gwyn’s equation is the most versatile model describing granular attrition and states that the elutriated weight fraction of sand at room temperature is proportional to time under constant strain rate conditions, Eq. (1). To estimate the weight changes of the sand in the spouted bed using Lee’s formula, Eq. (2), the definition of parameter Ka must be modified by assuming that it depends exclusively on the excess air velocity over that corresponding to
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Fig. 13. Comparison of experimental data for bed weight with those calculated using Lee’s model.
Fig. 12. Bed weight evolution for sands I and II for different air velocities and initial bed weights of (a) 6 kg and (b) 4.5 kg. (Points: experimental data; lines: calculated.)
minimum spouting velocity at the beginning of the run (u − ums )0 . The empirical correlation proposed for Ka is: Ka = k0 e−k1 /u(u−ums )0 ,
(4)
with constants k0 and k1 depending on the contactor geometry and various parameters related to the bed (solid density, porosity, sphericity, particle mean diameter and bed height). Scilab algorithms have been developed to fit the experimental data to the above mentioned models. The objective function (OF) is defined as:
m
OF =
i=1
(Wcal − Wexp )2n +
m
n+m
j=1
(Wcal − Wexp )2m
,
(5)
where Wcal and Wexp are the bed weights calculated with the model and the experimental values, respectively, and n and m are the number of measurements in each one of the runs carried out. In order to determine the model of best fit for the attrition kinetics in a conical spouted bed reactor, the data obtained for sand I at different velocities were fitted to both models. Fig. 11 shows the results for two air velocities, 48 m/s (Fig. 11(a)) and 54 m/s (Fig. 11(b)). By comparing the experimentally measured bed weights with those calculated using the equations, it is clear that both models are acceptable for predicting attrition kinetics in spouted beds, but the one based on that proposed by Lee et al. (1993) fits better the trend for long time runs due to its exponential term. Consequently, the model developed by Lee et al. (1993) was fitted to the experimental results obtained for different sand types, gas velocities and bed heights to compare experimental and calculated evolutions of
bed weight as shown in Fig. 12(a) for initial bed weight of 6 kg, and in Fig. 12(b) for 4.5 kg, both showing how faithfully the model predicts the experimental evolution of bed weight, with the attrition parameters k0 = 6.77 × 10–3 h–1 and k1 = −0.068 (m2 /s2 ). Fig. 13 compares experimental data for bed weight with those calculated using Lee’s model to show the adequacy of the model proposed. Despite satisfactory prediction of the model it should be noted that, similar to other attrition models, it is highly dependent on the geometric factors determining the hydrodynamic performance in the contactor. Moreover, the physical properties of the sand also play a significant role on the attrition process. Consequently, the attrition parameters calculated in this paper should be extrapolated with care to other experimental conditions.
5. Conclusions Two attrition kinetic models proposed by Gwyn (1969) and Lee et al. (1993) were fitted to experimental data obtained with two types of silica sand in a spouted bed contactor at room temperature. Both provide acceptable results but the second one predicts better results for long time runs due to the exponential term involved in the equation. The attrition rate constant (Ka ) depends exponentially on the excess of air velocity over that corresponding to the minimum spouting velocity at zero time. The values of the constants k0 and k1 are 6.77 × 10–3 h–1 and −0.068 m2 /s2 , respectively, and they depend on contactor geometry and bed material properties. As inlet air velocity is raised attrition rate increases until bed weight reaches a minimum value (Wmin ) at steady state. Furthermore, the higher the inlet air velocity, the lower the final bed weight. Consequently, in order to maintain a given weight of bed material in the contactor, and therefore the height of the bed, a certain amount of material will have to be added. Particle properties play an important role in the attrition process. Especially noteworthy is the effect of textural properties, that is, sand particles (type II) with sharp edges undergo more severe particle breakage than the more rounded. A sharp reduction in attrition has been observed when operating with a draft tube. The insertion of an internal device avoids the incorporation of particles into the spout at different heights and, consequently, avoids violent impacts in this region. Furthermore, attrition decreases as temperature is increased, which is attributed to the less vigorous gas–solid contact and the resulting reduction in particle breakage.
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