Bed-to-surface heat transfer in conical spouted beds of biomass–sand mixtures

Powder Technology 283 (2015) 447–454

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Bed-to-surface heat transfer in conical spouted beds of biomass–sand mixtures Juan F. Saldarriaga a,⁎, John Grace b, C. Jim Lim b, Ziliang Wang b, Nong Xu b, Aitor Atxutegi a, Roberto Aguado a, Martin Olazar a a b

Deparment of Chemical Engineering, University of the Basque Country UPV/EHU, PO Box 644, E48080 Bilbao, Spain Deparment of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC V6T 1Z3, Canada

a r t i c l e

i n f o

Article history: Received 27 April 2015 Received in revised form 28 May 2015 Accepted 30 May 2015 Available online 9 June 2015 Keywords: Heat transfer Spouted bed Sawdust Sand Conical Biomass

a b s t r a c t The effects of superficial gas velocity, bed composition and other operating parameters on bed hydrodynamics and bed-to-surface heat transfer were investigated the influence of bed height and gas velocity sawdust, sand and 30–70, 40–60 and 50–50 mixtures of biomass and sand in a conical spouted bed. Experiments were performed at minimum spouting velocity and 10 and 20% above this. At each superficial velocity, experiments were conducted at three heights, 0.1, 0.2 and 0.3 m above the inlet. A specially designed heat transfer probe measured the bed-to-surface heat transfer coefficient. The heat transfer coefficient increased from the wall to the spout, and also was observed to increase with increasing percentage of biomass in the blend. The experimental results are compared with published results, showing good agreement with the influence of height of the bed and gas velocity. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The spouted bed was developed by Mathur and Gishler for drying of wheat [1] and then applied in multiple processes. Spouted beds have been used in drying suspensions, solutions, and pasty materials [2–7]. Chemical reaction applications, such as catalytic polymerization [8], coal gasification [9,10], and waste pyrolysis [11–13], have also been under development. Since the 1990s, there has been considerable interest in the hydrodynamic regimes of conical spouted beds [14]. Although spouted beds have been widely used and studied during the last twenty years, expansion of laboratory scale units to industrial sizes is still a challenge, due to the instability in the expansion and operation with fine materials [15,16]. An important limitation is that there is a maximum spoutable bed height [16–18] which is usually controlled by one of three mechanisms: fluidization of the top annulus, choking of the spout, or growth of instabilities [18]. Another challenge is the formation of dead zones in the outer annulus ring of large columns. When operated in a continuous mode, heat transfer plays an important role in processes such as drying, pyrolysis and combustion [18,19]. For spouted beds, heat transfer is dependent on the character of the flow of particles in multiple zones [20]. Englart et al. [20] and Kmiec and Englart [21], assumed that spouted beds consist of two parts: spout and annulus. They argue that heat ⁎ Corresponding author. Tel.: +34 946015994; fax: +34 946013500. E-mail address: [email protected] (J.F. Saldarriaga).

http://dx.doi.org/10.1016/j.powtec.2015.05.046 0032-5910/© 2015 Elsevier B.V. All rights reserved.

transfer coefficients should be calculated separately for each region, taking into account the structure of the bed, although it is problematic to specify perfectly the boundary between zones. Spouted beds are typically used for drying, so that studies of heat transfer reported in the literature are mainly centered on that process [22–26]. Several authors have developed mechanistic models for heat transfer in spouted bed drying, requiring sophisticated numerical and mathematical methods for solving systems of differential equations [4, 21,27–30]. Similarly, the influence of temperature on spouted bed hydrodynamics in cylindrical columns has been studied by a number of authors [1,16,31,32]. Zabrodsky and Mikhalik [33] measured heat transfer coefficients using silica gel particles of 4 mm diameter and obtained values in the 190–260 W/m2 K range. Freitas and Freire [18] measured these coefficients for wall-to-bed transfer and obtained values below 100 W/m2 K for a draft tube conical spouted bed with glass beads. Makibar et al. [16] evaluated heat transfer coefficients for wall-to-bed transfer and obtained values around 170 W/m2 K for pyrolysis conditions, whereas, for an immersed sphere, values around 220 W/m2 K were obtained with silica sand of diameter 1.05 mm. Although some studies have reported the radial distribution of the concentration of solids, the relation between the average solids concentration of the cross-section and the local concentration of solids at the wall depends on the geometric configuration. Makibar et al. [16] recommend that more attention be paid to the annular region where particles spend most of their time. Knowing the wall-to-bed heat transfer coefficient is essential when part of the heat of reaction is supplied through

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the reactor wall, although particle convection also makes an important contribution. Previous studies evaluating heat transfer in spouted bed mostly made measurements using thermocouples located at different axial and radial positions in order to measure temperature changes within the reactor. Similarly, the air temperature was measured at the inlet of the system, to evaluate the changes in the various sections of the reactor and thereby determine the best correlations [16,20,22,34]. This kind of system is effective, but not accurate in determining the wall-to-bed heat transfer, whereas in fluidized beds different probes have been used, allowing measurements which are easier and more accurate. Different materials have been used for the probe, including copper, brass, stainless steel and bakelite. The probe is small in order to improve accuracy [35–38]. Other authors have worked with commercial probes [39–42]. Some authors have used platinum because it has better electrical properties, thus improving both heat flux measurement and probe temperature. Wu [43] developed a platinum film coated on a glass support to satisfy the above requirements. He designed a system that maintained a constant probe temperature and a less complex analog circuit than reported by other authors [44,45]. This probe successfully measured heat transfer in a circulating fluidized bed [43,46,47], proving that the heat transfer coefficient is determined by the arrival of particles at the probe. Not only did the results help elucidate the mechanisms of heat transfer, but they also provided valuable information on the hydrodynamics of circulating fluidized beds. There is little published literature with respect to heat transfer in conical spouted bed, so this work aims to evaluate the heat transfer coefficient from wall-to-bed, based on the methodology of Wu [43], but with platinum replaced by palladium, in order to improve the precision and accuracy of the data. Because of the importance of biomass for reducing greenhouse gas emissions, the tests were conducted with sawdust particles, both on their own and mixed with sand. 2. Experimental 2.1. Heat transfer measurement instrumentation For measuring the heat transfer coefficient, a probe and a control circuit were designed to maintain the probe at a constant temperature, in accordance with the previous work of Wu [43]. The heat transfer was measured based on the power required to hold the probe temperature constant. Other studies have used similar heat transfer probes, showing disadvantages as the temperature was not maintained constant or a complex analog circuit was needed [44,45]. Wu [43] improved the conditions, using simple digital control to maintain the probe temperature constant. Power Supply

Manufacture of the heat transfer probe included a thin palladium film deposited by electroless plating onto a glass disk of approximately 10 mm diameter and 1 mm thickness. In order to make accurate local heat transfer measurements, it was necessary that both the surface area of the probe and its mass be small. Both the heat flux from the probe and its temperature are determined with relative ease and high accuracy. Fig. 1 shows the circuit designed to maintain the probe temperature constant. It is connected at one end to a programmable direct current power supply (BK Precision model 1696) and at the other to a resistor of known resistance. Current from the power supply passes through the probe and the reference resistor before it is grounded. This causes the palladium film to heat up like any resistance heating element. Voltages before and after the probe, V1 and V2 respectively, are data-logged (at a frequency of 20 Hz) using an AD–DA interface card (Tecmar Labmaster TM-40) connected to a personal computer. Since the reference resistance, Rf, is known, the current passing through the circuit, I, can be calculated from I¼

ð1Þ

The resistance of the probe, Rpb, is given by Rpb ¼

ðV1 −V2 Þ I

ð2Þ

which, using Eq. (1), becomes Rpb ¼ R f

ðV1 −V2 Þ V2

ð3Þ

The probe is mounted at one end of guard heater consisting of a 57 mm long aluminum rod, 22 mm in diameter (Fig. 2). The heating element is a cartridge heater (High Density Cartridge Heater HDC00030) inserted into the middle of the rod from its other end. The temperature of this guard heater, measured by a thermistor, is maintained slightly lower than that of the probe. This minimizes and stabilizes heat loss from the back of the probe. It also stabilizes and limits the temperature variations of the glass support. During our experiments, the average temperatures of the probe and guard heater were maintained at 83 °C and 80 °C, respectively. The small difference was needed to maintain the stability of the controller. Allowance was made for this temperature difference in determining the heat transfer coefficient. The temperature of the probe, Tpb, can be obtained from the resistance of the probe which varies nearly linearly with temperature. Fig. 3 shows a typical plot of the variation of probe resistance with probe temperature, obtained by measuring the resistance of the probe immersed in a water bath at different temperatures. Tpb is controlled

H.T. Probe

Resistor

Rf

Rpb

V1

V2 Rf

Ground

V2

A/D-D/A Interface

Computer

Fig. 1. Schematic of circuit for controlling the temperature of the heat transfer probe.

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Fig. 2. Front and section views of probe assembly consisting of plastic film, palladium-coated glass, and guard heater. All dimensions are in mm.

by varying the voltage drop across the probe circuit through the programmable power supply. The computer continually monitors the voltage drop across the probe and computes the probe temperature. If the probe temperature is lower than the set point, the probe resistance will also be lower. The computer, employing simple feedback control action, then increases the programming voltage to the power supply, in turn raising the voltage across the probe circuit. This increases the power to heat the Pd film, restoring its temperature to the set point. The palladium film functions simultaneously as both a heater element and a temperature sensor. At any instant, the power dissipated by the film is given by q ¼ IðV1 −V2 Þ

ð4Þ

which, using Eq. (1), can be rewritten as q¼

V2 ðV1 −V2 Þ Rf

ð5Þ

6.4

Resistance (Ω)

6.2 6.0

The heat transfer coefficient can then be obtained from q  hi ¼
A Tpb −Tsusp

where A is the film surface area and Tsusp is the suspension temperature. 2.2. Experimental unit The experimental spouted bed column is shown in Fig. 4. The unit was constructed of polymethyl methacrylate with included cone angle, γ = 28° and an inlet diameter, D0 = 0.04 m. Other dimensions were as follows: Column diameter, Dc = 0.304 m; static bed height, H0 = 0.10, 0.20 and 0.30 m; probe center height z = 0.10, 0.20 and 0.35 m and Di = 0.06 m. Sawdust, sand and mixtures of sand and sawdust were used for the heat transfer study. The Archimedes number was estimated at room temperature. Key properties are provided in Table 1. The moisture content was measured following the ISO-589 standard using a halogen moisture analyzer (HR83, Mettler Toledo). Particle density was measured by a mercury porosimetry [48], and the average particle size (mean reciprocal diameter) was calculated from: 1 dp ¼ X

Xi dpi

5.8 5.6

ð6Þ

ð7Þ

5.4

3. Results and discussion 5.2 5.0 40

50

60

70

80

90

100

Temperature (°C) Fig. 3. Variation of electrical resistance of heat transfer probe as a function of probe temperature.

The heat transfer probe was used to obtain heat transfer coefficients at three heights and three radial positions in the conical spouted bed reactor. Measurements were made at three gas superficial velocities: minimum spouting velocity and 10 and 20% above this. The three axial levels were located z = 0.1, z = 0.2 and z = 0.35 m above the base of the reactor. The radial positions at z = 0.1 m height were at the wall

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DC

Table 2 Air velocities based on inlet cross-sectional area (0.008 m2) tested in the experiments (m/s). Material

Static bed height

ums

Sawdust

0.1 m 0.2 m 0.3 m 0.1 m 0.2 m 0.3 m 0.2 m 0.2 m 0.2 m

9.84 10.17 10.48 10.21 11.00 12.49 10.69 10.38 10.28

Sand

Sand 75%–sawdust 25% Sand 60%–sawdust 40% Sand 50%–sawdust 50%

H0

Probe Height

and 4%), and therefore the probe does not significantly alter bed hydrodynamics. Furthermore, visual observation confirmed that the insertion of the probe did not alter the shape or behavior of the fountain. Values of superficial gas velocities for each of the experiments with sawdust, sand and their mixtures are shown in Table 2. Before analyzing the results, it was necessary to establish that the heat transfer signals from the probe were both stationary and ergodic. This was established according to criteria by Bendat and Piersol [49] which require that the different sections of the same time history have similar means and variances within the limit of sampling variance. These conditions were satisfied, so the data can be analyzed using the values from a single continuous signal. Fig. 5 shows typical variations of heat transfer coefficient for the sawdust at three measurement levels on the wall of the reactor for a static bed height of 0.3 m. It is evident that at a greater height, the heat transfer coefficient increased. This was due to the motion of particles, impinging on the wall from the fountain just above the bed surface, then sliding along the inclined conical column surface below the bed surface. Similar behavior has been reported in drying different materials by other authors [18,20].

3.1. Heat transfer for sawdust For the sawdust, the time average heat transfer coefficient is plotted against u/ums at different heights in Figs. 6, 7 and 8. In each case, as the gas velocity increased, the heat transfer coefficient increased, as reported previously by Englart et al. [20] and Kalita et al. [50,51]. The results confirm the strong influence of gas velocity, bed height and overall operating conditions on the conical spouted bed heat transfer. For a bed height of 0.1 m (Fig. 6), values of heat transfer coefficient were higher than for bed heights of 0.2 m and 0.3 m due to greater particle–wall interaction. However, as shown in Figs. 7 and 8 for deeper

D0

Fig. 4. Spouted bed reactor.

and 0.02 m from the wall. For z = 0.2 and z = 0.35 m, the probe was placed at the wall and at 0.01 and 0.03 m from the wall. The volume of the probe is small compared to the volume of the bed (between ~1

Table 1 Properties of solid particles. Properties

Sawdust

Sand

Mean diameter, dp, mm Particle density, ρb, (kg/m3) Bulk density, ρs, (kg/m3) Moisture content, (wt.%, d.b.) Archimedes number, (Ar) Geldart classification

0.76 496 189 9.0 6.7 × 103 D

0.17 2650 1690 9.3 4.0 × 102 A

Instantaneous heat transfer coefficient hi, W/m2K

Di 250 Z=0.35 m

200

150

100 Z=0.20 m

50

0

Z=0.10 m

0

5

10 Time, t, s

15

20

Fig. 5. Typical traces of heat transfer coefficient measured at three heights for sawdust on the wall for H0 = 0.3 m.

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Fig. 6. Heat transfer coefficient at three superficial gas velocities for sawdust. H0 = 0.1 m, probe height z = 0.1 m. Fig. 8. Heat transfer coefficient at three superficial gas velocities for sawdust. H0 = 0.3 m, probe heights z = 0.1, 0.2 and 0.35 m.

beds, the time average heat transfer coefficients at z = 0.1 m above the bottom were similar at the two radial positions. Fig. 7 shows that between u/ums of 1 and 1.2 there were substantial increases in the heat transfer coefficient of 20 to 30 W/m2 K. For a bed height of 0.3 m, the heat transfer coefficient was observed to be homogenous at all three measuring levels. Fig. 8 shows a marked difference in the three radial measuring points, supporting the recommendation by Makibar et al. [16] that more attention should be paid to the annular region where particles spend most of their time. As the particles descend through the annular zone, transferring heat to the wall, coefficients on the surface of the probe ranged from 200 ± 20 W/m2 K at the top to 23 ± 20 W/m2 K near the bottom. At z = 0.1 m, values at the wall and inserted 0.02 m were similar, with a difference of only 6 W/m2 K, while at the other two levels, the heat transfer data were found to exhibit a more significant difference. 3.2 . Heat transfer for sand In the case of sand for a static bed height of 0.1 m, Fig. 9 shows that the heat transfer coefficient differed considerably from the sawdust data. This is due largely to different flow behaviors of the two materials. Sand was found to have a dead zone and irregular fluidization in the

Fig. 7. Heat transfer coefficient at three superficial gas velocities for sawdust. H0 = 0.2 m, probe heights z = 0.1 and 0.2 m.

annular zone, and sometimes even large bubbles. As the gas velocity increased, the coefficient of heat transfer increased significantly. For a bed height of 0.2 m, as shown in Fig. 10, the time average heat transfer coefficient at u = ums reached 308 W/m2 K at the wall, and, when inserted 0.03 m, 396 W/m2 K for a height of z = 0.2 m. It can also be seen that at z = 0.1 m, the time average heat transfer coefficients were much lower than at z = 0.2 m. Fig. 11 shows that the time average heat transfer coefficients exceeded 400 W/m2 K above the bed surface (z = 0.35 m), while at the z = 0.1 and 0.2 m levels, the time average heat transfer coefficient varied less with gas velocity than for the other two bed heights evaluated. This is because spouting for the 0.3 m bed height of sand was more uniform and homogenous. Furthermore, it can be seen that hi for sawdust with a bed height of 0.3 m was less than for sand, because the sand has a thermal conductivity much greater than pine sawdust (0.33 W/m K for sand compared with 0.06 W/m K for sawdust [52]). In Figs. 10 and 11, it is observed in the experiments with sand that the time mean heat transfer coefficient depends on the superficial gas

Fig. 9. Heat transfer coefficient at three superficial gas velocities for sand. H0 = 0.1 m, probe height z = 0.1 m.

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Fig. 10. Heat transfer coefficient at three superficial gas velocities for sand. H0 = 0.2 m, probe heights z = 0.1 and 0.2 m. Fig. 12. Heat transfer coefficient at three superficial gas velocities for 75–25 sand–sawdust mixture, Ho = 0.20 m.

500 ± 20 W/m2 K [53,54]. Likewise, as the gas flow rate increased, the heat transfer coefficient again increased, as reported earlier by Abid et al. [55] and Kim et al. [56]. 3.3. Heat transfer for mixture (sand–sawdust)

Fig. 11. Heat transfer coefficient at three superficial gas velocities for sand. H0 = 0.3 m, probe heights z = 0.1 m, 0.2 and 0.35 m.

In the literature there are few studies of mixtures of biomass with inert materials [50,51,57], especially in conical spouted bed. Figs. 12, 13 and 14 show that as the proportion of sawdust to sand increased, the heat transfer coefficient also decreased. For all cases examined, the time-average heat transfer coefficient increased with increasing superficial gas velocity, due to augmented renewal of particle at the transfer surface as particles traveled more quickly. This behavior is similar to those described by several authors who found that the heat transfer coefficient depends on the hydrodynamic conditions of the bed and the mass flow rate of particles in the vicinity of the heat transfer surface [20,50,51,58]. No previous works could be found in the literature with mixtures of biomass–sand in conical spouted bed reactors, so full comparisons cannot be made. Studies have, however, been reported on pressurized circulating fluidized beds where the optimum sawdust–sand mixture for heat transfer was 15% by weight sawdust, and for bubbling beds with 12% by weight sawdust [50,51,57]. 4. Conclusions

velocity and fluidization conditions of the material. A greater bed height gave more stable hi values. Table 3 compares the spouted bed measured heat transfer coefficients with studies in other types of gas–solid contactors. Coefficients were similar to those found in this study, with maximum values of

The effects of superficial velocity height and particle composition on bed-to-surface heat transfer in a conical spouted bed unit have been studied. Experiments were carried out at three superficial velocities ums, 1.10 ums and 1.20 ums, with sawdust, sand and three blends of sawdust with sand. The heat transfer coefficient was found to be smaller for

Table 3 Heat transfer coefficients in previous studies with sand in gas fluidized beds compared with those from this study for Ho = 0.30 m with probe flush with column surface. dp (μm)

hi (W/m2 K)

Authors

System

307 307 307 240 150 160 240 170

500 ± 20 320 ± 20 450 ± 20 500 ± 20 500 ± 20 450 ± 20 394 100–505

Kalita et al. [53] Kalita et al. [50] Kalita et al. [51] Kim and Kim [54] Abid et al. [55] Zhang and Koksal [59] Kim et al. [56] This work

Pressurized circulating fluidized bed Pressurized circulating fluidized bed Pressurized circulating fluidized bed Pressurized fluidized bed Fluidized bed Bubbling fluidized bed Fluidized bed Conical spouted bed

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ρs: Rf Rpb t T Tsusp Tpb u: ums: V1, V2 z:

453

bulk density, kg/m3 reference electrical resistance, Ω electrical resistance of heat transfer probe, Ω time, s temperature, K suspension temperature, K temperature of the probe, K air velocity referred to the inlet diameter, m/s minimum spouting velocity referred to the inlet diameter, m/s voltages before and after heat transfer probe respectively, V probe center height, m

Acknowledgment Juan F. Saldarriaga is grateful to the UBC Fluidization Research Center for facilitating this work and for a Ph.D. grant from the Administrative Department of Science, Technology and Innovation, COLCIENCIAS (Colombia). Fig. 13. Heat transfer coefficient at three superficial gas velocities for 60–40 sand–sawdust mixture, Ho = 0.20 m.

sawdust than for sand. The heat transfer coefficient increased when traversing the probe from the wall to the core. The bed-to-surface heat transfer coefficients averaged 203 W/m2 K for sawdust, 505 W/m2 K for sand and 262 W/m2 K for a 75–25 by weight mixture. These values are favorable enough for attaining high heating rates in combustion and other processes. Notation Ar Do: DC: dp: γ: Ho: hi: I q ρb:

Archimedes number, gd3pρ(ρs − ρ)/μ2 inlet diameter, m column diameter, m mean diameter, mm cone angle, ° static bed height, m heat transfer, W/m2 K electric current through probe circuit, A power dissipated by heat transfer probe, W particle density, kg/m3

Fig. 14. Heat transfer coefficient at three superficial gas velocities for 50–50 sand–sawdust mixture, H0 = 0.20 m.

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