Distribution of Cycle Times in Sawdust Conical Spouted Bed

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Distribution of cycle times in sawdust conical spouted beds equipped with fountain confiner and draft tube Juan F Saldarriaga, Idoia Estiati, Aitor Atxutegi, Roberto Aguado, Javier Bilbao, and Martin Olazar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03451 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Industrial & Engineering Chemistry Research

Distribution of cycle times in sawdust conical spouted beds equipped with fountain confiner and draft tube Juan F. Saldarriaga1,*, Idoia Estiati2, Aitor Atxutegi2, Roberto Aguado2, Javier Bilbao2, Martin Olazar2 1Universidad

de los Andes, Department of Civil and Environmental Engineering, Cr. 1Este #19A-40, Bogotá, Colombia.

2University

of the Basque Country (UPV/EHU), Department of Chemical Engineering, B. Sarriena s/n, Leioa, España. [email protected], phone +57 1 3394949 ext 1649

ABSTRACT

A study has been conducted on the joint influence of the fountain confiner and draft tube on the solid circulation in a conical spouted bed made up of sawdust. Knowledge of the performance of these fine particles of low density and irregular texture is required in both physical chemical and processes, such as drying, pyrolysis, or combustion of these materials in conical spouted beds. The study has been carried out according to a factorial design of experiments. The cycle times follow different distributions depending on the configuration of the contactor-draft tube-fountain confiner. The spouted beds equipped with only fountain confiner are those with the more uniform distribution and lowest cycles times. Therefore, this device provides high stability and ensures high turbulence to the beds formed by fine and irregular particles, with a behavior similar to that of stable beds made up of coarse particles, in which no internal device is required.

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

KEYWORDS Fountain confiner; conical spouted bed; biomass; nonporous draft tube; open-sided draft tube; hydrodynamic

HIGHLIGHTS 

The fountain confiner provides a great versatility in the cycle times



Cycle times have been determined for a conical spouted bed with fountain confiner



The cycle times for contactor-fountain confiner configurations are more uniform



The draft tube and fountain confiner provide high turbulence to the beds



The fountain confiner reduces short cycles and provides stability

INTRODUCTION

The use of biomass as a fuel for energy production has been intensively investigated for two or three decades, generating positive impacts on the environment, such as reduction of climate change. Based on the type of raw material used and the conversion technology applied, two main biofuel categories are contemplated, as are first and second generation biofuels, with a third potential generation based on specific crops, such as algae, being also increasingly considered1,2.

Currently, the challenge is associated with the production of biofuels and bio-based chemical products in an eco-efficient way1,3,4. Since the products and by-products are very numerous and diverse, a simple approach is required to estimate the economy of the production and the relative viability of the various production alternatives and routes. It is known that biomass mainly composed of hemicellulose, cellulose and lignin, can be decomposed at a temperature range


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between 225-325, 305-375 and 250-500 ºC, respectively5–8. Nowadays, the development of integrated processes that value the entire biomass, seeking to avoid the production of waste, has been promoted worldwide. Among the technologies used are the conversion of biomass to biooil, biochar, synthesis gas and others, which require thermochemical processes, such as roasting, carbonization, thermal liquefaction, pyrolysis and gasification. These processes receive great attention, with the challenge being to improve the quality of the products and scale up the process, which are essential requirements from the implementation of an industrial process2,3,5,9.

The combustion of biomass has become the main bioenergy route, which is responsible for 90% of the global contribution of bioenergy. The selection and design of a biomass combustion system is mainly determined by the characteristics of the fuel used, the local environmental legislation, the costs and equipment performance or availability, as well as the energy or capacity needed (heat, electricity)10. The fluidized bed technology is the best one to burn a fuel of low quality, high ash content and low heating value. Conical spouted bed reactors have proven to perform well at pilot plant, especially in applications that require physical and chemical treatment of biomass11–13. This is because it can achieve a good intensive gas-solid contact, which is an uttermost factor for biomass valorization by thermochemical processes1,10,14,15.

Thus, recent applications are those involving physical processes, as are: drying of seeds, pharmaceutical powders and biomaterials16–18, drying19,20, granulation21, coating22–24, heat carrier25–27 and solid mixing28. The applications in chemical processes include gasification29, combustion30–33 and pyrolysis34–36. Similarly, different modifications have been made to the original spouted bed in order to improve its performance, as are those involving the geometry of the contactor and the entrance of the gas to the bed. The aim of these modifications has been to



increase the capacity for handling sticky particles, decrease pressure drop, improve the cyclic movement of the particles and keep stable operation over a wide range of velocities37–41. Another modification to the system has been the insertion of draft tubes of different configuration (solid, porous and open-sided), with the aim being to allow operation with fine particles, precise control of the gas and solid residence times, improve the hydrodynamic performance and allow operation in a wide range of gas flow rates37–39,42,43. Recently, the spouted bed has been provided with a fountain confiner, which has been developed by our research in order to allow stable operation and reduce the fountain with fine and irregular materials and, especially, avoid the entrainment of fine particles44.

In order to determine the efficiency of each of these devices and improvements implemented in the spouted bed, different studies have been conducted, with one of special relevance being that aimed at the determination of the time needed by the particle to travel from the bed surface to the bottom and back again to the surface of the bed45–47. Saldarriaga et al.43 described the dependency on the solid circulation flow rate of the operating variables, such as diameter of the draft tube, particle size, entrainment zone length and spouting gas flow rate. Likewise, they determined the particle cycle time and the trajectory of particles for highly irregular materials, such as biomass, which is an essential information with the perspective of improving process efficiency. Altzibar et al.45 measured cycle times in spouted beds equipped with internal devices for beds made up of glass beds, and they found that cycle times (average, maximum and minimum) and solid circulation flow rates were influenced by the type of draft tube and its geometry. Furthermore, Saldarriaga et al.43 analyzed a spouted bed made up of pine sawdust particles provided with internal devices. They found a bimodal particle size distribution, in which one peak is due to short cycles in the upper section of the bed surrounding the spout and the other one to long cycle


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times followed by the particle from the upper surface to the bottom along the annulus and again to the bed surface through the spout. This bimodal distribution is caused by the formation of a crater on the surface of the bed.

There are no studies in the literature involving the use of a confiner for biomass treatment. In fact, this device has been developed by our research group in order to attain stability and improve the hydrodynamics of highly irregular texture materials, such as biomass particles, in conical spouted beds. Therefore, cycle times and particle trajectories will be determined in this study for a conical spouted bed provided with fountain confiner, with the aim being the reduction of operating costs and increase of operating capacity. Accordingly, a study has been conducted of the influence different geometric factors of the contactors, fountain confiners and internal devices have on particle cycle time (average, maximum and minimum), and therefore on the solid circulation flow rate. In fact, the solid circulation flow rate is inversely proportional to the average cycle time.

EXPERIMENTAL The experimental unit has been described in previous papers49,50. It consists of two blowers connected in parallel that supply a maximum air flow rate of 300 N m3 h-1 at a pressure of 1500 mm of water column. The flow rate is measured by two computer-controlled mass flow-meters in the ranges 0-30 m3 h-1 and 30-250 m3 h-1. The accuracy of this control is 0.5% of the flow rate measured.

The experimental unit main component is the conical contactor (Figure 1). The unit was operated with contactors of polymethyl methacrylate of different cone angles,  = 28, 36 and 45º, with the



inlet diameter being D0 = 0.04 m. The column diameter is Dc = 0.36 m and the static bed height H0 = 0.30 m.

Dc

γ/2 Ho

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Di

Do Figure 1. Geometric factors of the conical contactors.

Drafts tubes of different configurations have been inserted in the contactors, Figure 2. They are located along the axis of the contactor and fixed at its bottom, Figure 3. Their performance has already been studied for beds of sand and glass beads44,50. Two types of tubes have been used in this hydrodynamic study: nonporous draft tube, Figure 2a and open-sided draft tube, Figure 2b. In the case of the nonporous tubes, the main parameters governing hydrodynamics are the entrainment height (distance between the gas inlet nozzle and the lower end of the tube), LH, and the tube diameter, DT. The length of the tube is approximately the same as the bed height (Figure 2a). ACS Paragon Plus Environment

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The open-sided draft tubes have part of their lateral surface area opened in order to allow solid cross-flow from the annulus into the spout and gas diversion from the spout into annulus along the whole length of the spout (Figure 2b). They have been designed and studied in detail by Altzibar et al.51 and their main design parameters are the width of the faces (related to the aperture ratio), WH, and the tube diameter DT. The length of this tube is 0.50 m, which is 0.20 m above the bed surface. This length has been chosen according to previous experimentation with fine particles (lower than 1 mm)50,51, in which lower and denser fountains were observed when the tube end was above the bed surface (Figure 2b).

Figure 2. Geometry of the draft tubes. (a) nonporous draft tube and (b) open-sided draft tube



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The fountain confiner is made of polymethyl methacrylate and has a cylindrical shape, Figure 3. Three different fountain confiners have been used, with their diameters (DFC) being 0.15, 0.20 and 0.25 m, and they have denoted as D15, D20 and D25, respectively.

DFC HF

a

b

Figure 3. 3-D representation of the spouted bed with draft tube and fountain confiner (a). Location and factors of the fountain confiner in a 2-D representation: diameter of the device (DFC) and the distance between the bed surface and the lower end of the device (HF).

As determined by Altzibar et al.44, an essential factor for a suitable hydrodynamic performance is the distance between the bed surface and the lower end of the confiner, HF. The cycle time study has been conducted using only one value of this distance 0.09 m, which, based on preliminary runs, is the one for best performance, with pressure drop and air flow rate being minimum for this value. Likewise, high values of HF (the confiner located far from the surface), the effect of the


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device is not significant because it cannot confine the fountain and fine particles are entrained from the contactor with the gas.

Sawdust has been used for the hydrodynamic study. It is a Geldart’s group A material and its main properties are shown in Table 1. The moisture content has been measured following ISO589 standard using a halogen moisture analyzer (HR83, Mettler Toledo). Figure 4 shows the particle size distribution of the sand obtained by sieving (ISO-3310). Given that a direct measurement of the shape factor, ϕ, is not possible, it has been estimated based on bed voidage, according to the procedure described by Brown and Richards52. Particle density has been measured by mercury porosimetry53 and the average particle size (mean reciprocal diameter) determined according to the following expression:

dp =

1 Xi d pi

(1)



Table 1. Properties of the solid used Properties

Sawdust

Mean diameter dp (mm)

0.76

Density s (kg m-3)

496.4

Density b (kg m-3)

188.67

Moisture content (%)

8.96

Shape factor 

0.52

Archimedes number (Ar)

6.69 103

Geldart classification


A


70 60 50 Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0 0,5

1

2 3 Particle size (mm)

4

5

Figure 4. Particle size distribution of the sawdust.

The cycle time of the particles was measured by monitoring an isodromic particle of expanded polystyrene, whose hydrodynamic performance in the fountain is similar to sawdust particles. The size of the particle was chosen in order to have the same free-falling velocity as the biomass particles, and therefore the trajectories followed by the polystyrene particle in the fountain are similar to those by biomass particles43. This fact has been proven by monitoring the particles with a high speed camera (AOS technologies AG). Furthermore, visual observation through the wall clearly shows that the polystyrene particle descends along the annulus at the same velocity as the biomass particles, i.e., there is no particle percolation in the annulus. Therefore, the polystyrene particle follows the same trajectories as the biomass ones in the cycles described in the bed.

Thus, a 5 mm sphere of white expanded polystyrene (density 90 kg m-3) was visually monitored. This particle was clearly observed in the fountain and so the time between two successive


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appearances in the fountain is the time spent for describing a cycle. Furthermore, a series of preliminary runs have been carried out with the different draft tubes to ensure that the particle behaves as the other ones in the bed. Subsequently, runs have been carried out and 120 subsequent cycles times have been measured in each one. This number of cycles is the minimum required for a representative sample, which will be used to plot a suitable cycle time distribution function54. A statistical package (SPSS-21) has been used to draw the distribution and calculate the parameters. The maximum cycle time has been measured by monitoring a particle travelling along the wall from the bed surface to the bottom of the contactor. This is possible due to the transparent wall of the polymethyl methacrylate contactor.

The runs have been carried out following a design of factorial experiments in order to find the factors of greater influence on the cycle times. In addition, it also allows to draw conclusions about the effect of the angle of the contactor, the type of draft tube and the size of the fountain confiner on the sawdust hydrodynamics.

Tables 2, 3 and 4 show the factors and levels used in the experimental design involving both types of draft tubes and without draft tube.

Table 2. Factors and levels for the systems with nonporous tubes Factors Cone angle, γ (deg) Fountain Confiner diameter, DFC (m)

Levels 45

36

0.15

0.20


28


Static bed height, H0 (m)

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0.3

Draft tube diameter, DT (m)

0.053

Entrainment zone height, LH (m)

0.075

Minimum spouting velocity, ums (m s-1) 19.89 7.95 5.74

Table 3. Factors and levels for the systems with open-sided tubes Factors Cone angle, γ (deg)

Levels 45

36

Fountain confiner diameter, DFC (m)

0.15

0.20


0.3

Draft tube diameter, DT (m)

0.053

Width of the face, WH (m)

0.012

0.022

77

60

Aperture ratio % Minimum spouting velocity, ums (m s-1)

22.10

28

10.61 7.95

Table 4. Factors and levels for the systems without draft tube Factors Cone angle, γ (deg)

Levels 45

36

Fountain confiner diameter, DFC (m)

0.15

0.20


0.3

28

Minimum spouting velocity, ums (m s-1) 10,61 15.91 7.95


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The draft tubes used in this study are those chosen by Saldarriaga et al.39 for best performance, i.e., those that avoid large bubble formation or material blockage on the wall of the contactor, which, under certain conditions, are phenomena that cannot be avoided even with the fountain confiner.

RESULTS AND DISCUSSION Smooth spouting without any dead zone is attained when the sawdust beds are treated in a conical spouted bed provided with a confiner. Figure 5 shows the evolution of pressure drop with air velocity for a given system. As observed, the characteristic curve is very well delineated, as happens in beds made up of ideal spherical particles, which is an indication of the high spouting quality. Furthermore, a low operating pressure and high peak pressure drop are observed, which are typical features for light particles. 800 Increasing air velocity

700

Decreasing air velocity

600

Pressure drop (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500 400 300 200 100 0 0

1

2

3

4

5

6

Air velocity (m/s)


7

8

9

10


Figure 5. Evolution of pressure drop with air velocity for sawdust beds in a conical spouted bed with fountain confiner. System: H0 = 0.30 m, D0 = 0.05 m,  = 28°, DT = 0.053, DFC = 0.20 m, nonporous tube.

The fountain confiner clearly changes the particle circulation pattern, and the bimodal distribution (short and long cycles) observed by Saldarriaga et al.43 no longer occurs. The fountain confiner helps to stabilize the fountain with no crater formation on the upper surface of the bed, which in turn avoids the short cycles in the system. This is because the confiner collects the particles arriving to the fountain and avoids lateral fluctuations of the gaseous stream. In fact, the confiner straightens the spout, leading to a high upward circulation of the solid in the spout and its uniform distribution on the annular zone. In order to have a representative sample for statistical inference, 120 cycles have been taken in each run, with the runs carrying out in triplicate. The cycle times determined have been plotted in histograms, which allowed visualizing their distribution and determining the average cycle time.

Figure 6 shows as an example the distribution of cycle times determined for three types of draft tubes and without draft tube, for a given configuration provided with a fountain confiner of 0.20 m diameter. As observed, the distribution of cycle times without internal device, Figure 6a, ranges from 20 to 70 s and is more uniform than the others. This is specially preferred in any operation, because it is an indication of the absence of dead or quasi-dead zone in the bed. Furthermore, short and uniform cycles are the ideal situation when operating in a spouted bed. Draft tubes with high aperture ratios (70%) are the best internals to lead to these types of cycle times and nonporous draft tubes are the worst. The latter occurs because the descending velocity of biomass particles at a given cross-section in the annulus differs greatly. Thus, particle ACS Paragon Plus Environment

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velocities peak at an intermediate position in the annulus, and are much lower on both the contactor wall and the draft tube external wall. The distribution for the open-sided tube follows an exponentially decreasing function, with the distribution range being shorter as the aperture ratio is reduced from 77% to 60%, Figures 6b and 6c. Furthermore, the decreasing trend of the function is more pronounced as the aperture ratio is lower, i.e., the number of particles describing 10-20 s cycles is much higher. When the nonporous draft tube is used, Figure 6d, the trend is more similar to those for the open-sided tubes than for those without tube, but the distribution peaks for a cycle time of 30 seconds. Similar trends have also been observed by other authors43,45, who reported that the draft tube helps to stabilize the system and improve fluidization. Nevertheless, short cycles (10-20 s) prevail when the open-sided tubes (60 and 77% aperture ratio) are used, whereas those in the 30-40 s range prevail when nonporous tubes are used. These trends are explained by the fact that nonporous tubes force the particles to perform longer trajectories from the top to the bottom of the bed following different paths. 80

80

a

70

70

60

60

No. of observations

No. of observations

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 40 30

b

50 40 30

20

20

10

10 0

0 0

10

20

30

40

50

60

70

80

0

10

20

30

40

50

Time (seconds)

Time (seconds)


60

70

80


80

80

c

70

d

70 60

No. of observations

60

No. of observations

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 40 30

50 40 30

20

20

10

10

0

0 0

10

20

30

40

50

60

70

80

0

10

Time (seconds)

20

30

40

50

60

70

80

Time (seconds)

Figure 6. Cycle time distributions. System: H0 = 0.30 m, D0 = 0.05 m,  = 36° DT = 0.05 m, DFC = 0.20 m. (a) Without draft tube. (b) Open-sided draft tube with 77% aperture ratio. (c) Opensided draft tube with 60% aperture ratio. (d) Nonporous draft tube.

Figure 7 shows the average cycle times for the three contactors of angles 28°, 36° and 45°. All the runs have been carried out in contactors provided with fountain confiner and using different types of draft tubes and without draft tube. As observed, the average cycle times in the contactors with only fountain confiner (no draft tube) are lower than those in the contactors with internal devices. Concerning the internal devices, as the aperture ratio of the open-sided tube is decreased, cycle times increase, and the longer times correspond to the nonporous tube. This is explained by the fact that particles are forced to perform longer trajectories as the aperture ratio is lower, and the longest trajectories correspond to the nonporous tube. In this case, the particles must go down through the annular zone to the bottom of the contactor and then ascend through the spout to the fountain.


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80 70 60

Time (seconds)

50 40 30 20

Without Open- Open- Non tube sided sided porous 77% 60% 28º

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D20

D15

D20

D15

D20

D15

0

D20

10 D15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Without Open-sided Open-sided Non porous Without Open-sided Open-sided Non porous tube 77% 60% tube 77% 60% 36º

45º

Figure 7. Average cycle times for the all the systems studied.

Figure 8 shows the overall average cycle times and the average cycle times along the wall for all the systems studied. As observed, there is not a great different between the cycle times along the wall and the overall ones, which is an indication of the great positive influence of the confiner by improving the solid circulation along the wall. It should be noted that using only the confiner there is a dramatic improvement in bed performance by avoiding bubble or dead zone formation in the annulus. Furthermore, the shortest cycle times are attained using only the confiner and the draft tubes allow increasing this time, with the highest values being attained with the nonporous tube. Therefore, the operation with only a confiner is enough for attaining stability with high solid circulation flow rate.



80 Wall average

70

Overall average

60

Time (seconds)

50 40 30 20

Without Open- Open- Non tube sided sided porous 77% 60% 28

Without tube

Open-sided Open-sided Non porous Without 77% 60% tube 36

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D20

D15

D20

D15

D20

D15

0

D20

10 D15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Open-sided Open-sided Non porous 77% 60% 45

Figure 8. Average cycle times along the wall and overall average cycle times.

A statistical analysis using SPSS-21 package has been carried out to the results obtained by the experimental design. The factors and levels used in the experimental design and analysis are shown in Tables 2, 3 and 4 and are follows: Contactor angle (28, 36 and 45º), draft tube (no tube, open-sided tubes of 60 and 77% aperture ratio, and nonporous tubes) and confiner (0.15, 0.20 and 0.25 m in diameter). The analysis of the variance (ANOVA) of the results allows ascertaining the parameters and the binary interactions of highest significance. As observe in Table 5, the significance order of the main factors and interactions for a 95% confidence interval is as follows:  > draft tube > fountain confiner >  - draft tube >  - fountain confiner. Therefore, the three main factors analyzed are significant, i.e., have an influence on cycle times. Contactor angle is the factor of greatest influence and confiner diameter the one of lowest influence. It should be noted that no run without confiner has been considered because this device is essential for a


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suitable bed performance. The use of a fountain confiner reduces particle dispersion in the fountain. Consequently, this changes the radial flow of particles in the cone column. As the cone angle is decreased, the action of gravity in the direction of particle movement increases, reducing the cycle time. The interaction between the draft tube and fountain confiner is not significant for a 95% interval, which means that the influence of the draft tube on the cycle times does not depend on the size (diameter) of the confiner. Nevertheless, the other two interactions, namely, angle-draft tube and angle-confiner are significant, which means the influence of the angle depends on the type of internal device and on the confiner size, respectively. That is, the influence of the confiner and draft tube is different depending on the angle. In fact, when the contactor angle is small (28º), the annular zone is rather small and the dimensions of the confiner and type of draft tube hardy affect the descending trajectory, and therefore the cycle time. The reverse is true for greater angles.

Table 5. Analysis of variance to the average cycles corresponding to all the systems Source

Sum of squares

df

Mean squares

F

P-value

Corrected model

8154.012a

22

370.637

25.804

0.000

Intersections

20535.453

1

20535.453

1429.688

0.000

A. Angle

2276.359

2

1138.180

79.241

0.000

B. Draft tube

3289.904

3

1096.635

76.348

0.000

C. Fountain confiner

1244.681

2

622.341

43.328

0.000

721.677

6

120.279

8.374

0.003

MAIN EFFECTS

INTERACTIONS AB



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AC

291.824

3

97.275

6.772

0.011

BC

252.724

6

42.121

2.932

0.072

ERROR

129.272

9

14.364

TOTAL

32185.556

32

TOTAL CORRECTED

8283.284

31

a. R square = 0.984 (R set square = 0.946)

Figure 9 shows the average cycle times vs. the contactor angle for the different configurations of the fountain confined spouted bed (without tube and with different tubes). Each graph corresponds to a different fountain confiner (Figure 9a to 0.15 m diameter, Figure 9b to 0.20 m and Figure 9c to 0.25 m). It is clearly observed that all the main factors have great influence on the average cycle time. Furthermore, the systems without tube lead to the lowest cycle times with the confiners of 0.15 and 0.20 m (Figures 9a and 9b), but the open-sided ones (Figure 9c) lead to the lowest ones when the confiner of 0.25 m is used, which is probably related to the great stability ensured by the latter. In fact, operation with open-sided tubes and wide confiners is the closest situation to operation without any device. Overall, the fountain confiner of 0.20 m diameter gives way to the best performance, with the shortest cycle times being obtained operating with this device and without any draft tube.


Page 21 of 32

70

a

Cycle time (s)

60 50 40 30 20

Non porous Open-sided 60% Open-sided 77% Without tube

10 0 25

30

35

40

45

Angle  (º) 60

b

50

Cycle time (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 30 20


10 0 25

30

35

Angle  (º)


40

45


50

c

45 40

Cycle time (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 32

35 30 25 20 15


10 5 0 35

37

39

41

43

45

Angle  (º) Figure 9. Effect of angle on the cycle times for the four systems studied (without tube and with different tubes). Experimental conditions , 28, 36 and 45º. (a) DFC = 0.15 m, (b) DFC = 0.20 m, (c) DFC = 0.25 m.

Figure 10 shows the maximum and minimum cycle times for all the systems studied. As observed, the longer cycles are associated with combination of the nonporous draft tube and fountain confiner in a contactor of 45º angle. Likewise, as the diameter of the fountain confiner is increased the maximum cycle time decreases, and a more uniform distribution is attained, as also shown in Figure 7. It is also evident that the maximum and minimum cycle times are the smallest for the systems with only the fountain confiner, and therefore spouted beds equipped with only the confiner are suitable for the vigorous treatment of these irregular materials. Nevertheless, a nonporous tube should be used when long cycles are required. Therefore, a way to control the cycle time is the use of open-sided tubes of different aperture ratio.


Page 23 of 32

160

Maximun cycle

140

Minimun cycle

120 100 80 60 40

Without Open- Open- Non sided sided porous 77% 60%

Without

Open-sided Open-sided Non porous 77% 60%

28

36

Without

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D25

D20

D15

D20

D15

D20

D15

D20

D15

0

D20

20 D15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Open-sided Open-sided Non porous 77% 60% 45

Figure 10. Maximum and minimum cycle times for the systems studied.

CONCLUSIONS

A conical spouted bed equipped with draft tube, fountain confiner or their combination performs much better than a plain conical spouted bed in the treatment of biomass (sawdust) beds made up of fine and/or low-density particles of irregular texture. The results obtained based on an experimental design show that particle cycle times are influenced by bed and confiner geometry, as well as the draft tube configuration.

The cycle time distributions for a contactor equipped with a fountain confiner are more uniform than those for a contactor equipped with a fountain confiner and draft tube, with the latter having exponentially decreasing trends with most cycle times being short and a few ones long. This



trend is explained by the trajectories followed by the particles in each case. Thus, open-sided draft tubes force the particles to perform very short cycles close to the surface of the bed and long ones from the surface to the bottom of the bed, whereas there is a solid cross-flow from the annulus into the spout all over the spout when there is no draft tube. In the case of nonporous tubes, all the particles are force to travel from the surface to the bottom, but they follow different trajectories depending on the radial position (spout outer surface, contactor wall or intermediate radial positions), which explains the peak observed in the distribution. Therefore, the fountain confiner ensures stable spouting and both cycle time distribution and the average cycle time may be controlled by using a suitable draft tube configuration.

ACKNOWLEDGEMENTS

This work has received funding from Spain’s Ministry of Economy and Competitiveness of the Spanish Government (CTQ2016-75535-R (AEI/FEDER, UE)). J.F. Saldarriaga thanks the Universidad de los Andes for the Early-Stage Research Found -FAPA- (P3.2017.3830).

NOMENCLATURE Ar

Archimedes number, gdp3ρ(ρs-ρ)µ-2

Dc

Column diameter, m

DFC

Fountain confiner diameter, m

Di

Contactor base diameter, m

Do

inlet diameter, m

dp

Particle diameter, mm


Page 24 of 32

Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DT

Tube diameter, m

HF

Distance between the bed surface and the lower end of the device, m

Ho

Static bed height, m

LH

Entrainment zone height, m

LT

Length of the draft tube, m

ums

Minimum spouting velocity at the inlet orifice, m s-1

WH

Width of the faces, m

Greek letters γ

Cone angle, °

ρs

Bulk density, kg m-3

ρs

Particle density, kg m-3

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Graphical TOC


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Distribution of Cycle Times in Sawdust Conical Spouted Bed

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