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Jet penetration depth in a two-dimensional spout–fluid bed
Chemical Engineering Science 60 (2005) 315 – 327 www.elsevier.com/locate/ces
Jet penetration depth in a two-dimensional spout–fluid bed Wenqi Zhong∗ , Mingyao Zhang Key Laboratory on Clean Coal Power Generation and Combustion Technology of Ministry of Education, Thermal Energy Engineering Research Institute, Southeast University, Nanjing 210096, Peoples’s Republic of China Received 23 April 2004; received in revised form 28 July 2004; accepted 13 August 2004 Available online 25 September 2004
Abstract The jet penetration depth was proposed to be an important parameter to describe the jet action during the chemical process of spout–fluid bed coal gasification. A two-dimensional cold model of a spout–fluid bed coal gasifier with its cross section of 300 mm × 30 mm and height of 2000 mm was established to investigate the jet penetration depth. Four types of Geldart group D particles were used as bed materials. A multi-channel pressure sampling system and a high-resolution digital CCD camera were employed for experimental investigations. The effects of spouting gas velocity, spout nozzle diameter, static bed height, particle property and fluidizing gas flow rate on the jet penetration depth have been systematically studied by pressure signal analysis and image processing. Experimental results indicate that the jet penetration depth increases with increasing spouting gas velocity and spout nozzle diameter, while it decreases with increasing particle density, particle diameter, static bed height and fluidizing gas flow rate. Additional, a new correlation considered all of the above effects especially static bed height and fluidizing gas flow rate, was developed for predicting the jet penetration depth in spout–fluid beds. The correlation was compared with published experimental data or correlations, which was in well agreement with the present experimental results and some other references. 䉷 2004 Elsevier Ltd. All rights reserved. Keywords: Fluidization; Multiphase flow; Spout–fluid bed; Jet; Spout
1. Introduction The spout–fluid bed is an alternative gas–solid contactor, in addition to rejecting spouting gas through a central nozzle, fluidizing gas is introduced through a porous or perforated distributor surrounding the central nozzle, which can result in a higher rate of circulating of solids and fluid than either spouting or fluidization alone and reduce some of the limitations of both spouting and fluidization by superimposing the two types of systems. Compared with fluidized beds, spout–fluid beds can operate over a wider range of fluidizing flow rates without succumbing to slugging which generally reduce the efficiency of the system. With respect to spouted beds, the fluidizing gas increases the fluid–solid contact in
∗ Corresponding author. Tel.: +86-25-83795119; fax: +86-25-57714489. E-mail address: [email protected] (W. Zhong).
0009-2509/$ - see front matter 䉷 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.08.009
the annular dense region and reduces the likelihood of particle agglomeration, dead zone and sticking to the wall of the vessel (Pianarosa et al., 2000). Thus, spout–fluid bed coal gasifiers have been opted for a laboratory scale Advanced Pressurized Fluidized Bed Combustion—Combined Cycle (APFBC—CC) system (Zhang, 1998) and a Pressurized Partial Gasification—Combined Cycle (PPG—CC) system (Xiao and Zhang, 2002) by our laboratory. Previous experimental investigations on spout–fluid beds have described various flow regimes (Vukovic et al., 1984; Sutanto et al., 1985). The spout–fluid bed gasifier should be operated in the flow regime of “fluidization with a local spout” defined by Vukovic et al. (1984) or in the flow regime of “jet in the fluidized bed (JFI)” defined by Sutanto et al. (1985), which can prolong the resident time of steam and air in the bed, making it un-easy for much steam and air to pass through the bed and wasted, and enhancing the gas diffusion and particles mixing between the spout region and the annular dense region, obtaining more uniform axial
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temperature profiles in order to improve the gasification efficiency. In this view, the jet is playing an important role in mass transfer, heat transfer, momentum transfer during the chemical process of spout–fluid bed coal gasification. One of the features of gas injection in a spout–fluid bed through an upward nozzle (generally called as spout nozzle) is the distance to which fluid dynamic disturbances associated with the injection extend. Such a distance is known in the literature as the jet penetration depth Lj (Knowlton and Hirsan, 1980). Knowlton and Hirsan (1980) classified three types of definitions for the jet penetration depth: the deepest penetration depth of jet bubbles before losing their momentum, LB ; the penetration depth of a series of interpenetrating cavities, Lmax ; and the penetration depth of a cavity permanently attached to the nozzle, Lmin . The jet penetration depth can be considered as one of the key parameters to describe the jet action in spout–fluid beds. However, there has been little information concerning such a parameter in spout–fluid beds so far, many investigations paid attention to jetting fluidized beds. A lot of literatures (Behie et al., 1970, 1971; Yang and Keairns, 1978, 1979; Knowlton and Hirsan, 1980; Filla and Massimilla, 1984; Blake et al., 1990; Kimura et al., 1995; Vaccaro et al., 1997; Yang, 1998; Guo et al., 2001; Hong et al., 2003) have reported various measurement methods, measurement results and correlations of the penetration depth. Because of the markedly unsteady character of the jet, the different experimental conditions and measurement techniques, the kind of length measured and the subjectivity of some Lj measurements have often led to a spread of contrasting result (Vaccaro et al., 1997). In addition, since the configuration of spout–fluid beds and gas–solid interaction in spout–fluid beds are somewhat different from jetting fluidized beds, the jet penetration depths are still unveiled. Moreover, the spout–fluid bed coal gasifiers should be operated at proper conditions, too high spouting velocity will lead to spout, while too high fluidizing gas flow rate will dissipate the jet momentum resulting in forming large bubbles or even slugging. Thus, experimental as well as theoretical approaches aimed at grasping more trustful information on the jet penetration depth in spout–fluid beds are needed. The present work aimed to perform experimental investigations on the jet penetration depth in a two-dimensional spout–fluid bed. It focused on systematically examining the effects of particle density, particle diameter, spouting gas velocity, spout nozzle diameter, static bed height, and fluidizing gas flow rate on the jet penetration depth and its new correlation.
2. Experimental system The spout–fluid bed experimental system is schematically shown in Fig. 1, which consists of a spout–fluid bed body, a gas supplied system and measurement apparatus. The bed body has a cross section of 300 mm × 30 mm and height
7
1
8 12
5
2 A/D
13
9
3 4
6
11
Fluidizing gas inlet direction
10
6
Fig. 1. Schematic diagram of spout–fluid bed experimental system. 1—Computer; 2—A/D converter; 3—Multi-channel differential pressure signals transducer; 4—Roots-type blower; 5—Differential pressure sensor; 6—Flow meter; 7—Material adding tank; 8—Pressure measuring hole; 9—V type gas distributor; 10—Spout nozzle; 11—fluidization flux distributor; 12—Floodlight; 13—Digital camera & digital video.
of 2000 mm, which was made of 8 mm thick Plexiglas. The area of spout nozzle is 30 mm × 30 mm (can be adjusted to 20 mm × 30 mm, 10 mm × 30 mm and etc.). A V type gas distributor which has a 60◦ inclination angle was located at the bottom of the bed. The orifice in the air distributor is 1 mm in diameter, and the total area of all orifices is 1.1% to that of gas distributor. A Roots-type blower was used to supply with the spouting gas and the fluidizing gas. A pressure-reducing valve was installed to avoid pressure oscillations and achieve steady gas flow. The gas flow rates were measured by two electromagnetism flow meters. The spouting gas entered into bed directly through the upward spout nozzle. The fluidizing gas was divided into two equal fluxes by a flux distributor before it flowed into the gas chamber, respectively, and then entered into bed via the orifices in gas distributor. A multi-channel differential pressure signals sampling system was employed to investigate the pressure fluctuations. There are 15 pressure-measuring holes located in the back wall of the bed, either in spout or in annular dense regions, with their heights of 160, 245, 400, 600, 700, and 800 mm and 1000 mm above the bottom line of the bed. Every differential pressure sensor has two ports, one port was connected with the pressure-measuring hole in bed wall, and the other port was connected with a gas chamber in spouting gas pipe. The pressure differences were converted into voltage signals by differential pressure sensors with a scale of 0 ∼ 16 kPa, the voltage signals were sent to a computer through an A/D converter. A digital camera (Nikon 5700) and a digital video (Sony DCR-PC330E) was employed to photograph the flow regimes through transparent walls during the experiments. Two groups of 2000 W floodlights were used to enhance definition of photo when photographing.
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327 Table 1 Experimental conditions H0 (mm)
Qs (m3 /s)
Qf (m3 /s)
f (Hz)
t (s)
200–500
0–0.06
0–0.04
100
180
Table 2 Particle properties Particles
dp (mm)
S.D. (%)
s (kg/m3 )
(—)
umf (m/s)
Mung beans Polystyrene Millet Glass beans
3.2 2.8 1.6 1.3
±10.3 ±12.1 ±8.4 ±5.2
1640 1018 1330 2600
0.42 0.41 0.40 0.38
1.07 0.82 0.58 0.62
A summary of experimental conditions and the particle properties studied in this work were listed in Tables 1 and 2, respectively. 3. Results and discussion 3.1. Determination of the jet penetration depth Various measurement methods have been reported in literatures. The jet penetration depth were mainly determined by direct visual observation of the phenomenon (Zenz, 1968; Merry, 1975; Knowlton and Hirsan, 1980) and by photographic or high-speed cine film analysis technology (Markhevka et al., 1971; Tsukata and Horio, 1990). Besides, various probe techniques were employed including optical probes (Wen et al., 1982; Kimura et al., 1995), capacitance probe (Yutani et al., 1983), radiation dosimeters (Yates et al., 1986) and Pitot tubes (Behie et al., 1970, 1971; Raghunathan et al., 1988; Guo et al., 2001, Hong et al., 2003). Vaccaro et al. (1997) developed a pressure measurement technique, which was based on the power spectral analysis of the static pressure signals the simultaneously recorded at the jet axis and bed wall static during normal jetting fluidized bed operation. Their analysis also supported the evaluation of the characteristic depth by means of direct comparison between fluctuations of the static pressure at the axis and at the wall. According to the three types of the jet penetration depth classified by Knowlton and Hirsan (1980), techniques making use of direct visual observation access to the phenomenon might be indicated for the measurement of Lmax since they were unable to detect the residual momentum of gas and solids in the bubbles leaving the jet at Lmax . This technology was the least reliable because the intrinsic unsteadiness of the phenomenon renders the measurement too subjective. Determining Lj by an instantaneous picture representing the end of a cycle of jet development and explicitly acknowledged that the value Lj are consistent with Lmax . Lj measured by using optical probes or capacitance probes
317
in conjunction with statistical date treatment can correctly describe the geometrical boundaries of the dilute jet regime above the nozzle, therefore, the value of Lj conforms to Lmax . Similar to direct visual observation, being unable to detect the residual momentum of gas and solids in the jet bubbles, these probe techniques would not be able to yield the penetration depth LB . Among these measurement methods, reliable techniques suitable for LB measurement were limited substantially to three, each making use of probe or items of the measuring assembly inserted in the jet and also disturbing the phenomenon. They were the Pitot tube technique, the radiation dosimeters and simultaneous axis and wall static pressure measurement technique (Vaccaro et al., 1997). The depth corresponding to Lj measurements carried out using Pitot tube probes have lead to debate in literatures. Being based on the indirect measurement of gas momentum, Pitot tube probes technique should naturally yield a jet penetration depth conform to LB (Behie et al., 1970, 1971). While Raghunathan et al. (1988) gave a different interpretation that their Pitot tube technique applied to evaluate the penetration depth of an isolated jet yields Lj value conforming to Lmax rather than LB . They supported such a finding by frame-by-frame analysis of high-speed cine films take simultaneously with the Pitot tube measurements. Guo et al. (2001) also obtained the same conclusion that Pitot tube technique yield jet penetration corresponding to Lmax . For present work, a multi-channel difference pressure sampling system, a digital camera (Nikon 5700) and a digital video (Sony DCR-PC330E) were used to determine the jet penetration depth. Determining Lj by frame-by-frame analysis of instantaneous images represented the end of a cycle of jet development and explicitly acknowledged that the value Lj were consistent with Lmax , as presented in Fig. 2. As is known, the jet momentum flux decreases with the axis height, the difference pressure increases along the axis due to the dynamical pressure of jet converting into static pressure. After jet collapses, bubbles form, the momentum flux in jet region is equal to that in annular dense region, the bed is dynamically homogeneous and there is no difference between the jet and the rest of the bed at the same height. Here, the jet penetration depth Lj can be determined, as shown in Fig. 3. The jet penetration depth Lj determined by pressure signal analysis seems to conform to LB . While the pressure signal analysis are unable to fully detect the residual momentum of gas and solids in the bubbles leaving the jet at Lmax due to the fluctuation of pressure signal, Lj determination by this method may not corresponding to LB but a little large than Lmax . Fig. 4 shows the comparison of Lj determined by image analysis with pressure signal analysis under the same operation condition. By analyzing a series of date, the relation of these two methods have been obtained as photo
Lj = L j
signal
= Lmax = 0.92Lj
.
(1)
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Lj determined by image analysis (mm)
350
LB
bubble
Lmax
cavity
jet solids
250
200
150
200
250
300
350
Fig. 4. Comparison of Lj determined by image analysis with pressure signal analysis.
400 As=30mmx30mm,H0=400mm Millet Polystyrene Mung beans Glass beans
350
2000
300
1 1600
150
Lj determined by pressure signal analysis (mm)
Fig. 2. Lj determined by image analysis according to Knowlton and Hirsan’s (1980) theory.
2
Lj (mm)
Difference pressure ∆P (Pa)
300
100 100
Lj
Lmin
wake
determined by image analysis determined by pressure signal analysis
1200 1-Annular dense region 2-Axis jet region
250 200 150
800
100 Lj
400
50
5
10
15
20
25
30
35
40
us (m/s) 0
50
100
150
200
250
300
Bed height H (mm)
Fig. 3. Lj determined by pressure signal analysis.
The Lj determined by pressure signal analysis has been corrected by Eq. (1) for present work, which could make the jet penetration depth determined by these two methods both correspond to Lmax . Since the jet shape especially the boundary between the jet and the dense regime changes periodically as mentioned by Yang and Keairns (1982), Kimura et al. (1995) and Vaccaro et al. (1997), the jet penetration depth cannot be determined for one time only either by pressure signal analysis or by image analysis. In order to reduce the metrical errors, we calculated the arithmetic mean of Lj for multi-times measurements.
Fig. 5. Effect of spouting gas velocity on jet penetration depth (Qf /Qmf = 0).
condition of fluidizing gas flow rate equal to 0 and Qmf . The experimental results exhibit that the jet penetration depth is a function of spouting jet velocity, the jet penetration depth increases with increasing of spouting jet velocity. According Turner’s theory, the jet momentum flow rate can be expressed as M = ds2 f u2s . (2) 4 By increased spouting gas velocity, the jet momentum flow rate increases, which leads to the increasing of jet penetration depth.
3.2. Effect of spouting gas velocity on jet penetration depth
3.3. Effect of spout nozzle diameter on jet penetration depth
Figs. 5 and 6 show the effect of spouting gas velocity on jet penetration depth for various tested particles at the
The comparisons of jet penetration depth or three spout nozzles with their areas of 30 mm ×30 mm, 20 mm ×30 mm
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327 400
400 As=30mmx30mm,H0=400mm Millet Polystyrene Mung beans Glass beans
350
350
250 200
200 150
100
100 50 5
10
15
20
25
30
35
40
us (m/s) Fig. 6. Effect of spouting gas velocity on jet penetration depth (Qf /Qmf = 1).
H0=400mm, Qf / Qmf =1.0 Polystyrene, As=30mmx30mm Polystyrene, As=20mmx30mm Polystyrene, As=10mmx30mm Mung beans, As=30mmx30mm Mung beans, As=20mmx30mm Mung beans, As=10mmx30mm
300
250 Lj (mm)
250
150
200
150
100
50
H0=400mm,As=30x30mm,Qf /Qmf =0 Polystyrene,Φ=2.8mm Polystyrene,Φ=3.2mm
300 Lj (mm)
Lj (mm)
300
50
319
5
10
15
20
25
30
35
40
us (m/s)
Fig. 7. Effect of spout nozzle diameter on jet penetration depth.
and 10 mm × 30 mm (the hydraulic diameters are 30, 24 and 15 mm) are shown in Fig. 7, respectively. At a given spouting gas velocity, the jet penetration depth decreases with the decreasing of spout nozzle diameter, which due to the decreasing of spout gas momentum. The Eq. (2) indicates that the spout gas momentum at the spout nozzle exit is proportional to the square of spout nozzle diameter. The decreased magnitudes of jet penetration depth for mung beans are relatively larger than that for polystyrene with decreasing of spout nozzle diameter seems to resulting in the larger density of the mung beans. 3.4. Effect of particle properties on jet penetration depth The particle properties take a great effect on jet penetration depth. As shown in Figs. 5–7, the jet penetration depth decreases with the increasing of particle density for a given spouting gas velocity, static bed height and spout nozzle di-
5
10
15
20
25
30
35
40
us (m/s)
Fig. 8. Comparisons of jet penetration depth for polystyrene with two particle diameters.
ameter, which can be interpreted as following: if the spouting gas enters into an empty bed, the jet penetration depth would be the largest due to the least momentum of jet being dissipated. However, when the spouting gas enters into a bed with bed materials, the distinct difference properties of matter between gas phase and bed materials will restrict the development and extending of the jet, the jet momentum will increase when this difference becomes more and more distinct with increasing particle density. The jet penetration depth decreases with increasing particle diameter, as illustrated in Fig. 8. For a given spouting gas velocity, increasing particle diameter will lead to the increasing of particle inertial force, more jet momentum for jet schlepping particles will be dissipated when the jet ascends along axis. In addition, the bed viscosity will increase when the particle diameter increases, which dissipate more jet momentum to overcome larger resistance when the jet penetrate the bed materials. Therefore, the jet penetration depth decreases. 3.5. Effect of static bed height on jet penetration depth There has still little information concerning the effect of static bed height on the jet penetration in previous research (Chyang et al., 1997) so far, since generally considered that static bed height has little influence on the jet penetration depth for the vertical or horizontal single-jet system (Hong et al., 1996, 1997). However, the present research suggests that the jet penetration depth is significantly affected when the static bed height is low, while this effect is inconspicuous when the static bed height is high. The experimental data, as shown in Fig. 9, indicates that the jet penetration depth decreases with the increasing of static bed height. For a very shallow bed is used, the spouting gas plume pushes upwards to the surface very quickly. Thus, a suitable bed height is needed in order to get a fully developed jet.
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W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327 300
As=20mmx30mm,Qf / Qmf=1.0,Polystyrene us=6.9m/s us=10.4m/s us=13.8m/s us=17.2m/s us=20.7m/s
Lj (mm)
250
200
150
100
50
250
300
(a)
350 400 H0 (mm)
450
500
550
500
550
35
40
300 As=20mmx30mm,Qf / Qmf=1.0,Polystyrene us=6.9m/s us=10.4m/s us=13.8m/s us=17.2m/s us=20.7m/s us=24.2m/s
Lj (mm)
250
200
150
100
50 200
250
300
(b)
350 400 H0 (mm)
450
300 As=30mmx30mm,Qf /Qmf=1.0, Mung beans H0=245mm H0=300mm H0=400mm H0=470mm
250
Lj (mm)
200
150
100
50 (c)
5
10
15
20 25 us (m/s)
30
Fig. 9. Effect of static bed height on jet penetration depth.
3.6. Effect of fluidizing gas flow rate on jet penetration depth As is known, the spouting gas and the fluidizing gas should be well organized during the chemical process of coal gasification in spout–fluid bed, the fluidizing gas
plays an important role in jet penetration depth as well as the spouting gas. However, the effects of fluidizing gas flow rate on jet penetration depth have lead to debate in the literatures. Yates et al. (1986) employed two kinds of bed materials (aluminum, coke) to study the effect of fluidizing gas flow rate on the penetration depth, and found that the penetration depth decreased with increasing the fluidizing gas flow rate. Vaccaro et al. (1997) used glass beans (800.1200 m in diameter) to investigate this matter, and concluded that the jet penetration depth might either increase (d0 = 19 mm, 25 mm) or increase (d0 = 6.0 mm, 10 mm) due to the influence of different experimental conditions and operating variables. While Guo et al. (2001) employed the binary mixtures (80% sand and 20% millet, 30% coke and 70% slag) to investigate the effect of fluidizing gas flow rate on the penetration depth, and indicated that: at a given jet velocity, the penetration decreases when the fluidizing gas flow rate ranges from 1.0Qmf to 2.5Qmf , and then the jet penetration remains constant as the fluidizing gas flow rate increases from 2.5Qmf to 3.0Qmf . For present work, the jet penetration depth decreases with increasing fluidizing gas flow rate at a given spouting gas velocity, as presented in Fig. 10. The jet penetration depth varies smoothly when fluidizing gas flow rate is less than 1.0Qmf and more than 2.3Qmf , while it varies sharply when fluidizing gas flow rate ranges from 1.0Qmf to 2.3Qmf . The cause of the present observation has not been clarified, however the following discussion may be useful. In the cause when the fluidizing gas is added in to the bed, the bed voidage increases, which leads to the central jet gas diffusing to annular dense region, the jet momentum will be dissipated, thus the penetration decreases. When the fluidizing gas flow rate is less than 1.0Qmf , the voidage changes little, so the penetration depth varies smoothly. Bubbles form when the fluidizing gas flow rate is beyond the minimum fluidizing gas flow rate, the larger fluidizing gas flow rate, the more bubbles and the larger voidage in the annular dense region. Since the voidage in the annular dense region and fluidizing gas flow rate increases, the jet is easily dissipated in to the emulsion, moreover, as the bubbles form in the emulsion, bubbles and jets continuously coalesce, which dissipates much of the jet momentum, the penetration decreases sharply. Nevertheless, the penetration depth varies smoothly when the fluidizing gas flow rate is over 2.3Qmf . According to Guo et al. (2001), when the annular flow rate is too larger, the voidage in the annular dense region change much little, thus, the dissipation of jet momentum is nearly constant. For present study, the jet penetration depth is not constant but decreases smoothly seems to caused by discontinuous slugging observed in experiments with a frequency of about 0.3–0.7 Hz when the fluidizing gas flow rate is too larger. The formation and collapse of large bubbles when slugging would dissipate part of the jet momentum, thus, the jet penetration depth slightly decreases.
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
300
As=30mmx30mm,H0=400mm, Ploystyrene us=10.4m/s us=20.7m/s us=31.1m/s
250
Lj (mm)
200
150
100
50
0.0
0.5
1.0
1.5 2.0 Qf / Qmf
(a)
2.5
3.0
3.5
300 As=20mmx30mm,H0=515mm, Ploystyrene 10.4m/s
31.1m/s
20.7m/s
Lj (mm)
250
200
150
100
50
0.0
0.5
(b)
1.0
1.5 2.0 Qf / Qmf
2.5
3.0
300
Lj (mm)
250
As=30mmx30mm,H0=470mm, Mung beans us=13.8m/s us=24.2m/s us=34.5m/s
200
50 (c)
The typical comparison of experimental and calculated data showed that no correlation agrees well with the experimental results as shown in Fig. 11. Since it was difficult to determine a proper jet angle for the correlations proposed by Shakhova (1968) and Horio et al. (1990), a proper bed voidage εb for the correlations proposed by Merry (1971) and Zhou et al. (1999), and negative calculated values of Lj /dj at present calculating conditions by the two correlations (Zenz, 1968; Merry, 1971) when a low uj was used, and larger errors of the correlations of Merry (1975), the present work only employed the residual 14 correlations to perform the calculated data. The correlations from literatures almost did not consider the effect of the fluidizing gas flow rate on the penetration depth except the correlations proposed by sKnowlton and sHirsan, 1980, Luo et al. (1996) and Guo et al. (2001). The correlations not considered the effect of fluidizing gas flow rate might lead to errors between the calculated data and the experimental results, because the fluidizing gas takes effect on the jet penetration depth as illustrated in Fig. 10. The calculated data from the correlations (Knowlton and Hirsan, 1980; Luo et al., 1996; Guo et al., 2001), which considered the effect of fluidizing gas flow rate, are still not agree with present experimental results, as presented in Fig. 11(b). Moreover, the jet penetration depths are significantly affected by the static bed height especially a low static bed height as shown in Fig. 9, while no correlation considered the effect of static bed height among these correlations. The discrepancies, shown in Fig. 11, suggest that it might be difficult to find a generalized expression for the variety of working conditions encountered in practice. These correlations are not suitable for predicting the penetration depths for present work, thus a new correlation of the jet penetration depth in spout–fluid bed is expected. According to the previous research (Yang and Keairns, 1978, 1979; Luo et al., 1996, 1997; Guo et al., 2001; Hong et al., 2003), the jet penetration depth is the function of the two-phases Froude number, which can be expressed as
150
Lj =f dj
100
0.0
0.5
1.0 1.5 Qf / Qmf
2.0
2.5
3.0
Fig. 10. Effect of fluidizing gas flow rate on jet penetration depth.
3.7. New jet penetration depth correlation Main correlations of jet penetration depth in the literatures are listed in the Table 3. The typical comparison of our present experimental results with the prediction calculated by these correlations is shown in Fig. 11.
321
u2j g , = f (Fr , ). p − g gd j
(3)
The relation between the two-phases Froude number and the penetration depth can also be derived from the buoyancy theory of Turner (1973). The characteristic length scale, Lj , which is the jet penetration depth obtained by Turner for a buoyant fluid jet: 0.5 u2j g Lj J 3/4 ≈ 1/2 = = (Fr )0.5 . dj B dj 4 p − g gd j 4
(4)
For spout–fluid bed, since particles move down in the annular dense region and entrained into the jet, the flow dynamics is somewhat different from the fluid jets. However,
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Table 3 Main correlations of jet penetration depth in the literatures (1968–2003) Lj type
References and correlations Shakhova (1968)a L j uj j dj + 0.5 cot = 13 g gd p
(Lmax + Lmin )/2
Zenz (1968) L
0.044 d j + 1.3 = 0.5 log(g u2j ) j
Lmax
Basov et al. (1969)a Lj dj =
u0.35 j
0.919dp 0.0007+0.56dp
Lmax
dj0.3
Merry (1971)
2
g uj Lj dj + 4.5 = 5.25 (1−b )p gd p
0.4
0.2 g p
dp 0.2 dj
(Lmax + LB )/2
Vakhrushev(1972)a u 0.5 Lj j 1 − tan1 dj = 0.5 c + tan 4kut
(Lmax + Lmin )/2
0.5 c 4.21, 0.25 k 0.8 Turner (1973) Lj dj = 4
u2j p −g gd j g
0.5 Lmax
Merry (1975)
d 0.3 u2j Lj g j 1.3 dj = 5.2 p dp gd j
0.2
− 1
Lmax
Wen et al. (1977)a d −0.585 d u −0.654 Lj p p g j j dj = 814.2 g dj
u2j gd j
0.47 (Lmax + Lmin )/2
Yang and Keairns (1978) 0.5 u2j g Lj dj = 6.5 p −g gd j
Lmax
Yang and Keairns (1979) 0.187 u2j g Lj d = 15.0 − gd
Lmax
Hirsan et al.(1980)a 0.67 g uj Lj u −0.24 = 26.6 u d
LB
Knowlton and Hirsan (1980) 0.88 g uj Lj u −0.54 u d = 19.3
Lmax
j
j
j
p
p
p
g
j
gd j
mf
gd p
mf
Wen et al. (1982) 2 0.42 d (u −u ) −0.42 d 0.66 Lj mf g g p j p 4 (uj −umf ) d = 1.15 × 10 gd d j
p
p
j
(Lmax + Lmin )/2
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
323
Table 3 (Contd.,) Lj type
References and correlations Wen et al. (1982) 0.38 u2j Lj dj = 1.3 gd p
d u 0.13 0.56 d 0.25 g p j g j
Lmax
dp
p
Wu and Whiting (1988) 0.34 g 0.67 g dp (uj −umf ) −0.34 dp 0.43 Lj (uj −umf )2 = 821 d gd d j
p
p
Blake et al. (1990) 0.322 u2j Lj dj = 26.9 gd j
j
0.325 u d 2 −0.124 p j p g
Lmax
dj
p
Lmax
Horio et al. (1990)b −0.095 Lj 2u 0.857 1.2 dj = − 1 ln umf 1/2 pn P n
k
j
f F r 0.3 0.2 p j dn
dj dn = 1.56 k 1/2 tan
(Lmax + Lmin )/2
Pn
k = (1 − sin )(1 + sin ), fj = 0.02 Luo et al. (1996)
Lj u dj = 23.12 umf
−0.04808
Luo et al. (1997)
u2j g Lj dj = 55.6 p −g gd j
Zhou and Shen (1999)
u2j p −g gd j
0.31
2
p uj Lj dj = 408.299 (1−b )p gd p
g
0.2471
(p −g )uj dp2 dj
0.172
u d −0.08069 g j p
−0.124
0.412 g p
Lmax
Lmax
dp −0.000418 dj
(Lmax + LB )/2
, 0 < uf 2.5umf
Lmax
Guo et al. (2001)
Lj u dj = 19.18 umf
−0.3616
u2j g Lj D0 = 11.52 p −g gd j
Hong et al. (2003)
u2j g Lj dj = 26.47 p −g gd j
u2j p −g gd j g
0.2383
0.1966 , uf > 2.5umf
0.293
u d −0.1138 g j p
Lmax
a Cited from Davidson et al. (1985). b Cited from Kimura et al. (1995).
both Eqs. (3) and (4) indicate that the jet penetration depth strongly depends on the two-phase Froude number. The twophase Froude number has been successfully used (Yang and Keairns, 1978, 1979; Luo et al., 1996, 1997; Guo et al., 2001; Hong et al., 2003). Hence, we employed the two-phase Froude number to study the correlation of the jet penetration depth in spout–fluid bed. Present experimental results indicated that increasing particle diameter dp would lead to increasing particle inertial
force, more jet momentum for jet schlepping particles would be dissipated when jet ascends along axis. In addition, the bed viscosity would increase when increasing particle diameter, which dissipates more jet momentum to overcome larger resistance when penetrate the bed. Thus we employ the particle Reynolds number Rep rather than the dimensionless parameter of dp /dj to describe the effect of particle diameter on penetration depth. The particle Reynolds number Rep has ever used to perform the correlation of
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
Lj = f (us , ds , H0 , p , dp , Qf , ) ds g u2s H0 Qs + Qf dp g us , , , , =f p − g gd s Dt Qmf H0 u = f Fr , , , Rep , . Dt umf
14
12
4
12
10
0
(5)
where
14
11
2
−0.17 Lj u −0.11 0.39 H0 = 10.86(Fr ) (Rep )−0.03 ds Dt umf 0 uf < 3.0umf , (7)
dp g us
16
4
the coefficient a contains the parameter of jet expansion angle in Eq. (6), which is similar to previous research (Yang and Keairns, 1978, 1979; Filla and Massimilla, 1984; Blake et al., 1990; Luo et al., 1996, 1997; Guo et al., 2001; Hong et al., 2003). By regressing 606 groups of present experimental data using a commercial Package of Encyclopedias of Medical Statistic (PEMS) program, the correlation of the jet penetration depth was determined, as shown in Eq. (7).
Rep =
5
experimental data
6
b g Lj u2s H0 c u d dp g us e =a ds p − g gd s Dt umf c d H0 u = a(Fr )b (6) (Rep )e Dt umf
g u2s , p − g gd s
18
8
Since the dimensionless parameters of two-phase Froude number Fr , particle Reynolds number Rep , H0 /Dt and u/umf are employed to describe the correction of jet penetration depth shown in Eq. (5), Dt is the hydraulic diameter of bed, the jet penetration depth can be given as the following formation:
Fr =
20
Lj / dj
penetration depths (Blake et al., 1990; Luo et al., 1996; Hong et al., 2003). According to the analysis above, spouting gas velocity, spout nozzle diameter, static bed height, particle density, particle diameter and fluidizing gas flow rate take effect on the jet penetration depth. Thus, the jet penetration depth should be the function of these effects, which can be expressed as
.
Compared with various correlations of the jet penetration depth in previous literatures, the correlation proposed by present work has a development of considering more effects on the jet penetration depth, especially static bed height and fluidizing gas flow rate.
1
10
5
25 20 uj (m/s)
15
(a)
30
13 6 10 2 9 7 8 3
35
40
20 18
experimental data
16 14 12 Lj / dj
324
12 13
10 8 6 4
6
2 0 (b)
0.0
0.5
1.0
1.5 Qf / Qmf
2.0
2.5
3.0
Fig. 11. Typical comparison of present experimental data with calculated data from previous correlations (a) H0 = 470 mm, dj = 30 mm, Qf /Qmf =1, mung beans. (b) H0 =400 mm, dj =30 mm, us =20.7 m/s, polystyrene. 1—Basov et al. (1969); 2—Turner (1973); 3—Wen et al. (1977); 4—Yang and Keairns (1978); 5—Yang and Keairns (1979); 6—Knowlton and Hirsan (1980); 7—Wen et al. (1982); 8—Wen et al. (1982); 9—Wu and Whiting (1988); 10—Blake et al. (1990); 11—Luo et al. (1997); 12—Luo et al. (1996); 13—Guo et al. (2001); 14—Hong et al. (2003).
The correlation was put forward to predict the jet penetration depth at various operation conditions. Typical comparisons of present experimental data with calculated data from previous correlations and present correlation are presented in Fig. 12. The present correlation agrees with our experimental results qualitatively. The comparisons of experimental results at various operation conditions with the calculations by Eq. (7) are shown in Fig. 13. Besides of agreement with our present investigations, these results are consistent to those reported previous by Kimura et al. (1995), Vaccaro et al. (1997), Yang, 1998 and Guo et al. (2001), while are not accordant well with Chyang et al. (1997) resulting in the horizontal nozzle they used.
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
experimental data
18 16
12
14
4
20
13
6
(Lj/dj)exp
14
15 10 2
8
7
8 3 1
2 10
5
25 20 uj (m/s)
15
(a)
30
-30%
5
35
40
22
+30%
15 10
9
4
0
25
6
10
Present work Kimura et al.(1995) Vaccaro et al.(1997) Chyang et al.(1997) Yang et al.(1998) Guo et al.(2001)
11
12 Lj/dj
30
5
20
325
0
0
5
10
15 (Lj/dj)cal
20
25
30
5
20
experimental data
Fig. 13. Comparison of jet penetration depth experimental results with calculations by Eq. (7).
11
18 16 13
Lj/dj
14
4. Conclusions
12
14
4
6 15
12
10 8
10 2 9
7
6 4
8 3 1
2 0 5
10
25 20 uj (m/s)
15
(b)
30
35
40
18 experimental data
16 13 12
14
Lj/dj
12 10 6
8 15
6 4 (c)
0.0
0.5
1.0
1.5 Qf / Qmf
2.0
2.5
3.0
Fig. 12. Comparisons of present experimental data with calculated data from previous correlations and present correlation. (a) H0 = 400 mm, dj = 30 mm, Qf /Qmf = 0, polystyrene. (b) H0 = 515 mm, dj = 24 mm, Qf /Qmf =1.0, polystyrene. (c) H0 =515 mm, dj =24 mm, us =20.7 m/s, polystyrene. 1—Basov et al. (1969); 2—Turner (1973); 3—Wen et al. (1977); 4—Yang and Keairns (1978); 5—Yang and Keairns (1979); 6—Knowlton and Hirsan (1980); 7—Wen et al. (1982); 8—Wen et al. (1982); 9—Wu and Whiting (1988); 10—Blake et al. (1990); 11—Luo et al. (1997); 12—Luo et al. (1996); 13—Guo et al. (2001); 14—Hong et al. (2003); 15–present work.
Considering the operation of spout–fluid bed coal gasification, a proper spouting gas flow rate can not form a spout but a jet, which making it un-easy for much steam and air to pass through the bed and wasted, and enhancing the gas diffusion and particles mixture in bed, more uniform axial temperature profiles leads to improving the gasification efficiency. The jet penetration depth is an important parameter to describe the jet action during the chemical process of spout–fluid bed coal gasification. Pressure signal analysis and image processing have systematically studied the jet penetration depth in a twodimension cold model of a spout–fluid bed coal gasifier. The jet penetration depth increases with increasing spouting gas velocity and spout nozzle diameter, while it decrease with increasing particle density, particle diameter, static bed height and fluidizing gas flow rate. A new correlation of the jet penetration depth was developed and proposed for predicting the jet penetration depth, which had a development of considering more effects on the jet penetration depth, especially static bed height and fluidizing gas flow rate. The correlation was compared with published experimental data or correlations, which was in well agreement with the present experimental results and some other references. Besides of spouting gas velocity, spout nozzle diameter, static bed height, particle diameter, particle density and fluidizing gas flow rate, other parameters such as Geldart’s Group of particle, the configuration of fluidizing gas distributor, jet angle, and etc. can be expected to exert an influence on the jet penetration depth. This aspect clearly requires further experimental study. It is hoped that the present work will contribute to the further development of more rigorous and reliable prediction of jet penetration depth.
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W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
References
Notation As a, b, c, d, e Dt ds dj dp f Fr g H0 Lmin Lmax LB Lj p Qf Qs Qmf Rep t u uj us uf umf
area of spout nozzle, mm2 coefficient of penetration depth correlation bed hydraulic diameter, mm spout nozzle hydraulic diameter, mm jetting nozzle diameter, mm particle diameter, mm sampling frequency, Hz Two-phase Froude number gravitational accelerations, m/s2 static bed height, mm penetration depth of a cavity permanently attached to the nozzle, mm penetration depth of a series of interpenetrating cavities, mm the deepest penetration depth of jet bubbles before losing their momentum, mm jet penetration depth, mm pressure different measured by multi-channel signal sampling system, Pa fluidizing gas flow rate, m3 /s spouting gas velocity, m3 /s minimum fluidizing gas flow rate, m3 /s particle Reynolds number sampling time, s superficial gas velocity, m/s jet gas velocity, m/s spouting gas velocity, m/s fluidizing gas velocity, m/s minimum fluidizing gas velocity, m/s
Greek letters
b p g
particle packing voidage bed voidage Jet expansion angle, rad gas viscosity, kg/(m s) particle density, kg/m3 gas density, kg/m3
Acknowledgements Financial support from the National Key Program of Basic Research in China (NO. 199902210535), the Innovation Project for Graduate Student in University of Jiangsu Province (Hydrodynamic Characteristics and Scale-up Rules of Spout–fluid Beds) and the 985 Projects of Southeast University for Clean Energy Technology (SEU985-CET) were sincerely acknowledged. The authors also expressed sincere gratitude to Prof. J.R. Grace and Prof. S. Vaccaro for presenting us some of their valuable papers, which contributed to our research.
Basov, V.A., Markhevka, V.I., Melik-Akhnazarov, T.Kh., Orochko, D.I., 1969. Investigation of the structure of a nonuniform fluidized bed. Industrial Chemical Engineering 9, 263–272. Behie, L.A., Bergougnou, M.A., Baker, C.G., Bulani, W., 1970. Jet momentum at a grid of large gas fluidized bed. Canadian Journal of Chemical Engineering 48, 158–161. Behie, L.A., Bergougnou, M.A., Baker, C.G., Base, T.E., 1971. Further studies on momentum dissipation of grid jets in a gas fluidized bed. Canadian Journal of Chemical Engineering 49, 557–561. Blake, T.R., Webb, H., Sunderland, P.B., 1990. The nondimensionalization of equations describing fluidization with application to the correlation of jet penetration depth. Chemical Engineering Science 45, 365–371. Chyang, C.S., Chang, C.H., Chang, J.H., 1997. Gas discharge modes at a single horizontal nozzle in a two-dimensional fluidized bed. Powder Technology 90, 71–77. Davidson, J.F., Clift, R., Harrision, D., 1985. Gas Jet in Fluidized Bed: Fluidization, second ed. Academic Press, London, pp. 133–172. Filla, M., Massimilla, L., 1984. Analysis of the variables controlling gas jet penetration in fluidized beds. Industrial and Engineering Chemistry Fundamentals 23, 131–132. Guo, Q.J., Yue, G.X., Zhang, J.Y., 2001. Hydrodynamic behavior of a two-dimension jetting fluidized bed with binary mixtures. Chemical Engineering Science 56, 4685–4694. Hong, R.Y., Li, H.Z., Cheng, M.Y., Zhang, J.Y., 1996. Numerical simulation and verification of a gas–solid jet fluidized bed. Powder Technology 87, 73–81. Hong, R.Y., Li, H.Z., Li, H.B., Wang, Y., 1997. Studies on the inclined jet penetration length in a gas–solid fluidized bed. Powder Technology 92, 205–212. Hong, R.Y., Guo, Q.J., Luo, G.H., Zhang, J.Y., Ding, J., 2003. On the jet penetration height in fluidized beds with two vertical jets. Powder Technology 133, 216–227. Kimura, T., Horiuchi, K., Watanabe, T., Matsukata, M., Kojima, T., 1995. Experimental study of gas and particle behavior in the grid zone of a jetting fluidized bed cold model. Powder Technology 82, 135–143. Knowlton, T.M., Hirsan, I., 1980. The effect of pressure on jet penetration in semi-cylindrical gas-fluidized bed. In: Grace, J.R., Matsen, J.M. (Eds.), Fluidization. Plenum, New York, pp. 315–324. Luo, G.H., Zhang, J.Y., Zhang, B.J., 1996. The jet penetration depth in a fluidized bed of multi-component particles. Chinese Journal of Chemical Industry and Engineering 1, 91–99. Luo, G.H., Zhang, J.Y., Zhang, B.J., 1997. The behavior of gas flow ejected from two vertical nozzles in a fluidized bed. Chinese Journal of Chemical Engineering 3, 280–285. Markhevka, V.I., Basov, V.A., Melik-Akhnazarov, T.K., Orochko, D.I., 1971. The flow of a gas jet into a fluidized. Theoretical and Fundamental Chemical Engineering 5, 80–85. Merry, J.M.D., 1971. Penetration of a horizontal gas jet into a fluidized bed. Transactions of the Institute of Chemical Engineers 49, 189–196. Merry, J.M.D., 1975. Penetration of vertical jets into fluidized beds. A.I.Ch.E. Journal 21, 507–510. Pianarosa, D.L., Freitas, L.A.P., Lim, C.J., Grace, J.R., Dogan, O.M., 2000. Voidage and particle velocity profiles in a spout–fluid bed. Canadian Journal of Chemical Engineering 78, 132–142. Raghunathan, D., Mori, H., Writing, W.B., 1988. A technique for measurement of jet penetration in hot fluidized beds with a modified Pitot-tube probe. Industrial and Engineering Chemical Research 27, 1011–1016. Sutanto, W., Epstein, N., Grace, J.R., 1985. Hydrodynamics of spout–fluid beds. Powder Technology 44, 205–212. Tsukata, M., Horio, M., 1990. Gas motion and bubble formation at the distributor of a fluidized bed. Powder Technology 63, 69–74. Turner, J.S., 1973. Buoyancy Effects in Fluids, Cambridge University Press, Cambridge, England.
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327 Vaccaro, S., Musmarra, D., Petrecca, M., 1997. A technique for measurement of the jet penetration height in fluidized bed by pressure signal analysis. Powder Technology 92, 223–231. Vukovic, D.V., Hadzismajlovic, D.E., GrbaVcic, Z.B., 1984. Flow regimes for spout fluidized beds. Canadian Journal of Chemical Engineering 62, 825–829. Wen, C.Y., Deale, N.R., Chen, N.H., 1982. A study of jets in a threedimensional gas fluidized bed. Powder Technology 31, 175–184. Wu, C.S., Whiting, W.B., 1988. Interacting jets in a fluidized bed. Chemical Engineering Communications 73, 1–9. Xiao, J., Zhang, M.Y., 2002. Thermal performance analysis of pressurized partial gasification combined cycle (PPG-CC). Proceedings of the Eighth SCEJ Symposium on Fluidization, Japan, pp. 262–268. Yang, W.C., 1998. Comparison of the jet phenomena in 30-cm and 3-m diameter semicircular fluidized beds. Powder technology 100, 147–160. Yang, W.C., Keairns, D., 1978. Design and operating parameters for a fluidized bed agglomerating combustor/gasifier. In: Davidson, J.F., Keairns, D. (Eds.), Fluidization I. Cambridge University Press, Cambridge, England, pp. 208–213.
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Yang, W.C., Keairns, D., 1979. Estimating the jet penetration depth of multiple vertical grid jets. Industrial and Engineering Chemical Fundamentals 18, 317–329. Yates, J.G., Bejcek, V., Cheesman, D.J., 1986. In: Grace, JR., Sorensen, MA. (Eds.), The Effect Penetration into Fluidized Bed at Elevated Pressure. Foundation, New York, pp. 79–86. Yutani, N., Ho, T.C., Fan, L.S., Walawender, W.P., Song, J.C., 1983. Statistical study of the grid zone behavior in a shallow gas–solid fluidized bed using a mini-capacitance probe. Chemical Engineering Science 38, 575–582. Zenz, F., 1968. Bubble formation and grid design. Chemical Engineering Symposium series 30, 136–139. Zhang, M.Y., 1998. Pressurized Fluidized Bed Combustion Combined Cycle, Southeast University Press, Nanjing, China. Zhou, Y.M., Shen, X.L., 1999. An experimental investigation of the jet penetration depth in a two-dimensional fluidized bed. Chinese Journal of Boiler Technology 4, 1–5.
Jet penetration depth in a two-dimensional spout–fluid bed Wenqi Zhong∗ , Mingyao Zhang Key Laboratory on Clean Coal Power Generation and Combustion Technology of Ministry of Education, Thermal Energy Engineering Research Institute, Southeast University, Nanjing 210096, Peoples’s Republic of China Received 23 April 2004; received in revised form 28 July 2004; accepted 13 August 2004 Available online 25 September 2004
Abstract The jet penetration depth was proposed to be an important parameter to describe the jet action during the chemical process of spout–fluid bed coal gasification. A two-dimensional cold model of a spout–fluid bed coal gasifier with its cross section of 300 mm × 30 mm and height of 2000 mm was established to investigate the jet penetration depth. Four types of Geldart group D particles were used as bed materials. A multi-channel pressure sampling system and a high-resolution digital CCD camera were employed for experimental investigations. The effects of spouting gas velocity, spout nozzle diameter, static bed height, particle property and fluidizing gas flow rate on the jet penetration depth have been systematically studied by pressure signal analysis and image processing. Experimental results indicate that the jet penetration depth increases with increasing spouting gas velocity and spout nozzle diameter, while it decreases with increasing particle density, particle diameter, static bed height and fluidizing gas flow rate. Additional, a new correlation considered all of the above effects especially static bed height and fluidizing gas flow rate, was developed for predicting the jet penetration depth in spout–fluid beds. The correlation was compared with published experimental data or correlations, which was in well agreement with the present experimental results and some other references. 䉷 2004 Elsevier Ltd. All rights reserved. Keywords: Fluidization; Multiphase flow; Spout–fluid bed; Jet; Spout
1. Introduction The spout–fluid bed is an alternative gas–solid contactor, in addition to rejecting spouting gas through a central nozzle, fluidizing gas is introduced through a porous or perforated distributor surrounding the central nozzle, which can result in a higher rate of circulating of solids and fluid than either spouting or fluidization alone and reduce some of the limitations of both spouting and fluidization by superimposing the two types of systems. Compared with fluidized beds, spout–fluid beds can operate over a wider range of fluidizing flow rates without succumbing to slugging which generally reduce the efficiency of the system. With respect to spouted beds, the fluidizing gas increases the fluid–solid contact in
∗ Corresponding author. Tel.: +86-25-83795119; fax: +86-25-57714489. E-mail address: [email protected] (W. Zhong).
0009-2509/$ - see front matter 䉷 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.08.009
the annular dense region and reduces the likelihood of particle agglomeration, dead zone and sticking to the wall of the vessel (Pianarosa et al., 2000). Thus, spout–fluid bed coal gasifiers have been opted for a laboratory scale Advanced Pressurized Fluidized Bed Combustion—Combined Cycle (APFBC—CC) system (Zhang, 1998) and a Pressurized Partial Gasification—Combined Cycle (PPG—CC) system (Xiao and Zhang, 2002) by our laboratory. Previous experimental investigations on spout–fluid beds have described various flow regimes (Vukovic et al., 1984; Sutanto et al., 1985). The spout–fluid bed gasifier should be operated in the flow regime of “fluidization with a local spout” defined by Vukovic et al. (1984) or in the flow regime of “jet in the fluidized bed (JFI)” defined by Sutanto et al. (1985), which can prolong the resident time of steam and air in the bed, making it un-easy for much steam and air to pass through the bed and wasted, and enhancing the gas diffusion and particles mixing between the spout region and the annular dense region, obtaining more uniform axial
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W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
temperature profiles in order to improve the gasification efficiency. In this view, the jet is playing an important role in mass transfer, heat transfer, momentum transfer during the chemical process of spout–fluid bed coal gasification. One of the features of gas injection in a spout–fluid bed through an upward nozzle (generally called as spout nozzle) is the distance to which fluid dynamic disturbances associated with the injection extend. Such a distance is known in the literature as the jet penetration depth Lj (Knowlton and Hirsan, 1980). Knowlton and Hirsan (1980) classified three types of definitions for the jet penetration depth: the deepest penetration depth of jet bubbles before losing their momentum, LB ; the penetration depth of a series of interpenetrating cavities, Lmax ; and the penetration depth of a cavity permanently attached to the nozzle, Lmin . The jet penetration depth can be considered as one of the key parameters to describe the jet action in spout–fluid beds. However, there has been little information concerning such a parameter in spout–fluid beds so far, many investigations paid attention to jetting fluidized beds. A lot of literatures (Behie et al., 1970, 1971; Yang and Keairns, 1978, 1979; Knowlton and Hirsan, 1980; Filla and Massimilla, 1984; Blake et al., 1990; Kimura et al., 1995; Vaccaro et al., 1997; Yang, 1998; Guo et al., 2001; Hong et al., 2003) have reported various measurement methods, measurement results and correlations of the penetration depth. Because of the markedly unsteady character of the jet, the different experimental conditions and measurement techniques, the kind of length measured and the subjectivity of some Lj measurements have often led to a spread of contrasting result (Vaccaro et al., 1997). In addition, since the configuration of spout–fluid beds and gas–solid interaction in spout–fluid beds are somewhat different from jetting fluidized beds, the jet penetration depths are still unveiled. Moreover, the spout–fluid bed coal gasifiers should be operated at proper conditions, too high spouting velocity will lead to spout, while too high fluidizing gas flow rate will dissipate the jet momentum resulting in forming large bubbles or even slugging. Thus, experimental as well as theoretical approaches aimed at grasping more trustful information on the jet penetration depth in spout–fluid beds are needed. The present work aimed to perform experimental investigations on the jet penetration depth in a two-dimensional spout–fluid bed. It focused on systematically examining the effects of particle density, particle diameter, spouting gas velocity, spout nozzle diameter, static bed height, and fluidizing gas flow rate on the jet penetration depth and its new correlation.
2. Experimental system The spout–fluid bed experimental system is schematically shown in Fig. 1, which consists of a spout–fluid bed body, a gas supplied system and measurement apparatus. The bed body has a cross section of 300 mm × 30 mm and height
7
1
8 12
5
2 A/D
13
9
3 4
6
11
Fluidizing gas inlet direction
10
6
Fig. 1. Schematic diagram of spout–fluid bed experimental system. 1—Computer; 2—A/D converter; 3—Multi-channel differential pressure signals transducer; 4—Roots-type blower; 5—Differential pressure sensor; 6—Flow meter; 7—Material adding tank; 8—Pressure measuring hole; 9—V type gas distributor; 10—Spout nozzle; 11—fluidization flux distributor; 12—Floodlight; 13—Digital camera & digital video.
of 2000 mm, which was made of 8 mm thick Plexiglas. The area of spout nozzle is 30 mm × 30 mm (can be adjusted to 20 mm × 30 mm, 10 mm × 30 mm and etc.). A V type gas distributor which has a 60◦ inclination angle was located at the bottom of the bed. The orifice in the air distributor is 1 mm in diameter, and the total area of all orifices is 1.1% to that of gas distributor. A Roots-type blower was used to supply with the spouting gas and the fluidizing gas. A pressure-reducing valve was installed to avoid pressure oscillations and achieve steady gas flow. The gas flow rates were measured by two electromagnetism flow meters. The spouting gas entered into bed directly through the upward spout nozzle. The fluidizing gas was divided into two equal fluxes by a flux distributor before it flowed into the gas chamber, respectively, and then entered into bed via the orifices in gas distributor. A multi-channel differential pressure signals sampling system was employed to investigate the pressure fluctuations. There are 15 pressure-measuring holes located in the back wall of the bed, either in spout or in annular dense regions, with their heights of 160, 245, 400, 600, 700, and 800 mm and 1000 mm above the bottom line of the bed. Every differential pressure sensor has two ports, one port was connected with the pressure-measuring hole in bed wall, and the other port was connected with a gas chamber in spouting gas pipe. The pressure differences were converted into voltage signals by differential pressure sensors with a scale of 0 ∼ 16 kPa, the voltage signals were sent to a computer through an A/D converter. A digital camera (Nikon 5700) and a digital video (Sony DCR-PC330E) was employed to photograph the flow regimes through transparent walls during the experiments. Two groups of 2000 W floodlights were used to enhance definition of photo when photographing.
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327 Table 1 Experimental conditions H0 (mm)
Qs (m3 /s)
Qf (m3 /s)
f (Hz)
t (s)
200–500
0–0.06
0–0.04
100
180
Table 2 Particle properties Particles
dp (mm)
S.D. (%)
s (kg/m3 )
(—)
umf (m/s)
Mung beans Polystyrene Millet Glass beans
3.2 2.8 1.6 1.3
±10.3 ±12.1 ±8.4 ±5.2
1640 1018 1330 2600
0.42 0.41 0.40 0.38
1.07 0.82 0.58 0.62
A summary of experimental conditions and the particle properties studied in this work were listed in Tables 1 and 2, respectively. 3. Results and discussion 3.1. Determination of the jet penetration depth Various measurement methods have been reported in literatures. The jet penetration depth were mainly determined by direct visual observation of the phenomenon (Zenz, 1968; Merry, 1975; Knowlton and Hirsan, 1980) and by photographic or high-speed cine film analysis technology (Markhevka et al., 1971; Tsukata and Horio, 1990). Besides, various probe techniques were employed including optical probes (Wen et al., 1982; Kimura et al., 1995), capacitance probe (Yutani et al., 1983), radiation dosimeters (Yates et al., 1986) and Pitot tubes (Behie et al., 1970, 1971; Raghunathan et al., 1988; Guo et al., 2001, Hong et al., 2003). Vaccaro et al. (1997) developed a pressure measurement technique, which was based on the power spectral analysis of the static pressure signals the simultaneously recorded at the jet axis and bed wall static during normal jetting fluidized bed operation. Their analysis also supported the evaluation of the characteristic depth by means of direct comparison between fluctuations of the static pressure at the axis and at the wall. According to the three types of the jet penetration depth classified by Knowlton and Hirsan (1980), techniques making use of direct visual observation access to the phenomenon might be indicated for the measurement of Lmax since they were unable to detect the residual momentum of gas and solids in the bubbles leaving the jet at Lmax . This technology was the least reliable because the intrinsic unsteadiness of the phenomenon renders the measurement too subjective. Determining Lj by an instantaneous picture representing the end of a cycle of jet development and explicitly acknowledged that the value Lj are consistent with Lmax . Lj measured by using optical probes or capacitance probes
317
in conjunction with statistical date treatment can correctly describe the geometrical boundaries of the dilute jet regime above the nozzle, therefore, the value of Lj conforms to Lmax . Similar to direct visual observation, being unable to detect the residual momentum of gas and solids in the jet bubbles, these probe techniques would not be able to yield the penetration depth LB . Among these measurement methods, reliable techniques suitable for LB measurement were limited substantially to three, each making use of probe or items of the measuring assembly inserted in the jet and also disturbing the phenomenon. They were the Pitot tube technique, the radiation dosimeters and simultaneous axis and wall static pressure measurement technique (Vaccaro et al., 1997). The depth corresponding to Lj measurements carried out using Pitot tube probes have lead to debate in literatures. Being based on the indirect measurement of gas momentum, Pitot tube probes technique should naturally yield a jet penetration depth conform to LB (Behie et al., 1970, 1971). While Raghunathan et al. (1988) gave a different interpretation that their Pitot tube technique applied to evaluate the penetration depth of an isolated jet yields Lj value conforming to Lmax rather than LB . They supported such a finding by frame-by-frame analysis of high-speed cine films take simultaneously with the Pitot tube measurements. Guo et al. (2001) also obtained the same conclusion that Pitot tube technique yield jet penetration corresponding to Lmax . For present work, a multi-channel difference pressure sampling system, a digital camera (Nikon 5700) and a digital video (Sony DCR-PC330E) were used to determine the jet penetration depth. Determining Lj by frame-by-frame analysis of instantaneous images represented the end of a cycle of jet development and explicitly acknowledged that the value Lj were consistent with Lmax , as presented in Fig. 2. As is known, the jet momentum flux decreases with the axis height, the difference pressure increases along the axis due to the dynamical pressure of jet converting into static pressure. After jet collapses, bubbles form, the momentum flux in jet region is equal to that in annular dense region, the bed is dynamically homogeneous and there is no difference between the jet and the rest of the bed at the same height. Here, the jet penetration depth Lj can be determined, as shown in Fig. 3. The jet penetration depth Lj determined by pressure signal analysis seems to conform to LB . While the pressure signal analysis are unable to fully detect the residual momentum of gas and solids in the bubbles leaving the jet at Lmax due to the fluctuation of pressure signal, Lj determination by this method may not corresponding to LB but a little large than Lmax . Fig. 4 shows the comparison of Lj determined by image analysis with pressure signal analysis under the same operation condition. By analyzing a series of date, the relation of these two methods have been obtained as photo
Lj = L j
signal
= Lmax = 0.92Lj
.
(1)
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W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
Lj determined by image analysis (mm)
350
LB
bubble
Lmax
cavity
jet solids
250
200
150
200
250
300
350
Fig. 4. Comparison of Lj determined by image analysis with pressure signal analysis.
400 As=30mmx30mm,H0=400mm Millet Polystyrene Mung beans Glass beans
350
2000
300
1 1600
150
Lj determined by pressure signal analysis (mm)
Fig. 2. Lj determined by image analysis according to Knowlton and Hirsan’s (1980) theory.
2
Lj (mm)
Difference pressure ∆P (Pa)
300
100 100
Lj
Lmin
wake
determined by image analysis determined by pressure signal analysis
1200 1-Annular dense region 2-Axis jet region
250 200 150
800
100 Lj
400
50
5
10
15
20
25
30
35
40
us (m/s) 0
50
100
150
200
250
300
Bed height H (mm)
Fig. 3. Lj determined by pressure signal analysis.
The Lj determined by pressure signal analysis has been corrected by Eq. (1) for present work, which could make the jet penetration depth determined by these two methods both correspond to Lmax . Since the jet shape especially the boundary between the jet and the dense regime changes periodically as mentioned by Yang and Keairns (1982), Kimura et al. (1995) and Vaccaro et al. (1997), the jet penetration depth cannot be determined for one time only either by pressure signal analysis or by image analysis. In order to reduce the metrical errors, we calculated the arithmetic mean of Lj for multi-times measurements.
Fig. 5. Effect of spouting gas velocity on jet penetration depth (Qf /Qmf = 0).
condition of fluidizing gas flow rate equal to 0 and Qmf . The experimental results exhibit that the jet penetration depth is a function of spouting jet velocity, the jet penetration depth increases with increasing of spouting jet velocity. According Turner’s theory, the jet momentum flow rate can be expressed as M = ds2 f u2s . (2) 4 By increased spouting gas velocity, the jet momentum flow rate increases, which leads to the increasing of jet penetration depth.
3.2. Effect of spouting gas velocity on jet penetration depth
3.3. Effect of spout nozzle diameter on jet penetration depth
Figs. 5 and 6 show the effect of spouting gas velocity on jet penetration depth for various tested particles at the
The comparisons of jet penetration depth or three spout nozzles with their areas of 30 mm ×30 mm, 20 mm ×30 mm
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327 400
400 As=30mmx30mm,H0=400mm Millet Polystyrene Mung beans Glass beans
350
350
250 200
200 150
100
100 50 5
10
15
20
25
30
35
40
us (m/s) Fig. 6. Effect of spouting gas velocity on jet penetration depth (Qf /Qmf = 1).
H0=400mm, Qf / Qmf =1.0 Polystyrene, As=30mmx30mm Polystyrene, As=20mmx30mm Polystyrene, As=10mmx30mm Mung beans, As=30mmx30mm Mung beans, As=20mmx30mm Mung beans, As=10mmx30mm
300
250 Lj (mm)
250
150
200
150
100
50
H0=400mm,As=30x30mm,Qf /Qmf =0 Polystyrene,Φ=2.8mm Polystyrene,Φ=3.2mm
300 Lj (mm)
Lj (mm)
300
50
319
5
10
15
20
25
30
35
40
us (m/s)
Fig. 7. Effect of spout nozzle diameter on jet penetration depth.
and 10 mm × 30 mm (the hydraulic diameters are 30, 24 and 15 mm) are shown in Fig. 7, respectively. At a given spouting gas velocity, the jet penetration depth decreases with the decreasing of spout nozzle diameter, which due to the decreasing of spout gas momentum. The Eq. (2) indicates that the spout gas momentum at the spout nozzle exit is proportional to the square of spout nozzle diameter. The decreased magnitudes of jet penetration depth for mung beans are relatively larger than that for polystyrene with decreasing of spout nozzle diameter seems to resulting in the larger density of the mung beans. 3.4. Effect of particle properties on jet penetration depth The particle properties take a great effect on jet penetration depth. As shown in Figs. 5–7, the jet penetration depth decreases with the increasing of particle density for a given spouting gas velocity, static bed height and spout nozzle di-
5
10
15
20
25
30
35
40
us (m/s)
Fig. 8. Comparisons of jet penetration depth for polystyrene with two particle diameters.
ameter, which can be interpreted as following: if the spouting gas enters into an empty bed, the jet penetration depth would be the largest due to the least momentum of jet being dissipated. However, when the spouting gas enters into a bed with bed materials, the distinct difference properties of matter between gas phase and bed materials will restrict the development and extending of the jet, the jet momentum will increase when this difference becomes more and more distinct with increasing particle density. The jet penetration depth decreases with increasing particle diameter, as illustrated in Fig. 8. For a given spouting gas velocity, increasing particle diameter will lead to the increasing of particle inertial force, more jet momentum for jet schlepping particles will be dissipated when the jet ascends along axis. In addition, the bed viscosity will increase when the particle diameter increases, which dissipate more jet momentum to overcome larger resistance when the jet penetrate the bed materials. Therefore, the jet penetration depth decreases. 3.5. Effect of static bed height on jet penetration depth There has still little information concerning the effect of static bed height on the jet penetration in previous research (Chyang et al., 1997) so far, since generally considered that static bed height has little influence on the jet penetration depth for the vertical or horizontal single-jet system (Hong et al., 1996, 1997). However, the present research suggests that the jet penetration depth is significantly affected when the static bed height is low, while this effect is inconspicuous when the static bed height is high. The experimental data, as shown in Fig. 9, indicates that the jet penetration depth decreases with the increasing of static bed height. For a very shallow bed is used, the spouting gas plume pushes upwards to the surface very quickly. Thus, a suitable bed height is needed in order to get a fully developed jet.
320
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327 300
As=20mmx30mm,Qf / Qmf=1.0,Polystyrene us=6.9m/s us=10.4m/s us=13.8m/s us=17.2m/s us=20.7m/s
Lj (mm)
250
200
150
100
50
250
300
(a)
350 400 H0 (mm)
450
500
550
500
550
35
40
300 As=20mmx30mm,Qf / Qmf=1.0,Polystyrene us=6.9m/s us=10.4m/s us=13.8m/s us=17.2m/s us=20.7m/s us=24.2m/s
Lj (mm)
250
200
150
100
50 200
250
300
(b)
350 400 H0 (mm)
450
300 As=30mmx30mm,Qf /Qmf=1.0, Mung beans H0=245mm H0=300mm H0=400mm H0=470mm
250
Lj (mm)
200
150
100
50 (c)
5
10
15
20 25 us (m/s)
30
Fig. 9. Effect of static bed height on jet penetration depth.
3.6. Effect of fluidizing gas flow rate on jet penetration depth As is known, the spouting gas and the fluidizing gas should be well organized during the chemical process of coal gasification in spout–fluid bed, the fluidizing gas
plays an important role in jet penetration depth as well as the spouting gas. However, the effects of fluidizing gas flow rate on jet penetration depth have lead to debate in the literatures. Yates et al. (1986) employed two kinds of bed materials (aluminum, coke) to study the effect of fluidizing gas flow rate on the penetration depth, and found that the penetration depth decreased with increasing the fluidizing gas flow rate. Vaccaro et al. (1997) used glass beans (800.1200 m in diameter) to investigate this matter, and concluded that the jet penetration depth might either increase (d0 = 19 mm, 25 mm) or increase (d0 = 6.0 mm, 10 mm) due to the influence of different experimental conditions and operating variables. While Guo et al. (2001) employed the binary mixtures (80% sand and 20% millet, 30% coke and 70% slag) to investigate the effect of fluidizing gas flow rate on the penetration depth, and indicated that: at a given jet velocity, the penetration decreases when the fluidizing gas flow rate ranges from 1.0Qmf to 2.5Qmf , and then the jet penetration remains constant as the fluidizing gas flow rate increases from 2.5Qmf to 3.0Qmf . For present work, the jet penetration depth decreases with increasing fluidizing gas flow rate at a given spouting gas velocity, as presented in Fig. 10. The jet penetration depth varies smoothly when fluidizing gas flow rate is less than 1.0Qmf and more than 2.3Qmf , while it varies sharply when fluidizing gas flow rate ranges from 1.0Qmf to 2.3Qmf . The cause of the present observation has not been clarified, however the following discussion may be useful. In the cause when the fluidizing gas is added in to the bed, the bed voidage increases, which leads to the central jet gas diffusing to annular dense region, the jet momentum will be dissipated, thus the penetration decreases. When the fluidizing gas flow rate is less than 1.0Qmf , the voidage changes little, so the penetration depth varies smoothly. Bubbles form when the fluidizing gas flow rate is beyond the minimum fluidizing gas flow rate, the larger fluidizing gas flow rate, the more bubbles and the larger voidage in the annular dense region. Since the voidage in the annular dense region and fluidizing gas flow rate increases, the jet is easily dissipated in to the emulsion, moreover, as the bubbles form in the emulsion, bubbles and jets continuously coalesce, which dissipates much of the jet momentum, the penetration decreases sharply. Nevertheless, the penetration depth varies smoothly when the fluidizing gas flow rate is over 2.3Qmf . According to Guo et al. (2001), when the annular flow rate is too larger, the voidage in the annular dense region change much little, thus, the dissipation of jet momentum is nearly constant. For present study, the jet penetration depth is not constant but decreases smoothly seems to caused by discontinuous slugging observed in experiments with a frequency of about 0.3–0.7 Hz when the fluidizing gas flow rate is too larger. The formation and collapse of large bubbles when slugging would dissipate part of the jet momentum, thus, the jet penetration depth slightly decreases.
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
300
As=30mmx30mm,H0=400mm, Ploystyrene us=10.4m/s us=20.7m/s us=31.1m/s
250
Lj (mm)
200
150
100
50
0.0
0.5
1.0
1.5 2.0 Qf / Qmf
(a)
2.5
3.0
3.5
300 As=20mmx30mm,H0=515mm, Ploystyrene 10.4m/s
31.1m/s
20.7m/s
Lj (mm)
250
200
150
100
50
0.0
0.5
(b)
1.0
1.5 2.0 Qf / Qmf
2.5
3.0
300
Lj (mm)
250
As=30mmx30mm,H0=470mm, Mung beans us=13.8m/s us=24.2m/s us=34.5m/s
200
50 (c)
The typical comparison of experimental and calculated data showed that no correlation agrees well with the experimental results as shown in Fig. 11. Since it was difficult to determine a proper jet angle for the correlations proposed by Shakhova (1968) and Horio et al. (1990), a proper bed voidage εb for the correlations proposed by Merry (1971) and Zhou et al. (1999), and negative calculated values of Lj /dj at present calculating conditions by the two correlations (Zenz, 1968; Merry, 1971) when a low uj was used, and larger errors of the correlations of Merry (1975), the present work only employed the residual 14 correlations to perform the calculated data. The correlations from literatures almost did not consider the effect of the fluidizing gas flow rate on the penetration depth except the correlations proposed by sKnowlton and sHirsan, 1980, Luo et al. (1996) and Guo et al. (2001). The correlations not considered the effect of fluidizing gas flow rate might lead to errors between the calculated data and the experimental results, because the fluidizing gas takes effect on the jet penetration depth as illustrated in Fig. 10. The calculated data from the correlations (Knowlton and Hirsan, 1980; Luo et al., 1996; Guo et al., 2001), which considered the effect of fluidizing gas flow rate, are still not agree with present experimental results, as presented in Fig. 11(b). Moreover, the jet penetration depths are significantly affected by the static bed height especially a low static bed height as shown in Fig. 9, while no correlation considered the effect of static bed height among these correlations. The discrepancies, shown in Fig. 11, suggest that it might be difficult to find a generalized expression for the variety of working conditions encountered in practice. These correlations are not suitable for predicting the penetration depths for present work, thus a new correlation of the jet penetration depth in spout–fluid bed is expected. According to the previous research (Yang and Keairns, 1978, 1979; Luo et al., 1996, 1997; Guo et al., 2001; Hong et al., 2003), the jet penetration depth is the function of the two-phases Froude number, which can be expressed as
150
Lj =f dj
100
0.0
0.5
1.0 1.5 Qf / Qmf
2.0
2.5
3.0
Fig. 10. Effect of fluidizing gas flow rate on jet penetration depth.
3.7. New jet penetration depth correlation Main correlations of jet penetration depth in the literatures are listed in the Table 3. The typical comparison of our present experimental results with the prediction calculated by these correlations is shown in Fig. 11.
321
u2j g , = f (Fr , ). p − g gd j
(3)
The relation between the two-phases Froude number and the penetration depth can also be derived from the buoyancy theory of Turner (1973). The characteristic length scale, Lj , which is the jet penetration depth obtained by Turner for a buoyant fluid jet: 0.5 u2j g Lj J 3/4 ≈ 1/2 = = (Fr )0.5 . dj B dj 4 p − g gd j 4
(4)
For spout–fluid bed, since particles move down in the annular dense region and entrained into the jet, the flow dynamics is somewhat different from the fluid jets. However,
322
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
Table 3 Main correlations of jet penetration depth in the literatures (1968–2003) Lj type
References and correlations Shakhova (1968)a L j uj j dj + 0.5 cot = 13 g gd p
(Lmax + Lmin )/2
Zenz (1968) L
0.044 d j + 1.3 = 0.5 log(g u2j ) j
Lmax
Basov et al. (1969)a Lj dj =
u0.35 j
0.919dp 0.0007+0.56dp
Lmax
dj0.3
Merry (1971)
2
g uj Lj dj + 4.5 = 5.25 (1−b )p gd p
0.4
0.2 g p
dp 0.2 dj
(Lmax + LB )/2
Vakhrushev(1972)a u 0.5 Lj j 1 − tan1 dj = 0.5 c + tan 4kut
(Lmax + Lmin )/2
0.5 c 4.21, 0.25 k 0.8 Turner (1973) Lj dj = 4
u2j p −g gd j g
0.5 Lmax
Merry (1975)
d 0.3 u2j Lj g j 1.3 dj = 5.2 p dp gd j
0.2
− 1
Lmax
Wen et al. (1977)a d −0.585 d u −0.654 Lj p p g j j dj = 814.2 g dj
u2j gd j
0.47 (Lmax + Lmin )/2
Yang and Keairns (1978) 0.5 u2j g Lj dj = 6.5 p −g gd j
Lmax
Yang and Keairns (1979) 0.187 u2j g Lj d = 15.0 − gd
Lmax
Hirsan et al.(1980)a 0.67 g uj Lj u −0.24 = 26.6 u d
LB
Knowlton and Hirsan (1980) 0.88 g uj Lj u −0.54 u d = 19.3
Lmax
j
j
j
p
p
p
g
j
gd j
mf
gd p
mf
Wen et al. (1982) 2 0.42 d (u −u ) −0.42 d 0.66 Lj mf g g p j p 4 (uj −umf ) d = 1.15 × 10 gd d j
p
p
j
(Lmax + Lmin )/2
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
323
Table 3 (Contd.,) Lj type
References and correlations Wen et al. (1982) 0.38 u2j Lj dj = 1.3 gd p
d u 0.13 0.56 d 0.25 g p j g j
Lmax
dp
p
Wu and Whiting (1988) 0.34 g 0.67 g dp (uj −umf ) −0.34 dp 0.43 Lj (uj −umf )2 = 821 d gd d j
p
p
Blake et al. (1990) 0.322 u2j Lj dj = 26.9 gd j
j
0.325 u d 2 −0.124 p j p g
Lmax
dj
p
Lmax
Horio et al. (1990)b −0.095 Lj 2u 0.857 1.2 dj = − 1 ln umf 1/2 pn P n
k
j
f F r 0.3 0.2 p j dn
dj dn = 1.56 k 1/2 tan
(Lmax + Lmin )/2
Pn
k = (1 − sin )(1 + sin ), fj = 0.02 Luo et al. (1996)
Lj u dj = 23.12 umf
−0.04808
Luo et al. (1997)
u2j g Lj dj = 55.6 p −g gd j
Zhou and Shen (1999)
u2j p −g gd j
0.31
2
p uj Lj dj = 408.299 (1−b )p gd p
g
0.2471
(p −g )uj dp2 dj
0.172
u d −0.08069 g j p
−0.124
0.412 g p
Lmax
Lmax
dp −0.000418 dj
(Lmax + LB )/2
, 0 < uf 2.5umf
Lmax
Guo et al. (2001)
Lj u dj = 19.18 umf
−0.3616
u2j g Lj D0 = 11.52 p −g gd j
Hong et al. (2003)
u2j g Lj dj = 26.47 p −g gd j
u2j p −g gd j g
0.2383
0.1966 , uf > 2.5umf
0.293
u d −0.1138 g j p
Lmax
a Cited from Davidson et al. (1985). b Cited from Kimura et al. (1995).
both Eqs. (3) and (4) indicate that the jet penetration depth strongly depends on the two-phase Froude number. The twophase Froude number has been successfully used (Yang and Keairns, 1978, 1979; Luo et al., 1996, 1997; Guo et al., 2001; Hong et al., 2003). Hence, we employed the two-phase Froude number to study the correlation of the jet penetration depth in spout–fluid bed. Present experimental results indicated that increasing particle diameter dp would lead to increasing particle inertial
force, more jet momentum for jet schlepping particles would be dissipated when jet ascends along axis. In addition, the bed viscosity would increase when increasing particle diameter, which dissipates more jet momentum to overcome larger resistance when penetrate the bed. Thus we employ the particle Reynolds number Rep rather than the dimensionless parameter of dp /dj to describe the effect of particle diameter on penetration depth. The particle Reynolds number Rep has ever used to perform the correlation of
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
Lj = f (us , ds , H0 , p , dp , Qf , ) ds g u2s H0 Qs + Qf dp g us , , , , =f p − g gd s Dt Qmf H0 u = f Fr , , , Rep , . Dt umf
14
12
4
12
10
0
(5)
where
14
11
2
−0.17 Lj u −0.11 0.39 H0 = 10.86(Fr ) (Rep )−0.03 ds Dt umf 0 uf < 3.0umf , (7)
dp g us
16
4
the coefficient a contains the parameter of jet expansion angle in Eq. (6), which is similar to previous research (Yang and Keairns, 1978, 1979; Filla and Massimilla, 1984; Blake et al., 1990; Luo et al., 1996, 1997; Guo et al., 2001; Hong et al., 2003). By regressing 606 groups of present experimental data using a commercial Package of Encyclopedias of Medical Statistic (PEMS) program, the correlation of the jet penetration depth was determined, as shown in Eq. (7).
Rep =
5
experimental data
6
b g Lj u2s H0 c u d dp g us e =a ds p − g gd s Dt umf c d H0 u = a(Fr )b (6) (Rep )e Dt umf
g u2s , p − g gd s
18
8
Since the dimensionless parameters of two-phase Froude number Fr , particle Reynolds number Rep , H0 /Dt and u/umf are employed to describe the correction of jet penetration depth shown in Eq. (5), Dt is the hydraulic diameter of bed, the jet penetration depth can be given as the following formation:
Fr =
20
Lj / dj
penetration depths (Blake et al., 1990; Luo et al., 1996; Hong et al., 2003). According to the analysis above, spouting gas velocity, spout nozzle diameter, static bed height, particle density, particle diameter and fluidizing gas flow rate take effect on the jet penetration depth. Thus, the jet penetration depth should be the function of these effects, which can be expressed as
.
Compared with various correlations of the jet penetration depth in previous literatures, the correlation proposed by present work has a development of considering more effects on the jet penetration depth, especially static bed height and fluidizing gas flow rate.
1
10
5
25 20 uj (m/s)
15
(a)
30
13 6 10 2 9 7 8 3
35
40
20 18
experimental data
16 14 12 Lj / dj
324
12 13
10 8 6 4
6
2 0 (b)
0.0
0.5
1.0
1.5 Qf / Qmf
2.0
2.5
3.0
Fig. 11. Typical comparison of present experimental data with calculated data from previous correlations (a) H0 = 470 mm, dj = 30 mm, Qf /Qmf =1, mung beans. (b) H0 =400 mm, dj =30 mm, us =20.7 m/s, polystyrene. 1—Basov et al. (1969); 2—Turner (1973); 3—Wen et al. (1977); 4—Yang and Keairns (1978); 5—Yang and Keairns (1979); 6—Knowlton and Hirsan (1980); 7—Wen et al. (1982); 8—Wen et al. (1982); 9—Wu and Whiting (1988); 10—Blake et al. (1990); 11—Luo et al. (1997); 12—Luo et al. (1996); 13—Guo et al. (2001); 14—Hong et al. (2003).
The correlation was put forward to predict the jet penetration depth at various operation conditions. Typical comparisons of present experimental data with calculated data from previous correlations and present correlation are presented in Fig. 12. The present correlation agrees with our experimental results qualitatively. The comparisons of experimental results at various operation conditions with the calculations by Eq. (7) are shown in Fig. 13. Besides of agreement with our present investigations, these results are consistent to those reported previous by Kimura et al. (1995), Vaccaro et al. (1997), Yang, 1998 and Guo et al. (2001), while are not accordant well with Chyang et al. (1997) resulting in the horizontal nozzle they used.
W. Zhong, M. Zhang / Chemical Engineering Science 60 (2005) 315 – 327
experimental data
18 16
12
14
4
20
13
6
(Lj/dj)exp
14
15 10 2
8
7
8 3 1
2 10
5
25 20 uj (m/s)
15
(a)
30
-30%
5
35
40
22
+30%
15 10
9
4
0
25
6
10
Present work Kimura et al.(1995) Vaccaro et al.(1997) Chyang et al.(1997) Yang et al.(1998) Guo et al.(2001)
11
12 Lj/dj
30
5
20
325
0
0
5
10
15 (Lj/dj)cal
20
25
30
5
20
experimental data
Fig. 13. Comparison of jet penetration depth experimental results with calculations by Eq. (7).
11
18 16 13
Lj/dj
14
4. Conclusions
12
14
4
6 15
12
10 8
10 2 9
7
6 4
8 3 1
2 0 5
10
25 20 uj (m/s)
15
(b)
30
35
40
18 experimental data
16 13 12
14
Lj/dj
12 10 6
8 15
6 4 (c)
0.0
0.5
1.0
1.5 Qf / Qmf
2.0
2.5
3.0
Fig. 12. Comparisons of present experimental data with calculated data from previous correlations and present correlation. (a) H0 = 400 mm, dj = 30 mm, Qf /Qmf = 0, polystyrene. (b) H0 = 515 mm, dj = 24 mm, Qf /Qmf =1.0, polystyrene. (c) H0 =515 mm, dj =24 mm, us =20.7 m/s, polystyrene. 1—Basov et al. (1969); 2—Turner (1973); 3—Wen et al. (1977); 4—Yang and Keairns (1978); 5—Yang and Keairns (1979); 6—Knowlton and Hirsan (1980); 7—Wen et al. (1982); 8—Wen et al. (1982); 9—Wu and Whiting (1988); 10—Blake et al. (1990); 11—Luo et al. (1997); 12—Luo et al. (1996); 13—Guo et al. (2001); 14—Hong et al. (2003); 15–present work.
Considering the operation of spout–fluid bed coal gasification, a proper spouting gas flow rate can not form a spout but a jet, which making it un-easy for much steam and air to pass through the bed and wasted, and enhancing the gas diffusion and particles mixture in bed, more uniform axial temperature profiles leads to improving the gasification efficiency. The jet penetration depth is an important parameter to describe the jet action during the chemical process of spout–fluid bed coal gasification. Pressure signal analysis and image processing have systematically studied the jet penetration depth in a twodimension cold model of a spout–fluid bed coal gasifier. The jet penetration depth increases with increasing spouting gas velocity and spout nozzle diameter, while it decrease with increasing particle density, particle diameter, static bed height and fluidizing gas flow rate. A new correlation of the jet penetration depth was developed and proposed for predicting the jet penetration depth, which had a development of considering more effects on the jet penetration depth, especially static bed height and fluidizing gas flow rate. The correlation was compared with published experimental data or correlations, which was in well agreement with the present experimental results and some other references. Besides of spouting gas velocity, spout nozzle diameter, static bed height, particle diameter, particle density and fluidizing gas flow rate, other parameters such as Geldart’s Group of particle, the configuration of fluidizing gas distributor, jet angle, and etc. can be expected to exert an influence on the jet penetration depth. This aspect clearly requires further experimental study. It is hoped that the present work will contribute to the further development of more rigorous and reliable prediction of jet penetration depth.
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References
Notation As a, b, c, d, e Dt ds dj dp f Fr g H0 Lmin Lmax LB Lj p Qf Qs Qmf Rep t u uj us uf umf
area of spout nozzle, mm2 coefficient of penetration depth correlation bed hydraulic diameter, mm spout nozzle hydraulic diameter, mm jetting nozzle diameter, mm particle diameter, mm sampling frequency, Hz Two-phase Froude number gravitational accelerations, m/s2 static bed height, mm penetration depth of a cavity permanently attached to the nozzle, mm penetration depth of a series of interpenetrating cavities, mm the deepest penetration depth of jet bubbles before losing their momentum, mm jet penetration depth, mm pressure different measured by multi-channel signal sampling system, Pa fluidizing gas flow rate, m3 /s spouting gas velocity, m3 /s minimum fluidizing gas flow rate, m3 /s particle Reynolds number sampling time, s superficial gas velocity, m/s jet gas velocity, m/s spouting gas velocity, m/s fluidizing gas velocity, m/s minimum fluidizing gas velocity, m/s
Greek letters
b p g
particle packing voidage bed voidage Jet expansion angle, rad gas viscosity, kg/(m s) particle density, kg/m3 gas density, kg/m3
Acknowledgements Financial support from the National Key Program of Basic Research in China (NO. 199902210535), the Innovation Project for Graduate Student in University of Jiangsu Province (Hydrodynamic Characteristics and Scale-up Rules of Spout–fluid Beds) and the 985 Projects of Southeast University for Clean Energy Technology (SEU985-CET) were sincerely acknowledged. The authors also expressed sincere gratitude to Prof. J.R. Grace and Prof. S. Vaccaro for presenting us some of their valuable papers, which contributed to our research.
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