- Email: [email protected]
Experimental Study on a New Dual-Layer Granular Bed Filter for Removing Particulates
Jun. 2007
Journal of China University of Mining & Technology
Vol.17
No.2
J China Univ Mining & Technol 2007, 17(2): 0201–0204
Experimental Study on a New Dual-Layer Granular Bed Filter for Removing Particulates YANG Guo-hua, ZHOU Jiang-hua Maritime College, Ningbo University, Ningbo, Zhejiang 315211, China Abstract: A new dual-layer granular bed filter for hot gas cleanup was invented and studied experimentally. Fine sand, 0.5–1 mm grain size and about 1350 kg/m3 bulk density, was used as the lower layer of the filter. Expanded perlite particles, 2–5 mm grain size and about 70 kg/m3 bulk density, was used for the upper layer of the filter in this study. It was confirmed that the sizes and densities of these two media matched well; the binary media remained in complete segregation during regeneration by fluidization. Test results show that the filtration of the expanded perlite particle layer was characterized as “deep bed filtration.” Filtration of the fine sand layer was “surface cake filtration.” The expanded perlite particle layer contributed about 90% to the bed dust capacity, but only about 20% to the total bed pressure drop, which increased the bed dust capacity ten fold compared to a single-layer bed of the same sand and the same total bed pressure drop. The dust cake on the surface of the fine sand layer raised the collection efficiencies to over 99.99%. Key words: filtration; granular bed filter; particulate CLC number: X 51
1
Introduction
Advanced coal-fired combined power cycles, such as IGCC, require efficient removal of fine particles from high temperature, high pressure gas streams; hot gas filtration is a key component in advanced coal-fired power systems[1]. Research suggests that ceramic candle filters and granular bed filters are the two most promising approaches to hot gas cleanup for advanced coal conversion technologies[2]. Granular bed filters are attractive for hot gas filtration because of low construction and operating costs, high reliability at high temperature and great potential for simultaneous removal of solid and gaseous contaminants through the use of chemically reactive filter media. The basic principle of granular filtration is the removal of suspended particles in a gas-solid flow by passing the stream through filter media composed of granular material. As the gas-solid suspension flows through the filter media particles are deposited on the surfaces of the granules by interception, diffusion, inertial impaction, gravitational settling, electrical
migration or a combination of two or more of these effects. Granular bed filters have been used to filter particulates at temperature above 400 ℃ for over 80 years. Traditionally, several kinds of fixed bed filter were used but recently research has focused primarily on the moving bed[3–6]. In a fixed bed filter pressure drop increases continually, as more and more dust is trapped by the filter, until filtering must be stopped for regeneration. The smaller the size of the filter granules the higher the collection efficiency. But small granules also mean less dust capacity for the filter. Consequently, the size of the filter granules was chosen as a compromise between collection efficiency and bed dust capacity (or bed pressure drop). The granule size was usually no less than 1 mm in a single layer granular bed filter, e.g. 3–3.5 mm, 1–2 mm; 2–3 mm; 2–4 mm or 2–3 mm[7–9]. To raise collection efficiency and bed dust capacity simultaneously a new dual-layer granular bed filter was invented. Experimental results from testing such a filter are described in this paper.
Received 05 August 2006; accepted 10 September 2006 Projects 2006C23075 supported by the Key Research Project of Zhejiang Province and 02J20101-19 by the Science Foundation of Ningbo City Corresponding author. Tel: +86-574-87600550; E-mail address: [email protected]
202
2
Journal of China University of Mining & Technology
Principle
A new dual-layer granular bed filter consists of a fixed bed filter having two layers of granules along the flow direction, as shown in Fig. 1.
Fig. 1
(b) Regeneration
Principle of dual-layer granular bed filter
The lower filter media are small and heavy granules, while the upper filter media are big and light granules. The dirty gas first flows through the upper filter media, where the great majority of dust is trapped, and then passes through the lower filter media, where the rest of the fine particles are captured. The upper filter media have greater dust capacity and lower pressure drop because of their big size. The lower filter media can capture very fine particles of micron or sub-micron size. The lower layer provides very high filtration efficiency due to the small diameter of the filter media. The combination of these two layer of filer media provides both high filtration efficiency and high bed dust capacity (or low filter pressure drop), which is impossible for a single layer granular bed filter. Like all other fixed bed filters when bed dust-capacity or bed pressure-drop reaches a predetermined value the filter media need regeneration by fluidizing the bed. When a back flow passes vertically through the filter bed both layers of gran-
Experimental
The research was carried out in an experimental apparatus shown in Fig. 2. The cross sectional area of the granular bed was 120 mm×150 mm. Fine sand, 0.5–1 mm granule size and about 1350 kg/m3 bulk density, was used as the lower layer of the filter. Expanded perlite particles, 2–5 mm granule size and about 70 kg/m3 bulk density, was the upper filter layer. Air mixed with ground fly-ash, collected from an electrostatic precipitator, was the test gas stream. The test was conducted at ambient temperature. The size distribution of the ground fly ash, measured by MASTERSIAER 2000, is shown in Fig. 3. The d(0.1), d(0.5) and d(0.9) are 0.563 µm, 0.921 µm and 2.26 µm, respectively, by number, or 2.252 µm, 8.299 µm and 22.766 µm by volume
Fig. 2
(a) By population
4.1
Experimental apparatus
(b) By volume
Fig. 3
4
No.2
ules are fluidized. Consequently, the dust previously retained in the filter media is entrained in the fluidizing gas stream and the filter bed is regenerated in a few seconds. Because the two layers of granules have a large difference in bulk density the light granules float on the heavy granules when fluidized. The two layers of granules never mix during fluidization and the two layer bed structure is retained after regeneration. It is key that the two layers of filter media remain completely segregated after fluidization.
3
(a) Filtration
Vol.17
Size distribution of ground fly ash
Results and Discussion Filtration
The appearance of the filter after trapping the fly ash is seen in Fig. 4. The upper layer of the bed, the expanded perlite granule layer, was fully filled with fly ash in its upper part thus behaving as a typical
“deep bed filtration” as expected from other researches on coarse particle filters. This increased the bed dust capacity. Looking at the fine sand layer, only a thin layer of fly ash was observed on the surface of fine sand and no fly ash was observed inside this layer. Hence, the fine sand layer could be said to act as a “surface cake filtration.” This allows the filter to obtain very high
Experimental Study on a New Dual-Layer Granular Bed Filter for ……
YANG Guo-hua et al
collection efficiency. Table 1 summarizes some test data, where the collection efficiency is defined as: c −c Collection efficiency = 1 2 , c1 where c1 is inlet dust concentration, g/m3; c2 outlet dust concentration, g/m3.
Table 2 confirm that addition of a coarse granule layer to a fine layer is viable economically. The dual-layer bed added only 20 percent to the pressure drop but added more than 90 percent to the filter dust capacity. Table 2
Appearance of the filter after filtration Table 1
Test data of filtration Dual-layer
Single sand bed
Ⅰ
Sand layer height (mm)
75
75
75
Expanded perlite layer height (mm)
0
80
180
Ⅱ
Superficial velocity (m/s)
0.33
0.33
0.33
Bed pressure drop (9.8Pa)
212–505
224–525
215–511
19.7
60.0
113.0
Inlet dust concentration (g/m )
7.52
10.45
12.74
Outlet dust concentration (mg/m3)
1.20
1.05
0.89
Bed dust capacity (g)
52.4
224.6
515.5
99.984
99.99
99.99
Filtering time (min) 3
Collection efficiency (%)
From Table 1 the following can be seen: 1) Compared with a single-layer sand bed filter the dual-layer filter has a greater bed dust capacity. The dual-layer bed filter with a perlite granule layer 180 mm in height had a bed dust capacity ten times that of the single-layer sand bed under the same bed pressure drop. This shows the effect from “deep bed filtration” by the coarse granules. 2) Both the single-layer sand bed and the duallayer bed had very high collection efficiencies. This is the contribution of “surface cake filtration” provided by the fine sand layer. 3) The height of the perlite granule layer had a significant effect on bed dust capacity. The greater the depth of perlite granule layer the greater the bed dust capacity. This further confirms that the perlite granule layer worked as “deep bed filtration.” The combined effects of “deep bed filtration” and “surface cake filtration” gave the dual-layer bed filter both high collection efficiency and great bed dust capacity. This combination is impossible for a single-layer bed; coarse granule beds do not have high collection efficiency while fine granule beds do not have large capacity. The pressure-drop data shown in
4.2
Pressure drop distribution in bed
Ratio of sand layer pressure drop (%)
Ratio of perlite layer pressure drop (%)
Ratio of distributor pressure drop (%)
0
78.20
13.74
8.06
5
79.02
13.39
7.59
10
79.15
13.64
7.21
15
78.60
14.40
7.00
20
78.63
14.52
6.85
30
77.78
15.71
6.51
40
76.18
17.79
6.03
50
75.68
18.75
5.57
55
77.20
17.54
5.26
65
75.03
20.11
4.86
78
74.38
21.32
4.30
80
74.28
21.53
4.19
85
74.10
21.79
4.11
90
76.21
19.71
4.08
Filtering time (min)
Fig. 4
203
95
76.80
19.31
3.89
100
76.92
19.28
3.80
105
77.20
19.14
3.66
110
77.65
18.90
3.45
113
78.00
18.66
3.34
Regeneration
After the pressure drop of a fixed bed filter reaches some preset value the filter is renewed. The renewal of a single-layer bed filter can be achieved by either mechanical or pneumatic means. But a dual-layer bed filter can only be regenerated pneumatically lest the dual-layer bed become mixed. Pneumatic regeneration involves a fluidizing back-flow which entrains the previously retained dust and carries it away while fluidizing both the coarse and fine bed layers. This allows regeneration of the bed in a few seconds. Of course, it is key to keep the two layers segregated during the fluidization step otherwise the combined effects of “deep bed filtration” and “surface cake filtration” will be lost. To achieve this the two layers of granules should be carefully selected based on the following rules: First, both layers should be fluidized under a proper velocity; and second, the two layers should not mixed during fluidization, the upper layer should float on the lower layer of granules at all times. Following these rules fine, heavy sand and coarse, light, expanded perlite were selected by size and bulk density. The fine sand, 0.5–1 mm in size and 1350 kg/m3 bulk density had a minimum fluidization velocity of 0.34 m/s. This is the same minimum fluidization velocity as expected for expanded perlite 2–5 mm in size and 70 kg/m3 bulk density. When a
204
Journal of China University of Mining & Technology
back-flow passed through the bed vertically with a velocity between 0.432–0.671 m/s the two layers of granules were well fluidized but were not mixed, keeping completely segregated as shown in Fig. 5. Table 3 summarizes some test data concerning filter regeneration where the regeneration efficiency is defined as: Regeneration efficiency =
Vol.17
No.2
all more than 80% after 10 to 15 seconds of fluidization. This confirms that the dual-layer granular bed can be effectively regenerated by fluidization.
∆p2 − ∆p1 ∆p2 − ∆p0
where ∆p2 is the pressure drop of the layer or bed before regeneration, ∆p1 is the pressure drop of a layer or bed after regeneration and ∆p0 the pressure drop of a fresh layer or bed. From Table 3, it is seen that the regeneration efficiencies of the two layers, as well as the filter, were Table 3
Fig. 5
Complete segregation of two layers after regeneration
Test data of the filter regeneration
Sand layer height (mm)
Perlite layer height (mm)
Gas velocity (m/s)
Fluidizing time (s)
Regeneration efficiency of sand layer (%)
Regeneration efficiency of perlite layer (%)
Regeneration efficiency of bed (%)
75
0
0.56
10
82.50
—
82.50
75
80
0.49
10
85.14
84.62
85.10
75
135
0.49
10
85.31
85.45
85.33
75
180
0.49
15
88.74
80.76
87.05
5 Conclusion The dual-layer granular bed filter is a new concept for granular bed filtration. In this new filter most of the dust is captured in the upper layer which acts as a primary filter. The dust which escapes the upper layer is captured in the lower layer which has higher efficiency. This way both high filtration efficiency and great bed dust capacity were obtained in a single filter. Obviously, this is impossible for single layer granular bed filters. It was confirmed that the two layers of
granules should be carefully selected so that a combined effect of “deep bed filtration” from coarse granules in the upper layer and “surface cake filtration” from fine granules in the lower layer can be achieved. It was shown that the sizes and densities of the binary media could be chosen to keep the layers completely segregated during regeneration by fluidization. The author gratefully acknowledges the support of K C Wong Magna Foun in Ningbo University.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Smid J, Hsian S S, Hsian C Y, et al. Hot gas cleanup: new designs for moving bed filters. Filtration & Separation, 2005, 5: 36–39. Robert C B, Shi H W, Gerald C, et al. Similitude study of a moving bed granular filter. Powder Technology, 2003, 138(2–3): 201–210. Smid J, Hsian S S, Tsai F H, et al. Velocities in moving granular bed filters. Powder Technology, 2001, 114: 205–212. Hsian S S, Smid J, Tsai F H, et al. Placement of flow-corrective elements in a moving granular bed with louvered-walls. Chemical Engineering and Processing, 2004, 43(8): 1037–1045. Rodon I, Lee K C, Pfeffer R, et al. Granular bed filtration assisted by filter-cake formation, panel bed gas filter with puffback renewal of gas-entry surfaces. Powder Technology, 2005, 155: 52–61. Wu M S, Lee K C, Pfeffer R, et al. Granular bed filtration assisted by filter-cake formation, penetration of filter cakes by a monodisperse aerosol. Powder Technology, 2005, 155: 62–73. Rao S R. Developmental status of BHEL’S high temperature high pressure (HTHP) circulating bed granular filter (CBGF). Heat Recovery System & CHP, 1995, 15(2): 199–204. Gao S W, Xu S S, Wei S R, et al. Experimental study on moving granular bed filter for removing particulate at ambient temperature and high pressure. Journal of Fuel Chemistry and Technology, 2001, 29(6): 532–536. (In Chinese) Gao S W, Wei S R, Deng R Y, et al. Structure optimization and experimental study of moving granular bed filter for hot coal gas cleanup. Journal of Xian Jiaotong University, 2002, 36(9): 903–906. (In Chinese)
Journal of China University of Mining & Technology
Vol.17
No.2
J China Univ Mining & Technol 2007, 17(2): 0201–0204
Experimental Study on a New Dual-Layer Granular Bed Filter for Removing Particulates YANG Guo-hua, ZHOU Jiang-hua Maritime College, Ningbo University, Ningbo, Zhejiang 315211, China Abstract: A new dual-layer granular bed filter for hot gas cleanup was invented and studied experimentally. Fine sand, 0.5–1 mm grain size and about 1350 kg/m3 bulk density, was used as the lower layer of the filter. Expanded perlite particles, 2–5 mm grain size and about 70 kg/m3 bulk density, was used for the upper layer of the filter in this study. It was confirmed that the sizes and densities of these two media matched well; the binary media remained in complete segregation during regeneration by fluidization. Test results show that the filtration of the expanded perlite particle layer was characterized as “deep bed filtration.” Filtration of the fine sand layer was “surface cake filtration.” The expanded perlite particle layer contributed about 90% to the bed dust capacity, but only about 20% to the total bed pressure drop, which increased the bed dust capacity ten fold compared to a single-layer bed of the same sand and the same total bed pressure drop. The dust cake on the surface of the fine sand layer raised the collection efficiencies to over 99.99%. Key words: filtration; granular bed filter; particulate CLC number: X 51
1
Introduction
Advanced coal-fired combined power cycles, such as IGCC, require efficient removal of fine particles from high temperature, high pressure gas streams; hot gas filtration is a key component in advanced coal-fired power systems[1]. Research suggests that ceramic candle filters and granular bed filters are the two most promising approaches to hot gas cleanup for advanced coal conversion technologies[2]. Granular bed filters are attractive for hot gas filtration because of low construction and operating costs, high reliability at high temperature and great potential for simultaneous removal of solid and gaseous contaminants through the use of chemically reactive filter media. The basic principle of granular filtration is the removal of suspended particles in a gas-solid flow by passing the stream through filter media composed of granular material. As the gas-solid suspension flows through the filter media particles are deposited on the surfaces of the granules by interception, diffusion, inertial impaction, gravitational settling, electrical
migration or a combination of two or more of these effects. Granular bed filters have been used to filter particulates at temperature above 400 ℃ for over 80 years. Traditionally, several kinds of fixed bed filter were used but recently research has focused primarily on the moving bed[3–6]. In a fixed bed filter pressure drop increases continually, as more and more dust is trapped by the filter, until filtering must be stopped for regeneration. The smaller the size of the filter granules the higher the collection efficiency. But small granules also mean less dust capacity for the filter. Consequently, the size of the filter granules was chosen as a compromise between collection efficiency and bed dust capacity (or bed pressure drop). The granule size was usually no less than 1 mm in a single layer granular bed filter, e.g. 3–3.5 mm, 1–2 mm; 2–3 mm; 2–4 mm or 2–3 mm[7–9]. To raise collection efficiency and bed dust capacity simultaneously a new dual-layer granular bed filter was invented. Experimental results from testing such a filter are described in this paper.
Received 05 August 2006; accepted 10 September 2006 Projects 2006C23075 supported by the Key Research Project of Zhejiang Province and 02J20101-19 by the Science Foundation of Ningbo City Corresponding author. Tel: +86-574-87600550; E-mail address: [email protected]
202
2
Journal of China University of Mining & Technology
Principle
A new dual-layer granular bed filter consists of a fixed bed filter having two layers of granules along the flow direction, as shown in Fig. 1.
Fig. 1
(b) Regeneration
Principle of dual-layer granular bed filter
The lower filter media are small and heavy granules, while the upper filter media are big and light granules. The dirty gas first flows through the upper filter media, where the great majority of dust is trapped, and then passes through the lower filter media, where the rest of the fine particles are captured. The upper filter media have greater dust capacity and lower pressure drop because of their big size. The lower filter media can capture very fine particles of micron or sub-micron size. The lower layer provides very high filtration efficiency due to the small diameter of the filter media. The combination of these two layer of filer media provides both high filtration efficiency and high bed dust capacity (or low filter pressure drop), which is impossible for a single layer granular bed filter. Like all other fixed bed filters when bed dust-capacity or bed pressure-drop reaches a predetermined value the filter media need regeneration by fluidizing the bed. When a back flow passes vertically through the filter bed both layers of gran-
Experimental
The research was carried out in an experimental apparatus shown in Fig. 2. The cross sectional area of the granular bed was 120 mm×150 mm. Fine sand, 0.5–1 mm granule size and about 1350 kg/m3 bulk density, was used as the lower layer of the filter. Expanded perlite particles, 2–5 mm granule size and about 70 kg/m3 bulk density, was the upper filter layer. Air mixed with ground fly-ash, collected from an electrostatic precipitator, was the test gas stream. The test was conducted at ambient temperature. The size distribution of the ground fly ash, measured by MASTERSIAER 2000, is shown in Fig. 3. The d(0.1), d(0.5) and d(0.9) are 0.563 µm, 0.921 µm and 2.26 µm, respectively, by number, or 2.252 µm, 8.299 µm and 22.766 µm by volume
Fig. 2
(a) By population
4.1
Experimental apparatus
(b) By volume
Fig. 3
4
No.2
ules are fluidized. Consequently, the dust previously retained in the filter media is entrained in the fluidizing gas stream and the filter bed is regenerated in a few seconds. Because the two layers of granules have a large difference in bulk density the light granules float on the heavy granules when fluidized. The two layers of granules never mix during fluidization and the two layer bed structure is retained after regeneration. It is key that the two layers of filter media remain completely segregated after fluidization.
3
(a) Filtration
Vol.17
Size distribution of ground fly ash
Results and Discussion Filtration
The appearance of the filter after trapping the fly ash is seen in Fig. 4. The upper layer of the bed, the expanded perlite granule layer, was fully filled with fly ash in its upper part thus behaving as a typical
“deep bed filtration” as expected from other researches on coarse particle filters. This increased the bed dust capacity. Looking at the fine sand layer, only a thin layer of fly ash was observed on the surface of fine sand and no fly ash was observed inside this layer. Hence, the fine sand layer could be said to act as a “surface cake filtration.” This allows the filter to obtain very high
Experimental Study on a New Dual-Layer Granular Bed Filter for ……
YANG Guo-hua et al
collection efficiency. Table 1 summarizes some test data, where the collection efficiency is defined as: c −c Collection efficiency = 1 2 , c1 where c1 is inlet dust concentration, g/m3; c2 outlet dust concentration, g/m3.
Table 2 confirm that addition of a coarse granule layer to a fine layer is viable economically. The dual-layer bed added only 20 percent to the pressure drop but added more than 90 percent to the filter dust capacity. Table 2
Appearance of the filter after filtration Table 1
Test data of filtration Dual-layer
Single sand bed
Ⅰ
Sand layer height (mm)
75
75
75
Expanded perlite layer height (mm)
0
80
180
Ⅱ
Superficial velocity (m/s)
0.33
0.33
0.33
Bed pressure drop (9.8Pa)
212–505
224–525
215–511
19.7
60.0
113.0
Inlet dust concentration (g/m )
7.52
10.45
12.74
Outlet dust concentration (mg/m3)
1.20
1.05
0.89
Bed dust capacity (g)
52.4
224.6
515.5
99.984
99.99
99.99
Filtering time (min) 3
Collection efficiency (%)
From Table 1 the following can be seen: 1) Compared with a single-layer sand bed filter the dual-layer filter has a greater bed dust capacity. The dual-layer bed filter with a perlite granule layer 180 mm in height had a bed dust capacity ten times that of the single-layer sand bed under the same bed pressure drop. This shows the effect from “deep bed filtration” by the coarse granules. 2) Both the single-layer sand bed and the duallayer bed had very high collection efficiencies. This is the contribution of “surface cake filtration” provided by the fine sand layer. 3) The height of the perlite granule layer had a significant effect on bed dust capacity. The greater the depth of perlite granule layer the greater the bed dust capacity. This further confirms that the perlite granule layer worked as “deep bed filtration.” The combined effects of “deep bed filtration” and “surface cake filtration” gave the dual-layer bed filter both high collection efficiency and great bed dust capacity. This combination is impossible for a single-layer bed; coarse granule beds do not have high collection efficiency while fine granule beds do not have large capacity. The pressure-drop data shown in
4.2
Pressure drop distribution in bed
Ratio of sand layer pressure drop (%)
Ratio of perlite layer pressure drop (%)
Ratio of distributor pressure drop (%)
0
78.20
13.74
8.06
5
79.02
13.39
7.59
10
79.15
13.64
7.21
15
78.60
14.40
7.00
20
78.63
14.52
6.85
30
77.78
15.71
6.51
40
76.18
17.79
6.03
50
75.68
18.75
5.57
55
77.20
17.54
5.26
65
75.03
20.11
4.86
78
74.38
21.32
4.30
80
74.28
21.53
4.19
85
74.10
21.79
4.11
90
76.21
19.71
4.08
Filtering time (min)
Fig. 4
203
95
76.80
19.31
3.89
100
76.92
19.28
3.80
105
77.20
19.14
3.66
110
77.65
18.90
3.45
113
78.00
18.66
3.34
Regeneration
After the pressure drop of a fixed bed filter reaches some preset value the filter is renewed. The renewal of a single-layer bed filter can be achieved by either mechanical or pneumatic means. But a dual-layer bed filter can only be regenerated pneumatically lest the dual-layer bed become mixed. Pneumatic regeneration involves a fluidizing back-flow which entrains the previously retained dust and carries it away while fluidizing both the coarse and fine bed layers. This allows regeneration of the bed in a few seconds. Of course, it is key to keep the two layers segregated during the fluidization step otherwise the combined effects of “deep bed filtration” and “surface cake filtration” will be lost. To achieve this the two layers of granules should be carefully selected based on the following rules: First, both layers should be fluidized under a proper velocity; and second, the two layers should not mixed during fluidization, the upper layer should float on the lower layer of granules at all times. Following these rules fine, heavy sand and coarse, light, expanded perlite were selected by size and bulk density. The fine sand, 0.5–1 mm in size and 1350 kg/m3 bulk density had a minimum fluidization velocity of 0.34 m/s. This is the same minimum fluidization velocity as expected for expanded perlite 2–5 mm in size and 70 kg/m3 bulk density. When a
204
Journal of China University of Mining & Technology
back-flow passed through the bed vertically with a velocity between 0.432–0.671 m/s the two layers of granules were well fluidized but were not mixed, keeping completely segregated as shown in Fig. 5. Table 3 summarizes some test data concerning filter regeneration where the regeneration efficiency is defined as: Regeneration efficiency =
Vol.17
No.2
all more than 80% after 10 to 15 seconds of fluidization. This confirms that the dual-layer granular bed can be effectively regenerated by fluidization.
∆p2 − ∆p1 ∆p2 − ∆p0
where ∆p2 is the pressure drop of the layer or bed before regeneration, ∆p1 is the pressure drop of a layer or bed after regeneration and ∆p0 the pressure drop of a fresh layer or bed. From Table 3, it is seen that the regeneration efficiencies of the two layers, as well as the filter, were Table 3
Fig. 5
Complete segregation of two layers after regeneration
Test data of the filter regeneration
Sand layer height (mm)
Perlite layer height (mm)
Gas velocity (m/s)
Fluidizing time (s)
Regeneration efficiency of sand layer (%)
Regeneration efficiency of perlite layer (%)
Regeneration efficiency of bed (%)
75
0
0.56
10
82.50
—
82.50
75
80
0.49
10
85.14
84.62
85.10
75
135
0.49
10
85.31
85.45
85.33
75
180
0.49
15
88.74
80.76
87.05
5 Conclusion The dual-layer granular bed filter is a new concept for granular bed filtration. In this new filter most of the dust is captured in the upper layer which acts as a primary filter. The dust which escapes the upper layer is captured in the lower layer which has higher efficiency. This way both high filtration efficiency and great bed dust capacity were obtained in a single filter. Obviously, this is impossible for single layer granular bed filters. It was confirmed that the two layers of
granules should be carefully selected so that a combined effect of “deep bed filtration” from coarse granules in the upper layer and “surface cake filtration” from fine granules in the lower layer can be achieved. It was shown that the sizes and densities of the binary media could be chosen to keep the layers completely segregated during regeneration by fluidization. The author gratefully acknowledges the support of K C Wong Magna Foun in Ningbo University.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Smid J, Hsian S S, Hsian C Y, et al. Hot gas cleanup: new designs for moving bed filters. Filtration & Separation, 2005, 5: 36–39. Robert C B, Shi H W, Gerald C, et al. Similitude study of a moving bed granular filter. Powder Technology, 2003, 138(2–3): 201–210. Smid J, Hsian S S, Tsai F H, et al. Velocities in moving granular bed filters. Powder Technology, 2001, 114: 205–212. Hsian S S, Smid J, Tsai F H, et al. Placement of flow-corrective elements in a moving granular bed with louvered-walls. Chemical Engineering and Processing, 2004, 43(8): 1037–1045. Rodon I, Lee K C, Pfeffer R, et al. Granular bed filtration assisted by filter-cake formation, panel bed gas filter with puffback renewal of gas-entry surfaces. Powder Technology, 2005, 155: 52–61. Wu M S, Lee K C, Pfeffer R, et al. Granular bed filtration assisted by filter-cake formation, penetration of filter cakes by a monodisperse aerosol. Powder Technology, 2005, 155: 62–73. Rao S R. Developmental status of BHEL’S high temperature high pressure (HTHP) circulating bed granular filter (CBGF). Heat Recovery System & CHP, 1995, 15(2): 199–204. Gao S W, Xu S S, Wei S R, et al. Experimental study on moving granular bed filter for removing particulate at ambient temperature and high pressure. Journal of Fuel Chemistry and Technology, 2001, 29(6): 532–536. (In Chinese) Gao S W, Wei S R, Deng R Y, et al. Structure optimization and experimental study of moving granular bed filter for hot coal gas cleanup. Journal of Xian Jiaotong University, 2002, 36(9): 903–906. (In Chinese)