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Bigeminal tank for FGD wastewater treatment
36
Feature
Filtration+Separation March/April 2017
Water & wastewater
Bigeminal tank for FGD wastewater treatment A
bigeminal tank is a key part of the flue gas desulfurization wastewater treatment process for heavy metals and the precipitation and removal fine of particles. This article investigates treatment process simplification and equipment structure modification to reduce both operational costs and deposition of particles.
The burning of coal is one of the most important means of electricity generation. The flue gas, which contains a lot of dust, SO2, fluoride heavy metals and NOx, demands to be treated before discharge. In a wet flue gas desulfurization (FGD) process, the flue gas is forced into an absorption tower, where SO2 is absorbed by lime and limestone slurry and the gypsum is formed. Afterwards, the purified flue gas is discharged and gypsum slurry is dewatered. As a result of gypsum slurry dewatering, the FGD wastewater is formed which consists of heavy metals and fine particles [1].
Conventional chemical precipitation system The chemical precipitation method has been extensively used in FGD wastewater treatment for metals and the removal of fine particles. The conventional chemical precipitation (CCP) process includes hydroxide precipitation and sulfide precipitation. Heavy metals are normally precipitated by NaOH or Ca(OH)2. The minimum solubility of these metal hydroxides is usually in the pH value range of 8.0-11.0. www.filtsep.com
Compared with hydroxide precipitation, heavy metals precipitated by sulfides have lower solubility over a broad pH value range and can achieve a high degree of metal removal [2]. A conventional chemical precipitation (CCP) system mainly includes a pH adjust tank, a settling tank, a flocculation tank, a clarifier, a filter and five dosage devices, as shown in Figure 1. Typically, a pH adjust tank, settling tank and flocculation tank are applied as a group referring to a triple tank. In a pH adjust tank, NaOH or Ca(OH)2 is added to raise the pH of the wastewater to 8.0-11.0 and then the soluble metals are precipitated as insoluble hydroxides and oxyhydroxides. From the pH adjust tank, the wastewater flows to a settling tank, where sulfide is added to achieve a higher degree of metal precipitation. Then, the wastewater flows to flocculation tank, where flocculant is added for coagulation and co-precipitation. After that, polymer is added to flocculate fine suspended particles. The wastewater from flocculation tank flows to the clarifier.
The clarifier settles the particles that are initially present and formed during the chemical precipitation steps. The filter in the process further reduces particles and the filtrate is recycled back to the pH adjust tank. However, there are some disadvantages of the CCP system in a long term running process, including a long process line, complex dosage devices and high operation cost. Furthermore, a lot of particles in wastewater easily since to the bottom of the triple tank and clog the sludge outlets [3].
Modified chemical precipitation system To overcome the disadvantages of the CCP system, including complex dosage devices and high operation cost, a modified chemical precipitation (MCP) method has been reported [4]. In this method, a powder reagent replaces the five kinds of liquid reagents in the CCP system. To apply the MCP method, a modified system is presented in this article, as 0015 1882/17 ©2017 Elsevier Ltd. All rights reserved
Feature
Filtration+Separation March/April 2017
the values of particles’ concentration at the bottom of the tank do not change in 2 seconds, 40 time steps, the results are regarded to be available [6]. The main component of particles is CaSO4. Its density is 2960 kg/m3. Its mean size is 20μm in the MT and is 200μm in the FT. Its volume concentration is denoted as φ and the original value is φ0 = 0.684%. The density and viscosity values of wastewater are 998.2 kg/m3 and 0.001003 kg/m·s, respectively. Figure 1. Process flow diagram for a conventional chemical precipitation system.
Evaluating indicator
shown in Figure 2. Compared with the CCP system, a powder reagent device replaces the five reagents devices and the triple tank is simplified to a bigeminal tank. The bigeminal tank consists of a mixing tank (MT), a flocculation tank (FT). The wastewater with particles flows into the MT and is thoroughly mixed with the added powder reagent. Then, they overflow into the FT where the flocculation reaction and the flocs precipitation take place. Finally, the flocculated wastewater flows into the clarifier.
Design of bigeminal tank A CCP system of a power plant in Dalian (China) is used as a case study. The design and improvements of the bigeminal tank is based on the triple tank in this plant. For the bigeminal tank, the computational fluid dynamics (CFD) method is applied to predict the fluid flow and particles’ concentration distribution and to determine more suitable values of operation parameters. Finally, in order to improve the serious deposition of particles, as in the triple tank, a further improvement to the bigeminal tank is provided. The hydraulic retention time (HRT) is a measure of the average length of time that a compound remains in a storage unit and is HRT=60∙Ve/(10^9∙Q). Where Ve=S∙h is the effective volume of the storage unit, mm3, Q is the input flow rate, m/h3, S is the area of rectangular cross section, mm2, and h is the height of water which equals to the height of the baffle in this work, mm. HRT is usually expressed in minutes and its value usually equals to 23.5 min per
storage unit in the field of FGD wastewater treatment. The input flow rate of wastewater is 40 m3/h in this work. The HRT of the bigeminal tank is 46.9 min and that of the MT and the FT are 18.8 min and 28.1 min respectively. The dimension of the bigeminal tank is shown in Figure 3. The impellers used in the MT and FT are four-blade 45° downpumping pitched blade turbines (PBTd) and are located in the centre of each tank with diameters of D. The impeller off-bottom clearance is denoted as c and the rotational speed is denoted as r. The original parameters of the MT and FT are denoted as D1, D2, c1, c2, r1 and r2, respectively. Their values are set as D1=D2=950mm, c1=c2=850mm, r1=88 rpm, r2=60 rpm.
Numerical simulation method The investigated bigeminal tank is a fullscale model. The MT and FT are set as two single units to simulate. There are 1.5 million computational grids for the MT and 1.6 million for the FT, respectively. ANSYS FLUENT 15.0 is employed in the article. According to the reported methods [5], the two-phase flow system is simulated using Eulerian-Eulerian and dispersed standard k – ε model. Pressure and velocity are coupled by Phase Coupled SIMPLE method. A second order upwind difference scheme and standard wall function are used. The inlet of the tank is set as velocity inlet condition and the outlet is set as pressure outlet condition. The unsteady flow is computed with the Sliding Grid method. The time step is set as 0.05s. When the flow difference between the inlet and the outlet and
In this article, φmax is the maximum volume particles’ concentration of the tank. φave,v is the average volume particles’ concentration of the whole tank. φave,b is the average area particles’ concentration at the bottom plane of the tank. Generally, φave,v and φave,b change with the same tendency unless noted otherwise. The volume particles’ concentration at the bottom plane is more notable than the average volume particles concentration of the whole tank. Hence, φave,b is treated as the main evaluating indicator of operation parameters selection.
Operating conditions of MT Flow field distribution. When D/D1=1, c/c1=1, r/r1=1, the particles’ concentration φ near the wall and the bottom of the tank is slightly higher than that in the blade region where particles gain more energy. Particularly, deposition of particles often occurs at the centre and four bottom corners of the tank. φmax=1.2φ0 is obtained at the bottom centre under the impeller. The axial bulk flow caused by the impeller rotation drives particles to move upward and plays an active role in decreasing particles’ deposition. Some particles are propelled to the near wall region and then overflow out of the MT. Except for the bulk flow, the circulating movement of particles is formed at the centre and four bottom corners of the tank indicating that vortexes are formed. Particles in these regions can not gain enough energy from the liquid and can hardly escape, which leads to a higher concentration of particles. As a result, during continuous operation, more and more particles will accumulate at the bottom. www.filtsep.com
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Filtration+Separation March/April 2017
Table 1 Numerical simulation results with different impeller operating parameters
Case
Factor1
Factor2
1 2 3
c/c1=1 r/r1=1
D/D1
4 5 6 7
D/D1=1 r/r1=1
9
11
D/D1=1 c/c1=0.765
1.047
13
0.947
1.045
14
1
1.035
15
1.053
1.034
16
0.765
1.022
17
12
1.025
18
1
1.029
19
1.118
1.034
20
0.886
1.031
21
1 r/r1
Case
0.895
0.882 c/c1
8
10
ave,b/ 0
1.022
22
1.110
1.020
23
1.230
1.018
24
Factor1
c/c2=1
Factor2
D/D2
r/r2=1
D/D2=1.158 r/r2=1
c/c2
D/D2=1.158 c/c2=1
r/r2
ave,b/ 0
1
59.503
1.053
34.211
1.158
13.202
1.263
6.389
0.882
7.836
1
13.216
1.118
20.760
1.235
21.199
0.666
48.246
0.833
9.810
1
5.395
1.167
1.827
Table 2. The simulation results of the bigeminal tank
Case
ave,v/ 0
in MT
ave,v/ 0
in FT
ave,b/ 0
in MT
ave,b/ 0
in FT
(%)
OS
1.298
0.557
55.848
10.175
/
MS
0.731
0.162
29.094
19.444
82.3
Single factor experiments. In order to reduce particles deposition, the influence of the impeller paremeters are predicted and the results are summerized in Table 1. The ratios of D/D1 are set as 0.895, 0.947, 1 and 1.053 (c/c1=1, r/r1=1), respectively.
When D/D1 equals to 1 or 1.053, the values of φave,b are lower than the other two cases (Case 1 to 4). Under the similar mixture effect, the original parameter, D/D1=1, is a more suitable choice because it can save the equipment replacement cost.
The ratios of c/c1 are set as 0.765, 0.882, 1 and 1.118 (D/D1=1, r/r1=1). According to Case 5 to 8, when c/c1 equals to 0.765, φave,b is the lowest among four cases. The results mean the increase in axial velocity has a positive impact on decreasing the deposition of particles. The ratios of r/r1 are set as 0.886, 1, 1.11, and 1.23 (D/D1=1, c/c1=0.765). It is obvious that the mixing effect, the impeller displacement and the axial power will increase when the rotational speed increases. According to Case 9 to 12, φave,b decreases with an increase in the rotational speed, while the changes of these values are not significant in such low particles’ concentration situation. φave,b is at a minimum when r/r1=1.23. So the r/ r1=1.23 is regarded as a suitable case because it can provide a higher impeller displacement to ensure the mixing effect when the HRT of MT is lower than the common value. In summary, the recommended parameters for the MT are D/D1=1, c/c1=0.765, r/r1=1.23.
Operating conditions of FT
Figure 2. Process flow diagram for the modified chemical precipitation system.
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Flow field distribution. In this part, the investigation method for the FT is similar to that of the MT. The case (D/D2=1.158, c/c1=1, r/r1=1) is taken as an example to
40
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introduce the particles’ distribution and the flow field of the FT. φ near the bottom is much higher than that of the other regions. An amount of particles are at the corners of the bottom and φmax is approximately 64φ0. A stronger vortex is formed at the centre of the bottom. Single factor experiments. The ratios of D/D2 are set as 1, 1.053, 1.158 and 1.263 (c/c1=1, r/r1=1). According to Case 13 to 16, φave,b decreases with increasing of D/ D2. However, φave,v of FT continues to decline until D/D2=1.158 and the value increases when D/D2=1.263. Because the
Filtration+Separation March/April 2017
with φave,v/φ0=0.506 (c/c2=0.882). So c/c2=1 is more suitable. The ratios of r/r2 are set as 0.666, 0.833, 1, and 1.167 (D/D2=1.158, c/c2=1) in Case 21 to 24. When r/r2<1, φave,b are higher and the deposition of particles at the bottom is serious. When r/r2=1, 1.167, φave,v/φ0 =0.443, 0.526, respectively. It means more particles stay in the FT when r/r2=1.167. So the original rotational speed is still suitable for the FT after adjusting the impeller diameter. In summary, the recommended parameters for the FT are D/D2=1.158, c/c2=1, r/r2=1.
"In the bigeminal tank, the concentration of particles in the near-wall and bottom areas is higher than that of other areas." bigger diameter gives much greater axial flow and particles gain a longer residence time, the optimum value of D/ D2 is 1.158 for the FT. The ratios of c/c2 are set as 0.882, 1, 1.118 and 1.235 (D/D2=1.158, r/r2=1). According to Case 17 to 21, when c/c2=1.118 or 1.235, their φave,b are higher, which means the particles’ deposition is more serious at bottom than c/c2=0.882 or 1. Besides, φave,v/ φ0=0.443 (c/c2=1) is lower comparing
Modification of bigeminal tank The deposition of particles still exists at the bottom of the tank though more suitable operating conditions are adopted. It is necessary to modify the bigeminal tank structure. So the baffle between the FT and MT is modified. 11 holes with a radius of 25mm are placed at the bottom of the baffle and the centre distance is 250mm, which contains 9 semicircular holes in the middle and two quarter round holes at both ends. This modified scheme is denoted as MS. The bigeminal tank with
the original baffle is the initial scheme, denoted as OS. The bigeminal tank is treated as a whole part to run the simulations. The basic parameters are the aforementioned recommendation values and the size of the particles is 200μm. The particles’ concentration is shown in Table 2. α is defined as the percentage of particles’ flow rate at the holes from total flow rate. φave,v of the MT and FT in MS are lower than those in OS. That means, in MS, it is easier for particles to enter the FT and then leave the tank. φave,b/φ0 of the MT reduces from 55.848 to 29.094, a 47.9% fall, which means the concentration of particles decreases greatly at the bottom. The reason why φave,b of the FT in MS is higher than that in OS is that 82.3% of particles enter into the FT through the holes at the bottom of the baffle and increases particles’ concentration in these regions. Even so, φave,v is still lower after modification. Therefore, opening holes has a positive influence on the reduction of particles’ deposition during a long term process and cleaning the deposition of particles after shutdown will be more convenient.
Conclusions In the bigeminal tank, particles’ concentration in the near-wall and bottom areas is higher than that of other areas. The centre and four bottom corners of the tank are regions where deposition of particles easily forms. The recommended parameters of impellers are D/D1=1, c/c1=0.765, r/r1=1.23, D/ D2=1.158, c/c2=1, r/r2=1. Opening holes at the bottom edge of the baffle between the MT and FT can improve particles’ concentration distribution at the bottoms of the tank and equipment cleaning after shutdown will be more convenient.
•
References 1, 2, 3, 4, 5, 6, Please contact the author directly for full references.
Contact Yajing Tian, Yong Kang*, Jia Lu, Jicheng Yu School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350 China * Corresponding author Yong Kang: E-mail: [email protected] Figure 3. Structure dimension of the bigeminal tank. (units:mm)
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Feature
Filtration+Separation March/April 2017
Water & wastewater
Bigeminal tank for FGD wastewater treatment A
bigeminal tank is a key part of the flue gas desulfurization wastewater treatment process for heavy metals and the precipitation and removal fine of particles. This article investigates treatment process simplification and equipment structure modification to reduce both operational costs and deposition of particles.
The burning of coal is one of the most important means of electricity generation. The flue gas, which contains a lot of dust, SO2, fluoride heavy metals and NOx, demands to be treated before discharge. In a wet flue gas desulfurization (FGD) process, the flue gas is forced into an absorption tower, where SO2 is absorbed by lime and limestone slurry and the gypsum is formed. Afterwards, the purified flue gas is discharged and gypsum slurry is dewatered. As a result of gypsum slurry dewatering, the FGD wastewater is formed which consists of heavy metals and fine particles [1].
Conventional chemical precipitation system The chemical precipitation method has been extensively used in FGD wastewater treatment for metals and the removal of fine particles. The conventional chemical precipitation (CCP) process includes hydroxide precipitation and sulfide precipitation. Heavy metals are normally precipitated by NaOH or Ca(OH)2. The minimum solubility of these metal hydroxides is usually in the pH value range of 8.0-11.0. www.filtsep.com
Compared with hydroxide precipitation, heavy metals precipitated by sulfides have lower solubility over a broad pH value range and can achieve a high degree of metal removal [2]. A conventional chemical precipitation (CCP) system mainly includes a pH adjust tank, a settling tank, a flocculation tank, a clarifier, a filter and five dosage devices, as shown in Figure 1. Typically, a pH adjust tank, settling tank and flocculation tank are applied as a group referring to a triple tank. In a pH adjust tank, NaOH or Ca(OH)2 is added to raise the pH of the wastewater to 8.0-11.0 and then the soluble metals are precipitated as insoluble hydroxides and oxyhydroxides. From the pH adjust tank, the wastewater flows to a settling tank, where sulfide is added to achieve a higher degree of metal precipitation. Then, the wastewater flows to flocculation tank, where flocculant is added for coagulation and co-precipitation. After that, polymer is added to flocculate fine suspended particles. The wastewater from flocculation tank flows to the clarifier.
The clarifier settles the particles that are initially present and formed during the chemical precipitation steps. The filter in the process further reduces particles and the filtrate is recycled back to the pH adjust tank. However, there are some disadvantages of the CCP system in a long term running process, including a long process line, complex dosage devices and high operation cost. Furthermore, a lot of particles in wastewater easily since to the bottom of the triple tank and clog the sludge outlets [3].
Modified chemical precipitation system To overcome the disadvantages of the CCP system, including complex dosage devices and high operation cost, a modified chemical precipitation (MCP) method has been reported [4]. In this method, a powder reagent replaces the five kinds of liquid reagents in the CCP system. To apply the MCP method, a modified system is presented in this article, as 0015 1882/17 ©2017 Elsevier Ltd. All rights reserved
Feature
Filtration+Separation March/April 2017
the values of particles’ concentration at the bottom of the tank do not change in 2 seconds, 40 time steps, the results are regarded to be available [6]. The main component of particles is CaSO4. Its density is 2960 kg/m3. Its mean size is 20μm in the MT and is 200μm in the FT. Its volume concentration is denoted as φ and the original value is φ0 = 0.684%. The density and viscosity values of wastewater are 998.2 kg/m3 and 0.001003 kg/m·s, respectively. Figure 1. Process flow diagram for a conventional chemical precipitation system.
Evaluating indicator
shown in Figure 2. Compared with the CCP system, a powder reagent device replaces the five reagents devices and the triple tank is simplified to a bigeminal tank. The bigeminal tank consists of a mixing tank (MT), a flocculation tank (FT). The wastewater with particles flows into the MT and is thoroughly mixed with the added powder reagent. Then, they overflow into the FT where the flocculation reaction and the flocs precipitation take place. Finally, the flocculated wastewater flows into the clarifier.
Design of bigeminal tank A CCP system of a power plant in Dalian (China) is used as a case study. The design and improvements of the bigeminal tank is based on the triple tank in this plant. For the bigeminal tank, the computational fluid dynamics (CFD) method is applied to predict the fluid flow and particles’ concentration distribution and to determine more suitable values of operation parameters. Finally, in order to improve the serious deposition of particles, as in the triple tank, a further improvement to the bigeminal tank is provided. The hydraulic retention time (HRT) is a measure of the average length of time that a compound remains in a storage unit and is HRT=60∙Ve/(10^9∙Q). Where Ve=S∙h is the effective volume of the storage unit, mm3, Q is the input flow rate, m/h3, S is the area of rectangular cross section, mm2, and h is the height of water which equals to the height of the baffle in this work, mm. HRT is usually expressed in minutes and its value usually equals to 23.5 min per
storage unit in the field of FGD wastewater treatment. The input flow rate of wastewater is 40 m3/h in this work. The HRT of the bigeminal tank is 46.9 min and that of the MT and the FT are 18.8 min and 28.1 min respectively. The dimension of the bigeminal tank is shown in Figure 3. The impellers used in the MT and FT are four-blade 45° downpumping pitched blade turbines (PBTd) and are located in the centre of each tank with diameters of D. The impeller off-bottom clearance is denoted as c and the rotational speed is denoted as r. The original parameters of the MT and FT are denoted as D1, D2, c1, c2, r1 and r2, respectively. Their values are set as D1=D2=950mm, c1=c2=850mm, r1=88 rpm, r2=60 rpm.
Numerical simulation method The investigated bigeminal tank is a fullscale model. The MT and FT are set as two single units to simulate. There are 1.5 million computational grids for the MT and 1.6 million for the FT, respectively. ANSYS FLUENT 15.0 is employed in the article. According to the reported methods [5], the two-phase flow system is simulated using Eulerian-Eulerian and dispersed standard k – ε model. Pressure and velocity are coupled by Phase Coupled SIMPLE method. A second order upwind difference scheme and standard wall function are used. The inlet of the tank is set as velocity inlet condition and the outlet is set as pressure outlet condition. The unsteady flow is computed with the Sliding Grid method. The time step is set as 0.05s. When the flow difference between the inlet and the outlet and
In this article, φmax is the maximum volume particles’ concentration of the tank. φave,v is the average volume particles’ concentration of the whole tank. φave,b is the average area particles’ concentration at the bottom plane of the tank. Generally, φave,v and φave,b change with the same tendency unless noted otherwise. The volume particles’ concentration at the bottom plane is more notable than the average volume particles concentration of the whole tank. Hence, φave,b is treated as the main evaluating indicator of operation parameters selection.
Operating conditions of MT Flow field distribution. When D/D1=1, c/c1=1, r/r1=1, the particles’ concentration φ near the wall and the bottom of the tank is slightly higher than that in the blade region where particles gain more energy. Particularly, deposition of particles often occurs at the centre and four bottom corners of the tank. φmax=1.2φ0 is obtained at the bottom centre under the impeller. The axial bulk flow caused by the impeller rotation drives particles to move upward and plays an active role in decreasing particles’ deposition. Some particles are propelled to the near wall region and then overflow out of the MT. Except for the bulk flow, the circulating movement of particles is formed at the centre and four bottom corners of the tank indicating that vortexes are formed. Particles in these regions can not gain enough energy from the liquid and can hardly escape, which leads to a higher concentration of particles. As a result, during continuous operation, more and more particles will accumulate at the bottom. www.filtsep.com
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Feature
Filtration+Separation March/April 2017
Table 1 Numerical simulation results with different impeller operating parameters
Case
Factor1
Factor2
1 2 3
c/c1=1 r/r1=1
D/D1
4 5 6 7
D/D1=1 r/r1=1
9
11
D/D1=1 c/c1=0.765
1.047
13
0.947
1.045
14
1
1.035
15
1.053
1.034
16
0.765
1.022
17
12
1.025
18
1
1.029
19
1.118
1.034
20
0.886
1.031
21
1 r/r1
Case
0.895
0.882 c/c1
8
10
ave,b/ 0
1.022
22
1.110
1.020
23
1.230
1.018
24
Factor1
c/c2=1
Factor2
D/D2
r/r2=1
D/D2=1.158 r/r2=1
c/c2
D/D2=1.158 c/c2=1
r/r2
ave,b/ 0
1
59.503
1.053
34.211
1.158
13.202
1.263
6.389
0.882
7.836
1
13.216
1.118
20.760
1.235
21.199
0.666
48.246
0.833
9.810
1
5.395
1.167
1.827
Table 2. The simulation results of the bigeminal tank
Case
ave,v/ 0
in MT
ave,v/ 0
in FT
ave,b/ 0
in MT
ave,b/ 0
in FT
(%)
OS
1.298
0.557
55.848
10.175
/
MS
0.731
0.162
29.094
19.444
82.3
Single factor experiments. In order to reduce particles deposition, the influence of the impeller paremeters are predicted and the results are summerized in Table 1. The ratios of D/D1 are set as 0.895, 0.947, 1 and 1.053 (c/c1=1, r/r1=1), respectively.
When D/D1 equals to 1 or 1.053, the values of φave,b are lower than the other two cases (Case 1 to 4). Under the similar mixture effect, the original parameter, D/D1=1, is a more suitable choice because it can save the equipment replacement cost.
The ratios of c/c1 are set as 0.765, 0.882, 1 and 1.118 (D/D1=1, r/r1=1). According to Case 5 to 8, when c/c1 equals to 0.765, φave,b is the lowest among four cases. The results mean the increase in axial velocity has a positive impact on decreasing the deposition of particles. The ratios of r/r1 are set as 0.886, 1, 1.11, and 1.23 (D/D1=1, c/c1=0.765). It is obvious that the mixing effect, the impeller displacement and the axial power will increase when the rotational speed increases. According to Case 9 to 12, φave,b decreases with an increase in the rotational speed, while the changes of these values are not significant in such low particles’ concentration situation. φave,b is at a minimum when r/r1=1.23. So the r/ r1=1.23 is regarded as a suitable case because it can provide a higher impeller displacement to ensure the mixing effect when the HRT of MT is lower than the common value. In summary, the recommended parameters for the MT are D/D1=1, c/c1=0.765, r/r1=1.23.
Operating conditions of FT
Figure 2. Process flow diagram for the modified chemical precipitation system.
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Flow field distribution. In this part, the investigation method for the FT is similar to that of the MT. The case (D/D2=1.158, c/c1=1, r/r1=1) is taken as an example to
40
Feature
introduce the particles’ distribution and the flow field of the FT. φ near the bottom is much higher than that of the other regions. An amount of particles are at the corners of the bottom and φmax is approximately 64φ0. A stronger vortex is formed at the centre of the bottom. Single factor experiments. The ratios of D/D2 are set as 1, 1.053, 1.158 and 1.263 (c/c1=1, r/r1=1). According to Case 13 to 16, φave,b decreases with increasing of D/ D2. However, φave,v of FT continues to decline until D/D2=1.158 and the value increases when D/D2=1.263. Because the
Filtration+Separation March/April 2017
with φave,v/φ0=0.506 (c/c2=0.882). So c/c2=1 is more suitable. The ratios of r/r2 are set as 0.666, 0.833, 1, and 1.167 (D/D2=1.158, c/c2=1) in Case 21 to 24. When r/r2<1, φave,b are higher and the deposition of particles at the bottom is serious. When r/r2=1, 1.167, φave,v/φ0 =0.443, 0.526, respectively. It means more particles stay in the FT when r/r2=1.167. So the original rotational speed is still suitable for the FT after adjusting the impeller diameter. In summary, the recommended parameters for the FT are D/D2=1.158, c/c2=1, r/r2=1.
"In the bigeminal tank, the concentration of particles in the near-wall and bottom areas is higher than that of other areas." bigger diameter gives much greater axial flow and particles gain a longer residence time, the optimum value of D/ D2 is 1.158 for the FT. The ratios of c/c2 are set as 0.882, 1, 1.118 and 1.235 (D/D2=1.158, r/r2=1). According to Case 17 to 21, when c/c2=1.118 or 1.235, their φave,b are higher, which means the particles’ deposition is more serious at bottom than c/c2=0.882 or 1. Besides, φave,v/ φ0=0.443 (c/c2=1) is lower comparing
Modification of bigeminal tank The deposition of particles still exists at the bottom of the tank though more suitable operating conditions are adopted. It is necessary to modify the bigeminal tank structure. So the baffle between the FT and MT is modified. 11 holes with a radius of 25mm are placed at the bottom of the baffle and the centre distance is 250mm, which contains 9 semicircular holes in the middle and two quarter round holes at both ends. This modified scheme is denoted as MS. The bigeminal tank with
the original baffle is the initial scheme, denoted as OS. The bigeminal tank is treated as a whole part to run the simulations. The basic parameters are the aforementioned recommendation values and the size of the particles is 200μm. The particles’ concentration is shown in Table 2. α is defined as the percentage of particles’ flow rate at the holes from total flow rate. φave,v of the MT and FT in MS are lower than those in OS. That means, in MS, it is easier for particles to enter the FT and then leave the tank. φave,b/φ0 of the MT reduces from 55.848 to 29.094, a 47.9% fall, which means the concentration of particles decreases greatly at the bottom. The reason why φave,b of the FT in MS is higher than that in OS is that 82.3% of particles enter into the FT through the holes at the bottom of the baffle and increases particles’ concentration in these regions. Even so, φave,v is still lower after modification. Therefore, opening holes has a positive influence on the reduction of particles’ deposition during a long term process and cleaning the deposition of particles after shutdown will be more convenient.
Conclusions In the bigeminal tank, particles’ concentration in the near-wall and bottom areas is higher than that of other areas. The centre and four bottom corners of the tank are regions where deposition of particles easily forms. The recommended parameters of impellers are D/D1=1, c/c1=0.765, r/r1=1.23, D/ D2=1.158, c/c2=1, r/r2=1. Opening holes at the bottom edge of the baffle between the MT and FT can improve particles’ concentration distribution at the bottoms of the tank and equipment cleaning after shutdown will be more convenient.
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References 1, 2, 3, 4, 5, 6, Please contact the author directly for full references.
Contact Yajing Tian, Yong Kang*, Jia Lu, Jicheng Yu School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350 China * Corresponding author Yong Kang: E-mail: [email protected] Figure 3. Structure dimension of the bigeminal tank. (units:mm)
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