Experimental Study of Hydrocyclone Flow Field with Different Feed

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Experimental Study of Hydrocyclone Flow Field with Different Feed Concentration YanHong Zhang, Peng Qian, Yi Liu, and HuaLin Wang* State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: The influence of feed concentration on the solid/liquid two-phase flow in a mini-hydrocyclone was studied. A phase doppler particle analyzer was used to measure the two-phase flow pattern in a 25 mm hydrocyclone at three different feed concentrations (300 mg/kg (0.136% (v/v)), 800 mg/kg (0.364% (v/v)) and 1200 mg/kg (0.545% (v/v)). The measurements show that the feed concentration has remarkable influence on the velocities in a hydrocyclone. A higher concentration of solid particles leads to lower axial velocities and can suppress the turbulence of the liquid phase in the inner helical flow; in the outer helical flow, however, the influence was complex. In planes in eddy flow, the downward flow of the liquid phase was increased by a higher concentration of particles; at same time, the dimension of circular flow was also decreasing. In the pyramidal zone, however, the higher feed concentration corresponds to lower axial velocities at the wall region. In the whole experimental zone, the particles lead to the decreasing of tangential velocities. The presence of particles has little influence on the basic flow structure, but changes the size of the eddy flow in the cylindrical section. The correspondence between the higher feed concentration and the shift of the line of zero velocity value closer to the core is also observed which probably means more inlet particles would lead to more liquid leaving the hydrocyclone through the circular flow.

1. INTRODUCTION The experimental and theoretical study of multiphase flow has lagged behind that of single phase flow due to the the complexity and lack of effective measurement techniques for the multiphase flow. Yet the process industry in which many multiphase systems are involved demands urgent in-depth understanding of the mechanisms of multiphase flows so that the operation and optimization of multiphase processing equipment can be done on a sound basis. The hydrocyclone process was frequently used in mineral, pharmaceutical, and chemical industries to separate solid particles from liquid media, and the flow characteristics of the hydrocyclone have been attracting tremendous attention in the past 40 years. The first study on velocity profiles can be traced back to 1952.1 Most publications15 are focused on the flow structure in the hydrocyclone, revealing that in a hydrocyclone there exists an outer helical downward flow and an inner helical upward flow.6 In addition, there are circular and eddy flows close to the vortex finder.7 In some operating condition, there exists an air-core in the center,1,8,9 whose shape changes quasiperiodically.10 The separation efficiency is the most important factor of a hydrocyclone, which is closely related to the two-phase flow characteristics inside the hydrocyclone. Because of the limitations of the experimental technique, only a few papers reported the measurement of the solid/liquid flow field in a hydrocyclone. Dai et al.11 measured the radial and axial velocity components and the size distribution of solid particles in an 80 mm hydrocyclone by phase doppler particle analyzer (PDPA), and the solid particles used included polyspyrene, polyvinyl chloride, and quartz sand. The experimental results showed that the smaller particles are prone to be concentrated near the hydrocyclone center, and the concentration maximum is not near the hydrocyclone wall, but near the line of zero velocity value (LZVV) lines for the solid particles. Bergstr€om and r 2011 American Chemical Society

Vomhoff12 studied the flow field in opaque pulp fiber suspensions by ultrasonic velocity profiler (UVP). Their measurements showed that the pulp fibers had a strong influence on the liquid tangential velocity profile. At higher fiber concentration (7.5 and 11 g/L), the free-vortex-like behavior in the outer area was virtually suppressed. It seems that the study on solid/liquid twophase flow in cyclones is not enough, and further research was necessary, especially the influence of the particles on the velocities, LZVV, and particles distribution, etc. Chu et al.13 investigated the effects of geometric and operating parameters (inlet pressure, diameter of the underflow pipe) and feed characters (particle density, particle size) on the motion of solid particles in solid/liquid two-phase flow in an 80 mm hydrocyclone. Traditionally, the smallest hydrocyclones used had 50 mm diameter. In recent years however, small diameter (smaller than 50 mm) hydrocyclones have been used to collect fine particles in many industries. For example, 10 mm hydrocyclones have been reported to use for dewatering yeast.14 But the relevant researches are far from thorough. Finch15 found that small hydrocyclones exhibited a fish-hook partition curve and had a high bypass fraction. Many papers focus on the classification performance studies,1618 and modeling studies1823 of the small cyclones. Computational fluid dynamics (CFD) methods2426 were also employed to predict the cut-size predictions and the separation efficiency. Up to now, the effect solid phase concentration on the liquid/ solid flow field in small hydrocyclone has not been studied experimentally. In this work, solid/liquid two-phase flow in a Received: June 29, 2009 Accepted: May 31, 2011 Revised: May 20, 2011 Published: May 31, 2011 8176

dx.doi.org/10.1021/ie100210c | Ind. Eng. Chem. Res. 2011, 50, 8176–8184

Industrial & Engineering Chemistry Research

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Table 1. Relative Size of the Hydrocyclone

Figure 1. Schematic of experimental setup: 1, hydrocyclone; 2, stirred tank; 3, pump; 4, processor; 5, computer; 6, optic-fiber laser probe; 7, receiving optics.

DL/D

Do/D

Du/D

d/D

H1/D

H2/D

H3/D

h/D

0.68

0.24

0.08

1.2

0.8

1.4

0.92

8.76

2. EXPERIMENTAL SETUP The experimental facility used for the study was described in Figure 1. The hydrocyclone was operated at a constant inlet flow rate of 10 L/min and feed pressure of 0.15 MPa, with the pressure of both overflow and underflow being zero. A pump was employed to form a recycle loop. The split ratio of the overflow to the underflow was set at 10:1. Two flowmeters were used to measure the inlet and overflow rates. PDPA was used to measure the velocity profiles, the concentration, and the diameter of the particles of the liquid and solid phases. Three feed mass concentrations were investigated in experiments. A 25 mm plexiglass hydrocyclone was tested in the experiment. The hydrocyclone had an inlet with 6 mm diameter; the sectional view of the inlet was illustrated in Figure 2. To eliminate the influence of the curvature on the PDPA measurements, a series of 5 mm small holes were drilled on the wall in which little quartz glass windows were fixed to facilitate the measurement on a vertical plane. The geometry and dimensions of the hydrocyclone were illustrated in Figure 2 and Table 1, respectively. Z1Z7 indexes different axial coordinates of the hydrocyclone. The liquid phase was tap water at room temperature. The solid particles were catalyst used in catalytic cracking reactions with Fs = 2.2 g/cm3, and the inlet particle diameters were between 20 and 60 μm. The feed mass concentration was 300 mg/kg (0.136%(v/v)), 800 mg/kg (0.364%(v/v)), and 1200 mg/kg (0.545%(v/v)), respectively. Only half of the flow field was measured because an air-core existed. The cut size (d50) of this hydrocyclone was predicted by the follow correlation:27 d50 ¼

Figure 2. (a) The geometry of the hydorcyclone; (b) model of the inlet.

25 mm hydrocyclone was studied by PDPA, and the primary aim was to study the influence of the feed concentration on the characteristics of two-phase flow field. Although the hydrocyclones usually were used for dense systems, they were employed more and more to separate the minor solid contaminant from the liquid phase, such as catalysis recycling in methanol-to-olefins (MTO) technology. Therefore, relatively low concentration values were used in the experiment.

14:2D0:46 Din 0:6 Do 1:21 expð6:3cv Þ Du 0:71 ðH2 þ H3 H1Þ0:38 Q 0:45 ðFs FÞ0:5

When the feed mass concentration was 1200 mg/kg, the cut size (d50) was 14 μm. The shape of the particles was irregular and the distribution of the particle size is shown in Figure 3. The PDPA method was employed to examine the solid/liquid two-phase flow field in the hydrocyclone. The maximum solid concentration that the method can handle is 1.0 108. The range of the particle size that can be measured is 0.513000 μm, and for velocity, particle diameter, and concentration the measured error is 0.5%. In each experiment, the first radial direction was measured twice at an interval of 5 min. If the discrepancy between two measurements was less than 5%, then we thought that the flow reached steadiness and the measurement was started. Each parameter was obtained through 15 s time averaging. The collision of the particles to the inner wall of the hydrocyclone will lead to the overestimations of the measurement. In this work, however, the concentration of the solid was very low so the influence of the collisions can be neglected.

3. RESULTS AND DISCUSSION The tracer particles were made of polyamide with a diameter around 10 μm and a density of 1.01 g/cm3, which is almost equal to that of the water. Smaller tracer particles well follow the fluid; 8177

dx.doi.org/10.1021/ie100210c |Ind. Eng. Chem. Res. 2011, 50, 8176–8184


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Figure 3. Particle size distributions.

however, very tiny particles seriously scatter the laser, which may result in a too weak laser measurement. Because the solid particles in the study were between 20 and 60 μm, the solid phase can be distinguished through the particle size measurement. The influence of feed concentration on axial and tangential velocities, the root mean square (rms) velocities, the particle diameter, and the particle concentration are showed in Figures 49. 3.1. Axial Velocity Profiles. The radial distributions of the axial velocities of liquid phase with different feed mass concentration from Z1 to Z7 are shown in Figure 4. The x coordinate 12.5 mm in the figure was located in the axis of the hydrocyclone. For the dense system, besides the force between the continuous phase and the particles, there exists interacting force among the particles, which has important influence on the two-phase flow field. It can be seen that in the inner helical flow, the presence of particles decreases the liquid axial velocities, because higher concentration means greater drag force the particles exert on the continuous phase. For the outer helical flow, however, the influence was complex. In Z1 and Z2 planes, the downward flow of the liquid phase was increased by higher concentration of particles; at same time, the dimension of circular flow was also decreased. From Z3 to Z6, however, the higher feed concentration corresponds to lower axial velocities at the wall region; at the same time, the LZVV moves close to the core, because the downward flow rate was constant. Since the collisions between particles and the wall result in large shear force near the wall, the higher feed concentration resulted in lower axial velocities near the wall. The shift of the LZVV and the decrease of the axial velocities in the inner helical flow meant that the upflow in the hydrocyclone below Z1 decreased with the increase of the feed mass concentration. This phenomenon probably means that the existence of a particle phase leads to more liquid leaving the hydrocyclone through the circular flow. Figure 5 shows the radial distributions of the axial velocities of solid phase in Z1 and Z5. The influence of the feed concentration on the axial velocities of solid phase was similar with that of the liquid phase because the slip velocities in this zone were very small except near the air-core. 3.2. Tangential Liquid Velocity Profiles. The radial distributions of the tangential velocities of liquid phase were shown in Figure 6. The influence of the feed concentration was obvious, high particle concentration decreases the tangential velocities, probably due to more collisions between particles. The dispersed

particles in the liquid may increase the apparent viscosity of the suspension, thus enhancing the energy dissipation and ultimately reducing the tangential velocities of the two-phase flow. In the Z1 plane, the most reduction of tangential velocities appeared at r = 7.25 mm, where it decreased almost 24% from case 300 ppm to case 1200 ppm. In the region where radial coordinates were less than 4 mm, there was little difference among three cases in Z1 and Z2. In Z3, however, the most reduction of tangential velocities emerged in the wall area and it decreased almost evenly along the radial direction. In Z4, the tangential velocities around the air core decreased greatly. 3.3. Root-Mean-Square Velocities Distribution. Root mean square (rms) velocities represent the local turbulent intensity. The distributions of the rms velocities are shown in Figure 7 and Figure 8. The results showed that the particles suppress the turbulence of liquid phase close the core; near the wall, however, the influence was complex. In a comparison of Figures 7 and 8 to Figure 1, it was found that large axial rms velocities always corresponded to large axial velocities. Although the axial velocities near the wall were much less than those near the air core, the rms velocities were as much in these two regions from Z1 to Z3. It seemed that a great amount of violent turbulence accrued in the area near the wall. Because of the reverse of the axial velocity direction,28 the rms velocities near the air core increased suddenly from Z4. The distribution of tangential rms velocity had the same pattern with that of axial rms velocity; at the same time, the magnitude was a bit larger than that of axial rms velocity. 3.4. Concentration Profile and Diameter Distribution. PDPA gave the concentration with the dimension of particle numbers per cubic centimeter. The experimental results were displayed in Figure 9. The concentration has one or two maximum values between wall and axis, which correspond with the experimental results of Dai et al.10 At the lower sections, solid particles are distributed more uniformly in the radial direction. The higher the feed concentration, the larger the maximum value in the distribution curves. In Z1, particles cumulated in the low axial velocity zone which located in outer helical flow. The maximum values, which were not in proportion to the feed concentration, appeared at the inner helical flow just close to LZVV. In a comparison of Figure 9 with Figure 6, it was found that the local particle concentration played a very important role on the 8178



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Figure 4. Radial distributions of axial velocities of liquid phase.

tangential velocity. Higher local particle concentration leads to a large tangential velocity reduction. Figure 10 shows the distributions of average diameters for the solid particles. The larger particle was more prone to approach

the wall. In Z1, there was little difference among the three cases for the average diameter in the particle collective region. In the positions close to the wall, the average diameter for 1200 ppm was much larger than that for 300 ppm. Along with the downward 8179



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Figure 5. Radial distributions of axial velocities of solid phase.

Figure 6. Radial distributions of tangential velocities of liquid phase. 8180



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Figure 7. Axial rms velocity distributions for the liquid phase.

Figure 8. Tangential rms velocity distributions for the liquid phase.

flow of the fluid, the small particles move gradually in the radial direction, which leads to a decrease of the average diameter in

the outer helical flow (see Z4). Because the centrifugal force which acted on the particles for 1200 ppm was the smallest in the 8181



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Figure 9. Radial distributions of particle concentration.

three cases, the average diameters in outer helical flow were greater than these for 300 and 800 ppm (see Z5). This means that the separation efficiency for 1200 ppm was the lowest among the three cases.

The fluid which changed flow direction carried the large particles from outer helical flow to inner helical flow, which led to the large particles also appearing in the core (Figure 10, panels c and d). 8182



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Figure 10. Particle average diameter radial distribution.

4. CONCLUSIONS PDPA was employed to study the effect of the feed concentration on the solid/liquid two-phase flow field in a 25 mm hydrocyclone. Three feed concentrations were tested in the experiment. The measurement results suggest that although the feed concentration has little influence on the basic flow structure of both liquid and solid phases in a hydrocyclone, the concentration of the particles in feed makes much difference. Fitting to the expectation, the distributions of the velocities are quite similar in this work because the concentration of the solid phase was very low. A higher feed concentration corresponds to lower liquid axial velocities near the wall in the conic section and makes the LZVV shift to the central core. These changes in the inner helical flow may be because the upflow in the hydrocyclone is decreased with the increase of the feed mass concentration. The upflow rate decreases in the conic section because more liquid bypasses the hydrocyclone through circular flows than that in the case of single-phase liquid flow. The feed concentration has little influence on the concentration distribution in the radial direction; the concentration has one or two maximum values between wall and core. At the lower sections of the hydrocyclone, solid particles are distributed rather uniformly along the radial direction. The tangential velocities have a direct relationship with the local particle concentration. The existence of particles decreases the tangential velocities, with the largest reduction of 24%. As a result, the average diameters in the 1200 ppm case in the outer helical flow were almost 10% larger than those in the 300 ppm case. The decreasing of the tangential velocities was another factor that reduced the separation efficiency. The circular flows are disadvantageous to the separation because liquid leaves the hydrocyclone without passing the separation zone. High feed concentration would lead to more

circular flows and decreas the separation efficiency. More efforts should be made to reduce the circular flow capacity for better solid separation.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ86-21-64252748. Fax: þ86-21-64251894. E-mail: samwhl@ 163.com.

’ ACKNOWLEDGMENT This work was supported by Shanghai Scientific and Technological Innovation Project (10dz1201300), National Natural Science Foundation of China (No. 21076073) and Shanghai Rising-Star Program (No. 10QA1401700). ’ NOTATION cv = particle volume concentration D = diameter, cm z = axial coordinate, m u = velocity, m/s Q = flow rate, l/min Greek Letters

F = density, g/cm3 Subscripts

l = liquid s = solid w = axial coordinate θ = tangential coordinate rms = root-mean-square 8183



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Experimental Study of Hydrocyclone Flow Field with Different Feed

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