Study of particle's shape factor, inlet velocity and feed concentration on mini-hydrocyclone classification and fishhook effect

    Study of particle’s shape factor, inlet velocity and feed concentration on mini-hydrocyclone classification and fishhook effect Laleh Abdollahzadeh, Mahmoud Habibian, Rouhollah Etezazian, Soud Naseri PII: DOI: Reference:

S0032-5910(15)00384-8 doi: 10.1016/j.powtec.2015.05.007 PTEC 10998

To appear in:

Powder Technology

Received date: Revised date: Accepted date:

5 September 2014 5 May 2015 8 May 2015

Please cite this article as: Laleh Abdollahzadeh, Mahmoud Habibian, Rouhollah Etezazian, Soud Naseri, Study of particle’s shape factor, inlet velocity and feed concentration on mini-hydrocyclone classification and fishhook effect, Powder Technology (2015), doi: 10.1016/j.powtec.2015.05.007

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ACCEPTED MANUSCRIPT Study of particle’s shape factor, inlet velocity and feed concentration on minihydrocyclone classification and fishhook effect

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Laleh Abdollahzadeh1, Mahmoud Habibian*2, Rouhollah Etezazian3, Soud Naseri4 1

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Chemistry and Chemical Engineering Research Center of Iran, Faculty Oil Engineering, P.O.Box: 14335-186, Tehran, Iran.

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Corresponding author at: Chemistry and Chemical Engineering Research Center of Iran, Faculty Oil Engineering, P.O.Box: 14335-186, Tehran, Iran. 3

Isfahan University of Technology, Isfahan, Iran.

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Amirkabir University of Technology, 424 Hafez Ave, Tehran, Iran. P.O. Box: 15875-4413

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Abstract

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This research aims to study the influence of particle’s shape factor, inlet velocity and feed volume

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concentration on the particle classification and fishhook effect in a mini-hydrocyclone. To develop study of innovation, three particles with extensive range of sphericities (0.05, 0.34, and 0.82) were chosen. The maximum separation efficiency was determined at three conditions: the highest inlet velocity, most sphericity and lowest particle concentration. The high particle sphericity offer less drag for an equivalent particle diameter. The results revealed fishhook effect is more explicit for spherical particles. The presence of fishhook effect was described based on available theories.

Keywords: Hydrocyclone, Fishhook, Separation efficiency, Sphericity

Corresponding author: Tel.: + 982144580782, Fax: + 982144580753 E-mail address: [email protected]

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ACCEPTED MANUSCRIPT 1. Introduction The development of a simple and feasible solid–liquid separation device is urgent to further advancement

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in the use of micro-technology [1]. The mini-hydrocyclone, a micro-technology device with a simple geometry and lack of moving parts [2] is able for fractionating and separation of fines in feed streams [3].

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Hydrocyclone utilizes centrifugal force to separate solid from liquid streams in high capacity

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continuously [4]. This centrifugal force field brings about a rapid classification of particles based on particle size difference. Large particles are centrifuged outwards to the hydrocyclone wall and leave

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through the underflow orifice with the outer swirling flow. Fine particles dragged in by the fluid flow are

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removed by the inner swirling flow through the overflow in the vortex finder [5]. Parameters such as feed concentration, feed flow rate, pressure, particle size distribution, hydrocylone

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geometry all are important parameters which directly affect the separation efficiency and cut size of mini-

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hydrocyclone. The shapes of particles are very important in classification of sensitive industries such as fire resistant materials, but there is limited research on modeling, quantification of particle shape and its

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effect on hydro cyclone performance. The Heywood shape factor, Ø, as a valid quantity of particles’ shape is defined:



s S

(1.1)

Where, s is the surface area of a sphere having the same volume as the particle, and S is the actual surface area of the particle [6]. A Summary of particle’s shape literature in classification is given in table 1. Endoh et al. [7] used all types of particle’s shapes (plate, irregular and spherical) with aspect ratio definition instead of Ø. He 2

ACCEPTED MANUSCRIPT concluded that the dynamic behavior of flaky particles in a rotary flow depended greatly on their shape. In rotary flow by utilizing particle orientation it is believed the shape classification is possible.

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Kashiwaya et al. [8] used glass flake, PTFE and spherical glass particles. Approximated drag coefficient

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was calculated based on the settling velocity of the glass plate which was depended on the particle Reynolds number and the ratio of the particle diameter to thickness. The influence of the particle

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Reynolds number on the approximated drag coefficient neglected in the region of high particle Reynolds

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number, and the drag coefficient increased with an increase in the ratio of the particle diameter to thickness.

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Zhu and Liow [9] selected irregular and spherical types of particles and obtained the e approximate value of Ø by SEM. In the coarse particle range, the separation efficiency curve was steeper for the spherical

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particles showing that these particles were separated more efficiently than non-spherical particles. In this research work the exact values for the shape factor of three particles (spherical, flake aluminum

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and glass bead), determined using BET and SEM together with MIP software. The effect of feed concentration and hydrocyclone inlet velocity on separation efficiency was investigated. The discrepancy of separation efficiency with regards to shape factor was specified. The appearance of fish hook phenomena in small particle region with respect to the shape of particles were explored.

Table 1 Summary of particle’s shape literature in classification.

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ACCEPTED MANUSCRIPT Solids

Shape factor,

Size

Concentratio

Cyclone

Inlet

n

diameter(mm)

velocity(m/s)

Ø

Source

none found

10-100

none found

none found

none found

silica sand

0.77-0.91(1)

10-100

none found

none found

none found

Endoh et al. (1994)

glass powder

0.77-0.91(1)

10-100

none found

none found

none found

Endoh et al. (1994)

mica

0.04-0.07(1)

10-100

none found

none found

PTFE

none found

<249

0.1 wt.%

25.4

glass flake

none found

<88

0.1 wt.%

25.4

quartz

none found

<105

0.1 wt.%

spherical

0.98

<18 , <60

0.2 vol.%

glass

0.71

<18 , <60

0.2 vol.%

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none found

Endoh et al. (1994)

Endoh et al. (1994) Kashiwaya et al.(2012)

0.18 to1.01

Kashiwaya et al.(2012)

25.4

0.18 to1.01

Kashiwaya et al.(2012)

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1

Zhu and Liow (2014)

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1

Zhu and Liow (2014)

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0.18 to 1.01

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beach sand

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glass bead

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(1) Aspect ratio

2. Materials and methods 2.1. Materials

Spherical aluminum, glass sand and flake aluminum powder that have large differences on particle’s shape factor and very small differences in density and size distribution were used in this research. Spherical aluminum powder (parsianco, Iran) is a silvery powder with physical properties are given in table 2 and a microphotograph of figure 1a indicating that it has almost spherical shape. The glass sand particles (Glassbeedco, Iran) have an opalescent appearance which its size distribution and properties are given in table 2 and figure 1b. The flake aluminum powder (parsianco, Iran) has bright silvery color consisting of plate-like shape particles with specifications are given in figure 1c and table 2.

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Table2

Mean particle size, X50, (µm)

18.28

Maximum particle size (µm)

76.96

Shape factor from BET

0.8181

Shape factor from SEM

0.823

Specific surface area (m2/g)

0.1486

2500

2700

17.50

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2700

Flake aluminum

20.18

50.66

49.10

0.3447

0.0522

0.315

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0.4421

2.1073

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Density (Kg/m3)

Glass sand

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Spherical aluminum

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Properties

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Physical properties of particles.

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2.2. Experimental

Three factors: inlet velocity, solid concentration (operating parameters) and solid shape factor (particle

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solid property) were studied. The inlet velocity was adjusted to three levels, which are 3.25, 4.25 and 5.05 m/s. The feed concentration was set in three levels, 0.05, 0.1 and 0.2%v. The particle’s shape factors were 0.05226, 0.3447 and 0.8181 (wide range). All experiments were conducted in triplicates. Figure 2 illustrates a schematic of a mini-hydrocyclone with the characteristic dimensions in the vertical plane. The experiments in this Study were conducted using set-up shown in figure 3. Powders were dispersed in media (water) using an agitator. Suspension was pumped to the minihydrocyclone inlet. When system reached steady state conditions, samples from under flow and over flow was collected simultaneously. The particle size distribution was analyzed in wet phase. Samples were dried and weighed. 2.3. Analysis The two categories of analysis were performed: Firstly, to specify the shape factor and secondly, to measure the particle size disibution.

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ACCEPTED MANUSCRIPT 2.3.1. Determination of Shape factor The scanning electron microscope along with MIP software and specific surface area test were used to

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determine the shape factor of particles. 2.3.1.1. Scanning electron microscope test

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Figure 1 shows a micro-photograph of the samples taken with a Scanning electron microscope (SEM)

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Tescan, model Vega3 made in Czech Republic. SEM results were analyzed by MIP software to determine particle’s shape factor [10]. This software considered numerous particles and identified shape factor for

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each particle and finally averaged the values. Figure 4 shows reports of Spherical aluminum and Glass

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sand particles from MIP software. This software cannot determine the shape factor of flake aluminum due to a plate-like shape of particles. 2.3.1.2. Specific surface area test

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Belsorp-mini ii made in Japan was used to define the specific surface area tests (BET) of particle samples

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that it can determine the particle’s shape factor, Ø, using equation 2.1 and 2.2 [11].

(2.1)

(2.2)

where Sw is the powder specific surface area, ni and dvi are, the number and the equivalent volume diameter of particles in size class i (as measured by the Sympatec), ρ is the particle density. The accuracy of Equation 2.1 does not depend on any assumptions, being limited only by experimental conditions, such as the techniques employed to determine the material specific surface area. In the present work, the powder surface area was measured by nitrogen gas adsorption. Specific surface area values of samples were indicated in table 2. 6

ACCEPTED MANUSCRIPT 2.3.2. Particle size distribution test Particle sizes distributions of samples were obtained using laser method by a Sympatec particle size

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analyzer made in Germany. Spherical aluminum, glass sand and flake aluminum particle size distribution curves shows in figure 5. It is known that particle shape strongly affects the result of particle sizing [12].

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Also, Sympatec particle size analyzer has ability to consider particle’s shape factor as basic information to

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specify particle size distribution. So for more accuracy, firstly, the shape factors were determined by earlier mentioned methods and then results used for determination of corresponding particle size

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distribution.

3. Results 3.1. Effect of particle’s shape factor

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Spherical aluminum, glass sand and flake aluminum were used as feed to investigate the effect of

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0.1% and 3.25 m/s respectively.

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particle’s shape. The particle volume concentration and inlet velocity for all experiments were fixed at

Figure 6 shows the separation efficiency curves for all three particles. In the coarse particle range (>10 µm), the curves become smoother with decreasing sphericity around the cut size (the particle size at which 50% separation efficiency to a hydrocyclone under flow occurs) and the separation efficiency decreases with decreasing particle sphericity. At the particle size less than 6 µm, separation efficiency of spherical particles is less than glass sand. This strange behavior was observed by Zhu and Liow [9] at the particle size of 7 µm as well. 3.2. Effect of inlet velocity The experiments were performed for all three particles at a constant volume concentration (0.1%) and various inlet velocities (3.25, 4.25 and 5.05 m/s). The corresponding separation efficiency curves are shown in figure 7. The separation efficiency increases with increasing inlet velocity, because of increase in the centrifugal forces. The studied inlet velocities were in the turbulent regime so the centrifugal forces were considerable. The separation efficiency curves for flake and glass sand follow almost similar 7

ACCEPTED MANUSCRIPT patterns and the gap between these patterns is small. In the case of spherical particles there is a wide gap

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between the patterns of separation efficiency curves.

3.3. Effect of feed volume concentration

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Three feed volume concentrations of prepared samples were fed into the mini-hydrocyclone at an inlet

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velocity of 3.25 m/s and results are shown in figure 8. When particle concentration increases, the separation efficiency decreases slightly. There is a slight change in separation efficiency with respect to

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change in concentration (the change in concentration range is not wide).

4. Discussion

4.1. Coarse particle region (larger than 10 µm)

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The equation for forces balance is given by 4.1.

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(4.1)

This is a simplified Basset–Boussinesq–Oseen (BBO) equation for particle motion. On the right hand side of the equation,

virtual mass force and

is the fluid drag force,

is the gravitational force,

is the

is the pressure gradient force per unit particle mass (force/unit particle mass)

drag force is proportional to FD [13]. The fluid drag force factor, FD, is given by 4.2. (4.2)

Also FD is proportional to CD. CD is the drag coefficient for non-spherical particles given by 4.3 [6] as: h h

h

h

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(4.3)

ACCEPTED MANUSCRIPT Where

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(4.4)

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With Resph being the Reynolds number for the diameter of a sphere having the same volume as the nonspherical particle where

. Ø is inversely proportional to CD therefore drag force increases

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with decreasing Ø so separation efficiency decreases.

The large Solid particles exerted a so-called braking effect on the swirling flow due to the effect of

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friction drag caused by the shear stress between the particles and fluid flow in the swirling direction. For a

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particle moving in a swirling flow, it is subjected to a friction drag force in the swirling flow direction due

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to the relative motion between the solid and fluid phases. In return, a particle also exerts a friction drag force on the fluid phase, which dissipates the fluid momentum and slows down the main swirling flow [5]. The braking effect may be ignored for a low solid concentration flow as the amount of particles is relatively small, however, with an increase in solids concentration, the number of particles increases as well and the braking effect becomes more significant. The importance of the braking effect at a higher concentration could explain the larger decreases in the separation efficiency occurring at a solid concentration of 4% volume for the mini-hydrocyclone separation, which is due to the slowing down of the swirling resulting in a decreased centrifugal force on large particles [4]. Whereas at the concentration less than 2% vol. (in this investigation), the low frictional drag force between the solid and fluid phases causing little dissipation of fluid momentum which leads to a close separation efficiency curves (figure 8). The Cut sizes (d50 value) observed at a fixed inlet concentration of 0.1%vol. and the inlet velocities of 3.25, 4.25 and 5.05 m/s for spherical particles were 11, 9 and 4 µm and for the flake particles: 36, 21 and 18 µm and for the glass sand: 21, 15 and 10.5 µm (figure 7). At a force balance between the centrifugal 9

ACCEPTED MANUSCRIPT and drag forces acting in the hydrocyclone, there is an increase in centrifugal force when inlet velocity and particle sphericity rise which causes a decrease in cut size.

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The cut size values for different particle volume concentrations almost remain unchanged; therefore, low

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feed volume concentrations don’t have a noticeable impact on the balance of forces.

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4.2. Fish hook effect

The separation efficiency of particles below 10 µm increases unexpectedly with decreasing particle size

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leading to entrainment of the smalls to the hydrocyclone underflow [14, 15, 16, 17 and 18]. This effect is

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known as the fishhook effect and the fishhook dip is the particle diameter where minimum separation efficiency occurs [18]. This phenomenon occurs due to particle interaction while the finer particles are captured by the larger particles and the effect is more prominent as the fraction of large particle increases

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[17]. The smaller particles entrain by the wake region behind the large particles. The fish hook effect for spherical particles is most sharp and the fish hook dip appears at 2.38 µm. In the case of glass sand, the

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fish hook dip occurs at 6.4 µm. The flake particle shows a wide plateau region and there is a second fish hook dip. The fish hook dips for flake particles occur at 2.7 and 1.5 µm (figure 6). In general, positional differences of large particles in the flow field and their shapes are two principal reasons for differences in fishhook effect.

Figure 9A, B is a Schematic of the distribution of small particles around a large spherical particle. Fine particles can be transported to the far-wake away from the large particle (figure 9A), where the wake flow direction may be changed towards the hydrocyclone central region due to the influence of the free stream. However, in near wake, close to the large particle (figure 9B) the wake flow can engulf and entrain more particles over a wider field [19]. When the shape of particle changes with change in aspect ratio in the case of glass sand, the wake field (vortex shedding) is primarily influenced by the orientation and geometry [20]. As the aspect ratio value (AR) increases (shape factor decreases), the growth rate of recirculation length decreases also effective wake area declines so less numbers of small particles can entrain. On the other hand, while the main flow 10

ACCEPTED MANUSCRIPT approaches the base, no steady recirculation and eddies combine at the tail end of the geometry consequently. In the case of higher AR, the momentum loss induced by the bluff body tail region makes

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the flow steady. This enhances the stability of flow, despite the occurrence of flow separation at the corners of the triangle (figure 10, 11). Therefore these wake vortexes are not able to entrain more small

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particles. Based on these view points, fishhook effect reduces with decreasing in shape factor (figure 6).

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Zhu and Liow [9] performed experiment sand particles (V=4m/s, ρ=2650 kg/m3) and calculated by simulation the Reynolds number to be 150. This Reynolds number is alike to Reynolds number used in

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fig 10, 11. Indeed we can assume that our particles with similar properties have the same Reynolds

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number and behave in resembling manners.

5. Conclusion

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In this investigation we observed that the separation efficiency increases with increasing inlet velocity and particle sphericity significantly and decreasing feed volume concentration slightly. The maximum total

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separation efficiency obtained in this research work for spherical particles was 74.51% at 0.05% solid concentration and inlet velocity of 5.05 m/s. The minimum total separation efficiency of 47.52% was observed for flake particles at 3.25 m/s and volume concentration of 0.2%. The appearance of fish hook was clearly explained for spherical and irregular particles based on the available theories.

References: 11

ACCEPTED MANUSCRIPT [1] D. Roberge, L. Ducry, N. Bieler, P. Cretton, B. Zimmermann, Microreactor technology: a revolution for the fine chemical and pharmaceutical industries? Chem. Eng.Technol. 28 (2005) 318–323

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[2] L. Svarovsky, Hydrocyclones, Solid-Liquid Separation, , Butterworth& Co (Publishers) Ltd., London,4 (2000) 191–243.

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[3] L.Svarovsky, Hydrocyclones. Holt, Rinehart and Winston, London, Taneda, S., Experimental

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investigation of the wake behind a sphere at low Reynolds numbers.J.Phys.Soc.Jpn.11 (1984) 1104–1108. [4] G. Zhu, J. Liow, A. Neely, Computational study of the flow characteristics and separation efficiency

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in a mini-hydrocyclone, chem. Eng. Res. and des. 90 (2012) 2135–2147.

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[5] A. Hoffmann, L. Stein, Gas cyclones and swirl tubes: principles, design, and operation, Springer Verlag, Berlin. (2002) 77-93.

[6] A. Haider, O. Levenspiel, Drag coefficient and terminal velocity of spherical and nonspherical

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particles, Powder Technology 58 (1989) 63–70. [7] S. Endoh, H. Ohya, K. Masuda, S. Suzuki, H. Iwata, Study of the shape separation of fine particles

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using fluid fields: dynamic properties of irregular shaped particles in wet cyclones, Kona 12 (1994) 125–

[8] K. Kashiwaya, T. Noumachi, N. Hiroyoshi, M. Ito, M. Tsunekawa, Effect of particle shape on hydrocyclone classification, Powder Technology 226 (2012) 147–156. [9] G. Zhu, J. Liow, Experimental study of particle separation and the fishhook effect in a minihydrocyclone, Chem. Eng. Sci.111(2014)94–105. [10] O.P. Mills and W.I. Rose, Shape and surface area measurements using scanning electron microscope stereo-pair images of volcanic ash particles, Geosphere66 (2010) 805-811. [11] F.M. Barreiros, P.J. Ferreira, M.M. Figueiredo, Calculating Shape Factors from Particle Sizing Data, Part. Part. Syst. Charact.13 (1996) 368-373. [12] M. Naito, O. Hayakawa, K. Nakahira, H. Mori, J. Tsubaki, Effect of particle shape on the particle size distribution measured with commercial equipment, Powder Technology 100 (1998) 52–60.

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ACCEPTED MANUSCRIPT [13] C.T. Crowe, J.D. Schwarzkopf, M. Sommerfeld, Y. Tsuji, Multiphase Flows with Droplets and Particles, CRC Press.2011.

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[14] S.Pasquier,J.J. Cilliers,Sub-micron particle dewatering using hydrocyclones. Chem. Eng.J.80 (2000) 283–288.

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separation of fine particles.Miner.Eng.16 (2003) 1005–1007.

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[15] A.K. Majumder, P. Yerriswamy, J.P. Barnwal, The “fish-hook“ phenomenon in centrifugal

[16] T.Neesse, J. Dueck, L. Minkov, Separationof finest particles in hydro- cyclones,Miner.Eng.17 (2004)

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689–696.

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[17] H. Schubert, On the origin of “anomalous“ shapes of the separation curve in hydrocyclone separation of fine particles,Particul.Sci.Technol.22 (2004) 219–234. [18] A.K. Majumder, H. Shah, P. Shukla, J.P. Barnwal, Effect of operating variables on shape of “fish-

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hook“ curves incyclones,Miner.Eng.20 (2007) 204–206. [19] S. Taneda, Experimental investigation of the wake behind a sphere at low Reynolds numbers, J.

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Phys. Soc. Japan.11 (1956) 1104-1108.

[20] S. GangaPrasath, M.Sudharsan, V.VinodhKumar, S.V. Diwakar, T.Sundararajan, Effects of aspect ratio and orientation on the wake characteristics of low Reynolds number flow over a triangular prism, Journal of Fluids and Structures 46,2014,59-76.

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Fig.1. Microphotographs of a) aluminum spherical particles b) Glass sand particles and c) flake aluminum particles.

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Fig.2. Schematic representation of the hydrocyclone with the characteristic dimensions in the vertical plane.

Fig.3. Schematic representation of the test apparatus.

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Fig.4. Software report about shape factor quantity A) Glass sand particles is selected B) Spherical aluminum particles is selected C) software report for Glass sand particles D) software report for Spherical aluminum particles

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80

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Spherical Aluminum Glass sand Flaked Aluminum

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60

40

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CUMULATIVE DISTRIBUTION , %

100

0 1

10

100

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0.1

1000

PARTICLE SIZE, µm

3.25 m/s 0.1%

1.0

0.8

Flake Glass sand Spherical

0.6

0.4

0.2

0.0 0.1

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Separation efficiency

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Fig.5. Particle size distribution of the spherical aluminum, glass sand and flake aluminum.

1

10

100

1000

Particle size,µm

Fig.6. Comparison separation efficiency curves for flake aluminum, glass sand and spherical aluminum samples at the inlet velocity of 3.25 m/s and 0.1%vol.

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A 1.0

0.6

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3.25 m/s 4.25 m/s 5.05 m/s

0.4

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Separation efficiency

Flaked Aluminum 0.1% 0.8

0.0 0.1

1

10

100

1000

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Particle size, µm

B

5.05 m/s 4.25 m/s 3.25 m/s

0.8

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0.6

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Separation efficiency

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Glass sand 0.1%

1.0

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0.2

0.2 0.1

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0.4

1

10

100

1000

100

1000

Particle size, µm

C

Spherical Aluminum 0.1%

Separation efficiency

1.0

5.05 m/s 4.25 m/s 3.25 m/s

0.8

0.6

0.4

0.2 0.1

1

10

Particle size, µm

Fig.7. Separation efficiency curves at the 0.1% vol. for (A) Flake aluminum (B) Glass sand (C) Spherical aluminum.

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ACCEPTED MANUSCRIPT A flake Aluminum 3.25 m/s 0.05% 0.1% 0.2%

0.8

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Separation efficiency

1.0

0.6

0.2 0.1

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100

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Particle size, µm

B 0.8

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Glass sand 3.25 m/s 0.05% 0.1% 0.2%

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0.4

0.2 1

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Separation efficiency

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0.4

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1000

100

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Particle size,µm

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Spherical Aluminum 3.25 m/s

Separation efficiency

1.0

0.8

0.05 % 0.1 % 0.2 %

0.6

0.4

0.2 0.1

1

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Particle size, µm

Fig.8. Separation efficiency curves at the inlet velocity of 3.25 m/s for (A) Flake aluminum (B) Glass sand (C) Spherical aluminum.

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Fig.9. Schematic of the distribution of small particles in (A) far wake away from the large particle, and (B) near wake close to the large particle (The red circles denote small particles; the red arrows denote the large particle moving direction and the green arrows denote the direction of the wake flow) in Re=118[19]

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Fig.10. Instantaneous streamlines for three different aspect ratios when apex of the triangle faces the approaching flow (a) AR=0.5, (b) AR=1, (c) AR=3, for Re=150 [20].

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Fig.11. Instantaneous streamlines for three different aspect ratios when base of the triangle faces the approaching flow (a) AR=0.5, (b) AR=0.866, (c) AR=5, for Re=150 [20].

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Graphical abstract

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Highlights  We obtain exact particle’s shape factor by SEM and BET Analysis.  We examine changes in the particle sphericity on the separation efficiency and fishhook effect.  Increasing particle sphericity will increase separation efficiency.  Decreasing inlet velocity will decrease separation efficiency.  Increasing particle’s shape factor will increase Fishhook effect.

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