Inlet Particle-Sorting Cyclone for the Enhancement of PM2.5 Separation

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Inlet Particle-Sorting Cyclone for the Enhancement of PM2.5 Separation Pengbo Fu, Fei Wang, Xue-Jing Yang, Liang Ma, Xin Cui, and Hualin Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04418 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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Environmental Science & Technology

Inlet Particle-Sorting Cyclone for the Enhancement of PM2.5 Separation

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PENG-BO FU†, FEI WANG†, XUE-JING YANG†, LIANG MA†, XIN CUI‡, HUA-LIN

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WANG†*

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†State Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on

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Chemical Process, East China University of Science and Technology, Shanghai, 200237, P. R.

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China

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‡Shanghai Huachang Environmental Protection Co. Ltd., Shanghai, 201611, P. R. China

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*Corresponding author, E-mail: [email protected], Tel/Fax: +86-21-6425 1894

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For Table of Contents Only

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Abstract: Many cities are suffering from severe air pollution from fine particulate matter. Cyclone is

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an effective separator for particulate pollutant but has low efficiency for those with an aerodynamic

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diameter of 2.5 µm or less (PM2.5). In this research, four novel inlet particle-sorting cyclones were

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first developed to enhance the separation of PM2.5. The energy consumption, overall separation

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efficiency, particle grade efficiency ， outlet particle concentration and size distribution were

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compared with common cyclone (CM-C). It was found that the vertical reverse rotation cyclone

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(VRR-C), which made the smaller particles enter cyclone from radially outer side and axially lower

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side at the rectangular inlet, had the best separation performance, especially for PM2.5 separation.

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The mean diameter of inlet particles was 15.7 µm and the particle concentration was 2000 mg/m3,

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the overall separation efficiency of the VRR-C reached 98.3%, which was 6.4% higher than that of

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CM-C. PM2.5 grade efficiency of the VRR-C exceeded 80%, which was 15%~20% higher than that

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of CM-C. The PM2.5 content at the VRR-C outlet was 30.8 mg/m3, while that of CM-C was still

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118.4 mg/m3. The novel inlet particle-sorting cyclone is an effective separation enhancement for

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PM2.5 source control in the process of industrial production and environment protection.

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Introduction

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Particulate matter is the major medium that pollutes the atmospheric environment and does harm

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to people’s health,1-3 especially the particles with an aerodynamic diameter of 2.5 µm or less

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(PM2.5).4 These particles can enter the bronchi and disturb the gas exchange, which can result in

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diseases such as asthma, bronchitis, lung dysfunction and angiocardiopathy.5 The hazardous

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substances contained within PM2.5, such as harmful gases and heavy metals, can dissolve in the

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blood through the bronchi and pulmonary alveoli, which poses a direct threat to human health.6,7

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According to the investigation provided by the Global Burden of Disease (GBD) 2013, PM2.5 was

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identified as a leading risk factor for global disease burden with an estimated 2.9 million attributable ACS Paragon Plus Environment 3


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deaths in the year 2013.8 PM2.5 can also influence the acid deposition and the chemical equilibrium

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of air; as a result, fog and haze formed by polluted air reduces visibility, which impacts

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transportation.9 In short, PM2.5 is one of the biggest challenges facing the modern world, and the

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greatest difficulty is the low separation efficiency for these fine particles in industry.

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The crux of PM2.5 control is source control in the process of industrial production, thus the

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industrial capture and separation technology of fine particles is the key to solving this problem.5,10

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The major methods of separating particles from industrial gas include electrostatic separation,11

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settling separation,12 bag-type dust collection,13 and rotating centrifugal separation.14,15 However,

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electrostatic separation incurs high operation costs and has a potential security risk; the devices used

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in settling separation are always very large, with low efficiency and need a long settling time;

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Bag-type dust collection is hard to maintain in stable operation for long periods and the maintenance

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costs are very high. Therefore, with these technologies it will be difficult to obtain outstanding social

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and economic benefits in fine particle separation, especially in the PM2.5 separation.

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The cyclone is an important rotating centrifugal separation equipment that can effectively capture

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and separate the particles in the gas.16,17 In addition, the cyclone has the advantages of small

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structure size, enormous processing capacity, high separation efficiency, low energy consumption,

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easy operation and maintenance and is suitable for long periods of operation.18 Therefore, the

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cyclone has been widely used in various fields, such as in atmospheric pollution prevention and

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treatment,19,20 industrial gas purification,21 process optimization,22 particle grading,23 and particle

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sampling.24,25 However, the traditional cyclones usually have a larger cutting size and show low

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separation efficiency for particles with a small size (PM2.5).

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Many researchers have attempted to improve the separation efficiency of a cyclone by changing ACS Paragon Plus Environment 4

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the structural parameters, such as body diameter,26,27 inlet structure and size,28,29 vortex finder

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structure and size,30 lengths of the underflow straight pipe,31 and structure of the dust hopper.32 Some

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researchers attempted to enhance rotating centrifugal separation by improving the separable factors

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of materials, for example, to reduce the viscosity of materials by adjusting the temperature.33 Other

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researchers attempted to strengthen separation by introducing electric fields and magnetic fields

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based on the electrical and magnetic properties of the materials.34,35 However, these studies focus on

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the particle separation process as a whole; these techniques cannot extraordinarily improve the

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separation efficiency for the small particles, which are difficult to separate; as such, they can do little

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to face the challenge that more and more PM2.5 in the air is threatening the quality of our lives.

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To the best of our knowledge, particulate matter is always composed of different sized particles;

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the fine particles with a small size are hard to separate and capture is the main challenge for PM2.5

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control. For rotating centrifugal separation, Wang et al. first found that the initial position on the inlet

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section where particles enter the hydrocyclone influences the particle separation performance.36 Liu

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et al. introduced a volute chamber with a pre-sedimentation function before the inlet and effectively

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improved the classification efficiency for fine particles.37 Yang et al. found that particle

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arrangements at the entrance strongly influence the motion and distribution of dispersed phase

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particles by CFD simulations.38 Therefore, particle sorting to enhance the separation of fine particles

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is practical and feasible, but there is still much work that needs to be performed for PM2.5 separation

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enhancement in the atmospheric protection field.

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In this research, we designed four types of novel inlet particle-sorting cyclones by combining the

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sort function of a particle sorting classifier (PSC), which was reported in our previous studies.

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Different sized particles are in an ordered arrangement at the outlet of a PSC. 39 By changing the type



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of connection between the cyclone and PSC, we can control the smaller particles enter the cyclone

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from different positions at the inlet section of the cyclone. The results show that the inlet

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particle-sorting cyclone (VRR-C), which makes the smaller particles enter the cyclone from the

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radially outer side and the axially lower side of the inlet section, can effectively enhance the

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separation of fine particles, especially of PM2.5. This research is of great significance for the

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improvement of cyclone separation technology, as well as for the separation and capture of PM2.5 in

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the environmental protection field.

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Experimental section

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Particle sorting classifiers (PSCs). The sorting regular of different sized particles at the outlet of

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PSC is shown in Figure 1(a).39 For the purposes of this study, when the PSC makes the fluid medium

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rotate in the clockwise direction around the axis of the cavity, we call it the positive rotation PSC

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(PR-PSC), which is shown in Figure 1(c). When the PSC make the fluid medium rotate in the

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reverse direction, we call it the reverse rotation PSC (RR-PSC), which is shown in Figure 1(d). The

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height of the PSC (H) is 5 times as high as the rectangular inlet (a1), for the best sorting effects.39

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Figure S1 illustrates the structure models of PR-PSC and RR-PSC, and Table S2 shows the geometry

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parameters of PSCs.


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Figure 1. (a) The distribution of different sized particles at the outlet of PSC. The model of (b)

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cyclone, (c) PR-PSC, (d) RR-PSC. The model and the regular of particle sorting at the inlet of (e)

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CM-C, (f) PPR-C, (g) PRR-C, (h) VPR-C, (i) VRR-C.

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Inlet particle-sorting cyclones. Four different types of inlet particle-sorting cyclones were

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designed by changing the type of connection between the PSC and cyclone: (1) The PR-PSC and

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cyclone were connected in a parallel way; they form the parallel positive rotation cyclone (PPR-C).

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At the inlet section of the cyclone, the sorting regular of the different sized particles was that the

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smaller particles were close to the radially inner side and the axially upper side, as shown in Figure

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1(f); (2) The RR-PSC and cyclone were connected in a parallel way; they form the parallel reverse

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rotation cyclone (PRR-C). The sorting regular was that the smaller particles were close to the

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radially outer side and the axially upper side, as shown in Figure 1(g); (3) The PR-PSC and cyclone

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were connected in a vertical way; they form the vertical positive rotation cyclone (VPR-C). The

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sorting regular was that the smaller particles were close to the radially inner side and the axially



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lower side, as shown in Figure 1(h); (4) The RR-PSC and cyclone were connected in a vertical way;

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they form the vertical reverse rotation cyclone (VRR-C). The sorting regular was that the smaller

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particles were close to the radially outer side and the axially lower side, as shown in Figure 1(i). For

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the common cyclone (CM-C), the particles at the inlet section are in a disordered state, as shown in

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Figure 1(e).

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Figure 2. The geometry parameters of the adopted mini-cyclone

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The cyclone adopted in this research was a self-designed mini-cyclone that has high separation

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accuracy for the fine particle. The body diameter of the mini-cyclone was 75 mm, the inlet was


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rectangular tangential inlet and the dust hopper was equipped with an anti-reentrainment cone, which

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is used to reduce the back mixing of separated particles, thus improving the separation efficiency of

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cyclone, the cone angle of anti-reentrainment cone was 90° and the proportion of effective flow area

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was 30%. The cyclone was made of organic glass too. The geometry structure and parameters of the

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mini-cyclone adopted in this research are shown in Figure 2.

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Experimental equipment and procedure. A schematic diagram of the experimental setup is

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additionally shown in Figure 3. Particles were transported into a cyclone by air in the form of

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uniformly dispersed aerosols with a constant particle concentration of 2000 mg/m3; most of them

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were captured into the dust hopper. In our experiments, we used a variable frequency motor

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controlled discharge valve to add the fine particle, so the particle masses that entered the cyclone mi

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can be obtained by the constant feeding rate and feeding time. The dust hopper was connected with

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cyclone by a flange, so we can disassemble the hopper and take out the captured fine particles after

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the experiment, the particle mass that were captured into dust hopper mu can be obtained by an

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accurate electronic balance. All of our experiments were repeated three times.

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Figure 3. Schematic diagram of the experimental setup. 1-air filter; 2-air blower; 3-bypass valve;

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4-buffer tank; 5-discharge valve; 6-material tank; 7-main valve; 8-flow meter; 9-pressure meter;

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10-U-tube manometer; 11-reducer pipe; 12-particle sorting classifier; and 13-cyclone.

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The overall separation efficiency (E) was calculated by Eq. (1).

E=

mu × 100% mi

(1)

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To obtain the characteristics of particle size distribution at the inlet and outlet (dust hopper),

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particles were first qualitatively observed by a field-emission scanning electron microscope (NOVA

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NanoSEM450, FEI Inc., U.S.A), and then quantitatively measured using a Mastersizer 2000 laser

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particle size analyzer (Malvern instruments Ltd., England).

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Grade efficiency (G) refers to the separation efficiency of dispersed particles of different size

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grades in a suspension or the recovery rate of particles of all size grades.40,41 The specific grades are

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represented by dj (j = 1, 2, ...,n), and their grade efficiency can be obtained by Eq. (2).

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G(d j ) =

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where G(dj) refers to the separation efficiency of the particles with size dj; fu(dj) and fi(dj) refer to the

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mass percentage of particles with size dj at the dust hopper and at the inlet, respectively.

mu f u ( d j ) mi f i (d j )

×100%

(2)

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Materials. In the experiment, the continuous phase was the filtered air. The dispersed phase was

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the spherical silicon dioxide particles, which were used to simulate the aerosol particles in the

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atmospheric environment and industrial exhaust gases because both of them have the same

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characteristic of being composed of different sized particles. Figure 4 shows the size distribution of

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the spherical silicon dioxide particles. The volume mean diameter of the particles was 15.7 µm, the

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proportion of PM2.5 was 16.6%, and the proportion of PM10 was 41.4%, as shown in Figure 4(a). The ACS Paragon Plus Environment 10

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number-size distribution of inlet particles is shown in Figure 4(b). The scanning electron microscope

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(SEM) photograph, shown in Figures 4(c) and 4(d), indicated that the particles of silicon dioxide

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have a good degree of sphericity.

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Figure 4. Inlet silicon dioxide particle size distribution: (a) Volume distribution. (b) Number

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distribution. (c) Low magnification SEM photograph. (d) High magnification SEM photograph.

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Results and discussion

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Mean particle size distribution at the outlet of PSC. In our previous studies, we divided the

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rectangular outlet of PSC into 12 individual outlets with equal areas.39 The position of each outlet is

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denoted by ij (the outlet’s axial position is denoted by i, from the inlet side to the outlet side, i = 1, 2,

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3, 4; the outlet’s radial position is denoted by j, from the inside to the outside, j = 1, 2, 3). Figure S4

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shows the particle mass (mij) and mean particle size (dij) at 12 individual outlets when H = 5a1. The ACS Paragon Plus Environment 11


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results show that both particle mass and mean particle size increase progressively from radial inside

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to outside (i.e., from j = 1 to j = 3) and from axial inlet side to outlet side (i.e., from i = 1 to i = 4).

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This means that the smaller particles were close to the axial inlet side and the radially inner side,

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whereas the bigger particles were close to the axial outlet side and the radially outer side; particles

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were arranged in an ordered sorting at the outlet of PSC, as shown in Figure 1(a).

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Influence of gravity field on particle sorting. As shown in Figure 1(h) and Figure 1(i), the PSC

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assembled in VPR-C and VRR-C need to be positioned horizontally. To study the influence of the

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horizontal positioning of PSC on particle sorting, we studied the particle mass and mean particle size

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at the 12 individual outlets when the angle (θ) between the axis of PSC and the vertical are 0°, 45°,

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90°, 135° and 180°. We also calculated the radial sorting effect σ i and the axial sorting effect σ j ,

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which were defined in our previous studies.39 The results show that no matter the value of θ was, the

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regular of particle distribution at the rectangular outlet of PSC always presents in a consistent way.

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Moreover, the changes in σ i and σ j that were produced by changes in θ can be neglected

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(Figure S5). These results demonstrate that the gravity field has little effect on the particle sorting of

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the PSC. In other words, the PSC can be connected with a cyclone in any type without influencing

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the regular and efficiency of particle sorting at the outlet of the PSC.

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The pressure drop of PSC and cyclones. The pressure drop of cyclone mainly refers to the fluid

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pressure difference between the inlet and outlet, indicating the energy consumption that occurs while

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fluid is flowing through the cyclone. Pressure drop is a key technical parameter and operational

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indicator. Figure S6(a) shows the pressure drop (∆p1) of the individual PSC and Figure S6(b) shows

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the pressure drop (∆p2) of CM-C and four inlet particle-sorting cyclones under different inlet flow

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rates (Q). The results show that particle sorting led to a slight increase in cyclone pressure drop, and

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the VRR-C has the largest pressure drop among all the cyclones. However, because the pressure

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was not apparent. When the inlet flow rate was 40 m3/h, the pressure drop of the individual PSC was

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42 Pa, and the pressure drop of VRR-C was only 135 Pa higher than that of CM-C; hence, adding

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the PSC to compose an inlet particle-sorting cyclone does not lead to excessive additional energy

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consumption, the newly designed inlet particle-sorting cyclone has excellent economic performance.

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The dimensionless parameters Reynolds number (Re) and Euler number (Eu), which correspond

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to the inlet flow rate and pressure drop, respectively, are two important indices for the similar

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amplification criterion in engineering design and application of cyclone. The relationship of Re and

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Eu is an effective predictor of the cyclone energy consumption and processing capacity. Eu

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represents the value of the pressure drop coefficient. Therefore, particle sorting also led to a slight

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increase in Eu (Figure S7). In engineering scale-up design, the region where the value of Eu is lower

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should be selected.

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The overall separation efficiency of cyclones. Figure 5(a) shows the overall separation

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efficiency (E) of five types of cyclones under different inlet flow rates. The separation efficiency of

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each increased with the inlet flow rate at first, then began to decrease after the inlet flow rate reached

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40 m3/h. This occurred because when the flow rate was too small, the centrifugal force inside the

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cyclone that drives particles to move toward the sidewall was not large enough, resulting in low

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separation efficiency; when the inlet flow rate was excessively large, axial velocity became

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extremely large such that the residence time of the particles in the cyclone was shortened, and the

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excessively large flow also enlarged the short-circuit flow and turbulence intensity of the cyclone,

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which destroyed the relatively ordered state of the forced vortex and free vortex of the inside flow

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field; hence, the separation efficiency was decreased. Among the five types of cyclones, VRR-C has

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the highest separation efficiency under the same conditions. The highest overall separation efficiency

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(Emax) of VRR-C was up to 98.3%, which was 6.4% higher than that of CM-C. Figure 5(b) shows the ACS Paragon Plus Environment 13


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particle concentration at the outlet (Cout) of five types of cyclones when the inlet particle

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concentration was 2000 mg/m3; the lowest outlet particle concentration (Cmin) of CM-C was 160.6

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mg/m3, which was 5 times higher than the emission limits stipulated in the Chinese emission

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standard of air pollutants for the petroleum refining industry (GB 31570-2015, China) and thermal

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power plants (GB 13223-2011, China).42,43 In contrast, the lowest outlet particle concentration of

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VRR-C (Cmin) was just 34.1 mg/m3. Although the inlet particle concentration was as high as 2000

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mg/m3, the outlet gas of VRR-C nearly achieved the standard for direct emission. Therefore, VRR-C

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can be efficiently applied to the situations that require precise separation to increase the degree of air

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cleaning.

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Figure 5. The particle separating capacity of cyclones under different inlet flow rates: (a) Overall

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separation efficiency. (b) Particle concentration of the outlet gas.

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Figure 5 shows that the overall separation efficiency of VRR-C was the best; PRR-C and VPR-C

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also had the ability to improve the separating capacity of the common cyclone. As we all know,

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particles in the gas are not always of the same size but rather are a group of particles with different

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sizes; these particles with relatively small sizes are difficult to separate because their centrifugal

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separation and particulate matter pollution control. As shown in Figure 1(i), VRR-C controlled the

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smaller particles that entered cyclone from the radially outer side and the axially lower side of the

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inlet, which are the best inlet positions that facilitate particle separation.32 Therefore, the separation

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efficiency of the smaller particles was directly improved; although these relatively large particles

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were adjusted at inlet positions that are not good for separation, their centrifugal forces are large

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enough that they can easily move toward the sidewall and then be captured. Additionally, as shown

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in Figure 6, the initial position of relatively large particles in VRR-C were the radially inner side and

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axially upper side; in the process of these particles were driven to the radially outer side (sidewall)

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and the axially lower side (dust hopper) by field forces, these large particles can form a radially

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mobile membrane and an axially mobile membrane in VRR-C (Figure 6). The mobile membrane can

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further push the relatively small particles to move toward the sidewall and dust hopper where they

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can be easily separated, although the centrifugal forces of these small particles are not enough to

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support the separation process. Thus, the advantageous inlet positions for the smaller particles and

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the mobile membrane formed by the larger particles co-operate to improve the separation efficiency

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of the smaller particles (PM2.5) while exerting no reduction in the separation efficiency of the

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relatively large particles; therefore, the overall separation efficiency of VRR-C was significantly

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improved.

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Similarly, as shown in Figure 1 and Figure 6, the initial positions of the relatively large particles in

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PRR-C were the radially inner side and the axially lower side, i.e., it only formed a radially mobile

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membrane in the process of separation; the initial positions of the relatively large particles in VPR-C

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were the radially outer side and axially upper side, i.e., it only formed an axially mobile membrane.

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As a result, the mobile membrane also improved the separation efficiency of the small particles, and

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although PRR-C and VPR-C do not leave the smaller particles at the best inlet positions, the overall ACS Paragon Plus Environment 15


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separation of PRR-C and VPR-C was higher than that of CM-C but also lower than that of VRR-C

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(Figure 5). PPR-C cannot form any mobile membrane, and the smaller particles were at the most

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disadvantageous inlet positions (radially inner side and the axially upper side) for particle

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separation.32 Thus, the separation efficiency of small particles was reduced, resulting in the overall

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separation efficiency of PPR-C that was slightly lower than that of CM-C.

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Figure 6. Schematic diagram of radially and/or axially mobile membranes of the inlet

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particle-sorting cyclones

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The grading efficiency of cyclones. The overall separation efficiency (E) represents the overall

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separation capacity for particle groups but cannot indicate the separation capacity for each size grade,

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which is the function of particle grading efficiency (G). Therefore, G also can effectively predict the

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separation performance of a given cyclone. Figure 7(a) shows the grading efficiency curves of five

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types of cyclones when the inlet flow rate was 40 m3/h. The grading efficiency of the smaller


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particles (particularly PM2.5) shows the same regular as the overall separation efficiency. VRR-C had

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the highest PM2.5 grading efficiency at over 80%, which was 15%~20% higher than that of CM-C.

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As with the overall separation efficiency, the PM2.5 grading efficiencies of PRR-C and VPR-C were

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higher than that of CM-C, while that of PPR-C was lower. The grading efficiency curves further

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demonstrate that VRR-C improved the separation efficiency for PM2.5 while exerting no reduction in

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the separation of larger particles. Figure 7(b) shows that when the total inlet particle concentration

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was 2000 mg/m3, the PM2.5 concentration at VRR-C outlet was 30.8 mg/m3, which was 87.6 mg/m3

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lower than that of CM-C (118.4 mg/m3). The lower PM2.5 outlet concentration directly indicates that

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VRR-C possesses high value in controlling PM2.5 concentration in the atmospheric environment.

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Figure 7. (a) Grading efficiency curves of cyclones. (b) Particle size grade distribution at the outlet

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of cyclones.

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Results presented herein demonstrate that these novel inlet particle-sorting cyclones (especially

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the VRR-C) effectively enhanced the PM2.5 separation, and then improved the overall separation

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efficiency of the cyclone. The pressure drop and separation efficiency results also show that the

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VRR-C has excellent technic-economic performance, shown in Table 1. VRR-C not only provides a



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useful template for the higher efficiency cyclone design but also an effective enhancement for the

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separation and capture of PM2.5, which is the separation difficulty in the environmental protection

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field. The successful application of VRR-C in the industry could effectively reduce the PM2.5

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concentration for the source control in the atmospheric environment and play an important role in

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stopping the phenomenon of increasing levels of PM2.5 in the air threatening the quality of our lives,

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thus improving the quality of our atmosphere.

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Table 1 The technic-economic performance comparison of cyclones Technical performance Cyclones

Overall separation efficiency (%)

PPR-C

91.3

Outlet particle concentration (mg/m3)

PM2.5 grade efficiency (%)

173.6

Economic performance Outlet PM2.5 concentration (mg/m3)

~58

127.1

Pressure drop (40m3/h, Pa) 270

PRR-C

95.3

93.2

~75

72.9

288

VPR-C

94.1

117.3

~67

91.2

312

VRR-C

98.3

34.1

~81

30.8

343

CM-C

91.9

160.6

~63

118.4

235

291 292

Supporting Information

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The geometry models of PR-PSC and RR-PSC (S1); the detailed geometry parameters of PSCs

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(S2); the names and specifications of the main laboratory equipment and instruments (S3); the mass

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and size distribution of different sized particles at the outlet of PSC (S4); the influence of the

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positioning angle of PSC on particle sorting (S5); the pressure drop of individual PSC and cyclones

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under different inlet flow rates (S6); the relationship of Re and Eu at the inlet of cyclones (S7).

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Acknowledgments

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This research was supported by the sponsorship of the National Science Foundation for Distinguished Young Scholars of China (Grant No.51125032).


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Dentener, F.; Dingenen,

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R.V.; Estep, K.; Amini, H.; Apte, J.S.; Balakrishnan, K.; Barregard, L.; Broday, D.; V. Feigin,

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Inlet Particle-Sorting Cyclone for the Enhancement of PM2.5 Separation

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