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

Jan 11, 2017 - (8) PM2.5 can also influence the acid deposition and the chemical equilibrium of air; as a result, fog and haze formed by polluted air ...
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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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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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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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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).

298

Acknowledgments

299 300

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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