A New Fine Particle Removal Technology--Cloud-Air-Purifying

Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES

Thermodynamics, Transport, and Fluid Mechanics

A New Fine Particle Removal Technology--Cloud-Air-Purifying Bo Wang, Siqing Li, SIJIE DONG, Rubin Xin, Ruizhi Jin, Yumeng Zhang, Kejun Dong, and Yunchao Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03034 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

A New Fine Particle Removal Technology --Cloud-Air-Purifying BO WANG†*, SI-QING LI†, SI-JIE DONG†, RU-BIN XIN†, RUI-ZHI JIN‡, YU-MENG ZHANG†, KE-JUN DONG‡, YUN-CHAO JIANG†*

†Key Laboratory of Western China's Environmental Systems (Ministry of Education) and Engineering Research Center of Fine Particle Pollution Control Technology and Equipment, Gansu Province, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, PR China

‡Center for Infrastructure Engineering, Western Sydney University, Penrith, NSW 2751, Australia *Corresponding author: Name: Bo Wang Email: [email protected] Postal address: College of Earth and Environmental Sciences, Lanzhou University, Lanzhou, 730000, PR China. Phone number：+86 -15101311520 Name: Yun-Chao Jiang Email: [email protected] Postal address: College of Earth and Environmental Sciences, Lanzhou University, Lanzhou, 730000, PR China. Phone number：+86 -13919402572 1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: This paper presents an innovative dust removal technology--Cloud-Air-Purifying (CAP) which combines electro-acoustic ultrasonic nebulization with a cyclone dust collector. A humid environment is produced in the cyclone to imitate natural clouds in which the rain drops are formed. This facilitates the ultrafine particles in the environment to absorb moisture and agglomerate. Consequently, the ultrafine particles will significantly increase in size and thus be much more efficiently collected by the cyclone. Experimental results of the collection of two kinds of particles show that CAP significantly improves the collection efficiency of PM2.5 particles compared to traditional gas cyclones. Furthermore, numerical simulation is used to study the dynamics of the fine particles in CAP environment, which verifies the positive effect of particle growth on the collection efficiency. Finally, the successful industrial applications of CAP technology are demonstrated. This paper confirms CAP as a novel and effective method for controlling air pollution.

1. INTRODUCTION

Increasing population and industrialization has resulted in serious atmospheric pollution. Dust from construction, combustion and vehicle emissions contains many fine particles (PM2.5).1 Due to its small aerodynamic diameter, strong penetration and diffusivity, large specific surface area and easy adhesion of chemical and biological toxic substances, PM2.5 poses a great threat to human health and has been recognized by the World Health Organization as “a class 1 carcinogen”.2-5

Cyclone dust collectors are widely used in the industrial dust collection because of their low cost and stable operation.6 Centrifugal force generated by the high-speed rotation of air is used to separate 2


Page 2 of 37

Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


solid particles from waste gases. However, traditional cyclone dust collectors cannot effectively remove fine particles, even different optimizations have been conducted in their structures recently.7-11 Such a difficulty has been studied by microscopic numerical simulations,12-14 which have revealed that the motion of a particle in the cyclone is chiefly determined by the drag force and centrifugal force. For a small particle the former is normally greater than the latter, which makes the particle difficult to be separated from the gas. Therefore, to increase the sizes of the fine particles should be a way to improve the collection efficiency.

Two kinds of methods have been proposed to achieve such a goal, namely, heterogeneous vapor condensation and particle agglomeration.15-18 Theoretically, the first method can be realized by putting the fine particles in a supersaturated environment. Then the particles can act as condensing nuclei to have vapor condenses on the surfaces, and thus turn into larger droplets.19 However, normal humidification techniques cannot easily generate supersaturated vapor. Fortunately, electro-acoustic ultrasonic nebulization developed in recent years. It can transform high-frequency electromagnetic oscillation into mechanical oscillation of liquid, breaking liquid into ultrafine high-density droplets of 4~10 µm in diameter, which will rapidly be vaporized to form the supersaturation environment required for heterogeneous vapor condensation.20-21

On the other hand, the humid environment can also help the fine particles agglomerate when they collide, as when particles become wet they can be bonded by the capillary force.22-23 In particular, such agglomeration can be enhanced by a strong turbulent flow field such as in a cyclone dust 3



collector.24-27 This is observed in the formation of raindrops in natural clouds. The studies on this natural process showed that turbulent flow can promote the collision chance of the fine droplets,28-30 which can significantly accelerate the growth of raindrops by agglomeration.31-35

By exploiting the two techniques for particle growth, an innovative dust removal technology, Cloud-Air-Purifying (CAP), is put forward here. In CAP, the electro-acoustic ultrasonic nebulization is used to generate ultrasonic vapor, which will be mixed with the industrial waste gas and injected into a gas cyclone. The fine particles will grow in size due to the vapor condensation and agglomeration under the humid environment and turbulent flow field. The enlarged particles are then quickly separated from the gas by the centrifugal force field. Such a process is similar to artificial rainfall in a cyclone.36 The applications of CAP on two common industrial dust, molecular sieve and white carbon black particles, are investigated by combined physical and numerical experiments. The results confirm the promising features of this innovative technology.

2. EXPERIMENTAL METHODS AND MATERIALS

2.1. Experimental Platform and Procedure. The experimental set-up is shown in Figure 1. It consists mainly of the screw feeder (with a frequency converter to control the feed rate), feed pipe, nebulization box, ordinary Lapple cyclone,37 collection tank, PVC pipe and fan (with an inverter to control airflow). The geometric parameters of the cyclone separator are shown in Figure 2.

4


Page 4 of 37

Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. CAP technology experimental platform

Figure 2. The geometric parameters of the cyclone separator

Figure 1 also outlines the overall experimental process. At the beginning of an experiment, the fan and nebulizer were first switched on. After a certain period when airflow within the system was stabilized at 16 m/s, dust particles were added. The concentration of dust particles in the inlet (4026 mg/m³) was measured at the certain frequency (42Hz) of the screw feeder. The electro-acoustic ultrasonic nebulizer was used to generate vapor. In the pipe after the nebulization box, ultrasonic vapor and dust particles were mixed, such that particles agglomerate and increase in size. This mixture 5



Page 6 of 37

of vapor and dust followed the flow of air into the cyclone. Most of the dust particles were separated in the cyclone and collected in the dust hopper. A pressure gauge was inserted to measure the pressure drop between the inlet and the outlet of the cyclone. It should be noted here that the CAP cyclone is to remove the dust particles from the industrial waste gases, which are not aerosol particles.

The nebulizer used in the experiment has 7 levels of mass flow rates, i.e., 0, 1.5, 2.0, 2.5, 3.0, 4.0 and 5.0 kg/h. Under the current experimental conditions, the volume flow rate of air is 395 m3/h, so the ultrasonic vapor concentrations could be calculated as 0, 3.80, 5.06, 6.33, 7.59, 10.13 and 12.66 g/m3, respectively. The inlet particle concentration (C1) and the outlet particle concentration (C2) were measured by an automatic soot detector (Qingdao Lao Ying 3012H-01).

Overall removal efficiency was calculated by Eq. (1).

= 1 −

× 100%

(1)

Grade removal efficiency is the removal efficiency of dust particles in a certain range of particle sizes. It has the following relationship with overall dust removal efficiency.38

= 1 − (1 − )

(2)

where η is the grade efficiency, %; , are the particle mass fractions at the inlet and outlet,

respectively, %.

A filter was used at the system outlet to collect samples. The samples were subjected to particle size analysis using a Mastersizer 2000 laser particle size analyzer (detected size ranged from 6


Page 7 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.02-2000 µm), from which we can obtain the particle mass fractions at the inlet and outlet. The morphological changes of the particles were recorded by the scanning electron microscopy (SEM, JSM-6701F).

2.2. Materials. Molecular sieve and white carbon black were chosen in this study because they are common industrial dust. Moreover, they are composed mainly of fine particles, which results in poor performance for industrial tail gas treatment. Thus they are typical particles that can be used to testify the new technology. The apparent densities of the molecular sieve and white carbon black used in this study are 3875 kg/m3 and 2300 kg/m3, respectively. The particle size distributions of the two kinds of dusts are shown in Figure 3. In Table 1, D10, D50 and D90 are the particle diameters when cumulative particle size distribution reach 10%, 50% and 90%, respectively. The contact angle of the white carbon black is smaller than that of the molecular sieve, which represents the hydrophilicity of the former is better than that of the later.

Figure 3. The particle size distributions of the two kinds of dusts 7



Page 8 of 37

Table 1. Particle size distributions and contact angle Dust type

D10(µm)

D50(µm)

D90(µm)

Contact angle(°)

Molecular sieve

1.423

3.596

9.434

17.5

White carbon black

4.870

12.520

26.708

14.5

3. NUMERICAL SIMULATION METHODS

3.1. Numerical Modelling Process. For complex gas-solid flow in the cyclone separator, a computational fluid dynamics (CFD) model has been established and validated in the previous study.14 On the basis of this model, the droplet growth model is introduced to describe the particle growth due to vapor condensation.39 The modeling process is divided into two steps, as shown in Figure 4. In step 1, only air is considered. The governing equations for an incompressible fluid are the Navier-Stokes equations and the turbulent flow is modeled using the Reynolds stress model (RSM). In step 2, different sized particles are injected. The particle motions are traced using the Lagrangian particle tracking model (LPT). The details about the RSM and LPT models are introduced in Supporting Information (SI) S1. Note the particle growth due to vapor condensation is implemented by User-Defined-Function (UDF). The separation efficiency of each particle size is obtained as the ratio between the trapped and injected particles, and the separation efficiency curve is obtained.

8


Page 9 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Modeling process used in simulation

It should be noted that the numerical model in this work ignored the collisions between individual particles. This treatment has been widely accepted for dilute flows.40-41 For dense flows, the so-called combined approach of CFD and discrete element method (DEM) can be used, which is able to consider particle-particle and particle-fluid interactions.42-45 The computational effort of such a CFD-DEM approach is vast and deserves further studies.

3.2. Droplet Growth Model.

For a droplet in a humid environment with supersaturated vapor, according to mass conservation, its mass change rate should be equal to the mass flux flowing through the surface, given by:

= −

$

= −4 !" #

(3)

where is the mass flux, % is the mass of droplet, & is the time, ! is the density of liquid and " is the particle radius.

Based on previous studies, Kulmala established a complete condensational droplet growth model, considering the transition regime corrections and the Kelvin effect, as shown in Eq. (20):39

= −4 "

'( )'* +

,* -. /01 23(45 6.57

.57 , / 01 + * 01 40 9 :1; (57 )

A New Fine Particle Removal Technology--Cloud-Air-Purifying

Recommend Documents