High-Efficiency Particulate Air Filters Based on Carbon Nanotubes

CHAPTER

HIGH-EFFICIENCY PARTICULATE AIR FILTERS BASED ON CARBON NANOTUBES

26

Rufan Zhang, Fei Wei Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing, China

CHAPTER OUTLINE 1 Introduction ....................................................................................................................................643 2 Filtration Mechanism of Aerosol Particles ......................................................................................... 644 2.1 Classical Theories of Filtration Mechanism of Aerosol Particles .......................................... 644 2.2 In Situ Observation of the Capture of Aerosol Particles on Nanofibers ................................. 646 3 Performance Evaluation of Filters .....................................................................................................653 4 HEPA Filters Based on CNTs ............................................................................................................653 4.1 Free-Standing CNT Films ............................................................................................... 653 4.2 CNT Coating Filters ........................................................................................................ 654 4.3 Three-Dimensional CNT Sponge and Scaffold Filters ......................................................... 659 4.4 Hierarchical CNT Structure Filters ................................................................................... 659 4.5 Agglomerated CNT Fluidized Bed Filter ........................................................................... 661 5 Summary ........................................................................................................................................663 References ......................................................................................................................................... 663 Further Reading ..................................................................................................................................666

1 INTRODUCTION With the fast development of industries, aerosol particles caused haze pollution that has become a serious political, scientific, and public concern in many developing countries. This has a significant impact on living environments and public health [1–8]. Among the different types of haze pollution, PM2.5, defined as particulate matter (PM) with aerodynamic diameter below 2.5 μm, is one of the most serious threats to human health because they can penetrate the human bronchi and lungs and even enter alveolar cells due to its small size [9–15]. The direct filtration of PM2.5 by high-efficiency air filters is one of the most effective strategies to decrease the concentration of PM2.5 [8,16–23]. In addition, Nanotube Superfiber Materials. https://doi.org/10.1016/B978-0-12-812667-7.00026-4 Copyright # 2019 Elsevier Inc. All rights reserved.

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filtration is also currently extensively applied in many other industrial and domestic fields, such as in nuclear power, semiconductor manufacturing, airplane cabin air cleaning, pharmaceutical processing, and hospital environment [24]. When judging the performance of a filter, the filtration efficiency and pressure drop are the most important parameters [25]. A good filter is expected to have both a high filtration efficiency and a low pressure drop. However, these two parameters are usually in conflict against each other since the fibers that have high aerosol capture efficiency usually have high pressure drop. During the past several years, nanofibers have been widely used in air filtration due to the advantages that nanofibers have a much higher available specific surface area than microfibers and the diameter of nanofiber is comparable with the mean free path of the air molecules (66 nm under normal conditions) [8,17–19, 26], rendering the gas velocity being nonzero at the fiber surface because “slip” occurs [27,28]. The drag force caused by the nanofibers on the airflow is greatly reduced because of the “slip effect.” In comparison with nanofibers, carbon nanotubes (CNTs) exhibit more advantages to be used as high-efficiency filters due to their ultrathin diameters, ultrahigh specific surface areas, and excellent mechanical strengths. Particularly, air filtration based on CNTs can be conducted in the “free molecular flow” (FMF) regime, where the disturbance due to CNTs on the flow pattern is negligible [24,29]. Therefore, air filters based on CNTs can potentially overcome the inherent limitation in the trade-off between the filtration efficiency and pressure drop that traditional filters have. In this chapter, we give a brief introduction of filtration mechanism of aerosol particles, performance evaluation of filters, and the main progress of HEPA filters based on CNTs.

2 FILTRATION MECHANISM OF AEROSOL PARTICLES 2.1 CLASSICAL THEORIES OF FILTRATION MECHANISM OF AEROSOL PARTICLES According to the traditional theories of filtration [24,25], aerosol particles are captured from the gas stream by the fibers through direct interception, Brownian diffusion, inertia impact, gravity settling, or electrostatic deposition (Fig. 1A), which depend on gas velocity, particle sizes, fiber diameters, etc. Fig. 1B shows typical filter efficiencies for these mechanisms and the total efficiency. Because of the mass difference, the aerosol particles exhibit different motion from that of the gas when approaching the fibers. The particles are less subject to deviation from their course due to their momentum when the streamlines of the flow spread sideways past the fibers. Different mechanisms dominate for different particle-size ranges. For particles larger than 0.3 μm, the main interaction between them and the fibers are inertial impact and direct impact, while for small particles, especially for particles smaller than 0.1 μm, diffusion is usually the dominant mechanism [25,30]. All filters have a particle size that gives the minimum efficiency, which is called the most penetrating particle size (MPPS), which is typically around 0.3 μm or smaller. The filtration efficiency at the MPPS determines the classification of the air filter. In addition to these interactions, it was also found that the surface chemistry and electrostatic potential of fibers play key roles in the high-efficiency capture of aerosols [8]. For example, it was found that the polar chemical functional groups had strong binding affinity with aerosol particles [8].

2 FILTRATION MECHANISM OF AEROSOL PARTICLES

Electrostatic deposition

+ Fiber cross section

100 Collection efficiency%

Interception

Flow streamlines

Inertial impaction

Total filter

Settling Impaction Diffusion

50

Interception

MPPS

(B)

Surface area (m2/g)

10, 000

10–7

10–5

10–3

10–1

1.0

10.0

Particle diameter(mm)

101

Conventional textile fibers

1000

Conventional Textile textile microfibers fibers

100

Ultrafine textile fibers Electrospun nanofibers

10

Carbon nanotubes

Typical range of electrospun fibers

1 .1

0.1

0.01

Diffusion

(A)

645

Single and multiwalled carbon nanotubes

0.01 0.001

(C) Kn = O(10–3)

0.01

0.1

Free molecular Trasition flow flow

Diameter of human hair

1

10

100

0

(D)

Fiber diameter (mm) Kn = O(10–1)

13.2 10

Slip flow

528 0.25

Continuum flow

132 ´ 103 0.001

df (nm) Kn

Kn = O(10)

(E) FIG. 1 (A) Particle deposition on the filter structure. (B) Filter efficiency for individual single-fiber mechanisms and total efficiency. (C) Nanofibers have high surface areas. (D) Flow pattern around fibers of different diameters. (C) Kuwabara flow field about a fiber in the flow regimes with Kn ¼ 103, 101, and 10 from left to right. Part A: Reproduced with permission from Y. Chuanfang, Aerosol filtration application using fibrous media—an industrial perspective, Chin. J. Chem. Eng. 20 (2012) 1–9, Copyright 2012, Elsevier. Part B: Reproduced with permission from W.C. Hinds, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, John Wiley & Sons, 2012, Copyright 1999, John Wiley & Sons, Inc. Part C: Reproduced with permission from P. Gibson, H. Schreuder-Gibson, D. Rivin, Transport properties of porous membranes based on electrospun nanofibers, Coll. Surf. A: Physicochem. Eng. Asp. 187 (2001) 469–481, Copyright 2001, Elsevier. Part E: Reproduced with permission from B. Maze, H.V. Tafreshi, Q. Wang, B. Pourdeyhimi, A simulation of unsteady-state filtration via nanofiber media at reduced operating pressures, J. Aerosol Sci. 38 (2007) 550–571, Copyright 2007, Elsevier.

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Table 1 Relationship Between Knudsen Number, Fiber Diameter, and Flow Regime Knudsen Number (Kn)

The Diameter of Fiber (df)

Flow Regimes Around Fibers

Kn < 0.001 0.001 < Kn < 0.25 0.25 < Kn < 10 Kn > 10

df > 132 μm 528 nm < df < 132 μm 13.2 nm < df < 528 nm df < 13.2 nm

Continuum flow Slip flow Transition flow Free molecular flow

Note: The relationship between the fiber and Knudsen number is based on the mean free path of air molecular being 66 nm.

The flow field around isolated cylinders is an important aspect to understand the mechanisms by which the particles are collected on them. Researchers generally employ the Knudsen number, Kn, a dimensionless parameter to characterize the flow regime of the gas around the fibers: Kn ¼

2λ df

(1)

where λ is the mean free path of air molecules (about 66 nm under normal temperature and pressure [31]) and df is the diameter of the fiber. The flow regimes can be categorized into four different types: continuum flow regime, slip flow regime, transition flow regime, and FMF regime (Fig. 1C–E) [24]. Under normal conditions, fibers with different diameters have flow regimes as summarized in Table 1. For Kn < 0.001, the gas can be treated as a continuum fluid because the mean free path of the gas molecules is very small compared with the diameter of the fiber. The majority of air-filtration studies were conducted for flow in the continuum flow regime, where the factors that affect filter performance are well understood. For 0.001 < Kn < 0.25, the mean free path of gas molecules is comparable with the fiber diameter, the gas fluid will slip over the fiber surface, and this flow belongs to the slip flow regime where filtration theories were developed by modifying the continuum theories to include the slip effect. In the slip flow regime, the air-filtration theories predict an increase in filtration efficiency with an increase of the Knudsen number [32]. When the diameter of the fiber is much smaller than the mean free path of the gas molecules, the flow field around the fibers will be in the FMF regime. A fibrous filter consisting of electrospun fibers generally operates in the transition regime, and filters based on CNTs generally operate in the FMF regime under normal conditions. Thus, by using CNTs, especially single-walled carbon nanotubes (SWCNTs) that have diameters <10 nm, the air filtration will be in this regime under normal conditions. This is not possible with other fibers that can only get into this regime by increasing the mean free path of the gas molecules via reducing the pressure.

2.2 IN SITU OBSERVATION OF THE CAPTURE OF AEROSOL PARTICLES ON NANOFIBERS Although the classical theories can successfully interpret the filtration of aerosol particles, the essential mechanism and the dynamic capture and evolution processes of aerosols, especially at the nanoscale, are not yet fully understood. Additionally, according to the Knudsen number used for describing flow regimes, the gas around nanofibers with diameter of 100–300 nm is in the transition flow regime

2 FILTRATION MECHANISM OF AEROSOL PARTICLES

647

(Fig. 1C and D) [24,25]. The classical filtration theories are inadequate for describing filtration in the transition flow regime due to the inapplicability of the Navier-Stokes equation and the complexity of solving the Boltzmann equation [24]. Aerosols can be categorized into solid and liquid aerosols, for which the filtration mechanisms are also different. Zhang et al. conducted an in situ investigation on the nanoscale capture and evolution process of different aerosols on polyimide nanofibers [33]. Three types of aerosols were investigated: wetting liquid droplets, nonwetting liquid droplets, and solid particles. These aerosols showed distinct properties and capture behaviors. It was found that the wetting droplets had small contact angles on polyimide nanofibers and formed axisymmetric conformations on the nanofibers upon contact with them. In comparison, the nonwetting liquid droplets had large contact angles on polyimide nanofibers and only formed nonaxisymmetric conformations. The conformation difference between wetting and nonwetting droplets resulted from the difference between their surface tension [34]. In contrast, the solid aerosols did not have regular shapes and instead formed dendritic structures on the nanofibers. (1) The filtration of wetting liquid droplets Generally, three distinct cases of conformation can occur when a droplet is placed on a cylindrical fiber [35]: (i) film flow (Fig. 2A, but not very common, generally broken into distinct droplets due to the Plateau-Rayleigh instability to reduce the surface area and the surface energy) [36], (ii) a series of axisymmetric droplets (Fig. 2B, usually connected by a film with a thickness on the order of nm), and (iii) nonaxisymmetric droplets (Fig. 2C). When the contact angle between the droplets and the fiber is small, the conformation of the droplets is symmetrical with respect to the fiber axis. In contrast, when the contact angle is sufficiently large, a nonaxisymmetric conformation appears, which is more stable than the axisymmetric conformation. The volumes of oil droplets increased in two ways during the capture process. Firstly, small droplets from the source were continuously added to the existing larger ones on the nanofibers (Fig. 2D and E). Secondly, adjacent pairs of droplets often coalesced to form new ones with larger volumes (Fig. 2F and G). The diameter distribution of the droplets captured by the nanofibers was a typical Gaussian distribution (Fig. 2H). With more droplets captured and coalesced, the diameter distribution broadened, and the mean diameter increased. Besides, the strong capillary force of oil droplets between two adjacent nanofibers also caused the fibers to firmly adhere to each other (Fig. 2I and J). Moreover, for the nanofiber networks with high packing densities and the nanofiber separations on the same order of magnitude with the droplet sizes, the captured droplets were in the form of small pools covering nanofibers or in the form of a combination of both droplets and pools/ bridges between nanofibers after a long capture time (Fig. 2K). Contal et al. have studied the clogging phenomena of HEPA filters by liquid aerosol particles [37, 38]. They divided the dynamic filtration process of liquid aerosols into four stages. In the first stage, liquid aerosols are deposited on the fiber surface, and they surround the fiber. Frising et al. viewed the coverage of liquid aerosols to be uniform and that a “liquid tube” was formed around the fiber by coalescence [38]. The Plateau-Rayleigh instability caused an additional obstruction to the airflow while reducing the fiber collection area. As a result, the pressure drop increases slowly, and the penetration increases rapidly (particularly for submicrometer aerosols). In the second stage, “liquid bridges” between fibers and “liquid films” at fiber intersections appear with the accumulation of liquid aerosols. The specific area and collection area of the filter will be significantly decreased, resulting in an exponential increase in penetration and a further increase of pressure drop. The third stage is characterized by the formation of a large

FIG. 2 (A–C) Three distinct cases of conformation when a droplet with definite volume is placed on a cylindrical fiber with definite radius. (D) Real-time imaging of the capture, mobility, coalescence, and growth of oil droplets on polyimide nanofibers. (E and F) Schematic illustration showing the process of new droplets merging into the preexisting larger droplets on nanofibers. (G) The evolution of the diameter distribution of oil droplets. (H) Optical image of a filter with high packing density after a long time of wetting liquid droplet capture. (A–K) Reproduced with permission from R. Zhang, et al., In situ investigation on the nanoscale capture and evolution of aerosols on nanofibers, Nano Lett. 18 (2018) 1130–1138, Copyright 2018, American Chemical Society.

2 FILTRATION MECHANISM OF AEROSOL PARTICLES

649

liquid film on the surface of the filter. The airflow path will be clogged; thus, the pressure drop will increase exponentially. At the same time, the reduced space for airflow will accelerate the gas interstitial velocity, which will increase the impact mechanism for aerosol collection, resulting in an increase of the penetration of the filter. In the fourth stage, all the layers are saturated by the collected liquid. The collection and drainage of liquids then reach an equilibrium state. The pressure drop and penetration of the filter remain constant. (2) The filtration of nonwetting liquid droplets Zhang et al. selected water as an example to study the capture process of nonwetting droplets on polyimide nanofibers [33], since the polyimide nanofiber is a typical hydrophobic material. Fig. 3A and B shows an illustration and real-time images of how water droplets were captured by the polyimide nanofibers. The water droplets had a large contact angle on polyimide nanofibers. In contrary to the axisymmetric structure of oil droplets on polyimide nanofibers, the water droplets had only a small portion of their surfaces attached to the polyimide nanofibers and formed a nonaxisymmetric conformation. Meanwhile, the adjacent water droplets coalesced to form larger ones to minimize their surface energy (Fig. 3C–F). The coalescence of water droplets manifested differently from that of oil droplets. Adjacent but separated water droplets did not move toward each other, but instead grew large until they contacted and coalesced, suggesting that there was not a liquid film on the fiber between adjacent droplets because thin liquid films tend to be unstable when the liquid–vapor surface tension is high. The diameter distribution of water droplets was also a typical Gaussian distribution (Fig. 4A). After stopping the water vapor supply, most of the water droplets quickly evaporated into the air, and finally, almost nothing was left on the nanofibers (Fig. 4B and C). In addition, the water droplets also caused the adhesion of adjacent nanofibers but only within a much smaller region and with a different morphology from that caused by oil droplets. (3) The filtration of solid particles The capture of solid particles on nanofibers exhibited much difference from that of liquid droplets. Once the dust particles got into contact with the nanofibers, they immediately attached to and accumulated on the nanofibers (Fig. 5A). The dust particles were mainly captured by the nanofibers via van der Waals interaction. With continuous feeding of the dust flow, more and more solid particles were captured. The newly captured particles were attached to both the uncovered pairs of the nanofibers and the existing attached particles. Finally, numerous dendrite-like structures were formed with the continuous capture of dust particles. Unlike the liquid droplets, the solid particles could not move along the nanofibers and thus could not coalesce with each other. The morphology of the dust particles was stochastic and without a regular shape (Fig. 5B and C), which was different from that of liquid droplets. In addition, their size distribution was much broader and highly random (Fig. 5D). The adhesion of the dust particles with the nanofibers was weak, and some of the captured particles could even be blown away by the drag forces of a strong gas flow. The difference between the liquid and solid aerosols is related to the fundamental difference between liquids and solids, that is, the ability to flow. The liquid droplets can move and coalesce along the nanofibers. In contrast, the solid particles did not have regular shapes, and their size distribution was very broad. They created intrusive dendritic structures. In addition, Thomas et al. have studied the clogging behavior of HEPA filters by ultrafine particles during filtration [39]. The pressure drop across the filter increased with the deposited mass of aerosols.

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CHAPTER 26 HIGH-FFICIENCY PARTICULATE AIR FILTERS

FIG. 3 (A) Schematic illustration of water droplet capture on polyimide nanofibers. (B) Snapshots of real-time water droplet capture (i–iv) and evaporation (v–viii) process on polyimide nanofibers. (C) Schematic illustration and (D–F) real-time images of the coalescence of adjacent water droplets. (A)–(F) Reproduced with permission from R. Zhang, et al., In situ investigation on the nanoscale capture and evolution of aerosols on nanofibers, Nano Lett. 18 (2018) 1130–1138, Copyright 2018, American Chemical Society.

With vapor feedstock 400

Without vapor feedstock 400

20.8 s

200

24 s

0 400 200

0 400

0 400

0 400

11 s

Number

14 s

200

200 33.8 s

200 0 400

39.4 s

100 Number

200

300

30.8 s

0

200

200

400

0 400

0 400

300

5s

200

200

0 400

0 400

200 0

(A)

20 10 0 0 1 2 3 4 5

0.5 s

0

(B)

24 s 30.8 s 33.8 s 39.04 s 42.78 s 44 s

200 12 8 4 0 0

200

0 2 4 6 8 10 12 14 16 18 20 22 Diameter (mm)

42.8 s

Without vapor feedstock

0 2 4 6

44 s 4

100

8

0 0 2 4 6

8 10 12 14 16 18 20 22 Diameter (mm)

(C)

8 10 12 14 16 18 20 22 Diameter (mm)

FIG. 4 (A) Number and diameter distribution variation during the capture of water droplets with water vapor feeding. (B) Number and diameter distribution variation during the evaporation of water droplets without water vapor feeding. (C) Comparison of number and diameter distribution of water droplets with and without water vapor feeding. (A–C) Reproduced with permission from R. Zhang, et al., In situ investigation on the nanoscale capture and evolution of aerosols on nanofibers, Nano Lett. 18 (2018), 1130–1138, Copyright 2018, American Chemical Society.

2 FILTRATION MECHANISM OF AEROSOL PARTICLES

18 s

0.5 s 5s 11 s 14 s 18 s 20.8 s

400

200

0 400

Number

With vapor feedstock 400

651

652

CHAPTER 26 HIGH-FFICIENCY PARTICULATE AIR FILTERS

FIG. 5 The capture, morphology, and size distribution of dust particles. (A) Snapshots of the real-time capture of dust particles on polyimide nanofibers. (B) and (C) SEM images of captured dust particles. (D) Size distribution of dust particles on nanofibers. (A–D) Reproduced with permission from R. Zhang, et al., In situ investigation on the nanoscale capture and evolution of aerosols on nanofibers, Nano Lett. 18 (2018) 1130–1138, Copyright 2018, American Chemical Society.

The dynamic filtration process can be divided into three stages: depth filtration, transition area, and cake filtration. In the early stage of filtration, aerosol particles are deposited on the surface of individual filter fibers in the filter bed, and since their deposited amount is limited, they do not obviously affect the gas flow. As loading continues, aerosol particles are accumulated on the fiber surface and form dendritic structures. These dendritic structures grow relatively unhindered at first, but gradually, they will intercept each other’s growth paths and join together to form a cake on the front edge of the filter. At the point of the formation of a complete cake, the pressure drop across the filter will increase linearly with the mass of deposited aerosol particles. Then, the filtration of aerosol particles by fibrous filters will change from “depth filtration” to “surface filtration” because sieving now becomes the dominant filtration mechanism.

4 HEPA FILTERS BASED ON CNTs

653

3 PERFORMANCE EVALUATION OF FILTERS As mentioned above, the filtration efficiency and pressure drop are the most important parameters of filters. The criterion for judging a filter is the quality factor (Qf) [25], which was defined as Qf ¼

 ln N=N0  ln P lnE ¼ ¼ Δp Δp Δp

(2)

where N and N0 represent the concentration of aerosol particles upstream and downstream of the filter, respectively; P is the penetration, E is filtration efficiency, and Δp is the pressure drop of the air filter. This criterion contains the two most important aspects: efficiency and pressure drop. With the development of the air-filtration field, the dynamic filtration performance of an air filter received more attention.

4 HEPA FILTERS BASED ON CNTs CNTs have been applied in many fields since their discovery due to their superior properties, such as high electric conductivity, mechanical stability, and thermal conductivity. For example, their Young’s modulus is higher than 1 TPa [40–42], and tensile strength is higher than 100 GPa [43,44]. The weightspecific strength of CNTs is at least 400 times higher than that of steel. CNTs also exhibit remarkable electric [45] and thermal properties [46] and have exhibited great potential for use in numerous applications, such as actuators, nanoelectronics, ultrafast photonics, sensors, filters, and superstrong fibers [47,48]. CNTs show many advantages in fabricating HEPA filters due to their ultrathin diameters, high aspect ratio, high specific surface area, and high mechanical strength. Because of the nanoscale diameters of CNTs, the airflow around the CNTs is in the FMF regime during air filtration, which greatly reduces the pressure drop. Thus, by using CNTs, especially SWCNTs that have diameters <10 nm, the air filtration will be in this regime under normal conditions, which is not possible with other fibers that can only get into this regime by increasing the mean free path of the gas molecules via reducing the pressure. Research work on air filtration in the FMF regime is very scarce [29,49]. When the slip of the gas flow is large, there is less influence of the fibers on the flow field, and the gas streamlines are closer to the fiber surface. Based on this concept, Maze et al. further hypothesized that the disturbances to the airflow field caused by the fibers are negligible in the FMF regime, that is, the streamlines will have a negligible deviation from a straight line when approaching a fiber [29]. During the past years, numerous efforts have been taken in the fabrication of HEPA filters based on CNTs. Much attention has been paid on the following aspects of nanofibrous filters: thickness, fiber diameter, solid volume fraction of filters, and pore structure [50]. The structure design of CNT-based HEPA filters can be classified into freestanding CNT films, CNT coating filters, CNT sponges, hierarchical CNT structures, and agglomerated CNT fluidized bed filters.

4.1 FREE-STANDING CNT FILMS Aligned CNT sheets drawn from vertically aligned CNT arrays are a good candidate for fabricating freestanding CNT films. Yildiz et al. integrated aligned CNT sheets between microfiber polypropylene

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CHAPTER 26 HIGH-FFICIENCY PARTICULATE AIR FILTERS

nonwoven fabrics to produce filters with HEPA level efficiency, containing multiple CNT sheets that were stacked either parallel to the others or in a cross ply configuration (Fig. 6) [51]. To meet highefficiency particulate air (HEPA) filter requirements with a reasonable pressure drop, CNTs were laid in a cross plied structure within the filter. It was found that when the number of CNT layers increased, the filtration efficiency increased dramatically while the pressure drop also increased. The results demonstrated that the three-layer cross ply structure provided 99.98% filtration efficiency at 0.3 μm particle size at a 10 cm/s face velocity, making it a viable method for producing low basis weight HEPA filters utilizing CNTs as the main filtration component. The results show that these novel CNT filters have filtration properties that are comparable with electrospun fabrics, making them a viable option for future filtration applications. In addition to the aerosol removal, freestanding CNT films were also used in viral removal [52].

4.2 CNT COATING FILTERS Coating or directly growing CNTs on other mesh-type substrates is a common approach to fabricate CNT-based filters. Viswanathan et al. reported the fabrication of filters with efficiency >99% by coating multiwalled CNTs (MWCNTs) onto cellulose fiber filters (Fig. 7A–D) [53]. Porosity and distribution of pore sizes are important characteristics that affect filter performance. The random fiber morphology of CNT mats is ideal for use as fine particulate filters. The MWCNT-coated filters exhibited low pressure drop and better filter quality than cellulose filters even for very low MWCNT coverages (0.07 mg/cm [2]), which was comparable with the highest efficiency HEPA filter standards. Park et al. fabricated microstructures composed of CNTs on a conventional micron-sized fibrous metal filter by the direct growth of CNTs (Fig. 7E–J) [54]. The CNTs were synthesized on the metal filter by thermal chemical vapor deposition from acetylene gas and formed the microstructures of various morphologies such as bushes around the micron fibers or webs across the fibers. The filters on which the CNTs were grown directly exhibited higher filtration efficiency without significant increase in pressure drop compared with the raw metal filter. The filter quality was improved as the CNTs were grown like naps or weeds standing out beyond the surfaces of micron fibers. Li et al. fabricated depth-type hierarchical CNT/quartz-fiber (QF) filters through in situ growth of CNTs upon QF filters using a floating catalyst chemical vapor deposition method (Fig. 7K–Q) [16]. The filter specific area of the CNT/QF filters was >12 times higher than that of the pristine QF filters. Consequently, the penetration of submicron aerosols for CNT/QF filters was reduced by two orders of magnitude, which reaches the standard of HEPA filters. Meanwhile, the pore size of the hybrid filters only has a small increment due to the fluffy brushlike hierarchical structure of CNTs on QFs. The pressure drop across the CNT/QF filters only increased about 50% with respect to that of the pristine QF filters, leading to an obvious increased quality factor of the CNT/QF filters. It was found that CNTs were very efficient in capturing submicron aerosols. Moreover, the CNT/QF filters showed high water repellency, implying their superiority for applications in humid conditions. In addition to the aerosol removal, the CNT/OF filters also exhibited good ozone removal performance. Besides, Yang et al. reported the ozone conversion efficiency of the CNT/QF film for 10 h and compared with that of quartz film, activated carbon (AC), and a potassium iodide (KI) solution under the same conditions [55]. The results showed that the CNT/QF film had not only better ozone conversion efficiency but also higher pressure resistance than AC and the KI solution of the same weight. The ozone removal performance of the CNT/quartz-fiber film was comparable with AC at 20 times more

(A)

(B)

100 mm

a

b

(C)

1 cm

(D) 1.E+02

1 2 layer 3 layer 2 layer cross-ply

0.1

0.01 0.01

(E)

3 layer cross-ply

Particle penetration (%)

Particle penetration (%)

10

1.E+01 1.E+00 Control

1.E–01

1 layer 1.E–02

2 layer 3 layer

1.E–03

7 layer 2 layer cross-ply

1.E–04

3 layer cross-ply

1.E–05

0.1 Particle size (mm)

1

10

(F)

100

1000

Pressure drop (Pa)

FIG. 6 (A) Preparing filters by winding CNT sheets onto the polypropylene fabric. (B) Placing CNT sheet—polypropylene fabric layered structure onto a calendaring plate with hole in the middle. (C) SEM and (D) photographic images of the three-layer CNT filter structure with the cross ply geometry. (E) The filtration performance of filters consisting of two-layer, three-layer, two-layer cross ply, and three-layer cross ply CNT sheet orientations at a face velocity of 10 cm/s. (F) Particle penetration fraction of the different structures as a function of pressure drop at 0.3 μm particle size and 10 cm/s face velocity. (A–F) Reprinted with permission from O. Yildiz, P.D. Bradford, Aligned carbon nanotube sheet high efficiency particulate air filters, Carbon 64 (2013) 295–304, Copyright 2013, Elsevier.

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CHAPTER 26 HIGH-FFICIENCY PARTICULATE AIR FILTERS

FIG. 7 See figure legend on opposite page. (Continued)

4 HEPA FILTERS BASED ON CNTs

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weight. The CNTs played a dominant role in ozone removal by the CNT/quartz-fiber film. Park et al. grew MWCNTs on a glass fiber air filter using thermal chemical vapor deposition [56]. It was showed that the CNT deposition increased the filtration efficiency of nano- and submicron-sized particles, but did not increase the pressure drop across the filter. When a pristine glass fiber filter had no CNTs, the particle filtration efficiencies at particle sizes under 30 nm and near 500 nm were 48.5% and 46.8%, respectively. However, the efficiencies increased to 64.3% and 60.2%, respectively, when the CNT-deposited filter was used. The reduction in the number of viable cells was determined by counting the colony-forming units of each test filter after contact with the cells. The pristine glass fiber filter was used as a control, and 83.7% of the Escherichia coli were inactivated on the CNT-deposited filter. In addition, Parham et al. synthesized CNTs in the open pores of a commercial porous ceramic matrix consisting of mainly Al2O3 and SiO2 with an average pore size of 300 and 500 μm (Fig. 8A–E) [57]. The as-obtained composites containing 3 wt% of CNTs were used as a filter for the removal of yeast cells and different heavy metal ions from water and for the removal of particulates from air. The results showed that the CNTs containing composite filter demonstrated a high efficiency of yeast filtration (98%), c.100% heavy metal ion removal from water, and excellent particulate filtration from air. The composite filter also exhibited good reusability for these applications, owing to the excellent thermal and chemical stability of CNTs. Zhao et al. reported a chemical vapor deposition method to create CNT/ceramic composite filter by growing MWCNTs on a porous alumina ceramic membrane (Fig. 8F–I) [58]. Compared with the pristine alumina ceramic membrane, the resulting composite filter showed significant improvements in air-filtration performance, owing to the dramatic increase of specific area by two orders of magnitude and enhancement of wall slip flow effect over CNTs. The pressure drop across the composite filters decreased about 62.9% with respect to that of the pristine filters, while the filtration efficiency of the composite filters at the MPPS has been increased to 99.9999%, leading to an obvious higher quality factor. The presence of CNTs strongly inhibits the propagation of bacteria on the filters with an antibacterial rate of 97.86% and shows high water repellency (water contact angle of 148.2°). These results make the composite filter very promising for multifunctional air-filtration applications. Storti et al. also reported a similar fabrication of CNT-coated Al2O3-C filters for steel melt filtration [59]. Jung et al. prepared Ag-coated CNT hybrid nanoparticles (Ag/CNTs) using aerosol nebulization and thermal evaporation/condensation processes and tested their usefulness

FIG. 7, CONT’D (A–D) SEM images of MWCNT-coated filters. (E–G) SEM images of CNTs grown directly onto micron fibers along the different flow rates of hydrogen gas such as the synthesis conditions of four CNT filters. (I) Pressure drop versus filtration face velocities of the filters on which the CNTs are grown at different synthesis conditions. (J) Filtration efficiencies of the filters on which the CNTs are grown at different synthesis conditions. (K and L) SEM images of the pristine QF filters and the CNT/QF filter, respectively. (M and N) SEM images showing the cross section of the QF filter and CNT/QF filter, respectively. (O) Pressure drop versus face velocity of the QF filter and the CNT/QF filter. (P) Penetration of particles with different sizes in the QF filter and the CNT/QF filter. (Q) Quality factor versus particle size of the QF filter and the CNT/QF filter. (A–D) Reprinted with permission from G. Viswanathan, D.B. Kane, P.J. Lipowicz, High efficiency fine particulate filtration using carbon nanotube coatings, Adv. Matter. 16 (2004) 2045–2049, Copyright 2004, VCH Wiley. (E–J) Reprinted with permission from S.J. Park, D.G. Lee, Performance improvement of micron-sized fibrous metal filters by direct growth of carbon nanotubes, Carbon 44 (2006) 1930–1935, Copyright 2006, Elsevier. (K–Q) Reprinted with permission from P. Li, et al., In situ fabrication of depth-type hierarchical CNT/quartz fiber filters for high efficiency filtration of sub-micron aerosols and high water repellency, Nanoscale 5 (2013) 3367–3372, Copyright 2013, Royal Society of Chemistry.

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(A) Low-magnification SEM images of a porous ceramic matrix before CNT growth, (B) low-magnification SEM image of ceramic/CNT composite after CNT growth, (C) 3-D image of a ceramic substrate following a μ-CT scan showing the interconnectivity of pores, (D) SEM image exhibiting the high quality and quantity of CNTs grown on the ceramic disk, and (E) TEM image of CNTs collected from the filter. The hollow structural feature of the nanotubes can be clearly seen. (F–I) SEM images of the filters after gas filtration. (F and G) were SiO2 particles accumulated on the surface and clogged the pore of the pristine filters, respectively, while (H and I) were SiO2 particles accumulated on the surface of the composite filter and the MWCNTs grown around the pore of the composite filter with the captured SiO2 particles, respectively. (J–M) SEM images of (J) the pristine, (K) CNT-deposited, (L) Ag-nanoparticle-deposited, and (M) Ag/CNT-deposited filters. (A–E) Reprinted with permission from H. Parham, S. Bates, Y. Xia, Y. Zhu, A highly efficient and versatile carbon nanotube/ceramic composite filter, Carbon 54 (2013) 215–223., Copyright 2013, Elsevier. (F–I) Reprinted with permission from Y. Zhao, Z. Zhong, Z.-X. Low, Z. Yao, A multifunctional multi-walled carbon nanotubes/ceramic membrane composite filter for air purification, RSC Adv. 5 (2015) 91951–91959, Copyright 2015, Royal Society of Chemistry. (J–M) Reprinted with permission from J.H. Jung, G.B. Hwang, J.E. Lee, G.N. Bae, Preparation of airborne Ag/CNT hybrid nanoparticles using an aerosol process and their application to antimicrobial air filtration, Langmuir 27 (2011) 10256–10264, Copyright 2011, American Chemical Society.

CHAPTER 26 HIGH-FFICIENCY PARTICULATE AIR FILTERS

FIG. 8

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for antimicrobial air filtration (Fig. 8J–M) [60]. The CNT and Ag nanoparticle aerosols were mixed together and attached to each other, forming Ag/CNTs. Because of the attachment of Ag nanoparticles onto the CNT surfaces, the total particle surface area concentration measured by a nanoparticle surface area monitor was lower than the summation of each Ag nanoparticle and CNT generated. When Ag/ CNTs were deposited on the surface of an air filter medium, the antimicrobial activity against test bacterial bioaerosols was enhanced, compared with the deposition of CNTs or Ag nanoparticles alone, whereas the filter pressure drop and bioaerosol filtration efficiency were similar to those of CNT deposition only. These Ag/CNT hybrid nanoparticles are useful for applications in biomedical devices and antibacterial control systems.

4.3 THREE-DIMENSIONAL CNT SPONGE AND SCAFFOLD FILTERS Vertically aligned CNT arrays or agglomerated CNT sponges have ultrahigh specific surface areas and great potentials in fabricating HEPA filters. Srivastava et al. reported the fabrication of freestanding monolithic uniform macroscopic hollow cylinders having radially aligned CNT walls, with diameters and lengths up to several centimeters (Fig. 9A–C) [61]. These cylindrical membranes were used as filters for the elimination of multiple components of heavy hydrocarbons from petroleum and the filtration of bacterial contaminants such as E. coli or the nanometer-sized poliovirus ( 25 nm) from water. These macrofilters could be cleaned for repeated filtration through ultrasonication and autoclaving. The exceptional thermal and mechanical stability of CNTs and the high surface area, ease, and costeffective fabrication of the CNT membranes may allow them to compete with ceramic- and polymerbased separation membranes used commercially. Halonen et al. fabricated three-dimensional CNT scaffolds using micromachined Si/SiO2 templates as nanoparticulate filters and support membranes for gas-phase heterogeneous catalysis (Fig. 9D–I) [62]. The filtering efficiency of better than 99% was shown for the scaffolds in filtering submicrometer particles from air.

4.4 HIERARCHICAL CNT STRUCTURE FILTERS Hierarchical and gradient nanostructures are important to exploit the full potential of nanofibers in filtration applications. The introduction of a gradient into CNT/fiber hierarchical structures could result in a change of the particle capturing properties. Li et al. fabricated hierarchical CNT/quartz-fiber (QF) filters with gradient nanostructures where the content of CNTs decreases exponentially along the thickness direction of the filters (Fig. 10) [63]. The loading of catalysts for the growth of CNTs in the QF filter was achieved using an aerosol technique, which could be carried out on a large scale. With only 1.17 wt% CNT, the penetration of the CNT/QF filter at the MPPS was reduced by one order of magnitude, while the pressure drop only increased about 6% with respect to that of the pristine QF filter, leading to an obvious higher quality factor for the CNT/QF filter. More importantly, the lifetime of the CNT/QF filter with the CNT-rich side downstream has increased by 64% when compared with the pristine QF filter. In contrast, when the CNT-rich side was placed upstream, the lifetime of CNT/ QF filter was only 41.7% of that observed when placing the CNT-rich side downstream. It was found that the gradient nanostructure of the CNT/QF filter, together with the CNT/QF hierarchical structure, played very important roles in the simultaneous enhancement of the filtration efficiency and the service life of the air filters.

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FIG. 9 (A) Photograph of the bulk tube. (B) SEM image of the aligned CNTs with radial symmetry resulting in hollow cylindrical structure (scale 1 mm). (C) Schematics of the petroleum dynamics through the bulk tubes. (D–G) SEM images of various CNT membranes grown on microstructured and oxidized Si chips: synthesis times of (A) 0, (B) 20, (C) 30, and (D) 40 min were applied after the precursor prefeeding step. (H) Pressure drop versus flow rate of nanotube membranes. (I) Concentration of the particulates with a diameter of 0.3–2 μm in office air before and after filtering through the CNT membrane grown for 40 min and a sheet of commercial household HEPA filter of  200 μm thickness. (A–C) Reprinted with permission from A. Srivastava, O. Srivastava, S. Talapatra, R. Vajtai, P. Ajayan, Carbon nanotube filters, Nat. Mater. 3 (2004) 610–614, Copyright 2004, Nature Publishing Group. (D–I) Reprinted with permission from N. Halonen, et al., Threedimensional carbon nanotube scaffolds as particulate filters and catalyst support membranes, ACS Nano 4 (2010) 2003–2008, Copyright 2010, American Chemical Society.

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FIG. 10 An illustration of aerosols accumulating in the CNT/QF filter with different placement position and the QF filter. Reprinted with permission from P. Li, et al., Hierarchical carbon-nanotube/quartz-fiber films with gradient nanostructures for high efficiency and long service life air filters, RSC Adv. 4 (2014) 54115–54121, Copyright 2014, Royal Society of Chemistry.

4.5 AGGLOMERATED CNT FLUIDIZED BED FILTER Fluidization technology has been widely applied in chemical industries and is also a promising method in air filtration due to its high surface area and high mass-transfer efficiency. When the gravity of particles is balanced by the drag force of the fluidizing gas, the particles will suspend in a fluidization state, leading to the expanding of the fluidized bed. Compared with the confined structure of fibrous filters, the expanding bed results in the increase of space between fluidizing media, providing a larger capacity for holding dust. Because of the movement of the fluidizing media, the collected aerosol particles do not tend to form dendrites or dust cakes, avoiding the clogging problem of fibrous filters. Moreover, the pressure drop of the fluidized bed is theoretically in proportion to the weight in unit cross-sectional area, which can be optimized by adjusting the applicable density of fluidizing media and static bed height. On the other hand, the fluidizing media can be regenerated online through a circulating fluidized bed, which facilitates the practical application of fluidized bed filters. Wang et al. prepared a fluidized bed filter with agglomerated CNTs as the fluidizing media and investigated its air-filtration performance (Fig. 11) [64]. It was found that agglomerates of CNTs with diameters of 150–200 μm had fluidization characteristics similar to Geldart A particles. The penetration of aerosol particles for the fluidized bed filter with the above agglomerates as media and with a static bed height of 17.4 cm is 4  105 and 1.2  104 for 100 and 300 nm aerosol particles, respectively. Furthermore, the fluidization state of the filter media had a significant influence on the filtration performance. With the increase of gas velocity, the fluidized bed changed from agglomerate particulate fluidization (APF) to agglomerate bubbling fluidization (ABF), and its filtration efficiency decreased obviously. The filtration performance of fluidized bed filter was also compared with that of packed-bed filter. The fluidized bed filter had a slightly lower filtration efficiency and an obvious lower pressure drop than the

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FIG. 11 (A) Schematic diagram of the filtration performance test of the fluidized bed filter. (B) Schematic diagram of the filtration performance test of the packed-bed filter. (The porous plate on the top keeps stationary when the fluidizing gas goes through the bed.) (C) Filtration efficiency of particles with different sizes of fluidized bed filters with CNT agglomerates of 150–200 μm in APF and ABF and agglomerates of 355–600 μm in ABF. The inset is filtration efficiency with large-scale y-axis for fluidized bed with agglomerates of 150–200 μm in APF. (D) The fluidization curve of the fluidized bed with agglomerates of 150–200 μm as bed materials and a static bed height of 17.4 cm. (A–D) Reprinted with permission from C. Wang, et al., A high efficiency particulate air filter based on agglomerated carbon nanotube fluidized bed, Carbon 79 (2014) 424–431, Copyright 2013, Elsevier.

packed-bed filter, leading to a much higher quality factor, indicating that fluidized bed filters have superior advantages compared with packed-bed ones. The excellent filtration performance of the fluidized bed filter with agglomerated CNTs as bed materials indicates that it is a promising kind of HEPA filter.

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5 SUMMARY During the years, much progress has been made on the fabrication of HEPA filters based on CNTs. Various CNT-based air filters have been fabricated. Because of the extremely high specific surface areas of CNTs, high filtration efficiencies are easy to obtain. Meanwhile, it is also easy to get a low pressure drop via tuning the structures and thickness of CNT films. Freestanding CNT films and three-dimensional CNT sponges or arrays are simple ways to fabricate CNT-based HEPA filters. The main concern of this kind of filters lies in their mechanical stability and strength. In comparison, the CNT coating is a more effective way to fabricate HEPA filter with high filtration efficiency, low pressure drop, high mechanical strength, and long lifetime. Besides, a hierarchal structure and gradient nanostructure filtration provide new strategies for fabricating air filters with high filtration efficiency, low pressure drop, and long service life in the meanwhile. In addition, an agglomerated CNT fluidized bed provides another way to fabricate CNT-based filters. Air filtration involves different disciplines such as materials science, aerosol science, physics, environmental science, and engineering. To fully explore the advantages of air filtration in the FMF regime and CNTs in air filtration, it is necessary to conduct multidisciplinary research. A deeper understanding of the airflow pattern around a single nanofiber and the influence of nearby fibers on the flow pattern in the FMF regime is greatly required. It is also important to develop new models to describe the filtration efficiency and pressure drop across the filter in the FMF regime and to provide guidelines for the structure design of air filters. More work should be devoted to the research of the dynamic filtration process for more accurate knowledge of the evolution of filtration performance under continuous aerosol particle loading. The structure of CNT-based air filters can be further optimized using theoretical studies. Since CNTs have outstanding physical properties such as low electric resistivity, high thermal stability, and conductivity, multifunctional CNT-based air filters can be developed to meet the requirements of specific circumstances.

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FURTHER READING [65] W.C. Hinds, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, John Wiley & Sons, 2012. [66] P. Gibson, H. Schreuder-Gibson, D. Rivin, Transport properties of porous membranes based on electrospun nanofibers, Coll. Surf. A: Physicochem. Eng. Asp. 187 (2001) 469–481.