Carbon Nanotube Immobilized Composite Hollow Fiber Membranes

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Carbon Nanotube Immobilized Composite Hollow Fiber Membranes for Extraction of Volatile Organics from Air Smruti Ragunath, and Somenath Mitra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01213 • Publication Date (Web): 18 May 2015 Downloaded from http://pubs.acs.org on May 20, 2015

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The Journal of Physical Chemistry

Carbon Nanotube Immobilized Composite Hollow Fiber Membranes for Extraction of Volatile Organics from Air Smruti Ragunath and Somenath Mitra*

Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, USA. Fax: 973-596-3586, *email: [email protected].

ACS Paragon Plus Environment

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Abstract

This paper reports the development of novel carbon nanotube immobilized composite membranes (CNIM) for the extraction of volatile organics from air. The nanotubes were immobilized onto the selective layer of a composite membrane where they served as sorption sites and provided additional pathways for enhanced solute transport. Depending upon the process conditions, the presence of CNTs led to the organic removal with flux as high as 37.7E-05, 72.9E-05 and 8.22E-05 gm-mol/m2.min and an increase in mass transfer coefficient about 92.2%, 22.7% and 44.3% for toluene, dichloromethane and ethanol, respectively. The CNIM demonstrated several advantages including enhanced recovery at low concentrations and low temperature.

Highlights:

1. Development of novel carbon nanotube immobilized membrane for removal of VOCs from air.

2. The presence of CNTs led to an 80% enhancement in VOCs extraction efficiency and over 92% increase in mass transfer coefficient for toluene.

3. The CNTs serve as adsorption-desorption sites for rapid VOCs transport


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1. Introduction Volatile organic compounds (VOCs) have numerous industrial applications and have been a source of air pollution for decades (1, 2). Conventional, control technologies for the VOCs include thermal incineration, catalytic combustion, photo catalysis, adsorption and air stripping. Each technology has its own merits and limitations, and the applicability is situation-dependent on factors such as background matrix as well as concentration. Some of these processes are energy intensive, expensive for dilute streams and may also lead to the formation of secondary pollutants (3-5). Some are multi step processes, for example adsorption not only requires expensive sorbents but also regeneration (6-10). Membrane separation can be an effective VOCs control alternative where the organics are not exposed to high temperatures, there is no requirement of additional chemicals, the process can have small instrument footprint, and the compounds can be recovered (11-16). Other advantages of membrane methods are low energy requirements, high selectivity and the ability to handle high levels of moisture. These methods can be cost effective with the development of novel membranes that provide higher performance in terms of flux and selectivity. Membrane separation has undergone rapid developments in recent years with diverse applications in air and water treatment such as desalination, dialysis, ultrafiltration, gas separation, dehumidification, electro dialysis, and pervaporation(17, 18). Membrane separation can provide high selectivity and enrichment factors which can be used for capturing VOCs from dilute air stream (19-21). Various porous, non-porous as well as composite polymeric membranes made from polymers such as polyimide, polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polydimethylsiloxane have been used for separating VOCs from air as well as water(22). Recent efforts for enhancing selectivity and permeability have led to the development of thin film composite membranes(23, 24) and mixed matrix membranes consisting of interpenetrating polymeric


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materials with solid fillers(25, 26). The fillers often comprise of nanomaterials that can enhance membrane performance. The unique sorbent properties of CNTs have been utilized in different membrane separations where they offer several alternative mechanisms for solute transport(27, 28). Theoretical studies have shown that the permeation rate of certain liquids and gases through CNTs surpass that expected from classical diffusion models (29, 30) which has been attributed to the smooth CNT surface, frictionless rapid transport, molecular ordering (31) and increase in diffusivity(32, 33). Recently, we have reported the development of novel polymeric membranes by immobilizing CNTs on the membrane surface (34-36) , referred to as carbon nanotube immobilized membrane (CNIM), where the CNTs serve as a nano-sorbents or mediator for solute transport (37-40). The objective of this research is to study the extraction of VOCs from air streams using CNIM. This would have applications in air purification as well as concentrating the VOCs for environmental monitoring. 2. Experimental Section 2.1. Chemicals, materials and membrane modules Analytical grade toluene, dichloromethane, ethanol and acetone were used in the experiments and were obtained from Sigma Chemicals (St. Louis, MO). High purity N2 from Air Gas (Piscataway, NJ) and deionized water prepared using water purification system purchased from Barnstead Laboratory Apparatus (Model no 5023 Dubuque, Iowa). The raw multiwalled CNTs were purchased from Cheap Tubes Inc., (Brattleboro, VT). The CNTs were further purified in our laboratory using a microwave induced method described before (41, 42) . The average diameters of the CNTs were ∼30 nm and the length was as long as 15 μm. The above mentioned chemicals and materials were used in all the experiments. The base membrane used was 0.260 mm OD and 0.206 mm ID hollow fiber composite membrane purchased from Applied Membrane Technology (Minnetonka, MN) with 1-µm thick homogenous


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siloxane as the active layer deposited on microporous polypropylene as the support. The membrane module was constructed using ten 30-cm long hollow fibers in a 0.318 cm OD stainless steel casement. The membranes were held in the casing using ‘T’ connectors from Sigma Aldrich (St. Louis, MO) and sealed at both ends using fast setting epoxy purchased from Strobels Supply (Hornell, NY). This prevented the mixing of the two counter current streams. The effective surface area of the module was calculated to be 19.4 cm2. 2.2. Fabrication of CNT immobilized membrane Effective dispersal of CNTs and immobilization on the membrane surfaces were essential for CNIM fabrication. Ten mg of CNTs was dispersed in acetone and sonicated for 3 hours and 0.2 mg polyvinylidenedifluoride (PVDF) was dissolved in acetone and mixed with the CNTs dispersion. The mixture was then sonicated for another 30 min. CNIM composite membrane was fabricated by sonicating the composite hollow fiber membrane with a CNT suspension, which led to the introduction of CNTS on membrane surface (34, 35, 43). The PVDF served as a binder that held the CNTs in place. Later the membrane was washed with acetone to remove the PVDF as well as excess CNTs. The original hollow fiber membrane as well as one without the CNTs served as controls, both of which showed similar results (28). 2.3. Experimental procedure The schematic diagram for the experimental system used for VOCs removal is shown in Figure.1. Air stream containing VOCs was injected into the hollow fiber membrane module. The feed was mixed with a dry air stream to deliver pre-specified concentrations of VOCs. The feed flow rates were varied between 2 to 10 mL/min and the VOCs concentrations was maintained between 20 to 200 ppm. A countercurrent gas flow was used on the permeate side to transport the permeated VOCs from the membrane injected into a Gas Chromatograph (GC) at regular intervals, using a 6-port injection valve purchased from Valco Instruments Co Inc., (Houston, TX).


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Sample analysis was carried out using a portable SRI 8610 GC from SRI Instruments (Torrance, CA) equipped with a flame ionization detector. A 0.53 mm ID, 30m long, 3.0 µm thick open tubular capillary columns type Rxi-624 Sil MS was purchased from Restek Corporation (Bellefonte, PA) was used for separation. A Peak simple version 3.72 for Windows platform from SRI Instruments (Torrance, CA) was used for data acquisition and analysis. Each experiment was carried out in triplicates and the relative standard deviation was found to be in the range of ± 0.5 – 3.5%.

Figure.1: Schematic diagram of membrane separation system. 2.4. Membrane characterization


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Characterization of both unmodified composite membrane and the CNIM were carried out using scanning electron microscopy (SEM) Model Leo 1530 VP purchased from Carl Zeiss SMT AG Company (Oberkochen, Germany). The membranes were cut into 0.5 cm long pieces and coated with carbon film before SEM analysis. Thermogravimetric analysis (TGA) was used to investigate the thermal stability of the membrane. TGA was carried out using a Perkin-Elmer Pyris 7 TGA system at a heating rate of 10° C/ min in air. 3. Results and Discussion 3.1. Membrane characterization The SEM images of the CNIM and unmodified membranes are shown in Figure.2. The outer surface of the unmodified membrane was a dense homogenous siloxane layer which is shown in Figure 2 (a). The CNTs were coated on the siloxane layer. It is clear from Figure 2(b) that the CNTs were uniformly distributed on the membrane surface. (b)

(a)

Figure. 2. SEM images of (a) unmodified membrane (b) CNIM membrane.


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Figure 3. TGA for CNIM and unmodified membrane Figure 3 shows the thermogravimetric analysis of the membranes. The addition of CNTs to the membrane surface somewhat enhanced the thermal stability of the membranes. It is seen from the figure that the unmodified membrane degraded in the range of ~ 188°C – 511°C while CNIM degradation started around ~ 196 ° C and continued slowly to nearly ~ 680°C. On the basis of the TGA analysis, the CNT content of the membrane was estimated to be 0.543 wt. %. 3.2. Extraction of VOCs from air The rate of transport of the analytes through the membrane which is the flux across the membrane can be expressed by Fick’s law of diffusion,


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where J (gm-mol/m2.sec) is the flux, P is the permeability (gm-mol.m/m2.sec.Pa), and are the partial pressure of the VOCs at the inlet and permeate sides of the membrane, and is the membrane thickness. Permeability is dependent on the membrane/solute interactions and can be expressed as a product of solubility (S) (or partition coefficient) in the membrane and diffusivity (D). Permeability is expressed as, ∗ Since CNTs are excellent sorbents as well as molecular transporters, together these properties can increase permeability. During membrane extraction, interactions can take place via rapid solute exchange on the CNTs thus increasing the effective rate of mass transfer and thereby the flux. The high aspect ratio of CNTs can also dramatically increase the active surface area, which may contribute to enhanced flux. Extraction efficiency (EE) of VOCs for the membrane was determined as follows: %

∗ ! "## ∗ !

where $% , $' (mg/mL) are concentration of solute in the permeate and feed side, (% )*+ (' (mL) are the volume of permeate and feed streams respectively. Enhancement by carbon nanotube incorporation was measured as: ,- %

./0

1 22

1- 22

"##

where 34567 , 389:;?

∗ = @∗A

where 3B is the activation energy for diffusion and R is the gas constant. At the same time, the solubility or partition coefficient decreases with temperature which is given by Vant Hoff’s relationship DEF

∗ = C@∗A

where GHI is partial molar enthalpy for sorption and is a constant. The overall permeation is a tradeoff between these two phenomenon and there exists a maxima in the permeability curve (45, 46). Therefore, it is anticipated that if the temperatures were further increased there could be a decrease in flux. From the figure we can see that among the VOCs studied here, Dichloromethane showed highest flux which was followed by toluene and ethanol. However, at higher temperatures the ethanol flux was found to be higher than that of toluene. Figures 4 (b) show the VOCs removal in terms of flux attained as a function of feed flowrate in CNIM. The feed flow rate was varied from 2 to 10 mL/min at 25°C while the permeate/stripping gas flow rate was maintained constant at 5 mL/min. It was observed that the VOC flux increased with the increase in feed flow rate and finally began to plateau. This was attributed to the reduction in boundary layer with increase in flow rate. However, beyond a certain point it was no longer controlled by diffusing through the boundary layer and no further improvement in flux was observed with increase in flow rate.


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Figure 4. VOC flux attained (a) Effect of Temperature, (b) Effect of Feed flow rate on CNIM membrane.


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3.2.3.

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Extraction efficiency as a function of temperature

Figure 5. Effect of temperature on (a) extraction efficiency, and (b) enhancement with CNIM.


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Figures 5 (a) and (b) show extraction efficiency and enhancement attained for different VOCs as a function of temperature in the range of 25-70°C. Extraction efficiency increased with increase in temperature in both CNIM and unmodified membranes which could be attributed to its increased vapor pressure at elevated temperatures. It was observed from the figure that the enhancement obtained with CNIM membranes were much higher at lower temperatures and decreased with increase in temperature. This was due to the fact that at higher temperatures the diffusion coefficients were high to begin with and enhancement effects of CNTs were less pronounced. For instance, the enhancement for toluene decreased from 80% at 25° C to 56% at 70° C. The maximum enhancement was obtained for toluene (80%) followed by ethanol (26%) and dichloromethane (17%) respectively. It is evident from the results that CNIM demonstrated higher enhancement for toluene in comparison to other VOCs which was attributed to its nonpolar nature, structural symmetry and increased nonpolar interaction with CNTs (47, 48). The attainment of higher extraction efficiency by CNIM at lower temperature allows for a more energy efficient process. 3.2.3.

Extraction efficiency as a function of concentration

Figure 6 shows the extraction efficiency of CNIM and unmodified membrane in the concentration range of 25–200 ppm for toluene at constant temperature (25oC) and feed flow rate (6mL/min). The extraction efficiencies for both membranes increased with increase in concentration. For example with 200 ppm of toluene, CNIM showed extraction efficiency as high as 57%. At 50 ppm, the CNIM showed an extraction efficiency of 22% while the unmodified membrane reached the same efficiency at four times that concentration. Higher extraction efficiency at lower concentrations with CNIM opens up the possibility of extracting VOCs from low concentration streams.


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Figure 6. Extraction efficiency of toluene as a function of feed concentration. 3.3 Mass transfer Coefficient Vapor permeation through the membrane is known to follow a solution-diffusion model which involves sorption of the VOCs onto the membrane followed by diffusion through the polymer matrix and finally desorption into the permeate side. The overall mass-transfer coefficient is calculated as follows, assuming concentration of the permeate side to be zero(49), J

K K

where $LM is the feed concentration of individual VOC and NLM is the VOC flux in gmmol/m2.min. The VOC flux was computed based on the following equation


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K

O!P . R

where O!P is total mass of the permeate (gm-mol), is the permeation time (min), and R is the membrane area (m2). Table.1 Mass transfer coefficient of different VOCs for varying flow rate at 25°C Dichloromethane

Toluene

Ethanol

Flow rate (mL/min)

k x E-06(m/s)

Enhancement (%)

k x E-06(m/s)

Enhancement (%)

k x E-07(m/s)

Enhancement (%)

2

1.68

8.69

1.26

36.59

1.54

29.36

4

3.58

13.57

1.96

47.92

2.33

34.79

6

4.80

16.60

2.52

80.65

2.63

40.54

8

5.19

19.05

3.17

88.91

2.81

43.46

10

5.56

22.68

3.84

92.22

3.16

44.25

Table.2 Mass transfer coefficient of different VOCs for varying temperature at 6 mL/min flow rate Dichloromethane

Toluene

Ethanol

Temperature (oC)

k (m/s) x E-06

Enhancement (%)

k x E-06 (m/s)

Enhancement (%)

k (m/s) x E-06

Enhancement (%)

25

4.80

16.60

2.52

80.65

0.263

40.54

35

5.45

11.92

2.67

74.54

0.396

22.79

45

5.64

6.40

2.99

67.78

0.948

14.44

60

-

-

3.05

60.43

1.84

11.08

70

-

-

3.11

56.98

2.02

8.95

Table 1 & 2 represent the mass-transfer coefficients obtained at different flow rates and at different temperatures, respectively. The overall mass transfer is usually controlled by diffusion through the


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boundary layer at low flow rates. With increase in flow rate turbulence increases which reduces the boundary layer at the membrane interface. It was observed that as flow rate increased from 2 to 10 mL/min, overall mass transfer co-efficient with CNIM increased 231, 205 and 105% for dichloromethane, toluene, and ethanol, respectively. Similarly, increase in temperature led to higher diffusion coefficient and lower mass transfer resistance, thereby increasing the overall mass transfer coefficient. However, the increment in mass transfer coefficient with increase in temperature was not very significant for toluene and dichloromethane because the partition coefficients tend to be lower at elevated temperature. The effect of CNTs also reduced as the temperature increased. 3.4. Proposed mechanism The mechanism underlying enhanced VOC permeation is shown in Figure. 7. Typical permeation is given by solution diffusion model where dissolution of the solute on membrane is followed by diffusion of the solute and desorption on the permeate side (50, 51). This is applicable to the CNIM as well. The ability of the CNTs to provide high sorption capacity for the VOCs has been reported, where the interstitial spaces between CNTs also act as sorption sites (52). Therefore the presence of CNTs on the dense selective layer provided higher partitioning of the VOCs (27, 53). Additionally it has been reported that smooth CNT surfaces aid in frictionless movement of solute and rapid desorption of VOCs (54, 55). Therefor the CNTs not only increased the sorption capacity of the membrane but also aided in faster mass transfer to increase permeation flux.


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Figure.7: Mechanism of membrane separation process. 4. Conclusions Membrane extraction of volatile organics from air is demonstrated using CNIM. Presence of CNTs showed significant enhancement in membrane performances in terms of extraction efficiency and flux. Overall enhancement for both temperature and flowrate variation was observed for all volatile organics under study. CNIM membrane exhibited about 80% enhancement at 25 °C and nearly 92% enhancement at 10mL flowrate for toluene. Thus CNIM exhibited improved performance at lower temperatures and at


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higher flow rate, which implies that the presence of CNTs leads to higher permeation and faster mass transfer rate. Acknowledgement The authors would like to acknowledge Dr.Sagar Roy for technical guidance and Miss. Zheqiong Wu for performing TGA analysis. References (1) (2) (3)

(4) (5) (6)

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(16) (17) (18) (19) (20) (21) (22) (23)

(24) (25) (26)

(27) (28) (29) (30) (31) (32)

(33) (34)

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