Separation of gasoline vapor from nitrogen by hollow fiber composite membranes for VOC emission control

Journal of Membrane Science 271 (2006) 114–124

Separation of gasoline vapor from nitrogen by hollow fiber composite membranes for VOC emission control Yujing Liu a , X. Feng a,∗ , Darren Lawless b a

Department of Chemical Engineering, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 b Fielding Chemical Technologies Inc., Mississauga, Ont., Canada L5C 1T7 Received 28 April 2005; received in revised form 28 June 2005; accepted 1 July 2005 Available online 10 August 2005

Abstract This study deals with removal of volatile organic compounds (VOCs) from nitrogen by membranes for emission control. Hollow fiber composite membranes comprising of a thin layer of poly(ether block amide) supported on microporous poly(vinylidene fluoride) substrate were prepared for this purpose. The membranes were initially examined for the removal of representative VOCs from binary VOC/nitrogen mixtures. Then the separation of actual gasoline vapors, with and without gasoline additives, from nitrogen using the composite hollow fiber membranes was investigated. The separation performance of the membranes at various operating conditions (e.g. feed composition, operating temperature) was evaluated. It was found that the hollow fiber composite membranes were effective for the recovery of gasoline vapors from nitrogen. At ambient temperature, an overall permeate VOC concentration as high as 95 wt% was obtained, and the overall VOC flux was in the range of (2–4) × 10−2 g/(m2 s). The composition profile of the recovered gasoline was different from the gasoline vapor in the feed gas, and blending reformulation may be needed for the recovered gasoline to meet certain volatility and octane ratings. The membrane was found to be stable for gasoline vapor recovery during a 10-month period of testing. © 2005 Elsevier B.V. All rights reserved. Keywords: Hollow fiber; Composite membrane; Vapor permeation; VOC emission; Gasoline

1. Introduction The emission of volatile organic compounds (VOCs) into the atmosphere not only pollutes air, but also results in a significant economic loss. Industries producing solventcontaining waste gases have been under increasing regulatory and economic pressures. Many efforts have been devoted to recovering VOCs from the waste gas streams. While the best emission control technology depends on specific site conditions, membrane technology has become an attractive alternative to adsorption and condensation for recovering the organic compounds, especially when the VOC concentration is relatively high (>0.1 vol%) and the gas flow rate is relatively low (<100–1000 scfm) [1].

∗

Corresponding author. Tel.: +1 519 888 4567; fax: +1 519 746 4979. E-mail address: [email protected] (X. Feng).

0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.07.012

Among the many sources of organic compound emissions, petroleum refining and associated processes are the largest single emission source. When considering organic solvent emissions, gasoline and other light hydrocarbon emissions cannot be neglected. Gasoline is a mixture of hydrocarbons with chains containing 4–12 carbons, mostly paraffins, cycloparaffins and aromatics. Its composition depends on the gasoline specifications such as regular or premium, summer or winter quality. In addition, methanol, ethanol and methyl tert-butyl ether may also be present as oxygenates and octane number enhancers in lead-free gasoline. The recovery and reuse of evaporated gasoline from loading, unloading and other handling processes are of significant importance from both an economic and environmental points of view. Most of the prior work on membrane vapor recovery has focused on silicone rubber-based membranes (for example, [2–9]), and the separation of organic vapor–gas mixtures is often attributed to a solution diffusion mechanism.

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Poly(dimethyl siloxane) coated on a porous substrate (e.g. polysulfone and polyetherimide) is commonly used as the membranes for organic vapor recovery, and commercial systems are available in the market for a variety of organic vapor–air separations. However, one of the potential problems associated with silicone rubber-based materials is that they are not very resistant to some hydrocarbons (such as gasoline) [10], and thus the membrane stability will be a primary concern when silicone based membranes are to be used for gasoline vapor separation. There have been some efforts to modify silicone polymers by fluorination and cross-linking so as to improve the chemical resistance of the membranes for VOC separation [11]. Other attempts have also been made to explore the possibility of using microporous polyimide and polyetherimide membranes without coating to separate organic vapors from air or nitrogen [12–15]; presumably, the membrane permselectivity is due to surface diffusion of VOC molecules sorbed on the wall of the membrane pores. Previous studies showed that membranes based on elastomeric polymers generally have a good permselectivity to organic compounds. In this study, we report the use of poly(ether block amide) (PEBA) membranes for the separation of selected VOCs from nitrogen pertinent to gasoline vapor recovery. The rationale for selecting PEBA as a membrane material is based on the following. (i) PEBA is a family of thermoplastic elastomers with excellent chemical resistance. The copolymer has a regular linear chain where the rigid polyamide segments are trespassed with the flexible polyether segments; while the hard polyamide blocks provide the mechanical strength, the soft polyether blocks provide good permeation properties. (ii) PEBA polymers are shown to be a potential versatile rubbery material that can be used to make membranes for gas separation and pervaporation. PEBA membranes were found to compare very favorably with silicone rubber for the separation of organic compounds (especially ester compounds) from water by pervaporation. (iii) A variety of PEBA polymers with different kind and content of the polyamide and polyether segments are available commercially. In spite of the earlier work on the separation of polar/nonpolar gas mixtures [16–18], to the best of our knowledge, this material has not been studied as a membrane for VOC separation from air for emission control. In this study, hollow fiber composite membranes comprising of a thin layer of PEBA supported on a microporous poly(vinylidene fluoride) substrate were prepared. Poly(vinylidene fluoride) was selected as a substrate material because of its excellent chemical stability as well as good adhesion of PEBA coating layer on PVDF substrate. Gas permeation is a rate-controlled process, and a high permeation rate can be achieved by using thin membranes. Composite membranes consisting of a thin separation layer and a microporous substrate support are used in almost all industrially important gas separations. The hollow fiber composite PEBA membranes were tested for the removal of representative VOCs including hexane, heptane and cyclohexane, which are the main components of gaso-

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line, and dimethyl carbonate, ethanol, methanol and methyl tert-butyl ether that are the oxygenates and octane number enhancers of lead-free gasoline. The effects of feed VOC concentration and operating temperature on the membrane performance were evaluated in terms of permeance and permeate concentration. In addition, the separation of actual gasoline vapors from nitrogen using the hollow fiber composite membranes was also investigated. The membranes were tested extensively for ten months under varying conditions (e.g. at high or low feed concentrations and various operating temperatures with different organic compounds) and were found to be stable for gasoline vapor recovery.

2. Experimental 2.1. Materials Poly(vinylidene fluoride) (PVDF 741) and poly(ether block amide) (PEBA 2533) were supplied by Arkema Inc. (Philadelphia, PA). PEBA 2533 was comprised of 20 wt% nylon 12 as the amide segments and 80 wt% poly(tetramethylene oxide) as the ether segments. Lithium chloride was purchased from Aldrich Chemical. All solvents used in membrane preparation and VOC separation experiments were of reagent grade and supplied by BDH Chemicals, Fisher Scientific, Merck or Aldrich Chemical. Gasoline with an octane number of 87 was purchased from a local Esso gas station. Nitrogen gas (research grade, impurities <1 ppm) was supplied by Praxair Specialty Gases and Equipment. 2.2. Membrane preparation The PVDF substrate hollow fibers were fabricated from homogenous solutions of PVDF dissolved in N,N-dimethyl acetamide by the phase inversion technique. Lithium chloride was used as an additive. After degassing under vacuum, the dope solution was extruded through a tube-in-orifice spinneret with nominal inside and outside diameters of 0.5 and 1.0 mm, respectively. The external coagulation bath was filled with de-ionized water maintained at 33 ◦ C. De-ionized water was also used as the bore fluid, which was delivered at a constant flow rate to the tube side of the spinneret by a high pressure metering pump. The nascent hollow fibers emerging from the spinneret was partially solidified by the internal coagulation fluid. There was an air gap between the spinneret and the external coagulation bath so that the outer surface of the nascent hollow fiber was exposed to air for partial evaporation of the solvent prior to immersion into the coagulation bath, where coagulation began to occur on the outer surface of the membrane due to solvent–nonsolvent exchange. The as-spun hollow fibers were taken up by a spooling wheel, and the fibers remained in a water bath at room temperature for at least 7 days to complete coagulation, during which period the LiCl additive leached out from the membrane. The hollow

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Table 1 PVDF hollow fiber fabrication conditions Polymer solution composition PVDF (wt%) DMAc (wt%) LiCl (wt%) Temperature of dope solution (◦ C) Fiber extrusion speed (m/min) Flow rate of internal coagulant (m/min) Temperature of internal coagulant (◦ C) Temperature of external coagulant (◦ C) Air gap between spinneret and external coagulant (cm) Fiber take-up speed (m/min) Ambient conditions

75 20 5 50 2.7 0.96 22 33 10 8.3 22 ◦ C, 40–50% RH

fiber membrane was then thoroughly rinsed with de-ionized water before being air dried. The details on the dope solution composition and fiber spinning conditions are summarized in Table 1. The hollow fibers so prepared had an inside and outside diameters of 0.28 and 0.49 mm, respectively, as measured from the membrane cross-section under microscopy. To prepare the hollow fiber PEBA/PVDF composite membranes, the external surface of PVDF hollow fiber substrate membrane was coated with a 1 wt% PEBA in n-butanol solution at room temperature. After coating, the membranes were dried in a fume hood for 30 min. The coating process was repeated three times, and caution was exercised to ensure that the fibers coated with PEBA solution did not stick to each other. Finally, the membrane was completely dried. 2.3. Permeation experiments A miniature hollow fiber membrane module was assembled using a bundle of seven hollow fibers encased in a 1.27 × 10−2 m (1/2 in.) copper tubing. While one end of the fiber bundle was sealed with an epoxy resin, the other end was potted to form a gas-tight tube sheet, which was cut carefully to make the fiber bores fully open. The effective fiber length was 16.5 cm and the effective membrane area

based on the outer diameter was 17.7 cm2 . The experimental setup for VOC/nitrogen separation is shown in Fig. 1. A mixture of organic vapor and nitrogen was prepared by bubbling nitrogen through a porous sintered stainless steel diffuser immersed in a chosen organic liquid, which was placed in a thermal bath. The feed gas mixture was admitted to the shell side of the hollow fiber membrane module at atmospheric pressure. The membrane module was placed in a water bath whose temperatures could be controlled within ±0.5 ◦ C. The temperature of the module was kept at least 2 ◦ C higher than the temperature of the organic liquid reservoir to prevent the VOC from condensation in the membrane module. The permeate side (i.e. the bore side) of the membrane was connected to two cold traps that were immersed in liquid nitrogen. A vacuum pump was used to keep the permeate pressure below 1 kPa (10 mbar), which was monitored by a Pirani vacuum gauge (MKS Instruments). The permeated organic vapor was condensed and collected initially in one of the cold traps, and switched to the other when a steady state of permeation was reached. The permeation rate of VOC was determined by weighing the condensed VOC samples collected over a known period of time. The composition of VOC in the feed and residue streams were measured by a Varian CP 3800 gas chromatograph equipped with a thermal conductivity detector and a 60 m long capillary column. The feed flow rate was controlled in the range of 8–45 ml/s, which was found to be high enough that the variation in the concentration of the gas on the feed side was negligible. Thus, the feed flow rate could be approximated by the residue flow rate, which could be conveniently determined by a bubble flow meter. By changing the nitrogen flow rate and the temperature and liquid level of the organic solvent in the reservoir, different concentrations of VOC in the feed gas streams could be obtained. Before and after each series of VOC/nitrogen separation experiments, pure nitrogen permeance was measured to check the stability of the membrane. This was done in the following way. A bubble flow meter was connected to the residue outlet of the membrane module, and after the feed side of the membrane module was briefly purged with

Fig. 1. Schematic diagram of VOC separation experiment. (1) Nitrogen cylinder, (2) pressure gauge, (3) thermal bath, (4) liquid reservoir, (5) gas bubbler, (6) membrane module, (7) cold trap, (8) vacuum gauge and (9) vacuum pump.

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When the pressure of permeate side, which is under vacuum, is much lower than the feed pressure, the following approximations can be made to simplify the calculations if the feed contains more than one organic vapors JVi =

QVi PF Xi

and QN = JN PF (1 −

(5)


Xi )

(6)

where subscript i represents the organic vapor component i. The quantities QVi and Xi are obtainable from experiments, and the concentrations of VOCs in the permeate stream can be calculated by Fig. 2. Schematic diagram for nitrogen permeation measurement.

Yi =


nitrogen, the gas inlet valve was closed, as shown in Fig. 2. As the permeate side was evacuated by a vacuum pump, the soap film in the burette of the bubble flow meter was drawn to move downward due to the permeation of nitrogen through the membrane. The nitrogen permeance was determined from the speed of movement of the soap film.

3. Characterization of membrane performance for VOC separation

WV MtA

(1)

where WV is the mass of the permeate sample (i.e. the organic compound) collected in the cold trap over a period of t seconds, M the molar mass of the organic compound and A is the membrane area. The permeance of the organic compound through the membrane, JV , is given by JV =

QV PF X − P P Y

(2)

where X and Y are the mole fractions of the organic compound in the feed and the permeate, respectively; Y is related to the partial permeation fluxes of nitrogen and the organic compound by Y=

QV QV + Q N

(3)

The permeation flux of nitrogen, QN , is given by QN = JN [PF (1 − X) − PP (1 − Y )]

QVi + QN

(7)

i

Since only the organic compounds in the permeate are condensed and collected in the cold trap, the composition of the collected organic liquid can be calculated from QVi Yi =  QVi

(8)

The validity of this treatment had been verified by direct measurements of the composition of the organic liquid collected.

Considering the permeation of a binary organic vapor/nitrogen mixture, the flux of the organic component through the membrane, QV (mol/(m2 s)), can be obtained by QV =

QVi

(4)

Since QV , X, PF and PP are known quantities from the organic vapor/nitrogen separation experiments and JN can be obtained from pure nitrogen permeation measurement, the quantities Y and JV can thus be solved from Eqs. (2) to (4).

4. Results and discussion Hexane, cyclohexane and heptane are the main components of gasoline. They were used as model components to study VOC separation from nitrogen by the hollow fiber composite membranes. The separation of VOCs from their binary VOC/N2 mixtures was studied first to investigate the effects of feed VOC concentration on the membrane performance, and then the separation of the VOCs from mixtures containing all the three organic compounds was tested. In addition, the separation of methanol, ethanol, dimethyl carbonate and methyl tert-butyl ether, which are used as gasoline additives, from nitrogen was also investigated. Finally, the recovery of gasoline vapors with and without these additives by the hollow fiber composite membranes was evaluated. 4.1. Effect of feed concentration on membrane performance for VOC/N2 mixtures Fig. 3 shows the effects of feed VOC concentration on the VOC flux and the VOC concentration in the permeate for the separation of binary hexane/N2 , cyclohexane/N2 and heptane/N2 mixtures. The experiments were conducted at 22 ◦ C, at which the nitrogen permeance was determined to be 4.1 × 10−10 mol/(m2 s Pa). As expected, as the VOC concentration in the feed increased, both the VOC flux and

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Fig. 3. Effect of feed VOC concentration on the VOC flux and permeate VOC concentration for separation of binary hexane/nitrogen, cyclohexane/nitrogen and heptane/nitrogen mixtures.

VOC concentration in the permeate increased as well. In spite of their different concentrations in the feed gas, the permeation fluxes of the three organic compounds are similar in magnitude. It was shown that the VOC concentration in the permeate was greater than 50 mol% and could be as high as 90 mol% for all these vapors under the experimental conditions where the VOCs were 18–95% saturated in the feed. The permeance of the organic compounds through the membrane is presented in Fig. 4. It shows that the membrane perme-

Fig. 4. Effect of feed VOC concentration on the VOC permeance for the permeation of binary hexane/nitrogen, cyclohexane/nitrogen and heptane/nitrogen mixtures.

ance follows the order of heptane > cyclohexane > hexane. This is in the same order of penetrant condensability but opposite to the order of their molecular sizes. According to the solution–diffusion model, three steps are involved in the permeation through the membrane: sorption, diffusion and desorption. Since the permeate side was exposed to vacuum, desorption is normally not a controlling factor, and the membrane permeability is primarily determined by the solubility and the diffusivity of the penetrant. Smaller molecules tend to have a greater diffusivity. The permeance data observed here imply that the sorption aspect is dominant over the diffusion aspect as far as the membrane permeability is concerned. This means the VOC/nitrogen separation by the membrane is mainly governed by the solubility–selectivity. Similar results can be observed for other rubbery membranes for VOC/gas separations [4,7,9,19,20]. Deng et al. [21] tested a series of polyetherimide hollow fiber membranes for cyclohexane/nitrogen and heptane/nitrogen separations and reported that the most permselective membrane had a permeability of 3.9 × 10−9 and 2.2 × 109 mol/(m2 s Pa) for heptane and cyclohexane, respectively, while the nitrogen permeability was 3.9 × 10−11 mol/(m2 s Pa). In comparison, the PEBA/PVDF composite hollow fiber membrane had a permeability of an order of magnitude higher than the polyetherimide hollow fiber membranes, although a direct comparison of the membrane performance is difficult without knowing the detailed operating conditions of the polyetherimide membranes. The separation of mixtures of hydrocarbon vapors from nitrogen was also studied. The feed mixture was obtained by bubbling nitrogen through ternary liquid mixtures of hexane, cyclohexane and heptane with different compositions. Because these three organic compounds have different volatilities, the feed composition would change when the permeation proceeded for a sufficiently long period of time. The variation in the feed concentration was minimized by frequently replenishing the liquid reservoir with premixed liquid. The membrane performance at different feed compositions was evaluated, with the overall VOC concentration in the feed gas streams being maintained in a range of 5–10 mol%. The experimental results are summarized in Table 2. It is found that at the feed VOC concentrations tested, the overall VOC concentration in the permeate was 70–80 mol%, indicating that the VOCs were enriched substantially at the permeate side. However, the extent of enrichment for the three organic compounds is different. Fig. 5 shows the corresponding enrichment factor for the three VOCs defined as the ratio of the permeate to feed concentrations of the VOC component. In spite of the different feed compositions, the enrichment factor for heptane permeation was the highest, which was almost twice as much as the enrichment factor for cyclohexane. The order of the enrichment is consistent with the permeance of the organic vapors. Currently, methanol, ethanol and methyl tert-butyl ether (MTBE) are commonly used as oxygenates in gasoline, and dimethyl carbonate is considered to be a replacement for

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Table 2 Permeate flux and permeate composition separation of organic vapor mixtures from nitrogen Run no.

1 2 3 4 5 6 7 8 9

Total VOC flux (mol/(m2 s))

Feed composition (mol%) Nitrogen

Hexane

Cyclohexane

Heptane

89.63 92.50 93.89 94.14 93.56 93.91 93.99 94.75 93.06

5.85 1.96 3.12 1.22 3.83 3.41 0.97 0.98 1.93

2.87 5.2 1.97 3.21 1.82 1.68 4.54 3.44 4.49

1.65 0.34 1.01 1.44 0.79 1.00 0.50 0.83 0.51

1.68 1.73 1.17 1.51 0.96 1.45 1.37 1.16 1.29

Permeate composition (mol%) Nitrogen

Hexane

Cyclohexane

Heptane

19.89 19.47 26.25 21.64 30.32 22.42 23.39 26.56 24.48

36.34 17.68 26.10 10.64 28.56 35.65 9.39 9.48 18.00

20.85 55.19 22.42 34.92 20.70 20.24 55.08 45.16 47.73

22.92 7.65 25.23 32.80 20.41 21.69 12.13 18.80 9.79

methyl tert-butyl ether due to environmental concerns associated with the use of MTBE [22]. The separation of these vapors from nitrogen was also tested. Fig. 6 shows the VOC flux and VOC concentration in the permeate for the separation of binary organic vapor/nitrogen mixtures at room temperature. It can be seen that similar to the permeation of hexane, cyclohexane and heptane, with an increase in the feed VOC concentration, both the VOC flux and the VOC concentration in permeate increased. The permeance of these organic compounds through the membrane was shown in Fig. 7, which shows that the VOC permeance increased with an increase in the feed VOC concentration. 4.2. Effect of temperature on membrane performance In order to determine the effects of temperature on the membrane performance, the separation of binary VOC/nitrogen mixtures was carried out at different operating temperatures ranging from 0 to 40 ◦ C with constant feed VOC concentrations: hexane 5.0 mol%, cyclohexane 3.3 mol%, heptane 1.1 mol%, dimethyl carbonate 1.4 mol%, ethanol 1.4 mol%, methanol 3.4 mol% and MTBE 9.0 mol%. The permeation fluxes of the VOCs and their concentrations in the permeate are shown in Fig. 8 (for the separation of

Fig. 5. Enrichment factor of hexane, cyclohexane and heptane for separation of mixed VOCs from nitrogen.

Fig. 6. VOC flux and VOC concentration in permeate for separation of binary methanol/nitrogen, ethanol/nitrogen, dimethyl carbonate/nitrogen and MTBE/nitrogen mixtures.

Fig. 7. Permeance of methanol, ethanol, dimethyl carbonate and MTBE for the permeation of binary VOC/nitrogen mixtures.

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Fig. 8. Effect of temperature on VOC flux and permeate VOC concentration for separation of binary hexane/nitrogen, cyclohexane/nitrogen and heptane/nitrogen mixtures.

hexane, cyclohexane and heptane from nitrogen) and Fig. 9 (for the separation of methanol, ethanol, MTBE and dimethyl carbonate from nitrogen). It is shown that both the VOC flux and the VOC concentration in permeate tend to decrease

Fig. 9. Effect of temperature on VOC flux and permeate VOC concentration for separation of binary methanol/nitrogen, ethanol/nitrogen, dimethyl carbonate/nitrogen and MTBE/nitrogen mixtures.

Fig. 10. Permeance vs. reciprocal temperature.

with an increase in temperature; however, as the temperature increases, the effects of temperature on the permeation flux and permeate concentration are less significant. The permeance of nitrogen and the organic compounds is plotted in Fig. 10 as a function of reciprocal temperature. It is interesting to note that with an increase in temperature, the VOC permeance decreases, while the nitrogen permeance increases. Unlike the organic vapor permeation, the temperature dependency of nitrogen permeance follows the Arrhenius type of relation. These results can be explained qualitatively from the solubility and diffusivity aspects. In general, the sorption process is exothermic, and the solubility coefficient tends to decrease with an increase in temperature. On the other hand, an increase in temperature will increase the thermal motion of the segments of the polymer backbone, thereby enhancing the diffusion of the permeant through the membrane. The affinity between the polymer and nitrogen is relatively weak, and when the temperature increases, the increase in diffusivity is likely more significant than the decrease in solubility, leading to an increase in the permeability. In the case of VOC permeation, however, the solubility is high due to the strong permeant–membrane affinity. When the temperature is low, the sorption aspect dominates the permeation, and thus the VOC permeability decreases with an increase in temperature. When the temperature is high enough, both the diffusion and sorption become significant for VOC permeation. While the diffusivity tends to increase with an increase in temperature, the solubility decreases. As a

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Table 3 Experimental data for gasoline/nitrogen separation Run no.

Feed concentration (wt%) Nitrogen

1 2 3 4

59.19 61.08 66.59 78.33

Total VOC flux (g/(m2 s))

Gasoline 40.81 38.92 33.41 21.67

1.9 × 10−2 2.6 × 10−2 3.4 × 10−2 5.5 × 10−2

result, the temperature dependency of the VOC permeability is less significant at relatively high temperatures, as shown in Fig. 10. Obviously, a low operating temperature is favorable to the separation of organic vapors from nitrogen, and it is therefore desirable to operate the membrane system at ambient temperature from an application point of view. 4.3. Separation of gasoline vapors from nitrogen Gasoline is a complex mixture comprising of many hydrocarbon components that boil below 200 ◦ C. Chromatogram

Permeate concentration (wt%) Nitrogen

Gasoline

8.45 6.28 4.87 29.52

91.55 93.72 95.13 70.48

peaks were detected for the condensed liquid samples of gasoline vapor in the feed and permeate by the gas chromatograph in the experiment, and each peak in the chromatogram may represent several components that were not separated by capillary column of the gas chromatograph. The overall VOC mass flux and overall VOC mass fraction were used to characterize the separation performance, and identification of each and every component was thus not required in this work. As such, instead of using mass fractions to represent the actual concentration of each component in the mixture, the fractional peak area was reported. This is considered to be

Fig. 11. Comparison of composition profile of recovered gasoline in the permeate with that evaporated in the feed gas.

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Table 4 Experimental data for separation of gasoline (with 5 wt% additive) from nitrogen Run no.

Additive in gasoline

Feed concentration (wt%) Nitrogen

1 2 3 4

Dimethyl carbonate Methanol Ethanol MTBE

64.64 64.96 66.44

Total VOC flux (g/(m2 s))

Gasoline 35.36 35.04 35.04 33.56

adequate for the practical purpose of representing the composition profile of the samples. The separation experiments were carried out at 22 ◦ C. The feed gasoline concentration was varied from 22 to 41 wt%. Similar to the separation of ternary organic vapors from nitrogen described earlier, fresh gasoline liquid was added to the bubbling liquid reservoir frequently to minimize the composition variation caused by the different volatilities of

3.52 × 10−2

2.04 × 10−2 2.47 × 10−2 3.13 × 10−2

Permeate concentration (wt%) Nitrogen

Gasoline

4.80 6.54 6.28 5.23

95.20 93.46 93.72 94.77

the components in gasoline. Table 3 shows the permeation flux and permeate concentration of the gasoline vapor in the permeate. Obviously, the permeate is significantly enriched in the organic compounds, and a gasoline content of up to 95 wt% was obtained. Gasoline is a mixture of many hydrocarbons. During the experiment, the variation in the feed gasoline vapor concentration was achieved by adjusting the nitrogen flow rate. A variation in the overall organic content

Fig. 12. Comparison of composition profile of recovered gasoline in the permeate with that evaporated in the feed gas (original gasoline liquid contained 5 wt% gasoline additive).

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in the feed does not correspond to a uniform variation in the concentrations of all individual components due to their different volatilities. As shown in Table 3, the total organic mass flux tends to increase with a decrease in the overall mass concentration of the gasoline vapor in the feed, but no clear trend is observed for the overall concentration of the organic compounds in the permeate. This is caused by the different permeability of the various components, resulting in a variation in the concentration profile. After condensation in the cold trap, the recovered gasoline liquid can be reused. Because of the difference in the permeability of various organic components, the composition profile of the recovered gasoline is expected to be different from that of gasoline evaporated in the feed gas streams. It is thus of interest to see how the concentration profile has changed so that a proper blending of the gasoline can be done to meet certain fuel specifications. Fig. 11 presents the concentration of gasoline in feed and permeate (excluding N2 ) at the four different feed gasoline vapor concentrations shown in Table 3. As expected for solubility-selective membranes, the enrichment of larger and less volatile components in the permeate is more significant than the smaller and more volatile components. Consequently, the composition of the recovered gasoline is shifted to less volatile components. This should be taken into account in blend formulation of the recovered gasoline in order to retain proper volatility and octane ratings for reuse. As mentioned earlier, depending on the season and applications of gasoline, methanol, ethanol, MTBE and dimethyl carbonate are the main oxygenates and octane number enhancers. Experiments were also conducted for the recovery of gasoline vapors emitted by gasoline liquids containing 5 wt% of the additives. The gasoline concentration in the feed gas was 33.6–35.4 wt%. Table 4 lists the experimental data on the overall VOC flux and permeate VOC concentration. In all cases, the overall concentration of the organic compounds in the permeate was over 93%. The composition profiles of gasoline in the feed and permeate (excluding N2 ) are shown in Fig. 12. It is interesting to notice that among all the four additive components considered, methanol is enriched most significantly; the order of the enrichment factors is: methanol (9.4) > dimethyl carbonate (6.5) > ethanol (5.5) > MTBE (2.9). This is consistent with the results obtained from binary VOC/N2 separation except for DMC, which showed a high enrichment factor for dimethyl carbonate separation from nitrogen. This is presumably due to the fact that gasoline is a complex mixture and the interactions between the various components will also affect the permeation of individual component. More detailed studies, which is beyond the scope of the present study, are needed to verify this. The membrane has been tested extensively for organic compounds separation from nitrogen for a period of 10 months at various operating conditions (e.g. temperature, composition, presence of additives), and the membrane was found stable for gasoline vapor recovery.

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5. Conclusions Hollow fiber composite membranes comprising of a thin poly(ether block amide) skin layer supported on a microporous poly(vinylidene fluoride) substrate were developed to separate and recover gasoline vapors for emission control. The membranes were initially tested for the removal of representative VOCs including hexane, heptane and cyclohexane (which are the main components of gasoline) and dimethyl carbonate, ethanol, methanol and methyl-butyl ether (which are the oxygenates and octane number enhancers of gasoline) from binary VOC/nitrogen mixtures. Then the separation of gasoline vapor from nitrogen using the composite hollow fiber membranes was investigated. It was found that the PEBA/PVDF hollow fiber membranes are effective for the recovery of gasoline vapor from nitrogen for hydrocarbon emission control, but the composition profile of the recovered gasoline was different from the gasoline vapor in the feed gas, and blending reformulation may be needed for the recovered gasoline to meet volatility and other specifications for reuse. The separation performance of the hollow fiber membranes at various operating conditions (e.g. feed composition, operating temperature, presence of gasoline additives) was evaluated, and the membrane was found stable for gasoline vapor recovery.

Acknowledgements The poly(vinylidene fluoride) and poly(ether block amide) used in the study were generously supplied by Arkema Inc. (Philidelphia, PA). Research support from the Natural Sciences and Engineering Research Council (NSERC) of Canada is acknowledged.

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