Gas permeance properties of asymmetric carbon hollow fiber membranes at high feed pressures

Accepted Manuscript Gas permeance properties of asymmetric carbon hollow fiber membranes at high feed pressures Evangelos P. Favvas, George E. Romanos, Fotios K. Katsaros, Konstantinos L. Stefanopoulos, Sergios K. Papageorgiou, Athanasios Ch Mitropoulos, Nick K. Kanellopoulos PII:

S1875-5100(16)30208-6

DOI:

10.1016/j.jngse.2016.03.089

Reference:

JNGSE 1401

To appear in:

Journal of Natural Gas Science and Engineering

Received Date: 29 February 2016 Revised Date:

24 March 2016

Accepted Date: 29 March 2016

Please cite this article as: Favvas, E.P., Romanos, G.E., Katsaros, F.K., Stefanopoulos, K.L., Papageorgiou, S.K., Mitropoulos, A.C., Kanellopoulos, N.K., Gas permeance properties of asymmetric carbon hollow fiber membranes at high feed pressures, Journal of Natural Gas Science & Engineering (2016), doi: 10.1016/j.jngse.2016.03.089. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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CO2 permeance versus CO2 fugacity coefficient 900 Maximum Permeance Values

M3

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M1

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Permeance (GPU)

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Graphical Abstract

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Gas permeance properties of asymmetric carbon hollow fiber membranes at high feed pressures Evangelos P. Favvasa,b*, George E. Romanosa, Fotios K. Katsarosa, Konstantinos L. Nick K. Kanellopoulosa a

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Stefanopoulosa, Sergios K. Papageorgioua, Athanasios Ch. Mitropoulosb,

Membranes & Materials for Environmental Separations Laboratory, Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, Aghia Paraskevi, 153 41, Attica, Greece

b

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Hepheastus Laboratory, Department of Petroleum and Mechanical Engineering, Eastern Macedonia and Thrace Institute of Technology, 654 04 St. Lucas, Cavala, Greece

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Abstract

Αsymmetric carbon hollow fiber membranes (CHFs) were prepared and their gas permeance properties were investigated at high pressures up to ~60 bar. The main target of this work was to study the effect of the gas feed pressure on the permeance through carbon membranes.

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Specifically, nitrogen, carbon dioxide, methane and ethane were chosen for studying the permeance performance of the developed membranes at conditions of industrial interest. It has been observed that CO2 obtains a maximum permeance of 412 GPU at 27.5 bar and 765 GPU at

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22 bar for M1 and M3 membranes respectively while C2H6 presents maximum permeance values of 482 and 825 GPU at 22.45 and 16.13 bar for M1 and M3 membranes respectively. These

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maxima in permeances can be explained in terms of orientational correlations of the confined molecules, originating from inter–molecular quadrupolar interactions. Discussion on the diffusion mechanism of all the studied gases is also provided. Note that natural gas (NG) treatment, i.e. sweetening and purification processes, is energetically and economically convenient if it takes place under the conditions where the NG stream is extracted from the wells (i.e. pressure 30–60 bar). This practically means that if it is known the pressure where the used membranes provide the highest permeability factors, concerning the gas of interest, then the

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pressure can be fixed at this grade without pointless depressing actions. The presented testing methodology is very useful for identifying the optimum operation conditions towards an efficient

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gas separation process.

Keywords: carbon membranes; gas permeance; high-pressure gas permeances; fugacity; natural gas. Contact info: Evangelos P. Favvas (E-mails: [email protected] and [email protected], Tel.

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+302106503663, FAX: +302106511766).

1. Introduction

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The permeation of gases and liquids through microporous membranes has been studied for many decades. A landmark year was 1962 when Loeb and Sourirajan published a paper where highpermeable asymmetric cellulose acetate membranes were developed and studied for sea water demineralization based on the osmotic phenomenon (Loeb and Sourirajan, 1962). The first commercial scale separation process had been presented in the 1960s, achieving the desalination

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of brackish water by reverse osmosis via utilization of cellulose acetate membranes that selectively retained the salt and allowed pure water to permeate (Sourirajan, 1970). Since the late 60’s many types of polymeric and inorganic membranes have been reported as candidate

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materials for liquid and gas separation applications (Baker, 2004). Gas separation technology is an

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“outstanding” worldwide challenge for both industry and academic society which can also offer a significant reduction of the environmental impact and the cost in several industrial processes. However, several difficulties are usually encountered, especially in cases where molecules with similar sizes (kinetic diameters) should be separated. The study of gas separation has a long history from the age of Thomas Graham who gave the first description of the solution-diffusion model and his work on porous membranes led to Graham’s law of diffusion (Graham, 1866).

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During the last few years many types of membranes such as metal (Deveau et al., 2013), polymeric (Lee et al., 2011), ceramic and zeolitic (Smart et al., 2010; Varoon et al., 2011) and mixed matrix membranes (Mahajan and Koros, 2000; Mahajan and Koros, 2002; Song et al., 2008;

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Favvas et al., 2014a; Favvas et al., 2014b) have been studied and used in gas separation applications.

Polymeric membranes have been also studied for high-pressure permeability experiments, but with limited applicability (dependent on the membrane materials) due to swelling phenomena.

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As swelling phenomenon is described the process of dissolution of a polymer in a defined solvent, liquid or any fluid which provides good solvent properties. At first, the solvent

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molecules, for example the CO2, slowly diffuse into the polymer to produce a swollen gel. If the polymer–polymer intermolecular forces are high, thanks to crystallinity, crosslinking, or strong hydrogen bonding, this is all what happens. But, if these forces are overcome by the introduction of strong polymer–solvent interactions, a second stage, the relaxation even the dissolution of the

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polymer, can take place (Wessling et al., 1991; Izák et al., 2007). This phenomenon is also reported as membrane plasticization and is well-known to natural gas industrial membrane systems (Wind et al. 2002; Visser et al., 2005). The gas is typically treated at relatively high

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pressures of 30–60 bar. Note that at these high pressures of natural gas operations, membrane materials absorb 30–50 cm3(STP) of CO2/(cm3 polymer) (Baker and Lokhandwala, 2008).

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The reported studied materials are many, including polyimide, polyamide–imide, polyethersulfone/polyimide, polysulfone etc in which swelling is usually observed in streams of high carbon dioxide content (Wind et al., 2003a; Wind et al., 2003b; Kapantaidakis et al., 2003; Wind et al., 2004; Kosuri and Koros, 2008; Scholes, et al., 2010). On the other hand, inorganic membranes have become a promising material for gas separation technology with many potential applications such as the purification of hydrogen rich streams, the separation of olefin/paraffin

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mixtures, the recovery of CO2 from natural gas, the enrichment of syngas as well as the water treatment and the production of oxygen enriched air (Saufi and Ismail, 2004; Ismail and David, 2001; Koros and Mahajan, 2000; Favvas et al., 2007; Favvas et al., 2011). Nanoporous carbon

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membranes constitute another promising type of inorganic membranes for gas separation applications thanks to their good separation properties (Favvas et al., 2008b; Swaidan et al., 2013; Yoshimune and Haraya, 2013; Favvas et al., 2015). These membranes can be produced by

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carbonization, under inert atmosphere or vacuum. Lately, numerous synthetic precursors have been used to form carbon membranes, such as polyimide and its derivatives, polyacrylonitrile

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(PAN), phenolic resin, polyfurfuryl alcohol (PFA), polyvinylidene chloride–acrylate terpolymer (PVDC–AC), phenol formaldehyde, polyetherimide (PEI) and polyvinylpyrrolidone (PVP), cellulose, sulfonated poly(phenylene oxide) (SPPO) phenolic resols, numerous co-polymers and others (Song et al., 2008; Pandey and Chauhan, 2001; Salleh and Ismail, 2013; Yoshimune and Haraya,

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2013; Li et al., 2014). To this point of view, Salleh at al., in 2013, published a review paper

entitled “Precursor Selection and Process Conditions in the Preparation of Carbon Membrane for Gas Separation: A Review” where they presented condensed information regarding the sufficient

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materials and processes for the preparation of carbon gas separation membranes (Salleh at al., 2013). Certainly, Karvan et al. reported an interesting work regarding the carbon molecular sieve

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hollow fiber membrane manufacturing in a pilot-scale system (Karvan et al., 2013). Note that the one of the major reasons that the carbon hollow fiber gas separation membranes are not commercially available yet is the difficulty to be produced in a pilot-scale dimension. At this direction, from materials to application, another recent work was focused where cellulose acetate (CA) was chosen as carbon gas separation hollow fiber membrane precursor, while HYSYS

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simulation was also conducted to evaluate the process feasibility of CO2 capture by HFCMs in a post combustion process (He and Hägg, 2013). In the present work, two types of CHFs were examined for their permeance properties at

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high pressures. This work is a continuation of our recent studies where the permeation properties of carbon dioxide at pressures up to 55 bar had been investigated for the same type of membranes (Favvas, 2014). Particularly gas permeances of high purity N2, CO2, CH4 and C2H6

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gases were measured at pressures up to ~60 bar at 307 K. A maximum of the CO2 and C2H6 permeance vs pressure was observed, especially in the case of CHFs with wide micropores. The

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described methodology in this work is proved to be a very valuable tool for elucidating the gas diffusion mechanism in asymmetric CHFs and for defining the optimum operating window for

2. Experimental

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an efficient gas separation process.

2.1. Membrane preparation

BTDA-TDI/MDI, (P84), co-polyimide hollow fibers were developed by the dry/wet phase

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inversion process via the spinodal decomposition mechanism (Favvas and Mitropoulos, 2008b) in a spinning set up described previously (Chatzidaki et al., 2007; Favvas et al., 2008a). The

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carbon membranes were prepared through controlled pyrolysis of the polymeric co-polyimide precursor. In particular, membrane M1 was pyrolyzed at a heating rate of 5 K/min up to 1323 K with 150 mL/min of Ar sweeping the inner side of the tube, stabilization at the maximum temperature for 4 h and cooling down to 298 K with a rate of 10 K/min. The pyrolytic procedure followed for the activated membrane, Μ3, was similar to the one for M1. The subsequent

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activation process was carried out after completion of the isothermal pyrolysis step, by switching to 150 mL/min of CO2 for one minute (Favvas et al., 2011).

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2.2 TGA TGA thermographs were obtained in a TGA/DTA–DSC Thermogravimetric–Differential Thermal Analyzer (Setaram, Setsys Evolution 18) under inert environment (Ar, 150 mL/min)

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using platinum crucibles and a heating rate of 5 K/min. A mass of ~70 mg was typically used for all samples.

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2.3 XRD & SEM

The X-ray diffraction patterns were recorded on a Siemens XD-500 diffractometer using CuKα X-ray source. The dimensions and the asymmetric structure of both studied carbon hollow fiber membranes were investigated by Scanning electron microscopy (SEM). The instrument used was

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a FEI Scanning Electron Microscope. The specimens were mounted on the stub using a doubleside conductive carbon adhesive tape. In order to have better image analysis the carbon fiber

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samples were sputter coated using an ion-sputtering device.

2.4. Gas permeance measurements

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The experimental setup involved for the permeability measurement is described in detail elsewhere (Steriotis et al., 1996; Nitodas et al., 2008). In this work, we conducted single phase permeation experiments of four gases (N2, CO2, CH4 and C2H6) at high pressures in order to elucidate the mechanism of gas diffusion and define whether the developed materials could perform as molecular sieves or exhibit separation capacity through high-pressure adsorption or molecule orientation. The methodology and the devices for this type of experiments are also described in previous works (Katsaros et al., 1997; Favvas et al., 2015). The permeability values 6

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would be presented in the barrer unit after the multiplying of the gas permeance with the

2.5. High-pressure gas permeance measurements

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thickness of the membrane, l(µm).

Differential permeability is a very useful tool for the determination of the ideal working pressure of a membrane for a specific application. Studying the different gas species involved in a process

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and scanning a wide range of pressures, from mbar up to some tenths of bars, one can define the

specific gas separation application.

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optimum operating window where the membrane exhibits the highest yield and efficiency for the

In this type of experiment, each membrane is placed in a specially designed module, equipped with toggle valves, and the modules are mounted in permeability testing rigs that apply the dead-end mode of measurement. In this configuration, the membranes act as permeable

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the testing rig (Fig. 1).

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diaphragms between two closed volumes that correspond to the feed and the permeate section of

Fig. 1. High sensitive permeability apparatus for low and high pressure gas permeance measurements (Chatzidaki et al, 2007; Favvas et al., 2011).

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For our permeability experiments three fibers, each one about 5 cm long, were inserted into the stainless steel holder. The open ends of the fibers were sealed using a low vapor pressure epoxy resin (Torr seal®, Varian). The effective permeation area for the hollow fiber membrane modules

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was approximately 3.80 cm2. Due to the temperature limited performance of the epoxy resin (up to ~393 K), just before the sealing procedure, all membranes were heat treated at 423 K for at least 24 hours under helium atmosphere, in order to eliminate any trace of adsorbed molecules

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(i.e. H2O, CO2, HC etc). During the experiment, gas is introduced into both the feed and the permeate sections, in such way that both sides of the membranes are exposed to the same

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pressure (from 1 to 60 bar). The membranes are left overnight at the stable temperature of 307 K where they are equilibrated from both sides in consecutive increasing pressures. Prior to the measurement, the modules are isolated from the gas tanks by closing the respective toggle valves. Then, the feed section is kept at a slightly higher pressure (of the order of few hundreds

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of mbars) compared to that of sorption equilibrium, the toggle valves are opened and gas molecules cross the membrane toward the permeate section. The pressure increase rate in the permeate side is logged with a differential pressure manometer (Katsaros et al., 1997; Nitodas et

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al., 2008). Having accurately defined the volume of the permeate section, it is possible to convert

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the pressure increase rate to molar flux through the membrane.

3. Results and discussion 3.1. Membranes’ structural and morphological characterization The thermal stability of membrane precursors had been determined by TGA. The corresponding thermographs are shown in Fig. 2. Generally, there are two/three temperature regions of appreciable weight loss. The first weight loss below 373 K is attributed to the evaporation of ethanol (non-solvent) and any residual NMP solvent (boiling point: 477.3 K). 8

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53.2

90

Weight loss during CO2 activation

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ethanol, water and NMP removal

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55

1000

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Temperature (K)

Fig. 2. Thermogravimetric analysis of P84 precursor membrane for M1 (a) Weight loss under isothermal process at 1323 K for 4 hours (M1) and (b) weight loss under CO2 activation process for M3 membrane.

The second weight loss, at about 478 K, is attributed to the rearrangement of the polymeric

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chains packing. The third degradation stage, starting from around 673 K, is related to the decomposition of the polymer. A weight loss of 3% is observed during the isothermal treatment at 1323 K (see Fig. 2(a)), while the total weight loss is 45.42%. An additional weight loss of 1%

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takes place in membrane M3 during activation with the CO2 stream (see Fig. 2. inset (b)) due to

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the purification of the constrictions at the pore entrances and the surface roughness. From the studies often encountered in the literature, Argon is considered to be the best inert gas for polyimide precursors, since it leads to milder treatment than other gases, such as helium or nitrogen (Su and Lua, 2007). Moreover, high final pyrolysis temperatures (>873 K) resulted in the formation of carbon membranes with well structured pore network and smaller pore sizes (Favvas et al., 2007; Anderson et al., 2007). Based on these findings, and on the results of previous experiments where low permeance values were measured for membranes prepared at

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lower temperature (1173 K) (Favvas et al., 2008a), we decided to carbonise the membranes at 1323 K, under milder activation environment (Argon instead of Nitrogen) in order to enhance the pore structural characteristics and avoid the formation of larger pores.

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The XRD patterns of both carbon hollow fibers are presented in Fig. 3. It can be seen that there is not any shift in the Bragg peak position between the two membranes. Therefore the activation with CO2 does not change the crystalline structure of the developed materials. If this happens

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remarkable changes on the peaks’ positions will be observed (Peng et. al., 2013; Waluś et al.,

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2013).

d=3.72 Ǻ

M1 M3

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35 40 2 Theta

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d=2.06 Ǻ

Fig. 3. XRD patterns of M1 and M3 membranes (Favvas et al., 2013a).

The first peak at 23.86o with d-space of 3.72 Å reveals the occurrence of turbostratic structuring (Zhang et al., 2006; Dahn et al., 1997) (random layer lattice) while the second one at 44.4o coincides with the third characteristic X-ray diffraction peak for pure pyrolytic graphite powder (Jianqing, 1993) with d-space of 2.06 Å. Overall, the two samples can be classified as

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amorphous carbons, which consist of regions with graphite and turbostratic carbon conformations (Favvas et al., 2013a). Two SEM images are shown following in the Fig. 4 when the asymmetric structure of both

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membranes it is shown clear. An interesting morphological feature is the existence of a quite dense interface layer at a distance of about from 30–60 µm from the inner surface of the fiber (Fig. 1d). This continuous interface divides the bulk of the carbon hollow fiber membranes into

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two regions: the inner region with finger like voids (cavities) of ~30–60 µm in length and the outer region with similar cavities of ~60–100 µm in length. The occurrence of such an interface

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layer has also been observed in other recent studies (Yu et al., 2006a; Yu et al., 2006b; Favvas et al., 2011) and reflects the boundary region between the opposite streams of the non solvent (water), diffusing simultaneously from the inner (bore liquid) and outer (coagulation bath) surface of the fiber.

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The effect of the extra pyrolysis procedure, in the case of M3 membrane, on the morphological characteristics is evident when comparing the respective SEM images of the M1 to M3 membranes’ surface (see Fig. 4e and Fig. 4f). The extra roughness of the M3 membrane indicates

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that in fact CO2 could play the role of an activation agent. The increase of the gas permeation properties confirms this evidence. The ultramicroporous separating layer of the membrane is

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mentioned with red colored note (Fig. 4d). In conclusion, the SEM analysis is sufficient for the characterization of the bulk structure of asymmetric carbon porous hollow fibers and can be applied as a useful tool in order to investigate the effect of the pyrolysis treatment on the bulk morphology of the precursor material and of course couldn’t give information regarding the responsible for the gas diffusion pores.

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Fig. 4. SEM images of M1 (left) and M3 (right) carbon hollow fiber membranes (Favvas et al., 2011).

3.2. Gas permeation properties of the carbon hollow fiber membranes at low pressure The single-phase gas permeation properties of the CHFs at atmospheric pressure and different temperatures have already been reported in our previous work (Favvas et al., 2011), where we had also discussed, permselectivity properties in relation to the expected Knudsen separation

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factors. In this work the two membranes, the M1 and the M3, the ones with the larger pore sizes were selected for the high pressure gas permeance. The selected gases for the permeation

therefore the ones with poor selectivity values.

Activation Energy of permeation [Eact (kJ/mol)] CO2 Ar O2 CO N2

He

H2

2.6

2.89

3.3

3.4

3.46

3.62

0.9 – 3.6

— —

– 0.7 – 2. 5

— —

— – 1.9

— – 2.5

3.64

CH4

i-butene

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3.8

5.0

5.5

– 1.5 – 2.3

– 1.5 – 2.3

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Gas Kinet. Diameter (Å) M1 M3

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experiments were the gases with similar kinetic diameters (N2, CH4, C2H6 and CO2) and

— – 2.5

— – 2.6

Table 1. Activation energy of permeance for He, CO2, O2, CO, N2, CH4, i-butane and SF6 (Favvas et al., 2011).

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The activation energies of permeation were calculated by the slope of an Arrhenius plot and are presented in Table 1.

The characteristic property of microporous membranes is the activated gas transport. It has been found that the permeance J ( mol / m 2 ⋅ s ) through a microporous material increases as a

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 − Eact  function of temperature according to: J ∝ J 0 exp  , where Eact ( kJ / mol ) is the apparent  RT  activation energy. In all cases, the negative activation energy indicates that the gas permeance

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will decrease with the temperature increase (Dixon-Garrett et al., 2000; Pinnau and He, 2004; Favvas et al., 2013b). This kind of temperature dependence is characteristic for the dominance of

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Knudsen or Poiseuille mechanisms of gas transport and constitutes additional evidence for the existence of large mesopores. This can be also attributed to parallel fluxes in large micropores (5
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mainly ultra-microporous. However, the gas selectivity characteristics are poor because of the co-existence of a mesoporous network that can give rise to Knudsen or Poiseuille (viscous) flow. This phenomenon is more intense in the case of M3 membrane where the sizes of mesopores are

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greater than 200 nm (Favvas et al., 2011). This is the reason for the existing obviously maximum values at CH4, C2H6 and CO2 gas permeances contrary to the case of M1 where this phenomenon

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is observed weaker.

3.3. High-pressure gas permeation performance of carbon hollow fiber membranes

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The interest for enhancing the gas separation factors has fueled the membrane community towards the direction of preparing new membrane materials in order to improve the efficiency of the existent separation processes. In this point of view, the effect of the pressure conditions has been recently investigated at both polymeric, thermally-rearranged (TR) PIMs, carbon molecular sieve membranes and mixed matrix polymer membranes (Swaidan et al., 2013; Shahid and

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Nijmeijer, 2014) at feed pressures up to ~40 bar. The effect of feed pressure on gas permeation properties of each gas can be depicted by high pressure differential experiments. This principle of corresponding states can be used to correlate the solubility of both gases and vapors into a

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polymer over a wide range of temperatures (Stern et al., 1969). Differential permeance

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experiments for N2, CH4, C2H6 and CO2 were carried out at 307 K, which is higher temperature than the critical temperature for all gases in this study ( TC(N 2 ) = 126 K, TC(CH 4 ) = 190.3 K, TC(C2H6) = 305.2 K, TC(CO 2 ) = 304 K) and at a wide range of pressures, from 1 to 60 bar ( 1 ≤ P ≤ 60 bar). The presented data are the results of 3 different membrane batches and the permeance values have been confirmed after three times of measurements. Here it must be noted that the gas permeance measurements didn’t present fluctuations higher than 2-3% among them. In order to approach the phenomenon with a more realistic behavior we use the terms of

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fugacity, f, and fugacity coefficient, Φ, which describes the effective real “mechanical” pressure. The fugacity is equal to the pressure of an ideal gas which has the same chemical potential as the real gas (Lin and Freeman, 2005). The correlation between f and Φ is given by: f = Φ ⋅ P ,where

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P is the measured pressure and Φ is the dimensionless fugacity coefficient, which in general depends on the temperature, the pressure and the identity of the gas. For our calculations we use the NIST standard reference database for each gas (National Institute of Standards and

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Technology, Standard Reference Database, http://www.nist.gov/).

As it can be seen in Fig. 5, in the case of M1, an almost linear correlation between the N2

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and CH4 permeance and the fugacity coefficient is observed. This is a typical behavior for nonadsorbed or weakly adsorbed gases, where the increase permeance through the membrane can be attributed to the increase of viscous permeation flux. This finding is also in agreement with previous studies on the membranes (Favvas et al., 2011), where adsorption isotherms revealed

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very low adsorption capacity for these gases. On the contrary, the effect of pressure on permeance, in the case of strongly adsorbed molecules such as C2H6 and CO2, is completely different. In particular, an increase in permeance with the equilibrium pressure is observed for

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both gases below a critical pressure (below 20 bar). At higher equilibrium pressures (>20 bar) a

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maximum is observed, which is more pronounced for C2H6.

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1600

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Maximum Permeance Value

1000

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Permeance (GPU)

Permeance (GPU)

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Maximum Permeance Values

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CO2

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Fugacity(corrected pressure in bar)

(d)

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Fig. 5. Permeances of N2, CH4, C2H6 and CO2 gases for M1 and M3 membranes versus the gas’s fugacity coefficient.

On the other hand, in the case of M3, the permeance of all gases studied increases sharply at relatively low pressures (below 10 bar). Above a critical pressure, the rate of increase declines rapidly with the pressure, either reaching a plateau (N2 and CH4) or showing a maximum for C2H6 and CO2. The corresponded permeance values for each gas at both studied membranes M1 and M3 are presented in the following table (Table 2).

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Gas

Gas Permeance (GPU)

Maximum Permeance Value

Higher pressure tested (bar)

M3

M1

M3

N2

35 – 580

130 – 1160

–

–

~ 60

CH4

47 – 727

184 – 1414

–

–

~ 60

C2H6

42 – 482

154 – 825

482 GPU at f = 22.45 bar

825 GPU at f =16.13 bar

~ 40

CO2

32 – 412

120 – 765

412 GPU at f = 27.5 bar

765 GPU at f = 22 bar

~ 40

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M1

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Table 2. Gas permeance fluctuation and maximum measured values of each studied gas at M1 & M3 membranes.

As we can see the maximum measured values for C2H6 were 482 GPU at fugacity (f,

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corrected pressure magnitude), f = 22.45 bar and 825 GPU at f = 16.13 bar for M1 and M3 membranes respectively. For M1 membrane the CO2 permeance maximum value was 412 GPU at f = 27.5 bar while in the case of M3 membrane the maximum permeance value was 765 GPU for pressure f = 22 bar.

It must be noted that both membranes exhibit the same behavior for C2H6. However, the

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pressure where the permeance maximum is observed is different in each membrane: ~22.5 bar for M1 and ~ 18 bar for M3. The same phenomenon also occurs in the case of CO2 where the

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maximum in permeance is observed at ~22.5 bar, for M3. However, the drop in permeance is not so significant in M1. In this case a plateau like tendency is apparent at pressures higher than ~55

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bar. The effect of equilibrium pressure on gas permeance can also be depicted in detail in Table 3, where the permeance upgrade factor, which describes the aspect ration of current measured permeance over the corresponded permeance at 1 bar head pressure, is presented for both membranes of the four measured gases. The correlation between pressure, fugacity and fugacity coefficient is also presented in the Table 3. The totally different behavior of M3 in regards to M1 for N2 and CH4 can be attributed to increased adsorption of these gases on M3 even at room temperatures. These results are in accordance with previous adsorption studies on the membrane

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(Favvas et al., 2011). It must be noted that the kinetic diameters of all studied gases are very similar, i.e. 3.3, 3.64, 3.8 and 4.3 Å for CO2, N2, CH4 and C2H6 respectively; therefore the molecular size is not the crucial parameter for the different permeance behavior of these gases

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Knudsen diffusion is the main permeation mechanism.

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through the M1 and M3 carbon hollow fiber membranes. This is typical characteristic when the

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Pressure (P) [bar]

Fugacity (f) [bar]

CH4

Fugacity coefficient (Φ)

Permeance upgrade (times) M1

M3

Pressure (P) [bar]

Fugacity (f) [bar]

C2H6

Fugacity coefficient (Φ)

Permeance upgrade (times) M1

M3

Pressure (P) [bar]

Fugacity (f) [bar]

Fugacity coefficient (Φ)

CO2 Permeance upgrade (times)

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N2

Pressure (P) [bar]

Fugacity (f) [bar]

M1

M3

0.9926

1

1

0

0

Fugacity coefficient (Φ)

Permeance upgrade (times) M1

M3

---

1

1

0

---

1

1

0

0

---

1

1

0

---

1.08

1.0799

0.9999

1.7

2.0

1.08

1.0782

0.9983

1.8

2.0

1.08

1.072

0.9755

2.0

2.0

1

0.9955

0.9955

1.8

2.0

3.6

3.5984

0.9996

3.3

3.8

3.6

3.58

0.9944

3.6

3.7

3.6

3.5119

0.9499

4.3

3.5

3

2.9596

0.9865

3.3

3.3

7.4

7.3935

0.9991

5.1

5.3

7.4

7.3158

0.9886

5.7

5.0

7.4

7.0291

0.9191

6.7

4.5

7.6

7.3417

0.9660

7.1

4.9

11.95

11.934

0.9986

6.9

6.4

11.95

11.732

0.9817

7.7

6.0

12

11.029

0.8766

8.7

5.1

11.95

11.313

0.9467

8.6

5.6

18.4

18.364

0.9980

8.9

7.4

18.4

17.887

0.9721

10.0

6.9

18.4

16.129

0.8166

10.3

5.4

15.37

14.319

0.9316

9.7

6.0

22.95

22.896

0.9977

9.8

7.8

22.95

22.156

0.9654

10.1

7.3

27.5

22.456

0.7820

11.4

5.1

19.2

17.565

0.9148

10.5

6.2

27.55

27.477

0.9973

11.0

8.2

27.55

26.412

0.9587

12.1

7.4

32.75

25.609

0.7336

11.0

4.7

22

19.857

0.9026

11.1

6.2

33

32.901

0.9970

11.9

8.4

33

31.379

0.9509

13.3

7.7

40

29.343

0.6926

9.1

3.6

24.75

22.042

0.8906

11.8

6.3

40.3

40.165

0.9967

12.8

8.7

40.3

37.906

0.9406

14.0

27.85

24.427

0.8771

12.5

6.3

47.5

47.331

0.9964

14.5

8.7

47.5

44.206

0.9307

15.0

7.5

32

27.491

0.8591

13.0

6.3

59.4

59.181

0.9963

16.3

8.9

59.4

54.334

0.9147

15.4

7.7

36.6

30.714

0.8392

12.7

6.0

45.8

36.616

0.7995

12.8

5.6

54.64

41.591

0.7612

12.6

4.8

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0

EP

TE D

7.8

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Table 3. Correlation of equilibrium pressure, fugacity and fugacity coefficient with Ν2, CH4, C2H6 and CO2 permeances for Μ1 & Μ3 membranes at 307 K.

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Furthermore, the existence of a maximum in the permeance of a microporous network, in close analogy to mesoporous media (Steriotis et al., 1997), has already been reported in the literature (Katsaros et al., 1997) and can be attributed to the specific molecule-molecule and molecule/wall

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interactions.

This specific behavior (maximum) is only apparent for C2H6 and CO2 molecules, with similar shape and quadrupole moment, − 13.4 ± 0.4 ⋅ 10 −40 Cm2 and

− 3.34 ⋅ 10 −40 Cm2

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respectively (Adya and Wormald, 1992). The quadrupole moment could possibly result in orientational correlation effects, under special conditions, e.g. in bulk supercritical fluids or in

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fluids confined in nanoporous materials. For example, orientational correlations have been observed for carbon dioxide confined in MCM-41. Specifically, CO2 adsorption on MCM-41 took place under subcritical conditions, i.e. along an isotherm at 253 K and pressures up to 18 bar in conjunction with neutron diffraction measurements (Steriotis et al., 2008). The results

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showed that the main diffraction peak possessed a degree of asymmetry, which was attributed to intermolecular orientational correlations between neighboring CO2 molecules arising mainly from electrical quadrupolar interactions. Similar results were also observed by studying confined

2012).

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CO2 under supercritical conditions, at 308 K and at pressures up to 125 bar (Stefanopoulos et al.,

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The behavior of adsorbed CO2 on a microporous carbon at a temperature slightly above the critical point was also studied by adsorption in conjunction with in situ neutron diffraction. The results suggested that the confined phase, even at pressures well below the critical one, was in a state comparable to a high-pressure dense supercritical fluid or even bulk liquid (Steriotis et al., 2004). Furthermore, both the diffraction patterns and the radial distribution functions provided strong evidence for the presence of orientational correlations between adsorbed molecules. It is

20

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noteworthy that CO2 permeability measurements through this carbon membrane (which is reported in the current reference) have also revealed a maximum at 308 K and a main pressure of about 35 bar. In addition GCMC simulations followed a detailed examination of the local density

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profiles and angular distributions of molecular axes at different pressures within pores of different size at 308 K. The results showed an orientational transition of molecules confined in 1.15 nm slit graphitic pores between 30 and 40 bar. Note that the carbon membranes under study

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are rather similar because they have slit-like pores with almost same sizes (Favvas et al., 2013a). The assumption of the orientational transition is also valid for C2H6 molecules. It has been

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verified by neutron diffraction experiments at low temperatures, where the ethane patterns contain a large number of Bragg reflections (Coulomb et al., 1979). The best fits were obtained when the C–C bond was titled at an angle of 24o with the surface. The surface consisted of exfoliated graphite, a material with similar surface and bulk properties with M1 and M3

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membranes. Furthermore, in a molecular dynamic study the separation of ethylene/ethane mixtures was obtained using carbon nanotubes. The results showed that the different orientation configuration of ethane within the carbon nanotube holes probably played an important role in

EP

the molecule permeation and, finally, separation (Tian et al., 2013). All the aforementioned findings strongly suggest that the permeation behavior of CO2 and C2H6 can be explained in

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terms of orientational correlations between the confined molecules.

3.4. Fluid flow through the pores The fluid transport through a porous media can occur: i) by diffusive flux, ii) by viscous flow or iii) by both mechanisms. For isothermal steady state flow, in the absence of external fields, the flux in the x-direction (molecules per unit area per unit time) is given by the following

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expression (Nicholson and Cracknell, 1996): J = −

cDo  ∂µ  cBo  ∂p   −   where the first term in kT  ∂x  n  ∂x 

equation represents the diffusive flux J D which contains the diffusion coefficient, Do , and the

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molecular concentration, c (x) , inside the pore space while the second term, the viscous flux J V , is driven by the pressure gradient and contains the viscosity coefficient n and a geometrical term B0 . As Nicholson and his co-workers reported for a microporous system, the self-diffusion

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coefficient decreases with concentration, whilst the Darken transport diffusion coefficient passes

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through a maximum (Nicholson and Cracknell, 1996). These maxima in our data probably occur due to the confluence of the molecular diffusion and the surface adsorption phenomenon. The adsorption constitutes the overall phenomenon more complicated, because of the surface diffusion contribution, arising from the molecules that diffuse through the adsorbed into the pore walls layer. Αt lower temperatures, where the adsorbate fluid is at liquid phase, the total flow is

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the sum of the gas phase diffusion and the surface flow of the condensed phase. At higher temperatures, and especially when the system is at critical conditions, this “model” is also valid if we reckon that the adsorbate remains at the solid walls at a pseudo-liquid phase.

EP

In larger pores, the contribution of the viscous flow is higher than in smaller ones. Note that Bo depends on the square of the pore width (Bo=(PW)2/12, where PW is the internal pore width in the

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case of slit like pores). On the other hand, in micropores the diffusion of the gas molecules through successive jumps between adsorption sites on the pore walls is the main mechanism of the mass transfer. As a result one can say that for a definite pore size network the viscous flow contribution is lightweight at the beginning, while increases at higher pressures correlating the increase in the density of the adsorbate phase.

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According to this approximation, a qualitative estimation of the pressure dependence of permeance can be obtained. Therefore, the first area of the plot (see Fig. 5) where the permeance increased can be attributed to the diffusion flow since the contribution of the viscous flow is very

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low and the linear increase of the permeance can be attributed to the diffusion phenomena. On the other hand, the contribution of the viscous flow, which occurs because of the cooperative effects, starts above a critical fluid concentration inside the pores or above a critical differential

SC

pressure which is applied to the porous network. At higher pressures (hence densities) the permeance values decreased according to the decrease of the mean distances between the fluid’s

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molecules and the pore walls. At these conditions the fluid viscosity is recorded enhanced. It can be seen, that in the case of CO2 and C2H6, the phenomenon of permeance maximum is stronger in the M3 membrane. This happens because at the M3 membrane, the pore network which is responsible for the gas permeation phenomenon consists of larger pores than the M1

TE D

membrane, very close to mesoporous classification (Favvas et al., 2011). On the other hand, in the case of the M1 membrane, where the pore network is characterized as more microporous, the behavior is not very apparent and only in the case C2H6 permeance presents a maximum. It must

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be noted that the authors are planning to perform in situ adsorption and small-angle neutron scattering experiments in order to obtain a more realistic approach, relevant to the gas densities

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at the experimental conditions (Steriotis et al., 2004).

4. Conclusions

The experimental approach and the results derived from the high-pressure permeance study (at pressures up to ~60 bar) of four pure gases, CO2, C2H6, N2 and CH4 through two asymmetric carbon hollow fiber membranes are described and discussed. Their different permeance behavior is relevant to membranes’ pore size distribution. Specifically, an increase in flux is apparent for

23

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all cases while a reduction at higher pressures is obtained in cases of CO2 and C2H6. This phenomenon was more intense in the case of membrane with larger population of mesopores, M3, whereas it was weakened with the enhancement of microporosity. The permeance values

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fluctuated, in the case of CO2, from 32–412 and 120–765 GPU and presented the maximum value at 27.5 and 22 bar for M1 and M3 membranes respectively. For C2H6 the corresponded permeances were from 42–482 and 154–825 GPU and present the maximum value at 22.45 and

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16.13 bar for M1 and M3 membranes respectively. These maxima can be attributed to orientational correlations of the confined molecules mainly originating from quadrupolar

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interactions between neighboring molecules. Additionally, the exact behavior is also defined by the size of the pore network of the membranes and adsorption capacity of each gas. In general permeance experiments are a very useful tool for the characterization of asymmetric membranes. Low pressure measurements can provide structural information about membranes’ pore size and

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network. Furthermore, high pressure permeance experiments enable the determination of the optimum parameters at real industrial separation processes. In conclusion, this type of permeance experiments can be proved to be a useful tool for the investigation of the optimum working

EP

pressure conditions for the gas permeance or/and a gas separation process. For example, in the current study, the CH 4 / CO2 separation factor at elevated working pressures (~60 bar), although

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remarkable small, is higher than the corresponding factor at atmospheric pressures. Therefore, based on the specific process conditions (pressure, temperature, ranges of the requested fluxes) one can design and develop membrane systems with optimum pore sizes in order to achieve the optimum permeance coefficient and selectivity factors for a certain gas mixture. The investigation of the effect of feed pressure at membranes with high separation factors will be the next target of study.

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Acknowledgments The present work is a result in the framework of NSRF. The NANOSKAΪ Project (Archimedes

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Framework) of the Eastern Macedonia and Thrace Institute of Technology is co–financed by Greece and the European Union in the frame of operational program “Education and lifelong learning investing in knowledge society”, Ministry of Education and Religious Affairs, Culture

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and Sports, NSRF 2007–2013.

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Highlights

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1. High-pressure permeances of CH4, N2, CO2 & C2H6 through CHFMs were measured

2. For CO2 & C2H6 the increase of the feed pressure provides maxima in the permeance

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3. Maxima can be explained by orientational correlations of confined molecules

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4. Optimum operation condition: the pressure where a gas presents highest

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permeances