Separation and Purification Technology 131 (2014) 108–116 Contents lists available at ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur Simultaneous hydrogen sulphide and carbon dioxide removal from biogas by water–swollen reverse osmosis membrane Petr Dolejš a,b, Václav Poštulka a,b, Zuzana Sedláková a,c,⇑, Věra Jandová a, Jiří Vejražka a, Elisa Esposito c, Johannes Carolus Jansen c, Pavel Izák a a b c Institute of Chemical Process Fundamentals of the ASCR, v.v.i., Rozvojová 135, 165 02 Prague 6 – Suchdol, Czech Republic Institute of Chemical Technology, Faculty of Environmental Technology, Technická 5, 166 28 Prague 6 – Dejvice, Czech Republic Institute on Membrane Technology, ITM-CNR, Via P. Bucci 17/C, 87036 Rende, CS, Italy a r t i c l e i n f o Article history: Received 21 January 2014 Received in revised form 23 April 2014 Accepted 25 April 2014 Available online 6 May 2014 Keywords: Agro-biogas upgrading Biomethane Water vapour SEM and EDX analysis Porosimetry Carbon dioxide a b s t r a c t Biogas is a suitable alternative fuel if unwanted impurities are removed to avoid corrosion of the inner parts of an engine. A recent breakthrough in biogas purification showed that a thin hydrophilic composite membrane can create the selective water swollen barrier able to remove unwanted sour gases such as carbon dioxide and hydrogen sulphide owing to significantly higher water solubility of the latter in comparison to methane. This work presents the use of water–swollen membranes for the simultaneous removal of carbon dioxide, hydrogen sulphide and water vapour from agro-biogas. Up to 82 vol.% of carbon dioxide and 77 vol.% of hydrogen sulphide were successfully removed from the feed stream at a pressure of 220 kPa. The selection of the most suitable thin hydrophilic composite membrane based on the knowledge of its basic characteristics is discussed. SEM analysis showed that the surface of the best performing composites changed significantly upon swelling by water. It was found that a compact structure of the upper selective thin layer after the swelling by water is fundamental for obtaining a selective water–swollen membrane. The next key factor is a high porosity of the membrane support. A detailed comparison of various systems and their performance is presented. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction In spite of the increasing attention for alternative sources such as wind and solar energy, classical combustible energy carriers still play an essential role in the current society. The energy consumption has continuously risen [1–3] and the energy supply plays a fundamental role for the sustainability of the modern age to ensure the current quality of the human life [3,4]. Nowadays, the energy demand is supplied by fossil fuels for approximately 88% [5]. However, fossil fuels pollute the atmosphere by emissions of greenhouse gases like carbon dioxide, sulphur dioxide, and nitrogen oxides [6]. Fossil fuels reserves are limited resources. In this context, an intensive search for alternative renewable fuels is needed to find a solution to the growing energy challenges [3,7,8] from the economic as well as the environmental point of ⇑ Corresponding author at: Institute of Chemical Process Fundamentals of the ASCR, v.v.i., Rozvojová 135, 165 02 Prague 6 – Suchdol, Czech Republic. Tel.: +420 220390133. E-mail addresses: dolejsp@vscht.cz (P. Dolejš), postulkv@vscht.cz (V. Poštulka), sedlakova@icpf.cas.cz (Z. Sedláková), jandova@icpf.cas.cz (V. Jandová), vejrazka@icpf. cas.cz (J. Vejražka), izak@icpf.cas.cz (P. Izák). http://dx.doi.org/10.1016/j.seppur.2014.04.041 1383-5866/Ó 2014 Elsevier B.V. All rights reserved. view [9]. A biowaste such as a wastewater contains a lot of energy that might be exploited in the form of methane [10,11]. Biomass has also been recognized as a possible renewable energy source [6]. Biogas, an example of a gaseous biofuel, which can be obtained from biomass via a biochemical way, seems to be a very good candidate for the replacement of fossil fuels [12]. For example, natural gas can be replaced directly by biogas if the latter contains a sufficiently high amount of methane [5]. Raw biogas consists mainly of methane, carbon dioxide, and a small amount of various residual compounds, such as water vapour, hydrogen sulphide, ammonia, siloxanes, and mercaptanes [7]. Biogas contains typically 50–70 vol.% of methane and 30–50 vol.% of carbon dioxide, depending on its origin and on the season [7]. Biogas thus needs to be purified to become the ‘‘energy of the future’’ at engine-fuel quality [1]. Many different methods for carbon dioxide removal from biogas exist, namely water scrubbing, polyethylene glycol scrubbing, absorption of contaminants using molecular sieves, or pressure-swing absorption [7]. Carbon dioxide removal is an important operation to enhance the heating value of the gas [13]. Further, hydrogen sulphide has to be captured from biogas (i.e. by absorption or using active coal) both because of its high toxicity and because of its corrosive effect [7,13]. P. Dolejš et al. / Separation and Purification Technology 131 (2014) 108–116 Newly, biogas purification can be realized by the membrane separation technology [7,13,14]. The tested polymeric membranes have been made from silicone rubber, cellulose acetate, and polyimide [7,13–16]. At the current state of the art, upgrading of biogas with polymeric membranes is commercially competitive with the conventional technologies for carbon dioxide and hydrogen sulphide removal, such as pressure swing adsorption, temperature swing adsorption or amine scrubbing [17,18]. However, most of the membranes suffer damage by aggressive gases [7,13,14,18,19] and it is necessary to pre-treat the raw biogas and remove water vapour and also the potentially harmful compounds, namely hydrogen sulphide, ammonia and siloxanes. The need to minimize the costs of biogas upgrading leads a continuous search for new and more effective membrane materials [19]. One of the possibilities is the use of water–swollen membrane for simultaneous carbon dioxide and hydrogen sulphide removal from the biogas stream [7,8]. Conventional biogas purification methods require removal of water vapour from the biogas stream. However, it is well known that the solubility of quadrupolar carbon dioxide and polar gases in water is significantly higher than that of methane. Under appropriate conditions the polyamide layer of thin film composite reverse osmosis membranes is able to create a thin film of water, which can then perform as a perfect selective membrane for separation of polar gasses from methane [8]. The great advantage of this membrane separation is that unwanted and toxic gases, including water vapour itself, are removed from its continuously refreshed surface, thus avoiding contamination of the permselective membrane. Furthermore, the condensed water passing through the membrane ensures good permselectivity of the whole separation [8]. This method of biogas upgrading has been patented recently [20]. The contact of the thin hydrophilic composite (TFC) membrane surface with water causes swelling of the polyamide thin film. In order to achieve the spontaneous condensation of water, the temperature of the TFC membrane must be below the dew point of the raw biogas feed. Interestingly, the heat of evaporation of the liquid phase from the permeate side of the membrane helps to cool the membrane surface. The function of the water–swollen thin film composite membrane was previously proven for the high-pressure type of RO membrane and the subject of the present manuscript is to test also a low-pressure membrane meant for brackish water (much less expensive compression work would be used) and to compare the results. In particular, the possibility to use the polyamide composite membranes for simultaneous removal of both carbon dioxide and hydrogen sulphide from agro-biogas has been experimentally studied in the present manuscript. 2. Experimental 2.1. Materials and membrane preparation Commercial TFCs Reverse Osmosis (RO) membranes of two suppliers were tested. The TFC membrane specifications are displayed in Table 1. The first two TFCs supplied by Sterlitech Corporation (further denoted as Sterlitech I and Sterlitech II) were used for Table 1 The specifications of thin film hydrophilic composite (TFC) membranes. Supplier Product code Our designation Sterlitech Corporation Sterlitech Corporation Sterlitech Corporation Koch Membrane System Inc. YMAKSP3001 YMACM53001 YM70UBSP18 KM8011395 Sterlitech I Sterlitech II Low pressure membrane High pressure membrane 109 preliminary tests with binary mixtures of methane and carbon dioxide. The third TFC membrane supplied by Sterlitech Corporation was denoted as the low pressure RO membrane and was originally produced for low pressure brackish water desalination. The last used TFC membrane, supplied by Koch Membrane System Inc., was indicated as the high pressure RO membrane. A circular area of 124.6 cm2 was cut out from the flat sheet TFC membranes. Three millilitres of deionized water were spread with a brush on the membrane skin layer before closing the permeation cell, according to the previous experience to achieve water–swollen layer on the composite [8]. All single gases were supplied by Linde gas with a stated purity of at least 99.995%. Pure methane and pure carbon dioxide were mixed with a stream containing carbon dioxide and 10,000 ppm of hydrogen sulphide to obtain an agro-biogas model mixture with 940 ppm of hydrogen sulphide. 2.2. Membrane characterisation 2.2.1. SEM and EDX analysis Elemental analysis was carried out on the Tescan Indusem with Quantax 200 scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDX) analyzer from producer Bruker. The EDX system consists of an XFlash detector 5010 operated with an accelerating voltage of the electron beam in the range from 5 to 30 kV. The necessary accelerating voltage dependeds on the thickness of the deposited layer. SEM/EDX analysis provides a superior imaging quality and rapid and nondestructive quantitative elemental results directly in the live image. Due to the relatively high energy of the electrons, SEM/EDX equipment typically probes the surface at a depth between 1 and 2 lm. 2.2.2. Membrane support porosity Composite membranes were pre-dried at 40 °C before porosimetry measurements were carried out on a top-rating high-pressure mercury porosimeter AutoPore III (Micromeritics, USA), which performs high-pressure mercury intrusion up to 400 MPa. The porosimetry determination of the membrane support is described in detail in the literature [21]. 2.3. Apparatus and measurement The laboratory-scale biogas separation apparatus used in the present work is shown in Fig. 1. The feed stream is prepared by mixing CH4, CO2, and H2S using Bronkhorst mass flow controllers. Firstly, a preliminary test was carried out with a dry feed stream to check the separation performance of a virgin composite membrane. Secondly, the feed stream was saturated with water in a two-stage humidifier filled with a total of 24 mL deionized water and was fed through the first stage of the humidifier at laboratory temperature. The second stage of the humidifier was heated to reach a higher water saturation pressure. The temperature and humidity of the feed stream was measured by HygroFlex 4 system with HC2 sensor (Rotronic). The combined feed stream was then fed into the permeation cell. The water swollen surface was continuously refreshed during the measurement by the condensing water, thanks to the fact that the permeation cell (described in detail in the previous work [8]) was cooled. After leaving the permeation cell, the residual humidity in both permeate and retentate streams was removed in the cold traps (at 15 °C) to avoid humidity inside the backward pressure controller (Bronkhorst) and the IR gas analyser (Aseko Air LFÒ, CZ). Methane and carbon dioxide content were analysed by infrared sensors, and hydrogen sulphide and oxygen by electrochemical sensors, respectively. The maximal applied feed pressure was 500 kPa (absolute), while the permeate carrier gas (i.e. nitrogen) was always at atmospheric pressure. All 110 P. Dolejš et al. / Separation and Purification Technology 131 (2014) 108–116 Fig. 1. Scheme of the permeation apparatus. 1 – Permeation cell, 2 – CH4 flow controller, 3 – CO2 flow controller, 4 – flow controller for mixture containing CO2 with 10,000 ppm H2S, 5 – N2 flow controller, 6 – humidifier (first part 6a at laboratory temperature and second part 6b is heated), 7 – temperature and humidity sensor, 8, 9 – cold traps, 10 – back pressure regulator, 11– retentate flow meter, 12 – permeate flow meter, 13 – three-way electromagnetic valves, 14 – IR analyser. measurement parameters were controlled and collected by custom software developed in LabViewÒ environment (National Instruments, USA). by swelling of the upper layer. The gas permeation was therefore expressed in terms of the permeance, P/l, defined as the permeation flux (Ni) of a particular component, normalized for the partial pressure difference as the driving force: 2.3.1. Experimental conditions The first permeation experiments were carried out with binary mixtures simulated raw biogas obtained from a sewerage plant (Table 2). It was observed previously that there is no significant difference between raw biogas and a binary mixture composed of 40 vol.% CO2 and 60 vol.% CH4 [8]. Then the gas composition was changed to simulate an agro-biogas (last column in Table 2). The model of the agro-biogas mixture was composed of three gases, namely CH4, CO2 and H2S. This model mixture contained approximately 940 ± 15 ppm H2S. Nitrogen was used as the sweeping gas. The feed flow was 32 mL/min and the nitrogen flow was 10 mL/min, respectively. All experiments were carried out with a permeation cell temperature of 21 °C. Gas mixtures were saturated with water vapour in the humidifier to achieve a similar condition as in the real biogas plant operated under 37 °C. The experimental conditions were chosen on the basis of the previous experience [8]. At least three experiments were carried out at the same conditions. The data collection was started 1 h after the start of the permeation experiment to be sure that steady state was reached and to ensure that no hydrogen sulphide was lost by dissolving in the humidifier. Both permeate and retentate flows were recalculated to the binary or ternary mixtures eliminating the sweeping gas. Pi Ni Qi ¼ ; ¼ l pi;0  pi;p Aðpi;0  pi;p Þ 2.3.2. Evaluation of the experimental data The permeation through the TFCs was described by the solution-diffusion model [17]. The precise thickness (l) of the membrane could not be determined, due to the changes caused ð1Þ where pi,0 is the partial pressure of the component on the feed side, pi,p is the partial pressure of the component on the permeate side, Qi is the molar flux of the component defined per time unit, and A is the membrane area. The results were expressed in gas permeation unit (GPU), recalculated from SI units, as follows: 1 GPU ¼ 7:52  109 m3 : m2 s kPa ð2Þ The permselectivity between two gases ai/j is expressed as the ratio of their permeances: ai=j ¼ Pi : Pj ð3Þ 3. Results and discussion 3.1. Membrane characterisation 3.1.1. EDX and SEM analysis The main atoms present in the TFC membranes, namely C, O, S, and N, were determined by EDX analysis (Table 3). The atomic percentages were determined for both the surface and the cross section. The EDX analysis was done in at least five different places at the composite membranes. Table 2 The feed compositions for the permeation experiments. Feed flow was 32 mL/min and the sweeping gas nitrogen was 10 mL/min, respectively. Feed was saturated by water at 37 °C and the permeation cell was kept at 21 °C for all experiments. Component Methane Carbon dioxide Hydrogen sulphide a Binary mixtures (pre-tests) Model agro-biogas mixture vol.%a vol.%a 60.0 40.0 0 55.0 45.0 0 The experimental error was within ±0.05 vol.% for CH4 and CO2, and ±0.00015% for H2S. 53.0 47.0 0 53.3 46.6 0.1 111 P. Dolejš et al. / Separation and Purification Technology 131 (2014) 108–116 Table 3 Elemental composition of the TFC membranes determined by EDX analysis for the surface as well as the cross section of the membranes. Sterlitech I atom% atom% atom% atom% C O S N Sterlitech II Low pressure membrane High pressure membrane Surface Section Surface Section Surface Section Surface Section 82.86 7.56 4.34 0.00 77.15 18.82 2.53 0.00 79.41 14.85 3.46 0.00 77.14 20.98 1.19 0.00 72.13 13.33 3.15 4.81 79.49 8.72 0.00 0.00 73.96 14.85 3.44 7.49 69.54 24.12 0.88 5.12 No nitrogen was found by the elementary analyses in the upper layer of Sterlitech I and Sterlitech II membranes although all used TFC membranes were stated to be polyamides by the supplier. Since the EDX analysis typically probes the sample at a depth of 1.5 ± 0.5 lm, the skin layer in the Sterlitech membranes is probably too thin to be detected by this method and mainly the composition of the porous support membrane is determined. Further, a comparison of the neat dry composite surface with the used membrane surface was made by SEM analysis (Fig. 2). A visible difference between the neat and used composite was observed for the high pressure membrane, the former being smoother than the used membrane. Small changes were observed for the low pressure membrane, with a slight coarsening of the surface morphology upon use, while no significant changes appeared for both Sterlitech I and Sterlitech II membrane. The differences between the cross section structures of the membranes upon swelling by water were observed by SEM analysis (Fig. 2). A compact cross sectional structure of the upper selective layer was observed for both low pressure and high pressure membrane while the Sterlitech I and Sterlitech II membranes have a more porous structure. Two parts in all TFC membranes cross sections are shown in Fig. 2. The bottom parts are a non-woven fabric and the upper layer consists of a selective polyamide skin layer and its support. The Fig. 2. SEM analysis of the TFC membrane surface and cross section. Virgin membrane surfaces (top), used membrane surfaces (middle) and cross section (bottom) with the cross section of the virgin membrane shown as an inset. 112 P. Dolejš et al. / Separation and Purification Technology 131 (2014) 108–116 Table 4 The thicknesses of the upper layer of the membranes determined from the SEM analysis of the cross sections and the porosity of the membrane support. Membrane Thickness of the upper layer (lm) Porosity, e (%) Sterlitech I Sterlitech II Low pressure membrane High pressure membrane 50.48 ± 1.39 49.59 ± 0.75 45.45 ± 1.16 40.97 ± 0.54 42 41 48 54 thicknesses of the upper layer, was determined from SEM analysis with an experimental error lower than 5% and is given in Table 4. The upper layer of both Sterlitech I and Sterlitech II membranes was thicker than that of the low pressure and high pressure RO membranes. 3.1.2. Porosity of the membrane support The overall porosity of the membrane support is stated in Table 4. The low pressure and high pressure membranes had higher support porosity while a lower support porosity was observed for Sterlitech I and Sterlitech II membrane, respectively. The difference between the virgin TFC membrane and the used membrane is obvious from Fig. 3. After swelling by water, the biggest pores inside the support layer decreased for both low pressure and high pressure membrane whereas the opposite trend was observed for Sterlitech II. A negligible change in the amount of the biggest pores inside the support layer of Sterlitech I membrane was observed. The material characterisation, i.e. EDX/SEM analysis and the membrane support porosity will be discussed in the view of the transport properties below. 3.2. Gas transport properties 3.2.1. Preliminary binary permeation tests All dry virgin TFC membranes were found to be impermeable, as was reported previously [8]. This observation corresponds with the dense structure of all upper layer membranes determined by SEM analyses (Fig. 2). The further permeation measurements were therefore carried out exclusively with a humid feed stream. 3.2.1.1. Sterlitech I and Sterlitech II. The first permeation tests showed that the Sterlitech I and Sterlitech II membranes were not selective for the binary methane/carbon dioxide mixture (Fig. 4a and b). The permselectivity slightly increased if the composite was immersed into deionised water for two days before the Fig. 3. The distribution of the membrane support pore sizes obtained by porosimetry. The dashed lines represent the neat composite before use, while the solid lines represent the used membranes; Sterlitech I (a), Sterlitech II (b), low pressure membrane (c), and high pressure membrane (d). P. Dolejš et al. / Separation and Purification Technology 131 (2014) 108–116 (a) (b) (c) (d) 113 Fig. 4. Permselectivity as a function of a feed composition for the binary preliminary tests; Sterlitech I (a), Sterlitech II (b), low pressure membrane (c), and high pressure membrane (d). (a) (b) Fig. 5. Permeance of CO2, H2S, and CH4 (a) in the low pressure membrane and the corresponding permselectivities (b). Lines are plotted as a guide to the eye. experiments. However, the permselectivity of carbon dioxide over methane increased maximally to 2.0 after this treatment. This permselectivity was still far too low to be suitable for effective biogas purification. The surface structure of the composites did not change significantly after the usage (Fig. 2) and the cross section clearly demonstrated a porous structure, as discussed in 114 P. Dolejš et al. / Separation and Purification Technology 131 (2014) 108–116 Table 5 Permeate and retentate flow ratesa for the low pressure membrane at 21 °C. The feed was 32 mL/min and saturated by water vapour at 37 °C, nitrogen as the sweeping gas was 10 mL/min. Compound a b pabsolute Retentate (kPa) (mL min1) CH4 CO2 H2S 130 14.9 8.9 1.7  102 CH4 CO2 H2S 140 CH4 CO2 H2S CH4 CO2 H2S Permeate CH4 loss Retentate (vol.%) (vol.%, ppm)b Permeate 2.3 5.9 1.3  102 13.4 62.6 37.3 699 27.8 72.0 1645 14.8 8.3 1.5  102 2.4 6.5 1.5  102 14.0 64.0 35.9 654 27.2 72.6 1679 170 14.2 6.1 1.1  102 3.0 8.7 2.0  102 17.4 69.8 30.1 523 25.8 74.0 1665 220 13.3 3.4 5.4  103 3.9 11.4 2.5  102 22.7 79.6 20.3 325 25.6 74.2 1609 Sweeping gas eliminated. Methane and carbon dioxide in vol.% while hydrogen sulphide in ppm. (a) (b) Fig. 6. Permeance of CO2, H2S, and CH4 (a) in the high pressure membrane and the corresponding permselectivities (b). Lines are plotted as a guide to the eye. Table 6 Permeate and retentate flow ratesa for the high pressure membrane at 21 °C. The feed was 32 mL/min at 37 °C, nitrogen as the sweeping gas was 10 mL/min. Compound a b pabsolute Retentate (kPa) (mL min1) CH4 CO2 H2S 200 17.0 14.4 2.8  102 CH4 CO2 H2S 300 CH4 CO2 H2S CH4 CO2 H2S Permeate CH4 loss Retentate (vol.%) (vol.%, ppm)b Permeate 0.1 0.5 2.3  103 0.6 54.0 45.9 886 15.3 84.3 3658 16.8 12.9 2.4  102 0.2 2.0 6.5  103 1.2 56.6 43.3 793 9.3 90.4 2893 400 16.7 10.2 1.7  102 0.4 4.7 1.3  102 2.3 62.1 37.9 635 7.2 92.5 2538 500 16.5 7.7 1.3  102 0.5 7.2 1.7  102 2.9 68.1 31.8 540 7.0 92.8 2189 Sweeping gas eliminated. Methane and carbon dioxide in vol.% while hydrogen sulphide in ppm. Section 3.1. The low permselectivites indicated that the water was apparently not capable to form a fully dense film in the upper selective layer. Water bonding in hydrophilic polymers is usually a nonideal process leading to plasticization and/or clustering phenomena [22]. The first sorbed water molecules interact with polymer-specific sites [22,23], whereas water–water interactions become predominant at a high water activity [24]. Water tends to nucleate as ionic clusters at the membrane hydrophilic sites to an extent which depends on the ratio water/specific sides in the polymer [23]. In polyamides, the water molecules probably P. Dolejš et al. / Separation and Purification Technology 131 (2014) 108–116 associate with the amido groups in the skin layer. From the EDX analysis, it seems that a higher nitrogen content in the TFC membranes is more effective for carbon dioxide removal from biogas. The possible explanation of the inability to selectively separate the model biogas mixture is that no dense selective membrane was obtained for the first two composites. Either poor swelling or incomplete wetting of the TFC membranes caused the presence of open micropores in the TFC structure that were not filled by water. In general in terms of transport resistance, the support only becomes important for thin film composites based on very highly permeable polymers [25]. In terms of membrane integrity, the pores of the support must be sufficiently small to guarantee sufficient mechanical resistance, also in the case of significant swelling. The latter is a problem for the non-selective membranes Sterlitech I and Sterlitech II. Moreover, for the composite membranes that were not selective enough, the SEM analyses showed that the cross section morphology can be irreversibly changed during the drying (Fig. 2). The irreversibility of the morphology changes was checked and confirmed after the drying of a composite for one week and one month. 3.2.1.2. Low pressure and high pressure RO membranes. The preliminary binary tests with the low pressure and high pressure RO membranes showed that these membranes could be potentially used for biogas purification. The highest selectivities between methane and carbon dioxide were measured for the binary feed mixture containing higher amount of methane, a model for raw biogas obtained from sewage plants. The successful biogas upgrading by water–swollen membrane was reported previously [8] for biogas obtained from a sewerage plant. Binary mixtures containing from 55 to 60 vol.% methane yielded similar results (Table 2 and Fig. 4c and d). In contrast, the separation of the agro-biogas model mixture with 53 vol.% methane appeared to be more difficult, possibly due to the higher carbon dioxide concentration that caused a coupling effect or polarization phenomena. For further studies a representative agro-biogas model mixture was chosen. The mixture contained 940 ppm of hydrogen sulphide, which is a typical value for raw biogas [26]. The influence of the operating pressure was discussed for two representative TFC membranes. 3.2.2. Agro-biogas upgrading 3.2.2.1. Low-pressure RO membrane. The membrane performance was studied in the pressure range from 130 to 220 kPa (Fig. 5). The permeance of hydrogen sulphide and carbon dioxide significantly increases in the pressure range from 130 to 170 kPa, above which, a plateau is reached for both gases. Since the methane permeance is virtually pressure independent, the corresponding selectivities with respect to methane show the same trend as the permeance. Interestingly, H2S shows a nearly identical trend as CO2 the permselectivity of hydrogen sulphide over carbon dioxide is close to 1 and shows a marginal decrease, probably caused by a weak coupling effect. Thus, in a single step up to 65 vol.% of H2S and 59 vol.% of CO2 can be removed at 170 kPa, increasing to 82 vol.% of H2S and 77 vol.% of CO2 at 220 kPa. The methane concentration in the retentate increased with pressure, up to 80 vol.% (Table 5). The loss of methane to the permeate stream was still 20 vol.%, necessitating the use of more selective TFC membranes. 3.2.2.2. High-pressure RO membrane. The gas permeances and corresponding selectivities of the high pressure membrane in the pressure range from 200 to 500 kPa (the upper pressure limit of the apparatus) are displayed in Fig. 6. The permeance of carbon dioxide and hydrogen sulphide increased three and six fold, respectively, while the corresponding selectivities with respect to methane increased about two and three fold up to 400 kPa, then levelling 115 off. Under these conditions there is a modest increase of methane permeability with increasing pressure, whereas the H2S/CO2 permselectivity decreases because of the coupling effect. It is possible to remove from 43 vol.% of H2S and 32 vol.% of CO2 at 400 kPa to 57 vol.% of H2S and 48 vol. of CO2 at 500 kPa. The hydrogen sulphide concentration decreased from the initial value of 940 ppm to 540 ppm in the retentate, which is normally considered as a ‘‘safe value’’ for engines of cogeneration units [27]. The methane permeance was again the lowest due to its significantly lower solubility in water in comparison with CO2 and H2S. The methane concentration in the retentate increased with the feed pressure, up to a maximum of 68 vol.% at 500 kPa (Table 6). Due to its higher selectivity, for this membrane the loss of methane in the permeate stream was only 3 vol.%, much less than in the case of the low pressure membrane. In a previous study a methane content in the retentate stream of 95 vol.% was obtained for binary feed containing only CH4 and CO2 [8], using a TFC membrane with a thickness of the upper layer of 33 lm. It seems that the permselectivity improves when the thickness of the upper TFC layer decreases. The key parameter of the separation process is the chemical potential difference, which is a function of the pressure, temperature and concentration of the permeants. The different solubility of gases in selective water swollen polyamide layer on the membrane surface is the main responsible for the permselectivity of the membranes [8]. The water solubility reported in the NIST database at 21 °C for hydrogen sulphide, carbon dioxide, and methane is 111.0, 37.9  102, and 1.51 mmol of gas per 1 kg water at 1 bar, respectively. In this work, the occurrence of the solution-diffusion transport mechanism was thus supported by the higher hydrogen sulphide permeability compared to that of carbon dioxide, in accordance with its one order of magnitude higher solubility in water. It can be anticipated that solubility is the main factor contributing to the high selectivity for CO2 and H2S in the water swollen membranes. Secondly, based on the smaller kinetic diameters of CO2 (3.3 Å) and H2S (3.6 Å) compared to CH4 (3.8 Å), the membrane is also slightly size-selective, in favour of CO2. Upon further optimization, the present separation method with TFC membranes may offer cheaper future possibilities in comparison with conventional methods for simultaneous removal of H2S and CO2 by water scrubbing [28]. The water–swollen membrane can be used for complete purification or for biogas pre-treatment in order to protect other membranes used for the biogas upgrading because their separation properties can deteriorate by the longer exposure to hydrogen sulphide and carbon dioxide [29–32]. 4. Conclusion The simultaneous removal of carbon dioxide and hydrogen sulphide was studied for agro-biogas pre-treatment, using water– swollen hydrophilic thin film composites based on membranes supplied by two industrial producers. The hydrogen sulphide permeance was higher than that of carbon dioxide, owing to its higher solubility in water. The advantage of this membrane separation is that no pre-treatment of the feed gas is needed to remove the water vapour before the gas separation step, unlike other for instance glassy polymer membranes, which often lose performance due to plasticization by water vapour. The TFC membranes showed good permselectivities when the feed stream contains at least 85% relative humidity, which condenses onto the membrane surface and guarantees that the membrane remains continuously wet during its operation. Moreover, the presence of humidity in the feed stream is necessary for the separation mechanism, which relies on a much higher solubility of H2S and CO2 in the water sorbed into the TFC membrane in comparison with CH4. 116 P. Dolejš et al. / Separation and Purification Technology 131 (2014) 108–116 Comparison of obtained permselectivities with membrane characteristics suggests that a good water–swollen membrane should have the following features: – high porosity of the membrane support layer, – significant decrease of the size and number of the biggest pores caused by water swelling, – good swelling of the selective polyamide skin layer, – presence of a sufficient number of amido groups inside the upper layer of the hydrophilic composite. Acknowledgements P. Izák would like to acknowledge the financial support from the Czech Science Foundation by the grant No. 14-12695S. Z. Sedláková would like to acknowledge the Technology Agency of the Czech Republic (project TE01020080) and the Ministry of Education, Youth and Sports (project LH-14006). All gases were financed from the Grant Fecundus (RFCRCT-2010-00009) – Research Fund for Coal and Steel of EC and MŠMT (Ministry for Education, Youth and Sport) No.: 7C11009 and some parts in the equipment were bought thanks to the financial support of the MŠMT by the project LD-13018. P. Dolejš and J. Vejražka would like to acknowledge the grant P106/10/1194. Financial support of the Italian National Program, ‘‘Programma Operativo Nazionale Ricerca e Competitività 2007–2013’’, project PON01_01840 ‘‘MicroPERLA’’ is also gratefully acknowledged. Lenka Morávková and Hana Šnajdaufová are gratefully acknowledged for their assistance with some of the experiments. References [1] M.Y. Pavlov, A new energy paradigm for the third millennium, World Affairs Spring 10 (1) (2006) 12–27. [2] A.K. Agarwal, Biofuels (alcohols and biodiesel) applications as fuels for internal combustion engines, Prog. Energy Combust. Sci. 33 (2007) 233–271. [3] M. Asif, T. Muneer, Energy supply, its demand and security issues for developed and emerging economies, Renew. Sustain. Energy Rev. 11 (2007) 1388–1413. [4] S.R. Bull, Renewable energy today and tomorrow, Proc. IEEE 89 (8) (2001) 1216–1226. [5] P. Weiland, Biogas production: current state and perspectives (mini-review), Appl. Microbiol. Biotechnol. 85 (2010) 849–860. [6] A. Demirbas, Progress and recent trends in biofuels, Prog. Energy Combust. Sci. 33 (2007) 1–18. [7] M. Poloncarzova, J. Vejrazka, V. Vesely, P. Izak, Effective purification of biogas by a condensing-liquid membrane, Angew. Chem. Int. Ed. 50 (2011) 669–671. [8] M. Karászová, J. Vejražka, V. Veselý, K. Friess, A. Randová, V. Hejtmánek, L. Brabec, P. Izak, A water–swollen thin film composite membrane for effective upgrading of raw biogas by methane, Sep. Purif. Technol. 89 (2012) 212–216. [9] A. Demirbas, Biofuels sources, biofuel policy, biofuel economy and global biofuel projections, Energy Convers. Manage. 49 (2008) 2106–2116. [10] J. Wang, J. Zhang, H. Xie, P. Qi, Y. Ren, Z. Hu, Methane emissions from a fullscale A/A/O wastewater treatment plant, Bioresour. Technol. 102 (9) (2011) 5479–5485. [11] M.R.J. Daelman, E.M. van Voorthuizen, U.G.J.M. van Dongen, E.I.P. Volcke, M.C.M. van Loosdrecht, Methane emission during municipal wastewater treatment, Water Res. 46 (11) (2012) 3657–3670. [12] H. Ozgun, R.K. Dereli, M.E. Ersahin, C. Kinaci, H. Spanjers, J.B. van Lier, A review of anaerobic membrane bioreactors for municipal wastewater treatment: integration options, limitations and expectations, Sep. Purif. Technol. 118 (2013) 89–104. [13] S.A.M. Marzouk, M.H. Al-Marzouqi, M. Teramoto, N. Abdullatif, Z.M. Ismail, Simultaneous removal of CO2 and H2S from pressurized CO2–H2S–CH4 gas mixture using hollow fiber membrane contactors, Sep. Purif. Technol. 86 (2012) 88–97. [14] B. Kraftschik, W.J. Koros, J.R. Johnson, O. Karvan, Dense film polyimide membranes for aggressive sour gas feed separations, J. Membr. Sci. 428 (2013) 608–619. [15] P. Bernardo, E. Drioli, G. Golemme, Membrane gas separation: a review/state of the art, Ind. Eng. Chem. Res. 48 (2009) 4638–4663. [16] R.W. Baker, K. Lokhandwala, Natural gas processing with membranes: an overview, Ind. Eng. Chem. Res. 47 (2008) 2109–2121. [17] R.W. Baker, Membrane Technology and Applications, third ed., John Wiley & Sons Ltd., New York, United States, 2012. [18] A. Molino, M. Migliori, Y. Ding, B. Bikson, G. Giordano, G. Braccio, Biogas upgrading via membrane process: modelling of pilot plant scale and the end uses for the grid injection, Fuel 107 (2013) 585–592. [19] F. Bauer, T. Persson, C. Hulteberg, D. Tamm, Biogas upgrading – technology overview, comparison and perspectives for the future, Biofuels, Bioprod. Bioref. 7 (2013) 499–511. [20] P. Izák, M. Poloncarzová, J. Vejrazka; Institute of Chemical Process Fundamentals, Czech Republic. ‘‘The method and the apparatus for methane enrichment of biogas from sewage plant and agriculture’’ PCT-437 2011. [21] O. Šolcová, V. Hejtmánek, H. Šnajdaufová, P. Schneider, Liquid expulsion permporometry – a tool for obtaining the distribution of flow-through pores, Part. Part. Syst. Char. 23 (2006) 40–47. [22] S. Despond, E. Espuche, N. Cartier, A. Domard, Hydration mechanism of polysaccharides: a comparative study, J. Polym. Sci., Part B: Polym. Phys. 43 (2005) 48–58. [23] P.B. Balbuena, E.J. Lamas, Y. Wang, Molecular modeling studies of polymer electrolytes for power sources, Electrochim. Acta 50 (2005) 3788–3795. [24] A. Stathopoulos, P. Klonos, A. Kyritsis, P. Pissis, C. Christodoulides, J.C. Rodriguez Hernández, M. Monleón Pradas, J.L. Gómez Ribelles, Water sorption and polymer dynamics in hybrid poly(2-hydroxyethyl-co-ethyl acrylate)/silica hydrogels, Eur. Polym. J. 46 (2010) 101–111. [25] D.L. Meixner, P.N. Dyer, Characterization of the transport properties of microporous inorganic membranes, J. Membr. Sci. 140 (1998) 81–95. [26] N. Abatzoglou, S. Boivin, A review of biogas purification processes Biofuels, Bioprod. Bioref. 3 (2009) 42–71. [27] P. Lens, P.N.L. Lens, L.H. Pol, Environmental Technologies to Treat Sulfur Pollution: Principles and Engineering, IWA Publishing, 2000. [28] S. Basu, A.L. Khan, A. Cano-Odena, C. Liu, I.F.J. Vankelecom, Membrane-based technologies for biogas separations, Chem. Soc. Rev. 39 (2010) 750–768. [29] A. Makaruk, M. Miltner, M. Harasek, Membrane biogas upgrading processes for the production of natural gas substitute, Sep. Purif. Technol. 74 (2010) 83–92. [30] C.A. Scholes, S.E. Kentish, G.W. Stevens, Effects of minor components in carbon dioxide capture using polymeric gas separation membranes, Sep. Purif. Rev. 38 (2009) 1–44. [31] C.A. Scholes, G.W. Stevens, S.E. Kentish, The effect of hydrogen sulfide, carbon monoxide and water on the performance of a PDMS membrane in carbon dioxide/nitrogen separation, J. Membr. Sci. 350 (2010) 189–199. [32] Y. Xiao, B.T. Low, S.S. Hosseini, T.S. Chung, D.R. Paulc, The strategies of molecular architecture and modification of polyimide-based membranes for CO2 removal from natural gas – a review, Prog. Polym. Sci. 34 (2009) 561–580.