Polymer membranes for acid gas removal from natural gas

Polymer membranes for acid gas removal from natural gas

Accepted Manuscript Polymer Membranes for Acid Gas Removal from Natural Gas Gigi George, Nidhika Bhoria, Sama AlHallaq, Ahmed Abdala, Vikas Mittal PII...

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Accepted Manuscript Polymer Membranes for Acid Gas Removal from Natural Gas Gigi George, Nidhika Bhoria, Sama AlHallaq, Ahmed Abdala, Vikas Mittal PII: DOI: Reference:

S1383-5866(15)30402-0 http://dx.doi.org/10.1016/j.seppur.2015.12.033 SEPPUR 12758

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

28 September 2015 19 December 2015 21 December 2015

Please cite this article as: G. George, N. Bhoria, S. AlHallaq, A. Abdala, V. Mittal, Polymer Membranes for Acid Gas Removal from Natural Gas, Separation and Purification Technology (2015), doi: http://dx.doi.org/10.1016/ j.seppur.2015.12.033

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Polymer Membranes for Acid Gas Removal from Natural Gas Gigi George, Nidhika Bhoria, Sama AlHallaq, Ahmed Abdala, Vikas Mittal * The Petroleum Institute, Abu Dhabi, United Arab Emirates. Abstract The use of polymeric membrane technology is an exciting approach towards the removal of acid gases, namely, carbon dioxide and hydrogen sulfide, from natural gas streams. Polymer membranes exhibit good mechanical, thermal and chemical stabilities. These membrane materials can also possess desirable transport properties such as high permeability and selectivity. A number of studies have attempted to improve these properties without compromising the advantages of existing technologies. Various preparations and structures of polymer membranes were reviewed. The structure-property analyses of these polymer membranes used for acid gas removal followed by their selectivity-permeability relationship and economic aspects are considered here. Keywords: Acid gas removal, polymer membrane, hydrogen sulfide, carbon dioxide, natural gas

1. Introduction Natural gas has been a popular energy source for many decades, and its demand as a fuel is continuously increasing worldwide.[1] Many of the gas reserves worldwide contain low-quality natural gas with high levels of impurities or contaminants. Although some of these gas reserves were discovered decades ago, they were not developed due to the lack of economically feasible purification technologies. However, with the increase in natural gas prices and natural gas demands, many countries have directed their focus towards those low-quality gas reserves. The development of the new low-quality gas fields requires a more complicated series of processes to produce sales gas as per the market’s specifications. This in turn demands the development of new technologies that can cope with the increase in impurities while maintaining the economic feasibility of developing the gas plant. Commercial natural gas, mostly methane, has the highest heating value per unit mass (approximately 21,520 BTU/lbm = 50.1 MJ/kg, LHV) when compared to other hydrocarbon fuels such as butane, diesel fuel and gasoline. Additionally, it has the lowest carbon content per unit mass; thus, upon combustion, methane releases approximately 30% less carbon dioxide than oil and 43% less carbon dioxide than coal.[2] *Author to whom correspondence may be addressed: Vikas Mittal; E-mail: [email protected]

Thus, methane is the cleanest hydrocarbon fuel source. Raw natural gas, however, is composed of methane with a variety of other components, including higher hydrocarbons, water, acidic gases (carbon dioxide and hydrogen sulfide), and other impurities such as mercaptans (R-SH), helium, and nitrogen. For natural gas to reach sales gas specifications, the typical conditioning and processing chain includes inlet separation, sweetening, mercury removal, dehydration, natural gas liquids recovery, and, finally, compression for transportation. In addition, a condensate stabilization section is needed to recover the light gases from liquid hydrocarbons, and a sulfur recovery plant is needed to recover sulfur from the hydrogen sulfide (H2S) removed from the raw gas. There is a prediction of considerable growth in the natural gas sector over the next two decades; natural gas is also occasionally predicted to even overtake other conventional fossil fuels (e.g., oil) as the main fuel between 2020 and 2030.[3-5] At the end of 2013, the known worldwide natural gas reserves were reported to be approximately 185.7 trillion cubic meters, a volume that is estimated to be sufficient for 55.1 years of global demand.[4] Natural gas is therefore expected to have a significant impact on the industrial, transport, power, residential and commercial sectors.[6, 7] As H2S is a highly toxic gas[8], there were serious concerns about the safety tests to analyze the amount present in natural gas. Most of the literature reportedly used gas permeation test rigs under high levels of operation safety. We would like to highlight the standard American Society for Testing and Materials (ASTM) test method for the analysis of hydrogen sulfide in gaseous fuels (lead acetate reaction rate method), which is ASTM D4084-07(2012). This method is used in the industry to determine the concentration of hydrogen sulfide and to verify compliance with operational and environmental needs. The sales gas criteria as per the US pipeline specifications are as follows: (1) minimum gross calorific value of 950 BTU/SCF, (2) maximum water content of 7.0 lbs/mmcf, (3) maximum H2S content of 4 ppm vol., (4) maximum total inert gases content of 4% (maximum CO2 content of 2%), and (5) maximum hydrocarbon dew point of -10°C at operating pressure. The calorific value is the main parameter to represent the sales gas because it quantifies the energy that can be obtained from the gas as a fuel. Additionally, it determines the price of the produced gas in the market. To maintain the calorific value higher than the limit, inert gases such as nitrogen and carbon dioxide must be removed to a maximum of 4 mol%, of which only a maximum of 2 mol% carbon dioxide is allowed. Carbon dioxide has to be removed not only for the sake of the calorific value but also due to its corrosiveness in the presence of water, where it forms a weak acid. Similarly, hydrogen sulfide forms a weak acid in the presence of water and results in a highly corrosive environment. However, hydrogen sulfide creates a more serious problem; it is toxic to humans at ppm levels, and it causes instant death at 1000 parts per million (ppm). The water content should also be decreased to avoid water condensation, hydrate formation and blockage in the pipelines. Finally, the hydrocarbon dew point must be maintained to avoid selling heavier hydrocarbons with the gas because they can make higher revenue/profit if recovered and sold as natural gas liquids. All of the processes in the natural gas conditioning chain are important to achieve those four main criteria of pipeline specification. The removal of acid gases, including CO2 and H2S, is challenging, and the existence of these gases in the natural gas increases the risk associated with the gas plant and requires the usage of special materials that can withstand the corrosive environment. Various technologies have been identified for acid gas removal from natural gas; including using a liquid desiccant to absorb the acidic gases, using a solid desiccant to adsorb the acidic gases, cryogenic distillation, direct conversion by chemical reactions, and membrane 2

separation. The widely applied sweetening method is amine absorption, where alkaline amine solution is used to absorb the acidic gases in a high-pressure column. However, the focus of this review is gas sweetening using membranes. Gas separation by membranes initially emerged as an industrial application in the 1980s, and the first major membrane production was for hydrogen separation. A few years later, membrane technology was introduced for use in nitrogen separation from air and carbon dioxide removal from natural gas. Since then, many studies have been conducted to utilize membranes for gas separation in various applications, including acid gas removal from natural gas. As per the industrial expectations and due to the increase in demand for natural gas, membrane separation technology is expected to flourish more in the coming decade when low-quality gas reserves are expected to be developed. This is because membrane technology is an excellent candidate for removing high concentrations of acid gas, and it competes strongly with other technologies for bulk acid gas removal. Additionally, with regard to economics, the operating costs for the current absorption-based methods are directly proportional to the amount of acid gases in the feed gas.[9] However, for membrane systems, the acid gas concentration in feed affects only the capital cost of membrane modules, while the operating cost is minimal because the plant runs almost unmanned. Until recently, membranes have been limited to the removal of carbon dioxide from natural gas. Membranes are now becoming competitive for other applications (e.g., separation of nitrogen, hydrogen sulfide, and natural gas liquids) of natural gas processing.[10] New membrane materials and configurations can exhibit better efficiency and offer more stability towards the contaminants found in natural gas. When selecting a membrane material for a specific separation, a number of factors must be considered, including a favorable combination of the required permeability and selectivity and the mechanical and chemical properties of the membrane. Inorganic membranes can be categorized as porous or dense depending on the structure of the membrane material.[11, 12] In porous inorganic membranes, a thin layer of porous material is laid on top of a porous metal or ceramic support, which provides mechanical strength while offering minimum resistance to mass transfer. Carbon, glass, alumina, zeolite, silicon carbide, titania and zirconia membranes are the main candidates for use as porous inorganic membranes supported by substrates such as zirconia, zeolite, α-alumina, γ-alumina and porous stainless steel. There are various advantages and disadvantages of inorganic membranes compared with polymeric membranes.[13] Inorganic membranes are highly stable at high temperatures and can be resistant to corrosive and environmentally harsh conditions. For instance, zeolite membranes are microporous in nature, with crystalline alumina silicate membrane pores having uniform sizes. They separate gaseous components based on a molecular sieving mechanism. The molecular sieving principle requires pinhole and crack-free zeolite membranes. They have relatively lower gas fluxes in comparison to other inorganic membranes. Thus, thicker zeolite layers are required to obtain pinhole-free and crack-free zeolite layers. Thermal effects also present a disadvantage. At higher temperatures, the zeolite layer shrinks while the support which keeps expanding, usually resulting in thermal stress problems. An ideal zeolite membrane should be thermally stable, solvent resistant and have perfect shape selectivity. [14] For the polymeric membranes, main mechanism of gas transport is based on solution-diffusion mechanism. The model works in the way that the dissolution of membrane material happens followed by the diffusion. Different amounts of material get dissolved across the polymer 3

membrane and thus the separation occurs through a three-step process such as dissociation, diffusion and desorption[15, 16]. An important focus in the last decade was on utilizing membranes for carbon dioxide removal from flue gases to reduce its effect as a greenhouse gas and from natural gas to achieve pipeline specifications. According to the technical report from UOP Honeywell, the membranes for CO2 removal from natural gas are a fully established technology in the oil and gas industry. One of the world’s largest membrane plants for CO2 removal is designed to decrease the CO2 content in natural gas from 45% to 6% for 680 MMSCFD feed in Malaysia, and it began operation in 2007.[17] In addition, new designs are approaching one BCFD of natural gas as a feed to membrane plants.[18] This success in CO2 removal at high concentrations shows the potential of utilizing membranes for both H2S and CO2 separation. Many studies have been performed to identify the transport properties of CO2 through different polymeric membrane materials. Those polymeric materials usually had the same tendency to allow permeation of H2S along with CO2. However, limited research was carried out to identify the transport properties of H2S due to its toxicity. This forms an obstacle for H2S testing and necessitates special testing procedures and precautions in laboratories. Despite that and due to the increased interest in utilizing membranes for H2S gas removal, intensive safety precautions are taken, and research is being conducted to determine the transport properties of H2S with respect to promising membrane materials. Indeed, considerable focus has been applied recently to developing new membrane materials for H2S removal. Not only the membrane transport properties are considered but also the material processability, chemical and mechanical stability and the fabrication cost. The key required properties for a membrane to be suitable for gas separation are as follows:[19, 20] high gas permeation rate for the most permeable (the target of the separation) gas, high selectivity of the target gas against other compounds in the feed, manufacturing reproducibility, cost effectiveness and ability to be cheaply manufactured into different membranes modules, mechanical stability under high operation pressures (aging resistance), tolerance to contaminants and moderate temperature excursion (thermal and chemical stability) and plasticization resistance. The first two criteria are the most important ones that drive the rest; a high permeation flux requires a lower membrane area and leads to a lower-cost membrane system, and a high selectivity requires a lower driving force to achieve the desired separation and corresponds to a lower operating cost of the membrane system.[19, 21] The article by R.W. Baker represents the milestones in the development of membranes for gas separation, starting from defining Graham’s law of diffusion until the installation of membrane modules in operating plants. This article was limited to the generalized concepts of membrane structure and fabrication to propose the potential applications of membranes in natural gas treatment. However, the article left a gap with regards to material design, which we are addressing in this review, specifically for acid gas removal using polymer materials. [22] Baker’s book provides a detailed overview of the history of membranes for various applications.[23] A recent review published in 2015 by Jeon et al. focused on polyimide and polysulfone compounds for the separation of carbon dioxide/methane (CO2/CH4). [24] Our review examines the simultaneous separation of carbon dioxide, hydrogen disulfide and methane, which is important from a practical perspective for natural gas purification, using polymer membrane technology. Upon examining the literature published on membrane gas separation, four different fields can be identified: development of new membrane materials and the characterization thereof, membrane area calculation, process simulation and optimization, and membrane economics. Most of the research has been done on developing new membrane materials; as it 4

is the key element in the success of membrane separation. We have summarized this work in table 1, where various polymer membranes and membrane materials used for the acid gas removal from natural gas are tabulated with their respective permeability and selectivity data. 2.

Challenges of membrane technology for natural gas sweetening

Despite of the numerous advantages of current commercial membranes, these systems are blamed to perform at lower efficiency than amine systems for acid gas removal for a number of reasons such as the presence of contaminants, concentration polarization permeability/selectivity trade-off, physical aging and plasticization.[22] 2.1 Contaminants and pre treatment It is important for all membrane systems to have a proper pre-treatment of the natural gas feed stream entering to ensure better performance and efficiency. There are a number of contaminants present in natural gas which can lead to a decline in performance. One of them is the moisture content which causes the swelling and henceforth the destruction of the whole membrane integrity. If BETEX and heavy hydrocarbons (C6+) happen to present in membrane system, will lead to the formation of a film around the membrane surface and thus leading to drastic drop in permeation rates. There are several corrosion inhibitors and additives used for various offshore activities that are destructive for the membrane integrity. Particulate material, if present can block the membrane flow area which depends upon module to module. The spiral wound module is having fewer blockages than hollow fibre module. But it is expected that the long term particle flow into any membrane module will block it eventually. While considering the pre-treatment system for membrane gas separation, contaminants must be removed and must ensure that liquids will not be formed within the membranes themselves. Condensation can be prevented by achieving a predetermination of dew point before the membrane and then heating the gas enough to provide sufficient margin of superheat.[25] Uddin et al reported the influence on different impurities such as TEG, MEG, HHCs, and H2S on membrane performance [26]and their recently developed Hybrid FSC membranes shows great potential in natural gas sweetening. [27] 2.2 Concentration polarisation Concentration polarization is building up of concentration gradient in the boundary layer of membrane system. This phenomenon can be seen in all membrane separation processes because of the selective permeability of membrane. It leads to an inverse relation between the available driving force and the permeable species leading to adverse effects. This means that there is decrease in available driving force for species with more permeability and vice versa. This in turn reduces the overall efficiency of separation and raises the total cost including the capital and operation cost. The major factor which affects the concentration polarization is the permeation rate. With more permeation rate, there will be more serious concentration polarization. Also, for a given permeation rate, the membrane with higher separation factor will experience more concentration polarization. By increasing the gas feed velocity we can lower its effect but cannot be eliminated. For a fixed pressure across membrane, the effect is relatively small on actual operation pressure. But with change in gas feed pressure for a fixed pressure difference across membrane, there will be change in both the permeate flux as well as concentration polarization. Generally the gas composition has no effect on the overall concentration polarization. Thus the accountability of this phenomenon is important while doing membrane module designing.[28, 29] 5

2.3 Permeability/selectivity trade-off (the upper bound) The trade-off of permeability and selectivity is the main hurdle faced by polymeric membranes. An inverse relationship exists between permeability and selectivity, which indicates that the selectivity of a membrane to different gas pairs can be increased only with a corresponding decrease in gas permeability.[30-32] This permeability/selectivity trade-off was demonstrated by Robeson in 1991 and thus proving the major drawback in the commercialisation of polymeric membranes. It has been observed that there is an upper bound of permeability and selectivity in polymeric membranes. That study analysed a vast number of amorphous polymers with high glass transition temperature which were reported over a period of four to five decades of time. Figure 6 shows the CO2 /CH4 upper bound relationship for glassy and rubbery polymers as elucidated by Robeson in 1991 and revised in 2008. The study was based on the molecular diameter of gas molecules and it was concluded that the diffusion coefficient of the constituent gases governs the membrane separation competencies. The correlation between the polarity of gases and its relation to permselectivity properties were also discussed in the study. This inverse relationship has been reported in a number of studies.[33, 34] A graphical representation of this upper bound has been reported by Robeson, and this representation is referred to as a benchmark for the development of gas permeation membranes. [16, 35, 36] Until now, very few membranes have been able to override this benchmark.[37] A very recent 2015 study by Lin aimed at quantitatively interpreting and thereby redefining the upper bound of the separation performance using polymer membranes and free volume theory. Pure and mixture of gases were used and it was observed that the CO2 induced plasticization can result in the reduction in glass transition temperature and increase in free volume in the polymer–CO2 systems.[38] 2.4 Physical aging Physical aging of polymeric membranes reduces gas permeability and alters other polymer physical properties, including enthalpy, entropy and specific volume.[39-48] Aging slows with time for two main reasons; firstly when the excess free volume decreases the driving force for physical aging decreases, and secondly the reduction in free volume decreases the mobility of the polymer chain, which in turn decreases the segmental motions required to reorganise the polymer chains. The dependence of physical aging on polymer thickness is widely recognised, especially when the thickness is very less. The effects of thickness on aging are relevant concerns for polymeric gas separation membranes because the thicknesses of these membranes are often on the order of 0.1 mm [49]. The operations of current commercial membranes are limited to near-ambient temperatures. Additionally, the performance of commercial membranes degrades over time and corrosive and high temperature environments are not at all suitable.[50, 51] The decrease in the efficiency of polymeric membranes with time is dependent on a number of factors, including thermal instability, compaction, fouling and chemical degradation etc. 2.5 Plasticization Both CO2 and H2S are strong plasticization agents. A quantitative understanding of plasticization was studied by Zhang et al. The CO2 induced plasticization of polyimide membranes was studied where it performs in such a way that the permeability increases with time. It was shown that the preferential sorption sites are the imide groups. The ether and CF 3 groups are the other sorption sites. The plasticisation effects at different loadings were also determined and the effect of ether group in supressing the plasticization was also established. [52] Highly permeable cPIM-1, Torlon and cPIM-1/Torlon membranes where prepared by Yong where the improvements that are closer to Robeson upper bound were attributed to hydrogen bonding and 6

charge transfer complexes. These membranes exhibited high plasticization resistance up to 30 atm pressure. [53] On a recent study by Wang, the plasticization characteristics of thermally reduced (TR) polymers were traced using thin and thick films over hundreds of hours. It was observed that CO2 permeability of thick films did not show significant decline over time. The thick TR films showed accelerated ageing when exposed to C2H4, C2H6 and C3H8 and polymer matrix is rigorously plasticized by the condensable gases.[54] Plasticization in glassy polymeric membranes is a complex process because of the nonequilibrium state of the polymer used.[55-57] The extent of plasticization depends on a number of factors, including membrane material, morphology, thickness, temperature, pressure, feed composition and the types of permeating gases used. Low molecular weight compounds are often added to glassy polymers to improve the processability of the polymer.[58] These low molecular weight compounds are referred to as plasticisers, and their addition results in a more flexible polymer with an increased rate of segmental motion. This mechanism of increased chain flexibility is termed as ‘plasticization’.[59-62] Enhanced inter-segmental polymer mobility can severely affect separation performance, reduce the mechanical strength of the membrane and speed up the aging process, resulting in membrane failure.[63, 64] Some studies have also reported that plasticization helps during fabrication by increasing the fractional free volume of membrane.[62] Resistance against plasticization in polymeric membranes can be developed in a number of ways, including crosslinking, heat treatment, blending or reactively forming interpenetrating networks.[11, 65-67] Plasticization or swelling of the polymeric membrane matrix by highly plasticizing mixed gases is a big concern for most of the polymeric materials present so far. Scholes et al. reported an average decrease of 8% in CO2 permeability in presence of H2S. This was because of the polymer matrix plasticization by H2S which reduces the gas transport resistance leading to greater flux. [68] 2.6 Membrane compaction at high pressure The challenge of membrane compaction at high pressure may cause irreversible damage to the membranes. High feed pressures cause physical damage to the porous support membrane thereby damaging them. [69, 70] Most of the polymeric membranes are comprised of the porous structures either as separating layer or as mechanical support to composite membranes. The material characteristics as well as bulk porosity decides the mechanical stability of the porous membranes. Structures with micro voids are less affected by compaction than the sponge like structures. Higher operating pressures reorganize the macro void membrane structure resulting in increased hydraulic resistance as well as reduced void volume [71]. Bulk layer where most of the large pores and macro voids i.e. pore volume is present gets severely affected by the compaction.[72] Surface deposits in a membrane can be easily removed by chemical cleaning but membrane compaction is irreversible resulting in higher long-term operating costs.[73] Ebert et al. reported significant differences in compaction behavior using TiO2 with PVDF confirming that compaction of porous structure under pressure can be reduced by using organic-inorganic blends or composites to improve the mechanical strength.[70] 2.7 Membrane cost The cost of membrane depends on a number of factors such as like type of module, its material, labour, equipment, energy and quality cost [74]. This aspect is discussed in detail at the section 5 of this article; the economics and process optimization of membrane technology.

3. Polymeric membranes 7

Polymer membranes were traditionally developed for packaging applications. In the 1950s; polymer-based membranes made a large impact on natural gas treatment. Most of the available literature on acid gas transport properties, especially for H2S gas transport and polymer films, dates back to the 1950s and 1960s.[75-77] The available literature implies the need for novel materials that can efficiently remove acid gases and make natural gases that meets the US gas specifications.[78] The transport properties of acid gases, which are mainly based on their permeability, selectivity, solubility and diffusion coefficients in various polymers, need to be explored. Earlier studies reported by Robb in 1968 on acid gas permeability discussed the use of polydimethylsiloxane (PDMS) at 25°C.[79] Another approach in 1989 by Stern studied PDMS to determine the gas permeability coefficients at 55°C. These authors found H2S gas to be relatively more permeable than CO2, which was attributed to the higher solubility and condensability of hydrogen sulfide in the polymer. The gas permeability and the ideal selectivity of alkyl-substituted phenoxy-phosphazene-based membrane materials were studied by Orme to validate acid gas permeabilities. This study showed that the orderly chain packing of polymer membranes can be influenced by proper selection of phosphazenes as pendant groups, which in turn affects membrane performance.[80] Gas permeabilities in the range of 12.5 to 54 barrers and 16-20 barrers were obtained for CO2 and H2S, respectively. The selective permeability for CO2/CH4 was in the range of 4 to 12.3 barrers, while that of H2S/CH4 was 4-10 barrers. Gas separation with polymeric membranes emerged as a commercially viable approach in the early 1980s. The work by Vaughn also highlights the difficulty in designing materials with high selectivity towards both CO2 and H2S.[81] Over the last two decades, a significant amount of research was devoted to gas separation. Membrane researchers had to address various issues to attain higher stability, better performance and economic feasibility. Higher-order technologies are believed to contribute to the overall development of membrane technology and its commercial applications. Thus, the current techniques have led to the development of new commercially effective innovative materials for CO2 and H2S separation. A large amount of effort has also been invested into fabricating longer-lasting and defect-free membranes. Lower membrane flux and higher costs are closely related to the thickness of the membrane geometry, and these properties have been explored widely for the development of novel materials. Here, we discuss the various types of polymer membranes that have been used to remove acid gases from natural gas. Glassy polymers are widely used in studies of polymer membranes for acid gas removal. Cellulose acetate is the most widely used material for acid gas removal and has been reported to have a CO2/CH4 selectivity of 19 and a H2S/CH4 selectivity of 24 by Chatterjee et al.[82] These materials showed relatively low CO2/CH4 selectivities at the operating conditions that were typical of gas fields due to the presence of heavy hydrocarbons and swelling-induced plasticization. Polymer membrane technology and its use for natural gas separation are largely limited to chemical laboratories or pilot-scale research. In addition to cost concerns, further commercial use of gas separation membranes appears to be restricted by the membrane’s low selectivity for typical gas mixtures. Thus, there is a strong need for the continued development of gas separation polymer membranes, especially given that the existing technologies, processes and materials are inadequate to address the available opportunities to their fullest extent. It is assumed that using highly selective CO2 and H2S polymer membranes instead of either type of membrane alone can reduce the cost of membrane technology. However, polymer membranes that combine higher stability, flux and selectivity have not yet been experimentally realized. This literature review article shows that H2S gas is relatively more permeable than CO2 as a result of 8

H2S’s higher condensability and solubility in the polymers. Nevertheless, a balance between cost and separation efficiency has yet to be established. This technical review primarily describes polymer membranes that can address two key challenges: first, being capable of achieving higher permselectivity for acid gases, and second, being able to handle complex and aggressive feeds in the field while maintaining their physical and chemical material stability. A large number of studies have demonstrated polymer membrane routes for natural gas purification that are feasible, economic and environmentally friendly alternatives to existing technologies. Research on this aspect is in progress at various academic and industrial laboratories to improve the existing membranes or to invent novel polymer membranes that are highly efficient for natural gas purification while simultaneously reducing the overall cost of the separation process. 3.1 Dense polymeric membranes Most of the reported transport properties for polymer membranes are derived from dense film samples. The problem with most membrane materials is that they do not have adequate resilience to harsh industry conditions, such as high feed pressure/temperature, and as a result, they will fail. Most of the research in this field was thus focused on making resilient materials. Major research objectives include better mechanical and thermal stability along with high resistance to plasticization.[83] In addition to these properties, there is a major demand for dense polymer membrane materials with high selectivity and permeability towards acid gases. Polyamides in particular have excellent separation properties and durable mechanical properties to endure high-pressure natural gas feeds. Hao et al. used the fluorine-containing polyimide 6FDA-HAB to develop a glassy polymer for CO2- and H2S-selective membranes. The structure of the polymer is given below the figure 1. These membranes showed a CO2/CH4 selectivity of 60 and a H2S/CH4 selectivity of 15. Kraftschik reported dense copolymer membranes that are well suited for the instantaneous removal of CO2 and H2S from sour natural gas streams. The copolyimide used in this study was 6FDA-DAM:DABA (3:2), and gas permeation tests were performed at the representative aggressive conditions to replicate the field operations with observed CO2/CH4 selectivities up to 49. Cross-linkable dense films of polyimide-backboned 6FDA-DAM:DABA as a useful approach to stabilize the properties of the membranes were later explored.[84] The mechanism of using polyethylene glycol (PEG) as a cross-linking agent is shown in figure 2. It was observed that both the selectivity and the stability of membranes were improved by crosslinking. Attractive selectivities of 22 and 27 were obtained for H2S/CH4 and CO2/CH4, respectively, where triethyleneglycol and tetraethyleneglycol were used as cross-linking agents.

Figure 1: Structure of fluorine-containing polyimide 6FDA-HAB. Reproduced from reference [85] with permission from the Journal of Membrane Science.

Plasticization induced by H2S was not observed for feed pressures less than approximately 6–8 bars, whereas plasticization induced by CO2 occurred at feed pressures greater than approximately 25 bars. In another study, Scholes et al. demonstrated the performance of PDMS rubbery membranes. This study determined the interaction parameters of the system using the Flory–Huggins theory for a quaternary system.[68] 9

Figure 2: Cross-linking reaction scheme via thermally activated transesterification. Reproduced from reference [84] with the permission of Macromolecules.

As reported by Vaughn, a series of dense polymeric membranes consisting of polyamide−imides were synthesized, and these new polymers showed ideal CO2/CH4 selectivities of nearly 50.[81, 86, 87] As listed in table 1, the CO2 and H2S permeabilities of these polymers were shown to be higher than that of a commercial membrane (Torlon). However, these membranes showed lower resistance to plasticization for H2S while displaying enhanced stability towards CO2. The overall H2S/CH4 permselectivity in dense polymer membranes is governed by solubility, while diffusion selectivity governs CO2/CH4 separations. The work by Vaughn also highlights the difficulties in designing materials with high selectivity towards both CO2 and H2S. A polyamide‐imide, 6F-PAI-1, showed higher CO2/CH4 selectivity compared to rubbery commercial Pebax® membrane materials.[87] It was also observed that highly fluorinated polyamide‐imides can offer an economical and efficient platform for polymer membrane materials that can be used for natural gas purification. Thin film composites synthesized on ultra-porous polysulfone membrane substrates were studied to determine their gas transport properties, and the separation of CO2 and H2S from CH4 with these composites was reported by Sridhar. This study also used molecular dynamics simulations to verify the theoretical studies and calculate solubility parameters, cohesive energies and sorption values of CO2, H2S and CH4 gases in polyamide membranes.[88] The experimental study showed that these membranes can display permeances of 15.2 GPU and 51.6 GPU for CO2 and H2S, respectively, with selectivities of 14.4 for CO2/CH4 and 49.1 for H2S /CH4 systems. 3.2 Facilitated transport membranes Because facilitated transport membranes (FTMs) offer both enhanced selectivity to acid gas and increased flux, these types of membranes have received a great deal of attention.[89] A schematic of the facilitated transport mechanism is shown in figure 3.

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Figure 3: Schematic of the facilitated transport mechanism. Reproduced from reference [90] with the permission from Polymer Physics.

FTMs can be divided into three general categories: fixed carrier or chained carrier membranes, solvent-swollen polymer membranes and immobilized liquid membranes (ILMs). Fixed carrier membranes are prepared by attaching reactive functional groups to a polymer film, while ILMs are typically prepared by saturating the liquid carrier onto an inorganic membrane support material. Solvent-swollen membranes are prepared by solvent-aided swelling of the polymer followed by the introduction of a carrier species.[90] Typical flue gas streams may require preconditioning prior to acid gas purification. To maintain a high permeability and selectivity towards CO2 and H2S and the capability to retain this ideal characteristic without the need for preconditioning, a moisture removal step is generally required. Facilitated liquid membranes, which incorporate a number of liquid absorption stages, are different from any other conventional membrane system.[91] Better separation can be achieved when desired chemical reactions occur between the acid gases and ‘carriers’.[92] Ilconich demonstrated the production of an ionic liquid-facilitated membrane powered by amine functionalization followed by encapsulation on a polymeric support. [93] Promising performance for acid gas removal was demonstrated for cross-linked nylon66 and polysulfones. This study showed that the permeabilities of acid gases increased with decreasing feed partial pressures. High selectivities of 400 for CO2/CH4 and 800 for H2S/CH4 were observed. It was also determined that H2S transport is facilitated by the formation of the bisulfide ion, HS -. Membrane instability was observed with H2S testing by Shackley, who later devised a pressure gradient-based system to study the cross-membrane transport mechanism and stated that this mechanism is driven by partial pressures across the membrane.[94] Quinn prepared polyelectrolyte fixed carrier membranes consisting of poly(vinylbenzyltrimethylammoniumfluoride), PVBTAF, supported on a microporous support.[95] However, the membrane was relatively stable with CO2, suggesting that more work is needed on this membrane with respect to its mechanical stability. Carbon nanotube-reinforced polyvinylamine/polyvinyl alcohol was coated on a polysulfone support to prepare fixed-site-carrier membranes by Xuezhong et al. These membranes were tested with a permeation rig at 40 bar for a 10% CO2 to give permeance of 0.084–0.218 m3 (STP)/(m2 h bar) with CO2/CH4 selectivity of 17.9–34.7; no H2S study was performed using this system. [96, 97]

11

3.3 Polymer membrane contactors Marzouk et al. recently reported the use of microporous polymeric hollow fibers to prepare membrane contacts to investigate the absorption rate of acid gases and the removal of acid gases from gas streams, which were pressurized up to 50 bar.[98] Different concentrations of aqueous sodium hydroxide, amine solutions and distilled water were used as absorption liquids. Poly(tetrafluoroethylene) and poly(tetrafluoroethylene-co-perfluorinated alkyl vinyl ether) were employed as hollow fibers. This study demonstrated a significant reduction (nearly 10-fold) in membrane area and enhanced separation efficiency of the acid gases. The highly permeable glassy polymer PVTMS (poly-[vinyltrimethylsilane]) was used along with an absorption liquid (diethanolamine) to make a membrane contactor to study the chemical absorption of acid gases.[99] Chemical absorption was used for regeneration, and this material showed stable performance for at least one month. This study demonstrated the potential of gas-liquid membrane contactors for the efficient regeneration of absorption liquids loaded with acid gases. The use of alkanolamines in hollow fiber membrane contactors for the simultaneous separation of H2S and CO2 from natural gas was reported in 2011 by Hedayat; in this study, PVDF and PSf were used as hollow fibers.[100]

Figure 4: Membrane contactor setup for simultaneous absorption of H2S and CO2 from a gas mixture. Reproduced from reference [100] with the permission from the Journal of Membrane Science.

The membrane contactors were set up as shown in figure 4. The effects of the membrane material and absorbent liquid on acid gas removal efficiency and H2S to CO2 selectivity were studied. It was found that adding MEA enhanced the removal efficiency. Additionally, H2S removal was largely affected by the presence of CO2 in the feed gas; an increased CO2 concentration in the feed gas negatively influenced the efficiency of separation process. Conversely, increasing the MEA concentration negatively affected the total mass transfer. This study also confirmed that increased pressure favors acid gas removal, while changes in temperature did not significantly affect removal. Studies on gas-liquid membrane contractors will identify key opportunities and will lead to the invention of novel membranes with higher adsorbent compatibilities. Absorbance processes are highly affected by higher temperatures and pressures.[101] Kreulen studied acid gas removal using a liquid membrane filled with pure methyl-di-ethanol-amine (MDEA) and found no interaction with CO2 under these conditions.[102] A theoretical and experimental study on the effect of weak acids on the transport properties of carbon dioxide was performed by Meldon in 1977. They found that the transport mechanism is a function of the ionic and buffering characteristics of the weak acid; additionally, these findings were proven to be consistent with theoretical predictions [103]. Another study was performed by Matson et al. in an attempt to improve the selectivity of this 12

system by immobilizing solutions on the pores of membranes or by sandwiching solutions in between the membranes.[104, 105] 3.4 Mixed matrix membranes While inorganic membranes demonstrate higher performance, module fabrication is still very expensive compared to the cost of current polymeric modules. Only a few of the many hundreds of polymers developed in research laboratories have been commercialized, suggesting that a large number of polymeric materials are still available to be explored.[78] Even though there is a significant improvement in the field of polymeric gas separations, commercial polymeric membranes are still not in a position to overcome the permeability and selectivity trade-off. Conversely, although inorganic membrane materials offer attractive transport properties, they remain difficult and expensive to process.[106] The large-scale commercialization of inorganic membranes is still impractical due to the high fabrication costs of the materials involved. [63] Therefore, researchers suggested a new approach that can improve the performance of gas separation membranes.[107] Achieving the desired properties and performance for a particular membrane material remains a challenge, so scientists have accumulated the knowledge gained from various systems and used it to fabricate newer membranes. These materials are popularly known as mixed matrix membranes (MMMs). Most of the advancements in the fabrication of new membranes address the trade-off between permeability and selectivity.[36] Although these materials are expensive, their advantages over individual polymer and inorganic membranes have led to innovative approaches, such as gas separation processes for the purification of effluent streams and natural gas using MMMs. In one approach, the membranes consist of a molecular sieving material inserted into a polymer matrix; this system has the potential to perform economical and high performance gas separations. Mixed matrix membranes have both the processability of polymeric materials and the superior gas transport properties of inorganic materials. Some of these membranes have achieved CO2 separation performances above Robeson’s upper bound given in figure 6. The main drawback of mixed matrix membranes is that defects caused by poor contact may occur at the molecular sieve/polymer interface.[108] Additionally, the high cost of scale-up and brittleness are challenges faced by these membranes.[109] Inorganic membranes are more expensive than polymeric membranes, but have many advantages, including temperature and wear resistance, chemical inertness, high mechanical strength, well-defined stable pore structure and a long lifespan.[110] Polyimide-silica membranes were analyzed by Suzuki and Yamada to study their CO2 permeability. It was found that permeability is a function of silica content and that this trend can be used to significantly enhance CO2/CH4 selectivity.[111] Using a sol-gel method, fluorinated poly(amideimide) and TiO2-based nanocomposite membranes were synthesized by Hu et al., who found that lower concentrations of TiO2 led to higher CO2/CH4 selectivity.[112] Nanocomposite membranes of polyesterurethane and polyetherurethane were studied for their effect on silica nanoparticles incorporation and the associated permeability behavior with respect to CO2 and CH4. [113] Matrimid was used as base polymer to prepare dense and asymmetric membranes with metal organic frameworks (MOFs). Three different MOFs, [Cu3(BTC)2], ZIF-8 and MIL-53(Al), showed improved CO2/CH4 selectivity.[63] In another study, asymmetric membranes were prepared using 6FDA–ODA polyimide as the organic phase and grated zeolites with 3-aminopropylmethyldiethoxysilane as the inorganic phase. It was observed that 25 wt.% zeolite in the MMM increased the permeability and selectivity when compared to neat polyimide membrane.[114] Researchers evaluated the improvement in gas transport properties using nano-metal oxides incorporated into both rubbery and glassy polymers. The transport mechanism is largely influenced by the nature of the polymer matrix (i.e., rubbery or 13

glassy). Glassy polymers usually offer higher selectivity, which can be attributed to the condensability of the gas species.[115] Ahn prepared glassy polysulfone MMMs to study their mass transport properties and their effect on the filler.[116] Higher silica contents were demonstrated to favor gas permeability properties, which can be ascribed to the larger free volume created at the interface between the polymer and silica particles. Mesoporous silica materials were widely used as inorganic polymer membranes in the synthesis and functionalization of membrane materials. Synthesis conditions can be varied to obtain silica materials with various pore geometries and the desired range pore sizes of 2–30 nm. The transport mechanism of mesoporous materials is Knudsen diffusion, while solution-diffusion is dominant in the polymer matrix. It seems that the use of inorganic materials changes the transport mechanisms. Hydrothermal synthesis techniques or solvent evaporation techniques were widely used for the production of mesoporous membrane materials, which were usually grown on porous supports. Functionalization at various levels has been achieved in this field of research over the last 15 years. The general focus of research has been on fine-tuning the physiochemical properties of these materials so that they will be well suited for use as membranes for gas separation. This focus has greatly aided in the evolution of these materials for gas treatment applications such as natural gas stream purifications and other advanced applications.[115] In polymer nanocomposite based membranes, the ordered mesoporous thin films contribute significantly to the separation chemistry for gas mixture purification. Knudsen diffusion is the type of mechanism that occurs in ordered mesoporous membranes, where the pore diameters of the diffusing gas molecules are much smaller than the mean free path. Different gas components have different molecular masses, so the Knudsen diffusion regime and separation factor are proportional to √(MA/MB), where MA and MB are the molecular masses of components A and B, respectively. Inorganic polymeric membranes are very inexpensive, but reduced selectivity may result from contact with acid gases. McKeown recently reported an organic polymer membrane with intrinsic microporosity. This microporosity was achieved during the fabrication process, which involved contorted rod-like structures. This material provides a higher gas solubility because it has a high free volume[117]. Various approaches have been reported in the literature to help high temperature application of polymer membranes, such as zeolite-based polymer membranes, membranes made of silica, carbon molecular sieve membranes and modified alumina. [118-120] In another study, which was based on a physical adsorption-based mechanism, zeolite 13X (mesoporous) and zinox 380 (micropores) were used as adsorbents to remove H2S. Zeolite 13X showed a higher level of adsorption of H2S from natural gas. [121] A high silica CHA-type membrane supported on an αalumina support was used to remove acid gases from methane.[122] Thus, zeolites offer themselves as promising inorganic materials for MMMs. These microporous inorganic membranes exhibit high permeability and selectivity along with improved thermal, chemical and mechanical stability. Improving separation properties using polymer nanocomposite membranes is considered an interesting approach due to the unique properties exhibited by polymeric and inorganic materials, including chemical and thermal stability, good permeability, selectivity and mechanical strength.[123]

14

Figure 5: Illustration of nanogap formation in polymer nanocomposite membranes. Reproduced from reference [124] with the permission from Separation and Purification Technology.

As described by Jiang, porous polymethylmethacrylate can be used to immobilize arginine complexes to form polymer nanocomposites with high CO2 sorbent capabilities. This study also discussed complexation of arginine with polystyrene sulfonate, which resulted in enhanced CO2 adsorption capacity.[125] Polymers with intrinsic microporosity are materials with larger and more accessible surface areas and are of greater significance to adsorption and separation chemistry. The surface areas of these polymers can range from 300 to 1500 m2/g. Zeolites and inorganic microporous materials are widely used for adsorption and separations. The gas permeability capabilities of polymer nanocomposite membranes and the diffusion through these membranes were explained by Cong [124] with the help of a nanogap formation mechanism, as illustrated in figure 5. Poor compatibility of the polymer and nanoparticle can lead to nanogaps surrounded by nanoparticles within the polymer, resulting in increased gas diffusivity and permeability. This mechanism can also explain the unaltered selectivity associated with improved permeability in polymer nanocomposites. A similar mechanism for the formation of nanoporous cavities in the modified polyimides to improve polymer membranes is discussed here below in the thermally rearranged polymers section.[126] 3.5 Polymers of intrinsic microporosity

Polymers of intrinsic microporosity (PIMs), which are microporous materials with dimensions less than 2 nm with interconnected pores, have been compared to various inorganic microporous materials such as activated carbon and zeolites. PIM’s as membrane materials demonstrate high processability and good solubility. High-molecular-weight materials with solvent processability are decisive for the fabrication of free standing asymmetric membranes, thin film composites or isotropic films [127]. Till date there is a general lack of H2S information on these advanced materials. PIM’s and its derivatives possess very high CO2 permeabilities, making them potential candidate for acid gas removal in membrane based systems. They have already surpassed the Robeson’s upper bound plots from 1991 and 2008 (revisited version) for CO2/CH4 and have surface area exceeding 1000 m2/g [118, 128]. A great deal of research is being carried out regarding the modification of PIM’s, such as incorporating molecular units of different chain lengths to produce high molecular weight copolymers, [129] changing the width of the distribution of hole sizes and free volume of polymers [130], fuctionalizations. Another work by Swaidan et al suggests that improving the intermolecular hydrogen-bonding network can extensively improve the CO2/CH4 selectivity. Cross-linked PIMs membranes were formed at 175 °C by Du et al and they showed no CO2 plasticization for pure CO2 and CO2/CH4 mixtures 15

when tested at 20 atm pressure [131, 132]. Another approach towards manufacture of thermally cross linked membrane by treatment at 300 °C to obtain CO2 permeability of 4000 barrer and CO2/CH4 selectivity of 54.8 was done by Li in 2012 [133]. The PIM copolymers and its structures relationship were explained based on d-spacing and fractional free volume that was achieved through the pendant group substitution in a work by Du et al in 2008 [134]. Recently, Scholes et al. reported the effects of water vapor present on PIM-1 for CO2/N2 separation [135]. Seeing the higher permeabilities for CO2, we could assume higher permeabilities for H2S as well. This concept is in concordance with the idea that CO2 and H2S removal using polymer membranes may be complimentary to each other, which is discussed in figure 12. Highly selective PIMs with excellent permeabilities can be a step forward towards economical membrane based natural gas sweetening systems. Most of the research in the field of PIMs thus indicate some common themes and is mostly limited to CO2 and CH4 separation. However, further research in the line of functionalization with appropriate group to develop insights into the H2S transport properties would be beneficial. Thus, PIM’s are of great interest to explore with regards to acid gas transport properties in membranes systems. 3.6 Fluorinated polymers Since the discovery of poly(tetrafluoroethylene), fluoropolymers have been considered as strong candidates for material applications that require greater thermal and chemical stability. The capability of fluoropolymers to resist common organic solvents adds to their significance. Conventionally, fluoropolymers were not used as membrane materials due to their low cost effectiveness, processing difficulty and relatively low permeability due to their semicrystalline nature.[136-138] Over the last few decades, perfluorinated polymers have been discovered that are soluble, amorphous in nature and that exhibit good permeability. Teflon AF (DuPont), Hyflon AD (Solvay Solexis) and Cytop (Asahi Glass) are some examples of such perfluorinated polymers that can be prepared as membrane materials using a simple technique such as solution casting. In recently reports, these fluorinated polymers have been shown to have good transport properties. The solubility and selectivity of different fluorinated (TFE/PMVE49, Teflon AF 2400, and Cytop) and non-fluorinated polymers (PDMS, PTMSP and PC) have been compared and studied by accounting for comparisons between rubbery and glassy samples. It was observed that the measured H2S/CO2 selectivity of fluoropolymers was less than 1, while the solubility and selectivity of non-fluorinated polymers ranged between 2.9 and 3.3. Unfavorable interactions between the fluorinated polymer and H2S seemed to deprive H2S of its usual solubility advantage over CO2. The H2S permeabilities through fluoropolymer membranes were significantly lower than the expected values based on molecular predictions, while H2S permeabilities through non-fluorinated polymer membranes were in agreement with the standard correlations. Therefore, it may be concluded that membranes fabricated from fluoropolymers require further study to be proven useful to process designers in NG treatment. It was demonstrated in a recent study by Vaughn that the more highly fluorinated polymers such as 6F-PAI-1 polymer can exhibit better plasticization resistance against H2S.[87] A recent study by Saedi investigated the effect of preparation and operational parameters on PDMS coated PES membranes for natural gas sweetening.[139] Iovane has experimentally studied the performance of a PEEK membrane for CO2 and H2S removal from CH4 in a bio gas stream, which can be applied to natural gas removal.[140] Similarly, a recent agro-biogas based study by Dolejs has reviewed various thin film composite (TFC) membranes for removing CO2 and H2S simultaneously, which can also be explored for natural gas [141]. Amjad-Iranagh has studied molecular simulations of 16

PSF and its composites with chitosan, hyaluronic acid, poly(amidoamine) and hydroxyl poly(amidoamine) membranes for separation effects of acidic gases.[142] 3.7 Thermally rearranged polymers The material property transformations that occur in polymers upon thermal treatment are very interesting. Till date the literature has investigated on CO2 onlyand thus there is a general lack of H2S information on these advanced materials. Thermal treatment has been widely considered for its impact on membrane materials as well. The permeability and selectivity of polyimide membranes that had been pyrolyzed were excellent with regards to their acid gas removal capabilities, as discussed by Yampolskii.[77, 115] Improvements in the material properties of thermally rearranged polymers were ascribed to the structure-property relationships connected to the −OH and −SH side groups in the ortho position of the imide, which led to the formation of insoluble polybenzoxazoles or polybenzothiazoles. Few mechanisms were reported to explain the thermal rearrangements and molecular transformations, suggesting that further investigations on gas-separating membranes are needed.

Figure 6: CO2/CH4 upper bound relationship.[36, 118] Reproduced from reference [36] with the permission from the Journal of Membrane Science.

The permeability of a large number of polymer membranes that showed improved separation characteristics were collectively assessed by Park et al. in 2007.[143] Thermally rearranged polymers with benzoxazole-phenylene main chains exhibited exceptional CO2/CH4 separation capabilities above the prior and present upper limit of the Robeson plot, as shown in figure 6; these polymers are considered unique. The performance of these materials calls for a revision of the upper bound while leaving a gap in the literature for the further exploration of H2S transport properties. Hodgkin studied the effect of thermal treatments on hydroxyl-containing polyimide materials. Unique structural and chemical changes occur in the polymer backbone and at the side groups. It was found that these rearrangements can improve both the permeability and selectivity of polymeric membranes. The advantage was attributed to the formation of nanoporous cavities in the modified polymers.[126]

17

3.8 Polyurethanes Polyurethanes (PUs) are thermoplastic elastomers that have hard and soft domains at the micro phase level. The hard domain of polyurethanes consists of diisocyanate and chain extenders, while the soft segment consists of diols, which can derive from either polyesters or polyethers. The mechanical strength of PU polymers is driven by the hard segment, which forms the structural framework of the polymer. The soft segment forms flexible chain structures that can contribute to good gas permeability. The focus of research in this field has always been to find a good balance between the hard and soft segments because the ideal hard and soft block ratios can lead to a good separation process. Gas separation membranes formed from urethanes are useful in separating gases in gas mixtures. Polar gases containing polar and non-polar gas species were better separated when purified with polymer membranes. Some of the polymer membranes that showed desirable properties with industrial significance, such as permeability, permselectivity and durability, are discussed here. Polyurethane urea belongs to the same family of polymers, which are prepared by careful synthesis involving diisocynates, polyether diols and diamines. A study by Hua describes the transport characteristics of poly(urethane-urea)s obtained through synthesis steps, as shown in figure 7.[144] The properties of the synthesized polymers were compared with rubbery polyurethane urea polymer networks crosslinked with ethylene glycols and propylene glycols. It was discovered that propylene glycol based polyurethane urea has the highest permeability, but the CO2 selectivity was found to decrease gradually. Most of the techniques reported are patented, which included the use of polyurethane-polyether and polyurea-polyether block copolymers containing alternating polyether soft segments and either polyurethane or polyurea hard segments, as reported by Simmon.[145] These membranes revealed exceptionally good permeation rates with high selectivity. A patented work by Coady reported the use of a dried cellulose ester membrane with high permeability to H2S and CO2 to separate these gases from a natural gas stream.[146] The separation approach uses H2S selective rubbery polymer membranes made from poly ether urethane ureas for natural gas treatment. Chatterjee reports the possible use for membranes made of poly(ether urethanes) and poly(ether urethane ureas), with a structure as shown in figure 8, for the purification of low-quality natural gas streams. Dense membranes with either poly(propylene oxide) or poly(ethylene oxide) were used as the polyether component.

18

Figure 7: Typical polyurethane urea synthesis steps. Reproduced from reference [144] with the permission of the Journal of Membrane Science.

The permeability of these membranes was measured at 35°C and at pressures ranging from 4 to 13.6 atm. The polyurethane urea membranes showed a favorable and high H2S/CH4 selectivity (greater than 100 at 20°C) and a high H2S gas permeability. Different polyurethanebased membranes used in acid gas removal from natural gas are given in table 1, which compares the performance of these membranes based on the type of hard and soft segments present in the polyurethane membranes.

Figure 8: Structure of poly(ether urethane urea). Reproduced from reference [82] with the permission of the Journal of Membrane Science.

According to the comparison given in the table 1, the overall H2S /CH4 selectivity increased when poly(ethylene oxide) was used as the poly(ether) segment in the polyurethane. This trend is believed to be due to interactions between polar H2S molecules and the carbonyl groups of the membrane. This mechanism was also reported by Mohammadi to explain the influence of electron-donor amine groups.[147] A poly(ester urethane urea) membrane was used for acid 19

gas removal and was subsequently investigated to determine its permeability and selectivity for varying feed compositions, pressures and temperatures. Permeances of 45 barrers and 95 barrers for CO2 and H2S were measured, respectively, and selectivities of 43 barrers and 16 barrers for H2S/CH4 and CO2/CH4 were measured, respectively. Amani et al. recently explored molecular simulation as a reliable tool for predicting the gas transport behaviors of polymeric membranes. [148] Figure 9 shows the higher solubility of acid gases when compared to other gases that were tested at 298 K and 0-10 bar. The higher solubility and interaction energies were due to the higher affinity of H2S with the PUU membrane and thereby higher solubility of the H2S gas. The solubilities were in the order of H2S > CO2 > CH4 > O2 > N2. The higher solubility of H2S and CO2 was explained by the fact that, unlike CH4, H2S has dipolar and CO2 has quadrupolar properties.

Figure 9: Adsorption isotherm of acid gases in polyurethane urea. Reproduced from reference [148] with the permission of the Journal of Membrane Science.

Grand canonical Monte Carlo molecular dynamics were used for understanding the effect of the polyurethane urea membrane’s role on the gas separation properties. The study showed that the permeability of polymer membranes is a direct function of the urea linkages present in the polymer. With a higher the number of urea linkages, the polymer can have increased chain mobility and higher inter-chain distances. Additionally, a higher fractional free volume and an increased number of hydrogen bonds were also found to be the deciding factors in increasing the urea linkages. 3.9 Ionic liquid membranes Ionic liquids are molten salts at ambient temperature and pressure that have peculiar physical properties, such as negligible vapor pressure, a wide range of viscosities and high thermal and chemical stabilities. Pomelli first reported the solubility of hydrogen sulfide in Butyl methylimidazolium [BMIM]+ cation based ionic liquids. [149] The solubility of gases in ionic liquids is very significant for a variety of chemical processes, including gas separations, where solubility is strongly dependent on temperature.[136] Bulky, asymmetric organic cations include ammonium, pyrrolidinium, pyridinium, imidazolium and phosphonium, while organic/inorganic anions include bromide and tetrafluoroborate. Although ionic liquids exhibit many desirable properties, the use of ionic liquids for separations is limited so far due to the high cost and high energy consumption required to pump the liquids. Overcoming these difficulties remains a 20

challenge in this field of research. One attempt to address these difficulties led to the invention of supported ionic liquid membranes (SILMs), which are prepared by immobilizing ionic liquids onto the pores of a porous support. The adsorption and desorption of the permeate gas occurs simultaneously within the SILM.[93, 137, 138, 150, 151] Ionic liquids that can simultaneously separate both acid gases are restricted to the best performers in this study. The major drawback with these systems is that its reduced mechanical strength due to the presence of a liquid medium inside the polymer matrix may result in the long-term instability of membranes. The larger free volumes may be due to repulsions between ions and alkyl chains. An interesting study performed by Quinn suggested that H2S absorption occurs via the deprotonation of hydrogen sulfide to form bisulfide HS.-[137] A salt hydrate, tetramethylammonium fluoride tetrahydrate, [(CH3)4N]F·4O was prepared with a high capability to reversibly adsorb H2S quantities up to 0.30 mol H2S per mole of salt at 50°C and 100 kPa. The highest H2S/CH4 selectivity shown was 140, while the selectivity for CO2 was 8. The permeability–selectivity trade-off was also observed, as the increased H2S content in the gas reaction mixture suppressed CO2 permeability. Using a BMIM BF4 ionic liquid, Park reports a multi-phase separation process and the fabrication of a SILM using room temperature ionic liquids (RTILs) and polymers.[151] Polyvinylidene fluoride polymers and BMIM BF4 ionic liquids were used to fabricate the SILMs. The permeability coefficients obtained with these SILMs was 30-180 barrers for carbon dioxide and 160-1100 barrers for hydrogen sulfide. The selectivity of CO2/CH4 was in the range of 25–45, whereas the permeability of H2S/CH4 was in the range of 130–260. Jalili studied the solubility and diffusion of hydrogen sulfide in [EMIM][EtSO 4] ionic liquids and found them to be greater than those of carbon dioxide.[152] Huang described [EMIM][Pro] ionic liquids and reported that solubility correlates with anion alkalinity.[153] Huang also reported that H2S absorption capacity decreases as follows: [EMIM][Pro]>[EMIM][Ace]> [EMIM][Lac]. The potentials of SILMs for high pressure natural gas process due to the mechanical strength as most of literature results were reported at low pressure such as 1 bar.[154] SILMs with high mechanical strength and which are flexible dense films with sufficient mechanical resistance to support their potentials in high pressure natural gas feed need to be developed. A recent study by Jalil identifies [C2mim][OAc] ionic liquids that contain [OAc] anions (ethanoate or acetate) that can impart exceptionally high CO2 and H2S dissolving power. This study also reported the effect of –CF3 groups in the anion; the presence of –CF3 groups led to a proportional increase in acid gas solubility. A study by Sakhaeinia et al. on [HOemim][Tf 2N] confirmed the effect of –CF3 groups in the anion of the ionic liquid and a direct correlation with acid gas solubility.[152] The solubility of H2S is greater than that of CO2, and the solubility of carbon dioxide and hydrogen sulfide improves with the number of trifluoromethyl (–CF3) groups in the anion. The solubility behavior of acid gases follows the order: [HOemim][Tf2N]≥[HOemim][OTf]> [HOemim][PF6] >[HOemim][BF4]. The influence of anions in the ionic liquid on the permeability and selectivity of acid gas has been elucidated in studies reporting ionic liquids with high CO2 solubility. The usage of low-cost protic ionic liquids for H2S and CO2 absorption by Huang has demonstrated the high selectivity of H2S from CO2.[155] Mortazavi-Manesh has studied a large number of ionic liquids for their potential in the area of acid gas removal. Over 400 probable ionic liquids were categorized based on higher selectivity of absorption of H2S and CO2 over CH4. They concluded that the best 58 (15% of all ionic liquids studied) for acid gas removal predominantly contain anions BF4, NO3,CH3SO4 and cations N4111, pmg,tmg.[156] Almost all of the ionic liquid studies based on acid gas removal 21

have reported absorption data and solubility selectivity data. [157] These ionic liquids promise a good potential for further exploration of making membranes for natural gas sweetening.[158, 159]. An article by Scovazzo studied [emim][Tf2N], [emim][dca], [C3NH2mim][TfO] and [C3NH2mim][Tf2N] ionic liquids, which displayed high CO2 permeabilities in the range of 500 to 1780 barrers and CO2/CH4 selectivities in the range of 12 to 30.[160] The ionic liquid studied by Hanioka, [C3NH2mim][CF3SO3], displays a CO2 permeability of 2500 barrers and a CO2/CH4 selectivity of 120.[161] Polymerizable room temperature ionic liquids (RTILs), a source of polyRTILs, can be synthesized by incorporating polymerizable groups at either anionic or cationic sites on the ionic liquid structure, followed by polymerization. With these polymerizable functional groups, RTILs can be converted into polyRTILs, which can be used for gas separation membrane applications. Bara hypothesized that the permeability coefficients of polyRTILs can be tuned by varying the n-alkyl substituent and its chain length. Their adsorption capabilities were observed to be twice that of the liquid counterparts of polyRTILs. This may confirm the potential of polyRTILs for gas separations.[162] RTILs with polymerizable functional groups that can be attached to cationic sites include vinyl, acrylate, methyl acrylate, and styrene groups. PolyRTILs have larger free volumes compared with their molten states, making polyRTILs promising materials for CO2 sorption and separation.[152, 163-167] The various types of membranes and membrane materials that are reportedly used for the acid gas removal from natural gas streams are listed in table 1 with their respective permeability and selectivity data.

22

α

α

(H2S)

H2S /CH4

CO2/H2 S

Feed T °C

10-14 20-25 21 22.1 6.9

2.13 239

15-20 50 19 19.4 21

1.14 0.32

55.8

6.9

183

22.6

Poly(ether urethane) PU3

58.8

13

271

62.2

12.2

Poly(ether urethane urea) PU 2

197

α

40 35 35

Feed Pressure Bar 13.78 10 10

Gas composition Pure(P) or Mixed (M) (CH4/CO2/ H2S) % P M (65/29/6) M(70.8/27.9/1.3)

0.30

35

10

M(69.4/18.1/12.5)

58

0.22

35

10

M(70.8/27.9/1.3)

280

54.9

0.22

35

10

M(69.4/18.1/12.5)

6.1

613

19

0.32

35

10

M(70.8/27.9/1.3)

195

5.6

618

18

0.32

35

10

M(69.4/18.1/12.5)

44.7 50.8 22.4 25.4

17 15 22 20

199 223 102 123

74 66 102 95

0.22 0.22 0.22 0.21

35 35 20 20

10 10 10 10

M(70.8/27.9/1.3) M(69.4/18.1/12.5) M(70.8/27.9/1.3) M(69.4/18.1/12.5)

-

11

-

55

-

25

Up to 10

P

-

10.6 21.4 3.3 11.34 15.96 9.92 13.07

-

9.5 10.5 6.9 43 22 27 18

-

35 35 35 35

26.53 26.66 6.46 10 10 30 30

18000

-

-

-

0.85

23

1.38

M (Balance/4/800 ppm) M (Balance/4/800 ppm) M (Balance/4/800 ppm) M (91.6/5.4/3) M (93.2/6.2/0.6) M (91.6/5.4/3) M (93.2/6.2/0.6) M (46% CO, 10.5%CO2, 1.5% H2S, balance hydrogen) M (36.5 % CO, 11.7 % CO2, 0.7 % H2S, balance hydrogen) M (650ppm/4%CO2/bal CH4)

P* (CO2)

CO2/C H4

Cellulose Acetate Cellulose Acetate Cellulose Acetate Cellulose Acetate Poly(ether urethane) PU1

8.9 8.9 2.43 77.5

Polymer

Poly(ether urethane urea) PU4 Poly(ester urethane urea) Matrimid 5218 6 FDA-IPDA Silicone rubber Poly(ether urethane urea)

PTMSP

PDMS

PDMS coated PES PEEK module

P*

Ref. [168] [64] [169]

[82]

[170] [171]

[147]

[76]

4400

-

-

-

0.66

21

6.89

-

3.3

-

6.9

-

-

6.55

a

10.6

4.18

25

10

M (97.5/2.1/0.4)

[139]

-

56

-

-

7.5

M(53.5/40.2/0.2)

[140]

116.07 -

a

43.87 -

28.86

[171]

23

membrane TFC Sterlitech I TFC Sterlitech II TFC low P membrane TFC high P membrane Pebax 4011 Pebax MX 1657

Pebax MX 1074

Pebax MX 1041 Pebax 4033 SA00 Pebax 3533 SA00 Pebax 6333 SA00 Pebax 7233 SA00 Pebax MV 3000 SA 01 6FDA-DAM:DABA (3:2) PC

-

2 2

-

-

-

-

2.2 2.2

M ( 53.3/46.6/0.1) M ( 53.3/46.6/0.1)

19

5

21

3.5

0.9

-

5

M ( 53.3/46.6/0.1)

5.6

14.8

6.2

17.5

0.84

-

5

M ( 53.3/46.6/0.1)

-

16

-

70

-

-

26.66

[171]

69.1

-

-

-

0.27

21

13.1

69 89

14.1 -

248 126

50.6 -

0.27 0.71

35 25

10 3

122

-

-

-

0.22

21

190 psig

155 122 39.7 84.4 243 7.4 4.1

11.2 12 11 6.5 5.7 3.9 8.2

695 553 175 312 888 37.8 7.6

50.4 54 49 24 21 20 15

0.22 0.22 0.23 0.27 0.27 0.19 0.54

35 35 35 35 35 35 35

10 10 10 10 10 10 10

M (95.79/4.12/870ppm) M( 36.5 % CO, 11.7 % CO2, 0.7 % H2S, balance hydrogen) M (70.8/27.9/1.3) P M( 36.5 % CO, 11.7 % CO2, 0.7 % H2S, balance hydrogen) M (69.4/18.1/12.5) M (70.8/27.9/1.3) M (70.8/27.9/1.3) M (70.8/27.9/1.3) M (70.8/27.9/1.3) M (70.8/27.9/1.3) M (70.8/27.9/1.3)

100

10

487

49

0.21

35

4.48

P

[87]

55.6 50.8

32.1 31.1

25.4 23.6

14.7 14.4

0.58 0.61

35 35

49 49

6.5

-

-

-

4.3

21

6.89

M (70/20/10) M (70/20/10) M( 36.5 % CO, 11.7 % CO2, 0.7 % H2S, balance hydrogen) M( 36.5 % CO, 11.7 % CO2, 0.7 % H2S, balance hydrogen)

PSF

3.8

PSF

9.8±0.34

PSF/CST-HA(50) PSF/CSTHA(50)/PAMAM Nylon - 6

b

-

60.3±0.67

b

-

45.6±0.78

b

-

0.088

-

7.2±0. b 76 55.1± b 0.43 44.0± b 0.78 -

-

3.9

21

13

-

-

-

-

P

-

-

-

-

P

-

-

-

-

P

-

0.26

-

-

-

[141]

[76] [82] [172] [76]

[82]

[173]

[76]

[142]

[174]

24

Cytop

17

-

-

-

27

21

6.89

TFE/PMVE/8CNVE

28

-

-

-

8

37

6.89

Teflon AF 1600

680

-

-

-

6.8

23

1.38

Teflon AF 2400

2300

-

-

-

5.6

23

1.38

TFC polyamide

16.6 b 15.2 c 1.18 c 0.95 c 0.95 b 12.3

12.8 14.4 -

52.3 b 51.6 c 9.6 c 8.7 c 8.3 b 11.5

40.5 49.1 1920 1730 2100 -

0.32 0.29 0.12 0.11 0.11 -

30 30 30 25

10 10 4.85 6.22 7.59 3

For CO2 - M( 36.5 % CO, 11.7 % CO2, 0.7 % H2S, balance hydrogen) For H2S – M (15% H2S, 85% N2) M( 46% CO, 10.5%CO2, 1.5% H2S, balance hydrogen) M( 46% CO, 10.5%CO2, 1.5% H2S, balance hydrogen) M( 46% CO, 10.5%CO2, 1.5% H2S, balance hydrogen) P P M (79.7/10/10.3) M (79.7/10/10.3) M (79.7/10/10.3) P

-

196

-

-

25

3

P

0.98 8.1

-

25

3

P

5.29

35

4.48

P

PVBTAF

b

b

PEI/PEBA1657 PEI/PDMS/PEBA165 7/PDMS PDMS 6F-PAI-1

2700 a 32.8

42

2750 6.2

6F-PAI-2

14.2

a

49

3

10.3

4.73

35

4.48

P

6F-PAI-3

21.6

47

5.0

10.9

4.32

35

4.48

P

0.7 ± 8 ×

52.8 ±

0.2 ±

14.8 ±

10−3

0.6

0.02

1.9

35

4.55

P

-

-

-

6

-

-

6.89

4.8 -

4 -

12 2

10 -

0.4 -

30 25

2.1 1.59

-

-

12

-

-

25

1.59

b

157

a

b a

Torlon

Cardo type polyimide

PPOP

3.56

M (101-401 ppm CH4) P P P

[76]

[175] [95]

[81, 87, 172, 176]

[86]

H2S in

[177] [80] [178]

25

PTBP PDTBP Pure CA GCV- modified CA PBI composite membrane [(CH3)4N]F.4H2O) / celgard membrane 6FDA-HAB Modified vinylidene fluoride films

17 27

10 5.4

16 20

9.4 4

1.06 1.35

30 30

2.1 2.1

P P

8.66

29.5

8.71

29.7

0.99

35

34.5

M (60/20/20)

27.5 129.4 136

19.1 21.8 20

39.7 204 190

27.4 34.3 27.5

0.69 0.63 0.71

35 35 35

48.3 34.5 48.3

250

3.34

140 15

0.13 -

50 35

1.15 10

M (60/20/20) M (60/20/20) M (60/20/20) M (H2: 55%; CO2: 41%; CO: 1%; CH4: 1%; : N2 1% and H2S: 1%) M (89.77/5.1/5.3) P

5.3

-

-

23

-

M (5% H2S, (95%N2)

[182]

-

4.6

-

-

1

M (3.91% H2S, 96% CH4)

[183]

0.005

a

0.15

a

109 -

18.7 60

813 -

-

-

-

-

b

Seragel membrane b

-

-

-

3801140 cm Hg

a

78.8

-

-

-

b

N2, O2,CH4,CO2, and H2S in He

59.37 3.07 1.59 2.12 57.4 56.1 62.9 75.5 58.6 72.6 -

0.11 0.09 0.06 0.06 0.34 0.36 0.34 0.31 0.34 0.30 3.2-4 1.82 3.82

30 30 30 30 25 25 25 25 25 25 -

20 20 20 20 20 20 -

P P P P P P P P P P P P P P P P P

b

-

23.2

a

17.8

1130

6.4 34.6 7.27 3.66 19.6 20.1 20.13 23.8 20.6 22.3 -

1140 4 350 1195 -

3.75

[80]

[179]

[180] [137] [181]

P Mixture of 3% each Ar,

Polymer 3

250

a

GENERON PVTMS SILAR [MDEAH][Ac] [MDEAH][For] [DMEAH][Ac] [DMEAH][For] Bmim-BF4 Emim-CH3SO4 HOemim-NO3 N4111-Cl Pmg-L Tmg-TCA [bmim][PF6] [C2mim][eFAP] [C8mim][PF6]

226.7 45 1600 2000 -

-

[184, 185]

[186] [155]

[156] [187] [166] [188]

26

[P66614][NTf2] [C4mim][NTf2] [hmim][Tf2N] [emim][EtSO4] [omim][Tf2N]

-

5 13 -

-

12.5 10 -

0.47 0.62 0.67

80 80 40 30 30

-

P P P P P

2mHEAPr

-

42

-

27

-

80

-

P

[C4mim][CH3SO3]

-

45

-

40

-

80

-

P

[hemim][Tf2N]

-

-

-

-

0.43

30

-

P

[hemim PF6]

-

-

-

-

0.31

30

-

P

[hemim] [TfO]

-

-

-

-

0.38

30

-

P

[hemim][BF4]

-

-

-

-

30

-

P

[192]

[bmim][Tf2N] [bmim][BF4]

-

8.0 -

60 30

-

P P

25-45

130260

0.33 0.26

30-180

1601100

-

P

[159] [193] [151, 164]

PVDF + NaCO3

-

-

-

-

-

-

-

PPO

-

-

-

4

-

-

7.9

ePTFE fibers

-

-

-

-

-

-

Up to 50 bar

PVDF + BMIM BF4

0.29

-

M (17.9–1159 ppm H2S in CH4) M (101-401 ppm H2S in CH4) M (2% H2S from pressurized H2S–CH4)

[164, 189] [157] [152] [158] [164, 190] [164, 191]

[152]

[194] [177] [195]

α: selectivity P: permeability /permeance/flux *units of, P: with no superscript is Permeability in Barrer a : Permeance in GPU; b :Permeance in 10-6, cm3/cm2 s cm Hg, c: Flux in m3 (STP)/m2hr

Table1: Gas transport properties of various polymer membranes used for acid gas removal from natural gas.

27

4. Material Structure - gas transport performance analysis Polymeric membranes are great alternatives to existing technologies of acid gas removal from natural gas because of their ease of formation and scalability to modules of choice, lower cost, excellent mechanical stability at high pressure, etc. [196] Gas permeation through polymeric membranes, which are generally nonporous, is solution-diffusion controlled. This diffusion mechanism depends on the solubility of specific gases and their diffusion through membranes. Separation therefore depends on the molecular size of the gas and chemical interactions between the gas and the polymer membrane. Glassy and rubbery are the two main categories into which polymeric membranes can be divided. This classification is based on the glass transition temperature relative to ambient conditions. Rubbery membranes have glass transition temperatures lower than the ambient temperature, while glassy membranes have glass transition temperatures higher than the ambient temperature. Commercially, all industrial permselective gas separation membrane processes utilize glassy polymeric membranes because of their high selectivity and superior mechanical properties. The structure of glassy membranes is rigid and glass-like because these membranes are operated below their glass transition temperatures. Conversely, rubbery membranes are flexible and soft because these membranes are operated above their glass transition temperatures. Generally, rubbery polymers show high permeability but low selectivity, while glassy polymers behave oppositely (i.e., low permeability but high selectivity). Examples of rubbery polymers that are being used extensively in polymeric membranes for gas separation include silicone polymers and poly (dimethylsiloxane) (PDMS), while examples of glassy polymers used for gas separation include polyimides, polyamides, polyarylates, polyacetylenes, poly[1-(trimethylsilyl)-1-propyne] (PTMSP), polycarbonates, cellulose acetate, poly(phenylene oxide), polysulfones and cardotype polymers. Higher selectivity, permeability, competitive flux, long-term stability, low fouling rate and high mechanical, chemical and thermal stability are some of the main characteristics of an ideal membrane.[197] Changing the chain packing and chain rigidity can simultaneously increase both permeability and selectivity.[198, 199] The selection criterion for gas separation membrane material also depends on weakly and strongly interacting components in natural gas feeds.[186, 200] However, it is difficult for a single membrane to meet all of these requirements. The permeation properties of gas separation membranes are significantly affected by thermal treatment. Resistance against plasticization improves as the thermal treatment temperature is increased. Thermal rearrangement helps by increasing chain flexibility, which in turn greatly enhances the fractional free volume and permeability of the membrane while maintaining a high selectivity.[143] Table 1 presents an analysis of various types of membrane materials which were reportedly used for acid gas removal from natural gas streams with respect to their permeability and selectivity data. The test conditions were not always identical, and thus, it became difficult to provide a discussion to assess material selection. The feed temperature, which plays a major role in the performance of the membrane, was generally in the range of 25 to 40°C, whereas the feed pressure was in the range of 2 to 50 bar. The gas compositions used for testing permeability were either pure or a mixture of acid gases along with CH4 gas. By analyzing the reported H2S permeability performance given in table 1, we have summarized the findings in figure 10. A dotted line is added as a trendline in the plot to identify the polymer membranes that showed permeability above 200 barrers; it is expected to help the reader highlight polymer membrane materials that showed the highest permeability for H2S. Thus, the materials we highlight are polyetherurethane urea, polyether block amides (different Pebax grades), 28

supported ionic liquid membrane PVDF- BMIMBF4, SILAR and PBI composite membranes, etc. We thus generally observe that amine based functional polymers serve as superior candidates for acid gas removal.

Figure 10: H2S permeability of various polymer membranes

The selectivity of the gas pair H2S/CH4 as a function of H2S permeability is plotted in figure 11. Here, we introduce the plot to the study the permeability-selectivity relationship for H2S/CH4 as a function of H2S permeability. Each red triangle in the plot represents a data point that is representative of a reported value in the literature (table 1) that showed simultaneous separation properties for both acid gases.

Figure 11: selectivity- permeability relationship for H2S/CH4

The selectivity data interpretation showed us that materials such as polyetherurethane urea, polyether block amides (different Pebax grades), supported ionic liquid membrane PVDF29

BMIMBF4, modified cellulose acetate and PBI composite membranes, etc. simultaneously demonstrated higher H2S/CH4 selectivity and good permeability and can be classified as the ‘useful’ category of polymer materials for acid gas removal from natural gas. As our literature survey has extensively reviewed the available published data, an imaginary dotted line has been embedded in the picture as an H2S/CH4 upper bound for 2015. This observation is in line with Scholes et al. article which has reviewed about the simultaneous removal of CO2 and H2S to provide a plot of upper bound. [201] On comparison with Fig. 12, it becomes evident that the polymer material that has high H2S permeability also had high H2S/CO2 selectivity. Most of the highly H2S permeable polymer membranes also demonstrated high H2S/CO2 selectivity. Those polymer materials are consistently either polyetherurethane urea or different commercial grades of polyether block amides (different Pebax grades). It is evident that H2S permeability can be considered as a criterion for the selection of polymer membranes that can treat both the acid gases.

Figure 12: H2S permeability and H2S/CO2 selectivity relationship

We also highlight the necessity of further research on these suggested polymer materials and on the significance of polymer backbone modification by crosslinking, copolymerization, blending, etc. Blending and copolymerization were used widely as strategies to modify polymers to achieve ideal membrane characteristics. In heterogeneous blends, the two main factors that affect gas transport properties are the morphology of the two phase structure and the nature of interface between the phases [202, 203]. In homogeneous miscible blends, the gas transport properties depend on the strength of the interactions between the two components to be blended.[204, 205] The copolymers can be categorized in terms of block, rigid-flexible block and random [206] based on their micro-phase separation morphology.[207-209] Another study by Car described the use of blended thin film membranes made from Pebax and polyethylene glycol for CO2 separation.[210] The activation energy required for single gas permeation was calculated using the Arrhenius equation after testing the membrane at various pressures and temperatures. In a work by Yave, carbon dioxide-selective copolymer membrane materials were synthesized from polyether block copolymer and polyethylene glycol-dimethylether (PEG30

DME).[211] In a commercial process used by UOP, PEG-DME are used under the trade name Genosorb® in the Selexol® process for natural gas treatment and acid gas removal. The use of this copolymer material increased the carbon dioxide (CO2/H2) permeability and selectivity. In a recent study, Pebax copolymer SA01 MV 3000 was used for the simultaneous separation of CO2 and H2S from CH4. When studied under rich and lean feed streams of acid gases, Pebax showed selectivities of 40 and 10 for H2S/CH4 and CO2/CH4, respectively. [87] The use of commercial copolymer membrane materials such as the Pebax polymers as separation membranes for the purification of low-quality natural gas streams has been studied and reported. These membrane materials are thermoplastic elastomers made of flexible polyether and rigid polyamides; these materials exhibit H2S /CH4 selectivity and high H2S permeability, potentially allowing natural gas to be upgraded to meet US pipeline specifications. This study compared two different types of commercial Pebax membrane materials, namely, the MX and SA series, both of which are polyether block amides. It was observed that the MX series of Pebax polymers showed H2S/CH4 selectivities from 49 to 54 and CO2/CH4 selectivities from 11 to 14 when tested at 35°C and 10 atm. Pebax 4011 was reported to have a CO2/CH4 selectivity of 16 and a H2S/CH4 selectivity of 70.[147] This article also reported another commercial material, Matrimid 5218, which has a CO2/CH4 selectivity of 10.16 and a H2S/CH4 selectivity of 9.5. For comparison, silicone rubber exhibits a CO2/CH4 selectivity of 3.3 and a H2S/CH4 selectivity of 6.9. In various other studies, nanocomposite membranes were prepared using Pebax 1657, multiwalled carbon nanotubes and quaternary ammonium compounds; these membranes showed high CO2 solubility coefficients when tested.[212] Cross-linking is a polymer structure modification that may help in reducing the chain mobility, thus resulting in an increase of the glass transition temperature. Cross-linking has been used to produce membranes that have designable properties with improved efficiencies and increased resistance to plasticization and aging. There have been many studies on the synthesis of uncross-linked and cross-linked membrane materials, especially polyimides (because of their good gas separation and physical properties), to determine the effect of the degree of crosslinking on swelling and plasticization due to CO2. These studies found an increase in CO2/CH4 selectivity with an increasing degree of cross-linking.[39, 66, 213-224] Modified polymers can be made by introducing bulky substituent groups along the polymer backbone, which increases the free volume and thereby increases the permeability. However, introducing bulky substituent groups also reduces selectivity. The substitution of rigid monomer units or less-mobile linkages helps reduce segmental motion along the chain backbone, thereby reducing the size of diffusional gaps and increasing selectivity. Additionally, the introduction of large, polar substituents helps increase the selectivity by increasing inter-segmental interactions. The simultaneous use of these changes during synthesis helps increase both permeability and selectivity. There are different classes of nanoporous polymer networks, including covalent organic frameworks, hyper-crosslinked polymers, conjugated microporous polymers and polymers of intrinsic microporosity, which are based on synthetic materials of the lighter elements of the periodic table.[225] The growing variety of synthetic routes to these materials allows a range of different polymer networks to be formed, including crystalline and amorphous structures. It is also possible to incorporate many different types of functional groups in a modular fashion. Mixed matrix membranes (MMMs) were prepared with Pebax and polyhedral oligomeric silsesquioxane (POSS) and were shown to have an improved CO2 permeability, which may be due to the large cavity of POSS and the cavity’s effect on polymeric chain packing.[226]

31

The upper bound plots given in figure10-12 have helped to identify the ‘useful’ class of materials which was suggested based on the upper bound plots for H2S/CO2. However, the target of the separation is preventing CH4 permeation and thus figure 13 reconsiders the data available from table 1 to extract some additional conclusions regarding useful classes of materials. It was observed that membranes with highest H2S permeability are least permeable for CH4. The materials with lowest CH4 permeability were plotted against materials with high CO2 and H2S permeability. The result was in line with the depiction of kinetic diameter and condensability of the main constituents of natural gas as described by Baker[21] in 2008. The useful category of materials were screened based on the criteria that it has permeability of CH4 < 20 barrers; permeability of CO2 > 100 Barrers and permeability of H2S > 600. The messages that can be taken away from this point of view of structure and property relationship is that there are few reported materials that has very high potential on the simultaneous acid gas removal from naturl gas. The materials thus identified were polyetherurethane urea, polyether block amides (Pebax grade 3533), and PBI composite membranes.

Figure 13: Upper bound plots with respect to lowest CH4 permeability

5. Economics and process optimization of membrane technology Process optimization and economics are the most important aspects to consider when studying membrane-based gas separation. This is because they are related to the implementation of membranes on an industrial scale. Cost estimation is crucial not only for determining the feasibility of membranes but also for determining its potential to replace the conventional acid gas removal technologies, mainly amine sweetening. To study the economics of membranes, the first step is to estimate the cost. Analogous to the cost estimation of any chemical process, the cost of a membrane gas separation process is divided into capital cost and operating cost. Capital cost includes the cost of the membrane material, the frame (casing, valve and tubing), pretreatment equipment and compression equipment, a heat exchanger, and a vacuum pump. Operating and maintenance cost includes product losses, power expenses for operating compressors and pumps, membrane replacement, and maintenance expenses. Optimization of the process based on the cost is the second step, where the different parameters affecting the cost are studied and optimized. Various comprehensive studies have been conducted on the 32

economics of membrane systems and the various factors affecting the processing cost. Most of these papers focused on membranes for CO2 removal from natural gas stream; however, some considered both CO2 and H2S removal. Several papers have studied the economics of CO2 removal by membranes and the membranes-amine hybrid system introduced in the 1980s when membranes first emerged for gas separation.[227] However, each studied the system for a specific case only, which makes it difficult to compare the different papers. Additionally, the different papers/researchers used different cost elements and factors to estimate the processing cost; thus, the quantitative comparison is not applicable. For example, different papers have studied systems with specific feed flow rates, compositions and pressures; with specific membrane configuration without considering the other configurations; and with specific retentate or permeate conditions. Moreover, the membrane system is not yet fully developed for the economic parameters and the estimated cost values to be definite. B.D. Bhide and S.A. Stern have eliminated those deficiencies in their published papers and published two comprehensive papers in 1993.[228] In those papers, the different membrane configurations were studied and optimized to achieve the lowest processing cost. The effects of the different process and economical parameters were discussed, including feed composition, flow rate and pressure, membrane stage cut, pressure ratio, and recycle ratio, as well as membrane material selectivity and membrane material cost and the cost of methane losses. Additionally, the presence of H2S and H2O were addressed, but with very low concentrations (1 mol% H2S only), as the cellulose acetate membrane would be plasticized with higher concentrations. The separation cost of the membrane system was compared to the cost of the conventional DEA absorption process. In 1998, Bhide published a paper in studying the hybrid membrane-amine system with the selected optimum membrane configuration. The paper examined the economics of the hybrid system and studied the effect of the different process and economical parameters on the total processing cost. The base case for the three published papers is given in table 2. Feed composition Feed flow rate Feed pressure Permeate pressure Membrane thickness Retentate composition Membrane material CH4 permeabilitya CO2 permeabilitya C2H6 permeabilitya N2 permeabilitya : (cm3(STP).cm)/(s.cm2.cmHg)

5-40 mol% CO2, 1 mol% N2 , 1 mol% C2H6, Balance CH4 35 MMscfd 55.2 bar 1.4 bar 1000 Å 2 mol% CO2 Cellulose Acetate 0.4257 x 10-10 9 x 10-10 0.18 x 10-10 0.4257 x 10-10

a

Table 2: Feed composition as summarized by Bhide. Reproduced from reference [169] with permission from the Journal of Membrane Science.

33

As per the results from those three papers, membrane separation has a lower processing cost when compared to amine absorption for high compositions of acid gas in the feed. This is explained by the fact that the operating cost of amine absorption is directly related to the amount of acid gas in the feed. That is, the higher the acid gas content, the higher the required amine circulation rate and the higher the energy required for circulating and regenerating it. Conversely, the diffusion rate increases for membranes at high concentrations of acid gas in the feed [169]. The total separation cost was represented as a function of the mole fraction of CO2 in the feed, and the product specification for was CO2 ≤ 2 mol%. Additionally, it can be seen that the hybrid system has an intermediate cost as per their findings. Later, in 2002 and 2008, J. Hao, P.A. Rice and S.A. Stern published two similar comprehensive papers studying membranes for both CO2 and H2S separation from natural gas utilizing two types of polymeric membrane materials with high CO2/CH4 and H2S/CH4 selectivities.[85] Various process configurations were studied with and without recycling under a wide range of acid gas concentration in the feed (0-40 mol% CO2 and 0-40 mol% H2S). However, the amine sweetening technology was not considered for comparison in a hybrid system. The target separation was to reach the pipeline specifications of 4 ppm H2S and 2% CO2.The best configurations for the different acid gas content scenarios were then identified based on the economical assessment. The two papers by Hao included area calculation, cost estimation and optimization method. The papers investigated the co-removal of CO2 and H2S with membrane systems, where two types of membranes were contained in one module and with modules of one type of membrane material but in different configurations. The idea of combining two membrane materials in one module was first introduced by Ohno; Stern and co-workers verified the concept theoretically and experimentally.[229-231] As per their results, no single configuration was assigned as the optimum. Rather, they defined an optimum configuration for each different feed composition.

Figure14: Processing costs as a function of both CO2 and H2S concentrations in feed for the optimal process configurations with and without recycling. (A) Single stage with CO2/CH4-selective membranes. (B) Two stages in series without recycling, with H2S/CH4-selective membranes in first stage and CO2/CH4-selective membrane in second stage. (C) Single stage with H2S/CH4-selective membranes. (D) Two stages in series with recycle streams. Reproduced from reference [232] with the permission from the Journal of Membrane Science.

34

Figure 14 below shows one of the main figures representing the processing cost per MSCF of product for the range of CO2 and H2S studied. Ahmed presented the simulation of membrane systems with a new two-dimensional cross-flow model in Aspen HYSYS for CO2 removal from natural gas, which was published in 2012.[233] Visual basic was used to simulate the twodimensional cross-flow model as a user defined unit operation in Aspen HYSYS. In their paper, different process configurations were considered and optimized and the process economics were studied to determine the optimum configuration. Although similar configurations were studied by F. Ahmed et.al. and B.D. Bhide, along with similar operating conditions and cost estimation methods, each study selected a different design as the optimum. This might be because F. Ahmed et.al. included a feed compression step, which influenced the cost. Additionally, each study set some parameter before optimizing the configurations, which might have created some differences. Notably, B.D. Bhide et al. represented the processing cost per MSCF of feed, while J. Hao et al. represented it per MSCF of product. This again makes it difficult to directly compare their results quantitatively.The operating and feed conditions and the membrane properties used in the study are summarized in the table 3 below: Feed composition Feed flow rate Feed pressure Permeate pressure Membrane thickness Membrane material Methane permeabilitya CO2 permeabilitya H2S permeabilitya Membrane material Methane permeabilitya CO2 permeabilitya H2S permeabilitya a :(cm3(STP).cm)/(s.cm2.cmHg)

0-40 mol% CO2, 0-40 mol% H2S, Balance CH4 35 MMscfd 55.2 bar 1.4 bar 1000 Å Poly(ether urethane urea) , H2S selective 2 x 10-10 32 x 10-10 150 x 10-10 Fluorine-containing polyimide (6FDA-HAB) , CO2 selective 0.1 x 10-10 6 x 10-10 1.5 x 10-10

Table 3: Feed condition and the membrane properties. Reproduced from reference [85] with permission from the Journal of Membrane Science.

After those comprehensive papers, later publications on membranes studied only one process configuration. Recently, in 2011, L. Peters studied both amine sweetening and membranes acid gas removal and estimated the total capital investment of each system. Each system was studied and optimized separately to achieve 2% CO2 in sweet gas and above 90% CO2 in acid gas.[234] As would be expected, the membranes’ total capital investment was lower than that of the amines. In this paper, three different feed cases were studied; one of the cases flow rate was 373 MMSCFD. This was the highest studied flow rate, as all of the previous work was on a 35 MMSCFD feed flow. Ahmed, in 2013, proposed a model incorporated into Hysys that takes into account the temperature and pressure dependence of membrane permeances.[235] The model was validated by experimental results, and it was concluded that neglecting the temperature and pressure effect would lead to lower processing cost estimations. However, only one configuration was studied. Using the HYSYS simulation integrated with 35

ChemBrane, an economic feasibility analysis was performed by Xuezhong et al. for evaluating the techno-economic feasibility. The system was studied at lower CO2 concentrations, such as 10%.The results showed that FSC membranes can be used at the cost of 5.73E−3 $/Nm3 of sweet NG produced. [96] In a very recent report by Lock in 2015, a high CO2 content was reported, considering the fact that most natural gas fields have a high CO2 concentration. They studied a double-staged membrane system with a recycling system, which can alter the recycling ratio. By increasing the recycling ratio, the membrane area and compressor power were increased. Thus, increasing the recycling ratio led to a substantial impact of low selectivity and high CO2 feed concentration.[236]

6. Conclusions Although many polymeric membranes and their potential applications for gas separation have been reported, only a limited number of these membranes have been explored for the simultaneous removal of both CO2 and H2S from natural gas streams. The permeability, selectivity and mechanical stability of polymer membranes are the decisive factors in the separation of acid gases from natural gas streams. Among the polymer membranes considered in this review, the membranes that exhibited higher potential to treat both CO2 and H2S are mainly polyetherurethane urea, polyether block amides, supported ionic liquid membranes, modified cellulose acetate and PBI composite membranes. The functional polymers with amide functional groups are better candidates for acid gas removal. Polymer backbone modification by crosslinking, copolymerization, and blending are useful strategies to develop durable materials. These polymer membrane materials have the potential to set the benchmark for further improvement. Continuous progress will be made possible by improving the polymer membrane fabrication processes and modifying the physical and chemical structures. Concerns over the high cost of membranes can be reduced by creating polymer membranes with higher permeability and selectivity for acid gases relative to CH4. Current research and development efforts should focus on resolving the key challenges that have been identified and making new polymer membranes which can have high gas permeability while at the same time retaining membrane selectivity along with physical and chemical stability. Moreover, various types of polymer membranes discussed in this review can be used on their own or in combination with other techniques to achieve natural gas separation with desired selectivities. Therefore, we recommend that the natural gas industry around the globe, particularly in the Middle Eastern region, consider applying this technology without delay. Acknowledgments The authors thank ADNOC Gas Subcommittee for funding the project “development of twostage membrane/adsorption acid gas removal process”. Notes *Author to whom correspondence may be addressed: Vikas Mittal; E-mail: [email protected] Chemical Engineering Department, The Petroleum Institute, Abu Dhabi, United Arab Emirates.

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46

Polymer Membranes for Acid Gas Removal from Natural Gas Gigi George, Nidhika Bhoria, Sama AlHallaq, Ahmed Abdala, Vikas Mittal * The Petroleum Institute, Abu Dhabi, United Arab Emirates.

1.

Figure 1: Structure of fluorine-containing polyimide 6FDA-HAB. Reproduced from reference [81] with permission from the Journal of Membrane Science.

47

2.

Figure 2: Cross-linking reaction scheme via thermally activated transesterification. Reproduced from reference [80] with the permission of Macromolecules.

48

3.

Figure 3: Schematic of the facilitated transport mechanism. Reproduced from reference [86] with the permission from Polymer Physics.

49

4.

Figure 4: Membrane contactor setup for simultaneous absorption of H2S and CO2 from a gas mixture. Reproduced from reference [96] with the permission from the Journal of Membrane Science.

50

5.

Figure 5: Illustration of nanogap formation in polymer nanocomposite membranes. Reproduced from reference [120] with the permission from Separation and Purification Technology.

51

6.

Figure 6: CO2/CH4 upper bound relationship.[34, 114] Reproduced from reference [34] with the permission from the Journal of Membrane Science.

52

7.

Figure 7: Typical polyurethane urea synthesis steps. Reproduced from reference [140] with the permission of the Journal of Membrane Science.

53

8.

Figure 8: Structure of poly(ether urethane urea). Reproduced from reference [78] with the permission of the Journal of Membrane Science.

54

9.

Figure 9: Adsorption isotherm of acid gases in polyurethane urea. Reproduced from reference [144] with the permission of the Journal of Membrane Science.

55

10.

Figure 10: H2S permeability of various polymer membranes

56

11.

Figure 11: selectivity- permeability relationship for H2S/CH4

57

12.

Figure 12: H2S permeability and H2S/CO2 selectivity relationship

58

13.

Figure 13: Upper bound plots with respect to lowest CH4 permeability

59

14.

Figure14: Processing costs as a function of both CO2 and H2S concentrations in feed for the optimal process configurations with and without recycling. (A) Single stage with CO2/CH4-selective membranes. (B) Two stages in series without recycling, with H2S/CH4-selective membranes in first stage and CO2/CH4-selective membrane in second stage. (C) Single stage with H2S/CH4-selective membranes. (D) Two stages in series with recycle streams. Reproduced from reference [228] with the permission from the Journal of Membrane Science.

60

P* Polymer (CO2)

CO2/ H2S

Feed Press ure Bar

H2S) %

P*

α

α

CO2/ CH4

(H2

H2S /CH4

S)

Gas composition

Fe ed T °C

α

Pure(P) or Mixed (M) (CH4/CO2/

Ref .

Cellulose Acetate

-

10-14

-

1520

-

40

13.78

P

[16 8]

Cellulose Acetate

8.9

20-25

-

50

-

-

-

-

[64 ]

Cellulose Acetate

8.9

21

-

19

-

-

-

-

[16 9]

Cellulose Acetate

2.43

22.1

2.1 3

19.4

1.14

35

10

M (65/29/6)

77.5

6.9

239

21

0.32

35

10

M(70.8/27.9/1.3)

55.8

6.9

183

22.6

0.30

35

10

M(69.4/18.1/12.5)

58.8

13

271

58

0.22

35

10

M(70.8/27.9/1.3)

62.2

12.2

280

54.9

0.22

35

10

M(69.4/18.1/12.5)

197

6.1

613

19

0.32

35

10

M(70.8/27.9/1.3)

195

5.6

618

18

0.32

35

10

M(69.4/18.1/12.5)

44.7

17

199

74

0.22

35

10

M(70.8/27.9/1.3)

50.8

15

223

66

0.22

35

10

M(69.4/18.1/12.5)

22.4

22

102

102

0.22

20

10

M(70.8/27.9/1.3)

25.4

20

123

95

0.21

20

10

M(69.4/18.1/12.5)

Poly(ester urethane urea)

-

11

-

55

-

25

Up to 10

P

Matrimid 5218

-

10.6

-

9.5

-

-

26.53

M (Balance/4/800 ppm)

Poly(ether urethane) PU1

Poly(ether urethane) PU3

Poly(ether urethane urea) PU 2

Poly(ether urethane urea) PU4

6 FDA-IPDA

-

21.4

-

10.5

-

-

26.66

M (Balance/4/800 ppm)

Silicone rubber

-

3.3

-

6.9

-

-

6.46

M (Balance/4/800 ppm)

[82 ]

[17 0]

[17 1]

61

Poly(ether urethane urea)

-

11.34

-

43

-

35

10

M (91.6/5.4/3)

-

15.96

-

22

-

35

10

M (93.2/6.2/0.6)

-

9.92

-

27

-

35

30

M (91.6/5.4/3)

-

13.07

-

18

-

35

30

M (93.2/6.2/0.6)

[14 7]

M (46% CO, PTMSP

18000

-

-

-

0.85

23

1.38

10.5%CO2, 1.5%

H2S, balance hydrogen) M (36.5 % CO,

4400

-

-

-

0.66

21

6.89

PDMS

[76 ]

11.7 % CO2, 0.7 % H2S, balance hydrogen) M

-

3.3

-

6.9

-

-

6.55

(650ppm/4%CO2 /bal CH4)

PDMS coated PES

116.07

PEEK module membrane

[17 1]

43.87

28. a 86

10.6

4.18

25

10

M (97.5/2.1/0.4)

[13 9]

-

-

-

56

-

-

7.5

M(53.5/40.2/0.2)

[14 0]

TFC Sterlitech I

-

2

-

-

-

-

2.2

M ( 53.3/46.6/0.1)

TFC Sterlitech II

-

2

-

-

-

-

2.2

M ( 53.3/46.6/0.1)

a

[14 1]

TFC low P membrane

19

5

21

3.5

0.9

-

5

M ( 53.3/46.6/0.1)

TFC high P membrane

5.6

14.8

6.2

17.5

0.84

-

5

M ( 53.3/46.6/0.1)

Pebax 4011

-

16

-

70

-

-

26.66

M (95.79/4.12/870pp m)

Pebax MX 1657

[17 1]

M( 36.5 % CO, 69.1

-

-

-

0.27

21

13.1

11.7 % CO2, 0.7 % H2S, balance hydrogen)

[76 ]

62

69

14.1

248

50.6

0.27

35

10

M (70.8/27.9/1.3)

[82 ]

89

-

126

-

0.71

25

3

P

[17 2]

M( 36.5 % CO, 11.7 % CO2, 0.7

122

-

-

-

0.22

21

190 psig

155

11.2

695

50.4

0.22

35

10

M (69.4/18.1/12.5)

122

12

553

54

0.22

35

10

M (70.8/27.9/1.3)

Pebax MX 1041

39.7

11

175

49

0.23

35

10

M (70.8/27.9/1.3)

Pebax 4033 SA00

84.4

6.5

312

24

0.27

35

10

M (70.8/27.9/1.3)

Pebax 3533 SA00

243

5.7

888

21

0.27

35

10

M (70.8/27.9/1.3)

Pebax 6333 SA00

7.4

3.9

37. 8

20

0.19

35

10

M (70.8/27.9/1.3)

Pebax 7233 SA00

4.1

8.2

7.6

15

0.54

35

10

M (70.8/27.9/1.3)

Pebax MV 3000 SA 01

100

10

487

49

0.21

35

4.48

P

55.6

32.1

25. 4

14.7

0.58

35

49

M (70/20/10)

31.1

23. 6

Pebax MX 1074

6FDADAM:DABA (3:2)

50.8

14.4

0.61

35

49

% H2S, balance hydrogen)

M (70/20/10)

[76 ]

[82 ]

[87 ]

[17 3]

M( 36.5 % CO, PC

6.5

-

-

-

4.3

21

6.89

11.7 % CO2, 0.7 % H2S, balance hydrogen) M( 36.5 % CO,

PSF

3.8

-

-

-

3.9

21

13

PSF

9.8±0.3 b 4

-

7.2 ±0.

-

-

-

-

[76 ]

11.7 % CO2, 0.7 % H2S, balance hydrogen) P

[14 2]

63

76

PSF/CSTHA(50)

60.3±0. b 67

-

b

55. 1±0 .43

-

-

-

-

P

-

-

-

-

P

-

0.26

-

-

-

b

PSF/CSTHA(50)/PAMA M

Nylon - 6

45.6±0. b 78

-

44. 0±0 .78 b

0.088

-

-

[17 4]

For CO2 - M( 36.5 % CO, 11.7 % CO2, 0.7 %

H2S, balance Cytop

17

-

-

-

27

21

6.89

hydrogen) For H2S – M (15% H2S, 85%

N2) M( 46% CO, TFE/PMVE/8C NVE

28

-

-

-

8

37

6.89

10.5%CO2, 1.5%

H2S, balance hydrogen) M( 46% CO,

Teflon AF 1600

680

-

-

-

6.8

23

1.38

10.5%CO2, 1.5%

H2S, balance hydrogen)

M( 46% CO, Teflon AF 2400

2300

-

-

-

5.6

23

1.38

10.5%CO2, 1.5%

[76 ]

H2S, balance hydrogen)

TFC polyamide

16.6

15.2 PVBTAF

12.8

52. b 3

b

14.4

51. b 6

c

-

9.6

b

1.18

c

40.5

0.32

49.1

0.29

1920

0.12

10

30

P

10

P

4.85

M (79.7/10/10.3)

[17 5]

64

0.95

c

-

8.7

0.95

c

-

b

-

b

-

PEI/PEBA1657

12.3

PEI/PDMS/PEB A1657/PDMS

157

a

c

1730

0.11

30

6.22

M (79.7/10/10.3)

8.3

c

2100

0.11

30

7.59

M (79.7/10/10.3)

11. b 5

-

-

25

3

P

-

-

25

3

P

196 b

-

275 a 0

0.98

-

25

3

P

42

6.2

8.1

5.29

35

4.48

P

49

3

10.3

4.73

35

4.48

P

5.0

10.9

4.32

35

4.48

P

3.56

35

4.55

P

PDMS

2700

6F-PAI-1

32.8

a

6F-PAI-2

14.2

a

6F-PAI-3

21.6

47

0.7 ± 8

52.8 ±

[95 ]

[81 , 87, 17 2, 17 6]

0.2 Torlon

14.8 ±

× 10−3

0.6

± 1.9

[86 ]

0.02

Cardo type polyimide

M (101-401 ppm

-

-

-

6

-

-

6.89

4.8

4

12

10

0.4

30

2.1

P

-

-

2

-

-

25

1.59

P

-

-

12

-

-

25

1.59

P

PTBP

17

10

16

9.4

1.06

30

2.1

P

PDTBP

27

5.4

20

4

1.35

30

2.1

P

8.66

29.5

8.7 1

29.7

0.99

35

34.5

M (60/20/20)

27.5

19.1

39. 7

27.4

0.69

35

48.3

M (60/20/20)

129.4

21.8

204

34.3

0.63

35

34.5

M (60/20/20)

136

20

190

27.5

0.71

35

48.3

M (60/20/20)

PPOP

Pure CA

GCV- modified CA

H2S in CH4)

[17 7] [80 ] [17 8]

[80 ]

[17 9]

65

M (H2: 55%; PBI composite membrane

0.15

0.0 a 05

a

250

3.34

CO2: 41%; CO: 1%; CH4: 1%;

[18 0]

: N2 1% and

[(CH3)4N]F.4H2 O) / celgard membrane

H2S: 1%) 109

18.7

813

140

0.13

50

1.15

M (89.77/5.1/5.3)

[13 7]

6FDA-HAB

-

60

-

15

-

35

10

P

[18 1]

Modified vinylidene fluoride films

-

-

5.3

-

-

23

-

-

-

-

4.6

-

-

1

-

23. b 2

-

-

-

3801140 cm Hg

Seragel membrane

3.75

b

b

M (5% H2S, (95%N2)

[18 2]

M (3.91% H2S, 96% CH4)

[18 3]

P

Mixture of 3% each Ar, N2,

17.8

113 a 0

78.8

-

-

-

6.4

114 b 0

59.37

-

-

-

P

45

34.6

4

3.07

-

-

-

P

PVTMS

1600

7.27

350

1.59

-

-

-

P

SILAR

2000

3.66

119 5

2.12

-

-

-

P

[MDEAH][Ac]

-

-

-

-

0.11

30

-

P

[MDEAH][For]

-

-

-

-

0.09

30

-

P

[DMEAH][Ac]

-

-

-

-

0.06

30

-

P

[DMEAH][For]

-

-

-

-

0.06

30

-

P

Bmim-BF4

-

19.6

-

57.4

0.34

25

20

P

20.1

-

56.1

0.36

25

20

P

Polymer 3

250

a

226.7 GENERON

Emim-CH3SO4

a

O2,CH4,CO2, and H2S in He

[18 4, 18 5]

[18 6]

[15 5]

66

HOemim-NO3

20.13

-

62.9

0.34

25

20

P

N4111-Cl

23.8

-

75.5

0.31

25

20

P

Pmg-L

20.6

-

58.6

0.34

25

20

P

Tmg-TCA

22.3

-

72.6

0.30

25

20

P

[15 6]

[bmim][PF6]

-

-

-

-

3.2-4

-

-

P

[18 7]

[C2mim][eFAP]

-

-

-

-

1.82

-

-

P

[16 6]

[C8mim][PF6]

-

-

-

-

3.82

-

-

P

[18 8]

[P66614][NTf2]

-

5

-

12.5

-

80

-

P

[C4mim][NTf2]

-

13

-

10

-

80

-

P

[hmim][Tf2N]

-

-

-

-

0.47

40

-

P

[15 7]

[emim][EtSO4]

-

-

-

-

0.62

30

-

P

[15 2]

[omim][Tf2N]

-

-

-

-

0.67

30

-

P

[15 8]

P

[16 4, 19 0] [16 4, 19 1]

2mHEAPr

-

42

-

27

-

80

-

[16 4, 18 9]

[C4mim][CH3S O3]

-

45

-

40

-

80

-

P

[hemim][Tf2N]

-

-

-

-

0.43

30

-

P

[hemim PF6]

-

-

-

-

0.31

30

-

P

[hemim] [TfO]

-

-

-

-

0.38

30

-

P

[hemim][BF4]

-

-

-

-

0.29

30

-

P

[19 2]

[bmim][Tf2N]

-

8.0

-

0.33

60

-

P

[15

-

[15 2]

67

9] [bmim][BF4]

PVDF + BMIM BF4

-

30-180

-

-

-

25-45

160 110 0

130260

0.26

30

-

-

-

P

[19 3]

P

[15 1, 16 4]

M (17.9–1159 PVDF + NaCO3

-

-

-

-

-

-

-

ppm H2S in

CH4) PPO

ePTFE fibers

-

-

-

-

-

-

4

-

-

-

-

7.9

-

Up to 50 bar

M (101-401 ppm

H2S in CH4) M (2% H2S from pressurized H2S–

CH4)

[19 4]

[17 7]

[19 5]

α: selectivity P: permeability /permeance/flux *units of, P: with no superscript is Permeability in Barrer a : Permeance in GPU; b :Permeance in 10-6, cm3/cm2 s cm Hg, c: Flux in m3 (STP)/m2hr

Table1: Gas transport properties of various polymer membranes used for acid gas removal from natural gas.

68

Feed composition

5-40 mol% CO2, 1 mol% N2 , 1 mol% C2H6, Balance CH4

Feed flow rate

35 MMscfd

Feed pressure

55.2 bar

Permeate pressure

1.4 bar

Membrane thickness

1000 Å

Retentate composition

2 mol% CO2

Membrane material

Cellulose Acetate 0.4257 x 10-10

a

CH4 permeability

9 x 10-10

a

CO2 permeability

0.18 x 10-10

a

C2H6 permeability N2 permeabilitya

0.4257 x 10-10 a

: (cm3(STP).cm)/(s.cm2.cmHg)

Table 2: Feed composition as summarized by Bhide. Reproduced from reference [169] with permission from the Journal of Membrane Science.

69

Feed composition

0-40 mol% CO2, 0-40 mol% H2S, Balance CH4

Feed flow rate

35 MMscfd

Feed pressure

55.2 bar

Permeate pressure

1.4 bar

Membrane thickness

1000 Å

Membrane material

Poly(ether urethane urea) , H2S selective

Methane permeabilitya

2 x 10-10

CO2 permeabilitya

32 x 10-10

H2S permeabilitya

150 x 10-10

Membrane material

Fluorine-containing polyimide (6FDA-HAB) , CO2 selective

Methane permeabilitya

0.1 x 10-10

CO2 permeabilitya

6 x 10-10

H2S permeabilitya

1.5 x 10-10 a

:(cm3(STP).cm)/(s.cm2.cmHg)

Table 3: Feed condition and the membrane properties. Reproduced from reference [85] with permission from the Journal of Membrane Science.

70

Highlights

1. Polymer membranes for simultaneous separation of CO2 & H2S from natural gas. 2. Examining simultaneously polymer membrane material, performance and economics. 3. Polymer material selection for CO2 & H2S removal (Table 1). 4. Material - gas transport performance analysis (Section 4).

71