State of the Art - Industrial

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Ind. Eng. Chem. Res. 2009, 48, 4638–4663

Membrane Gas Separation: A Review/State of the Art P. Bernardo,† E. Drioli,*,†,‡ and G. Golemme†,‡ National Research Council-Institute for Membrane Technology (ITM-CNR), Via Pietro Bucci, c/o UniVersity of Calabria, cubo 17/C, 87030 Rende, Italy, and UniVersity of Calabria, Department of Chemical Engineering and Materials and INSTM Consortium, cubo 45/A, Via Pietro Bucci, 87036 Rende, Italy

In the last years membrane processes for gas separation are gaining a larger acceptance in industry and in the market are competing with consolidated operations such as pressure swing absorption and cryogenic distillation. The key for new applications of membranes in challenging and harsh environments (e.g., petrochemistry) is the development of new tough, high performance materials. The modular nature of membrane operations is intrinsically fit for process intensification, and this versatility might be a decisive factor to impose membrane processes in most gas separation fields, in a similar way as today membranes represent the main technology for water treatment. This review highlights the most promising areas of research in gas separation, by considering the materials for membranes, the industrial applications of membrane gas separations, and finally the opportunities for the integration of membrane gas separation units in hybrid systems for the intensification of processes. 1. Introduction Modern membrane engineering is an important way to implement the process intensification (PI) strategy by innovative design and process development methods aimed at decreasing production costs but also equipment size, energy utilization, and waste generation.1 Membrane science and technology are recognized today as powerful tools in solving some important global problems, developing new industrial processes needed for a sustainable industrial growth. In seawater desalination, membrane operations or their combination in integrated systems are already a successful approach for solving the situation of freshwater demand in many regions of the world, at lower costs and minimum environmental impact. Membranes are a factor of 10 times more energetically efficient than thermal options for water desalination.2 The major production cycles consume as much as 40-50% of the energy used just for separations, often carried out by inefficient thermally driven separation processes. Membrane gas separation (GS) is a pressure-driven process with different industrial applications that represent only a small fraction of the potential applications in refineries and chemical industries. Since 1980, when the serial production of commercial polymeric membrane was implemented, membrane GS has rapidly become a competitive separation technology. Differently from conventional separation unit operations (e.g., cryogenic distillation and adsorption processes), membrane GS does not require a phase change. Moreover, the absence of moving parts makes GS systems particularly suited for use in remote locations where reliability is critical; in addition, the small footprint makes them very attractive for remote applications such as offshore gas-processing platforms. The use of membranes in separation processes is growing at a slow but steady rate.3 Baker in 2002 estimated the market * To whom correspondence should be addressed. E-mail: e.drioli@ itm.cnr.it. Tel.: +39 0984 492029. Fax: +39 0984 402103. † National Research Council-Institute for Membrane Technology. ‡ University of Calabria.

scale of membrane GS technology in year 2020 to be five times of that of year 2000.3 It is expected that membrane GS will play an increasingly important role in reducing the environmental impact and costs of industrial processes,2 particularly in the present scenario in which, although energy cost is volatile (oil price is below $50 per barrel after a maximum of $147 last summer), the global hydrocarbon reservoirs are destined to reduce dramatically during this century. There are many opportunities to extend markets for membrane GS. In some cases, the membrane materials, membrane configuration, and preparation routes are inadequate to fully exploit these new fields. A broad range of materials was investigated and different improvements were achieved by the time in this field. Today, much of the research work is being addressed to the investigation of new materials and to the development of new membrane structures that exhibit both higher selectivity and permeability to specific gases. This review addresses current state-of-the art materials and the major efforts in the development in the membrane GS field at Research Institutions and major involved Companies. 2. Membrane GS Material Science Progress The choice of a membrane material for GS applications is based on specific physical and chemical properties, since these materials should be tailored in an advanced way to separate particular gas mixtures. Moreover, robust (i.e., long-term and stable) materials are required to be applied in a membrane GS process. The GS properties of membranes depend upon: • the material (permeability, separation factors), • the membrane structure and thickness (permeance), • the membrane configuration (e.g., flat, hollow fiber) and • the module and system design. Both membrane’s permeability and selectivity influence the economics of a GS membrane process. Permeability is the rate at which any compound permeates through a membrane; it

10.1021/ie8019032 CCC: $40.75  2009 American Chemical Society Published on Web 04/22/2009

Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009

depends upon a thermodynamic factor (partitioning of species between feed phase and membrane phase) and a kinetic factor (e.g., diffusion in a dense membrane or surface diffusion in a microporous membrane). The selectivity is the ability of a membrane to accomplish a given separation (relative permeability of the membrane for the feed species). Selectivity is a key parameter to achieve high product purity at high recoveries. Membrane GS has the potential to grow enormously if more selective membranes will become available. Process design aspects for membrane GS were discussed recently in detail by Baker:4 different system configurations were described from an industrial point of view, together with the more suitable membrane system, depending on the selectivity of the membrane and on the target performance of the process considered. In multistage membrane systems there is a tradeoff between permeate composition and permeate pressure and therefore, recompression costs. GS commercial modules employ generally organic polymers as asymmetric nonporous membranes based on solutiondiffusion transport mechanism. Both gas separation and vapor permeation membrane processes are mostly based on the same mechanism: sorption of the permeant into the membrane, permeation by diffusion through the membrane, desorption at the low pressure side of the membrane. An important polymer feature for preparing GS membranes is its processability into hollow fiber membranes (“spinnability”). Hollow fiber modules (each module contains thousands of fibers) are of interest for large-scale industrial applications, due to the high membrane area to module volume ratio (>1000 m2/m3) which results in high productivity per volume unit and costefficient production.5 Examples of such membranes are PRISM (Air Products) and MEDAL (Air Liquide). Polymers cannot withstand high temperatures and aggressive chemical environments; moreover, when applied in petrochemical plants, refineries, and natural gas treatment, heavy hydrocarbons in feed gas streams can be a problem, particularly in hollow fiber modules. Many polymers can be swollen or plasticized when exposed to hydrocarbons or CO2 with high partial pressure, even in low concentrations: their separation capabilities can be dramatically reduced or the membranes irreparably damaged. Therefore, pretreatment selection and condensate handling are critical decision factors for a proper operation of GS modules. Typically, polymeric membranes show high selectivities and low throughput when compared to porous materials, due to their low free-volume. Polymeric membranes generally undergo a trade-off limitation between permeability and selectivity:6 as selectivity increases, permeability decreases, and vice versa. Robeson,6 collecting a large number of permeation data for different polymeric membranes, showed as for small gaseous molecules (e.g., O2, N2, CO2, and CH4) a superior limit (upper bound) exists in a selectivity/permeability diagram. The presence of an upper bound can be theoretically rationalized on the basis of the transition state theory.7 Chain stiffness and interchain separation increases are recognized as ways to systematically improve separation performance until the interchain separation becomes large enough that the polymer segmental motion no longer controls penetrant diffusion. Unless significant enhancement in solubility selectivity could be achieved, the upper bound would represent the asymptotic end point in the performance of polymeric membranes whose separation properties are related to solution-diffusion transport mechanisms. To achieve higher selectivity/permeability combinations, materials that do not obey these simple rules would be required.

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Table 1. Most Important Glassy and Rubbery Polymers Used in Industrial Membrane GS rubbery polymers

glassy polymers

poly(dimethylsiloxane) ethylene oxide/propylene oxide - amide copolymers

cellulose acetate polyperfluorodioxoles polycarbonates polyimides poly(phenylene oxide) polysulfone

The development of inorganic membranes (e.g., silica, zeolites, etc.) and carbon -based molecular sieves is particularly interesting because they can withstand aggressive chemicals as well as high temperatures. Also these materials present drawbacks: high cost, modest reproducibility, brittleness, low membrane area to module volume ratio, low permeability in the case of highly selective dense membranes (e.g., metal oxides at temperatures below 400 °C) and difficult sealing at high temperatures (greater than 600 °C). The design of new polymers with enhanced selectivity offers slow progress. Existing challenges of low selectivity and permeability in polymer membranes are being addressed with some degree of success with advanced materials. Two types of nanoengineered materials have widely been studied also in terms of their high performance in presence of aggressive agents: cross-linked polymers and mixed-matrix materials.8 A better understanding of the transport mechanism of gases in polymers on a molecular level is needed to develop also new materials. There is an increasing interest in the predictive power of the computational methods to calculate gas diffusion coefficients and solubilities in polymeric membranes. Computer simulation is widely used for the analysis of the molecular structure of amorphous and semicrystalline polymers and the diffusion of small gas molecules through these materials.9-11 First relatively simple flexible chain polymers (e.g., polydimethylsiloxane and poly(isobutylene), poly(ethylene) and poly(propylene)) were studied to obtain qualitative information about relations between transport mechanisms and structural properties, later the focus shifted also to stiff-chain and other complex polymers (polyelectrolytes or catalytically active polymers) and toward a more quantitative approach for the characterization of structure property relations (e.g., relationships between freevolume and gas transport properties). Molecular dynamics (MD) simulations are successful only when detailed atomistic models are used for both the polymer matrix and the gas molecules, and if the thermal vibrations of the polymer matrix are taken into consideration.12 2.1. Polymers. Despite the large number of polymeric materials investigated and developed for GS applications, the number of polymers used in commercial system is still limited.13 The main rubbery and glassy polymers employed for GS membranes are listed in Table 1. Within a polymer membrane, pores and channels have a wide range of sizes and topologies. Polymer free-volume, the fraction of the volume not occupied by the electronic clouds of the polymer, plays an important role in the transport properties of low molecular weight species and gases. Not only the overall amount of free-volume, but also the distribution of the effective micropore size if the free-volume elements are interconnected, is likely to have a significant influence on polymer properties. Some experimental techniques (probe methods such as e.g., Positron Annihilation Lifetime Spectroscopy, Inverse Gas Chromatography, and 129Xe NMR) are available for determining the average radius (1-10 Å) and the size distribution of the

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free-volume elements.14 Atomistic modeling can only explain the topology and structure of free-volume elements in polymers. An amorphous polymer kept above its glass transition temperature (Tg) is in a rubbery state. It presents a relatively large amount of free-volume, owing to transient voids between the highly mobile polymer chains. When the temperature is lowered below its Tg, the polymer behaves as a rigid glass: the fractional free-volume decreases, resulting in insufficient space for large-scale co-operative movements of the polymer backbone. Typically, rubbery polymers present high permeabilities and their selectivity is mainly influenced by differences in the condensability of the gas species. When applied to separate an organic vapor from nitrogen, rubbery membranes preferentially permeate the organic molecules. In this area, poly(organosiloxanes) were studied in detail because of the vast utility of polydimethylsiloxane (PDMS), which has high permeability coefficients, owing to its large free-volume, and high selectivities for condensable gases. Silicone rubber is extremely permeable and has adequate vapor/inert gas selectivities for most applications; composite membranes of silicone rubber are used in almost all of the installed vapor separation systems.15 Almost all industrial membrane GS processes utilize glassy polymers because of high gas selectivity and good mechanical properties. Usually in glassy polymers the more permeable species are those with low molecular diameter and selectivity is due to differences in molecular dimension. Usually, the glassy state is characterized by a relatively small fraction of freevolume. A larger amount of free-volume (up to 20%) can be “frozen-in” by cooling or by a rapid removal of a solvent in some polymers with stiff molecular structures. These voids are not interconnected, and a low accessible surface area can be measured by gas adsorption. Medium to high free-volume glassy polymers (e.g., polyimides, polyphenyleneoxides, etc.) are used to produce membranes since the voids aid the transport of gas or liquid through the material. Polyimides (PI) are rigid, high-melting point, high Tg and thermally stable polymers obtained by polycondensation reactions of dianhydrides with diamines.16 The polymer separation properties can be tailored by using different type of dianhydrides and diamines. The introduction of -C(CF3)2- groups is believed to increase the chain stiffness which in turn reduces the intrasegmental mobility, and reduces and limits the degree of chain packing by increasing the free-volume, serving as molecular spacers and chain stiffeners in the polymer. As a consequence, aromatic polyimides that contain -C(CF3)2groups tend to have higher CO2/CH4 selectivities. However, fluorinated polyimides showed the tendency to plasticization or physical aging.13 Postsynthesis modification by H+ ion beam irradiation was reported for polyimide (Matrimid)-ceramic composite membranes.17 Following irradiation at high ion fluencessthe number of ions per unit membrane surface (H+/ cm2)sdue to an increase in permeability for different gases (e.g., He, CO2, O2, and N2), the Matrimid approached the 1991 Robeson’s upper-bound. The small molecules (e.g., He and O2) exhibited larger increase, resulting in an overall increase in selectivity for most gas pairs (e.g., He/N2, He/CH4, CO2/CH4, and O2/N2). Cross-linking appeared to dominate the modification of microstructure at the lowest irradiation ion fluence, while the formation of small molecular size defects became increasingly important at higher ion fluence. H+ ion beam irradiation reduced the presence of polar carbonyl group of Matrimid, which might result in the decrease in sorption of CO2 and in smaller increase in CO2 permeability relative to other small molecules.

Table 2. Fractional Free-Volume (FFV) and Permeation Properties of “Very High Free-Volume” Polymers

polymer

FFV (%)

PTMSP PMP Teflon AF 2400 PIM-1 PIM-7

32-34 28 33 22-24a

O2 permeability (Barrer)

O2/N2 selectivity (-)

reference

6100 2700 1600 370 190

1.8 2.0 2.0 4.0 4.5

23 24 25 26 26

a FFV accessible to helium, from the PIM-1 density and the PIM-1 framework density.27

PEEK-WC is a phenolphthalein based poly(etheretherketone), having a lactone group sticking out of the backbone. The good thermal and mechanical properties of the semicrystalline PEEK is preserved, the polymer is amorphous and therefore soluble in chlorohydrocarbons, amides, and ethers. Therefore, it is well suitable for the preparation of polymeric membranes by phase separation techniques. CO2/N2 and O2/N2 selectivity of 33 and 6, respectively, comparable to typical commercial membranes, but at a slightly lower permeance (2.3 × 10-11 m3 m-2 s-1 Pa-1 for CO2 and 4.3 × 10-12 m3 m-2 s-1 Pa-1 for O2) than typical commercial membranes, like polyimide composites, were measured for asymmetric PEEK-WC membranes prepared by the dry-phase inversion method.18 Some interesting cases of the few “very high free-volume” polymers are the polyalkynes poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(4-methyl-2-pentyne) (PMP), and the perfluoropolymer Teflon AF 2400 (obtained by copolymerization of tetrafluoroethylene and perfluoro-2,2-dimethyl-1,3-dioxole).19 These high Tg materials can be classified as microporous: they show a high surface area in N2 adsorption experiments (BET), their free-volume is interconnected20 and, therefore, they show extremely high gas permeabilities, which can be 2-3 orders of magnitude higher than those measured for conventional high free-volume polymers (Table 2). As for nanoporous solids, it is believed that these large free-volume elements (0.68 nm radius, as determined by PALS in PTMSP21 and PMP22), about twice the size of free-volume elements in conventional glassy polymers, act as sorption sites that can be occupied by condensable gases, hindering the passage for smaller molecules. PTMSP was first described by Masuda28 and, owing to its outstanding gas permeability and also vapor/gas selectivity, it was the focus of considerable fundamental and applied research for membrane GS.23 Merkel et al.29 reported a n-C4H10/CH4 mixed gas selectivity of 35 in PTMSP membranes, which is the highest value reported for this gas pair. PTMSP and PMP are even more permeable and more selective to condensable vapors than silicone rubber, the industry benchmark polymer for vapor/gas separation. However, the practical utility of PTMSP is limited by a fast physical aging (gradual relaxation of nonequilibrium excess free-volume in glassy polymers) and also by its solubility in many organic compounds which results in potential membrane dissolution in process streams of greatest interest. Some studies were carried out to address the PTMSP aging: Jia et al.30 cross-linked PTMSP with bis azides to stabilize the large excess free-volume elements and observed an improved physical stability, but reduced O2 and N2 permeability. Freeman et al.31 observed a strong enhancement of PTMSP chemical resistance by cross-linking (cross-linked PTMSP becomes insoluble in common PTMSP solvents such as toluene and cyclohexane). The initial permeability of the cross-linked PTMSP membranes decreased when compared to un-crosslinked PTMSP due to free-volume reduction; an increase in the


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Figure 1. Repeat units of glassy and amorphous perfluoropolymers used in the preparation of GS membranes. Teflon AF 2400: x ) 87, Tg ) 240 °C. Teflon AF 1600: x ) 65, Tg ) 160 °C. Hyflon AD80X: x ) 80, Tg ) 134 °C. Hyflon AD60X: x ) 60, Tg ) 130 °C. Cytop, Tg ) 108 °C. Table 3. Permeation Properties of Glassy and Amorphous Perfluoropolymer Membranes permeability (barrer) polymer

O2

N2

Teflon AF 2400a 1600 780 Teflon AF 1600 270 110 Hyflon AD 80 67 24 Hyflon AD 60 57 20 Cytop 16 5.0 a

selectivity (-)

CH4 CO2 C2H6 C3H8 CO2/CH4 reference 600 80 12 10 2.0

3900 370 200 520 150 130 35

6.5 6.5 13 13 18

25 36 36 36 36

Pure gases, P Feed ) 3.5 bar; thickness, 20 µm.

O2/N2 selectivity confirmed that the cross-linked membranes were more size selective. Cross-linking was not successful in maintaining the permeability and vapor/gas selectivity of PTMSP over time, unless 10 wt % of polysiloxysilsesquioxane was added to the polymer: N2, O2, CH4 permeabilities were constant over 100 days. In a different study, it was shown that n-butane permeability decreased for a cross-linked PTMSP membrane, however its C4/CH4 selectivity was higher than for a PTMSP membrane containing 30% nanosized fumed silica (20-30 vs 13-18 at 35 °C).32 PMP, even with lower permeabilities than PTMSP, presents a much better chemical stability which can be useful in hydrocarbon separation.13 Amorphous Teflon AF2400 possesses many advantages as a membrane material, including good film forming properties, extremely high chemical stability, low susceptibility to swelling, and, differently from other high permeability polymers, no detectable aging. In addition, it is insoluble in common organic solvents but shows good solubility in perfluorinated solvents, which are used in the preparation of composite membranes.33 Teflon AF2400 and Teflon AF1600 (DuPont) are the most permeable among perfluoropolymers (Figure 1 and Table 3), a family with excellent thermal and chemical resistance, melt stability, good mechanical properties, usable in a broad temperature range. These features originate from the high energy of C-F (485 kJ/mol) and C-C (360 kJ/mol) bonds. Other perfluoropolymers are the copolymers of tetrafluoroethylene (TFE) and 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD), known commercially as Hyflon AD, (produced by Ausimont, now Solvay Solexis, Italy), and Cytop,34 produced by the Asahi Glass Company (Japan). The FFV of Hyflon AD60X (23%)35 is lower than that of Teflons AF, but still much higher than that of common polymers (e.g., polysulfone, poly(ether sulfone) and polyimide). Hyflon AD60X is a compromise of a moderately high selectivity and a still interesting permeability, in comparison with the more permeable but less selective Teflon AF. It is particularly suitable for use in dense GS membranes, especially in the presence of organic vapors. In perfluoropolymers the solubility selectivity substantially changes, therefore they represent materials suited for challenging separations (e.g., olefin/paraffin or natural gas treatment). Experimental GS data obtained with membranes prepared with TTD-TFE (Hyflon AD) copolymers and data from the literature on Teflon AF membranes revealed an interesting linear relationship between permeation and Tg.37 Perfluoropolymers

present an unusually low hydrocarbon-vapor sorption, and a high resistance toward swelling and plasticization.36 In the case of Hyflon AD38 very high CO2 permeability (260-280 barrer) are reported and CO2/CH4 selectivity around 9-11 in the case of streams containing 20% of CO2 and up to 5.5 MPa. Membranes prepared using Cytop and Hyflon AD present selectivity factors CO2/CH4 of ca. 10-15 also in presence of important amounts of C3+ hydrocarbons and/or CO2, and also at high feed pressure values.36 A composite membrane made by a porous asymmetric hollow fiber with a perfluoropolymer coating was patented by Praxair Technology.39 Prior to coating, the substrate is impregnated with a fluid immiscible with the perfluorinated solvent. Membranes formed by depositing an ultrathin, dense separation layer of an amorphous perfluoropolymer on top of a porous polyethersulfone substrate, and used for the volatile organic compounds (VOCs) separation and recovery were described in a successive patent to Praxair.40 Dense solution-cast membranes of Hyflon AD 60X present an unusually high tendency to retain the solvent;41 this reduces the thermo-mechanical stability and may cause foaming of the polymer films upon strong heating under vacuum. Plasticization of the polymer by the residual solvent decreases the Tg, reduces the permselectivity of the membranes, and increases the gas permeability and the diffusion coefficients of the larger gas species. In comparison, melt-pressed films have a higher selectivity, mostly due to a stronger size-sieving effect, penalizing the diffusion rate of the larger gas species (e.g., CO2, O2, nitrogen, and methane). These differences may be related either to the thermomechanical history of the samples but also to the effect of the casting solvent. Complete removal of the residual solvent from solution-cast films may require conditions (e.g., drying temperatures near or above the Tg) which are normally prohibitive for the membrane integrity. The origin of the solvent retention in Hyflon is not yet fully understood; it could be related to the particularly high FFV, to the free-volume distribution of Hyflon, and to specific polymer-solvent interactions. Polymers of intrinsic microporosity (PIMs) were recently synthesized by McKeown et al.26,27,42,43 These materials are obtained by forming a backbone that has no conformational freedom, but is sufficiently contorted to prevent an effective packing. As conventional molecular sieves, they represent a new class of microporous material (with interconnected pores less than 2 nm in size), generated through polymer chemistry and offering processability combined with control over surface functionality and surface properties. Differently from conventional nonsoluble microporous materials, they have good solubility and easy processability. These materials swell reversibly in the presence of a nonsolvent such as methanol. In PIM polymers the permeability order is CO2 > H2 > He > O2 > Ar > CH4 > N2 > Xe, while in many glassy polymers the typical behavior is He > CO2. The permeation data reported for CO2 and CH4 in these membranes27 are located in between the 1991 and the 2008 Robeson’s upper bounds (e.g., CO2 permeability of 2300 barrer and CO2/CH4 selectivity 18.4) (Figure 3).

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Park et al.44 demonstrated that some dense glassy polybenzoxazoles and polybenzothiazoles permit outstanding molecular and ionic transport and separation performance, beyond the limits of conventional polymers. The peculiar free-volume microstructure in these so-called “thermally rearranged polymers” is created by the thermal decarboxylation of precursor polyimides between 350 and 450 °C, which induces a change in the chain conformations and spatial location of rigid moieties. The size of free-volume elements can be rationally tailored by controlling the degree of the rearrangement and the flexibility of the original chain. CO2 and other small molecules go through hourglass-shaped pores connected by size selective throats which hinder the passage of larger molecules (e.g., CH4): CO2 moves through them hundreds of times faster than in conventional membranes. Thermally rearranged polymers work much better than conventional membranes at separating out CO2 from methane, since they are very permeable (1600 barrer for CO2) and, differently from cellulose acetate and polyimides, maintain CO2/CH4 selectivity high (in excess of 40) even with large CO2 partial pressures. If this material is used instead of conventional cellulose acetate membranes to natural gas process plants, it would require 500 times less space, and would lose less natural gas in their waste products.45 2.2. Solubility-Controlled Membranes. Solubility controlled membranes, sometimes referred to as “reverse-selective membranes”, preferentially allow large gas or vapor molecules to permeate in a gaseous mixture containing smaller molecules. These membranes find application in the separation of CO2 from lighter gases, in VOCs removal from permanent gas streams, in monomer recovery from the exhaust of polymerization reactors, and are studied for the removal of higher hydrocarbons from methane. An interesting application is represented by solubility controlled H2 separation (e.g., CO2/H2 separation). Poly(ethylene oxide) (PEO) is recognized as an interesting material for CO2 membrane separation46 owing to polar ether oxygens that interact with quadrupolar CO2, molecules resulting in high solubility selectivity. The affinity of the membrane material for CO2 must not be too high such that its mobility is reduced, otherwise the CO2 flux will suffer. Copolymers containing PEO blocks, highly flexible, weak size-sieving and with high diffusion coefficients, present high CO2 permeability and high CO2/H2 selectivity.47 PEO, however, shows very low CO2 permeability (ca. 12 barrer at 35 °C and infinite dilution) due to high crystallinity levels;48 the crystalline regions reduce the polymer chain mobility in the amorphous phase and increase size-sieving ability, thus decreasing CO2/H2 selectivity. To hinder crystallization, Freeman et al.49 developed a series of cross-linked and highly branched PEO, introducing short non-PEO segments into the PEO backbone. These membranes were demonstrated to have outstanding separation performance for H2 purification by removing acid gases (CO2 and H2S) from feed streams of practical interest, as well as SO2 and NH3 from nonpolar gases. Differently from conventional membrane materials, the presence of moisture and high-pressure CO2 in the feed improves permeability and selectivity. CO2/H2 separation is also exploited in a hydrocarbon synthesis process (e.g., Fischer-Tropsch), from which both hydrocarbons and high purity hydrogen are obtained. The use of a reverseselective membrane for the hydrogen-depleted stream will provide a CO2-enriched permeate and, advantageously, a hydrogen-containing retentate at elevated pressure.50,51 Solubility is the key factor that enables CO2 to preferentially permeate over the less soluble components including H2.

Solubility controlled membranes based on polyether-polyamide block copolymers (e.g., Pebax) and selectively permeable to CO2 are available from MTR (U.S.A.). The permeation properties of these membranes are optimized for separating CO2 and polar gases from nonpolar gases. Poly(propylene glycol) and poly(ethylene oxide) copolymers and blends with PEO, eventually containing nanofiller particles, havebeenconsideredforsolubility-controlledCO2 separations.52-54 The ethylene oxide copolymers studied in the literature for this purpose include polymers in which the rigid moieties are made of polyimide, polyurethane,55 polystyrene,53 polyesters.56,57 Other polymeric materials that may be of interest for CO2/ H2 separation include polyphosphazenes.58,59 PDMS and PTMSP are more permeable to condensable gases such as CO2 or H2S only at lower temperatures (25 °C); at higher temperatures sorption selectivity decreases and the more mobile H2 becomes more permeable.60 Very high free-volume polymersssuch as poly(4-methyl-2pentyne) (PMP) and PTMSPscontaining nanometric filler particles, mainly fumed silica, have shown interesting n-butane/ CH4 reverse perm-selectivity; these hybrids membrane materials will be discussed later in paragraph 2.4 with the other mixedmatrix membranes. 2.3. Carbon-Based Membranes. Carbon-based membranes can be classified into two classes: (1) carbon molecular sieve membranes (CMSs) and (2) carbon nanotubes (CNTs) membranes. 2.3.1. Carbon Molecular Sieve (CMS) Membranes. Carbon molecular sieve membranes have a very long history. As early as 1955 Barrer and Strachan produced a pioneering work on the adsorption and the diffusion of six permanent gases in microporous plugs made of compressed, high specific surface carbon powder, evidencing the importance of surface flow for the most polarizable species.61 In the next years Barrer and coworkers produced an extensive, detailed, and sophisticated analysis of gas and vapor sorption and diffusion through those early compacted carbon membranes.62 CMSs show excellent intrinsic performance for GS applications. These micro- to nanosize materials are obtained today through the pyrolysis (at high temperature in an inert atmosphere) of polymeric precursors already processed in the form of membranes, and may be considered as just a type of “very high free-volume” materials. Carbon membranes are believed to contain slit-shaped pores among planar aromatic moieties. The mechanism of separation in carbon membranes depends on the pore size, which determines the degree of interaction between molecules and pores. Molecular sieving is dominant when the effective pore diameters are on the molecular scale (3-5 Å). For larger pore sizes in which both components in a mixture can be accommodated, selective sorption is the key factor in determining the real separation properties of the membrane (e.g., the surface selective flow membranes described later in this paragraph). Extensive studies were carried out on the preparation of CMS membranes from both rubbery and glassy polymers.63-69 Polyimides are the most used precursors,70-74 but their use results in high production costs. Therefore, efforts were made to use less expensive starting materials (e.g., polyacrylonitrile75). Other precursors investigated are poly(furfuryl alcohol)76 and phenolic resin.77 Carbon membranes were prepared in both unsupported (typically capillary tubes or hollow fibers and flat membranes) and supported (typically flat or tubular) forms. Supported membranes grafted onto macroporous materials were developed to overcome the poor mechanical stability of unsupported carbon


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Table 4. O2/N2 Selectivities Reported for Carbon Membranes starting material cellulosic or phenolic resins hollow fibers hexafluoroisopropylidene (6FDA)-based copolyimide hollow fibers with O2/N2 selectivities of 4 Kapton polyimide Flat homogeneous films Kapton hollow fibers phenol formaldehyde resin Flat dense films polyimide film coated on porous alumina tubes poly(furfuryl alcohol)

pyrolysis conditions

O2/N2 selectivity

reference

800 and 950 °C (postpyrolysis steps required to open the structure of the 950 °C membranes) 550 °C

7-8

86

11-14

70

800 °C up to 800 °C up to 1000 °C 800-950 °C with a postpyrolysis activation 700 °C post pyrolysis carbon deposition step 450 °C, 120 min

4.2 11 23 11 9.7 14 30.4

72 87

membranes.78 The options for coating the supports with thin polymeric films are ultrasonic deposition,79,80 dip coating,71 vapor deposition,81 spin coating,82 and spray coating.83 Detailed descriptions of the steps involved in carbon-based membrane fabrication (precursor selection, polymeric membrane preparation, pretreatment of the precursor, pyrolysis process, posttreatment of pyrolyzed membranes, and module construction) have been presented in the Ismail and David’s review.84 The manipulation of the pretreatment variables, pyrolysis process parameters, and post-treatment conditions was shown to provide an opportunity to enhance the separation performance of carbon membranes.82,85 Referring to the O2/N2 separation, interesting selectivities were reported for carbon membranes, as summarized in Table 4. One important example is that of the surface selective flow (SSF) membranes developed by Air Products and Chemicals.89 The membrane consists of a thin layer (2-3 µm) of a nanoporous carbon (6-7 Å pore diameter) supported on the bore side of a macroporous alumina tube. This technology is able to produce enriched hydrogen streams at high pressure. Carbon hollow fibers were developed and commercially produced by Israel’s Carbon Membranes Ltd.90 Carbon membranes combine improved gas transport properties for light gases (gases of molecular sizes smaller than 4.0-4.5 Å) with thermal and chemical stability. However, they are not suitable to separate some gas mixtures of interest to industry, such as branched pentanes versus linear hydrocarbon molecules or gas-vapor mixtures (e.g., H2/hydrocarbons).8 A prepurifier is required for removing traces of strongly adsorbing vapors, which can clog up the membrane pores. This pretreatment is typical for many industrial adsorption separators and may be avoided by operating at sufficiently high temperatures.91 One major disadvantage that hinders their commercialization is their brittleness, making them require a careful handling.8,92-94 This may be prevented to a certain degree by optimizing precursors and preparation methods.93 When compared to polymeric membranes, the cost of carbon-based membranes is 1 to 3 orders of magnitude greater per unit area. Only when they achieve a much better performance than polymeric membranes might this high investment cost be justified.95 Despite the great experimental effort devoted to the development of CMS membranes from the 1950s, aging, pore blocking by higher organics, and brittleness are the reasons that induced several companies to give up with their development programs. To the best of our knowledge there is today no producer of commercial CMS membrane modules. 2.3.2. Carbon Nanotubes as Membranes. The possibility of using carbon nanotubes (CNTs) as membranes for GS was initially proposed by MD simulations that predicted transport of gases inside single-walled carbon nanotubes with diameter of ca. 1 nm, orders of magnitude faster than in any other known

88 71 80

materials with nanometer-scale pores (e.g., zeolites).96-99 CNTs are different from other ceramic membranes with atomic-scale pores, owing to the smoothness of the inner surface,100 the high rigidity of the graphene plane and the nonpolar nature of the sp2 carbon network.101 Some of the theoretical predictions were verified experimentally with larger carbon nanotubes.102,103 Hinds et al.102,104 and then Holt et al.103 reported the synthesis of free-standing and silicon-chip supported vertically aligned carbon nanotubes membranes by complex multistep synthesis procedures. They used chemical vapor deposition to grow oriented carbon nanotube carpets. Hinds and co-workers102 developed polymer-nanotube composite membranes using multiwalled carbon nanotubes (MWNTs) with large diameters (6-7 nm) and verified that the transport of liquids (alkanes, water) is orders of magnitude faster than can be accounted for by conventional hydrodynamic flow.104 Holt et al.103 have developed nanotube-Si3N4 composite membranes. They produced dense parallel arrangements of double-walled carbon nanotubes (DWNTs) having a diameter of about 1.6 nm with both sides open in a supported membrane structure. To develop this highly perfect geometry they used advanced microfabrication facilities: they surrounded the nanotubes with a matrix of silicon nitride, before ion milling to remove excess silicon nitride, and reactive ion etching to open up the ends of the nanotubes. Gas flow is 1-2 orders of magnitude faster than through a commercial polycarbonate nanoporous membrane with 15 nm pore size. The water flow through their membranes was more than 3 orders of magnitude faster than expected from hydrodynamic flow calculations. Moreover, these nanotube membranes exhibited extraordinarily high size exclusion selectivity for 2 nm colloidal gold particles. The move to devices suitable for large-scale applications requires to scale up the fabrication techniques to economically produce membranes with large surface areas. Chemical vapor deposition, while producing well-aligned carbon nanotubes, is expensive, long, and is limited to fabricating samples with small areas (e.g., subcm2). CNTs were aligned in the pores of a hydrophilic polyamide filter105 although to date this approach was successful only with multiwalled nanotubes (MWNTs). Lin et al.106 developed vertically aligned MWCNT membranes using CNTs grown above the surface of a porous R-Al2O3 disk; the method can be scaled up for practical applications. These membranes have lower areal tube density as compared to CNT membranes on dense silicon and quartz support, and their gas permeance is inversely proportional to the squared root of the gas molecules. However, the diffusivity of gases is only about four times larger than predicted by the Knudsen model. Compared with other CNT membranes,103 the enhancement in permeability or diffusivity in CNT membranes due to atomically smooth CNT pore wall decreases with increasing CNT pore size.

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The use of single-walled carbon nanotubes (SWNT) with smaller diameter as membranes is particularly interesting because, in addition to fast transport rates, the 4-12 Å pore openings might be size-selective for gas mixtures. However, they have to be aligned vertically relative to the penetrant stream. This is the most important challenge facing the fabrication of SWNT membranes. Marand et al. produced vertically aligned SWNT membranes embedded in polysulfone.107 The authors believe that SWNTs orient in the shear flow produced during filtration and propagate the perpendicular alignment to the PTFE filter via long-range repulsive forces between zwitterions attached to the carbon nanotube surface. This filtration method facilitates the orientation of carbon nanotubes on porous supports and can be adapted to large-scale membrane formation. The gas transport properties of the nanotube/polymer membranes are analogous to those of CNT membranes grown by chemical vapor deposition. The authors obtained the first data for transport of CO2/CH4 mixtures through CNT membranes; the comparison with the predictions of atomistic simulations of mixture diffusion seems to indicate the presence of transport resistances for molecules entering and leaving the nanotube ends. Transport of gas and water through the nanotubes was much higher than predicted by classical diffusion or hydrodynamics models. The transport rates observed by Holt et al.103 for different gases are one to 2 orders of magnitude larger than would be predicted by assuming a Knudsen description, which is in quantitative agreement with predictions from simulations.108 Alternative transport mechanisms were proposed that emphasize transfer limitations at the tube entrance. The very efficient transport in the tubes makes the kinetics of surface transfer becomes more important.101 MD simulations for mixture separation in SWNT membranes predict the preferential adsorption of CH4 relative to H2 and CH4/H2 selectivities as high as 10-20.109 Despite the exceptionally fast mass transport for both gas and water, the enhancement of the selectivity of CNT membranes is a big challenge. The DWNT membranes may not be sufficiently cost-effective for large-scale applications but might be developed for the separation of valuable molecular species by size exclusion. The road to useful industrial applications of CNT membranes may be a long and arduous one, due to the selectivity and cost requirements, but the potential payoff is very large. Atomistic simulations can provide some help and guidance. These preliminary results should also motivate other approaches to membrane fabrication that use CNTs in a composite nanotube/polymer membrane which can result in the mass production of very efficient devices. 2.4. Mixed-Matrix Membranes. Mixed-matrix membranes (MMMs) are a well-known route to enhance the properties of polymeric membranes.110 Their microstructure consists of an inorganic material in the form of micro- or nanoparticles (discrete phase) incorporated into a polymeric matrix (continuous phase). The use of two materials with different flux and selectivity provides the possibility to better design a GS membrane, allowing the synergistic combination of polymers’ easy processability and the superior GS performance of inorganic materials. In principle, the incorporation of the inorganic component can be seen as a relatively easy modification of existing methods for fabricating large-surface area polymeric membranes; therefore, MMMs possess an economic advantage over inorganic membranes. In addition, they may offer enhanced physical, thermal, and mechanical properties for aggressive environments and could be a way to stabilize the polymer

membrane against change in permselectivity with temperature.111 These membranes offer very interesting properties; however, their cost, difficulties for commercial scale manufacture, and brittleness remain important challenges. The successful development of MMMs depends on several factors, such as the proper selection of polymeric matrix and inorganic filler and the elimination of interfacial defects between the two phases. It is also important to control filler concentration, shape, and dimensions in order to reach the expected performance. Koros et al.112,113 proposed some criteria for material selection and preparation protocols in order to match the necessary transport characteristics of materials to prepare high GS performance MMMs. A recent comprehensive review on this topic was presented by Chung et al.114 The authors underline that the major GS applications addressed by researchers working on MMMs include air separation and natural gas separation, while the investigations on high added value separations are limited. Moreover, research on fabrication of MMMs hollow fibers is still partial.115 For the formation of thin, defect-free mixedmatrix structures as asymmetric membranes, several aspects are relevant: the rheological properties of the dope solutions, the spinning conditions, and the shape, size, and wettability of the particles. Several inorganic materials, porous and nonporous, were considered for the preparation of MMMs. These inorganic particles can improve the separation (e.g., by molecular sieving mechanism) or increase the membrane free-volume.110 Molecular sieves were chosen to enhance selectivity for a given gas mixture by increasing the sorption of the desired gas component. In particular, zeolites were widely used in rubbery and glassy polymers.116-120 Pioneer work was carried out by Paul and Kemp118 and further expanded by Kulprathipanja et al.121 using rubbery polymers. According to Paul and Kemp,118 the incorporation of zeolite 5A into silicone rubber did not improve the separation properties of the polymer. Initial success (significantly improved performance for separation of O2/N2 and CO2/CH4) was then achieved incorporating zeolites in highly flexible rubbers such as polydimethylsiloxane and ethylenepropylene diene rubber (EPDM),116 and in glassy flexible polymers, such as cellulose acetate121 and polyvinyl acetate (PVAc).113 Jia et al.117 also found higher O2 permeabilities (from 571 to 655 barrer) and O2/N2 selectivities (from 2.14 to 2.92) in silicalite-1 filled silicone rubber (PDMS) membranes than unfilled membrane. Duval et al.116 showed that zeolites (silicalite-1, 13X and KY) improve, to a large extent, the separation performances of poorly selective rubbery polymers for a CO2/ CH4 mixture, while the CMSs did not enhance the separation performance. The flexibility of the rubbery polymers was suggested as the primary cause of good polymer-zeolite interaction and stress-free interfaces with the zeolite.113 An adequate contact between the dispersed phase of the molecular sieve and the polymeric phase was generally observed when using rubbery polymers as membrane matrix. However, the high gas fluxes of polymeric rubber matrices can lead to low improvement in the MMM selectivity. The improvements in separation performance found by adding zeolites to rubber polymers stimulated other research to the use of rigid glassy polymers. This theme has faced some difficulties, particularly in controlling the adhesion between the polymer phase and the external surface of the particles.122-124 Initial efforts at fabricating MMMs using glassy polymers and zeolites resulted in the formation of voids at the interface between the polymer and the filler due to the weak polymer-filler interaction125 that reduced the separation performance of the composite

Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009 113,113

membrane with respect to the unfilled polymer. To eliminate the unselective gaps between the polymer and zeolite, different strategies were proposed:120,125-128 (i) surface modification of the zeolite external surface, (ii) use of a silane coupling agent to introduce favorable interactions between polymer and zeolite, (iii) addition of a plasticizer to increase the flexibility of the polymer or chemically linking the two components together, and (iv) high temperature casting and removal of the dense films from the casting surface at solution T g. Vankelecom et al.,129 investigating the incorporation of zeolites in polyimide matrices, observed a weak adhesion between the two phases. The same authors then proposed the silylation of the zeolite outer surface to improve their incorporation in PI films,130 with nonappreciable improvement of the polymer-filler contact. However, the use of modification techniques has resulted in progress in this field:131,132 some efforts seemed to eliminate the presence of voids between the polymer and the zeolite, but the resulting permeability of the MMM was often sacrificed. Marand et al.133 used a block copolymer containing both flexible and rigid segments. The polydimethyl siloxane (PDMS) segments provide flexibility and good contact with the zeolite surface, removing the need of coupling agents, plasticizers, or chemical reactions. The addition of PDMS moieties to the rigid polyimide backbone (with higher selectivity than the PDMS) resulted in increased permeability and decreased selectivity. However, the further addition of zeolite L (0-20%) to the copolymer systems resulted in a loss of both permeability and selectivity. These effects were attributed to the blocking of the zeolite pores by the flexible PDMS segments. An increase of n-butane/i-butane selectivity was observed instead after the addition of silicalite-1 (MFI) crystals to a PTMSP matrix.134 Interesting results for MMMs from polyethersulfone and hydrophilic zeolites (13X and 4A) were reported by Suër et al.135 For both zeolites, gas permeabilities (N2, O2, Ar, CO2, H2) and selectivities increase with the zeolite loading. However, a high amount of zeolite induces formation of microporous cavities and channelling, demonstrating the weak interactions and incompatibility of these materials. To reduce the voids between polymer and filler, 2,4,6-triaminopyrimidine (TAP) was used by Yong et al.126 and different zeolites of LTA and FAU topology (4A, 5A, 13X, Na-Y) were dispersed in a Matrimid polyimide matrix, resulting in very high CO2/CH4 separation factors (617 in the case of PI/4A/TAP). In the past few years it has been demonstrated experimentally that in some cases mixed-matrix membranes containing welldispersed “flakes” obtained by exfoliation of molecular sieves136-138 show improved separation performances and sieving capabilities. If the platelets are selective for only one species in a mixture, and their larger surface is parallel to the membrane skin, that is, normal to the direction of the flux, then theoretical results indicate that minor amounts of well-dispersed sieving “flakes” (3-5%) can enhance dramatically the separation performance.139,140 Very thin (2-3 nm), porous, exfoliated zeolite sheets have been obtained already for incorporation in polymeric matrices.141 These developments are particularly interesting since in real industrial GS membranes the separating layer is only 100-200 nm thick. The industrial interest in MMMs can be also appreciated from four recent patents, two to UOP, one each to Air Liquide and Chevron. The first UOP patent142 presents high flux MMMs that are prepared by incorporating porous inorganic fillers (e.g.,

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Table 5. CO2 and CH4 Pure Gas Permeation for AlPO-18-PIM1 MMMsa 142 Pe CO2 ∆Pe Pe CH4 CO2/CH4 ∆CO2/CH4 membrane dense film (Barrer) CO2 (%) (Barrer) (-) (%) PIM1 30% AlPO-18-PIM1 a

4792 8202

71

590 630

8.13 13.0

60

Tests at 50°C and 690 kPa.

Table 6. Propylene and Propane Pure Gas Permeation for AlPO-14-PIM1 MMMsa 142 membrane dense film

Pe C3) (Barrer)

∆Pe C3 (%)

Pe C3 (Barrer)

C3)/C3 (-)

∆C3)/C3 (%)

PIM1 30% AlPO-14-PIM1

2046 3111

52.1

657 759

3.12 4.10

31

a

C3), propylene; C3, propane. Tests at 50°C and 207 kPa.

microporous and mesoporous molecular sieves, carbon molecular sieves, porous metal-organic frameworks) into the high flux PIMs. Some permeation data for these membranes are presented in Tables 5-6, showing improved separation performance in the CO2/CH4 and propylene/propane separation. MMMs of polyimide and SSZ-13, a zeolite of CHA topology with narrow pore size, were patented by Air Liquide143 for the production of O2-enriched air or N2-enriched-air, for the separation of CO2 from hydrocarbons or nitrogen, and the separation of helium or hydrogen from various streams. The MMMs provide improved performance compared to pure polymer membranes, particularly when used to form asymmetric film membranes or hollow fiber membranes and do not interact with organic solvents present in the process streams. Chevron144 patented MMMs by using silica molecular sieves washed in water at 95 °C for 18 h prior to calcination, a process called “super water washing”. It is claimed that “super washing” the sieves removes foreign amorphous materials adhering to the outer surface and most alkali metals from the sieves, and therefore enhances, compared to conventional preparation techniques, the ability of the molecular sieves to bond with an organic polymer to form a highly selective and permeable MMM. PDMC, a fluorinated polyimide containing dangling carboxylic acid groups, can be cross-linked with 1,3-propane diol. When SSZ-13 is treated with aminopropyldimethylethoxysilane and dispersed in cross-linked PDMC, an increase is observed in both mixed gas CO2/CH4 selectivity (up to 47) and CO2 permeability (up to 89 barrer) compared to the pure cross-linked PDMC, surpassing the 1991 Robeson’s upper bound,145 even though these membranes did not display the selectivity enhancements forecast by the Maxwell model.146 In addition, these membranes proved resistant to CO2 plasticization up to 30 bar. Among the works involving the addition of CMS to polymeric matrices, Vu et al.147 prepared MMMs by using two commercial glassy polymers: Ultem 1000 (polyetherimide) and Matrimid 5218 (polyimide). These MMMs exhibited excellent polymerfiller contact and a remarkable higher performance compared with those of the pure polymer. For example, Ultem-CMS and Matrimid-CMS MMM, showed enhancements by up to 40-45%, respectively, in CO2/CH4 selectivity (pure gas) with respect to the pure polymers. Marchese et al.148 developed MMMs for CO2/CH4 separation using an acrylonitrile-butadiene-styrene copolymer and two micro-mesoporous activated carbons. The composite membranes did not exceed the 1991 Robeson’s upper bound, but they displayed improved performances within the attractive region. The best GS performance at 20 °C (PeCO2 )

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11-21 barrer; CO2/CH4 selectivity ) 35-51) were achieved without compromising their mechanical stability. Metal oxides nanoparticles were also considered for preparing MMMs for GS applications. Moaddeb and Koros149 introduced SiO2 particles into six high performance polymers and found improved GS performance of the selective layer, particularly for O2 and N2. This was mainly attributed to the enhanced rigidity of polymer matrix due to the adsorption of polymer chains to the surface of silica. Hu et al.111 developed MMMs based on fluorinated poly(amide-imide) and TiO2 via sol-gel technique. Their composite membranes showed higher permselectivities for selected gas pairs when compared to the pure polymer. These results were attributed to the denser and more rigid structure of the composite membranes which was detected by the increase in apparent activation energy of permeation. Lin et al.150 introduced the use of MgO nanoparticles as fillers into a cross-linked poly(ethylene glycol) diacrylate matrix for CO2/H2 separation. In the optimum condition, they achieved significantly improved CO2 permeability while maintaining selectivity by loading ca. 32 wt % of MgO. The enhanced CO2 permeability was ascribed to the effect of nanoparticles on gas diffusivity. Hosseini et al.151 introduced MgO nanoparticles into a Matrimid matrix followed by treatment with silver ions. The best performance (CO2/CH4 and H2/N2 selectivity increased by 50% and 35%, respectively, compared to the pure Matrimid membrane) was observed for nanocomposite membranes with 20 wt % MgO and after a 10-day silver ion treatment. In principle, the gas permeation properties of nonporous inorganic filled MMMs can be predicted by the Maxwell equation146 as a decreasing permeability with increasing filler content, provided that the filler particles do not interact with each other (diluted phase), and the bulk properties in one phase are not influenced by the other phase. Extended Maxwell equations can be devised in order to take into account relatively high volume fractions of fillers, both porous and nonporous.152 An enhancement of the gas permeability with nanosized particles was reported by different groups, and explained as a result of an increase in free-volume because of the inefficient chain packing as well as the presence of extra void volume at the interface between polymer and filler. This behavior was observed for polysulfone/silica nanoparticles MMMs.153 Moreover, the permeability of large gases is preferentially enhanced by the addition of silica resulting from the increase in freevolume, which strongly increases the diffusion coefficient and permeability, and results in a reduction in pure-gas selectivity. Nanoscale, nonporous fumed silica (FS) particles were reported to systematically increase penetrant permeability also in Teflon AF2400, similar to the behavior observed in vapor-selective polyacetylenes, but contrary to results in traditional filled polymer systems.154 This is the result of increased diffusion coefficients, as already observed for filled PMP and PTMSP. PALS revealed that FS addition increases the size of free-volume elements, resulting in augmented penetrant permeability and diffusion coefficients, similar to the behavior of FS-filled polyacetylenes. Permeability of large penetrants increases more than those of small molecules in filled Teflon AF2400, thereby decreasing the size selectivity of this polymer. As a result, AF2400 exhibits a selectivity reversal for the vapor-gas pair n-C4H10/CH4, becoming n-butane selective above 18 wt % FS. Moreover, while AF2400 is readily plasticized by n-butane, AF2400 containing 40 wt % FS exhibits an antiplasticization behavior. The addition of nanoscale FS particles to Teflon AF2400 has no detectable effect on the polymer’s Tg, indicating that the filler has little impact on polymer chain stiffness or

mobility. FS-induced permeability enhancement decreases with increasing base polymer permeability. The incorporation of FS nanoparticles to cross-linked PTMSP increased the initial permeabilities;32 however, these composite membranes did not show stable permeability over time. The addition of hydrophobic silica (up to 50 wt %) to PTMSP was reported to increase permeability and decrease the propane/H2 selectivity.155 Besides an increase in large free-volume size (determined by PALS) with increasing filler content, interstitial mesopores (located among the particles of a silica agglomerate) were observed. The size of these interstitial cavities was increasing with filler concentration. The presence of mesopores in the MMMs explains also the reduced selectivity compared with unfilled PTMSP membranes. Thermal analysis showed that FS has a retarding effect on polymer decomposition. Few discussions are present in the open literature on the spinning of asymmetric hollow fiber MMMs for GS. Jiang et al.156 described the formation of zeolite-polymer hollow fiber MMMs which were defective. Successful mixed-matrix hollow fibers for GS were presented by Husain and Koros.157 The authors increased the hydrophobicity of the zeolite surface by capping surface hydroxyls with hydrophobic organic chains via a Grignard-type reaction. The fibers incorporating the treated zeolites showed a selectivity enhancement of 10% for O2/N2, 29% for He/N2, and 17% for CO2/CH4 pure gas pairs, respectively, and 25% for CO2/CH4 mixed gas. The success of the “Grignard treated” zeolites in the polymer matrix also highlights that coupling of the polymer to the sieve surface is not a prerequisite for successful MMM formation. 2.5. Zeolite Membranes. In the last two decades, zeolite membranes were extensively studied, considering preparation techniques and separation features because of their higher thermal and chemical resistance compared with those of polymer membranes. Zeolite membranes in principle might separate continuous mixtures on the basis of differences in the molecular size and shape,158 but also on the basis of different adsorption properties.159 The separation ability of a microporous membrane can be described by the interplay of the mixture adsorption equilibrium and the mixture diffusion in a way similar to the solubility-diffusion model established for describing the permeation behavior of dense organic polymer membranes. Different methods for the controlled preparation of supported zeolite membranes have been established.160 Basically, they can be distinguished into one-step methods and in secondary growth (seeding techniques) methods. The latter, decoupling zeolite nucleation from crystal growth, allows optimizing the conditions of each step independently, reducing or suppressing any secondary nucleation.161 Seeding, the first step, consists in the deposition of the crystal seeds on the surface of a support, followed by a crystal growth by means of a hydrothermal treatment. The advantage of the membranes made by secondary growth is the high flux and the possibility to orient the seed crystals;162 the drawback is the ease of formation of defects or non-zeolitic pores, which form intercrystalline pathways, larger than the zeolite pores and therefore not selective. In an interesting one-step method (pore-plugging synthesis) zeolite crystals are grown within the pores of a macroporous support, yielding very robust, defect-free MFI membranes,163 whose permeance however is not very high. Researchers from Colorado164 have recently demonstrated that MFI zeolite membranes swell when exposed to normal alkanes (C3-C6) and SF6, but not to CO2 or benzene. The experimental evidence indicates that MFI membranes with significant flow through defects can be highly selective if the defects are small


enough and one of the molecules in the mixture swells the crystals, thereby shrinking the defect size. Usually, the transport of a gas through a zeolite membrane as a function of temperature goes through a maximum, then decreases, and rises again at higher temperatures.165 For several years the transport of gases in zeolite membranes has been interpreted by the superposition of the Stefan Maxwell approach at low temperatures, which takes into account only adsorption,166,167 and of the so-called activated Knudsen diffusion at higher temperatures.168,169 Interestingly, the transport of gases through MFI membranes prepared by plugging the pores of the ceramic support, rather than growing them on top, do not show any increase of permeance after the lower temperature maximum and can be perfectly fitted by the Maxwell-Stefan model alone.170 The authors believe that the increase in the gas permeance observed at higher temperatures through film-like membranes on top of a support is due to the reversible opening of defects, probably because of the mismatch of the thermal expansion coefficients of zeolite and support. Many types of zeolite membrane have been studied (e.g., MFI, LTA, MOR, FAU). The first commercial application is that of LTA zeolite membranes for solvent dehydration by pervaporation.171 Some other plants were installed since 2001, but no industrial applications are reported for zeolite membranes in the GS field.160,172 The reasons for this limited application in industry might be due to economical feasibility (development of higher flux membranes should reduce both costs of membranes and modules) and poor reproducibility. Little progress is seen in the development of small-pore zeolite membranes for H2 separation.160 A H2/N2 separation factor of 24 was reported for the small-pore zeolite A.173 No higher values were reported, owing to the presence of intercrystalline spaces in the zeolite layer. Interesting methods to improve the separation performance of zeolite membranes, particularly for small gas molecules, are related to postsynthesis modification by silylation to decrease the pore size and to increase the hydrophobicity.174,175 A MFI membrane with a H2/ N2 separation factor of 1.4-4.5 reached values of 90-140 after silane catalytic cracking.174 Y-type zeolite membranes were demonstrated as potential candidates for CO2/N2 separation.176 The separation of light hydrocarbon (C3H6 and C3H8)/N2 mixtures was investigated using FAU membranes,177 considering a ternary mixture having a composition of a typical polypropylene vent stream (12.5% C3H6, 12.5% C3H8, 75% N2). Only C3H6 can be recovered from such a mixture. This result is very important because C3H6 is the valuable monomer and if recovered with suitable purity it can be recycled back to the polymerization reactor. With the development of capillaries, fibers, and multichannel modules the membrane area to module volume ratio could be improved. Current estimated costs per zeolite membrane GS module are of ca. $4,000/m2, comparing favorably with that of metal membranes and modules (ca. $16,000/m2). It can be expected that, once in mass use and production, those costs will reduce significantly to less than $1,000/m2, allowing zeolite membranes to compete both in economics and on performance.95 Challenging separations as paraffin/olefin, CO2/other gases, and aromatics/aliphatics might be solved using zeolite membranes.160 2.6. Facilitated Transport Membranes. Facilitated transport membranes (FTMs) have also shown promising results for GS. These membranes comprise a carrier (typically metal ions) with

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a special affinity toward a target gas molecule and this interaction controls the rate of transport. FTMs can be categorized into four major types:178 (1) immobilized liquid membranes, (2) ion-exchange membranes, (3) polymer/metal ion dispersions, and (4) modified polymer membranes. Selection of suitable carrier with high affinity toward the desired molecules, in conjunction with the adoption of appropriate strategy for design and fabrication of membranes, are the principal criteria to use the advantages of facilitated transport for GS. The primary drawback to supported liquid membranes is that over time the aqueous carrier solution evaporates or is pushed out of the membrane pores, resulting in a nonselective transport.4 Thick membranes will present improved stability and reasonable lifetimes but a lower gas flux. Even though other means of improving the stability of supported liquid FTMs have been explored, including adding dense sealing layers, or less volatile carrier solvents, their inherent instability limits their commercial exploitation. Room temperature ionic liquids (RTILs), with negligible vapor pressure, overcome the problem of volatility. Suitable polymerizable surfactant RTILs form a variety of lyotropic liquid crystalline phases when mixed with water or simple RTILs. The polymerization of RTILs may yield polymeric materials with ordered nanopores, which are of great interest for CO2 separations from N2 and CH4.179,180 Kovvali and Sirkar reported an excellent CO2/N2 selectivity at atmospheric pressure for the viscous and nonvolatile liquid poly(amidoamine) (PAMAM) dendrimer, as an immobilized liquid membrane.181,182 The PAMAM dendrimers kept the good CO2 selectivity also in the presence of water vapor. This is very interesting for the application in CO2 separation from combustion flue gases, since exhausted gases usually contain saturated water vapor. However, this immobilized liquid membrane has insufficient tolerance to handle the large trans-membrane pressure differences often required for obtaining sufficiently large gas fluxes. To overcome this problem, PAMAM dendrimer composite membranes comprising a gas selective layer on a porous substrate were developed.183 The composite membrane showed a CO2 permeance of 4.6 × 10-10 m3 (STP) m-2 s-1 Pa-1 (61 GPU) and a CO2/N2 selectivity of 230 at a pressure difference of 97 kPa at 40 °C, meeting the conditions for practical application. The PAMAM dendrimer composite membrane also showed CO2/CH4 selectivity similar to that of CO2/ N2. Improvements to the mechanical properties and CO2 separation capabilities of the PAMAM dendrimer composite membrane are required. Limitations of immobilized liquid membranes in real operating conditions184,185 led to the development of ion-exchange membranes. Way et al. prepared ion-exchange membranes containing organic amine counterions which showed enhanced performance for CO2 removal from methane streams.186 A FTM for CO2 separation was patented by Ha¨gg et al.187 The membrane has a support coated with cross-linked polyvinylamine; a fixed carrier in the membrane helps so that the CO2 molecules in combination with moisture form HCO3- ions, which are then quickly and selectively transported through the membrane.188 The technology is effective and eco-friendly, and can be used for CO2 removal in coal-powered plants. Its effectiveness increases proportionally to the CO2 feed concentration. Within a 5 year period, these membranes will be tested in four large power plants in Europe. These membranes are

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under further development within the European project NanoGloWa (Nanostructured Membranes against Global Warming).189 Silver ions were widely investigated as carriers for the production of FTMs: they facilitate the transport of olefins, forming reversible π-complexation with molecules containing double-bonds, leading to a subsequent solubility enhancement. This mechanism was successfully exploited for the laboratory scale olefin/paraffin separation.190,191 Long-term instability were recognized for polyether-polyamide block copolymer (Pebax 2533) membranes containing high loadings (>30%) of AgBF4.192 This study highlights as olefin permeation rates and olefin/ paraffin selectivities in silver salt membranes are time dependent, both decreasing over time. This behavior is mainly related to the reduction of the available silver carrier ions (e.g., owing to hydrogen, hydrogen sulfide, and acetylene) and to olefin-induced structural changes that occur in the membranes over time. Almost all aged and some poisoned Pebax +AgBF4 membranes (except those exposed to H2S) can be regenerated by a periodic contact with an oxidizing vapor or liquid treatment, similarly to back-flushing treatments for ultra- or microfiltration membranes to manage fouling. Today sulfur poisoning is the major problem that makes Ag+ containing membranes unfit for industrial use. An interesting recent paper on new materials for natural gas separation presented the development of polyethersulfone (PES)NaA zeolite and PES-AgA zeolite MMMs fabricated at high temperatures.193 The CO2 permeability of PES-AgA zeolite MMMs is higher than that of PES-zeolite NaA, whereas their CH4 permeability is lower. This trend might be the result of a reversible complexation reaction between the silver ion and the CO2 molecule. CO2 and CH4 permeability of PES-zeolite AgA MMMs decreases with increasing zeolite content owing to the partial pore blockage of the zeolite and to the polymer chain rigidification, whereas their CO2/CH4 selectivity increases with zeolite loadings: the highest value reaches 59.6 at 50 wt % zeolite loading, probably because of a combined effect of the facilitated transport mechanism of the silver ion and the molecular sieving mechanism of the zeolite. Both CO2-induced plasticization test and CO2/CH4 mixed gas measurement through this PES-zeolite AgA MMMs proved suitable for the separation of natural gas. 3. Summary of Literature Data For comparison purposes of different materials suited for a particular separation, a selectivity vs permeability diagram can be employed. Membrane permeance provides a figure of merit for membrane performance for practical applications, but permeability data are often reported and for research purposes can be more interesting. In the following, some literature data are presented in selectivity vs permeability representation for the main gas separation applications. An upper bound for polymeric materials is also represented. Figure 2 reports some experimental results reported for different polymers, PIM materials, MMMs, and carbon membranes in the case of O2/N2 separation, together with the 1991 and the 2008 Robeson’s upper bound.6,194 An improvement in selectivity was observed for Hyflon membranes,195 together with a reduced permeability when the membranes are heated up to 200 °C for the removal of any residual solvent. PIMs data are close to the “present” upper bound; some MMMs and carbon MMMs overcome this limit. Figure 3 reports some data for the separation CO2/CH4, together with the upper bound presented in the Robeson’s papers

in 19916 and 2008.194 When the performances of MMMs and the one of the pure polymer are compared (Figure 2) the latter is always worse. The data available for PIMs are close to the last reported upper bound, while the points over the upper bounds are related to MMMs prepared with PIMs, other types of MMMs, and the XLPEO operated at -20 °C. The performance of commercial membranes used to separate carbon dioxide from high-pressure natural gas, reported by Baker,208 is enclosed in the shaded area below the 1991 upper bound. In Figure 4 are collected different data for the case of CO2/ N2 separation. No upper bound was reported by Robeson for this separation in 1991; the limits in Figure 4 are taken from three different literature papers206,221,222 together with the 2008 upper bound.194 Also in this case the materials showing better transport properties are carbon membranes and MMMs. 4. Industrial Applications of Membrane GS The main industrial applications developed for membrane GS are summarized in Table 7. In the following paragraphs some of the industrial processes will be discussed. 4.1. Hydrogen Recovery. Hydrogen recovery was among the first large-scale commercial applications of membrane GS technology. A comprehensive review on this topic was recently presented by Ockwig and Nenoff.95 The commercial success in the mid-1970s of the Permea hollow-fiber prism system for in-process recycling of hydrogen from ammonia purge gases was the starting point of the penetration of membrane technology in large-scale manufacturing. (Formerly a Monsanto Company, Permea, Inc. is now owned by Air Products and Chemicals, Inc.) A two-step membrane design (as that in Figure 5a) was chosen for this ideal application for membrane technology. In fact, the ammonia reactor operates at high pressures (ca. 130 bar), thus providing the necessary driving force for separation; the H2/N2 membrane selectivity is high and the feed gas is free of contaminants (e.g., heavier hydrocarbons). This technology has been extended to other situations for recovery of hydrogen from gas mixtures (H2/CO or H2/CH4 ratios adjustment for syngas production) and has been successfully competing with cryogenic distillation and pressure swing adsorption (PSA) processes. H2 recovery from refinery streams is an emerging field for membrane GS in the petrochemical industry; it is a key approach to meet the increased demand of hydrogen (for hydrotreating, hydrocracking, or hydrodesulfurization processes) owing to new environmental regulations. An example is the H2 recovery from high pressure purge gas of a hydrotreater (Figure 5b). A lower investment cost than PSA or cryogenic separation was estimated by Spillman for the H2 recovery from refinery offgas by polymeric membranes (polyimide membranes, Ube).230 This comparison is from 1989; since then membrane capital prices have dropped and the separation performances have improved. China’s first hydrogen recovery unit by membrane technology from refinery’s hydrocracking dry gas and PSA resolving gas ran a successful trial production in Sinopec Zhenhai Refining Plant.231 The membrane in the Prism system is a polysulfone hollow fiber with a thin silicone film on it. Polyimide membranes are currently successfully applied for H2-recovery in refineries, due to their stability and interesting separation factors (H2/N2 of ca. 100-200).233,234


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Figure 2. Robeson’s diagram for the O2/N2 separation.

Commercial polymeric membrane modules (e.g., MEDAL)235 can operate at high pressure (120 bar), with flow rates up to 330 000 Nm3/h; hydrogen recovery can reach 98% in volume, with hydrogen purity as high as 99.9%. Hydrogen is largely produced by natural gas steam reforming which results in a gaseous mixture containing mainly hydrogen

and also carbon dioxide and carbon monoxide. Conventional membranes, which allow H2 to pass through while retaining other gases, would decrease hydrogen pressure. Solubility controlled membranes that permit larger gas molecules like CO2 and polar molecules pass through, allow the bulk gas to be retained at feed pressure, thereby minimizing or avoiding

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Figure 3. Robeson’s diagram for the CO2/CH4 separation.


Figure 4. Robeson’s diagram for the CO2/N2 separation.

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Table 7. Main Industrial Applications of Membrane GS separation H2/N2 H2/CO H2/hydrocarbons O2/N2 CO2/hydrocarbons (CH4) H2O/hydrocarbons (CH4) H2S/hydrocarbons He/hydrocarbons He/N2 hydrocarbons/air H2O/air volatile organic species (e.g., ethylene or propylene)/ light gases (e.g., nitrogen)

process ammonia purge gas syngas ratio adjustment hydrogen recovery in refineries nitrogen generation, oxygen-enriched air production natural gas sweetening, landfill gas upgrading natural gas dehydration sour gas treating helium separation helium recovery hydrocarbons recovery, pollution control air dehumidification polyolefin purge gas purification

H2recompression, even if it potentially adds a CO2 compression step. Hydrogen membrane rejection and contaminate permeation is currently explored for H2 purification, primarily using carbonbased membranes.95 4.2. Air Separation. The small difference between kinetic diameters of nitrogen (3.64 Å) and oxygen (3.46 Å) molecules makes air separation very difficult by a simple size effect. The

Figure 5. Photographs showing H2 membrane separation plants (from PRISM brochure232). Top photograph: two-stage ammonia installation operated since 1979, that recycles 90% pure H2 to the reactor. Bottom photograph: refining installation operated since 1980 for upgrading a portion of a naphtha hydrotreater off-gas to produce make-up H2 (>97%) for a hydrocracker. Reprinted with permission from ref 232. Copyright Air Products and Chemicals, Inc.

Table 8. Progresses of the Membranes for the O2/N2 Separation (25°C)236-240

year

polymer

1970s 1982 1985 1986 1989 1989 1996 2005

poly(vinyltrimethylsilane) poly(4-methyl-1-pentene) ethyl cellulose polysulfone poly(phenylene oxide) halogenated polycarbonates (35 °C) polyimides polyimides

O2/N2 O2 permeability selectivity (Barrer) (-) 47 24 15 1.2 16 2.3-1.4 4-0.1 18-0.079

4.3 3.6 3.4 6.0 4.0 6.4-7.4 6-9 9-19.8

membrane performance for separation of air components has advanced during the past 15 years, as shown in Table 8 that evidences the trade-off between permeability and selectivity with polymeric membranes. As pointed out by Baker,3 improvements in membrane O2/ N2 selectivity are beginning to reach a point of diminishing returns. Increasing the O2/N2 membrane selectivity from the actual value of 8 to 12 (at the same permeation rate), will reduce compressor size by about 20%, resulting in a 10-15% reduced nitrogen production costs.3 In today’s highly competitive world, such an improvement is unlikely to greatly expand the total market. Nitrogen production by membrane systems was a great success and today is the largest GS process in use. Nitrogen gas is used in many applications related to safety (e.g., to prevent fires and explosions in tanks and piping systems and to prevent equipment degradation, during shutdown periods, in compressors, pipelines, and reactors). Membranes are dominating for applications of less than 50 tons/day and relatively low purity (0.5-5% O2). As pretreatment, particulates, water, and oil in compressed air are removed by coalescing/particle filters. Singlestage membrane operation is preferred. Air is pressurized and fed into the membrane separators; faster gases (O2, CO2, water vapor) permeate through the polymeric fiber walls and are collected and vented to the atmosphere while the slower, nonpermeate N2 gas is available at the other end of the separator. Oxygen production by membrane systems is still underdeveloped, since most of the industrial O2 applications require purity higher than 90%, which is easily achieved by adsorption or cryogenic technologies but not by single-stage membranes. However, membrane are applied to the production of oxygenenriched air (OEA), to be used in chemical and related industries, in the medical field, food packaging, etc. In industrial furnaces and burners, OEA (25-35% O2) injection results in higher flame temperatures and reduces the volume of parasite nitrogen to be heated; this means lower energy consumption. Belyaev et al.241 proposed to use OEA produced by membrane systems in low quality coal gasification. Membranes are economically convenient when the final O2 concentration is in the range 25-50%;242 in this case, a single stage is the most economic configuration.243 The separation is carried out with the permeate under vacuum (with a flow rate lower than the feed stream side). Thousands of compact on-site membrane systems generating nitrogen gas are installed in offshore and petrochemical industry. Air Products Norway has delivered more than 670 PRISM systems producing N2 for different ship applications, and more than 160 PRISM systems for offshore installations.244 In October 2008, Air Products announced to expand its manufacturing capacity (40000 membrane modules per year) by over 50% with a new facility near its PRISM production plant in Missouri (U.S.A.).245 In addition to generating N2, Air Products’ modules


are also used in enriching oxygen for passengers on the world’s longest and highest highland railway (Qinghai-Tibet). Air Liquide is installing in Dalian (China) a new air separation unit with a capacity of 550 ton/day of oxygen to supply gaseous O2, nitrogen, and argon by pipeline to the steelworks (Dongbei Special Steel Group).246 In Japan, Ube Industries, originally focused on membranes for hydrogen recovery in the petroleum refining and chemical industries, has scheduled to double its module production, passing from 20000 to 40000 units by October 2007.247 The company is increasing its production of polyimide hollow fibers to meet the demand for nitrogen separation membranes and alcohol dehydration membranes. Membranes produced by this technology are being introduced at a number of ethanol refining plants, mainly in the USA and Europe, driven by the rapid increase in the demand for bioethanol as an additive for petrol. Dense inorganic membranes (ion transport membranes, ITM), able to be permeated only by oxygen (or hydrogen), are being developed to operate at high temperature, typically greater than 700 °C.248,249 High-temperature air separation process has better synergy with power generation systems. Commercial-scale ITM oxygen modules have been fabricated by Air Products (0.5 ton/ day of oxygen); this technology requires 35% less capital (much simpler flow sheet) and 35-60% less energy (less compression energy associated with oxygen separation) than cryogenic air separation.250 4.3. CO2 Separation. Depending on the specific use of the CO2 membrane in the production of power or hydrogen, the necessary membrane properties may be significantly different. Polymeric membranes are today more developed and commercially available for CO2 separation. Rubbery polymers have attracted greater interest for the separation of CO2 from H2 due to higher flux rates and high selectivity (solubility controlled or reverse selective H2 separation membranes). A limited number of reports is related to this topic; polyacetylenes24 and organosilane modified porous glass251 were studied for CO2/ H2 separation at low temperatures (25-40 °C). Glassy polymers (mainly cellulose acetate and polyimides) dominate industrial CO2 separation applications such as in the separation of CO2 from CH4, CO, N2 or other hydrocarbons.24 Carbon dioxide removal from natural gas (natural gas sweetening) is mandatory to meet pipeline specifications (e.g., down to 2% vol. in U.S.A.), since CO2 reduces the heating value of natural gas, is corrosive, and freezes at a relatively high temperature, forming blocks of dry ice that can clog equipment lines and damage pumps. Membrane technology is attractive for CO2 and H2S removal, because many membrane materials are very permeable to these species (enabling a high recovery of the acid gases without significant loss of pressure in the methane pipeline product gases), and because treatment can be accomplished using the high wellhead gas pressure as the driving force for the separation. A high natural gas recovery (>95%) can be achieved in multistage systems. Membrane systems are typically installed for small size applications (less than 6000 Nm3/h) and remote locations, since amine processes are too complicated for small productions. Membrane and amine systems become competitive for a size of 6000-50000 Nm3/h, while bigger plants are installed for offshore platforms or for enhanced oil recovery. Membrane GS presents significant advantages for the offshore industry; moreover, it is an environmentally friendly alternative to traditional amine absorption.

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Table 9. Permeation Data for Cellulose Acetate Commercial Membranes permeability (Barrer)

separation factor (i/CH4) (-)

CO2

H2O

CO2

H2S

N2

C2H6

reference

8.9 8.9

500 500

20-25 21

50 19

1 1

0.42 0.42

230 253

Table 10. Cynara Hollow Fiber Modules diameter (in) length (in)

5 40

12 40

16 72

30 72

Cellulose acetate, initially developed for membrane reverse osmosis, is the most widely used and tested material for natural gas sweetening (Table 9), as in UOP’s membrane systems.252 Cynara-NATCO produces hollow fiber modules in cellulose triacetate254 and has recently provided a membrane system (16 in. modules) for the natural gas sweetening in an offshore platform in the Thailand gulf (830000 Nm3/h), which is the biggest membrane system for CO2 removal.255 The plant reduces the CO2 concentration from 36 down to 16% (in southeast Asia a CO2 concentration of 23% can be acceptable for using the gas in power stations). The geometric characteristics of the Cynara modules are reported in Table 10. The 16 in. module has a 17.5 times higher feed capacity than the flow-rate allowed by a 5 in. module, while the 30 in. module (the last development) allows a feed capacity 62.5 times higher than the smallest one, reducing weight and footprint of more than 90%. This is extremely important on offshore platforms and other spaceconstrained facilities. PRISM (Air Products) are other commercial membranes for CO2 separation. Figure 6 represents a plant installed in July 2004 with a capacity of 9,500 Nm3/h of natural gas and reducing the CO2 concentration from 4.5% down to ca. 2%.256 In the field of the natural gas treatment, commercially proven membranes (NitroSep process, MTR)257 are beginning to be used to separate hydrocarbons from nitrogen. These systems, using composite membranes, allow production from highnitrogen gas reserves. The injection of gases miscible with oil, like CO2 and/or N2, increases the recovery rates of oil and/or gas from a petroleum reservoir (enhanced oil recovery). Membranes are useful for recovering CO2 from natural gas in enhanced oil recovery applications because of the high CO2 concentrations (typically >50%) and high pressure (up to 140 bar) involved in these processes. Polymeric membranes (e.g., MEDAL)258 are applied to separate these streams into natural gas and a CO2 enriched

Figure 6. Natural gas treatment membrane plant. Reprinted with permission from ref 256. Copyright Air Products and Chemicals, Inc.

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Figure 7. (a) Natural gas treatment (one stage); (b) natural gas treatment (two stage); (c) CO2 enrichment [from ref 263].

stream, which is recompressed and injected in the wells. Cynara, in 1983, installed the first membrane plant of this type in Texas (U.S.A.). The plant, initially designed to reduce CO2 from 45% to 28%, treating 60000 Nm3/h in 160 modules (5 in × 12 in), is still working and today has a capacity of more 120000 Nm3/ h, decreasing the CO2 content from 80% to less than 10%.259 The high solubility of CO2 and H2S in cellulose acetate induces plasticization, causing the polymer to swell with disruption of the polymer matrix, with an increase in the mobility of the polymer chains, thereby adversely changing the membrane permeation characteristics. To overcome plasticization problems, polyimides, in particular, have attracted considerable attention due to their relatively high thermal, chemical, and mechanical stability combined with high selectivity and permeability for CO2.260 Polyimide membranes were initially used for hydrogen recovery, but were then modified for CO2 removal. Cross-linking was investigated as a method for reducing the incidence of swelling and plasticization resulting from the exposure of polyimides to CO2. This approach has led to significant

improvement without reducing CO2 permeability.261,262 Ube industries commercialize aromatic polyimide modules, suggesting 100 °C as the maximum operating temperature and a pressure of 150 bar. Some examples of the use of these membranes in CO2 separation applications263 are reported in Figure 7. The other important field where membranes can contribute is CO2 capture. The aim is to reduce CO2 emissions into the atmosphere, due to combustion of fossil fuels and contributing to global warming. The key issue for CO2 capture is minimizing the energy required in the overall process (capture + sequestration). CO2 capture in power generation processes that involve fossil fuels can be divided into three strategies: Precombustion fuel decarbonization involves the initial conversion of a fossil fuel into H2 and CO2 (through, e.g., partial oxidation, methane-steam reforming, or autothermal reforming, followed by water-gas shift). Therefore, carbon of the fuel is removed prior to combustion, whereby the fuel heating value is transferred to hydrogen. In these cases, the CO2 separation


will happen at very high pressures (up to 7 MPa ∆P) and high temperatures (300-700 °C). Postcombustion capture involves the separation of CO2 from the flue gas coming from the combustion of fossil fuels (from a standard gas turbine combined cycle, or a coal-fired steam power plant). This technology involves CO2 separation at a relatively low temperature, from a gaseous stream at atmospheric pressure and with low CO2 concentration (ca. 5-15% if air is used during combustion). SO2, NO2 and O2 may also be present in small amounts. Oxy-fuel combustion is the process of firing a fossil-fueled power plant with an oxygen-enriched gas mixture (95% oxygen) instead of air. Usually, oxygen comes from a cryogenic air separation unit, since this is a technology that is applicable for large-scale plants. To keep combustion temperatures at a permissible level, the mixture is diluted with recycled flue gas (CO2). Oxy-fuel combustion produces approximately 75% less flue gas than air-fueled combustion and produces exhaust consisting primarily of CO2 and H2O and, therefore, ready for sequestration. However, because of the energy and economic costs of producing oxygen, an oxy-fuel power plant is less efficient than a traditional air-fired plant. Established methods for CO2 capture are absorption, adsorption, and membrane processes. Amine absorption is today’s best available technology for postcombustion capture, with an energetic cost in the range 4-6 GJ/ton CO2 recovered (mainly due to significant energy consumption in the regeneration step). This option requires large-scale equipment for the CO2 removal and chemicals handling. It also consumes heat and results in additional exhaust-gas pressure losses, producing a reduced thermal efficiency. In postcombustion capture, CO2 must be separated from waste gases (where N2 is the main component). Due to the low CO2 concentration and pressure in this gas, the commercial membranes used to separate CO2 from natural gas at high pressures are not suited. Their use will result in a large membrane area and in high compression costs. Favre et al.264 presented a simulation study on the separation performances and energy cost of a single-stage membrane module for postcombustion applications, considering coal-fired power plants (10% CO2 in the flue gas) and binary dry CO2/N2 feed mixtures. The authors found that the membranes currently available (CO2/ N2 selectivities less than 50) are not adequately selective to produce the recovery ratios and permeate compositions called for by government regulations, but membranes with a selectivity above 100 are required. However, for flue streams containing 20% or more CO2, reasonable recoveries and permeate compositions (that do not exceed 80%) can be attained with currently available materials and the related cost will be only about 0.5-1 GJ/ton CO2 recovered, which is lower than amine absorption (steel or cement production). Quite recently researchers from MTR have presented a twostage membrane process design for the postcombustion separation of CO2 from the flue gas of a 500 MWe coal fired power plant.265 The design is based on bench scale test results of MTR Polaris membranes with CO2 permeance of 1000 GPU and CO2/ N2 selectivity of 50. The flue gas is emitted at low pressure; therefore, in order to reduce the compression costs, a blower is used, and the driving force for the first membrane stage is a downstream low vacuum (0.11 bar), whereas in the second stage it is a sweep of air which then is sent to the combustion. In turn, the CO2 entrained in the air stream and recycled increases the driving force (19% CO2) for the separation in the first stage. A 90% recovery of CO2 requires 18% of the energy produced by the power plant, which compares well with the 30% of power

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required by an amine absorption process. The membrane surface required for one single power plant, producing 500 m3/s of flue gas and 460 ton/h of CO2, is about 5-10 × 105 m2: this huge surface needs to be packed in spiral wound modules of very large diameter, which require only 135 skids. The modest pressures involved will not demand steel pressure vessels, thereby reducing the investment costs. According to the researchers of MTR, in real operations one of the main problems that can be forecast is the effective removal of ashes with a reasonably low pressure drop, and, in general, the removal of CO2 from combustion flue gases remains an economical challenge. Amine absorption is the best option when a high purity is desired, as for natural gas treatment, where methane losses must be minimized. High purities systematically induce higher energy consumption. Since N2 entrainment during capture has a negligible impact on CO2 sequestration and only moderately increases energy consumption due to compression, a less selective process such as membrane permeation could be of interest in order to minimize the overall energy penalty (capture + sequestration).264 These observations support the need for materials development, but also show that membranes are already promising for CO2 capture in certain industries. When it comes to the reuse of CO2 separated from combustion flue gases, one option is the dry reforming of methane, that is the use of carbon dioxide instead of substantial parts of steam for the production of synthesis gas. Since reforming is carried at 600-800 °C, the economy of the process dictates that CO2 must be separated hot.266 Therefore, for the power generation process the membranes are required to be stable at higher temperatures, in excess of 300 °C. When coal is gasified, CO in the syngas can be converted CO2 by means of the water gas shift reaction (CO + H2O / H2 + CO2) at 350∼600 °C, whose conversion is severely limited by thermodynamics. The membrane removal of CO2 from the mixture during the reaction would improve the yields, and at the same time would reduce the costs of postcombustion CO2 separation. Even in this case a CO2 selective membrane able to withstand high temperatures is required. Yet an other case in which CO2 (and O2) selective membranes resistant at high temperatures are needed is the treatment of combustion flue gases, containing residual oxygen, in order to obtain the oxidant of the oxyfuel combustion process.267 Membranes based on alumina, zeolites, silica, and carbon have received considerable attention in both amorphous and crystalline forms for separating CO2 at high temperature.268 Materials which conduct CO32- ions (e.g., molten Li2CO3 formed from the reaction of Li2ZrO3 with CO2) have also been studied for CO2/CH4 separation up to 600 °C.268,269 These membranes may provide an economically efficient, stable, and robust separation membrane in applications where excess heat/ energy is readily available to melt the carbonate. Inorganic membranes can find application in high-temperature membrane reactors for integration in power generation cycles with CO2 capture.270 Integrated H2, O2, or CO2 membrane separation is analyzed, and it is concluded that development of power plant concepts including membrane technology is not yet fully explored. Significant design optimization would be required in order to identify efficient, feasible, and environmentally sound technical solutions. In addition, further development and validation of performance of membranes in real applications are needed.

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Table 11. Main Industrial Applications of Vapour/Gas Membrane Separation process polyolefin plant resin degassing gasoline vapor recovery systems at large terminals polyvinyl chloride manufacturing vent gas ethylene recovery in ethylene oxide manufacturing plants ethylene recovery in vinyl acetate manufacturing plants natural gas processing/fuel gas conditioning

recovery (%)

system cost ($)

91 (hydrocarbon) 50 (N2)

-

95-99 (hydrocarbons)

-

99 (VCM)

-

75 (ethylene)

550,000

70 (ethylene)

700,000

80 (C3+ hydrocarbons)

-

4.4. Separation and Recovery of Organics from Gas Streams. The separation and recovery of organic solvents from gas streams is also rapidly growing at the industrial level. This is an interesting field, which created a new market for membranes, coupling economic and environmental benefits. This area concerns the recovery of high-value organic vapors and off-gas treatment. The main industrial applications of vapor/ gas membrane separation are listed in Table 11.15 The introduction of more stringent emission standard in Germany at the end of the 1980s and in the United States at the beginning of 1990s271 was an important driving force for this advance. The first small membrane units for separating organic vapors from air were installed in the late 1980s. By the mid-1990s, MTR (U.S.A.) was selling million-dollar systems (spiral-wound modules) to petrochemical plants to recover olefins from polyolefin production plants. In the same period, in Europe GKSS and its licensees began installing large systems (plateand-frame modules) to capture gasoline vapors from tank farms and fuel terminals. Baker15 in 2006 reported the following data: the market for vapor separation systems was at least $20-30 million per year; more than 100 large systems, with a cost of $1-5 million each, were installed; there were at least 500 small systems, with a cost of $10,000-$100,000 each, operating to capture vapor emissions from retail gasoline stations, industrial refrigerator units, and petrochemical process vents. For economic reasons, membranes for this application should be organic vapor selective materials such as polymeric rubbery membranes that selectively permeate VOCs (due to their high solubility) from air or nitrogen. PDMS is used. Such systems typically achieve greater than 99% removal of VOC from the feed gas and reduce the VOC content of the stream to 100 ppm or less. Membrane systems are competitive with carbon adsorption or condensation for streams containing more than 5000 ppm, particularly if high VOC recovery is required. MTR and OPW Fueling Components have developed a membrane vapor recovery system for the fuel storage tanks of retail gasoline stations. The OPW Vaporsaver system, fitted with MTR’s membranes, reduces hydrocarbon emissions by 95-99%272 and pays for itself with the value of the recovered gasoline. Recovery of natural gas liquids (NGL) is also attractive because these hydrocarbons (C3+) have a better value as a chemical feedstock than as a fuel and their removal is mandatory in order to meet pipeline specifications for dew point and heating value. Typically, NGL recovery is done in refrigeration and turbo-expander plants, complicated to operate, with moving parts and with high capital and operating costs. Membrane systems

(e.g., VaporSep, MTR272) are simple and low-cost solutions for this application. Membranes are competitive with propane refrigeration for flows up to 0.6-0.8 million Nm3/day.3 Heavy hydrocarbons permeate faster than CH4 in these membranes (at ambient temperature); they are recovered as a liquid after recompression and condensation. In addition, since the membrane also preferentially permeates water vapor, the residue gas will also be dehydrated, reducing or eliminating the need for a conventional dehydration system. Glassy PTMSP and PMP are the most permeable and selective polymers for the separation of n-butane from CH4. Fumed silica as a filler proved effective in enhancing both the permeability and the selectivity of PMP due to an increase of free-volume.22,110,273 In PTMSP, instead, an increase of the already large free-volume determines an increasingly larger contribution of a methane selective Knudsen type gas transport, which lowers the n-butane/CH4 selectivity. 4.5. Air and Natural Gas Dehydration. Finally, the air and natural gas dehydration are placed among the most interesting applications of membrane separation processes. In these operations the heavier species are present in dilute concentrations and preferential permeation of the heavier species results in lower surface area requirements; furthermore, the “cleaned” lighter component is kept on the high-pressure side, which may be advantageous. An efficient membrane system for air dehumidification has been commercialized by Permea (Cactus Membrane Air Dryer).274,275 The membrane removes water, in the form of water vapor, from compressed air, while nitrogen, argon, and most of the oxygen continue through the hollow core of the fibers to the end of the separator. Liquids should be removed by coalescing filters upstream of the membrane. A small amount of the slower gases passes through the fiber, and this is used to sweep the water vapor through the separator. Cactus membranes work on the principle of dew point depression. If inlet conditions change, for example, compressed air with a lower inlet dew point is supplied, the separator will provide dry air at an even lower dew point. A refrigerated dryer dehydrates air by cooling it down to condense the water, and with an electricity consumption. Membrane dryers have a wide range of drying capability, while desiccant dryers dehydrate air by adsorbing water on a solid granular desiccant. Because the desiccant must be regenerated, two desiccant towers are used, and the air to be dried is periodically switched between the two towers. Maintenance is required on the switching valves, and there is the issue of desiccant carryover into the dried air as well as desiccant life. 5. Hybrid Systems Hybrid membrane processes are integrated processes in which the membrane system operates in combination with another unit operation, or processes in which the basic functioning of the membrane is joined with another physical or chemical process in a single unit operation. A properly designed hybrid process will balance the drawbacks of the specific process and favorably combine their advantages. The result will be a better separation, contributing to a sustainable process improvement by allowing the reduction of investment and operational costs. Typically, membranes provide a moderately pure product at low cost (bulk separation) that may be economically upgraded by a subsequent process. An example is the pilot BP distillation/ membrane hybrid process for propane/propylene separation,276 where a part of the vapor mixture from the cryogenic distillation column is sent in the membrane system which extracts the


propylene using an aqueous solution of silver nitrate (which forms a reversible complex with propylene). The integration of membranes with other separation processes well-established in the chemical and petrochemical industries, such as PSA, was considered in different works.277-281 Membrane permeation can be an effective aid in the pressurization and high-pressure adsorption steps of a typical PSA process; the pressure difference available from the PSA can be used for operating the membrane incorporated into the blowdown step of the PSA cycle.282 Usually, the combination membranes + PSA is considered in H2 separation, while hybrid membranes + amine absorption are applied to the CO2 separation. A comparison of the separation cost for the membrane process with diethanolamine (DEA) absorption showed that the membrane process is more economical for CO2 concentrations in the feed in the range 5-40 mol %.283 When the feed also contains H2S, the cost for reducing the CO2 and H2S concentrations to pipeline specifications increases with increasing H2S concentration (1000-10000 ppm). If membrane processes are not economically competitive because of the high H2S concentration in the feed, the separation cost could be significantly lowered by using hybrid membrane processes. In such processes, the bulk of CO2 and H2S is separated from sour natural gas with membranes and the final purification is performed by means of suitable gas-absorption processes.283 A hybrid system (Cynara membranes + amine absorption) is operating since 1994 in Mallet (Texas, U.S.A.) to perform the bulk removal of CO2 from associated gas (90% CO2 and heavy hydrocarbons), before downstream treating. The membrane system offered a 30% reduction in operating cost when compared with a methyl diethanolamine (MDEA) system and significantly reduced the size of the subsequent operations.284 In the process industry the final choice of a separation process is the result of a balance between the economics, the desired purity and recovery of the product, and other conditions and restrictions, such as the desired capacity and the composition of the feed and the possibility of integration with other processes. In this logic, the integration of commercial membrane separation units (i.e., GS for hydrogen recovery, microfiltration for coke removal, membrane contactors for water purification) within an existing ethylene production cycle by steam cracking was proposed.285 GS membranes for H2/CH4 separation reduce significantly the energy required in the cryogenic distillation section, allowing less severe operating conditions with respect to the typical cycle. Other membrane units could be introduced in the future, such as membrane systems for olefin/paraffin separation which are very likely to be developed at a commercial stage in the next few years.3 In a subsequent analysis, ethylene production by a catalytic process (ethane oxidative dehydrogenation) was considered. The introduction of a membrane reactor with an oxygen conducting membrane for the separate feeding of oxygen was proposed.286 Moreover, the integration of a GS membrane system, producing an O2-enriched stream, with the traditional reactor was proposed. The membrane reactor process, compared to the traditional one, achieves essential targets as improved safety, the reduction of plant size and complexity, energy and exergy savings, increased flexibility. These are important aspects to be taken into account (e.g., by new metrics287) in the evaluation of the overall performance in the PI logic. In the PI strategy, the process quality evaluation should take into account not only economical aspects, but also safety, environmental and technical aspects, such as fire and explosion risk, noise, emissions/waste, energy efficiency, flexibility and controllability, reliability, and maintainability.

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6. Conclusions In the chemical process industry, membrane-based gas and vapor separations are today well consolidated and compete with cryogenic distillation, absorption, and pressure swing adsorption. Membranes compete with other separation processes on the basis of overall economics, safety, and environmental and technical aspects. Originally, these systems were developed for hydrogen recovery. Presently, there are several industrial applications (production of nitrogen and oxygen enriched gases, H2 recovery from refinery streams and CO2 separation from natural gas). Membrane units are smaller than the conventional plants of corresponding capacity, therefore they are suited for off-shore natural gas treatment. The recovery of CO2 from flue gas streams for environmental reasons is also under development. Emerging fields are vapor/gas separation or N2/methane separation. There are already developed applications of membrane units in the petrochemical field; others might come with the availability of new membrane materials able to withstand more demanding process conditions of temperature, pressure, and aggressive media. The permanent challenge in membrane GS technology is to improve the current processes, extending the range of application of this technology. Research in polymeric membrane technology in recent years has resulted in the development of systems which offer the potential for generation or separation of gases for use in a range of applications at attractive costs. The significant positive results reached in GS membrane systems are, however, still far away to realize the potentialities of this technology. Problems related to the pretreatment of the streams, to the membrane lifetime, to their selectivity and permeability slow down the growth of large scale industrial applications. A great deal of research was carried out on improved membrane materials for GS, since materials science is a critical issue. The essential focus for R&D is on developing robust membranes, in order to meet the key criteria of reliability and durability for industrial applications. Promising materials, such as carbon hollow fibers and hybrids containing carbon nanotubes, still require other work. Inorganic membranes, when industrially available, can find application at high temperature, or in new applications involving hydrocarbon vapors, contributing to the success of this technology. However, a better engineering (design of supermolecular membrane morphologies and economical modules) are equally critical to the full field development. Besides the experimental search for better membrane materials, molecular dynamics studies, leading to better insights into the separation processes, might help to develop molecularly engineered chemical structures with high fluxes at high selectivity. The application of the membrane processes together with other separation techniques in hybrid processes may be advantageous. Despite all advantages, however, the potential of hybrid separation processes in industrial scale is not fully exploited because of the lack of general design methodologies and detailed process know-how. In this respect, modeling of membrane processes (integrated process simulator to model complete hybrid process, predictive membrane and module performance models) can be very useful. With the development of new process concepts, new membrane applications will emerge. Proper demonstration projects, such as some in progress, will help this development. Acknowledgment The Italian Ministry of Education, University and Research (Progetto “FIRB-CAMERE RBNE03JCR5sNuove membrane

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catalitiche e reattori catalitici a membrana per reazioni selettive come sistemi avanzati per uno sviluppo sostenibile (New catalytic membrane reactors and catalytic membranes for selective reactions such as advanced systems for sustainable development)”) is gratefully acknowledged for cofunding this work. Nomenclature P ) pressure, kPa Pe ) permeability, m3(STP) m m-2 s-1 Pa-1, 1 Barrer ) 10-10 cm3(STP) cm cm-2 s-1 cmHg-1 ) 7.6 × 10-18 m3(STP) m m-2 s-1Pa-1 T ) temperature, °C Tg ) glass transition temperature, °C Acronyms CMS ) carbon molecular sieve CNT ) carbon nanotube DEA ) diethanolamine DWNT ) double-walled nanotube FFV ) fractional free-volume FS ) fumed silica FTM ) facilitated transport membrane GPU ) gas permeation unit, 1 GPU ) 10-6 cm3(STP) cm-2 s-1 cmHg-1 ) 7.6 × 10-12 m3(STP) m-2 s-1 Pa-1 GS ) gas separation ITM ) ion transport membrane MD ) molecular dynamics MDEA ) Methyl diethanolamine MMM ) mixed-matrix membrane MWNT ) multiwalled nanotube NMR ) nuclear magnetic resonance NGL ) natural gas liquids OEA ) oxygen-enriched air PALS ) positron annihilation lifetime spectroscopy PAMAM ) poly(amidoamine) PDMS ) polydimethylsiloxane PEEK-WC ) poly(etheretherketone PEO ) poly(ethylene oxide) PES ) polyethersulfone PI ) polyimide PIM ) polymer of intrinsic microporosity PMP ) poly(4-methyl-2-pentyne) PSA ) pressure swing adsorption PTMSP ) poly(1-trimethylsilyl-1-propyne) RTILs ) room temperature ionic liquid SSF ) surface selective flow STP ) standard conditions: temperature 273.15 K and pressure 105 Pa (IUPACsCompendium of Chemical Terminology, 2nd ed., 1997) SWNT ) single-walled nanotube TFE ) tetrafluoroethylene TTD ) 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole VOC ) volatile organic compound

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ReceiVed for reView December 10, 2008 ReVised manuscript receiVed March 6, 2009 Accepted March 9, 2009 IE8019032

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