Membranes for Gas Separation

As energy costs rise, membrane technology for separating gases is likely to play an increasingly important role in reducing the environmental impact and costs of industrial processes.

Gas separation membranes offer a number of benefits over other gas separation technologies, according to Benny D. Freeman, professor of chemical engineering at the University of Texas, Austin. "Conventional technologies such as the cryogenic distillation of air, condensation to remove condensable organic vapors from gas mixtures, and amine absorption to remove acid gases such as carbon dioxide from natural gas require a gas-to-liquid phase change in the gas mixture that is to be separated," he explains. "The phase change adds a significant energy cost to the separation cost. Membrane gas separation, on the other hand, does not require a phase change.

"In addition, gas separation membrane units are smaller than other types of plants, like amine stripping plants, and therefore have relatively small footprints," Freeman continues. "A small footprint is important in environments such as offshore gas-processing platforms. The lack of mechanical complexity in membrane systems is another advantage."

Currently, gas separation membranes are most widely used in industry for:

◾ Hydrogen separation, for example, hydrogen/nitrogen separation in ammonia plants and hydrogen/hydrocarbon separations in petrochemical applications;

◾ Separating nitrogen from air;

◾ CO₂ and water removal from natural gas;

◾ Organic vapor removal from air or nitrogen streams.

At the moment, the most widely used membrane materials for gas separation are polymers. They are attractive as membranes because they can be processed into hollow fibers with high surface areas. The relatively low cost of manufacturing the fibers makes them of interest for large-scale industrial applications. Examples of such membranes are the MEDAL and PRISM membranes produced, respectively, by Air Liquide and Air Products for wide-ranging gas separation applications. Each device contains thousands of fibers.

Membrane devices for gas or vapor separation usually operate under continuous steady-state conditions with three streams. The feed stream--a high-pressure gas mixture--passes along one side of the membrane. The molecules that permeate the membrane are swept using a gas on the other side of the membrane in the so-called permeate stream. The nonpermeating molecules that remain on the feed-stream side exit the membrane as the retentate stream. A pressure difference across the membrane drives the permeation process.

The economics of a gas separation membrane process is largely determined by the membrane's transport properties--that is, its permeability and selectivity for a specific gas in a mixture. Ideally, membranes should exhibit high selectivity and high permeability. For most membranes, however, as selectivity increases, permeability decreases, and vice versa. That's the trade-off.

A membrane is a selective semipermeable barrier that allows different gases, vapors, or liquids to move through it at different rates, explains Richard D. Noble, chemical engineering professor at the University of Colorado, Boulder. "A membrane is defined by what it does and not what it is," he says. "The membrane restricts the motion of molecules passing across it so that some molecules move more slowly than others or are excluded. A wide range of mechanisms are available for this restriction; for example, size variability of the molecules, affinity for the membrane material, and permeation driving forces--typically concentration or pressure difference."

Each gas component in a feed mixture has a characteristic permeation rate through the membrane. The rate is determined by the ability of the component to dissolve in and diffuse through the membrane material.

Freeman and coworkers have been developing novel polymeric solubility-selective gas separation membrane materials. Some of their recent work has focused on materials in which CO₂ is highly soluble and can therefore be used to remove CO₂ from natural gas.

"Our approach is to use materials that separate gases based primarily on high solubility selectivity," Freeman says. "We believe that, in chemically challenging environments, such materials could have better separation property profiles than conventional materials.

"Membranes will have a large role to play in important environmental and energy-related processes such as the cost-effective purification of hydrogen and methane."

"MOST RESEARCH has focused on using very rigid polymers for CO₂ removal from natural gas," he adds. "Such materials gain high selectivity from high diffusion selectivity. The issue they face is that, in the presence of high partial pressures of CO₂ or higher hydrocarbon contaminants, their separation properties can deteriorate to levels that are not useful relative to conventional separation technologies."

Aliphatic and aromatic hydrocarbons, which are present in small amounts in natural gas, are highly soluble in hydrocarbon-based polymers. As a result, they can plasticize polymer membranes and therefore reduce the membranes' diffusion selectivity.

"This problem with stability of separation properties in the process environment can lead to rigorous and expensive pretreatment of the gas using conventional technologies to remove the higher hydrocarbons to protect the membranes from them," Freeman explains. "Extensive pretreatment increases the cost of the membranes and makes them less competitive relative to conventional technologies."

In yet-to-be-published work, Freeman's group has demonstrated that a family of rubbery materials based on cross-linked poly(ethylene oxide) shows promise for overcoming this problem. The materials are strongly solubility selective for the removal of CO₂ from natural gas. The group has also demonstrated that the materials are strongly selective for CO₂ in CO₂/H₂ mixtures.

"We have shown that CO₂ separation performance can be improved by introducing methoxy chain ends to the polymers or by adding MgO nanoparticles to the polymer matrix," Freeman says. "Decreasing temperature can lead to further enhancement of the separation properties. The materials could also potentially be used to remove polar molecules such as H₂O, H₂S, and NH₃ from mixtures with CH₄, N₂, H₂, and other light gases."

THE GROUP also has shown that hydrocarbons such as methane, ethane, and propane are significantly less soluble in fluoropolymers than expected (Macromolecules 2004, 37, 7688). "We discovered that fluoropolymers exhibit unusual separation properties due to their anomalous solubility selectivity," Freeman says. "This property means that such materials exhibit a low tendency to plasticize relative to hydrocarbon-based materials."

Alan R. Greenberg, professor of mechanical engineering at the University of Colorado, Boulder, points out that polymeric membranes used in gas separation applications often experience compaction--that is, a decrease in membrane thickness over time. "Compaction and plasticization are simultaneous and competing effects in these applications," he says. "Plasticization usually leads to swelling and an increase in the permeability of glassy polymers." Compaction, on the other hand, is due to mechanical deformation--that is, creep--when a large pressure drop is applied across the membrane.

Compaction, he adds, has been reported to cause a significant decrease in the performance of a membrane over time. This is possibly a result of a decrease in the free volume of the membrane skin layer or an increase in the thickness of the skin. Increased skin thickness occurs if the porous sublayer of the membrane in the region adjacent to the skin compacts and becomes as dense as the skin.

Greenberg's group is currently developing a method for simultaneously measuring mechanical and transport properties of membranes made of cellulose acetate, polybenzimidazole, and other polymers in pressurized CO₂/N₂ feed streams. "We plan to use the results to develop a better understanding of the fundamental factors governing the relationship between creep and gas permeability in dense polymer films at elevated temperatures," he says.

Dense films have limited free volume for the transport of gas molecules through the materials. If the amount of free volume is increased so that it becomes interconnected, the material takes on the characteristics of a porous material.

"Polymer membranes are amorphous and possess a wide distribution of micropore sizes," observes Neil B. McKeown, a chemistry professor at Cardiff University, in Wales. "Polymers with larger free volumes allow lots of molecules through but do not efficiently block the passage of larger molecules, whereas more tightly packed polymers are size-selective but flux is too small to be practical."

McKeown; Peter M. Budd, a senior lecturer in chemistry at the University of Manchester, England; and coworkers have developed a series of porous polymer materials, with pore diameters of less than 2 nm, that combine high gas permeability and high selectivity for separating pairs of gases such as N₂/O₂ and CH₄/CO₂. They call the materials "polymers of intrinsic microporosity," or PIMs. "A PIM is special because its molecular structure is neither flexible enough for its backbone to change shape significantly nor regular enough for its molecules to pack together," Budd says. "In the solid state, it traps a great deal of space and so behaves like a molecular sieve."

The PIMs are prepared by the dioxane-forming polymerization of hydroxylated aromatic monomers with fluorinated or chlorinated aromatic monomers. In collaboration with chemist Detlev Fritsch at the GKSS Research Centre in Geesthacht, Germany, the group has tested thin-film composite membranes consisting of a gas-selective PIM layer on a porous poly(acrylonitrile) support (J. Membr. Sci. 2005, 251, 263).

"FOR PRACTICAL application in gas separation membranes, the selective layer is usually made as thin as possible to have as high a flux as possible," Budd explains. "What is really exciting is that, for important gas mixtures like N₂/O₂, PIMs are much more selective than any other polymer of comparable permeability."

Meanwhile, at Imperial College London, a group led by Kang Li, a reader in chemical engineering, has tested microporous polyvinylidene fluoride (PVDF) hollow-fiber membranes for odor control (AIChE J. 2005, 51, 1367). The authors note that odorous gas streams are characterized by low concentrations of odor compounds, large gas volumes, and low gas pressures.

The chemical engineers selected the removal of hydrogen sulfide from N₂ gas streams as an example of odor control. They point out that H₂S--a highly toxic, corrosive, acidic, and odorous gas--is a common impurity in natural gas, refinery gas, and coal gas. These gases are used for industrial and domestic heating and for chemical processing.

"Our results indicate that H₂S at very low concentrations can be removed from gas streams effectively and economically at high removal rates using PVDF hollow-fiber membrane modules," Li says. "The modules could also potentially be fitted in air conditioners for indoor air purification."

A group led by chemical engineering professor Kamalesh K. Sirkar at New Jersey Institute of Technology, Newark, has been developing polymer membranes for removing volatile organic compounds (VOCs) from gas mixtures such as air. "Conventional vapor permeation devices for VOC removal and recovery from gas mixtures remove the VOCs by selective permeation through a composite membrane of a rubbery polymer like polydimethylsiloxane on a porous glassy polymeric support," Sirkar explains. "Flat membranes are used in spiral-wound or envelope-type modules. However, the membrane packing densities are somewhat low in such configurations.

"We have developed a technique using silicone-coated polypropylene hollow fibers where the feed vapor/gas mixture is exposed to the substrate below the membrane and the permeate vapors are in contact with the top of the rubbery coating," he continues. "The technique eliminates the pressure drop in the permeate through the porous substrate, providing a higher driving force. Other advantages include easy scale-up, compactness of the device, and significant swelling resistance."

At the University of Wyoming, Laramie, Youqing Shen, assistant professor of chemical and petroleum engineering, and colleagues have shown that poly(ionic liquid)s, such as poly(p-vinylbenzyltrimethylammonium tetrafluoroborate) and poly[2-(methacryloyloxy)ethyltrimethylammonium tetrafluoroborate] are potentially useful membrane materials for separating CO₂ (Chem. Commun. 2005, 3325). "We unexpectedly found that polymers prepared from ionic liquid monomers have much faster sorption/desorption rates and higher CO₂-absorption capacities than room-temperature ionic liquids," Shen says. "We are currently making the polymers into membranes to test their ability to separate CO₂."

Materials used or being developed for gas separation membranes span the whole organic-inorganic spectrum from polymers at one end to ceramics, metals, and other inorganic materials at the other. Inorganic membranes generally exhibit far better chemical, mechanical, thermal, and pressure stability than polymer membranes. They do not swell, for example.

"THE BEAUTY of inorganic membranes such as ceramic membranes is that they show an absolute selectivity for separation," explains Li at Imperial College London. "A highly purified product, oxygen or hydrogen, can therefore be obtained in a single operation."

Li's group has been working on perovskite membranes. He points out that, to date, much of the research work on these materials has focused on disk-shaped membranes because methods to process ceramics into other configurations, such as hollow fibers, have not been available. Disk-shaped membranes have limited membrane area, however, and keeping the disks sealed inside the membrane module at the high temperatures at which the membranes operate is a problem. Perovskite membranes work by oxygen conduction, with transport of O₂ occurring by conduction of oxide ions, O^2-, through the perovskite oxide network. Temperatures in excess of 800 °C are necessary to obtain sufficient ion conductivity.

Li's group has prepared hollow fiber ceramic membranes for air separation from lanthanum-strontium-cobalt-iron oxide perovskites (AIChE J. 2005, 51, 1991). The technique involves spinning precursors of the hollow fibers using a polymer solution containing suspended perovskite powders. The precursors are then heated at 1,280 °C for several hours.

"Compared with disk-shaped membranes, our hollow-fiber membranes possess a much larger membrane area per unit volume for oxygen permeation," Li says. "By adopting long hollow fibers and keeping the two sealing ends away from the module's high-temperature zone, the problem of high-temperature sealing no longer exists. The fibers have an asymmetric structure--that is, a thin separating dense layer integrated with porous layers on one side or both sides. This structure significantly reduces the membrane's resistance to oxygen permeation compared with symmetric membranes prepared by conventional methods."

Inorganic membranes could play a large role in the emerging hydrogen economy, according to Tina M. Nenoff, distinguished member of the technical staff at Sandia National Laboratories.

In the steam-reforming process, for example, H₂ and CO₂ are formed at high temperature by the catalyzed reaction of steam and CH₄ in natural gas. "Production of H₂ from reforming gas requires the separation of H₂ from CH₄, CO, CO₂, N₂, H₂O, and impurities," Nenoff notes. "To do this energy efficiently, the separations should be run at or close to the reforming temperatures, which are over 450 °C. Inorganic membranes are robust at these temperatures and exhibit high selectivity."

At the Center for Inorganic Membrane Studies at Worcester Polytechnic Institute, in Massachusetts, chemical engineering professor Yi Hua Ma and coworkers have been developing palladium and palladium-alloy membranes for the separation of hydrogen produced by the steam reforming process. "As the demand for hydrogen will undoubtedly increase immensely in the future, dense composite palladium--especially palladium-alloy--membranes will play an important and essential role in pure hydrogen production," Ma notes. "Currently, many technical issues need to be addressed, such as the development of thin membranes with good separation and long-term thermal and mechanical stability."

THE GROUP has developed unique technologies to synthesize palladium membranes that are stable for thousands of hours (Ind. Eng. Chem. Res. 2004, 43, 2936). "One of our membranes was tested for 6,000 hours under steam-reforming conditions in a Shell laboratory," Ma says.

In recent work, Ma and colleagues investigated the hydrogen-permeability and hydrogen-selectivity stability of a low-copper-content Pd-Cu membrane with a thickness of less than 10 µm supported on a porous stainless steel substrate. "Porous stainless steel supports, unlike porous ceramic or glass supports, resist cracking," he observes. "Palladium-porous stainless steel membranes are also easily assembled, and the thermal expansion coefficient of stainless steel is very close to that of palladium, ensuring good mechanical properties of the composite membrane during temperature cycling.

"Composite Pd-Cu alloys have the advantage over other types of palladium membranes of being mechanically stable and sulfur resistant," Ma adds. "Pd-Cu alloys with a relatively high copper content have higher hydrogen permeability but lose their hydrogen transport properties at temperatures above 450 °C because of a phase transformation of the alloy at high temperatures."

The group measured the hydrogen permeation rate of a porous stainless-steel-supported, 10-µm-thick Pd-Cu membrane containing around 10% copper by weight and observed no decline in the rate at 450 °C over 500 hours.

Chemical engineering professor S. Ted Oyama and research professor Yunfeng Gu at Virginia Polytechnic Institute & State University, Blacksburg, have been working on highly permeable and selective silica-alumina ceramic membranes for hydrogen separation. "Our permselective membranes are inorganic and therefore robust and can be used at high temperatures and pressures," Oyama says. "They have higher permeability than palladium, yet are composed of inexpensive materials: alumina and silica."

The membranes are prepared by chemical vapor deposition of a thin SiO₂ layer on a porous alumina substrate. The support is prepared by dip-coating a commercial macroporous alumina tube with a series of boehmite (AlOOH) sols of decreasing particle sizes.

"The membranes have multilayer structures, with size-graded layers of alumina deposited on a porous support," Oyama explains. "The topmost layer is a 2030-nm-thick layer of permselective silica. The size gradation allows the structure to be thin--less than 1 µm and defect free. The resulting silica-on-alumina composite membrane has excellent permeability for hydrogen over CO₂, N₂, CO, and CH₄.

"The membrane does not have continuous pores, but rather consists of a network of solubility sites," he adds. "The permeation mechanism is different from all other membranes. The permeating molecules jump between adjacent solubility sites."

Oyama and colleagues have also studied the antagonistic effects of pressure on reaction equilibrium and permeability in membrane reactors. In these reactors, a reaction and separation are carried out simultaneously. The system they studied was the catalytic dry-reforming of methane with carbon dioxide (CH₄ + CO₂ s 2CO + 2H₂) using an alumina-support rhodium catalyst and a silica-on-alumina composite membrane.

The group showed that as pressure increased, the enhancement of H₂ and CO yields in the reactor went through a maximum and then declined (Top. Catal. 2004, 29, 45). "This occurred because, although the rate of hydrogen separation increased with increasing pressure, the conversions of the reactants decreased with increasing pressure," the authors note. "Thus, the maximum was due to a trade-off between a transport property (hydrogen separation) and a thermodynamic quantity (hydrogen production), which had opposing pressure dependencies."

Nenoff's team at Sandia has been developing zeolite thin-film membranes for gas separations in hydrogen fuel production from CH₄ or water sources, for separating CO₂/N₂ mixtures, and for other applications. "Zeolites are crystalline silicate-based molecular sieves that bring a chemically, thermally, and mechanically stable inorganic matrix to the membrane," she explains. "They have well-defined subnanometer pores that can be tuned for size-exclusion separations.

"We envision zeolite membranes as replacements for the costly and energy-intensive cryogenic distillation processes used in production today," she says. "The membranes could probably be easily and cheaply retrofitted into existing production plant designs."

Nenoff's team has recently developed a synthetic route for preparing porous alumina-supported defect-free zeolite membranes and investigated their ability to separate CO₂/N₂ mixtures under dry or moist conditions (Ind. Eng. Chem. Res. 2005, 44, 937). The group showed that the presence of water vapor significantly enhanced CO₂ selectivity at 110-200 °C but drastically lowered the selectivity below 80 °C.

In other work, Nenoff, Junhang Dong at the New Mexico Institute of Mining & Technology, and coworkers developed a catalytic zeolite membrane for the nonoxidative conversion of methane to higher hydrocarbons, such as ethane and propane, and hydrogen (Catal. Lett. 2005, 102, 9). The disk-shaped, alumina-supported membrane has bimetallic clusters of a platinum-cobalt catalyst loaded inside the zeolite channels. CH₄ flows on one side of the membrane (the feed side) and H₂ on the other side (the sweep side). The higher hydrocarbons are produced continuously on the H₂ sweep side. Because methane is renewable through bioprocesses, the conversion of CH₄ to higher hydrocarbons and hydrogen is a challenging research topic with tremendous industrial interest, the authors note.

In related work, the same group has shown that "excellent" separation of p-xylene from m-xylene/p-xylene vapor mixtures can be achieved with surface-modified zeolite membranes synthesized on tubular alumina substrates. p-Xylene is used for the synthesis of terephthalic acid, which is used to produce polyester resins and fibers.

"Industrial production of pure p-xylene is costly to a large extent because of the difficulties associated with the separation of xylene isomers," Dong, Nenoff, and coauthors note in a yet-to-be-published paper. "Currently, xylene isomers are separated mainly by fractional crystallization and zeolitic adsorption processes, both of which are energy intensive and require batch operations."

At the University of Colorado, Noble, John L. Falconer, professor of chemical and biological engineering, and coworkers have been investigating the use of zeolite membranes on stainless steel tubular supports for high-pressure CO₂/CH₄ separation. Separating the two gases is important in natural gas processing because CO₂ reduces the energy content of natural gas. Also, CO₂ in water is acidic and corrosive in transportation and storage systems. CO₂ is currently removed from natural gas in amine plants that are complex and costly or in membrane plants that use CO₂-selective cellulose acetate membranes.

THE ZEOLITE used by the Colorado team, known as SAPO-34, contains silica, alumina, and phosphate, and it has an eight-membered ring. CO₂ is the smaller molecule and adsorbs more strongly than CH₄ in this type of zeolite. "These SAPO-34 membranes are 5 µm thick, and their pores are similar in size to the CH₄ molecule so that CO₂ preferentially permeates," Falconer notes. "We have obtained high CO₂ selectivities at 30 atm and 50 °C."

In a recent paper, the team showed that as the pressure drop across the membrane increases, both CO₂ flux and CO₂ permeate concentration increase for a 50-50 mixture of the two gases (Ind. Eng. Chem. Res. 2005, 44, 3220). However, the separation selectivity decreases as temperature increases.

A zeolite with a 10-membered ring known as MFI-type is also potentially useful for gas separations. Associate professor of chemical engineering Joaquín Coronas, professor of chemical engineering Jesús Santamaría, and coworkers at the University of Zaragoza, Spain, demonstrated that MFI-type zeolite membranes on porous alumina or stainless steel supports can be used to remove volatile organic pollutants from indoor air (J. Membr. Sci. 2004, 240, 159).

"We used n-hexane, formaldehyde, and benzene as model pollutants and showed that our membranes successfully remove low concentrations of the pollutants from indoor air," Coronas says. "Although the permeation fluxes obtained were low due to the low driving forces involved, they fall in the range of interest for domestic applications of indoor quality control."

In another project, the group synthesized a 5-µm-thick membrane of microporous titanosilicate, K₂TiSi₃O₉·H₂O, on commercial porous TiO₂ tubular supports and showed that it is able to separate H₂/N₂ mixtures with high selectivities for H₂ (Chem. Commun. 2005, 3036). The membrane material has the structure of the mineral umbite, K₂ZrSi₃O₉·H₂O. The work was carried out in collaboration with a group at the University of Aveiro, Portugal, led by professor of inorganic and materials chemistry João Rocha.

"These materials have pore sizes of around 0.3 nm," Coronas says. "They do not adsorb N₂ and display very slow diffusion of water."

The collaboration has also led to the recent synthesis of pure titanosilicate and vanadosilicate membranes (Stud. Surf. Sci. Catal. 2005, 158, 423). The materials, known as ETS-10 and AM-6, respectively, are prepared on commercial tubular supports.

"We have used ETS-10 and AM-6 membranes for the separation of propylene/propane mixtures," Rocha says. "Propylene is one of the most important raw materials in the petrochemical industry since it finds applications in the synthesis of products such as polymers and propylene oxide. The main processes to obtain propylene yield a propylene/propane mixture. While distillation has been used to separate these products, it consumes much energy. Membrane separation is an attractive alternative in this context."

Organic-inorganic hybrid materials consisting of highly selective rigid phases, such as zeolites, dispersed in a continuous polymeric matrix are leading candidates for challenging membrane applications, according to William J. Koros, professor of chemical and biomolecular engineering at Georgia Institute of Technology, Atlanta. "Our group is doing research on membranes that are simultaneously more selective and more durable than the current state-of-the-art modules needed for handling high-volume feed streams," Koros says. "We work on cross-linked polymers and blends of nanoscale molecular sieving zeolites or carbons within a matrix of a conventional polymer--such as polyimide-based polymers. With the 'mixed matrix composite' membranes, we aim to achieve ultrahigh permeation membrane properties without sacrificing the ease of membrane formation associated with conventional polymers.

"The work is important for gas and vapor separations where the effective sizes of the molecules involved are quite similar," he continues. "A good example is the removal of CO₂ from high-pressure natural gas to upgrade product quality."

The gas separation membrane field is highly competitive both within itself and with other gas separation technologies, according to Cardiff University's McKeown. "The key desirable properties for membranes are high flux (permeability), selectivity, processability, stability, and cost," he says. "All of these factors need to be favorable before you are onto a winner. After a frantic burst of activity in the 1980s, research in the membrane field went into a bit of a lull because industry believed that there were enough suitable materials around and that any incremental advances were not worth the considerable investment necessary.

"The current rise in energy costs makes membrane processes, which are generally low in such costs, more attractive," he concludes. "Membranes will have a large role to play in important environmental and energy-related processes such as the cost-effective purification of hydrogen and methane."