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{{short description|Technology for splitting specific gases out of mixtures}}
Gas mixtures can be effectively separated by [[synthetic membranes]] made from polymers such as [[polyamide]] or [[cellulose acetate]], or from ceramic materials.<ref name = handbook/>
[[File:Flux distribution inside the fiber.jpg|thumb|Membrane cartridge]]
 
While polymeric membranes are economical and technologically useful, they are bounded by their performance, known as the Robeson limit (permeability must be sacrificed for selectivity and vice versa).<ref name=":0">{{Cite journal|lastlast1=Jang|firstfirst1=Kwang-Suk|last2=Kim|first2=Hyung-Ju|last3=Johnson|first3=J. R.|last4=Kim|first4=Wun-gwi|last5=Koros|first5=William J.|last6=Jones|first6=Christopher W.|last7=Nair|first7=Sankar|date=2011-06-28|title=Modified Mesoporous Silica Gas Separation Membranes on Polymeric Hollow Fibers|journal=Chemistry of Materials|volume=23|issue=12|pages=3025–3028|doi=10.1021/cm200939d|issn=0897-4756}}</ref> This limit affects polymeric membrane use for CO<sub>2</sub> separation from flue gas streams, since mass transport becomes limiting and CO<sub>2</sub> separation becomes very expensive due to low permeabilities. Membrane materials have expanded into the realm of [[Silicon dioxide|silica]], [[zeolite]]s, [[metal-organic framework]]s, and [[perovskite]]s due to their strong thermal and chemical resistance as well as high tunability (ability to be modified and functionalized), leading to increased permeability and selectivity. [[Membrane]]s can be used for separating gas mixtures where they act as a permeable barrier through which different compounds move across at different rates or not move at all. The membranes can be nanoporous, polymer, etc. and the gas molecules penetrate according to their size, [[Mass diffusivity|diffusivity]], or solubility.
 
==Basic process==
Gas separation across a membrane is a pressure-driven process, where the driving force is the difference in pressure between inlet of raw material and outlet of product. The membrane used in the process is a generally non-porous layer, so there will not be a severe leakage of gas through the membrane. The performance of the membrane depends on permeability and selectivity. Permeability is affected by the penetrant size. Larger gas molecules have a lower diffusion coefficient. The polymer chain flexibility and free volume in the polymer of the membrane material influence the diffusion coefficient, as the space within the permeable membrane must be large enough for the gas molecules to diffuse across. The solubility is expressed as the ratio of the concentration of the gas in the polymer to the pressure of the gas in contact with it. Permeability is the ability of the membrane to allow the permeating gas to diffuse through the material of the membrane as a consequence of the pressure difference over the membrane, and can be measured in terms of the permeate flow rate, membrane thickness and area and the pressure difference across the membrane. The selectivity of a membrane is a measure of the ratio of permeability of the relevant gases for the membrane. It can be calculated as the ratio of permeability of two gases in binary separation.<ref name="Chong et al 2016" />
 
The membrane gas separation equipment typically pumps gas into the membrane module and the targeted gases are separated based on difference in diffusivity and solubility. For example, oxygen will be separated from the ambient air and collected at the upstream side, and nitrogen at the downstream side. As of 2016, membrane technology was reported as capable of producing 10 to 25 tonnes of 25 to 40% oxygen per day.<ref name="Chong et al 2016" />
 
==Membrane governing methodology==
 
[[File:Mechanisms of transport in membranes.jpg|thumb|upright=3.4|right|(a) Bulk flow through pores; (b) Knudsen diffusion through pores; (c) molecular sieving; (d) solution diffusion through dense membranes.]]
There are three main [[diffusion]] mechanisms. The first (b), [[Knudsen diffusion]] holds at very low pressures where lighter molecules can move across a membrane faster than heavy ones, in a material with reasonably large pores.<ref name="Smit">{{Cite book|title=Introduction to Carbon Capture and Sequestration|author1=Berend Smit |author2=Jeffrey A. Reimer |author3=Curtis M. Oldenburg |author4=Ian C. Bourg |publisher=Imperial College Press|year=2014|isbn=978-1-78326-328-8|location=|pages=281–354}}</ref> The second (c), [[Molecular sieve|molecular sieving]], is the case where the pores of the membrane are too small to let one component pass, a process which is typically not practical in gas applications, as the molecules are too small to design relevant pores. In these cases the movement of molecules is best described by pressure-driven convective flow through capillaries, which is quantified by [[Darcy's law]]. However, the more general model in gas applications is the solution-diffusion (d) where particles are first dissolved onto the membrane and then diffuse through it both at different rates. This model is employed when the pores in the polymer membrane appear and disappear faster relative to the movement of the particles.<ref>{{Cite book|title=Membrane Technology and Applications|last=Richard W. Baker|publisher=John Wiley & Sons Ltd|year=2004|isbn=978-0-470-85445-7|location=|pages=15–21}}</ref>
 
In a typical membrane system the incoming feed stream is separated into two components: permeant and retentate. Permeant is the gas that travels across the membrane and the retentate is what is left of the feed. On both sides of the membrane, a gradient of [[chemical potential]] is maintained by a pressure difference which is the driving force for the gas molecules to pass through. The ease of transport of each species is quantified by the [[Permeation|permeability]], P<sub>i</sub>. With the assumptions of ideal mixing on both sides of the membrane, [[ideal gas law]], constant diffusion coefficient and [[Henry's law]], the flux of a species can be related to the pressure difference by [[Fick's laws of diffusion|Fick's law]]:<ref name="Smit" />
 
:<math>JJ_i=\frac{D_i K_i(p_i'-p_i'')}{l}=\frac{P_i(p_i'-p_i'')}{l}</math>
 
where, (J<sub>i</sub>) is the [[Mass flux|molar flux]] of species i across the membrane, (l) is membrane thickness, (P<sub>i</sub>) is permeability of species i, (D<sub>i</sub>) is diffusivity, (K<sub>i</sub>) is the Henry coefficient, and (p<sub>i</sub><sup>'</sup>) and (p<sub>i</sub><sup>"</sup>) represent the partial pressures of the species i at the feed and permeant side respectively. The product of D<sub>i</sub>K<sub>i</sub> is often expressed as the permeability of the species i, on the specific membrane being used.
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The flow of a second species, j, can be defined as:
 
:<math>JJ_j=\frac{P_j(p_j'-p_j'')}{l}</math>
[[File:Membrane separation process.jpg|thumb|upright=2|A simplified design schematic of a membrane separation process]]
 
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This coefficient is used to indicate the level to which the membrane is able to separates species i from j. It is obvious from the expression above, that a membrane selectivity of 1 indicates the membrane has no potential to separate the two gases, the reason being, both gases will diffuse equally through the membrane.
 
In the design of a separation process, normally the pressure ratio and the membrane selectivity are prescribed by the pressures of the system and the permeability of the membrane . The level of separation achieved by the membrane (concentration of the species to be separated) needs to be evaluated based on the aforementioned design parameters in order to evaluate the cost -effectiveness of the system.
 
==Membrane performance==
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The material of the membrane plays an important role in its ability to provide the desired performance characteristics. It is optimal to have a membrane with a high permeability and sufficient selectivity and it is also important to match the membrane properties to that of the system operating conditions (for example pressures and gas composition).
 
Synthetic membranes are made from a variety of polymers including [[polyethylene]], polyamides[[polyamide]]s, polyimides[[polyimide]]s, [[cellulose acetate]], [[polysulphone]] and polydimethylsiloxan[[polydimethylsiloxane]].<ref name="gas_sep">{{cite book | title = Separation of Gases | last1 = Isalski | first1 = W. H. | year = 1989 | publisher = Oxford University Press | location = New York | series = Monograph on Cryogenics | volume = 5 | pages = 228–233 }}</ref>
 
===Polymer membranes===
[[Polymeric membranes]] are a common option for use in the capture of CO<sub>2</sub> from flue gas because of the maturity of the technology in a variety of industries, namely petrochemicals. The ideal polymer membrane has both a high [[Reactivity–selectivity principle|selectivity]] and [[Semipermeable membrane|permeability]]. Polymer membranes are examples of systems that are dominated by the solution-diffusion mechanism. The membrane is considered to have holes which the gas can dissolve (solubility) and the molecules can move from one cavity to the other (diffusion).<ref name="Smit" />
 
It was discovered by Robeson in the early 1990s that polymers with a high selectivity have a low permeability and opposite is true; materials with a low selectivity have a high permeability. This is best illustrated in a Robeson plot where the selectivity is plotted as a function of the CO<sub>2</sub> permeation. In this plot, the upper bound of selectivity is approximately a linear function of the permeability. It was found that the solubility in polymers is mostly constant but the diffusion coefficients vary significantly and this is where the engineering of the material occurs. Somewhat intuitively, the materials with the highest diffusion coefficients have a more open pore structure, thus losing selectivity.<ref>{{cite journal|last1=Robeson|first1=L.M.|title=Correlation of separation factor versus permeability for polymeric membranes|journal=Journal of Membrane Science|date=1991|volume=62|issue=165|pages=165–185|doi=10.1016/0376-7388(91)80060-j}}</ref><ref>{{cite journal|last1=Robeson|first1=L.M.|title=The upper bound revisited|journal=Journal of Membrane Science|date=2008|volume=320|issue=390|pages=390–400|doi=10.1016/j.memsci.2008.04.030}}</ref> There are two methods that researchers are using to break the Robeson limit, one of these is the use of glassy polymers whose phase transition and changes in mechanical properties make it appear that the material is absorbing molecules and thus surpasses the upper limit. The second method of pushing the boundaries of the Robeson limit is by the facilitated transport method. As previously stated, the solubility of polymers is typically fairly constant but the facilitated transport method uses a chemical reaction to enhance the permeability of one component without changing the selectivity.<ref name="Merkel">{{Cite journal|lastlast1=Merkel|firstfirst1=Tim C.|last2=Lin|first2=Haiqing|last3=Wei|first3=Xiaotong|last4=Baker|first4=Richard|date=2010-09-01|title=Power plant post-combustion carbon dioxide capture: An opportunity for membranes|url=http://www.sciencedirect.com/science/article/pii/S0376738809007832|journal=Journal of Membrane Science|series=Membranes and CO2 Separation|volume=359|issue=1–2|pages=126–139|doi=10.1016/j.memsci.2009.10.041}}</ref>
 
===Nanoporous membranes===
[[File:Microscopic model of a nanoporous membrane.jpg|thumb|upright=2|right| Microscopic model of a nanoporous membrane. The white open area represents the area the molecule can pass through and the dark blue areas represent the membrane walls. The membrane channels consists of cavities and windows. The energy of the molecules in the cavity is U<sub>c</sub> and the energy of a particle in the window is U<sub>w</sub>.]]
Nanoporous membranes are fundamentally different thanfrom polymer-based membranes in that their chemistry is different and that they do not follow the Robeson limit for a variety of reasons. The simplified figure of a nanoporous membrane shows a small portion of an example membrane structure with cavities and windows. The white portion represents the area where the molecule can move and the blue shaded areas represent the walls of the structure. In the engineering of these membranes, the size of the cavity (L<sub>cy</sub> x L<sub>cz</sub>) and window region (L<sub>wy</sub> x L<sub>wz</sub>) can be modified so that the desired permeation is achieved. It has been shown that the permeability of a membrane is the production of adsorption and diffusion. In low loading conditions, the adsorption can be computed by the Henry coefficient.<ref name="Smit" />
 
If the assumption is made that the energy of a particle does not change when moving through this structure, only the entropy of the molecules changes based on the size of the openings. If we first consider changes the cavity geometry, the larger the cavity, the larger the entropy of the absorbed molecules which thus makes the Henry coefficient larger. For diffusion, an increase in entropy will lead to a decrease in free energy which in turn leads to a decrease in the diffusion coefficient. Conversely, changing the window geometry will primarily effect the diffusion of the molecules and not the Henry coefficient.
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==== Silica membranes ====
Silica membranes are [[Mesoporous material|mesoporous]] and can be made with high uniformity (the same structure throughout the membrane). The high porosity of these membranes gives them very high permeabilities. Synthesized membranes have smooth surfaces and can be modified on the surface to drastically improve selectivity. Functionalizing silica membrane surfaces with amine containing molecules (on the surface [[silanol]] groups) allows the membranes to separate CO<sub>2</sub> from flue gas streams more effectively.<ref name=":0" /> Surface functionalization (and thus chemistry) can be tuned to be more efficient for wet flue gas streams as compared to dry flue gas streams.<ref>{{Cite journal|lastlast1=Chew|firstfirst1=Thiam-Leng|last2=Ahmad|first2=Abdul L.|last3=Bhatia|first3=Subhash|date=2010-01-15|title=Ordered mesoporous silica (OMS) as an adsorbent and membrane for separation of carbon dioxide (CO2)|url=http://www.sciencedirect.com/science/article/pii/S0001868609001092|journal=Advances in Colloid and Interface Science|volume=153|issue=1–2|pages=43–57|doi=10.1016/j.cis.2009.12.001|pmid=20060956}}</ref> While previously, silica membranes were impractical due to their technical scalability and cost (they are very difficult to produce in an economical manner on a large scale), there have been demonstrations of a simple method of producing silica membranes on hollow polymeric supports. These demonstrations indicate that economical materials and methods can effectively separate CO<sub>2</sub> and N<sub>2</sub>.<ref name=":1">{{Cite journal|lastlast1=Kim|firstfirst1=Hyung-Ju|last2=Chaikittisilp|first2=Watcharop|last3=Jang|first3=Kwang-Suk|last4=Didas|first4=Stephanie A.|last5=Johnson|first5=Justin R.|last6=Koros|first6=William J.|last7=Nair|first7=Sankar|last8=Jones|first8=Christopher W.|date=2015-04-29|title=Aziridine-Functionalized Mesoporous Silica Membranes on Polymeric Hollow Fibers: Synthesis and Single-Component CO2 and N2 Permeation Properties|journal=Industrial & Engineering Chemistry Research|volume=54|issue=16|pages=4407–4413|doi=10.1021/ie503781u|issn=0888-5885}}</ref> Ordered mesoporous silica membranes have shown considerable potential for surface modification that allows for ease of CO<sub>2</sub> separation. Surface functionalization with [[amine]]s leads to the reversible formation of [[carbamate]]s (during CO<sub>2</sub> flow), increasing CO<sub>2</sub> selectivity significantly.<ref name=":1" />
 
==== Zeolite membranes ====
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Zeolites are crystalline [[aluminosilicate]]s with a regular repeating structure of molecular-sized pores. Zeolite membranes selectively separate molecules based on pore size and polarity and are thus highly tunable to specific gas separation processes. In general, smaller molecules and those with stronger zeolite-[[adsorption]] properties are adsorbed onto zeolite membranes with larger selectivity. The capacity to discriminate based on both molecular size and adsorption affinity makes zeolite membranes an attractive candidate for CO<sub>2</sub> separation from N<sub>2</sub>, CH<sub>4</sub>, and H<sub>2</sub>.
 
Scientists have found that the gas-phase enthalpy (heat) of adsorption on zeolites increases as follows: H<sub>2</sub> < CH<sub>4</sub> < N<sub>2</sub> < CO<sub>2</sub>.<ref>{{Cite journal|lastlast1=Poshusta|firstfirst1=Joseph C|last2=Noble|first2=Richard D|last3=Falconer|first3=John L|date=2001-05-15|title=Characterization of SAPO-34 membranes by water adsorption|url=http://www.sciencedirect.com/science/article/pii/S0376738800006669|journal=Journal of Membrane Science|volume=186|issue=1|pages=25–40|doi=10.1016/S0376-7388(00)00666-9}}</ref> It is generally accepted that CO<sub>2</sub> has the largest adsorption energy because it has the largest [[Quadrupole|quadrupole moment]], thereby increasing its affinity for charged or polar zeolite pores. At low temperatures, zeolite adsorption-capacity is large and the high concentration of adsorbed CO<sub>2</sub> molecules blocks the flow of other gases. Therefore, at lower temperatures, CO<sub>2</sub> selectively permeates through zeolite pores. Several recent research efforts have focused on developing new zeolite membranes that maximize the CO<sub>2</sub> selectivity by taking advantage of the low-temperature blocking phenomena.
 
Researchers have synthesized Y-type (Si:Al>3) zeolite membranes which achieve room-temperature separation factors of 100 and 21 for CO<sub>2</sub>/N<sub>2</sub> and CO<sub>2</sub>/CH<sub>4</sub> mixtures respectively.<ref>{{Cite journal|lastlast1=Kusakabe|firstfirst1=Katsuki|last2=Kuroda|first2=Takahiro|last3=Murata|first3=Atsushi|last4=Morooka|first4=Shigeharu|date=1997-03-01|title=Formation of a Y-Type Zeolite Membrane on a Porous α-Alumina Tube for Gas Separation|journal=Industrial & Engineering Chemistry Research|volume=36|issue=3|pages=649–655|doi=10.1021/ie960519x|issn=0888-5885}}</ref> [http://america.iza-structure.org/IZA-SC/framework.php?STC=DDR DDR-type] and [http://www.acsmaterial.com/sapo-250.html SAPO-34] membranes have also shown promise in separating CO<sub>2</sub> and CH<sub>4</sub> at a variety of pressures and feed compositions.<ref>{{Cite journal|lastlast1=Himeno|firstfirst1=Shuji|last2=Tomita|first2=Toshihiro|last3=Suzuki|first3=Kenji|last4=Nakayama|first4=Kunio|last5=Yajima|first5=Kenji|last6=Yoshida|first6=Shuichi|date=2007-10-01|title=Synthesis and Permeation Properties of a DDR-Type Zeolite Membrane for Separation of CO2/CH4 Gaseous Mixtures|journal=Industrial & Engineering Chemistry Research|volume=46|issue=21|pages=6989–6997|doi=10.1021/ie061682n|issn=0888-5885}}</ref><ref>{{Cite journal|lastlast1=Li|firstfirst1=S.|last2=Falconer|first2=J. L.|last3=Noble|first3=R. D.|date=2006-10-04|title=Improved SAPO-34 Membranes for CO2/CH4 Separations|journal=Advanced Materials|language=en|volume=18|issue=19|pages=2601–2603|doi=10.1002/adma.200601147|bibcode=2006AdM....18.2601L | s2cid=96222879 |issn=1521-4095}}</ref> The SAPO-34 membranes, being nitrogen selective, are also strong contender for natural gas sweetening process.<ref>{{Cite journal|last1=Alam|first1=Syed Fakhar|last2=Kim|first2=Min-Zy|last3=Kim|first3=Young Jin|last4=Rehman|first4=Aafaq ur|last5=Devipriyanka|first5=Arepalli|last6=Sharma|first6=Pankaj|last7=Yeo|first7=Jeong-Gu|last8=Lee|first8=Jin-Seok|last9=Kim|first9=Hyunuk|last10=Cho|first10=Churl-Hee|date=2020-05-01|title=A new seeding method, dry rolling applied to synthesize SAPO-34 zeolite membrane for nitrogen/methane separation|url=https://www.sciencedirect.com/science/article/pii/S0376738819333137|journal=Journal of Membrane Science|language=en|volume=602|pages=117825|doi=10.1016/j.memsci.2020.117825| s2cid=213870612 |issn=0376-7388}}</ref><ref>{{Cite journal|last1=Wu|first1=Ting|last2=Diaz|first2=Merritt C.|last3=Zheng|first3=Yihong|last4=Zhou|first4=Rongfei|last5=Funke|first5=Hans H.|last6=Falconer|first6=John L.|last7=Noble|first7=Richard D.|date=2015-01-01|title=Influence of propane on CO2/CH4 and N2/CH4 separations in CHA zeolite membranes|url=https://www.sciencedirect.com/science/article/pii/S0376738814007145|journal=Journal of Membrane Science|language=en|volume=473|pages=201–209|doi=10.1016/j.memsci.2014.09.021|issn=0376-7388}}</ref><ref>{{Cite journal|last1=Li|first1=Shiguang|last2=Zong|first2=Zhaowang|last3=Zhou|first3=Shaojun James|last4=Huang|first4=Yi|last5=Song|first5=Zhuonan|last6=Feng|first6=Xuhui|last7=Zhou|first7=Rongfei|last8=Meyer|first8=Howard S.|last9=Yu|first9=Miao|last10=Carreon|first10=Moises A.|date=2015-08-01|title=SAPO-34 Membranes for N2/CH4 separation: Preparation, characterization, separation performance and economic evaluation|url=https://www.sciencedirect.com/science/article/pii/S0376738815002768|journal=Journal of Membrane Science|language=en|volume=487|pages=141–151|doi=10.1016/j.memsci.2015.03.078|issn=0376-7388}}</ref>
 
Researchers have also made an effort to utilize zeolite membranes for the separation of H<sub>2</sub> from hydrocarbons. Hydrogen can be separated from larger hydrocarbons such as C<sub>4</sub>H<sub>10</sub> with high selectivity. This is due to the molecular sieving effect since zeolites have pores much larger than H<sub>2</sub>, but smaller than these large hydrocarbons. Smaller hydrocarbons such as CH<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, and C<sub>3</sub>H<sub>8</sub> are small enough to not be separated by molecular sieving. Researchers achieved a higher selectivity of hydrogen when performing the separation at high temperatures, likely as a result of a decrease in the competitive adsorption effect.<ref>{{Cite journal |last1=Cao |first1=Zishu |last2=Anjikar |first2=Ninad D. |last3=Yang |first3=Shaowei |date=February 2022 |title=Small-Pore Zeolite Membranes: A Review of Gas Separation Applications and Membrane Preparation |journal=Separations |language=en |volume=9 |issue=2 |pages=47 |doi=10.3390/separations9020047 |issn=2297-8739|doi-access=free }}</ref>
 
==== Metal-organic framework (MOF) membranes ====
There have been advances in [[Zeolitic imidazolate framework|zeolitic-imidazolate frameworks]] (ZIFs), a subclass of [[metal-organic framework]]s (MOFs), that have allowed them to be useful for carbon dioxide separation from flue gas streams. Extensive modeling has been performed to demonstrate the value of using MOFs as membranes.<ref>{{Cite journal|lastlast1=Gurdal|firstfirst1=Yeliz|last2=Keskin|first2=Seda|date=2012-05-30|title=Atomically Detailed Modeling of Metal Organic Frameworks for Adsorption, Diffusion, and Separation of Noble Gas Mixtures|journal=Industrial & Engineering Chemistry Research|volume=51|issue=21|pages=7373–7382|doi=10.1021/ie300766s|issn=0888-5885}}</ref><ref>{{Cite journal|lastlast1=Keskin|firstfirst1=Seda|last2=Sholl|first2=David S.|date=2009-01-21|title=Assessment of a Metal−Organic Framework Membrane for Gas Separations Using Atomically Detailed Calculations: CO2, CH4, N2, H2 Mixtures in MOF-5|journal=Industrial & Engineering Chemistry Research|volume=48|issue=2|pages=914–922|doi=10.1021/ie8010885|issn=0888-5885}}</ref> MOF materials are adsorption-based, and thus can be tuned to achieve selectivity.<ref>{{Cite journal|lastlast1=Zornoza|firstfirst1=Beatriz|last2=Martinez-Joaristi|first2=Alberto|last3=Serra-Crespo|first3=Pablo|last4=Tellez|first4=Carlos|last5=Coronas|first5=Joaquin|last6=Gascon|first6=Jorge|last7=Kapteijn|first7=Freek|date=2011-09-07|title=Functionalized flexible MOFs as fillers in mixed matrix membranes for highly selective separation of CO2 from CH4 at elevated pressures|journal=Chemical Communications (Cambridge, England)|volume=47|issue=33|pages=9522–9524|doi=10.1039/c1cc13431k|issn=1364-548X|pmid=21769350}}</ref> The drawback to MOF systems is stability in water and other compounds present in flue gas streams. Select materials, such as ZIF-8, have demonstrated stability in water and benzene, contents often present in flue gas mixtures. ZIF-8 can be synthesized as a membrane on a porous alumina support and has proven to be effective at separating CO<sub>2</sub> from flue gas streams. At similar CO<sub>2</sub>/CH<sub>4</sub> selectivity to Y-type zeolite membranes, ZIF-8 membranes achieve unprecedented CO<sub>2</sub> permeance, two orders of magnitude above the previous standard.<ref>{{Cite journal|lastlast1=Venna|firstfirst1=Surendar R.|last2=Carreon|first2=Moises A.|date=2010-01-13|title=Highly Permeable Zeolite Imidazolate Framework-8 Membranes for CO2/CH4 Separation|journal=Journal of the American Chemical Society|volume=132|issue=1|pages=76–78|doi=10.1021/ja909263x|pmid=20014839|issn=0002-7863}}</ref>
[[File:CMR1.PNG|thumb|Structure of a perovskite. A membrane would consist of a thin layer of the perovskite structure.]]
 
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[[Perovskite]] are mixed metal oxide with a well-defined cubic structure and a general formula of ABO<sub>3</sub>, where A is an [[Alkaline earth metal|alkaline earth]] or [[lanthanide]] element and B is a [[transition metal]]. These materials are attractive for CO<sub>2</sub> separation because of the tunability of the metal sites as well as their stabilities at elevated temperatures.
 
The separation of CO<sub>2</sub> from N<sub>2</sub> was investigated with an α-alumina membrane impregnated with BaTiO<sub>3</sub>.<ref>{{Cite journal|url=http://www.sciencedirect.com/science/article/pii/037673889400109X|title=Separation of CO2 with BaTiO3 membrane prepared by the sol—gel method |language=en|access-date=2017-05-05|doi=10.1016/0376-7388(94)00109-X|volume=95|issue=2 |journal=Journal of Membrane Science|pages=171–177 | last1 = Kusakabe | first1 = Katsuki|date=1994-10-24 }}</ref> It was found that adsorption of CO<sub>2</sub> was favorable at high temperatures due to an endothermic interaction between CO<sub>2</sub> and the material, promoting mobile CO<sub>2</sub> that enhanced CO<sub>2</sub> adsorption-desorption rate and surface diffusion. The experimental separation factor of CO<sub>2</sub> to N<sub>2</sub> was found to be 1.1-1.2 at 100<sup>o</sup>&nbsp;°C to 500<sup>o</sup>&nbsp;°C, which is higher than the separation factor limit of 0.8 predicted by [[Knudsen diffusion]]. Though the separation factor was low due to pinholes observed in the membrane, this demonstrates the potential of perovskite materials in their selective surface chemistry for CO<sub>2</sub> separation.
 
== Other membrane technologies ==
In special cases other materials can be utilized; for example, [[palladium]] membranes permit transport solely of hydrogen.<ref>{{Cite journal | last1 = Yun | first1 = S. | last2 = Ted Oyama | first2 = S. | doi = 10.1016/j.memsci.2011.03.057 | title = Correlations in palladium membranes for hydrogen separation: A review | journal = Journal of Membrane Science | volume = 375 | issue = 1–2 | pages = 28–45 | year = 2011 | pmid = | pmc = }}</ref> In addition to palladium membranes (which are typically palladium silver alloys to stop embrittlement of the alloy at lower temperature) there is also a significant research effort looking into finding non-precious metal alternatives. Although slow kinetics of exchange on the surface of the membrane and tendency for the membranes to crack or disintegrate after a number of duty cycles or during cooling are problems yet to be fully solved.<ref>{{cite journal|last1=Dolan|first1=Michael D.|last2=Kochanek|first2=Mark A.|last3=Munnings|first3=Christopher N.|last4=McLennan|first4=Keith G.|last5=Viano|first5=David M.|title=Hydride phase equilibria in V–Ti–Ni alloy membranes|journal=Journal of Alloys and Compounds|date=February 2015|volume=622|pages=276–281|doi=10.1016/j.jallcom.2014.10.081}}</ref>
 
==Construction==
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* Recovery of hydrogen in [[oil refinery]] processes
* Separation of methane from the other components of [[biogas]]
* [[Oxygen concentrator|Enrichment of air by oxygen]] for medical or metallurgical purposes. One of the methods used for commercial production of [[nitrox]] breathing gas for [[underwater diving]].
* Enrichment of [[ullage]] by nitrogen in [[inerting system]]s designed to prevent fuel tank explosions
* Removal of [[water vapor]] from [[natural gas]] and other gases{{further|Air dryer#Membrane dryer|l1=Membrane driers}}
* Removal of [[Sulfur dioxide|SO<sub>2</sub>]], [[Carbon Dioxide|CO<sub>2</sub>]] and [[hydrogen sulfide|H<sub>2</sub>S]] from natural gas (polyamide membranes)
* Removal of [[Volatility (chemistry)|volatile]] organic liquids (VOL) from air of exhaust streams
 
=== Air separation ===
Oxygen-enriched air is in high demanded for a range of medical and industrial applications including chemical and combustion processes. Cryogenic distillation is the mature technology for commercial air separation for the production of large quantities of high purity oxygen and nitrogen. However, it is a complex process, is energy-intensive, and is generally not suitable for small-scale production. Pressure swing adsorption is also commonly used for air separation and can also produce high purity oxygen at medium production rates, but it still requires considerable space, high investment and high energy consumption. The membrane gas separation method is a relatively low environmental impact and sustainable process providing continuous production, simple operation, lower pressure/temperature requirements, and compact space requirements.<ref name="Han et al" >{{cite journal|title=Highly Selective Oxygen/Nitrogen Separation Membrane Engineered Using a Porphyrin-Based Oxygen Carrier |first1=Jiuli |last1=Han |first2=Lu |last2=Bai |first3=Bingbing |last3=Yang |first4=Yinge |last4=Bai |first5=Shuangjiang |last5=Luo |first6=Shaojuan |last6=Zeng |first7=Hongshuai |last7=Gao |first8=Yi|last8=Nie |first9=Xiaoyan |last9=Ji |first10=Suojiang |last10=Zhang |first11=Xiangping |last11=Zhang |journal=Membranes |volume=9 |issue=115 |date=3 September 2019 |page=115 |doi=10.3390/membranes9090115 |pmid=31484439 |pmc=6780238 |doi-access=free }}</ref><ref name="Chong et al 2016" >{{cite journal|url=http://jestec.taylors.edu.my/Vol%2011%20issue%207%20July%202016/11_7_8.pdf |journal=Journal of Engineering Science and Technology |volume=11 |issue=7 |date=2016 |pages=1016–1030 |title=Recent progress of oxygen/nitrogen separation using membrane technology |first1=K. C. |last1=Chong |first2=S. O. |last2=Lai |first3=H. S. |last3=Thiam |first4=H. C. |last4=Teoh |first5=S. L. |last5=Heng }}</ref>
 
==Current status of CO<sub>2</sub> capture with membranes==
A great deal of research has been undertaken to utilize membranes instead of absorption or adsorption for carbon capture from flue gas streams, however, no current{{when|date=June 2019}} projects exist that utilize membranes. Process engineering along with new developments in materials have shown that membranes have the greatest potential for low energy penalty and cost compared to competing technologies.<ref name="Smit" /><ref name="Merkel" /><ref name="Brunetti" />
 
=== Background ===
Today, membranes are used for commercial separations involving: N<sub>2</sub> from air, H<sub>2</sub> from ammonia in the [[Haber process|Haber-Bosch process]], [[Natural-gas processing|natural gas purification]], and tertiary-level [[Enhancedenhanced oil recovery|Enhanced Oil Recovery]] supply.<ref name="Bernardo">{{Cite journal|last=Bernardo P., Clarizia G.|title=30 Years of Membrane Technology for Gas Separation|url=https://pdfs.semanticscholar.org/af85/934fa668a312d3e71c284ced478c12edfd16.pdf|archive-url=https://web.archive.org/web/20170901202824/https://pdfs.semanticscholar.org/af85/934fa668a312d3e71c284ced478c12edfd16.pdf|url-status=dead|archive-date=2017-09-01|journal=The Italian Association of Chemical Engineering|volume=32 | year = 2013 |pagess2cid=6607842}}</ref>
 
Single-stage membrane operations involve a single membrane with one selectivity value. Single-stage membranes were first used in natural gas purification, separating CO<sub>2</sub> from methane.<ref name="Bernardo" /> A disadvantage of single-stage membranes is the loss of product in the permeate due to the constraints imposed by the single selectivity value. Increasing the selectivity reduces the amount of product lost in the permeate, but comes at the cost of requiring a larger pressure difference to process an equivalent amount of a flue stream. In practice, the maximum pressure ratio economically possible is around 5:1.<ref name="Baker">{{Cite journal|last=Baker|first=Richard W.|date=2002-03-01|title=Future Directions of Membrane Gas Separation Technology|journal=Industrial & Engineering Chemistry Research|volume=41|issue=6|pages=1393–1411|doi=10.1021/ie0108088|issn=0888-5885}}</ref>
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=== Need for multi-stage process ===
Single -stage membranes devices are not feasible for obtaining a high concentration of separated material in the [[permeate]] stream. This is due to the pressure ratio limit that is economically unrealistic to exceed. Therefore, the use of multi -stage membranes is required to concentrate the permeate stream. The use of a second stage allows for less membrane area and power to be used. This is because of the higher concentration that passes the second stage, as well as the lower volume of gas for the pump to process.<ref name="Baker" /><ref name="Merkel"/> Other factors, such as adding another stage that uses air to concentrate the stream further reduces cost by increasing concentration within the feed stream.<ref name="Merkel" /> Additional methods such as combining multiple types of separation methods allow for variation in creating economical process designs.
 
=== Membrane use in hybrid processes ===
Hybrid processes have long -standing history with gas separation.<ref>{{Cite journal|last=Bernardo P., Clarizia G|title=30 Years of Membrane Technology for Gas Separation|url=https://pdfs.semanticscholar.org/af85/934fa668a312d3e71c284ced478c12edfd16.pdf|archive-url=https://web.archive.org/web/20170901202824/https://pdfs.semanticscholar.org/af85/934fa668a312d3e71c284ced478c12edfd16.pdf|url-status=dead|archive-date=2017-09-01|journal=The Italian Association of Chemical Engineering|volume=32 | year = 2013 |pagess2cid=6607842}}</ref> Typically, membranes are integrated into already existing processes such that they can be retrofitted into already existing carbon capture systems.
 
MTR, Membrane Technology and Research Inc., and [[University of Texas at Austin|UT Austin]] have worked to create hybrid processes, utilizing both absorption and membranes, for CO<sub>2</sub> capture. First, an [[Carbon dioxide scrubber|absorption]] column using [[piperazine]] as a solvent absorbs about half the carbon dioxide in the flue gas, then the use of a membrane results in 90% capture.<ref name="Freeman">{{Cite journal|url=http://www.sciencedirect.com/science/article/pii/S1876610214018803|title=Hybrid Membrane-absorption CO2 Capture Process |last=Brice Freeman, Pingjiao Hao, Richard Baker, Jay Kniep, Eric Chen, Junyuan Ding, Yue Zhang Gary T. Rochelle.|date=January 2014|language=en|archive-url=|archive-date=|dead-url=|access-date=2017-04-28|doi=10.1016/j.egypro.2014.11.065|volume=63|journal=Energy Procedia|pages=605–613|doi-access=free}}</ref> A parallel setup is also, with the membrane and absorption processes occurring simultaneously. Generally, these processes are most effective when the highest content of carbon dioxide enters the amine absorption column. Incorporating hybrid design processes allows for retrofitting into [[fossil fuel]] power plants.<ref name="Freeman" />
 
Hybrid processes can also use [[Air separation|cryogenic distillation]] and membranes.<ref name="Lin">{{Cite journal|lastlast1=Lin|firstfirst1=Haiqing|last2=He|first2=Zhenjie|last3=Sun|first3=Zhen|last4=Kniep|first4=Jay|last5=Ng|first5=Alvin|last6=Baker|first6=Richard W.|last7=Merkel|first7=Timothy C.|date=2015-11-01|title=CO2-selective membranes for hydrogen production and CO2 capture – Part II: Techno-economic analysis|url=http://www.sciencedirect.com/science/article/pii/S0376738815001635|journal=Journal of Membrane Science|volume=493|pages=794–806|doi=10.1016/j.memsci.2015.02.042|doi-access=free}}</ref> For example, [[hydrogen]] and [[carbon dioxide]] can be separated, first using cryogenic gas separation, whereby most of the carbon dioxide exits first, then using a membrane process to separate the remaining carbon dioxide, after which it is recycled for further attempts at cryogenic separation.<ref name="Lin" />
 
=== Cost analysis ===
Cost limits the pressure ratio in a membrane [[Carbon dioxide|CO<sub>2</sub>]] separation stage to a value of 5; higher pressure rationsratios eliminate any economic viability for CO<sub>2</sub> capture using membrane processes.<ref name="Merkel" /><ref name="Huang">{{Cite journal|lastlast1=Huang|firstfirst1=Yu|last2=Merkel|first2=Tim C.|last3=Baker|first3=Richard W.|date=2014-08-01|title=Pressure ratio and its impact on membrane gas separation processes|url=http://www.sciencedirect.com/science/article/pii/S037673881400194X|journal=Journal of Membrane Science|volume=463|pages=33–40|doi=10.1016/j.memsci.2014.03.016}}</ref> Recent studies have demonstrated that multi-stage CO<sub>2</sub> capture/separation processes using membranes can be economically competitive with older and more common technologies such as amine-based [[Carbon dioxide scrubber|absorption]].<ref name="Merkel" /><ref name="Lin" /> Currently, both membrane and amine-based absorption processes can be designed to yield a 90% CO<sub>2</sub> capture rate.<ref name="Brunetti">{{Cite journal|lastlast1=Brunetti|firstfirst1=A.|last2=Scura|first2=F.|last3=Barbieri|first3=G.|last4=Drioli|first4=E.|date=2010-09-01|title=Membrane technologies for CO2 separation|url=http://www.sciencedirect.com/science/article/pii/S0376738809008540|journal=Journal of Membrane Science|series=Membranes and CO2 Separation|volume=359|issue=1–2|pages=115–125|doi=10.1016/j.memsci.2009.11.040}}</ref><ref name="Merkel" /><ref name="Huang" /><ref name="Hao">{{Cite journal|lastlast1=Hao|firstfirst1=Pingjiao|last2=Wijmans|first2=J. G.|last3=Kniep|first3=Jay|last4=Baker|first4=Richard W.|date=2014-07-15|title=Gas/gas membrane contactors – An emerging membrane unit operation|url=http://www.sciencedirect.com/science/article/pii/S0376738814002257|journal=Journal of Membrane Science|volume=462|pages=131–138|doi=10.1016/j.memsci.2014.03.039}}</ref><ref name="Freeman" /><ref name="Lin" /> For [[Carbon capture and storage|carbon capture]] at an average 600 MW coal-fired power plant, the cost of CO<sub>2</sub> capture using amine-based absorption is in the $40–100 per ton of CO<sub>2</sub> range, while the cost of CO<sub>2</sub> capture using current membrane technology (including current process design schemes) is about $23 per ton of CO<sub>2</sub>.<ref name="Merkel" /> Additionally, running an amine-based absorption process at an average 600 MW coal-fired power plant consumes about 30% of the energy generated by the power plant, while running a membrane process requires about 16% of the energy generated.<ref name="Merkel" /> CO<sub>2</sub> transport (e.g. to [[Carbon capture and storage|geologic sequestration]] sites, or to be used for [[Enhanced oil recovery|EOR]]) costs about $2–5 per ton of CO<sub>2</sub>.<ref name="Merkel" /> This cost is the same for all types of CO<sub>2</sub> capture/separation processes such as membrane separation and absorption.<ref name="Merkel" /> In terms of dollars per ton of captured CO<sub>2</sub>, the least expensive membrane processes being studied at this time are multi-step [[Countercurrent exchange|counter-current]] flow/sweep processes.<ref name="Brunetti" /><ref name="Merkel" /><ref name="Huang" /><ref name="Hao" /><ref name="Freeman" /><ref name="Lin" />
 
==See also==▼
* {{annotated link|Barrer}}
* [[{{annotated link|Industrial gas]]}}▼
* {{annotated link|Primary life support system}}
* [[{{annotated link|Nitrogen generator#Membrane technology|Membrane technology for nitrogen generation]]}}▼
 
==References==
{{reflist}}
* {{cite book|last=Vieth|first=W.R.|year=1991|title=Diffusion in and through Polymers|publisher=Munich: Hanser Verlag|isbn=9783446155749}}
 
▲==See also==
* [[Barrer]]
▲* [[Industrial gas]]
* [[Primary Life Support System]]
▲* [[Nitrogen generator#Membrane technology|Membrane technology for nitrogen generation]]
 
{{Fuel Transport}}
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[[Category:Separation processes]]
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