Carbon Dioxide Separation from Flue Gases: A Technological Review Emphasizing Reduction in Greenhouse Gas Emissions

Abstract

Increasing concentrations of greenhouse gases (GHGs) such as CO₂ in the atmosphere is a global warming. Human activities are a major cause of increased CO₂ concentration in atmosphere, as in recent decade, two-third of greenhouse effect was caused by human activities. Carbon capture and storage (CCS) is a major strategy that can be used to reduce GHGs emission. There are three methods for CCS: pre-combustion capture, oxy-fuel process, and post-combustion capture. Among them, post-combustion capture is the most important one because it offers flexibility and it can be easily added to the operational units. Various technologies are used for CO₂ capture, some of them include: absorption, adsorption, cryogenic distillation, and membrane separation. In this paper, various technologies for post-combustion are compared and the best condition for using each technology is identified.

1. Introduction

There are ten primary GHGs including water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) that are naturally occurring. Perfluorocarbons (CF₄, C₂F₆), hydrofluorocarbons (CHF₃, CF₃CH₂F, and CH₃CHF₂), and sulfur hexafluoride (SF₆), are only present in the atmosphere due to industrial processes. Water vapor is the most important, abundant and dominant greenhouse gas, and CO₂ is the second-most important one (Table 1). Concentration of water vapor depends on temperature and other meteorological conditions, and not directly upon human activities. So it was not indicated in Table 1 [1–3].

Table 1.

The main greenhouse gases and their concentration [2, 3].

Compound	Preindustrial concentration (ppmv)	Concentration in 2011 (ppmv)	Atmospheric lifetime (years)	Main human activity source	GWP**
Carbon dioxide (CO2)	280	388.5	~100	Fossil fuels, cement production, land use	1
Methane (CH4)	0.715	1.87/1.748	12	Fossil fuels, rice paddies, waste dumps, livestock	25
Nitrous oxide (N2O)	0.27	0.323	114	Fertilizers, combustion industrial processes	298
CFC-12 (CCL2F2)	0	0.000533	100	Liquid coolants, foams	10,900
CF-113 (CCl2CClF2)	0	0.00000075	85	n.a.	6,130
HFC 23 (CHF3)	0	0.000018	270	Electronics, refrigerants	11,700
HCFC-22 (CCl2F2)	0	0.000218	12	Refrigerants	1,810
HFC 134 (CF3CH2F)	0	0.000035	14	Refrigerants	1,300
HCFC-141b (CH3CCl2F)	0	0.00000022	9.3	n.a.	725
HCFC-142b (CH3CClF2)	0	0.00000020	17.9	n.a.	2,310
HFC 152 (CH3CHF2)	0	0.0000039	1.4	Industrial processes	140
Perfluoromethane (CF4)	0.00004	0.00008*	50,000	Aluminum production	6,500
Perfluoroethane (C2F6)	0	0.000003*	10,000	Aluminum production	9,200
Sulfur hexafluoride (SF6)	0	0.00000712*	3,200	Dielectric fluid	22,800

 *Concentration in 2011.

**Global warming potentials (GWPs) measure the relative effectiveness of GHGs in trapping the Earth's heat.

CO₂ is the primary anthropogenic greenhouse gas, accounting for 77% of the human contribution to the greenhouse effect in recent decade (26 to 30 percent of all CO₂ emissions). Main anthropogenic emissions of CO₂ come from the combustion of fossil fuels. CO₂ concentration in flue gases depends on the fuel such as coal (12–15 mol-% CO₂) and natural gas (3-4 mol-% CO₂). In petroleum and other industrial plants, CO₂ concentration in exhaust stream depends on the process such as oil refining (8-9 mol% CO₂) and production of cement (14–33 mol-% CO₂) and iron and steel (20–44 mol-%). From 2004 to 2011, global CO₂ emissions from energy uses were increased 26% (Figure 1) [4–10]. Figure 2 indicates that power plant (55% of global CO₂ emissions), transportation (23%), and industry (19%) have highest share in the CO₂ emission in USA. Cement and petrochemical plants are two major industries for CO₂ emission, such that cement industry contributes about 5% to global anthropogenic CO₂ emissions. Also, petrochemical industries are a large share of CO₂ emission; for example, only in Iran, petrochemical industries emission was about 15 Mton CO₂/year [11–16].

Figure 1.

Global CO₂ emissions from fossil fuel combustion and cement production [23].

Figure 2.

U.S. GHG Emissions Allocated to Economic Sectors [2].

The Kyoto Protocol is the first international agreement on emissions of GHGs. In this protocol, industrialized countries agreed to stabilize or reduce the GHGs emissions in the commitment period 2008–2012 by 5.2% on average (compared to their 1990 emissions level). Overall, the result of global CO₂ emissions (Figure 1) shows the failure of Kyoto protocol; therefore, in 2011 Durban COP meeting, this protocol was extended until 2017. Several countries with high GHGs emission like China, India, Brazil, and even Iran have added to this Protocol. Intergovernmental Panel on Climate Change (IPCC) predicted the atmosphere may contain up to 570 ppmv CO₂ by the year 2100, causing a rise of mean global temperature and sea level around 1.9°C and 38 m, respectively [15, 17–20]. Given that the earth's average temperature continues to rise, Intergovernmental Panel on Climate Change (IPCC) stated, global GHG emissions must be reduced by 50 to 80 percent by 2050 to avoid dramatic consequences of global warming [21–23].

Carbon capture and storage (CCS) is the most indicated technology to decrease CO₂ emission from fossil fuels sources to atmosphere. Also, CO₂ separated from flue gases can be used in enhanced oil recovery (EOR) operations where CO₂ is injected into oil reservoirs to increase mobility of oil and reservoir recovery [24, 25]. Pure CO₂ has many applications in food/beverage and different chemical industries such as urea and fertilizer production, foam blowing, carbonation of beverages and dry ice production, or even in the supercritical state as supercritical solvent [26–28].

From this definition, CCS consists of three basic stages: (a) separation of CO₂, (b) transportation, and (c) storage. Operating costs of these stages have been estimated in 2008:

1. CO₂ separation from exhausting gases: 24 to 52 €/ton-CO₂,

2. transportation to storage location: 1 to 6 €/ton-CO₂ per 100 km,

3. storage: −28 to 42 €/ton-CO₂.

Therefore, CO₂ separation is a major stage in CCS. The CCS total costs can vary from −3 to 106 €/ton-CO₂ (negative values are expected for the injection of CO₂ in EOR). There are three major approaches for CCS: pre-combustion capture, oxy-fuel process, and post-combustion capture (Figure 3) [25, 30, 31].

Figure 3.

Three basic approaches of CO₂ capture [29].

Pre-combustion capture involves reaction of a fuel with oxygen or air and in some cases steam to produce a gas mainly composed of carbon monoxide and hydrogen, which is known as synthesis gas (syngas) or fuel gas. The produced carbon monoxide is reacted with steam in a catalytic reactor, called shift converter, to give CO₂ and more hydrogen. CO₂ is then separated, usually by cryogenic distillation or chemical absorption process, resulting in a hydrogen-rich fuel that can be used in many applications, such as furnaces, gas turbines, engines and fuel cells [32, 33].

A main advantage of post-combustion is the higher CO₂ concentration and pressure achieved in the output stream. The main disadvantage of pre-combustion capture is system needs long-term development in a number of enabling technical areas to achieve targeted efficiency towards a hydrogen economy. This disadvantage has limited application of this approach and increased investments costs of pre-combustion capture [34, 35].

In oxy-fuel combustion, nearly pure oxygen is used for combustion instead of ambient air, and this results in a flue gas that is mainly CO₂ and H₂O, which are separated by condensing water. Three major advantages of this method are high CO₂ concentration in output stream (above 80% v/v), high flame temperature, and easy separation of exhaust gases. The major disadvantages of oxy-fuel combustion are high capital cost and large electric power requirement to separate oxygen from air [36–38].

The principle of post-combustion capture is CO₂ separation from flue gas after combustion. Generally, the CO₂ in flue gas is diluted (8–15% CO₂) with inert gases such as nitrogen, argon, and water in addition to oxygen. Flue gases are normally at atmospheric pressure and high temperatures (between 320 K and 400 K) [39–41]. Post-combustion capture does not require expensive technologies such as syngas separation, hydrogen turbine, fuel cell. Post-combustion capture is the most important to prevent CO₂ emissions, because it offers flexibility and does not need to change combustion cycle. If the capture plant shuts down, the power plant can still operate [42, 43]. Major disadvantage of this method is unfavorable condition of flue gases.

Because of the importance in selecting suitable process for CO₂ separation, in this research various technologies for this purpose have been focused.

2. CO₂ Separation Technologies

Based on economical and environmental considerations, it is necessary to apply efficient and suitable technology for CO₂ separation with low operating cost and energy consumption. Up to now, there are several gas separation technologies being investigated for post-combustion capture, namely, (a) absorption, (b) adsorption, (c) cryogenic distillation, and (d) membrane separation (Figure 4) [39, 44]. Although various new methods were suggested for CO₂ separation, Granite and Brien [45] reviewed some of the most novel methods for carbon dioxide separation from flue and fuel gas streams, such as use of electrochemical pumps and chemical looping for CO₂ separation.

Figure 4.

Different technologies for CO₂ separation [29].

2.1. Absorption

Absorption stripping is an important technology for CO₂ capture from fuel gas; in this technology desired component in mixed gases are dissolved in a solvent (bulk phase) [46]. The general scheme of this process is depicted in Figure 5.

Figure 5.

Schematic diagram of CO₂ absorption pilot plant.

The flue gas (containing CO₂) is cooled (between 318 and 323 K), and fed to the absorption column (scrubber) where the solvent absorbs CO₂. The CO₂-rich solution is fed into a heater to increase the temperature of solution, then to a stripper column to release the CO₂. The released CO₂ is compressed, and the regenerated absorbent solution is cooled and recycled to the absorber column [47, 48].

Energy required for post-combustion CO₂ capture is an important issue. Thus, recent studies suggest that reduction of the cost of this capture could be achieved by finding suitable solvents that could process larger amounts of CO₂ for a given mass and require less energy for stripping stage [49, 50].

2.1.1. Solvents

In absorption process, flue gas is contacted with a liquid “absorbent” (or “solvent”), and CO₂ is absorbed by this solvent [21]. However, the absorbent should have a suitable capacity for CO₂ absorption, high kinetic rate for CO₂ absorption, negligible vapor pressure, and high chemical and thermal stability and should be harmless for labor persons [51–53].

The solvents used for CO₂ absorption can be divided into two categories: physical and chemical solvents. Physical solvent processes use organic solvents to physically absorb acid gas components rather than reacting chemically, but chemical absorption depends on acid-base neutralization reactions using alkaline solvents [54, 55]. In the recent years, many studies have compared the performance of different solvents as listed in Table 2.

Table 2.

Various solvents suggested for CO₂ separation.

Group of solvents	Advantage	Disadvantage	Application	Reference
Physical
Dimethyl ether of polyethylene glycol (Selexol)	(i) Require low energy for regeneration (less than 20% of the value for chemical absorbent) (ii) Low vapor pressure, low toxicity, and less corrosive solvent	(i) Dependent on temperature and pressure; therefore they are not suitable for post-combustion process (ii) Low capacity for CO2 absorption	Natural gas sweetening	[29, 39, 57, 62, 63]
Glycol			Capturing CO2 and H2S at higher concentration
Glycol carbonate			Separating CO2 from other gases
Methanol (Rectisol)			CO2 removal from various streams
Fluorinated solvent			(i) CO2 removal from various streams (ii) Separating CO2 from other gases
Chemical
Alkanolamines: monoethanolamine (MEA), diethanolamine (DEA), and methyl diethanolamine (MDEA)	(i) React rapidly (ii) High selectively (between acid and other gases) (iii) Reversible absorption process (iv) Inexpensive solvent	(i) Low CO2 loading capacity (ii) Solvent degradation in existence of SO2 and O2 in flue gas (concentrations must be less than 10 ppm and 1 ppm) (iii) High equipment corrosion rate (iv) High energy consumption	Important for removing acidic components from gas streams	[58, 60, 61, 64–66]
Amino acid and aqueous amino acid salt	(i) The possibility of adding a salt functional group. (ii) The nonvolatility of solvents (iii) Having high surface tension (iv) Having better resistance to degradation than other chemical solvents (v) Better performance than MEA of the same concentration for CO2 absorption	Decreased performance in the presence of oxygen	Suggested for CO2 separation from flue gases	[65, 67–69]
Ammonia	(i) No degradation in the presence of SO2 and O2 in the flue gases (ii) No corrosion effect (iii) Require low energy to regeneration (1/3 that required with MEA) (iv) Low costs with aqueous ammonia, respectively, 15% and 20% less than with MEA	(i) Reversible at lower temperatures (not suitable for post-combustion) (ii) Production of solid products and their operating problems (iii) Explosion of dry CO2-NH3 reaction in the high concentration of CO2 in the flue gas (explosive limit for NH3 gas is 15–28%)	Suggested for CO2 separation from flue gases	[39, 70]
Ionic liquid (IL)	(i) Very low vapor pressure (ii) Good thermal stability (iii) High polarity (iv) Nontoxicity	Increased viscosity with CO2 absorption	Suggested for CO2 separation from flue gases	[71–74]
Aqueous piperazine (PZ)	(i) Fast absorption kinetics (CO2 absorption rate with aqueous PZ is more than double that of MEA) (ii) Low degradation rates for CO2 separation (iii) Negligible thermal degradation in concentrated PZ solutions (iv) Favorable equilibrium characteristics (v) Very low heat of absorption (10–15 kCal/mol CO2), 80–90% energy required for aqueous amine system	Lower oxidative degradation of concentrated PZ (i.e., 4 times slower than MEA in the presence of the combination of Fe2+/Cr3+/Ni2+ and Fe2+/V5+)	(i) Effective for treating syngas at high temperatures (ii) Application of additional amine promoters for natural gas treating and CO2 separation from flue gases	[29, 66, 75, 76]

(1) Alkanolamines. Between various solvent groups, alkanolamines group is the most important and more used for CO₂ separation. A major problem in the usage of amines for CO₂ absorption is equipment corrosion, so Albritton et al. [56] examined corrosion rate of various amine solvents and suggested corrosion rate could reduce in the following order: monoethanolamine (MEA) > 2-amino-2-methyl-1-propanol (AMP) > diethanolamine (DEA) > methyl diethanolamine (MDEA).

On the other way, MEA can react more quickly with CO₂ than MDEA, but MDEA has higher CO₂ absorption capacity and requires lower energy to regenerate CO₂ [39, 57, 58]. Thus, it can be concluded that MEA is one of the best amine solvents for CO₂ separation. Idem et al. [59] reported substantial reduction in energy requirements and modest reduction in circulation rates for amine blends relative to the corresponding single amine system of similar total amine concentration. Wang et al. [57] found that when MEA and MDEA are mixed at the appropriate ratio, the energy consumption for CO₂ regeneration is reduced significantly. Dave et al. [28] compared the performance of several amine solvents and ammonia solutions at various concentrations. They showed that 30 wt% AMP based process has the lowest overall energy requirement among the solvents considered in their study (30% MEA, 30% MDEA, 2.5% NH₃, and 5% NH₃) [28, 60].

Knudsen et al. [61] studies showed that it is possible to run the post-combustion capture plant continuously while achieving roughly 90% CO₂ separation levels and CASTOR-2 (blended amine solvents), operated in pilot scale with lower steam requirement and liquid-to-gas ratio (L/G) than the conventional MEA solvent.

Besides alkanolamines, carbonate-bicarbonate buffers and hindered amines are used in the bulk removal of CO₂ owing to the low steam requirement for its regeneration. Mitsubishi Heavy Industries and Kansai Electric have employed other patented chemical solvents—strictly hindered amines called KS-1, KS-2, or KS-3. The regeneration heat of KS solvents is said to be ~3 GJ/t CO₂, that is, 20% lower than that of MEA with ~3.7 GJ/t CO₂ [60, 64, 77]. Generally, the overall cost of amine absorption/stripping technology for CO₂ capture process is 52–77 US$/ton CO₂ [71].

(2) Amino Acid. Amino acids have the same functional groups as alkanolamines and can be expected to behave similarly towards CO₂ but do not deteriorate in the presence of oxygen. Based on the results of tests, the aqueous potassium salts (composed of sarcosine and proline) are considered to be the most promising solvents. The most common amino acids used in the gas treating solvents are glycine, alanine, dimethyl glycine, diethyl glycine, and a number of sterically hindered amino acids [65, 67, 68].

Results of many research groups showed that these solvents are suitable for application in membrane gas absorption units, because these solvents have better performance and degradation resistance than other chemical solvents [78]. Amino acid salts formed by neutralization of amino acids with an organic base such as amine showed better CO₂ absorption potential than amino acid salts from neutralization of amino acid salts from an inorganic base such as potassium hydroxide [79, 80]. Aronu et al. [69] studied the performance of amino acids neutralized with 3-(methylamino)propylamine (MAPA), glycine, β-alanine, and sarcosine. Their results indicated that sarcosine neutralized with MAPA has the best CO₂ absorption performance. Its performance is also enhanced by promoting with excess MAPA [69].

(3) Ammonia. Since ammonia is a toxic gas, prevention of ammonia “slip” to the atmosphere is a necessity. Despite this disadvantage, chilled ammonia process (CAP) was used for CO₂ separation (Figure 6). In the CAP, CO₂ is absorbed in an ammoniated solution at a lower absorption temperature (275–283 K) that reduced ammonia emissions from the CAP absorber. Ammonium carbonate solution resulted in approximately 38% carbon regeneration compared to MEA solution [70, 81, 82].

Figure 6.

Schematic layout of CO₂ separation block based on the chilled ammonia process [92].

(4) Aqueous Piperazine (PZ). Piperazine (PZ) is as an additive used for amine systems to improve kinetics of CO₂ absorption, such as MDEA/PZ or MEA/PZ blends. Because PZ solubility in water is low, concentration of PZ is between 0.5 and 2.5 M. As indicated in Table 2, increasing the concentration of PZ in solution allows for increased solvent capacity and faster kinetic. The presence of potassium in solution increases the concentration of CO₃ ²⁻/HCO₃ ⁻ in solution; therefore, solution has buffering property. These competing effects yield a maximum fraction of reactive species at potassium to piperazine ratio of 2 : 1 [75, 83, 84].

2.2. Adsorption

Adsorption operation can reduce energy and cost of the capture or separation of CO₂ in post-combustion capture. To achieve this goal, it is necessary to find adsorbents with suitable properties. In general, CO₂ adsorbent must have high selectivity and adsorption capacity and adequate adsorption/desorption kinetics, remain stable after several adsorption/desorption cycles, and possess good thermal and mechanical stability [51, 85–88]. The adsorbents used for CO₂ separation placed into two main categories: physical and chemical adsorbents.

2.2.1. Chemical Adsorption

Chemisorption is a subclass of adsorption, driven by a chemical reaction occurring at the exposed surface. Adsorption capacities of different chemical adsorbents are summarized in Table 3.

Table 3.

Adsorption capacity of chemical adsorbents for post-combustion CO₂.

Sorbent	Operating temperature (K)	Operating pressure (kPa)	CO2 capture capacity (mol CO2/kg sorbent)	Regeneration cycles, n	CO2 capture capacity remained after n cycles (%)	Reference
Mesoporous (MgO)	298	101	1.8	3	100	[93]
CaO nanopods	873	101	17.5	50	61.1	[94]
CaO derived from nanosized CaCO3	923	101	16.7	100	22.2	[93]
CaO-MgAl2O4 (spinel nanoparticles)	923	101	9.1	65	84.6	[93]
Nano CaO/Al2O3	923	101	6.0	15	61.7	[93]
Lithium silicate nanoparticles	883	101	5.77	n.a.	n.a.	[93]
Nanocrystalline Li2ZrO3 particles	843	101	6.1	8	100	[93]
CaO/Al2O3	923	101	6.02	n.a.	n.a.	[93]
Lithium silicate	993	n.a.	8.18	n.a.	n.a.	[17]
Lithium zirconate	673	100	5.0	n.a.	n.a.	[93]
Lithium orthosilicate	873	100	6.13	n.a.	n.a.	[93]
Calcium oxide	873	100	17.3	n.a.	n.a.	[93]
Magnesium hydroxide	473	1034	3.0	n.a.	n.a.	[93]
Mesoporous magnesium oxide	373	100	2.27	n.a.	n.a.	[93]
Lithium Silicate nano particles	873	101	5	n.a.	n.a.	[95]
HTI-HNa	573	134	1.109	50	93.3	[93]

A wide range of metals have been studied including [89]

1. metal oxides: CaO, MgO,

2. metal salts from alkali metal compounds: lithium silicate, lithium zirconate to alkaline earth metal compounds (i.e., magnesium oxide and calcium oxide),

3. hydrotalcites and double salts.

In general, one mole of metal compound can react with one mole of CO₂ with a reversible reaction. The process consists of a series of cycles where metal oxides (such as CaO) at 923 K are transformed into metal carbonates form (such as CaCO₃) at 1123 K in a carbonation reactor to regenerate the sorbent and produce a concentrated stream of CO₂ suitable for storage [90, 91].

Considerable attention was paid to calcium oxide (CaO) as it has a high CO₂ adsorption capacity and high raw material availability (e.g., limestone) at a low cost. Lithium salts was recorded a good performance in CO₂ adsorption, but it gained less focus due to its high production cost. Although double salts can be easily regenerated due to low energy requirement, their stability has not been investigated [93, 96].

The reaction of CO₂ adsorption with Li₂ZrO₃ is reversible in the temperature range of 723–863 K. The capacity of lithium silicate (8.2 moL CO₂/kg sorbent at 993 K) is larger than that of lithium zirconate (4.85 moL/kg sorbent) [17].

Hydrotalcite (HT) contains layered structure with positively charged cations balanced by negatively charged anions [97, 98]. Adsorption and final capacity of different adsorption/desorption cycles are listed in Table 3.

One way for improving CO₂ adsorption efficiency is application of nanomaterials. Different nano-materials can be used for CO₂ separation (Table 3). However, nanomaterials always have high production cost with complicated synthesis process such as carbon nanotubes and graphite nanoplatelets [99, 100].

The main disadvantage of chemical adsorbents is difficult regeneration process, and application of these adsorbents needs more studies for finding new adsorbents [88, 95].

2.2.2. Physical Adsorption

Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption. If the CO₂ adsorption capacity of solid adsorbents reaches 3 mmoL/g, the required energy for adsorption will be less than 30–50% energy for absorption with optimum aqueous MEA [101]. The major physical adsorbents suggested for CO₂ adsorption include activated carbons and inorganic porous materials such as zeolites [102, 103]. The adsorption capacities of various physical adsorbents are summarized in Table 4.

Table 4.

Adsorption capacity of physical adsorbents for post-combustion CO₂.

Sorbent	Operating temperature (K)	Operating pressure (kPa)	CO2 capture capacity (mol CO2/kg sorbent)	Regeneration cycles, n	CO2 capture capacity remained after n cycles (%)	Reference
Activated carbon	303	110	1.58	n.a.	n.a.	[93]
AC (4% KOH)	303	30	0.55	n.a.	n.a.	[93]
AC (EDA + EtOH)	303	30	0.53	n.a.	n.a.	[93]
AC (4% KOH + EDA + EtOH)	303	30	0.64	n.a.	n.a.	[45, 70, 79]
NiO-ACs	298	101	2.227	n.a.	n.a.	[104]
13X	393	15.198	0.7	n.a.	n.a.	[105]
5A	393	15.198	0.38	n.a.	n.a.	[105, 106]
4A	393	15.198	0.5	n.a.	n.a.	[105]
WEG-592	393	15.198	0.6	n.a.	n.a.	[105]
APG-II	393	15.198	0.38	n.a.	n.a.	[105]
Na-Y	273	10.132	4.9	n.a.	n.a.	[105]
Na-X	373	101.32	1.24	2	n.a.	[105]
NaKA	373	101.32	3.88	—	n.a.	[105]
NaX-h	323	101.32	2.52	2	n.a.	[105]
NaX-h	373	101.32	1.37	2	n.a.	[105]
Na-X-c	323	101.32	2.14	2	n.a.	[105]
Na-X-c	373	101.32	1.41	2	n.a.	[105]
Cs-X-h	323	101.32	2.42	2	n.a.	[105]
Cs-X-h	373	101.32	1.48	2	n.a.	[105]
Cs-X-c	323	101.32	1.76	2	n.a.	[105]
Cs-X-c	373	101.32	1.15	n.a.	n.a.	[105]
MCM-41	298	100	0.62	n.a.	n.a.	[93]
MCM-41 (DEA)	348	100	1.26	n.a.	n.a.	[93]
MCM-41 (50% PEI)	348	100	2.52	n.a.	n.a.	[93]
Activated carbon	303	30	0.35	n.a.	n.a.	[93]
MCM-41 (50% PEI) “molecular basket”	348	100	2.95	n.a.	n.a.	[93]
PE-MCM-41	298	100	0.50	n.a.	n.a.	[93]
PE-MCM-41 (TRI)	298	100	2.85	n.a.	n.a.	[93]
PE-MCM-41 (DEA)	348	100	2.36	n.a.	n.a.	[93]
MCM-48	298	100	0.033	n.a.	n.a.	[93]
MCM-48 (APTS)	298	100	0.639	n.a.	n.a.	[93]
MCM-41	298	100	0.62	n.a.	n.a.	[93]
Molecular basket' MCM-41 (50% PEI)	348	100	2.5	8	96.0	[93]
PE-MCM-41 (TRI)	298	100	1.8	10	94.4	[93]
PE-MCM-41 (DEA)	298	100	2.9	7	96.6	[93]
MWNT	303	101	1.7	20	n.a.	[4, 93]
Unmodified [(Cu3(btc)2]*	298	1818	6.7	n.a.	n.a.	[101]
CNT@ (Cu3(btc)2)	298	1818	13.52	n.a.	n.a.	[101]
MIL-101**	298	1010	0.84	n.a.	n.a.	[101]
MWCNT@MIL-101	298	1010	1.35	n.a.	n.a.	[101]
MOF-2	298	4545	3.20	n.a.	n.a.	[107]
MOF-177	298	4545	33.5	n.a.	n.a.	[107]
Zr-MOFs	273	988	8.1	n.a.	n.a.	[107]
Ca-Al LDH with ClO4 −	406	1	3.55	n.a.	n.a.	[108]
Pd-GNP nanocomposite	298	1111	5.1	n.a.	n.a.	[109]
f-GNP	298	1111	4.3	n.a.	n.a.	[109]
Pd-GNP nanocomposite	298	1111	4.5	n.a.	n.a.	[109]
f-GNP	298	1111	3.8	n.a.	n.a.	[109]
Pd-GNP nanocomposite	298	1111	4.1	n.a.	n.a.	[109]
f-GNP	298	1111	3.3	n.a.	n.a.	[109]
Ceria-based oxides doped with 5% gallium (III)	298	101	0.282	n.a.	n.a.	[110]
Amine modified layered double hydroxides (LDHs)	298–353	101	0.74–1.75	n.a.	n.a.	[108]

 *Cu₃(btc)₂; btc: 1,3,5-benzene-tricarboxylate.

**MIL-101 or Cr₃(F,OH)(H₂O)₂O[(O₂C)C₆H₄(CO₂)]₃ ·nH₂O (n ≈ 25) is one of the metal organic frameworks with Lewis acid sites that can be activated by removal of guest water molecules.

Coal is one of the adsorbents being suggested for CO₂ separation. The total amount of CO₂ that can be adsorbed in coal depends on its porosity, ash, and affinity for this molecule [111, 112]. Sakurovs et al. [113] showed that the ratio of maximum sorption capacity between CO₂ and methane decreases with increasing carbon content. The average CO₂/CH₄ sorption ratio is higher for moisture-equilibrated coal and decreases with increasing coal rank (1.4 for high rank coals to 2.2 for low rank coals) [114–116].

Activated carbon (AC) has a number of attractive characteristics, such as its high adsorption capacity, high hydrophobicity, low cost, and low energy requirement for regeneration [117–119]. Activated carbons are inexpensive, insensitive to moisture, and easy for regeneration. These adsorbents have well developed micro- and mesopore structures that are suitable for high CO₂ adsorption capacity at ambient pressure [120–122].

However, activated carbon CO₂/N₂ selectivities (ca. 10) are relatively low; zeolitic materials offer CO₂/N₂ selectivities 5–10 times greater than those of carbonaceous materials. The adsorption capacity and selectivity of zeolites are largely affected by their size, porous diameter, charge density, and chemical composition of cations in their porous structures. The average value of heat adsorption on zeolites (36 kJ/moL) is larger than for activated carbon (30 kJ/moL), confirming the mentioned affirmation. Moreover, activated carbon can be regenerated easily and completely. Also its capacity did not decay after 10 consecutive processes cycles [122–124].

Due to the increase in cost of raw materials, growing research interest has been focused on producing AC from agricultural wastes. Some of the agricultural wastes include the shells and stones of fruits, wastes resulting from the production of cereals, bagasse, and coir pith [100]. Rosas et al. [125] prepared hemp-derived AC monolith by phosphoric acid activation. The activated carbons from hemp stem are microporous materials and therefore suitable ones for hydrogen storage and CO₂ capture [126].

Siriwardane et al. [127] studied CO₂ adsorption on the molecular sieve 13X, 4A and activated carbon. The molecular sieve 13X showed better CO₂ separation than molecular sieve 4A. At lower pressures (<50 psi), activated carbon had a lower CO₂ separation than the molecular sieves, but adsorption was higher for activated carbon than molecular sieves at higher pressures [127, 128].

Liu et al. [129] indicated that zeolite 5A has higher volumetric capacities and less severe heat effect of the zeolite 13X. Chabazite zeolites were prepared and exchanged with alkali cations: Li, Na, K and alkaline-earth cations: Mg, Ca, Ba. Zhang et al. [130] studied the potential of these zeolites for CO₂ separation from flue gas by vacuum swing adsorption. It was found that NaCHA and CaCHA hold comparative advantages for high temperature CO₂ separation whilst NaX showed superior performance at relatively low temperatures [130]. In physical adsorption, the size and volume of the pores are important. Micropores are defined as pores, 2 nm in size, mesopores between 2 and 50 nm, and macropores, 50 nm in size. The micropores make better selective adsorption of CO₂ over CH₄ [131, 132].

Carbon nanotubes (CNTs) are the most famous among nano-hollow structured materials and their dimension ranges from 1 to 10 nm in diameter and from 200 to 500 nm in length [133]. Cinke et al. [134] indicated that purified single-walled carbon nanotubes (SWNTs) adsorbed CO₂ better than unpurified SWNT. In addition, multiwalled carbon nanotubes (MWNTs) showed stability for 20 cycles of adsorption and regeneration [135].

More recently, nanosystems researchers have synthesized and screened a large number of zeolitic-type materials known as zeolitic imidazolate frameworks (ZIFs). CO₂ capacities of the ZIFs are high, and selectivity against CO and N₂ is good [136, 137]. The results of researchers (Burchell and Judkins [138], Dave et al. [28], and Yong et al. [139]) indicated that the CO₂ adsorption efficiency of the honeycomb monolith is twice than activated carbon and 1.5 times greater than ZIF material [29]. Results of Kimber et al. [140] showed that CO₂ selectivity of honeycomb monolithic composite decreased with increasing in burn-off.

Graphite nanoplatelets (GNP) were prepared by acid intercalation followed by thermal exfoliation of natural graphite. Functionalized graphite nanoplatelets (f-GNP) were prepared by further treatment of GNP in acidic medium. Palladium (Pd) nanoparticles were decorated over f-GNP surface by chemical method [109, 141, 142]. Adsorption capacity of this adsorbent is presented in Table 4.

The presence of several impurity gases (SO_x/NO_x/H₂O) greatly complicates the CO₂ separation processes. Therefore, conventional adsorption-based CO₂ separation processes rely on using a pretreatment stage to remove water, SO_x, and NO_x, which adds considerably to the overall cost. Also this prelayer can be used before the amine absorption column [143, 144]. Deng et al. [145] showed that the adsorption capacities follows the order SO₂ > CO₂ > NO > N₂ on both zeolites (5A and 13X). Comparing two different adsorbents, the better separation efficiency can be achieved by 5A zeolite [145].

Zhang et al. [130] focused on the effect of water vapour on the pressure/vacuum swing adsorption process. The selected adsorbents in this study were CDX (an alumina/zeolite blend), alumina, and 13X zeolite as these adsorbents are either the prelayer for water adsorption or the main CO₂ adsorption layer in the packed bed [130].

Metal-organic framework (MOF) materials are crystalline with two- or three-dimensional porous structures that can be synthesised with many of the functional capabilities of zeolites. Several MOFs have been proposed as adsorbents for CO₂ separation processes, and among these Cu-BTC [polymeric copper (II) benzene-1,3,5-tricarboxylate] has proved to be dedicated with CO₂ adsorption performances that are higher than those of typical adsorbents such as 13X zeolite [105, 107, 146, 147].

The MCM-41 material is one of the mesoporous products which was prepared by the hydrothermal method from mobil composition of matter (MCM) powders. Lu et al. [148] showed that mesoporous silica spherical particles (MSPs) can be synthesized using low-cost Na₂SiO₃ thus they can be cost-effective adsorbents for CO₂ separation from flue gas [149, 150].

Layered double hydroxides (LDHs) have general formula [M_1−x ^IIM_x ^III(OH)₂][X_x/C ^g− · nH₂O] with x typically in the range between 0.10 and 0.33. These materials can be readily and inexpensively synthesized with the desired characteristics for a particular application such as CO₂ adsorption [108, 151].

2.2.3. Adsorbent Modification

The role of CO₂ as a weak Lewis acid is well established. Because of the nature of CO₂, the surface of the physical adsorbents can be modified by adding basic groups, such as amine groups and metal oxides to improve CO₂ adsorption capacity or selectivity [152–154]. Three different methods for the production of these adsorbents were investigated: activation with CO₂, heat treatment with ammonia gas (amination and ammoxidation), and heat treatment with polyethylenimine (PEI). However, it has been suggested that amine modification can produce better and cheaper CO₂ adsorbents [24, 104, 155, 156].

Xu et al. [157, 158] designed selective “molecular basket” by grafting polyethylenimine (PEI) uniformly on MCM-41. CO₂ adsorption capacity of the adsorbent was 24 times higher than MCM-41 and 2 times higher than PEI [93]. The addition of ammonium hydroxide resulted in the Zr-MOF with a slight lower adsorption of CO₂ and CH₄; however, the selectivity of CO₂/CH₄ is significantly enhanced [159, 160]. Results of Abid et al. [107] showed that the selectivity of CO₂/CH₄ on Zr-MOF is between 2.2 and 3.8, while for Zr-MOF-NH₄ selectivity is between 2.6 and 4.3.

A nitrogen-rich carbon with a hierarchical micro-mesopore structure exhibited a high CO₂ adsorption capacity (141 mg/g at 298 K, 1 atm), excellent separation efficiency (CO₂/N₂ selectivity is ca. 32), and excellent stability [161]. Plaza et al. [162] results showed that CO₂ adsorption capacity of the DETA-impregnated alumina (≥2.3 mmoL/g) exhibited is the highest.

Amine modified layered double hydroxides (LDHs) have been prepared by several different methods. Park et al. [163] used dodecyl sulfate (DS) intercalated LDH as precursor and added (3-aminopropyl) triethoxysilane (APTS) together with N-cetyl-N,N,N-trimethylammonium bromide (CTAB) [164]. The highest adsorption capacity of amine modified LDHs for CO₂ was achieved at 1.75 mmoL/g by MgAl N3 at 353 K and 1 bar. According to data in Table 4, this adsorbent has high CO₂ capacity at high temperature; therefore, this adsorbent is suitable for post-combustion CO₂ capture [108].

Wang et al. [114] reported that porous carbons with well-developed pore structures were directly prepared from a weak acid cation exchange resin (CER) by the carbonization of a mixture with Mg acetate in different ratios [108]. The main parameters of this adsorbent (such as CO₂ capacity) are indicated in Table 4.

Shafeeyan et al. [165] prepared different adsorbents based on the central composite design (CCD) with three independent variables (i.e., amination temperature, amination time, and the use of preheat treated (HTA) or preoxidized (OXA) sorbent as the starting material). They demonstrated that the optimum condition for obtaining an efficient CO₂ adsorbent is using a preoxidized sorbent and amination at 698 K for 2.1 h [165].

Table 4 compares CO₂ adsorption capacities and stability of different absorbents, which were studied for post-combustion CO₂ capture.

2.2.4. Different Cycles for CO₂ Adsorption

Five different regeneration strategies were demonstrated in a single-bed CO₂ adsorption unit: pressure swing adsorption (PSA), temperature swing adsorption (TSA), vacuum swing adsorption (VSA), electric swing adsorption (ESA), and a combination of vacuum and temperature swing adsorption (VTSA). The difference between these technologies is based on the strategy for regeneration of adsorbent after the adsorption step (Figure 7). In PSA applications, the pressure of the bed is reduced. VSA is preferred to the special PSA application where the desorption pressure is below atmospheric, whereas in TSA, the temperature is raised while pressure is maintained approximately constant, and in ESA the solid is heated by the Joule effect [166–169].

Figure 7.

Schematic diagrams of various adsorption cycles, (a) TSA, (b) PSA, (c) VSA, and (d) ESA; thin lines indicated operation streams in regenerated step.

For the single-bed cycle configurations, the productivity and CO₂ recovery followed the sequence:

$$\begin{array}{c}\text{ESA}<\text{TSA}<\text{PSA}<\text{VSA}<\text{VTSA}.\end{array}$$ (1)

The performances of PSA, TSA, VSA, VTSA, and ESA processes for CO₂ separation are reported in Table 5. Since application of adsorption process for CO₂ capture in industrial scale is very important, in recent years some researches have been focused on this area; for example, Lucas et al. [170] studied the scale-up CO₂ adsorption with activated carbon.

Table 5.

Comparison between several adsorption cycles for CO₂ separation process [166].

Process	CO2 feed molar fraction (%) (other gases present)	CO2 purity (%)	CO2 recovery (%)
PSA	13 (O2)	99.5	69
TSA	10	95	81
TSA	17	n.a.	40
ESA	10	23.33	92.57
VSA	15	90	90
VSA	17	n.a.	87
3-bed VSA	12	90–95	60–70
PSA/VSA	20	58–63	70–75
PSA/VSA	15 (H2O)	59	87
VPSA	17	99.5–99.8	34–69
VPSA	16 (O2)	99	53–70
PTSA	10	99	90
2-bed-2-step PSA	n.a.	18	90
VTSA	17	n.a.	97

2.3. Cryogenic Distillation

Cryogenic method utilized low temperatures for condensation, separation, and purification of CO₂ from flue gases (freezing point of pure CO₂ is 195.5 K at atmospheric pressure). Therefore, under the cryogenic separation process, the components can be separated by a series of compression, cooling, and expansion steps. It enables direct production of liquid CO₂ that can be stored or sequestered at high pressure via liquid pumping [171–173].

The advantages of this technology can be summarized as follows [6, 8, 174].

1. Liquid CO₂ is directly produced, thus making it relatively easy to store or send for enhanced oil recovery.

2. This technology is relatively straightforward, involving no solvents or other components.

3. The cryogenic separation can be easy scaled-up to industrial-scale utilization.

The major disadvantages of this process are the large amount of energy required to provide the refrigeration and the CO₂ solidification under a low temperature, which causes several operational problems [176–178]. Therefore, more studies are required for reducing the cost of cryogenic separation.

Clodic et al. [179] indicated that the energy requirement for cryogenic process was in the range of 541–1119 kJ/kg CO₂. Zanganeh et al. [6] have constructed a pilot-scale CO₂ capture and compression unit (CO₂ CCU) that can separate CO₂ as liquid phase from the flue gas of oxy-fuel combustion. Their results showed that cryogenic is the most cost effective when the feed gas is available at high pressure. Therefore, cryogenic is not suitable for post-combustion and it is well effective for separation stream with high CO₂ concentration such as oxy-fuel combustion. Amann et al. [180] reported that conversion of O₂/CO₂ cycle was more efficient than amine scrubbing but more difficult to implement because of the specific gas turbine.

Xu et al. [175] studied a novel CO₂ cryogenic liquefaction and separation system (Figure 8). In this system, two-stage compression, two-stage refrigeration, two-stage separation, and sufficient recovery of cryogenic energy were adopted. The energy consumption for CO₂ recovery is only 0.395 MJ/kg CO₂. Furthermore, this CO₂ cryogenic separation system is more suitable for gas mixtures with high initial pressure and high CO₂ concentration [175].

Figure 8.

Novel CO₂ cryogenic liquefaction and separation system [175].

Song et al. [181] developed a novel cryogenic CO₂ capture system based on Stirling coolers (SC). The operation of Stirling cooler contains four processes: isothermal expansion, refrigeration under a constant volume, isothermal compression, and heating under a constant volume condition. This novel cryogenic system can condense and separate H₂O and CO₂ from flue gas. Their results showed that under the optimal temperature and flow rate, CO₂ recovery of the cryogenic process can reach 96% with 1.5 MJ/kg CO₂ energy consumption.

Tuinier et al. [182] exploited a novel cryogenic CO₂ capture process using dynamically operated packed beds (Figure 9). By the developed process, above 99% of CO₂ could be recovered from a flue gas containing 10 vol.% CO₂ and 1 vol.% H₂O with 1.8 MJ/kg CO₂ energy consumption [181].

Figure 9.

Schematic axial temperature and corresponding mass deposition profiles for the cryogenic; (a) capture, (b) recovery, and (c) cooling cycles [182].

Chiesa et al. [183] proposed an advanced cycle that a molten carbonate fuel cell (MCFC) was used to separate the CO₂ from the gas turbine exhaust of a natural gas fired combined cycle power plant. In this cycle, gas turbine flue gases actually are used as cathode feeding for MCFC. While CO₂ is moved from the cathode to anode side, concentrate CO₂ in the anode exhaust. Then the CO₂ is concentrated on the anode side of MCFC allowing to easily treat this spent fuel stream in a cryogenic process to split combustible species (routed back to gas turbine combustor) from the CO₂ addressed to storage (Figure 10) [183].

Figure 10.

Plant layout showing the integration of the MCFC in a combined cycle, with cryogenic CO₂ separation after oxygen combustion of the cell an anode exhaust [183].

2.4. Membrane Separation

The membrane separation method is a continuous, steady-state, clean and simple process, and ideal as an energy-saving method for CO₂ recovery. Gas separation using membranes is a pressure-driven process. Due to the low pressure of flue gases, driving force is too low for membrane processes in post-combustion (low pressure and low CO₂ concentration). Membrane processes offer increased separation performances when CO₂ concentration in the feed mixture increases [184–186].

Membrane separation processes have several advantages over other CO₂ separation technologies. The required process equipment is very simple, compact, relatively easy to operate and control, clear process and easy to scale up [187, 188].

The energy required for the recovery of CO₂ by membrane processes depends on the target purity, flue gas composition, and membrane selectivity for CO₂. Howevre membrane processes require too much energy for post-combustion CO₂ capture; therefore, low partial pressure of CO₂ in the flue gas is a possible disadvantage for the application of membranes. Another disadvantage of membrane process is that the membrane selectivity for the separation of CO₂ from SO_x and NO_x is very low. Membrane process is not useful for high flow rate applications [189–191].

Therefore, the useful membrane for post-combustion CO₂ capture should have some specification such as [192, 193]

1. high CO₂ permeability,

2. high selectivity for CO₂ separatation from flue gases,

3. high thermal and chemical stability,

4. resistant to plasticisation,

5. resistant to aging,

6. cost effective,

7. low production cost for different membrane modules.

Many efforts have been made to find new material with suitable properties (Table 6).

Table 6.

Carbon dioxide and nitrogen gas permeability data for different membranes.

Name	Feed pressure (atm)	Temperature (K)	P* (CO2) (barrer)	P* (N2) (barrer)	α (CO2/N2)	Reference
Ion-exchanged zeolites membrane
Y (FAU) with α-A12O3 support	n.a.	308	n.a.	n.a.	139	[197]
ZSM-5 (MFI) with α-A12O3 support	n.a.	n.a	n.a.	n.a.	3	[197]
ZSM-5/polymeric silica	n.a.	373	1140	n.a.		[198]
Stainless steel support infiltrated with a eutectic molten carbonate mixture (Li/Na/K)	n.a.	923	7780	n.a.	16	[199]
Y-type	n.a.	303–403	35900–89800	n.a.	5	[200]
NaY	n.a.	313	359000	n.a.	5	[200]
Li(20%)Y	n.a.	308	210000	n.a.	3	[200]
K(30%)Y	n.a.	308	269000	n.a.	9	[200]
K(62%)Y	n.a.	313	150000	n.a.	6	[200]
Rb(38%)Y	n.a.	313	150000	n.a.	3	[200]
Cs(32%)Y	n.a.	313	59900	n.a.	2	[200]
20% K2CO3, 80% Li2CO3	n.a.	798	2990	n.a.	4	[199]
MCM-48	n.a.	n.a.	10200	n.a.	0.8	[189]
PEI-modified MCM-48	n.a.	363	14100	n.a.	80	[201]
Chitosan	1.75	295	100	n.a.	100	[192]
Swollen chitosan	1.5	383	482	n.a.	250	[192]
Arginine salt-chitosan	1.5	383	1500	n.a.	852	[194]
Polyacetylene
Polytrimethyl-prop-1-ynyl-silane	n.a.	298	19000	1800	10.6	[193]
Poly-3,3-dimethyl-but-1-yne	n.a.	298	560	43	13.0	[193]
Poly-1-(dimethyl-trimethylsilanylmethyl-silanyl)-propyne	n.a.	298	310	21	14.8	[193]
Poly-1-[dimethyl-(2-trimethylsilanyl-ethyl)-silanyl]-propyne	n.a.	298	150	14	10.7	[193]
Polytrimethyl-(2-prop-1-ynyl-phenyl)-silane	n.a.	298	290	24	12.1	[193]
Poly-1-prop-1-ynyl-2-trifluoromethyl-benzene	n.a.	298	130	7.3	17.8	[193]
Poly-dec-2-yne	n.a.	298	130	14	9.3	[193]
Poly-1-chloro-dec-1-yne	n.a.	298	170	16	10.6	[193]
Poly-1-chloro-oct-1-yne	n.a.	298	130	11	11.8	[193]
Poly-1-chloro-hex-1-yne	n.a.	298	180	10	18	[193]
Polyhexyl-dimethyl-prop-1-ynyl-silane	n.a.	298	71	4.3	16.5	[193]
Polytrimethyl-(1-pentyl-prop-2-ynyl)-silane	n.a.	298	120	8.7	13.8	[193]
Polyhexyl-dimethyl-(1-propyl-prop-2-ynyl)-silane	n.a.	298	70	6.3	11.1	[193]
Polyprop-1-ynyl-benzene	n.a.	298	25	2.2	11.4	[193]
Polybut-1-ynyl-benzene	n.a.	298	40	4.5	8.9	[193]
Polyoct-1-ynyl-benzene	n.a.	298	48	5.5	8.7	[193]
Polychloroethynyl-benzene	n.a.	298	23	1.0	23.0	[193]
Poly-1-ethynyl-2-methyl-benzene	n.a.	298	15	3.0	5.0	[193]
Polydimethyl-phenyl-(1-propyl-prop-2-ynyl)-silane	n.a.	298	54	2.5	21.6	[193]
Polyarylene ether
6FPT-6FBPA	1.0	308	25.29	2.18	11.6	[193]
6FPT-BPA 1.0 35	1.0	308	18.53	1.37	13.5	[193]
6FPPy-6FBPA	1.0	308	29.46	2.39	12.32	[193]
6FPPy-BPA	1.0	308	21.44	1.70	12.6	[193]
Fixed site carrier membrane (FSCM)
Polarix	2.0	303	107	n.a.	50	[202]
PAAM-PVA/PS	10	298	2.4 × 105	n.a.	80	[203]
PVAm/PVA blend	1.45	298	2.12 × 106	n.a.	145	[204]
PEI/PVA	n.a.	298	104	n.a.	230	[184]
PDMA/PS	2	296	3 × 105	n.a.	53	[143]
Polyamine
PA12	10	308	120	n.a.	51	[152]
PA6	10	308	66	n.a.	56	[152]
Polyethyleneimine/polyvinyl butyral	0.132	318	380	n.a.	32	[193]
Poly[(2-N,N-dimethyl) aminoethyl methacrylate]	0.237	298	370	n.a.	111	[193]
Poly(vinylbenzyltrimethyl ammonium fluoride)	0.224	296	113	n.a.	983	[193]
Polyethyleneimine/poly(vinyl alcohol)	0.355	298	650	n.a.	235	[193]
PEI/PDMS/PEBA1657/PDMS	5	298	1.57 × 106	n.a.	64	[205]
Polyarylate
BPA/IA	10	308	5.4	0.24	22.5	[193]
BPA/tBIA	10	308	24.2	1.20	20.2	[193]
HFBPA/IA	10	308	19.1	1.11	17.2	[193]
HFBPA/tBIA	10	308	56.9	3.88	14.7	[193]
PhTh/IA	10	308	6.74	0.28	24.1	[193]
PhTh/tBIA	10	308	23.8	1.09	21.8	[193]
FBP/IA	10	308	12.4	0.57	12.4	[193]
FBP/tBIA	10	308	36.8	1.93	19.1	[193]
TBBPA/IA	10	308	4.93	0.18	27.4	[193]
TBBPA/tBIA	10	308	21.5	0.90	23.9	[193]
TBHFBPA/IA	10	308	25.6	1.07	23.9	[193]
TBHFBPA/tBIA	10	308	85.1	4.47	19.0	[193]
TBPhTh/IA	10	308	8.34	0.29	28.8	[193]
TBPhTh/tBIA	10	308	30.6	1.28	23.9	[193]
TBFBP/IA	10	308	20.4	0.70	29.1	[193]
TBFBP/tBIA	10	308	69.5	2.94	23.6	[193]
DMBPA/IA	10	308	1.24	0.063	19.7	[193]
DMBPA/Tbia	10	308	8.0	0.39	20.5	[193]
TMBPA/IA	10	308	12.0	0.58	20.7	[193]
TMBPA/tBIA	10	308	44.6	2.52	17.7	[193]
DiisoBPA/IA	10	308	5.16	0.27	19.1	[193]
DiisoBPA/tBIA	10	308	16.1	1.08	14.9	[193]
DBDMBPA/IA	10	308	5.45	0.22	24.8	[193]
PhAnth/IA	10	308	9.0	0.36	25	[193]
PhAnth/tBIA	10	308	25.9	1.35	19.2	[193]
FBP/IA	10	308	12.4	0.57	21.8	[193]
FBP/tBIA	10	308	36.8	1.93	19.1	[193]
Polycarbonates
PC	1–10	308	6.0–6.8	0.289–0.32	21	[193]
TMPC	1–10	308	17.58–18.6	1.0	18.6	[193]
TCPC	1	308	6.66	0.36	18.5	[193]
TBPC	1	308	4.23	0.182	23.2	[193]
HFPC	10	308	24	1.6	15.0	[193]
TMHFPC	10	308	111	7.4	15.0	[193]
NBPC	10	308	9.1	0.47	19.4	[193]
PCZ	10	308	2.2	0.105	21.0	[193]
PC-AP	2	308	9.48	0.361	26.3	[193]
FBPC	2	308	15.1	0.592	25.5	[193]
Polyethylene oxide
PEO	7.8	298	8.1	0.07	140	[193]
PEO	4.4–14.6	308–318	13–52	0.24–1	55	[193]
PEO-PBT	n.a.	308	120	2	60	[193]
EO/EM/AGE (80/20/2)	n.a.	308	773	16.8	46	[193]
EO/EM/AGE (77/23/2.3)	n.a.	308	680	15.5	44	[193]
EO/EM/AGE (96/4/2.5)	n.a.	308	580	12.1	48	[193]
Polyimides
Amine modified polyimide	0.368	308	186	n.a.	38	[193]
PMDA-BAPHF	6.8	308	11.8	0.66	17.8	[193]
PMDA-3BAPHF	6.8	308	6.12	0.29	21.1	[193]
PMDA-4,4′-ODA	6.8–10	308	1.14–2.7	0.049–0.1	23.3	[193]
PMDA-3,3′-ODA	6.8–10	308	0.50–3.55	0.018–0.145	24.5–27.8	[193]
PMDA-MDA	10	308	4.03	0.20	20.2	[193]
PMDA-IPDA	10	308	29.7	1.50	19.8	[193]
PMDA-BAPHF	10	308	17.6	0.943	18.7	[193]
PMDA-BATPHF	10	308	24.6	1.50	16.4	[193]
BPDA-BAHF	1–10	298–308	23–27.7	0.6–1.39	19.9–37.7	[193]
BPDA-mTrMPD	10	308	137	8.42	16.3	[193]
BTDA-4,4-ODA	10	308	0.625	0.0236	26.5	[193]
BTDA-BAPHF	10	308	4.37	0.195	22.4	[193]
BTDA-BAHF	10	308	10.1	0.45	22.4	[193]
BTDA-mTrMPD	10	308	30.9	1.55	19.9	[193]
BTDA-BAFL	1	298	15	0.39	38.5	[193]
PI	10	308	2.00	0.063	31.7	[193]
oMeCat-durene	1	303	27	0.83	33	[193]
mMeCat-durene	1	303	20	0.59	34	[193]
DMeCat-durene	1	303	63	2.05	31	[193]
mtBuCat-durene	1	303	71	2.55	28	[193]
oMeptBuCat-durene	1	303	67	2.5	27	[193]
TMeCat-durene	1	303	200	8.1	25	[193]
mMetCat-MDA	1	303	22	0.65	34	[193]
mtBuCat-MDA	1	303	63	2.2	29	[193]
TMeCat-MDA	1	303	110	3.8	30	[193]
TMeCat-TMB	1	303	39	1.2	33	[193]
DBuCat-TMB	1	303	95	4.9	19	[193]
mtBuCat-DMOB	1	303	6.7	0.21	32	[193]
TMeCat-6FiPDA	1	303	54	1.9	28	[193]
6F	3	n.a.	114	5.8	19.6	[193]
TMMPD	3	n.a.	600	35.1	17.1	[193]
IMDDM	3	n.a.	196	10.8	18.1	[193]
ODA	3	n.a.	25	0.97	25.8	[193]
Matrimid 5218	10	308	6.5	0.25	25.6	[193]
6FDA-based polyimides
6FDA-pPDA	10	308	15.3	0.80	19.12	[193]
6FDA-pDiMPDA	10	303	42.7	2.67	16.0	[193]
6FDA-durene	10	308	440	35.60	12.4	[193]
6FDA-durene	10	303	456	35.50	12.85	[193]
6FDA-mPDA	6.8–10	308	8.23–9.20	0.36–0.447	20.6–22.7	[193]
6FDA-mMPDA	6.8–10	303	40.1–42.5	2.12–2.24	17.9–20.1	[193]
6FDA-mTrMPDA	10	308	431	31.6	13.6	[193]
6FDA-DATr	6.8	303	28.63	1.31	21.9	[193]
6FDA-DBTF	6.8	308	21.64	1.17	18.5	[193]
6FDA-PHDoeP	6.8	303	8.59	4.50	1.91	[193]
6FDA-PEPE	6.8	308	6.88	0.255	27.0	[193]
6FDA-PBEPE	6.8	303	2.50	0.099	25.3	[193]
6FDA-PMeaP	6.8	308	2.41	0.086	28.0	[193]
6FDA-3,4′ODA	10	303	6.11	0.259	23.6	[193]
6FDA-APAP	10	308	10.7	0.473	22.6	[193]
6FDA-pp′ODA	10	303	16.7	0.733	22.8	[193]
6FDA-BAPHF	10	308	19.1	0.981	19.5	[193]
6FDA-BATPHF	10	303	22.8	1.30	17.5	[193]
6FDA-BAHF	10	308	51.2	3.11	16.5	[193]
6FDA-1,5-NDA	10	308	23	1.1	21	[193]
6FDA-durene 24 h amidation	10	n.a.	11.6	1.33	8.75	[193]
6FDA-durene/mPDA (50/50)	10	n.a.	84.6	5.18	16.4	[193]
6FDA-durene/mPDA (50/50) 4 h amidation	10	n.a.	54.9	3.38	16.2	[193]
6FDA-durene/mPDA (50/50) 6 h amidation	10	n.a.	49.1	3.27	15.0	[193]
6FDA-durene/mPDA (50/50) 12 h amidation	10	n.a.	46.0	2.94	15.6	[193]
6FDA-durene/mPDA (50/50) 24 h amidation	10	n.a.	36.0	2.06	17.5	[193]
6FDA-durene/mPDA (50/50) 48 h amidation	10	n.a.	24.5	1.38	17.8	[193]
6FDA-FDA/HFBAPP (1/1)	1.1 kg/cm2	303	465.0	19.9	23.4	[193]
6FDA-ODA	10	308	23	0.83	27.7	[193]
6FDA-4,4-ODA	6.8	303	22.0	0.94	23.4	[193]
6FDA-MDA	10	308	19	0.81	23.5	[193]
6FDA-4BDAF	6.8	303	19	0.98	19.4	[193]
6FDA-3,3′-ODA	6.8	308	2.1	0.10	21	[193]
6FDA-3BDAF	6.8	303	6.3	0.24	26.3	[193]
6FDA-IPDA	10	308–328	24.3–27.4	0.87–1.39	19.7–27.9	[193]
6FDA-DAF	10	308–328	19.5–21.3	0.81–1.15	18.5–24.1	[193]
PI-1	1	303	32	1.4	22.9	[193]
PI-3	1	303	360	16.5	21.8	[193]
PI-4	1	303	62	2.4	25.8	[193]
PI-5	1	303	190	7.3	26.0	[193]
6FDA-BAFL	1	298	98	3.3	29.7	[193]
Poly(phenylene oxide)
PPO (hollow fiber)	4	308	106		21	[205]
PPS	1.5	308	1.60	0.046	34.8	[193]
PDMPO	1.5	308	65.5	3.5	18.7	[193]
PDPPO	1.5	308	39.9	1.5	26.6	[193]
PDMPO	6.891	295	90.0	3.7	24.3	[193]
PDMPO (20.0% brominated)	6.891	295	93.6	3.8	24.6	[193]
PDMPO (37.4% brominated)	6.891	295	97.1	3.7	26.2	[193]
PDMPO (60.0% brominated)	6.891	295	159.9	8.0	20.0	[193]
Polypyrrole
6FDA-TAB	10	308	54.0	2.6	20.8	[193]
6FDA-TADPO	10	308	27.6	1.2	23.0	[193]
BBL	10	308	0.12	0.003	46.3	[193]
Polysulfones
PSF	10	308	5.6	0.25	22.4	[193]
TMPSF	10	308	21	1.06	19.8	[193]
HFPSF	10	308	12	0.67	17.9	[193]
TMHFPSF	10	308	72	4.0	18	[193]
PSF-F	10	308	4.5	0.20	22.5	[193]
PSF-O	10	308	4.3	0.20	21.5	[193]
PSF-P	10	308	6.8	0.32	21.3	[193]
TMPSF-F	10	308	5.5	0.61	9.0	[193]
TMPSF-P	10	308	13.2	0.57	23.2	[193]
BIPSF	10	308	5.6	0.24	23.3	[193]
TMBIPSF	10	308	31.8	1.21	26.3	[193]
1,5-NPSF	10	308	1.6	0.057	28.1	[193]
2,6-NPSF	10	308	1.5	0.051	29.4	[193]
2,7-NPSF	10	308	1.8	0.074	24.3	[193]
DMPSF	10	308	2.1	0.091	23.1	[193]
HMBIPSF	10	308	25.5	1.2	23.3	[193]
DMPSF-Z	10	308	1.4	0.057	24.6	[193]
PSF-AP	2	308	8.12	0.278	29.2	[193]
FBPSF	2	308	13.8	0.484	28.5	[193]
PSF-M	1	308	2.8	0.11	25.5	[193]
TMPSF-M	10	308	7.0	0.28	25.0	[193]
PSF-BPFL	1	308	10	0.25	40	[193]
3,4′-PSF	1	308	1.5	0.066	22.7	[193]
1,3-ADM PSF	35	308	7.2	0.33	21.8	[193]
2,2-ADM PSF	35	308	9.5	0.46	20.6	[193]
PSF (6% Br, 92% C≡CSiMe3)	1	308	36.5	2.1	17.4	[193]
PSF (3% Br, 47% C≡CSiMe3)	1	308	18.5	1.24	14.9	[193]
PSF (21% Br, 77% C≡CSiMe3)	1	308	28.2	1.7	16.6	[193]
PSF (5% Br, 45% C≡CSiMe3)	1	308	16.4	0.9	18.2	[193]
PSF	1	308	5.6	0.25	22.4	[193]
PSF-s-HBTMS	1	308	21	0.96	22.2	[193]
PSF-o-HBTMS	1	308	70	3.29	21.3	[193]
PSF-CH2-TMS	1	308	18	0.95	18.9	[193]
EM3	1	308	29	1.3	22	[193]
EM2	1	308	6.2	0.24	26	[193]
EM1	1	308	4.8	0.16	30	[193]
SM3 (degree of substitution = 2.0)	1	308	18	0.77	23	[193]
SM3 (degree of substitution = 1.0)	1	308	10	0.38	26	[193]
SM1	1	308	5.1	0.17	30	[193]
PPSF	1	308	3.2	0.10	32	[193]
RM3	1	308	27	1.9	14	[193]
RM2	1	308	6.7	0.60	11	[193]
RM1	1	308	6.9	0.61	11	[193]
HFPSF	1	308	12.0	0.67	17.9	[193]
HFPSF-o-HBTMS	1	308	105	5.63	18.6	[193]
HFPSF-s-TMS	1	308	41	2.0	20	[193]
HFPSF-o-TMS	1	308	84	4.7	18	[193]
HFPSF-TMS	1	308	110	6.3	18	[193]
TM6FPSF	1	308	72	4.0	18	[193]
TM6FPSF-s-TMS	1	308	96	5.2	19	[193]
TMPSF-TMS	1	308	32	1.51	21.3	[193]
TMPSF-s-TMS	1	308	66.3	3.07	21.6	[193]
TMPSF-HBTMS	1	308	72	3.36	21.4	[193]
Other membranes
HQDPA-PDA	7	303	0.598	0.016	37.4	[193]
HQDPA-PDA	7	373	1.70	0.111	15.3	[193]
HQDPA-DBA	7	303	0.683	0.015	45.5	[193]
HQDPA-DBA	7	373	2.10	0.125	16.8	[193]
HQDPA-MDBA	7	303	1.18	0.034	34.7	[193]
HQDPA-MDBA	7	373	2.37	0.160	14.8	[193]
HQDPA-EDBA	7	303	2.26	0.077	29.4	[193]
HQDPA-EDBA	7	373	4.18	0.292	14.3	[193]
12H	5	308	4.6	0.21	21.9	[193]
6H6F	5	308	8.6	0.44	19.5	[193]
6F6H	5	308	8.9	0.42	21.2	[193]
12F	5	308	12.9	0.76	17.0	[193]
PBK	10	308	3.3	0.13	25.4	[193]
PBK-S	10	308	3.27	0.11	29.7	[193]
PBSF	10	308	10.8	0.47	23.0	[193]
PES/PI	4	308	1.15 × 105	n.a.	30	[193]
PPES	n.a.	273	0.92	0.027	34	[193]
PPESK	n.a.	273	0.75	0.042	18	[193]
20 percent DEA immobilized in 25.4 μm microporous polypropylene supports	0.16–1.67	298	974–4825	n.a.	56–276	[200]
Copolymers and polymer blend
PEBA 2533 (hollow fiber)	6.8	273	260	n.a.	32	[206]
PEBA/PSF composite	3.4	273	6.1 × 105	n.a.	30	[206]
COPNA	n.a.	373	2990	n.a.	14	[200]
Pebax	n.a.	303	73	n.a.	15.6	[207]
Pebax/PEG10	n.a.	303	75	n.a.	15.8	[207]
Pebax/PEG20	n.a.	303	80	n.a.	15.9	[207]
Pebax/PEG30	n.a.	303	105	n.a.	15.1	[207]
Pebax/PEG40	n.a.	303	132	n.a.	15.1	[207]
Pebax/PEG50	n.a.	303	151	n.a.	15.5	[207]
Pebax/PEG-DME10	n.a.	303	123	n.a.	44	[208]
Pebax/PEG-DME20	n.a.	303	206	n.a.	45	[208]
Pebax/PEG-DME30	n.a.	303	300	n.a.	46	[208]
Pebax/PEG-DME40	n.a.	303	440	n.a.	42	[208]
Pebax/PEG-DME50	n.a.	303	606	n.a.	43	[208]
6FDA-TAB	10	308	54.0	2.8	19.3	[193]
6FDA/PMDA-TAB (50 : 50)	10	308	15.8	0.70	22.6	[193]
6FDA/PMDA-TAB (25 : 75)	10	308	3.13	0.098	31.9	[193]
6FDA/PMDA-TAB (10/90)	10	308	1.11	0.036	30.8	[193]
6FDA-TAB/DAM (75/25)	3	308	73.7	3.1	23.8	[193]
6FDA-TAB/DAM (50/50)	3	308	155	6.6	23.5	[193]
6FDA-DAM	3	308	370	29.5	12.5	[193]
6FDA/TMPDA	n.a.	308	400	23.5	17.02	[193]
6FDA/PMDA (1 : 6)-TMMDA (CH2Cl2 cast)	10	308	187	11.7	16.0	[193]
6FDA/PMDA (1 : 6)-TMMDA (NMP cast)	10	308	144	8.76	16.4	[193]
6FDA/PMDA (1 : 6)-TMMDA (DMF cast)	10	308	88.6	5.16	17.2	[193]
MDI-BPA/PEG (75)	2	308	31	0.70	44	[193]
MDI-BPA/PEG (80)	2	308	48	1.0	47	[193]
MDI-BPA/PEG (85)	2	308	59	1.20	49	[193]
L/TDI (20)-BPA/PEG (90)	2	308	47	0.92	51	[193]
L/TDI (40)-BPA/PEG (85)	2	308	35	0.72	48	[193]
IPA-ODA/PEO3 (80)	2	308	58	1.1	53	[193]
BPDA-pp′ODA	n.a.	303	18000	n.a.	31	[155]
BPDA-ODA/DAT (oxidized)	n.a.	308	599	n.a.	40	[155]
BPDA-ODA/DABA/PEO1 (75)	2	308	2.7	0.048	56	[193]
BPDA-mDDS/PEO1 (80)	2	308	3.8	0.066	58	[193]
BPDA-ODA/DABA/PEO2 (70)	2	308	14	0.25	57	[193]
BPDA-ODA/DABA/PEO2 (80)	2	308	36	0.64	56	[193]
BPDA-ODA/PEO3 (75)	2	308	75	1.4	52	[193]
BPDA-mDDS/PEO3 (75)	2	308	72	1.4	53	[193]
BPDA-mPD/PEO4 (80)	2	308	81	1.5	54	[193]
BPDA-ODA/PEO4 (80)	2	308	117	2.3	51	[193]
PMDA-ODA/DABA/PEO1 (80)	2	308	14	0.27	52	[193]
PMDA-ODA/PEO2 (75)	2	308	40	0.74	54	[193]
PMDA-mPD/PEO3 (80)	2	308	99	2.0	50	[193]
PMDA-APPS/PEO3 (80)	2	308	159	3.1	51	[193]
PMDA-APPS/PEO4 (70)	2	308	136	2.6	53	[193]
PMDA-mPD/PEO4 (80)	2	308	151	2.9	52	[193]
PMDA-ODA/PEO4 (80)	2	308	167	3.2	52	[152]
PMDA-pDDS/PEO4 (80)	2	308	238	4.9	49	[152]
PMDA/BTDA-BAFL (50 : 50)	1	298	43	1.3	33	[193]
PMDA/BTDA–BAFL (90 : 10)	1	298	130	3.8	34	[193]
BPDA-BAFL/HMDA (50 : 50)	1	298	0.54	0.014	39	[193]
PPES	n.a.	298	0.92	0.027	34	[193]
PPES/PPEK (3 : 1)	n.a.	298	2.94	0.074	40	[193]
PPES/PPEK (1 : 1)	n.a.	298	4.12	0.089	46	[193]
PPES/PPEK (1 : 3)	n.a.	298	2.06	0.026	39	[193]
PPES/PPEK (1 : 4)	n.a.	298	1.77	0.052	34	[193]
PPEK 18	n.a.	298	0.75	0.042	18	[193]
HQDPA-DPA/MDPA	7	303	0.957	0.023	41.2	[193]
HQDPA-DPA/MDPA	7	373	2.34	0.147	15.9	[193]
HQDPA-DPA/EDPA	7	303	1.334	0.036	37.6	[193]
HQDPA-DPA/EDPA	7	373	3.25	0.207	15.7	[193]
PI	10	308	2.00	0.063	31.7	[193]
PI/10PS	10	308	2.33	0.085	27.4	[193]
PI/15PS	10	308	2.32	0.09	25.8	[193]
PI/20PS	10	308	2.90	0.91	3.19	[193]
PI/25PS	10	308	4.29	0.91	4.71	[193]
PI/10PSVP	10	308	3.58	0.13	28.4	[193]
PI/15PSVP	10	308	3.71	0.14	26.5	[193]
PI/20PSVP	10	308	5.65	0.15	38.4	[193]
PI/25PSVP	10	308	6.55	1.55	4.31	[193]
NTDA-BDSA (30)/CARDO/ODA	3	303	70	1.7	41	[193]
NTDA-BDSA (30)/CARDO]	3	303	164	4.5	36	[193]
NTDA-BDSA (30)/BAPHF	3	303	23	0.64	36	[193]
NTDA-BDSA (30)/ODA	3	303	5.2	0.1	52	[193]
6FDA-FDA/HFBAPP (1/1)	1.1 kg/cm2	303	465	19.9	23.4	[193]
6FDA-durene/pPDA (80/20)	10	308	230	16.88	13.62	[193]
6FDA-durene/pPDA (50/50)	10	308	126	7.74	16.28	[193]
6FDA-durene/pPDA (20/80)	10	308	59.26	2.81	21.09	[193]
6FDA-durene/3,3′-DDS (75/25)	10	308	84.7	5.91	14.3	[193]
6FDA-durene/3,3′-DDS (50/50)	10	308	19.8	1.09	18.2	[193]
6FDA-durene/3,3′-DDS (25/75)	10	308	5.12	0.26	19.7	[193]
6FDA-3,3′-DDS	10	308	1.84	0.08	22.7	[193]
6FDA-6FpDA-DABA-12.5	4	308	34.0	2.01	16.9	[193]
6FDA-6FpDA–DABA-12.5 annealed	4	308	70.8	4.50	15.7	[193]
6FDA-6FpDA-DABA-12.5 (22.5% TMOS)	4	308	30.9	1.70	18.2	[193]
6FDA-6FpDA-DABA-12.5 (22.5% TMOS) annealed	4	308	47.6	3.16	15.1	[193]
6FDA-6FpDA-DABA-12.5 (15.0% MTMOS)	4	308	44.0	2.53	17.4	[193]
6FDA-6FpDA-DABA-12.5 (15.0% MTMOS) annealed	4	308	110	7.07	15.6	[193]
6FDA-6FpDA-DABA-12.5 (15.0% PTMOS) 4 35	4	308	32.3	1.80	17.9	[193]
6FDA-6FpDA-DABA-12.5 (15.0% PTMOS) annealed	4	308	91.8	5.59	16.4	[193]
6FDA-6FpDA-DABA-12.5 (22.5% PTMOS)	4	308	30.7	1.88	16.3	[193]
6FDA-6FpDA-DABA-12.5 (22.5% PTMOS) annealed	4	308	90.9	5.87	15.5	[193]
6FDA-6FpDA-DABA-25	4	308	20.3	1.20	16.9	[193]
6FDA-6FpDA-DABA-25 annealed	4	308	77.3	4.85	15.9	[193]
6FDA-6FpDA-DABA-25 (22.5% TMOS)	4	308	15.7	1.06	14.8	[193]
6FDA-6FpDA-DABA-25 (22.5% TMOS) annealed	4	308	79.8	4.87	16.4	[193]
6FDA-6FpDA-DABA-25 (15.0% MTMOS)	4	308	16.6	1.07	15.5	[193]
6FDA-6FpDA-DABA-25 (15.0% MTMOS) annealed	4	308	81.1	5.07	16.0	[193]
6FDA–6FpDA-DABA-25 (22.5% MTMOS)	4	308	16.6	1.07	15.5	[193]
6FDA-6FpDA-DABA-25 (22.5% MTMOS) annealed	4	308	60.1	3.837	15.7	[193]
6FDA-6FpDA-DABA-25 (15.0% PTMOS)	4	308	18.4	0.94	19.6	[193]
6FDA-6FpDA-DABA-25 (15.0% PTMOS) annealed	4	308	104	6.25	16.6	[193]
6FDA-6FpDA-DABA-25 (22.5% PTMOS)	4	308	19.1	0.98	19.5	[193]
6FDA-6FpDA-DABA-25 (22.5% PTMOS) annealed	4	308	104	6.25	16.6	[193]
Poly(5 : 5 BPA/BN)	5	308	5.71	0.19	30.1	[193]
Poly(7 : 3 BPA/BN)	5	308	4.62	0.16	28.9	[193]
Cross-linking polymers
Poly(ethylene oxide-co-epichlorohydrin) (1 : 1) 1.1%	300	298	15.0	2.3	6.52	[193]
Poly(ethylene oxide-co-epichlorohydrin) (1 : 1) 2%	300	298	14.9	1.0	14.9	[193]
Poly(ethylene oxide-co-epichlorohydrin) (1 : 1) 5%	300	298	16.1	0.5	32.2	[193]
DM14/MM9 (100/0)	0.967	298	45	0.66	68	[193]
DM14/MM9 (100/0)	0.967	323	107	2.8	38	[193]
DM14/MM9 (90/10)	0.967	298	62	0.90	69	[193]
DM14/MM9 (90/10)	0.967	323	133	3.4	39	[193]
DM14/MM9 (70/30)	0.967	298	96	1.5	66	[193]
DM14/MM9 (70/30)	0.967	323	195	5.4	36	[193]
DM14/MM9 (50/50)	0.967	298	144	2.25	64	[193]
DM14/MM9 (50/50)	0.967	323	260	7.2	36	[193]
DM14/MM9 (30/70)	0.967	298	210	3.3	63	[193]
DM14/MM9 (30/70)	0.967	323	350	10.6	33	[193]
DB30/MM9 (100/0)	0.967	298	93	1.5	63	[193]
DB30/MM9 (100/0)	0.967	323	200	5.7	35	[193]
DB30/MM9 (90/10)	0.967	298	105	1.6	64	[193]
DB30/MM9 (90/10)	0.967	323	210	5.8	36	[193]
DB30/MM9 (70/30)	0.967	298	141	2.1	67	[193]
DB30/MM9 (70/30)	0.967	323	270	7.7	35	[193]
DB30/MM9 (50/50)	0.967	298	179	2.9	62	[193]
DB30/MM9 (50/50)	0.967	323	330	9.7	34	[193]
DB30/MM9 (30/70)	0.967	298	250	4.2	60	[193]
DB30/MM9 (30/70)	0.967	323	410	12.4	33	[193]
DM9/MM9 (90/10)	0.967	298	18.3	0.3	68	[193]
DM9/MM9 (90/10)	0.967	323	51	1.3	38	[193]
DM23/MM9 (90/10)	0.967	298	145	2.2	66	[193]
DM23/MM9 (90/10)	0.967	323	290	7.6	38	[193]
DB10/MM9 (90/10)	0.967	298	6.7	0.11	61	[193]
DB10/MM9 (90/10)	0.967	323	27	0.79	34	[193]
DB69/MM9 (90/10) (cooling)	0.967	298	240	4.3	56	[193]
DB69/MM9 (90/10) (cooling)	0.967	323	510	14.2	36	[193]
DB69/MM9 (90/10) (heating)	0.967	298	98	1.6	62	[193]
DB69/MM9 (90/10) (heating)	0.967	323	400	11.4	35	[193]
DM14/MM23 (30/70) (cooling)	0.967	298	240	3.9	62	[193]
DM14/MM23 (30/70) (cooling)	0.967	323	420	12	35	[193]
DM14/MM23 (30/70) (heating)	0.967	298	250	4.0	62	[193]
Matrimid 5218	10	308	6.5	0.25	25.6	[193]
Matrimid 5218, 1-day cross-linking	10	308	7.4	0.29	25.6	[193]
Matrimid 5218, 3-day cross-linking	10	308	6.0	0.24	25.2	[193]
Matrimid 5218, 7-day cross-linking	10	308	5.1	0.21	24.6	[193]
Matrimid 5218, 14-day cross-linking	10	308	4.7	0.19	24.1	[193]
Matrimid 5218, 21-day cross-linking	10	308	3.4	0.15	22.2	[193]
Matrimid 5218, 32-day cross-linking	10	308	1.9	0.13	15.0	[193]
6FDA-durene, 5 min cross-linked	10	308	136	11.1	12.3	[193]
6FDA-durene, 10 min cross-linked	10	308	91.8	6.53	14.1	[193]
6FDA-durene, 15 min cross-linked	10	308	70.0	6.05	11.6	[193]
6FDA-durene, 30 min cross-linked	10	308	30.3	2.87	10.6	[193]
6FDA-durene, 60 min cross-linked	10	308	2.14	0.40	5.35	[193]

 *Permeability.

Various groups of materials have been already proposed and experimentally investigated for post-combustion CO₂ capture with membrane process. By modifying membrane their properties can be improved. For example, when amine functional groups are randomly dispersed in the silica matrix, this membrane can separate CO₂ with high selectivity. On the other hand, membrane structure can be modified by adding arginine salts [194–196].

2.4.1. Inorganic Membranes

Based on structure, inorganic membranes can be classified into two categories: porous and dense. In porous inorganic membranes, a porous thin top layer is supported on a porous metal or ceramic support. Zeolite, silicon carbide, carbon, glass, zirconia, titania, and alumina membranes are mainly used as porous inorganic membranes supported on different substrates, such as α-alumina, γ-alumina, zirconia, zeolite, or porous stainless steel [17, 199, 209, 210].

Zeolite membrane is the most important group of inorganic membranes. Zeolite membranes are considered more expensive than polymeric membranes, and therefore their unique properties of size selectivity and thermal and chemical stability should be exploited for successful application [211–213].

The dense inorganic membranes (nonporous material) consist of a thin layer of metal, such as palladium and its alloys (metallic membrane), or solid electrolytes, such as zirconia. Another form of inorganic membrane is the liquid-immobilized membrane, where the pores of a membrane are completely filled with a liquid, which is permselective for certain compounds. Recently, attempts have been made to develop dense molten carbonate selective membranes for CO₂ separation at high temperatures (>723 K). The inorganic membranes have high thermal stability for CO₂ separation, but their selectivity and permeability are very low [200, 214, 215].

2.4.2. Polymeric Membranes

In polymeric membranes, the selective layer is generally a nonporous film that transports gases across by the solution-diffusion mechanism. Polyacetylenes, polyaniline, polyarylene ethers, polyarylates, polycarbonates, polyetherimides, polyethylene oxide, polyimides, polyphenylene oxides, polypyrroles, polysulfones, and amino groups such as polyethyleneimine blends, polymethacrylates are examples of polymeric membranes used for CO₂ separation [17, 216–218].

Selective polymeric membranes can be divided into two basic categories: glassy and rubbery. Almost glassy polymeric membranes are more suitable than rubbery polymeric membranes for CO₂ separation because of their high gas selectivity and good mechanical properties. On the other hand, rubbery membranes are flexible and soft and they have a high permeability but a low selectivity, whereas glassy polymers exhibit a low permeability but a high selectivity [206, 219–221].

Several advantages of polymeric membranes are (i) low cost of production; (ii) high performance separation; (iii) ease of synthesis; and (iv) mechanical stability. Although the polymeric membranes have high selectivity and permeability for CO₂ separation, but their thermal stability is very low, and these membrane may be plasticized with influence of CO₂ in membrane. Therefore, application of these membranes for post-combustion capture is limited, and flue gas must first be cooled down to 313–333 K for membrane process [184, 222, 223].

Ren et al. [205] prepared polymeric membranes with block copolymers; the balance of the hard and soft blocks can provide a good CO₂ separation performance without loss of its permeability.

Improved polymeric membrane materials with superior separation performance can be obtained by synthesizing new polymers or modification or blending existing commercial polymers with organic or inorganic compounds [208, 224].

Due to high operating cost of membrane processes, it is necessary to perform more researches and studies about preparation of suitable membranes.

2.4.3. Mixed Matrix Membranes

Zeolites, carbon molecular sieves (CMS), and many polymeric materials offer attractive transport properties for CO₂ separation. By mixing membrane material, excellent membrane with high performance for CO₂ separation (selectivities of CO₂/N₂ = 17.8–39.6) can be prepared [200]. A group of scientists proposed the use of membrane based on polymer/immobilized liquid system especially polymerized ionic liquid membrane (PILM) or gelled ionic liquid membrane. ILMs consisting of aqueous solutions of 20% DEA immobilized in 25.4 μm microporous polypropylene supports have low permeability and suitable selectivity (974 barrer, 276, resp.) in 2 atm at 298 K [225–228].

2.4.4. Hollow Fiber Membrane

Most industrially important membranes for gas separations are hollow fiber ones. Asymmetric hollow fiber membranes (such as polyvinylidene difluoride (PVDF)) with inner skinless structures are favourable for CO₂ separation and absorption in gas-liquid membrane by low mass-transfer resistance and high permeability. In addition, this process can achieve significantly high adsorption efficiencies due to the much larger surface area for gas-liquid interface than conventional gas absorption processes [206, 229–232].

According to data in Table 6, inorganic membranes have high permeability (about 150000 barrer) and low selectivity (about 15). Of course, some of inorganic membranes such as Y (FAU) with α-A1₂O₃ support and chitosan group are highly selective for CO₂/N₂ separation (selectivity (α) ≈ 100–800). Among polymeric membranes, polyamines have high permeability and selectivity (10⁶ (barrer) and 980, resp.), and the second FSCM membranes have high permeability and fine selectivity (10⁵ (barrer), 230, resp.). Other polymeric membrane groups are not selective for CO₂/N₂ separation, and maximum selectivity of these membranes is about 30.

2.5. Novel CO₂ Capture Technologies

These methods include electrochemical pumps and chemical looping approaches to CO₂ separation. The molten carbonate and aqueous alkaline fuel cells have been studied for use in separating CO₂ from both air and flue gases. Electrochemical pumps discussed include carbonate and proton conductors. Molten carbonate is nearly 100% selective for CO₂ separation, but major disadvantage in the application of molten carbonate electrochemical cells for CO₂ separation is that this process is not repeatedly. Other disadvantages of these technologies are: corrosion, difficult operating condition (T = 873 K), and sensitivity to the presence of  SO_x [45, 233].

In chemical looping combustion, the oxygen for combustion of the fuel is provided by a regenerable metal oxide catalyst. The chemical looping scheme can be presented in the general form [45]:

$$\begin{array}{c}\text{HC}+\text{metal}\mathrm{\hspace{0.17em}\hspace{0.17em}}\text{oxide}\\ \hspace{1em}⟶{\text{CO}}_{\mathrm{2}}+{\text{H}}_{\mathrm{2}}\text{O}+\text{lower}\mathrm{\hspace{0.17em}\hspace{0.17em}}\text{oxide}\mathrm{\hspace{0.17em}\hspace{0.17em}}(\text{and}/\text{or}\mathrm{\hspace{0.17em}\hspace{0.17em}}\text{metal})\end{array}$$ (2)

$$\begin{array}{c}\text{lower}\mathrm{\hspace{0.17em}\hspace{0.17em}}\text{oxide}\left(\text{and}/\text{or}\mathrm{\hspace{0.17em}\hspace{0.17em}}\text{metal}\right)+{\text{O}}_{\mathrm{2}}⟶\text{metal}\mathrm{\hspace{0.17em}\hspace{0.17em}}\text{oxide}\end{array}$$ (3)

Nickel oxide is one main candidate for the chemical looping combustion of methane, as low as 673 K, because it is extensive and effective for the chemical looping combustion [45].

2.6. Discussion

Various technologies such as absorption, adsorption, cryogenic distillation, and membrane have been suggested for CO₂ separation from flue gases (Table 7). In this paper, various technologies for different feed conditions were investigated. Absorption is an important technology for CO₂ separation. Although physical solvents required low energy for regeneration, they have low absorption capacity and selectivity for CO₂ separation. Selexol is the best physical solvent and suitable for sweetening natural gas. However, physical absorption is not economical for flue gas streams with CO₂ concentration lower than 15 vol% (95 US$/ton CO₂ [234]).

Table 7.

Comparison between various technologies used for CO₂ capture.

Technology	Advantages	Disadvantages	Scale
Absorption	(i) React rapidly (ii) High absorption capacities (iii) Very flexible	(i) Equipment corrosion (ii) High energy required for regenerating solvent	Industrial
Adsorption	(i) Low energy consumption and cost of CO2 capture (ii) Suitable for separating CO2 from dilute stream	Low adsorption capacities (in flue gases conditions)	Pilot
Cryogenic distillation	(i) Liquid CO2 production (ii) Not requiring solvents or other components (iii) Easy scaled-up to industrial-scale application	Require large amount of energy	Pilot
Membrane separation	(i) Clean and simple process (ii) Continuous, steady-state technology	Require high energy for post-combustion CO2 capture	Experimental

Chemical solvents are classified in main groups such as alkanolamines, ammonia, aqueous piperazine (PZ), and amino acids. Chemical absorbents such as monoethanolamine (MEA) have high absorption capacities and are very flexible for CO₂ separation; therefore, these solvents are usually preferred to physical solvents. Chemical absorption with alkanolamines is the only technology that is used in an industrial scale for post-combustion capture. Amines react rapidly, selectively, and reversibly with CO₂ and are relatively nonvolatile and inexpensive solvents. In this process, the corrosion is the main problem; therefore, in recent studies, new amines and various mixtures of them were proposed and compared with previous ones to find suitable solvents. Suitable solvents for CO₂ separation must have high CO₂ absorption capacity, less corrosion, less viscosity and less regeneration energy. These studies showed that CASTOR 1 and 2, which are blended amine solvents (MEA/MDEA), are the best chemical adsorbents so far proposed for post-combustion CO₂ capture. Experimental results indicated that amine amino acid salts (AAAS) have better performance than MEA of the same concentration for CO₂ absorption, but do not deteriorate in the presence of oxygen. However, absorption has several disadvantages such as it requires high energy to regenerate solvents (3.0 GJ/ton CO₂ for absorption with 40%wt MEA in 210 kPa [235]), therefore need more efforts in the future to reduce energy consumption in post-combustion CO₂ capture with chemical absorption.

Adsorption is the one effective technology that can reduce energy and cost of the capture or separation of CO₂ in post-combustion capture. Adsorption is suitable for separating CO₂ from dilute and low flow rate stream, but flue gases conditions are the main problem against industrialization adsorption process. The CaO-MgAl₂O₄ and nano CaO/Al₂O₃ are the best chemical adsorbents. Although, the chemical adsorbents have high capacity and selectivity, but their regeneration is difficult. Physical adsorption is the most suitable for CO₂ capture at high pressures and low temperatures. At higher pressure (above 4 bar) activated carbons are more efficient than zeolites. The energy and cost of adsorption for activated carbons are nearly half of that of zeolites. On the other hand, zeolites (particularly 13X and 5A) have high selectivity for CO₂ separation. Generally, zeolite 5A may have better adsorption efficiency at co-adsorption of SO₂, NO and CO₂ than zeolite 13X.

In order to achieve more selective CO₂ separation from flue gases, the modified adsorbent surface was considered. New adsorbents such as honeyncomb monolith, MOFs, CHAs (NaCHA and CaCHA), PMO (MCM and SBA) and MSPs (Na₂SiO₄) are suitable adsorbents for selective CO₂ separation but they require more researches and studies. However, the development of suitable adsorbents with high CO₂ adsorption capacity, which can be replaced absorption with chemical adsorbent, is still demanded.

Cryogenic distillation separation can be used for CO₂ separation but its major disadvantage is the large amount of energy required to provide the refrigeration. Many new processes have been proposed for using cryogenic, but generally this technology is not suitable for post-combustion capture and is appropriate for oxy-fuel combustion method and CO₂ separation from exhaust of cement industry (stream with high CO₂ concentration).

The membrane separation method is a continuous, steady-state, clean and simple process for CO₂ recovery. Since the pressure drop is driving force for membrane process, the flue gas stream must compress. Since compressing flue gas is very difficult and expensive, membrane separation is suitable for high pressure stream with high concentration (>10 vol%). Inorganic membrane have high thermal and chemical stability but their selectivity is lower than polymeric membranes. Although Y (FAU) with α-Al₂O₃ support and arginine salt chitosan are the best inorganic membrane, zeolite mambranes are suitable ones for CO₂ separation. Polymeric membranes are very selective for CO₂ separation but they have low thermal stability. Therefore, polymeric membranes are suitable for application in pre-combustion processes. Glassy polymeric membranes have higher selectivity, while the rubbery polymeric membranes have higher thermal stability. Perfect membrane with high performance for CO₂ separation (selectivities of CO₂/N₂ = 17.8–39.6) can be prepared by mixing various membranes.

Because of operating problems and high cost of compressing, membrane separation is not suitable for post-combustion capture, but membrane technology is suitable for producing oxygen-enriched streams from air, in oxy-fuel combustion systems.

Electrochemical pumps and chemical looping are two new technologies suggested for CO₂ capture. Now these technologies are not effective in comparison with other technologies. Therefore, application of electrochemical pumps and chemical looping in CCS needs more research.

3. Conclusion

Because of economical and environmental incentives, researchers have mainly focused on CO₂ separation from different process streams, especially from the flue gases. In recent years, post-combustion capture has been the topic of many researches, because it is more flexible and can be easily added to the fossil fuel power plants.

Based on above findings, it can be concluded that flue gases properties (mainly concentration of CO₂, temperature and pressure) are the most effective factors for selection of suitable process for CO₂ separation.

Since flue gases have high temperature (about 373 K), low pressure, and low CO₂ concentration (1 atm and 10–15% moL), bulk absorption and adsorption processes may be the best suitable process for CO₂ separation from these streams. Due to simplicity of absorption process, this process has been applied in industrial plants, although many researches have been focused on preparation of adsorbents with high selectivity and capacity, in recent years. For industrial application, more studies about adsorbents are necessary. Cryogenic distillation and membrane processes are efficient for gas streams with high CO₂ concentration. Therefore, these process are economically efficient for pre-combustion capture. In recent years, different studies have been performed to optimize cryogenic cycles and preparation of suitable membrane for CO₂ separation from post-combustion flue gases.

By the result of this study, future research direction on the scale-up and industrialization of adsorption (with modified adsorbent), and membrane process for CO₂ separation is suggested. Therefore more studies must be focused on modeling and simulation of these processes (membrane and adsorption), although research for finding new adsorbent, suitable mambrane (with mixed or modified present membrane) and blending amine solvents can reduce CCS cost.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.