Gas Membranes for CO 2 /CH 4 (Biogas) Separation: A Review

Abstract

Gas separation by membrane carries many advantages over other methods of gas separation, including the amenability of the gas separation process to simplification and scaling up, and the elimination of the need for phase change of the gas. Material composition is, by far, the most important factor in determining the gas separation effectiveness of a membrane. This article is a review of the existing research on polymers as used in the separation of carbon dioxide/methane (CO₂/CH₄), generally focusing on polyimide and polysulfone compounds. Data on thermally rearranged (TR) polymer membranes and mixed matrix membranes were also included. In this study, the membrane performance of different polymeric materials was examined by comparing the data on CO₂/CH₄ selectivity obtained through a literature review and the upper bound proposed by Robeson in 2008. Results showed that polyimide and polysulfone membranes are highly applicable to the separation of CO₂/CH₄ gas mixtures: polyimide showed particular promise for commercial implementation. Polysulfone membranes exhibited high selectivity but low CO₂ permeability that failed to exceed the upper bound. However, case studies suggested that polysulfones in the hollow fiber membrane form have a wide variety of applications in an industrial setting. On the other hand, the TR polymer membranes, which are currently used for research, had a CO₂ permeability of 5–5,903 Barrer and CO₂/CH₄ selectivity of 3.8–124, which exceeded the upper bound proposed by Robeson. We, thus, came to the conclusion that future research on CO₂/CH₄ gas separation should focus on polysulfone, polyimide, and TR membranes as materials for economically efficient and highly effective gas separation membranes.

Introduction

The discovery of asymmetric membranes in the mid-1960s sparked an enormous interest in membrane technology, which has evolved from early reverse osmosis techniques to modern gas separation technology, and found applications in wide industrial fields (Paul and Yampolskii, 1994). In particular, the application of membrane technology to gas separation has attracted much attention in recent years, as the use of a membrane allows an isolation of the target gas without phase change and the membrane processes for gas separation are highly amenable to scaling up. In recent years, glassy and rubbery polymers have exhibited over one thousand gas permeation properties, the characteristics of each membrane varying according to the molecular structures particular to the polymers used. It follows, therefore, that quantitative effect analysis, based on existing gas permeation data, can be used to set material selection criteria for the development of high-quality gas separation membranes. Such analysis also promises to be useful in the engineering and selection of industrially useful polymers for membranes designed to isolate particular gaseous compounds.

Gas separation by membrane

There are two main mechanisms of gas separation by membrane; the first is the separation by a membrane riddled with micropores (porous membrane), while the second mechanism occurs with a membrane that has no pores (nonporous membrane). They are well explained by the pore flow model and the solution–diffusion model, respectively. The mechanism of separation by the nonporous membrane—the most commonly used membrane type for gas separation—can be explained by the solution–diffusion model. The solution–diffusion model explains that physical properties of a membrane, such as its rigidity, degree of cross linkage, and attractive forces between molecules of the constituent polymers, affect its gas permeability such that these properties are the primary factors that determine the rate at which a given gas molecule permeates the membrane. Below is shown the Equation (1) expressing the permeability of membranes using the solution–diffusion mechanism (Paul and Yampolskii, 1994; Yampolskii et al., 2006). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} P = DS \tag{1}\end{align*} \end{document}

Here, P is the permeability coefficient of the gas, expressed in units of Barrer, 1 Barrer being defined as 1×10⁻¹⁰ [cm³ (STP)·cm/cm²·s·cmHg]. The permeability coefficient P for a given gas is a product of the solubility coefficient (S) and diffusion coefficient (D) of the target gas through the membrane, as shown in Equation (1). Each membrane's permeability coefficient varies according to the polymer composition of the membrane used (Yampolskii et al., 2006). The characteristic of a gas separation membrane is determined by its selectivity, with ideal selectivity expressed as the below Equation (2) (Wijmans and Baker, 1995): \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \alpha_ {AB} = \frac {P_A} {P_B} , \tag {2} \end{align*} \end{document}

where P_A and P_B are the permeability coefficients of gas A and gas B, respectively. The gas A of P_A in the numerator is generally with the superior permeability coefficient and, thus, α_AB is a value greater than 1. By substituting Equation (1) into Equation (2), α_AB can be expressed as a product of diffusion selectivity (α_D) and solubility selectivity (α_S) as follows: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \alpha_ {AB} = \frac {P_A} {P_B} = \frac {D_A S_A} {D_B S_B} = \alpha_D \alpha_S , \tag {3} \end{align*} \end{document}

where D and S stand for diffusion coefficient and solubility coefficient, respectively. An independent analysis of α_D and α_S is useful in the assessment of the gas separation characteristics of a particular membrane. These characteristics determine which polymers are suitable for membranes to separate mixed gases. Research actively continues on the application of such gas membrane modules to industrial manufacturing processes.

Polymer membranes (materials)

Nonporous membranes are most commonly used in gas separation and they are usually composed of polymers that are generally classified as either glassy or rubbery. Glassy polymers show a low permeability, but tend to maximize differences in diffusivity rates between different gases, according to the molecular size and, thereby, generally exhibit high specificity. In contrast, rubbery polymers achieve superior permeability and diffusivity owing to the flexibility of their molecular chains, while selectivity is generally low (Won et al., 1999). The selectivity of rubbery polymers is very sensitive to the solubility selectivity, and rubbery polymers are, therefore, used in membranes designed to separate vapor/gas mixtures and isolate carbon dioxide (CO₂). Glassy polymer, on the other hand, is a diffusion-selective polymer because of its low solubility. Therefore, glassy polymers are often applied to the separation of O₂/N₂, H₂/N₂, and H₂/methane (CH₄) (Mulder, 1996). Glassy polymers are commonly applied to gas separation, including polyimides (PI), polysulfones (PSF), and polycarbonates (PC). PI and PSF, in particular, exhibit superior mechanical strength, heat resistance, and membrane-forming qualities and, therefore, find many applications in gas separation. Most polymers are formed in flat sheets in themselves, but they are usually manufactured as hollow fibers and tubes.

CO₂/CH₄ (Biogas)

Biogas is generated by the anaerobic digestion of organic matter in sewage plants, landfills, industrial wastes, and agricultural production (Scholz et al., 2013). Biogas from anaerobic digestion is a gaseous mixture of 55–70% CH₄, 30–45% CO₂, and small amounts of other gases, including hydrogen sulfide (H₂S) and ammonia (NH₃). Of these, CH₄ is a valuable source of energy and has been utilized in many ways over the years. Separation and purification techniques are a prerequisite to use CH₄ as a fuel and CO₂ in many technological applications and they are hot research topics. Upgrading the raw biogas, which is often referred to as biomethane, and supplying this gas to the natural gas grid seem an attractive alternative to its utilization in combined heat and power engine (Scholz et al., 2013). Absorption or adsorption still forms the mainstay of CO₂ recovery, but economical inefficiency incurred by phase conversion and chemical treatment has provoked interest in gas separation membranes as an alternate means of recovery, for which active research is underway and application measures have been partially implemented. This article reviews the current body of research on the application of polymer membranes to the gas separation of CO₂ and CH₄ from biogas.

Upper bound (1991, 2008)

The upper bound, proposed by Robeson (1991 and 2008), is a log-log graph expression of the relationship between separation factor and permeability and a determination of an upper limit value derived from experimental data. Generally, permeability and selectivity values for mixed gases comprised two main discrete gases plotted on a graph, which is called Robeson's upper bound. The main parameters of gas separation are the composition of the gaseous mixture and the permeability of a particular gas to be recovered. Robeson's upper limit is an expression of the limitations of gas separation membranes and requires a trade-off between permeability and selectivity (Moghadam et al., 2011).

The upper bound correlation follows the relationship in the Equation (4) below: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} P_i = k \alpha_{ij}^n \tag{4}\end{align*} \end{document}

here, P_i is the permeability of the fast gas, α_ij (P_i/P_j) is the separation factor, k is referred to as the front factor, and n is the slope of the log–log plot of the noted relationship. Below this line, on a plot of log α_ij versus log P_i, virtually all the experimental data points exist (Robeson, 2008). The limit of the upper bound relationship is determined by the diffusivity coefficient, while the upper bound correlation can be used to qualitatively determine where the permeability process changes from solution–diffusion to Knudsen diffusion (Zimmerman et al., 1997; Husain and Koros, 2007; Robeson, 2008).

Robeson's upper bound was first proposed in 1991 and revisited in 2008. The data used in calculating the upper bound in 1991 and 2008 have been organized in Tables 1 and 2, respectively. The data used in 1991 showed a CO₂ permeability ( \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $$P_{CO_{2}}$$ \end{document} ) of 0.5–18,000 Barrer and a CO₂/CH₄ selectivity of 4.3–140 (Table 1). However, the 2008 data, for the same compounds, showed a CO₂ permeability ( \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} $$P_{CO_{2}}$$ \end{document} ) of 0.029–29,000 Barrer and a CO₂/CH₄ selectivity of 4.42–2,200 (Table 2), a significant increase in the upper bound compared to that proposed in 1991. Robeson's upper bound was and continues to be a useful standard in the research of CO₂/CH₄ separation by the membrane.

Table 1.

Experimental Data Close to the 1991 Upper Bound for CO₂/CH₄ Separation^a

Polymer	P(CO₂) (Barrer)	P(CH₄) (Barrer)	α(CO₂/CH₄)	Ref.
Poly(trimethylsilylpropyne)	18,000	4,186	4.3
Poly(tert-butylacetylene)	1,360	160	8.5	Takada et al. (1985)
Polyimide(6-FDA-ODA) (10 atm)	23.0	0.381	60.3	Kim et al. (1988b)
Polyimide(6-FDA-DAF) (10 atm)	32.2	0.631	51.0	Kim et al. (1988a)
Poly(methyl methacrylate)	0.65	0.005	130	Hoover et al. (1987)
Poly(methyl methacrylate)	0.50	0.003	140	Chiou and Paul (1988)
Poly(tetramethyl bis L sulfone)	65	1.728	37.6	Pilato et al. (1975)

Source: Robeson (1991).

CO₂, carbon dioxide; CH₄, methane.

Table 2.

Experimental Data Close to the 2008 Upper Bound for CO₂/CH₄ Separation ^a

Polymer	P(CO₂) (Barrer)	P(CH₄) (Barrer)	α(CO₂/CH₄)	Ref.
PVSH doped polyaniline	0.029	0.000013	2,200	Hachisuka et al. (1995)
Polypyrrole 6-FDA/PMDA (25/75)-TAB	3.13	0.022	140	Zimmerman and Koros (1999)
Polyimide TADATO/DSDA (1/1)-DDBT	45	0.75	60	Yang et al. (2001)
Poly(diphenyl acetylene)3a	110	2.30	47.8
Poly(diphenyl acetylene)3f	330	12	27.5	Shida et al. (2006)
Poly(diphenyl acetylene)3e	290	9.20	31.5
Polyimide 6-FDA-TMODA/DAT (1/1)	130.2	3.34	38.9
Polyimide 6-FDA-TMODA/DAT (3/1)	187.6	5.53	33.9	Wang et al. (2007)
Polyimide 6-FDA-TMPDA	555.7	24.48	22.7
Polyimide PI-5	190	5.60	33.9	Al-Masri et al. (1999)
Polyimide 6-FDA-durene	677.8	33.58	20.18	Lin and Chung (2001)
6-FDA-based Polyimide (8)	958	39.91	24	Nagel et al. (2002)
PIM-7	1,100	62.14	17.7
PIM-1	2,300	125	18.4	Budd et al. (2005)
PTMSP	19,000	4298.64	4.42
PTMSP	29,000	6,502.24	4.46	Mizumoto et al. (1993)

Source: Robeson (2008).

PI, polyimides.

Polymer Membranes for Separation of CO₂/CH₄

Despite the recent proliferation of newly researched and developed polymers, the number of polymers that have widespread industrial use is limited. Most industrial fields require membranes with the properties of mechanical strength and superior gas selectivity provided by glassy polymers (Wang et al., 2005). This section is a review of the literature on polymer membranes applied to the separation of CO₂ and CH₄ gases and documented cases where polymer membranes were applied to the biogas separation process. Through this review, we aimed at assessing the applicability and suitability of polymeric membranes to the separation of the CO₂/CH₄ mixtures, commonly known as biogas.

Polyimide

Polyimide, with properties of high mechanical strength and heat resistance, was developed by Sroog of DuPont in the late 1950s and has, ever since, been a subject of extensive research and application in fields ranging from aeronautics to electronics. Polyimide exhibits a low permeability coefficient and high selectivity as well as heat stability and the ability to form very thin membranes, which have attracted the attention of researchers from diverse fields (Wang et al., 2005). Polyimide has many applications, including but not limited to the separation of oxygen and nitrogen from air and the purification of natural gas (Yampolskii et al., 2006). Much research has been carried out to enhance the permeability and selectivity of polyimide membranes with a particular focus on altering polyimide structure for enhanced diffusivity (Hirayama et al., 1996; Wang et al., 2005). The study by Wang et al. (2005) especially demonstrated that the performance of membrane gas separation could be improved by varying the polymer structure. Currently, polyimide membranes have been applied to CO₂ separation. Such application, however, requires stepwise and multistage design (Dinello et al., 1989; Rautenbach and Welsch, 1994). The high cost of PI also limits their use in wide applications (Kapantaidakis et al., 1996). Nevertheless, their superior physical properties and high selectivity draw the attention of researchers seeking novel applications of this material.

Data on CO₂ permeability and selectivity of CO₂/CH₄ separation by polyimide membranes are charted in Table 3. Wang et al. (2007) conducted research on the systematic transformation of the diamine ratio of 6-FDA-TMPDA/DAT copolyimides. All copolyimides showed a tendency to dissolve entirely or partially in solvents, and their permeability and the diffusivity coefficient were seen to decrease while the selectivity of CO₂/CH₄ was seen to increase with increased DAT content. The permeability and selectivity data published in this study exceeded the upper bound.

Table 3.

Experimental Data on CO₂ Permeability and Selectivity of CO₂/CH₄ Separation of Polyimide Membranes as Reported in Literature

Membrane	Pressure (atm)	Temp. (°C)	P(CO₂) (Barrer)	P(CH₄) (Barrer)	α(CO₂/CH₄)	Ref.
6-FDA-DMB	6.8–20	22	219	6.7	33	White et al. (1995)
BDCDA-MPD			3.7	0.10	37
BDCDA-DDSO			4.2	0.12	35
HDCDA-6F	3	30	4.1	0.10	41	Ayala et al. (2003)
PDCDA-6F			5.0	0.16	31.3
BDCDA-6F			15.6	0.40	39
6-FDA-Durene (2/2)			612	45.0	14
6-FDA-Durene+plntA (2.0/1.6/0.4)	10	35	317	15.0	21	Xiao et al. (2007)
6-FDA-Durene+plntA (2.0/1.2/0.8)			181	6.8	27
6-FDA-NDA			22.6	0.47	48.1
6-FDA-NDA/Durene (75/25)			70.0	1.65	42.4
6-FDA-NDA/Durene (50/50)	10	35	96.4	3.93	24.5	Chan et al. (2003)
6-FDA-NDA/Durene (25/75)			274.0	12.90	21.2
6-FDA-Durene			423.0	28.00	15.1
6-FDA-mPD			11.03	0.19	58.0
6-FDA-mPD/DABA 9/1			6.53	0.10	65.3
6-FDA-mPD/DABA 9:1 (crosslinked)	10	35	9.50	0.15	63.3	Staudt-Bickel and Koros (1999)
6-FDA-DABA (crosslinked)			10.40	0.12	87.0
ODPA/TAP/ODA (1/1/0) NH₂-HBPI			0.096	0.002	48
ODPA/TAP/ODA (1/0.95/0.05) NH₂-HBPI			0.156	0.003	39
ODPA/TAP/ODA (1/0.75/0.25) NH₂-HBPI			0.246	0.004	41
ODPA/TAP/ODA (1/0.5/0.5) NH₂-HBPI			0.352	0.007	26
ODPA/TAP/ODA (1/0.25/0.75) NH₂-HBPI			0.451	0.006	45
ODPA/TAP/ODA (1/0/1) Linear PI	1.5	30	0.595	0.010	35	Peter et al. (2009)
ODPA/TAP/ODA (2/0.95/0.05) ANH-HBPI			0.140	0.003	50
ODPA/TAP/ODA (2/0.75/0.25) ANH-HBPI			0.135	0.004	27
ODPA/TAP/ODA (2/0.5/0.5) ANH-HBPI			0.257	0.004	37
ODPA/TAP/ODA (2/0.25/0.75) ANH-HBPI			0.285	0.006	36
6-FDA-TMPDA			555.72	24.49	22.70
6-FDA-MPDA/DAT (3/1)			187.63	5.54	33.86
6-FDA-MPDA/DAT (1/1)	1.97	35	130.21	3.35	38.85	Wang et al. (2007)
6-FDA-MPDA/DAT (1/3)			74.69	1.56	47.86
6-FDA-DAT			39.59	0.69	57.41
6-FDA-Durene (original)			612.0	45.1	13.6
CHBA: 30 min			55.2	2.86	19.3
CHBA: 30 min Annealed: 100°C	—	35	37.2	1.87	19.9	Shao et al. (2005)
CHBA: 30 min Annealed: 150°C			54.5	2.40	22.7
CHBA: 30 min Annealed: 200°C			59.8	2.28	26.2
Un-crosslinked PDMC			18.0	0.52	34.50
Crosslinked PDMC	4.42	35	60.4	1.60	37.75	Hillock and Koros (2007)
Crosslinked PDMC			60.4	1.33	45.41
6-FDA-mPD			9.2	0.160	57.6
6-FDA-mMPD			40.1	0.877	45.7
6-FDA-mTrMPD			431	26	16.6
6-FDA-pPD			15.3	0.286	53.5
6-FDA-pDiMPD	10	35	42.7	1.07	39.9	Tanaka et al. (1992)
6-FDA-pTeMPD			440	28.2	15.6
BPDA-mTrMPD			137	8.08	17.0
BTDA-mTrMPD			30.9	1.25	24.7
Original PI-200			235.28	10.42	22.58
Original PI-300			326.82	17.04	19.18
Original PI-350			392.86	22.80	17.23
Original PI-400			519.68	28.85	18.01
Original PI-425			1,302.21	62.52	20.83
PI-g-α-CD-200			241.23	11.98	20.14
PI-g-α-CD-300			373.61	22.25	16.79
PPM-α-CD-350			416.64	25.95	16.06
PPM-α-CD-400			568.65	30.72	18.51
PPM-α-CD-425			2,423.03	111.67	21.70
PI-g-β-CD-200	10	35	238.75	11.55	20.67	Askari et al. (2012a)
PI-g-β-CD-300			403.46	25.14	16.05
PPM-β-CD-350			593.25	37.29	15.91
PPM-β-CD-400			772.33	41.42	18.65
PPM-β-CD-425			3,112.26	140.25	22.19
PI-g-γ-CD-200			251.38	11.85	21.21
PI-g-γ-CD-300			416.38	23.85	17.46
PPM-γ-CD-350			663.35	40.59	16.34
PPM-γ-CD-400			929.97	48.43	19.28
PPM-γ-CD-425			4,211.12	187.66	22.44

Polysulfone

Polysulfones (PSF) are high-functioning glassy polymers with properties of transparency, heat resistance, chemical resistance, and electrical insulation (Julian and Wenten, 2012). Their ease of processing makes PSF suitable for a wide range of applications, including food service tools, electricity, electronics, medical devices, plumbing, automobile parts, and, most recently, gas separation membrane technology. Extensive research has been conducted on the permeation properties of both pure and mixed gases through polysulfone membranes, and PSF are already extensively used throughout the gas separation industry. Subsequent research has focused on the manufacture and modulation of polysulfone hollow fibers and the adaptation of these to an industrial scale and multistage design (Zimmerman et al., 1997; Baker, 2002).

Industrial hollow fibers that are used in gas separation are generally composed of tetramethyl bisphenol-A polysulfone, showing a lower permeability but a four to five times higher selectivity than polysulfone. Thus, the performance of polysulfone membranes is highly dependent on membrane formulations and manufacturing parameters (Yampolskii et al., 2006; Julian and Wenten, 2012). Shaping PSF into the abovementioned hollow fibers and mixed matrix membranes (MMM) is a recent effort to overcome the traditional drawbacks of polysulfone polymers, such as low permeability, while making use of their advantages.

Table 4 shows data on CO₂ permeability and selectivity of CO₂/CH₄ separation of polysulfone membranes. McHattie et al. (1991a, 1991b; 1992), who conducted research on methyl-substituted polymers, reported that polysulfone membranes exhibit high to low CO₂ permeability in the order of TMHFPSF>TMPSF>TMPSF-F. This effect was achieved by the subtle structural changes that resulted from tetramethyl substitution about hexafluoro bisphenol-A polysulfones (HFPSF), PSF, and bisphenol-F PSF. Aitkin and Paul (1993) reported that PSF that were based on naphthalene exhibited a high selectivity but a CO₂ permeability difference 30% lower compared with other isomeric polymers.

Table 4.

Experimental Data on CO₂ Permeability and Selectivity of CO₂/CH₄ Separation of Polysulfone Membranes as Reported in Literature

Membrane	Pressure (atm)	Temp. (°C)	P(CO₂) (Barrer)	P(CH₄) (Barrer)	α(CO₂/CH₄)	Ref.
PSF			4.6	0.21	21.9
DMPSF	10	30	2.2	0.07	29.0	Kim and Hong (1997)
PSF			5.5	0.24	23
PSF-NH₂ (16%)			2.7	0.11	24
PSF-NH₂ (38%)	10	35	3.2	0.13	25	Ghosal et al. (1996)
PSF-CH₂-NH₂ (51%)			1.95	0.11	18
PSF-CH₂-imide (51%)			2.12	0.08	26
PSF			5.6	0.25	22
TMPSF			21.0	0.95	22
DMPSF	10	35	2.1	0.07	30	McHattie et al. (1991a)
DMSF-Z			1.4	0.041	34
HFPSF			12.0	0.54	22
PSF-F	10	35	4.5	0.19	24	McHattie et al. (1991b)
PSF-O			4.3	0.17	24
TMPSF-F			15	0.58	26
TMHFPSF	10	35	72	3.00	24	McHattie et al. (1992)
3,4-PSF			1.5	0.05	29
PSF-P			6.8	0.34	20
PSF-M			2.8	0.11	25
TMPSF-P			13.2	0.60	22
TMPSF-M	10	—	7.0	0.28	25	Aitken et al. (1992)
BIPSF			5.6	0.25	22
TMBIPSF			31.8	1.27	25
HMBISPSF			25.5	0.94	27
1,3-ADM PSF			7.2	0.32	22
2,2-ADM PSF	10	—	9.5	0.39	24	Pixton and Paul (1995)
PSF/MCM-48 (10 wt%)			6.1	0.25	24.0
PSF/MCM-48 (20 wt%)	3.94	25	7.7	0.28	27.2	Jomekian et al. (2011)
PSF/MCM-48 (40 wt%)			13.2	0.40	32.7
1,5-NPSF			1.6	0.03	44
2,6-NPSF	10	—	1.5	0.03	41	Aitkin and Paul (1993)
2,7-NPSF			1.8	0.05	36
TM-NPSF			5.17	0.13	38.0
HF-NPSF	2		5.25	0.15	34.2
TMHF-NPSF			6.97	0.23	29.3
TM-NPSF	4	35	4.85	0.13	36.7	Camacho-Zuniga et al. (2009)
HF-NPSF			4.89	0.14	33.5
TMHF-NPSF			6.60	0.21	30.4
BPSF			3.2	0.11	27
MPSF	10	—	2.2	0.07	29	Kim and Hong (1999)
TMSPSF			15.1	0.94	16
PSF-PPHA	10	35	6.5	0.28	23.0	Houde et al. (1994)
PSF-NO₂ (50%)			3.39	0.14	24.2
PSF-NO₂ (98%)	10	35	2.30	0.08	28.7	Ghosal and Chern (1992)

HFPSF, hexafluoro bisphenol-A polysulfones; PSF, polysulfones; PSF-F, bisphenol-F polysulfones.

PSF are effective in gas separation membranes, but low permeability of CO₂ is an issue that needs to be addressed. Therefore, future research yielding formulations and manufacturing protocols that will allow the development of membranes to surpass Robeson's upper bound is promising and in demand.

Polycarbonates thermally rearranged, and miscellaneous polymers (cellulose acetate, polyphenylene oxide, etc.)

Polycarbonates (PC) mostly based on bisphenol-A (4,4-dihydroxy-2,2′-diphenylpropane), exhibit superior mechanical properties, transparency, and electric properties and they are synthesized by interfacial polycondensation; they have been used in studies of CO₂/N₂ gas transport properties (Lee and Lee, 1993; Powell and Qiao, 2006). Most PC tend to have a CO₂ permeability of under 40 Barrer and selectivities range from 15 to over 25. One notable exception to this is the PC, TMHEPC, which possess a CO₂ permeability of 111 and a CO₂/N₂ selectivity of 15.0 (Powell and Qiao, 2006). PC membranes exhibit a high O₂/N₂ selectivity but a low permeability coefficient for O₂. Likewise, PC membrane's CO₂/CH₄ selectivity is high and its CO₂ permeability is low. Active research is underway to increase the permeability of PC while maintaining the high selectivity for which these polymers are known.

Thermally rearranged (TR) polymer membranes are created by applying heat to solid glassy polymers, and the resulting deformation transforms molecular chains and produces micropores within the polymers, increasing the permeability of gas molecules. Such polymers, when used in membranes, have the potential to augment selective permeability (i.e., high permeability and selectivity) for the target gas by acting as a molecular sieve, and the membranes have a large free volume and an enhanced mass transfer rate due to micropores within the polymers themselves. Kim et al. (2012) prepared TR polybenzoxazole (TR-PBO) hollow fiber membranes. Asymmetric hollow fiber membranes were spun from a hydroxyl poly(amic acid) precursor and subsequently converted to TR-PBO hollow fiber membranes after thermal treatment above 400°C. The skin structure and porous substructure in TR-PBO hollow fiber membranes were maintained even after thermal treatment at above glass transition temperature (T_g) of precursor polymers. These TR polymer membranes exhibit excellent gas separation performance, especially in CO₂ separations, without any evident plasticization effect. TR polymers are also a solution to the problematic plasticization that occurs during processing other types of gas separation membranes as a result of permeation of CO₂ through the membrane in high-pressure conditions (Kim et al., 2012).

Cellulose acetate membranes began attracting attention in the 1960s and were commercialized in the 1980s. They make up 80% of the market for natural gas separation membranes. However, the frequent plasticization of such membranes, due to the action of CO₂ or heavy hydrocarbons, is a nagging issue with cellulose acetate membranes (Scholes et al., 2012). Currently, extensive research on cellulose acetate is underway, but the low mechanical strength of cellulose acetate, which requires it to be combined with other synthetic polymers to form a membrane, continues to pose a problem.

Data on CO₂ permeability and selectivity of CO₂/CH₄ separation by PC membranes, TR polymer membranes, cellulose acetate membranes, polyphenylene oxide (PPO) membranes, and other polymer membranes are charted in Tables 5 and 6.

Table 5.

Experimental Data on CO₂ Permeability and Selectivity of CO₂/CH₄ Separation of Polycarbonate and Miscellaneous Polymer Membranes as Reported in Literature

Membrane	Pressure (atm)	Temp. (°C)	P(CO₂) (Barrer)	P(CH₄) (Barrer)	α(CO₂/CH₄)	Ref.
TBBA-PC			3.6	0.10	35
TABBA:BA (50/50)-PC			6.4	0.22	29
TABBA:BA (30/70)-PC			3.6	0.11	32
TABBA:TMBA (1/1)-PC			7.0	0.23	30
TCBA-PC			2.6	0.10	25
TCBA-BA (50/50)-PC	—	30	5.5	0.22	25	Chan et al. (1978)
TCBA-BA (30/70)-PC			3.6	0.11	32
TMBA-PC			16.3	0.60	27
TMBA-BA (50/50)-PC			5.7	0.23	24
TMBA-BA (50/50)-PC			5.7	0.19	29
PC/pNA/zeolite 4A	3.7	25	4.61	0.08	51.8	Şen et al. (2007)
PVC/SBR/C-MWCNT	2	25	16.31	0.26	62.73	Rajabi et al. (2013)
CTA	—	25	3.94	0.14	28.14	Koh et al. (2011)
TMSP	—	25	33,100	15,990	2.07	Barbari et al. (1988)
TMSP	—	35	28,000	13,023	2.15	Barbari et al. (1989)
PPO	—	35	61	4.29	14.2	Barrer (1984)
Pure PVAc (low-pressure case)	2.7		2.15	0.064	33.5
Pure PVAc (high-pressure case)	30		11.4	0.457	25.0
PVAc MMM (low-pressure case)	2.7	35	4.33	0.088	49.4	Adams et al. (2011)
PVAc MMM (high-pressure case)	30		11.5	0.284	40.6
PVA/PEG	—	30	80.2	2.43	33	Xing and Ho (2009)
SBA-15/PPO	1.9	—	48.54	0.91	53.3	Weng et al. ( 2011)
Matrimid 5218/TiO₂	2	35	8	0.57	13.8	Moghadam et al. (2011)
MOF-5/Matrimid^®	2	35	20.2	0.45	44.7	Perez et al. (2009)
ZSM-5/Matrimid	1.3	—	14.61	0.25	56.48	Zhang et al. (2008)
PSF			6.3	0.22	28.6
PSs/silica MMMs (0.2Øf)	4.4	35	19.7	1.10	17.9	Ahn et al. (2008)
PSF			2.0047	0.4848	4.1351
PSF/PI (70/30)-10 wt% ZSM-5			1.7535	0.4133	4.2417
PSF/PI (50/50)-10 wt% ZSM-5	2.0–4.5	35	1.5130	0.3418	4.4253	Dorosti et al. (2011)
PSF/PI (30/70)-10 wt% ZSM-5			1.6755	0.3810	4.3976
PI			1.5929	0.3616	4.4051
PSF			3.9	0.165	23.55
5 wt% SWNT/PSF			5.12	0.27	18.82
10 wt% SWNT/PSF	4	35	5.19	0.28	18.41	Kim et al. (2007)
15 wt% SWNT/PSF			4.52	0.28	16.09
PES/SAPO-34	2	35	5.77	0.15	37	Cakal et al. (2012)
PES/zeolite 4A	10	—	1.62	0.035	46.28	Ismail et al. (2008)
APTES-MCM41/PSF	4	34.9	7.3	0.26	28	Kim and Marand (2008)

MMM, mixed matrix membrane; PC, polycarbonates; PPO, polyphenylene oxide.

Table 6.

Experimental Data on CO₂ Permeability and Selectivity of CO₂/CH₄ Separation of Thermally Rearranged Polymer Membranes as Reported in Literature

Membrane	Pressure (atm)	Temp. (°C)	P(CO₂) (Barrer)	P(CH₄) (Barrer)	α(CO₂/CH₄)	Ref.
TR350			35.3	0.78	45
TR400	10	35	160	5.51	29	Sanders et al. (2012)
TR450			410	17.08	24
TR-PAI (250)			24	0.3	91.5
TR-PPL (300)			73	2.0	34.8
TR-PPL (350)			82	6.0	14.0
TR-PPL (400)	—	25	126	4.0	32.3	Han et al. (2010)
TR-PPL (450)			234	8.0	29.0
TR-iPBI			11	0.1	93.1
TR-PBI (450)			1,624	35	46.1
Spiro TR-PBO (6F)			675	33.75	20
Spiro TR-PBO (PM)			263	14.61	18
Spiro TR-PBO (BP)	1	35	87	5.8	15	Li et al. (2013)
Spiro TR-PBO (BPA)			8.8	0.46	19
TR-1-450	10	35	1,610	26.83	60	Park et al. (2007)
HAB-6-FDA-EA (TR400)			51	1.41	36
HAB-6-FDA-Ac (TR400)	10	35	174	5.10	34.1	Guo et al. (2013)
HAB-6-FDA-Pac (TR400)			211	11.40	18.5
PBO			296	9.22	32.1
CPBOc (95/5)			1,079	41.66	25.9
CPBOc (90/10)			1,539	64.93	23.7
CPBOc (85/15)	3.5	35	1,306	58.82	22.2	Yeong et al. (2012)
Oc (70/30)			1,238	60.39	20.5
CPBOc (50/50)			935	44.10	21.2
CPBO			255	9.37	27.7
HPI (300)			9.9	0.07	124
PBO (450)			4,201	150.03	28
PBO-co-PPL82 (450)			1,874	50.64	37
PBO-co-PPL55 (450)	1	25	1,805	46.28	39	Choi et al. (2010)
PBO-co-PPL28 (450)			525	6.73	78
PPL450 (450)			234	8.06	29
HPEI			8.6	0.13	66.2
400-1			23.3	0.62	37.6
400-2			27.0	1.35	20.0
400-3			22.5	0.80	28.1
450-1	—	—	41.4	1.45	28.6	Calle and Lee (2011)
450-2			118.6	3.90	30.4
450-3			485.8	17.00	28.6
aPBO			398	12	34
tPBO			4,201	150.35	28
cPBO	—	—	5,568	253.09	22	Misdan (2010)
sPBO			5,903	256.65	23
TR-1-HCl			912	14.80	61.6
TR-1-HCl (dedoping)			759	20.37	37.2
TR-1-HCl (redoping)			702	16.99	41.3
TR-1-HNO₃	—	25	325	55.08	5.9	Han (2010)
TR-1-H₃PO₄			295	3.29	89.4
TR-1-HPF₆			5	1.31	3.8

TR, thermally rearranged; TR-PBO, TR polybenzoxazole.

New upper bounds for CO₂/CH₄

Robeson proposed an upper bound, in 1991 and 2008, based on the data shown in Tables 1 and 2, respectively. Figure 1 depicts four graphs for comparison sake, between Robeson's upper bound and the CO₂ permeability and CO₂/CH₄ selectivity data for PI, PSF, PC, and other polymers obtained from our review of the literature.

FIG. 1.

New upper bounds for carbon dioxide/methane (CO₂/CH₄) with different polymer membranes: (a) polyimides, (b) polysulfones, (c) polycarbonate and miscellaneous polymers, and (d) thermally rearranged (TR) polymers.

PI showed a high CO₂ permeability of 0.096–4,211.12 Barrer and a CO₂/CH₄ selectivity of 10.8–87, which exceeded the Robeson's upper bound. MMMs incorporating polyimide with other compounds showed similar values for permeability and selectivity. Polysulfone membranes exhibited CO₂ permeability of 1.4–72 Barrer and a CO₂/CH₄ selectivity of 16–44, values inferior to that of polyimide. These results are explained by the fact that polysulfone membranes have high selectivity but low CO₂ permeability. The membranes based on glassy polymer such as PSF membrane are known to have high selectivity but low permeability, whereas the rubber-based membrane such as polydimethylsiloxane membrane has high permeability but low selectivity (Adewole et al., 2013). Further research aimed at increasing the CO₂ permeability of polysulfone membranes is underway.

Other membrane materials, such as cellulose acetate and PC, were shown to have CO₂ permeability and CO₂/CH₄ selectivity values that were not yet near the upper bound described by Robeson; however, many researchers have reported MMMs based on such polymers to successfully approach the upper bound. Additionally, the TR polymer membranes, which are currently under active research, had a CO₂ permeability of 5–5,903 and CO₂/CH₄ selectivity of 3.8–124, which exceeded the upper bound proposed by Robeson in 2008; their high CO₂ permeability in particular implies its high potential for future industrial use.

The resulting graph shows that PI and TR polymers have many instances where CO₂/CH₄ selectivity exceeds 50, the value necessary for commercialization to take place. It is implied that this is due to recent advancements in research and development on PI and TR materials, the superior CO₂ permeability and selectivity of these materials, and their greater potential in comparison to other materials.

CO₂/CH₄ Separation by Hollow Fiber Membranes

Hollow fiber membrane

Polymers increase permeability when they are used in a hollow fiber membrane, and module arrangements can be varied to obtain desired values for permeability and selectivity. Obtained values were thus arguably adequate for commercialization and demonstrated that the formation of modules enables hollow fibers to be applied to diverse fields. The modern gas separation membrane no longer represents a flat plate of film but is shaped as hollow fibers. The largest membrane area per volume is obtained with hollow fibers, and currently, fibers are almost exclusively used in gas separations (Scholz et al., 2013). Hollow fiber membranes are generally manufactured by phase inversion method, and they are formed by extrusion molding and using a dope solution composed of a polymer, solvent, and a volatile nonsolvent. Hollow fiber membranes are also versatile, having the potential to be created as either uniformly dense or microporous structures (Baker et al., 1991).

The process by which hollow fiber membranes are manufactured is largely composed of five steps. The first and foremost step is the forming of the dope solution, of which its composition and viscosity determine the underlying structure of the finished product. The second step is the initial extrusion of the hollow fiber. Air gap formation, the most important parameter in the second step, is largely determined by the flow velocity of both the dope and bore solutions, the coagulation temperature, and the spinning velocity. Then, the hollow fiber undergoes a washing process, drying process, and a final post-treatment stage. Hollow fibers that are prepared in this way are bundled together in units of hundreds to thousands and potted in a housing structure designed to withstand high pressure, thus forming a module that can maximize treatable gas volume. Separation facilities that use hollow fiber membrane modules are therefore space efficient and lend themselves well to scaling up.

Separation characteristics of hollow fiber membrane for CO₂/CH₄ separation

The greater part of the literature that we reviewed deals with membranes in the form of hollow fibers, as does the industry in practice. The use of hollow fibers facilitates the increase of effective membrane area and the application of membrane technology to commercial ends.

Table 7 shows the permeance and selectivity of various hollow fiber membranes, which were made of polyimide, polysulfone, and other materials. Of the polyimide hollow fibers, the 6-FDA-durene exhibits good thermal and mechanical properties and superior gas separation performance, enabling wide industrial use. According to the research by Chung et al. (2000), CO₂ permeance is high at 373 GPU; this result is explained by the better packing of molecular chains that were induced by the shear stress of 6-FDA-durene polyimide interaction with CO₂. The TR-PBO membrane module achieved the highest degree of CO₂ permeance, over 1,000 GPU higher than that observed with any other module, by decreasing the thickness of the skin layer of the membrane (Kim et al., 2012). Li and Chung (2008, 2010) reported achieving selectivity over two to three times greater than normal polymers with a dual-layer hollow fiber membrane, which was manufactured by using pyrolysis and electrostatic crosslinking. The body of research performed on hollow fibers implies great promise and a myriad of possible applications for this technology to industrial ends.

Table 7.

CO₂ Permeance and Selectivity of CO₂/CH₄ Separation of Various Hollow Fiber Membranes as Reported in Literature

Hollow fiber membrane	Pressure (atm)	Temp. (°C)	P(CO₂) (GPU)	P(CH₄) (GPU)	α(CO₂/CH₄)	Gas type	Ref.
6-FDA-2,6-DAT	13.6	18	59	1.46	40	Mixed gas	Cao et al. (2002)
6-FDA-2,6-DAT/1% (w/v) m-xylenediamine in methanol	2.0	23	25	0.42	59	Mixed gas	Cao et al. (2003)
6-FDA-Durene/mPDA 50/50 copolyimide	13.6	20	53.3	1.24	42.9	Pure gas	Qin et al. (2005)
Matrimid	14.8	20	11.2	0.26	47	Mixed gas	Dong et al. (2010)
6-FDA/DAM: DABA (3/2)	13.6	35	117	3.16	37	Mixed gas	Ma and Koros (2013)
PI-g-β-CD/6-FDA-Durene dual-layer (heat treatment 400°C)	13.6	25	130	8.4	15.5	Pure gas	Askari et al. (2012b)
Silicon rubber-coated 6-FDA-durene	2.7	25	373	19	19.7	Pure gas	Chung et al. (2000)
PSF/4-PVP/SR	13.6	25	92.0	3.19	28.8	Pure gas	Qin and Chung (2006)
Cellulose/PSF dual layer	13.6	25	3.05	0.08	38	Pure gas	Mao et al. (2011)
PSF mixed matrix (xerogels)/silicon coating	2.0	35	103.3	3.88	32.7	Pure gas	Magueijo et al. (2013)
PSF-FS	2.0	35	90.04	2.75	32.74	Pure gas	Wahab et al. (2012)
PES dual-layer+Ag⁺ form	13.6	25	10.5	0.113	87.9	Mixed gas	Li and Chung. (2010)
Torlon®	50.0	35	0.84	0.02	44	Pure gas	Kosuri et al. (2008)
PES/SR coating	6.8	25	31.6	0.635	49.8	Mixed gas	Li et al. (2008)
TR-PBO (module)	1.0–2.0	—	1,938	137.1	14	Pure gas	Kim et al. (2012)
Carbon (PEI/PVP-based)	7.0	—	0.69	0.01	69	Pure gas	Salleh and Ismail (2011)
SR/P4VP/PEI multilayer	6.8	25	7.44	0.12	62	Pure gas	Shieh et al. (2001)
Dual-layer PES-zeolite beta/P84 (800°C)	5.0	35	1.67	0.011	152	Pure gas	Li and Chung (2008)

Case studies of biogas separation using hollow fiber membranes

Table 8 shows several cases that illustrate the separation performance and operational environment of biogas separation using hollow fiber membranes. Case 1 showed the results of CO₂/CH₄ separation using a polyimide hollow fiber membrane. CH₄ recovery efficiency was 58% at 30°C and 70% at 50°C, and both the elimination efficiency of CO₂ and the concentrate of CH₄ in retentate improved as temperature and pressure conditions increased (Park et al., 2011). Case 2 demonstrated the use of a polysulfone membrane to recover CH₄ from biogas generated by the anaerobic digestion process and assess CH₄ permeability properties. A double stage membrane module was used to elevate CH₄ concentration in retentate up to 93%, and CH₄ concentration in the second stage permeate was up to 60%, allowing the permeate to return to the feed (Hwang and Jeong, 2011). Case 3 achieved CH₄ concentration of 90.1%, under temperature of −5°C, pressure of approximately 8 atm, gas composition of 60:40 (CH₄/CO₂), and gas flow rate of 5 L/min conditions (Son, 2010). Case 4 achieved CH₄ concentration of almost 95% by upgrading the natural gas and landfill gas with polyimide membranes (Harasimowicz et al., 2007). Case 5 demonstrated that energy consumption of compressor could be decreased with higher CO₂/CH₄ selectivity, as well as CH₄ concentration of up to 96.6% (Watanabe, 1999). Case 6, a simulation study based on experimental data on PVA/PVAm blend FSC membranes, successfully attained a CH₄ concentration of 89.5% and CO₂ purity of 83% (Deng and Hägg, 2010).

Table 8.

Case Studies of Biogas Separation Using Hollow Fiber Membranes as Reported in Literature

Case no.	Membrane type	Experimental conditions	Test results	Ref.
1	Polyimide	CO₂/CH₄=33–35:65–67Temp.=30, 50°CPressure=6.77 atm	CH₄ recovery efficiency: 58% at 30°C, 70% at 50°C	Park et al. (2011)
2	Polysulfone	CO₂/CH₄=80:20Temp.=20°CPressure=5.92 atm (feed)	CH₄ conc. in retantate: 93.4%	Hwang and Jeong (2011)
3	Polyacetylene	CO₂/CH₄=40:60Temp. = −5°CPressure=7.89 atm	CH₄ conc. in retantate: 90.1%	Son (2010)
4	Polyimide	CO₂/CH₄/H₂S=30:68:2Temp.=40°CPressure=5.92 atm (feed)	CH₄ conc. in retantate: 93.45%	Harasimowicz et al. (2007)
			CH₄ conc. in permeate: 35.75%
5	Polyimide	CO₂/CH₄=18–21:74–79Temp.=30°CPressure=1 atm	CH₄ conc. in retantate: 96.6%	Watanabe (1999)
6	PVAm/PVA	CO₂/CH₄=35:65Temp.=25°CPressure=0.986 atm	CH₄ conc. in retantate: 89.5%	Deng and Hägg (2010)
			CO₂ conc. in permeate: 83%

H₂S, hydrogen sulfide.

Conclusions and Recommendations

Gas separation by membrane carries many advantages, such as the elimination of the need for phase change of the gas in the separation process, ease of scaling up, and potential for simplification in the separation process. Perhaps, the most important part of such a gas separation process is the selection of the material for the membrane. This review sought to assess the suitability of various polymeric materials for CO₂/CH₄ (biogas) separation membrane. Permeability data for CO₂ and selectivity data for CO₂/CH₄ for each polymer material were collected from the literature and assessed against Robeson's upper bound.

Our findings demonstrated that polyimide and polysulfone membranes are highly applicable to the separation of CO₂/CH₄ gas mixtures: polyimide showed an especial promise for commercial implementation. Polysulfone membranes exhibited high selectivity but low CO₂ permeability that failed to exceed the upper bound. However, case studies suggested that PSF in a hollow fiber membrane form find a wide variety of applications in an industrial setting. Data for TR membranes were also collected from the literature. Values exceeding the upper limit were frequently seen, suggesting that the properties of TR materials render them highly applicable to industrial implementation. We came to the conclusion therefore that future research on CO₂/CH₄ gas separation should focus on polysulfone, polyimide, and TR membranes as materials for economically efficient and highly effective gas separation membranes.

Footnotes

Acknowledgments

This research was supported by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry, and Fisheries (IPET) (project 312041-3) and by the 2013 sabbatical year research grant of the University of Seoul.

Author Disclosure Statement

No competing financial interests exist.

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Gas Membranes for CO 2 /CH 4 (Biogas) Separation: A Review

Abstract

Abstract

Introduction

Gas separation by membrane

Polymer membranes (materials)

CO2/CH4 (Biogas)

Upper bound (1991, 2008)

Polymer Membranes for Separation of CO2/CH4

Polyimide

Polysulfone

Polycarbonates thermally rearranged, and miscellaneous polymers (cellulose acetate, polyphenylene oxide, etc.)

New upper bounds for CO2/CH4

CO2/CH4 Separation by Hollow Fiber Membranes

Hollow fiber membrane

Separation characteristics of hollow fiber membrane for CO2/CH4 separation

Case studies of biogas separation using hollow fiber membranes

Conclusions and Recommendations

Footnotes

Acknowledgments

Author Disclosure Statement

References

CO₂/CH₄ (Biogas)

Polymer Membranes for Separation of CO₂/CH₄

New upper bounds for CO₂/CH₄

CO₂/CH₄ Separation by Hollow Fiber Membranes

Separation characteristics of hollow fiber membrane for CO₂/CH₄ separation