In Gas Separation Unit

Membrane processes for biofuel separation: an introduction

A. Gugliuzza , A. Basile , in Membranes for Clean and Renewable Power Applications, 2014

3.3.1 Gas separation: basics and transport mechanisms

Gas separation in membranes takes place due to differences in transport of the different species flowing through the membrane itself. Both porous ¹ inorganic and dense polymeric membranes can be used as selective gas separation barriers, but most of the membranes used in gas separation are made of polymers, like cellulose derivatives, polysulfone, polyimides and polyamides. Depending on the pore size, gas transport in inorganic membranes may occur via different mechanisms. Figure 3.7 illustrates some of the different mechanisms of gas permeation through porous and dense membranes.

3.7. Gas transport mechanisms through porous and dense membranes.

With large pores – from 0.1 to 10   μm – gases permeate the membrane by convective flow (and no separation occurs). This is the case of permeation through macroporous membranes. Vice versa, mesoporosity takes into account the collisions of the molecules with the pore walls, which are more frequent than the collisions among molecules and so the molecular diffusion is predominant. In this case, the mean free path of the gas molecules is greater than the pore size. Such a mesoporous membrane is a conglomerate of capillarities, large and small, straight and tortuous; diffusion is governed by the Knudsen mechanism and, in accordance with kinetic theory, the transport rate of any gas is inversely proportional to the square root of its molecular weight (Graham's law of diffusion). Finally, if the membrane pores are extremely small (< 2   nm), then gases are separated, for example, by molecular sieving. The transport in these membranes is very complex and involves diffusion in the gas phase and (single or multilayer) surface diffusion that occurs when the permeating species exhibit a strong affinity for the membrane surface and adsorb along the pore walls.

On the other hand, in dense polymeric materials, the behaviour is primarily controlled by solubility and diffusivity relationships. Solution–diffusion is widely accepted as the main mechanism of transport of gases through dense polymeric membranes. As already said, this mechanism is generally considered to be a three-step process: during permeation, the gas absorbs onto one surface of the membrane (on the upstream side), diffuses through the polymer matrix and desorbs from the downstream side. The permeation of the gases depends on both the diffusion and the concentration gradient of the species along the membrane. Generally, the driving force for the selective transport of a species across a membrane is typically associated with a gradient of concentration, pressure, temperature, electric potential, and so on. In the following section, by referring to the solution–diffusion mechanism, the partial pressure difference will be considered, expressed conveniently in terms of mole fractions of the retentate and permeate phases.

The key parameters, dictating membrane performances, are permeability and selectivity.

The permeability or permeability coefficient (P) (mol   ·   m   ·   m^{−   2}  ·   s^{−   1}  ·   Pa^{−   1}), which represents the proportionality coefficient with the flux at steady state of a specific gas through the membrane:

[3.1] $\frac{\mathit{P}}{\mathit{\delta }}=\frac{\mathit{Q}}{\mathit{A}\cdot \Delta \mathit{p}}$

where δ is the effective thickness of the membrane, Q the gas permeation rate through the membrane, A the membrane surface area and Δp the pressure difference across the membrane. This is a phenomenological law and represents a measure of the quantity of a component that permeates through the membrane; generally it is accepted as unit of the permeability coefficient 1 barrer, which is equal to 10^{−   10}  cm³(STP) cm/(cm²  s cmHg) or mol/(m   s   Pa) in SI units (Yampolkii, 2011).

The permeability depends, in the most general case, on the temperature, the nature of gas and the chemical and physical structure of the polymer. In particular, it increases with increasing condensability of the gas, because of the highest sorption in the polymer.

The ideal gas selectivity, which is defined as the ratio of the permeability coefficients, as the permeation of different gases proceeds independently (for example in the case of single gas permeation):

[3.2] ${\mathit{\alpha }}_{\text{ij}}=\frac{{\mathit{P}}_{\mathit{i}}}{{\mathit{P}}_{\mathit{j}}}$

This parameter is a measure of the ability of a membrane to separate two gases. As a general rule, the productivity of a membrane is associated with the permeability, whereas the number of membrane stages required for a given purity and/or the quantity of product that is lost during process is related to selectivity. The permeability coefficient is related to both the diffusivity coefficient (D, which measures the mobility of the molecules within the membrane) and solubility coefficient (S, which measures the solubility of gas molecules within the membrane). The relationship between permeability, diffusivity and solubility for a generic component i is described by:

[3.3] ${\mathit{P}}_{\mathit{i}}={\mathit{D}}_{\mathit{i}}\cdot {\mathit{S}}_{\mathit{i}}$

where D is the diffusivity coefficient (m²/s) and S the solubility coefficient (mol/m³⋅Pa).

As said, the diffusion coefficient, D, reflects the mobility of the individual molecules in the membrane; whereas the solubility coefficient, S, reflects the number of molecules dissolved in the membrane. Solubility mostly increases with increasing molecular weight while the diffusivity is decreased.

Often the membrane thickness is not known because of measurement difficulties and also may not be constant throughout the membrane. In these cases the quantity that characterizes the gas permeation rate through a membrane is the permeance, P/δ (cm³(STP) cm/(cm²  s cmHg) or mol/(m   s   Pa)), defined as the flux through the membrane normalized by the driving force. Sometimes the permeance is also expressed in GPU (gas permeation units): 1 GPU   =   10^{−   6}  cm³(STP) cm/(cm²  s cmHg).

Because the permeability is equal to the product of the diffusivity and solubility coefficients of the gas species (Equation [3.3]), the selectivity can be also written as:

[3.4] ${\mathit{\alpha }}_{\text{ij}}=\frac{{\mathit{S}}_{\mathit{i}}}{{\mathit{S}}_{\mathit{j}}}\frac{{\mathit{D}}_{\mathit{i}}}{{\mathit{D}}_{\mathit{j}}}$

It should be stressed that it is not generally possible to predict the mass transport behaviour of a mixture starting from single component measurements. Future research on gas and vapour mixture separation is of great importance, because the separation efficiency of the membrane for practical applications is a crucial parameter. Nevertheless, the ideal perm-selectivity is convenient because in the absence of strong interactions between the permeating gases, the permeability coefficients of the pure gases can be used. Vice versa, in the case of mixtures, strong interactions between the permeating components are generally present and so another parameter is more important for the design of a membrane plant: the separation factor, SF. For a binary mixture of components i and j it is defined as:

[3.5] $\mathit{S}{\mathit{F}}_{\text{ij}}=\frac{\left({\mathit{Y}}_{\mathit{i}}/{\mathit{Y}}_{\mathit{j}}\right)}{\left({\mathit{X}}_{\mathit{i}}/{\mathit{X}}_{\mathit{j}}\right)}$

where Y and X are the molar concentrations in the permeate and feed sides, respectively, and the subscripts i and j refer to the two components in the mixture. During experiments, both X_i and X_j are fixed by the experimental conditions, whereas Y_i and Y_j must be determined by gas chromatography or mass spectrometry. The separation factor is defined to be always   >   1 and depends on the experimental conditions, such as pressure difference or the absolute pressure of the supplied gas. In real systems, the diffusion coefficient D and the solubility coefficient S may both be functions of concentration, so the theoretical analysis becomes more complicated. D_i / D_j , the ratio of the diffusion coefficients of the two gases, is often called diffusivity selectivity and is related to the different size of the molecules. On the other hand, S_i / S _j , the ratio of the solubility coefficients of the two gases, reflects the relative sorption of the gases and can be viewed as the solubility selectivity. This term determines the overall selectivity in rubber polymers.

The diffusion coefficient decreases with increasing molecular size in polymer materials. In fact, large molecules interact with more segments of the polymer chain with respect to small molecules; whereas the mobility selectivity always favours the transport of small molecules with respect to the large ones. When membrane selectivity is too low, more than one membrane stage will be necessary. This will increase capital and operating cost due to the additional membrane area required and the recompression between the stages. Investment costs for compressor, vacuum pump and membrane area have to be balanced with the operating costs (Vansant, 1990; Baker, 2004; Javaid, 2005).

A rubbery membrane is an amorphous polymeric material that operates above its T_g under conditions of thermodynamic equilibrium. In these membranes, the sorption of low molecular weight is typically described by Henry's law for cases in which the sorbed concentrations are low:

[3.6] ${\mathit{C}}_{\mathit{D}}={\mathit{K}}_{\mathit{D}}\mathit{f}$

where C_D is the concentration of gas in the membrane matrix, K_D the Henry's law constant and f the fugacity (a measure of the chemical potential) of the gas considered. For rubbery polymers and low concentrations of penetrant, the diffusion coefficient D_D is typically constant and P is independent of the feed pressure. In the case of the presence of high activity of gases or vapours, deviation from Henry's law sorption is observed. In rubbery membranes, the transport of molecules is typically described by a solution–diffusion mechanism, whereby the solution of low molecular weight in rubbery polymers is similar to penetrant sorption into low molecular weight liquids.

A glassy membrane is an amorphous polymeric material that operates below its T_g under conditions far from thermodynamic equilibrium. The polymer chains are packed imperfectly, leading to excess free volume in the form of microscopic voids in the polymeric matrix. Within these voids, Langmuir adsorption of gases occurs that increases the solubility. Unlike rubbery membranes, glassy membranes are able to discriminate effectively between extremely small differences in the molecular dimensions of common gases (e.g., 0.2–0.5 angstrom). In glassy membranes, the transport of molecules is typically described by the so-called dual-mode model. Some of the gas molecules are absorbed in the polymer matrix and follow Henry's law, whereas some are adsorbed into microscopic voids and their concentration, C_H , is described by the following equation:

[3.7] ${\mathit{C}}_{\mathit{H}}=\frac{{\mathit{C}}_{\mathit{H}}^{\text{'}}\mathit{bf}}{\left(1+\mathit{b}\mathit{f}\right)}$

where C_H′ is the maximum adsorption capacity, and b the ratio of rate coefficients of adsorption and desorption. The total sorption for glassy polymers is then described by the sum of the two components of gas molecules adsorbed in the polymeric matrix (Paul, 1994):

[3.8] $\mathit{C}={\mathit{C}}_{\mathit{D}}+{\mathit{C}}_{\mathit{H}}$

The success of the dual-mode sorption model in describing penetrant sorption in glassy polymers is due to the physical significance that can be related to model parameters.

For both glassy and rubbery membranes, the transport properties for gases are almost similar and the relationship between temperature and the transport of small molecules is generally viewed as an activated process and obeys an Arrhenius relationship:

[3.9] $\mathit{P}={\mathit{P}}_{0}exp\left(\frac{-{\mathit{E}}_{\mathit{P}}}{\mathit{RT}}\right)$

[3.10] $\mathit{D}={\mathit{D}}_{0}exp\left(\frac{-{\mathit{E}}_{\mathit{D}}}{\mathit{RT}}\right)$

[3.11] $\mathit{S}={\mathit{S}}_{0}exp\left(\frac{-{\mathit{H}}_{\mathit{S}}}{\mathit{RT}}\right)$

where P ₀, D ₀ and S ₀ are the initial conditions, E_P and E_D the activation energies for permeation and diffusion, respectively, H_S the heat of sorption, R universal constant gas and T absolute temperature.

From these equations, it follows that for both glassy and rubbery polymers, an increase in the temperature produces an increase in the permeability and a decrease in the selectivity of a membrane. Vice versa, the activation energy is generally smaller in glassy polymers.

For many membranes, there is a trade-off between selectivity and permeability: membranes with a high selectivity show a low permeability, and vice versa. In other words, a highly permeable membrane tends to have low selectivity and vice versa. For example, in the case of polymeric membranes, Robeson (2008) suggested that this trade-off may be represented by a graph, where the logarithm of the selectivity is represented against the logarithm of the permeability of the more permeable gas for a binary mixture, as schematically shown in Fig. 3.8.

3.8. Upper bound correlation for general binary mixture separation.

In this figure, the upper bound represents the limit for achieving a high selectivity combined with a high permeability. The upper bound relationship can be expressed by the following Equation [3.12]:

[3.12] ${\mathit{P}}_{\mathit{i}}=\mathit{k}{\mathit{\alpha }}_{\text{ij}}^{\mathit{n}}$

where P_i is the permeability of the more permeable gas and n is the slope of the log–log limit.

Since the paper published by Robeson, only a few examples of polymeric membranes which exceed the upper bound have been published, and overcoming it is the focus of many recently awarded patents in polymeric membranes. In fact, achieving both high carbon dioxide permeability and high selectivity is desirable. It should be also said that exceeding the Robeson limit is not a rigid rule. In fact, Koros and Mahajan (2000) suggested the possibility of exceeding the upper bound by using the so-called mixed-matrix membranes. Recently, Berchotold (2006) found that the polybenzimidazole membrane exceeds the Robeson upper bound for H₂/CO₂ selectivity versus H₂ permeability in the range of temperature 100–400   °C.

Apart from permeability and selectivity, other membrane properties are also very important, such as their thermal, chemical and plasticization resistance, as well as the aging effects for ensuring continual performance over long periods. Moreover, it is also important that the manufacture of standard membrane modules is cost effective. Considerable experimental research has been addressed at meeting these aims. An extensive review describing both polymeric and inorganic membrane patents was recently published by Scholes et al. (2008), with particular attention paid to carbon dioxide separation through polymeric membrane systems for flue gas applications. This review is particularly interesting because it focuses on recent novel approaches in polymeric membranes that achieve separation performance above Robeson's upper bound and are therefore possibly more commercially competitive than current membrane gas separation technologies.

Depending on the composition of gaseous streams and operating conditions, the development of polymeric membranes for desired gas separation necessarily requires a careful assessment of both materials and assembly procedures.

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28th European Symposium on Computer Aided Process Engineering

Xinyan Liu , ... Rafiqul Gani , in Computer Aided Chemical Engineering, 2018

Abstract

Gas separation processes are among the most important operations in the oil and gas related industries. The most common separation technology applied is distillation, which consumes large amounts of energy. Because of the good stability, non-volatility, tunable viscosity and designable properties, ionic liquids (ILs) are regarded as novel potential solvents and alternative media for gas absorption. Therefore, a strategy for hybrid gas separation process synthesis where distillation and IL-based absorption are employed for energy efficient gas processing has been developed. In this work, a three-stage methodology proposed for hybrid gas separation process design and evaluation is proposed: IL screening, where a systematic screening method together with a database tool is established to identify suitable ILs; process design, where the important design issues (amounts of solvent needed, operating temperatures and pressures, evaporation conditions, etc.) are determined; process simulation and evaluation, where the final separation process results are concluded.

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Membranes and Membrane Processes

Norfazliana Abdullah , ... Ahmad F. Ismail , in Current Trends and Future Developments on (Bio-) Membranes, 2018

2.5.3.1 Gas Separation

Gas separation mostly uses nonporous/isotropic membranes (Pandey and Chauhan, 2001; Seo et al., 2006). In the process of preparing the membrane for gas separation, defects can occur, which reduce the membrane performance. Therefore, the top surface of the membrane is coated with a very thin layer to seal the defects. The coating layer has the function to remove defect but does not contribute to the intrinsic separation properties of the membranes. There is another membrane for gas separation, which uses a composite coating as a selective layer. The material is used to facilitate the intrinsic separation properties of the membranes but the original microporous membrane is only a support for the material (Budd et al., 2005). This type of membrane is mainly applied for the separation of O₂/N₂ and volatile organic compounds (VOCs/N₂). Compared with conventional technology used in gas separation, membrane technology offers minimal operating cost. Gas separation using membranes does not require gas-to-liquid (GTL) phase change in the gas mixture, thus, significantly decreasing the energy costs. Gas separation membranes have been successfully applied in hydrogen separation, CO₂ capture, organic vapor removal, and natural gas separation.

For gas separation, the gas permeation mechanism can be explained using the Knudsen diffusion, surface diffusion, multilayer diffusion, capillary condensation, or molecular sieving (i.e., configurational diffusion). These mechanisms are strongly dependent on the pore size, the pore size distribution of the membrane, operating temperature and pressure, and the nature of the membrane and the permeating molecules (Pandey and Chauhan, 2001; Baker, 2002). Knudsen diffusion is a likely mechanism to control the transport rate if the pore diameter is smaller than the mean free path of the molecule involved. In this case, the gases permeate in proportion to their molecular velocity, and hence in inverse proportion to the square root of their molecular weight. If the gasses are strongly adsorbed in the membrane pores, surface diffusion will enhance the permeation rate relative to the Knudsen diffusion. Molecular sieving mechanism may take place if the membrane pores are roughly similar to the diameter of the gas molecule. This mechanism is characterized by strong temperature dependence. Through this mechanism, larger gas molecules will experience a sharp decline in permeability. The different mechanisms for gas separation are shown in Fig. 2.12 (Khatib and Oyama, 2011).

Figure 2.12. Different mechanisms of gas separation through porous membrane and dense membrane. (A) Convective flow. (B) Knudsen diffusion. (C) Molecular sieving (D) Solution-diffusion.

From Khatib and Oyama, 2011. With permission from Elsevier.

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Ceramic Membranes, Preparation, Properties, and Investigation on CO2 Separation

Dimitris E. Koutsonikolas , ... Sotiris P. Kaldis , in Current Trends and Future Developments on (Bio-) Membranes, 2018

1 Introduction

Gas separation processes play a crucial role in many industrial applications and plants. Gas separation with membranes is a potential alternative to conventional gas separation processes (such as absorption, adsorption, and distillation), which showed a major breakthrough in the 1970s. Membrane separation processes provide several advantages over other conventional gas separation techniques because they do not require the addition of extra chemicals or solvents and also do not involve any energy-intensive phase transformation processes. Moreover, the necessary process equipment is very simple, with no moving parts, compact, relatively easy to operate and control, and easy to scale up. Currently, gas separation membranes have found commercial applications mainly for oxygen separation from air, natural gas purification, and hydrogen recovery in oil refineries. These application fields are dominated by polymeric membranes because polymeric materials are significantly cheaper than inorganic ones and they can be easily fabricated and scaled up in large area membrane modules. Regarding CCS applications, gas separation membranes have not yet been tested at industrial scale, but current research efforts are moving from lab scale to pilot testing for a range of applications and membrane materials, with emphasis to be given again in polymeric membranes. However, the application of polymeric membranes to CO₂ separation sometimes is argued as limited because of their insufficient thermal, mechanical, and chemical stabilities and their intrinsic low permeabilities. Moreover, many polymers can be swollen or plasticized when exposed to CO₂, causing dramatic reduction of their separation efficiency or even the irreparable damage of the membranes.

For all these reasons, the past few years, research focusing on the development and application of ceramic membranes is gaining increasing attention. Although ceramic membranes for gas separation are certainly still in an early technological stage, they show high potential mainly for precombustion CO₂ capture. Ceramic membranes show higher thermal, chemical, and mechanical stabilities than polymeric ones, offering potentially longer lifetimes and reduced maintenance costs. Moreover, they exhibit much higher fluxes than polymeric membranes, requiring less membrane area for a specific application. However, these materials possess also a number of drawbacks compared with polymers, such as higher cost, reduced reproducibility due to defects formation, and increased brittleness. Moreover, they usually can be fabricated into modules with lower specific membrane area (area to volume ratio), which also present difficulties in sealing at high temperatures (higher than 300°C) (Abanades et al., 2015; Pera-Titus, 2014; Bernardo et al., 2009; Shekhawat et al., 2003).

Ceramic membranes include a number of different materials, such as α- and γ-alumina (α- and γ-Al₂O₃), zirconia (ZrO₂), titania (TiO₂), silica (SiO₂), and various types of perovskites. Perovskite membranes are dense materials with crystal vacancies, which are able to act as high-temperature H⁺ or O⁻ conductors. Therefore, this type of membranes is out of the main scope of this chapter. On the contrary, microporous alumina, zirconia, titania, and silica membranes are potential candidates for various gas separation applications. Among them, microporous silica membranes have shown the highest potential for CO₂ separation applications. This chapter addresses current state of the art regarding the research and development of ceramic membranes for CO₂ separation, with emphasis given on microporous silica membranes.

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Membrane Gas Separation

Endre Nagy , in Basic Equations of Mass Transport Through a Membrane Layer (Second Edition), 2019

18.11 Concluding Remarks

Membrane gas separation is a promising technology, which is rapidly growing, and new types of membranes offer new application possibilities for both the chemical and biochemical industries. This chapter surveyed the fundamental properties of membrane gas separation processes, focusing on the transport mechanisms and the characteristic mass transfer rates during membrane gas separation. It summarized the solution/diffusion model based on thermodynamics, the characteristic sorption isotherms, and discussed the solution/diffusion transport model, mass transport with bulk convective velocity induced by the diffusive flow of components, and/or by the transmembrane pressure difference as well as mass transport in the presence of a polarization layer. At the end of the study, the mass transport equations through an asymmetric membrane layer were briefly shown with and without the feed phase polarization layer.

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Advanced membrane separation processes and technology for carbon dioxide (CO2) capture in power plants

A. Basile , ... P. Morrone , in Developments and Innovation in Carbon Dioxide (CO2) Capture and Storage Technology, 2010

7.3 Performance of membrane systems

Gas separation using membranes is a highly attractive, energy efficient technique for CO₂ capture. ^{20 , 21} A membrane is a physical device able to remove selectively one or more components from a mixture while rejecting others. Membrane gas separation shows different advantages over conventional processes and has been well described in many excellent reviews (see for example References 22–24). Membrane separation processes are used today in bulk chemistry as well as in the petrochemical sector.

Membranes offer several advantages, including their small size, simplicity of operation and maintenance, compatibility, diversity and lack of pollutant by-products. The main membrane separation techniques are: reverse osmosis, nano- ultra- and microfiltration, pervaporation, gas separation, vapour permeation and electrodialysis. The 'standard' membrane processes (reverse osmosis, nano- ultra- and microfiltration) are now reasonably commonplace in the majority of chemical sectors. ²⁵ Gas separation membranes are used in many industrial processes, such as the production of air enriched with oxygen, separation of CO₂ and H₂O from natural gas, purification of H₂ and so on. Various reviews and books can be found in the literature. ^{21 , 26–29}

In 1992, studying the application of polymeric membranes for recovering CO₂ from the flue gas of a power plant, Feron et al. ³⁰ and Van Der Sluijs et al. ³¹ showed that up to 76   % CO₂ removal is achievable and, moreover, that the economic competitiveness of the process depends on the membrane used, in particular, on its selectivity to gas transport.

In separating one or more gases from a feed mixture and generating a specific gas-rich permeate, a membrane acts as a 'filter' that allows the preferential passage of certain substances. In particular, a membrane will separate gases only if some components of the mixture are able to pass through the membrane more rapidly than others. In other words, the flux of the gas to be separated (in our case CO₂, preferentially in CO₂-rich feed mixture) should be higher than all the others (under the same conditions).

Membranes can be separated into two types: porous and non-porous (or dense) membranes. Porous membranes separate gases through small pores in the membrane based on molecular size. Non-porous or asymmetric membranes, which separate based on solubility and diffusivity, are more commonly used for gas separation, e.g. in natural gas applications. For both porous and non-porous membranes, there are many possible separation mechanisms, but only six of them are considered important for gas separation: Knudsen diffusion, molecular sieving, surface diffusion, facilitated transport, capillary condensation and solution-diffusion separation. ^{28 , 32–36} Among them, solution-diffusion is the most appropriate process for CO₂ separation in polymeric membranes. ³⁵ In dense membranes, the gas transport is based on a solution-diffusion mechanism and results in selective transport of gases and, consequently, their separation. It is interesting to observe that there is a trade-off between selectivity and permeability: membranes with a high selectivity show low permeability, and vice versa. ³⁷ Figure 7.2 indicates the ideal CO₂/N₂ selectivity versus fast component CO₂ permeability of polymeric membranes. Compared to other separations, such as O₂–N₂ and CO₂–CH₄ mixtures, CO₂–N₂ mixture appears to be an easier separation.

7.2. CO₂/N₂ selectivity versus the CO₂ permeability of polymeric membranes.

In 1991, Robeson suggested that this trade-off possesses an upper bound. ³⁷ Figure 7.3 shows an example of this upper bound, for a range of glassy and rubbery membranes involved in CO₂/CH₄ separations. Since Robeson's paper, only a few examples of polymeric membranes that exceed the upper bound have been published, and overcoming it is the focus of many recently awarded patents in polymeric membranes. The aim is to achieve both high CO₂ permeability and selectivity. For example, the polybenzimidazole membrane exceeds the Robeson upper bound for H₂/CO₂ selectivity versus H₂ permeability in the temperature range 100–400   °C. Recently, Koros and Mahajan suggested the possibility of exceeding the upper bound using mixed-matrix membranes. ³⁸

7.3. Comparison of Robeson's curve for CO₂/CH₄ separation by glass (o) and rubber (□) membranes.

Apart from permeability and selectivity, other membrane properties are also very important, such as their thermal, chemical and plasticisation resistance, and ageing affects when continued performance is required over long periods of time. Cost-effectiveness in manufacturing standardised membrane modules is also important. Considerable experimental research has been addressed to meeting these aims. An extensive review describing original polymeric and inorganic membrane patents was recently published by Scholes et al. ³⁹ with particular attention paid to CO₂ separation through polymeric membrane systems in flue gas applications. This review is particularly interesting because it focuses on recent novel approaches in polymeric membranes that achieve separation performance above Robeson' s upper bound and therefore are potentially more commercially competitive than present membrane gas separation technologies.

Another important extensive review of polymeric CO₂/N₂ gas separation membranes for the capture of CO₂ from power plant flue gases was recently published by Powel and Qiao. ³⁶ In this review, a chemist's perspective is taken, i.e. the gas permeability properties of dense membranes are seen in the light of their chemical structure. The Carapellucci and Milazzo ¹ paper already cited above investigates the engineering aspects of using membranes for CO₂ separation in flue gases. Some important aspects of these three papers regarding polymeric membranes will be considered here.

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Membrane-based technology for methane separation from biogas

Birgir Norddahl , ... K.V. Christensen , in Emerging Technologies and Biological Systems for Biogas Upgrading, 2021

6.3.2 Inorganic membranes for gas separation

Gas separation inorganic membranes are mainly divided into ceramic membranes and zeolites. Ceramic membranes are produced by coating of ceramic layers on top of a support and are typically formed of aluminum, titanium, zirconium or silicon oxides, as well as silicon carbide.

Zeolites are three-dimensional networks of silicate and aluminosilicate that can contain pores in the range of 3–8   Å (Baker, 2004). Both can withstand high temperatures and have good chemical resistance. However, they are expensive and brittle. Zeolites have been used dispersed in polymers in the form of mixed-matrix membranes (Pechar et al., 2006; Bastani et al., 2013) or as support for gas–liquid membrane contactors (Zhang et al., 2018).

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Integrated Design and Simulation of Chemical Processes

Alexandre C. Dimian , ... Anton A. Kiss , in Computer Aided Chemical Engineering, 2014

9.2.2 Selector analysis

Gas separation manager makes use of three selectors, enrichment, sharp separation and purification, as presented in Table 9.3 together with suitable separation methods.

Table 9.3. Selectors and methods for gas separations (+ means applicable, − n/a)

Separation method	Enrichment	Sharp separation	Purification
Condensation	+	–	–
Cryogenic distillation	+	+	–
Physical absorption	+	+	–
Chemical scrubbing	–	+	+
Molecular sieve adsorption	+	+	+
Equilibrium-limited adsorption	+	+	+
Membranes	+	+	–
Catalytic oxidation	–	–	+
Catalytic hydrogenation	–	–	+
Chemical treatment	–	–	+

1.

Enrichment consists of a significant increase in the concentration of one or several species in the desired stream, although by this operation neither high recovery nor purity is achieved. Condensation, physical absorption, membrane permeation, cryogenic distillation and adsorption are convenient separation techniques.

2.

Sharp separation consists of splitting the mixture in products with high recovery of target components. The sharpness is defined as the ratio of key component concentrations in products. This should be more than 10. Potential techniques are physical absorption, chemical scrubbing, cryogenic distillation, molecular sieving and equilibrium adsorption when the molar fraction of adsorbate is less than 0.1.

3.

Purification deals with the removal of impurities with the goal of achieving very high concentration of the dominant component. The initial concentration of impurity in mixture should be less than 2000   ppm, while the final concentration of impurity in product should be less than 100   ppm. Suitable separation methods are equilibrium adsorption, molar sieve adsorption, chemical absorption and catalytic conversion.

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Novel Multilayered Structures and Applications

Deepak Langhe , Michael Ponting , in Manufacturing and Novel Applications of Multilayer Polymer Films, 2016

6.4.1 Multilayered Gas Separation Membranes

Gas separation membranes are used in diverse applications such as gas purification, separation of byproducts, modified atmosphere packaging (MAP) applications [17]. Ceramic and alloy-based membrane technologies have historically been used for their excellent chemical resistance and stability. The polymer-based technologies use polymer separation membranes supported on porous substrates, which provide mechanical durability and stability. Fabrication of ultrathin separation membranes is typically achieved by solution casting or interfacial polymerization, while the porous support is fabricated by phase inversion techniques. Porous support materials can also be nonwoven mats produced by various airlaid, wetlaid, electrospinning, or melt spinning techniques [18]. It is obvious that multiple energy intensive fabrication steps and significant handling of organic solvents are necessary to create polymer separation membranes with good mechanical properties. To address these challenges, an approach of coextrusion processing was explored to create membranes using a two-polymer multilayer film system, where the first material was selected as a gas separation membrane polymer (highly selective polymer) and the second polymer created porous structures [19].

A poly(ether-block-amide) (PEBA) polymer with 86 mol% polytetramethyl oxide (PTMO) and 14 mol% polyamide-12 (PA-12) was used for its high CO₂/O₂ selectively and high gas permeability. Two approaches of creating porous layers with alternating gas separation layers were explored. In the first approach, PEBA was coextruded with PP blends containing CaCO₃, which created porous PP layers during uniaxial stretching of the layer composite. In the second approach, a multilayered film of PEBA with β-phase PP, achieved by blending it with a β-phase nucleating agent quinacridonequinone (QQ), was extruded. The β-nucleated PP transformed into α-phase during stretching and created porous features due to void formation between the lamellae during this transformation.

The multilayered structures, PEBA/(PP + CaCO₃) and PEBA/β-PP were produced in 30/70 and 10/90 volume composition films with varying number of layers from 2 to 17. An example of a three-layered extruded film with PEBA as outside layers and PP + CaCO₃ nonporous core layer is shown in Figure 6.7a . Film orientation by uniaxial stretching induced porosity in PP layers at CaCO₃ sites, while reducing PEBA layer thickness, Figure 6.7b. As the film orientation induced porosity, the oxygen, and CO₂ flux increased due to porous PP layers. The membrane composites showed the CO₂/O₂ selectivity of at least seven. The effect of number of layers on the oxygen permeance values is shown in Figure 6.7. The oxygen permeance in the layered composites below 6 μm PEBA layer thickness was less than the PEBA control. Postorientation annealing techniques were used to improve the oxygen permeance of the membranes. For example, annealing PEBA/PP + CaCO₃ films above PEBA melting temperature increased the oxygen and CO₂ flux by 100% and showed selective improvement from 8.7 to 9.6. The permeability values of the multilayered membrane structure were not significantly different in porous membranes created using CaCO₃ filler or beta phase transformation. Figure 6.8 shows the oxygen permeability and the CO₂/O₂ selectivity of the PEBA/β-PP membranes [20]. The multilayered membranes showed oxygen permeability values close to 5 GPU (GPU = Barrer/μm), which was significantly less than 30 GPU required for a standard MAP package [21]. The 3-layered structures showed highest oxygen permeabilities and the permeance than the 17-layered films possibly due to reduced porosity at higher number of layers. Further structural evaluation revealed the inconsistent pore formation in the layers resulting into higher selectivities but lower permeance values. The modeling results revealed the nonporous areas in PP layers negatively impacting the transport properties. For PEBA/β-PP multilayered composites, the CO₂/O₂ selectivity varied between six and nine in these multilayered films.

Figure 6.7. (a) As-extruded symmetric three layer films of 30/70 volume composition PEBA/PP + CaCO₃; (b) uniaxiallyoriented PEBA/PP + CaCO₃ films; (c) comparison of oxygen permeance for PEBA/PP + CaCO₃ multilayed (3-, 9-, and 17-layered) membranes.

Figure 6.8. O₂ permeability of multilayered films as a function of total PEBAX thickness (top) and multilayered film CO₂/O₂ selectivity as a function of film permeance (bottom). The permeability and selectivity of a single layer 80PTMO-PA12 (Pebax) film are shown. Results for 3L-10, 3L-30, 17L-10, and 17L-30 films are presented. (80PTMO-PA12 contains 80 wt% of poly(tetramethyloxide) and PA12 is a polyamide block. 3L-10 represents three layers and 10% PEBAX, 3L-represents 3-layered films with 30% PEBAX, 17L-10 represents 17-layered film with 10% PEBAX, and 17L-30 represents 17-layered film with 30% PEBAX.)

The coextrusion technology demonstrated a solvent-free approach to create supported polymer membranes. Although, this technology is in early research stage, it offers a unique approach in creating MAPs. Process optimization to create porous alternating layers will be useful toward producing commercial membranes [19,20].

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https://www.sciencedirect.com/science/article/pii/B978032337125400006X

Challenges for CO2 capture by membranes

Colin A. Scholes , in Advances in Carbon Capture, 2020

Abstract

Membrane gas separation has many advantages that make the technology ideal for carbon dioxide capture from a range of industrial processes. These advantages, however, have not been translated into wide scale adoption of membrane technology, with only natural gas sweetening currently commercialized. This is due to challenges that carbon capture applications present membranes, which need to be addressed before the technology can expect wider industry acceptance. These issues include the stability of separation performance, due to plasticization, competitive sorption, vapor accumulation, and chemical degradation as well as the modest selectivity of commercial membranes requiring multiple membrane stages to achieve the necessary purity. Here, the challenges facing membrane gas separation in carbon capture are analyzed relative to the various industrial processes, current strategies to address these challenges presented, and the economics needed for membranes to be competitive with alternative technologies discussed.

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https://www.sciencedirect.com/science/article/pii/B9780128196571000165

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Source: https://www.sciencedirect.com/topics/engineering/separation-gas

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