Materials selection guidelines for membranes that remove CO2 from gas mixtures
Introduction
Carbon dioxide is an impurity that must be removed from mixtures with light gases such as CH4, N2 and H2, and the scale of these separations is enormous [1]. For example, the annual US production of natural gas is 5.6×1011 m3 (STP), and approximately 20% of this gas contains CO2 at concentration above the allowable US pipeline specification, which is 2 vol% or less.[2] Similarly, hydrogen is a basic chemical in the fertilizer and refinery industries, and its annual production is 8.1×109 kg in the US alone [3]. H2 is usually produced via steam reforming of hydrocarbons and, as such, it is contaminated with CO2 during production; the CO2 must be removed to produce highly purified H2 [1]. Hydrogen production is expected to increase as refinery demands for H2 increase and as H2 applications (e.g. fuel cells) increase. Additionally, CO2 recovery from flue gas (primarily in mixtures with N2) is becoming more important due to global warming, and there are initiatives that might eventually require CO2 removal from flue gas [4]. These applications could sharply increase the demand for more energy-efficient, cost-effective strategies for CO2 removal from gas streams.
Currently, CO2 is removed from gas mixtures mainly by absorption technology (such as amine or hot potassium carbonate aqueous solutions), pressure swing adsorption and membrane technology [1]. Economically, membranes may be advantageous in small and medium scale separations and when product purity requirements are not extremely stringent [5]. Membrane technology enjoys inherent advantages, such as small footprint, mechanical simplicity, and high energy efficiency, relative to traditional acid gas treatment technologies [6]. For membrane-based separations in the applications mentioned above, it is highly desirable to selectively remove CO2 from mixtures with light gases such as H2, N2 and CH4, thereby maintaining the light gas at or near feed pressure (in the case of H2 and CH4) to avoid expensive recompression of the desired light gas product. For CO2 removal from N2, selectively removing CO2 avoids permeation of the major component (N2) across the membrane, which could vastly reduce membrane area requirements.
The emergence of membrane technology for CO2 removal from natural gas in the 1980s resulted from several breakthroughs. Although gas permeation in membranes has been studied since the 1940s, membrane fabrication technology was not sufficiently developed to provide high enough gas flux for industrial applications until Loeb and Sourirajan introduced techniques to prepare high flux anisotropic membranes with selective layer thicknesses of less than 0.5 μm and often less than 0.1 μm [5]. The second breakthrough came with the development of high surface-to-volume membrane module designs, such as spiral-wound and hollow-fiber modules [5]. These designs accommodate large membrane area in small volumes, significantly reducing the footprint of membrane systems. These two advances contributed to the successful development of reverse osmosis membrane systems. However, Loeb–Sourirajan type anisotropic membranes could not be directly used for gas separations, because pinholes or defects are always introduced during the membrane preparation process, and they diminish selectivity substantially. Henis and Tripodi [7] resolved this limitation by applying a thin layer of silicon rubber to the membrane (e.g. polysulfone) to eliminate non-selective convective flow through pinholes. The composite membrane exhibited separation and permeation properties similar to those of polysulfone since silicon rubber has much higher gas permeability than polysulfone. These achievements allowed membrane technology to become a viable alterative to conventional gas separation technologies such as absorption and adsorption. Recent efforts have focused on membrane materials optimization to achieve better separation performance and better stability in process environments [8].
The steady-state permeability of gas A, PA, through a film of thickness l is defined as [9]where NA is the steady state flux of gas through the film (cm3 (STP)/cm2 s), l is the film thickness (cm), and p2A and p1A are the upstream (i.e. high) and downstream (i.e. low) partial pressures (cmHg), respectively. Permeability coefficients are commonly expressed in units of Barrers, where 1 Barrer=10−10 cm3 (STP) cm/(cm2 s cmHg). If diffusion obeys Fick's law and the downstream pressure is much less than the upstream pressure, the permeability can be expressed as [9]where DA is the average effective diffusivity, and SA=C2A/p2A is the ratio of gas concentration sorbed in the upstream face of the polymer, C2A, to the upstream pressure, which is also called the apparent solubility of penetrant A in the polymer. The ideal selectivity of a membrane for gas A over gas B is the ratio of their pure gas permeabilities [5]where DA/DB is the diffusivity selectivity, which is the ratio of the diffusion coefficients of gases A and B. The ratio of the solubility of gases A and B, SA/SB, is the solubility selectivity.
Generally, penetrant solubility increases with increasing condensability (i.e. higher critical temperature or higher normal boiling point) and more favorable interactions with the polymer, while gas diffusivity is enhanced by decreasing penetrant size, increasing polymer fractional free volume, increasing polymer chain flexibility, and decreasing polymer–penetrant specific interactions. Table 1 summarizes the condensability and molecular size of CO2 and several other gases of interest. In polymers and liquids, CO2 typically exhibits higher solubility than light gases in large measure due to its higher condensability (as characterized by Tc). Based on relative molecular size difference alone, CO2 diffusivity should be higher than that of CH4, similar to that of N2, and lower than that of H2. For CO2/CH4 separation, membrane materials with high diffusivity selectivity have been extensively pursued by designing relatively rigid polymers with high glass transition temperatures, and high CO2 permeability has been sought by maintaining or increasing fractional free volume [10]. Another avenue, which has received much less attention, is materials with higher values of CO2/light gas solubility selectivity. This strategy is absolutely required for CO2/H2 separation, which exhibits unfavorable diffusivity selectivity, and may be necessary for CO2/N2 separation where the penetrant size difference is not large. This report describes structure–transport property guidelines for designing polymers with high CO2 permeability and CO2/light gas selectivity; it focuses mainly on materials that achieve high selectivity as a result of high solubility selectivity. CO2 is the target gas in these examples because there is great interest in CO2 separations and also because there are more experimental data for CO2 than for any other acid or polar gas. However, the materials design considerations discussed here may also be applicable for removing other acid or polar gases (e.g. H2S, SO2, H2O, NH3, etc.) from mixtures with light gases. In this regard, special attention will be paid to H2S, since it is an acid gas (like CO2) and is often a contaminant that must also be removed from the gas streams of interest. More specifically, the effect of various polar groups on CO2 solubility, diffusivity and permeability and CO2/light gas selectivity is discussed. The present report focuses primarily on rubbery polymers. Because ethylene oxide units provide the best combinations of CO2 permeability and CO2/N2 and CO2/H2 selectivity known to date, we review various strategies of incorporating high concentrations of ethylene oxide groups or poly(ethylene oxide) in polymers while avoiding crystallization of ethylene oxide units, which substantially decreases gas permeability.
Section snippets
Structure–gas solubility correlation
Penetrant solubility in rubbery polymers is often described using the Flory–Huggins model [11]where a is penetrant activity, χ is the Flory–Huggins interaction parameter, m is the ratio of polymer to penetrant partial molar volumes , and ϕ2 is the volume fraction of gas dissolved in the polymer matrix. For ideal gases, the activity is p/psat, where psat (atm) is the penetrant saturation vapor pressure at the temperature of the experiment. The gas volume
Structure–gas diffusivity and permeability correlations
Gas diffusion in polymers is often qualitatively understood to depend sensitively on free volume [40]where A0 is a pre-exponential factor, γ is a numerical factor introduced to account for possible overlap of free volume elements, is the minimum free element size needed to accommodate a gas molecule, which is dependent on penetrant size, and 〈Vf〉 is the average free volume in the polymer. Based on this model, higher free volume would generally increase gas diffusion
Structural design of poly(ethylene oxide) containing polymers
The unique property of polar ether oxygens for CO2 separation has attracted much interest. There have been numerous efforts to design polymers containing poly(ethylene oxide) (PEO) for CO2/N2 and CO2/H2 separations [4], [29], [30], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], in part because ethylene oxide units have a high concentration of ether oxygens and are relatively easy to fabricate. In comparison, poly(methylene oxide) would have an even higher ether oxygen
Discussion
Table 3 summarizes CO2 permeability and CO2/N2 selectivity in various PEO containing materials at 35 °C along with the glass transition temperature, estimated CO2 permeability and fractional free volume of the PEO phase. For consistency, all of the permeability data are reported at 35 °C, based on either direct measurement or interpolation using data from the corresponding references. While CO2/N2 selectivity remains relatively constant, indicating that PEO is continuous and provides the dominant
Conclusions
Structure/property guidelines have been extensively explored in an effort to improve the separation performance of polymer membranes for gas separation by increasing polymer size sieving ability (i.e. diffusivity selectivity) [68], [69]. However, favorable solubility selectivity has not been fully pursued as a route to enhance gas separation properties, probably due to the fact that penetrant diffusion coefficients are often more sensitive than solubility to polymer structure, and diffusion
Acknowledgements
The authors gratefully acknowledge partial support of this project by the United States Department of Energy under grant number DE-FG02-99ER14991. This research work was also partially supported with the funding from the United States Department of Energy's National Energy Technology Laboratory under a subcontract from Research Triangle Institute through their Prime Contract No.: DE-AC26-99FT40675.
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