Water vapor permeation through cellulose acetate membranes and its impact upon membrane separation performance for natural gas purification
Introduction
Natural gas production is estimated to be over 3300 billion cubic meters per year worldwide [1] and the removal of acid gases from the raw gas is the largest industrial market for membrane gas separation. While the composition of raw natural gas is different from reservoir to reservoir, it primarily consists of methane (70–90%), with small amounts of C1–C4 hydrocarbons such as ethane, propane, butane, along with higher molecular weight fractions. Water vapor, carbon dioxide, acidic gases, mercury, hydrogen sulfide, helium and nitrogen are also present in raw natural gas as impurities. A glycol dehydration unit is typically used for removing water from the natural gas feed streams prior to membrane processing [2]. However, some water vapor may still reach the membrane if the glycol unit is not working effectively. Further, glycol units are not always used in smaller installations such as biogas processing.
Cellulose acetate was the original membrane material used for CO2 separation from natural gas, building on the experience gained using this material for the desalination of saline water. Cynara® (part of Cameron) and UOP Separex® (part of Honeywell) are the two major membrane manufacturers currently supplying cellulose acetate based modules. Depending on the degree of acetylation, which corresponds to the extent of substitution of the hydroxyl groups in the glucoside repeating unit with acetyl groups (Fig. 1), the polymer is named as cellulose acetate, cellulose diacetate or cellulose triacetate.
Dehydration units (mostly diethylene or triethylene glycol dehydrators) are widely used upstream of the membrane unit to prevent hydrate formation [3], typically reducing the water content to ~100 ppm at around 70 bar, 60 °C [2]. It has previously been reported that the presence of water and some hydrocarbon derivatives (e.g. acetone) can damage and dissolve the membrane [4], but the sensitivity of the separation performance of cellulose acetate membranes to water vapor is not well understood. Almost all studies on the effects of water vapor on cellulose acetate membranes were conducted in the 1980 s [4], [5], [6], [7], [8]. The permeation measurements in this period often assumed ideal mixing of water vapor and gas and neglected the impact of concentration polarization upon the permeation properties.
Due to its small size and high hydrogen bonding affinity [9], [10], [11], water vapor can have a very significant effect upon membrane performance [6], [12], [13], [14]. It is argued that the clustering of water into multimolecular groups can act to ‘fill’ the free volume within the polymer structure, resulting in a loss of fractional free volume [15], [16]. It has been reported that these water clusters result in a decrease in the penetrant diffusivity as water vapor activity increases in a phenomenon referred to as ‘antiplasticization’ [17], [18], [19], [20], [21], [22], [23]. Conversely, water molecules can plasticize or swell the membrane structure, causing an increase in free volume and a corresponding increase in diffusivity with increasing vapor concentration [24], [25], [26], [27]. Depending on the hydrophobicity of the membrane and the activity of water vapor, these phenomena may coexist in a multi-component separation system, in both cases causing a loss of selectivity.
In this work., the mixed vapor/gas permeation system developed within our group to eliminate concentration polarization effects [14] combined with Positron Annihilation Lifetime Spectroscopy (PALS) [28] is applied to quantify the permeation of water/CO2 mixtures through cellulose acetate membranes more accurately, allowing for a better understanding of the behavior of water vapor in a simulated natural gas sweetening environment. Two types of cellulose acetate with different degree of acetylation are investigated during exposure to humidified methane and wetted carbon dioxide and methane mixtures.
Section snippets
Materials
Cellulose acetate with acetylation degrees of 51.6% (CDA) and 61.6% (CTA) (equivalent to degrees of substitution of 2.2 and 2.85, respectively) were kindly supplied by Daicel Corporation, Japan, in a powder state. The chemical structure of a typical cellulose acetate is presented in Fig. 1. Prior to membrane fabrication, the polymers were dried at 100 °C under vacuum overnight to ensure the complete removal of moisture.
Membrane preparation
Dense cellulose acetate membranes were prepared by a casting method using
Characterization
The physical properties of the cellulose acetate membranes are summarized in Table 1. CDA is more dense than CTA due to both the more bulky nature of the acetyl groups, which results in greater free volume as the acetylation degree increases; and the increased hydrogen bonding between hydroxyl groups at lower degrees of acetylation. The water uptake of cellulose acetate membranes was also greater at lower degrees of acetylation, further reflecting the greater hydrogen bonding with more hydroxyl
Conclusions
As gas humidity increases, the permeability of water vapor also increases for CDA and CTA, respectively. However, the rate of increase changed significantly at a water vapor partial pressure of ~2.5 kPa. The change at this point was confirmed by PALS analysis to be due to the onset of membrane swelling effects or plasticization. For vapor activities above 0.8 (a water partial pressure of 4.6 kPa), membrane swelling was severe enough to influence the gas separation performance for more than 8 h.
Acknowledgments
The authors would like to thank the Particulate Fluids Processing Centre (PFPC), a Special Research Centre of the Australian Research Council for access to equipment. Funding for this project is provided by the Cooperative Research Centres (CRC), Australian Government Department of Industry Australia. CMD and AJH acknowledge the support of CSIRO׳s Office of the Chief Executive Science Leader program and CMD would like to acknowledge the ARC DECRA support (DE140101359). The authors would also
References (42)
- et al.
The effects of water vapor on the separation of methane and carbon dioxide by gas permeation through polymeric membranes
J. Memb. Sci.
(1983) - et al.
The effect of degree of acetylation on gas sorption and transport behavior in cellulose acetate
J. Memb. Sci.
(1989) - et al.
Mathematical model and experimental validation of water cluster influence upon vapour permeation through hydrophilic dense membrane
J. Memb. Sci.
(2004) - et al.
Operating temperature effects on the plasticization of polyimide gas separation membranes
J. Memb. Sci.
(2007) - et al.
Second component effects in sorption and permeation of gases in glassy polymers
J. Memb. Sci.
(1983) - et al.
Gas permeation and separation properties of sulfonated polyimide membranes
Polymer
(2006) - et al.
Water vapor permeation in polyimide membranes
J. Memb. Sci.
(2011) - et al.
Thermal, mechanical, physical, and transport properties of blends of novel oligomer and thermoplastic polysulfone
Polymer
(2000) - et al.
Mixed gas water vapor/N2 transport in poly(ethylene oxide) poly(butylene terephthalate) block copolymers
J. Memb. Sci.
(2005) - et al.
Kinetics of water vapor sorption in sulfonated polyimide membranes
Desalination
(2002)
Sorption and diffusion of water vapour in hydrogen-bonded polymer blends
Polymer
Solubility and diffusion of binary water—methyl alcohol vapor mixtures in cellulose acetate membranes
J. Memb. Sci.
Sorption, diffusion and vapor permeation of various penetrants through dense poly(dimethylsiloxane) membranes: a transport analysis
J. Memb. Sci.
Water vapor permeability and diffusivity through methylcellulose edible films
J. Memb. Sci.
Water vapor/propylene sorption and diffusion behavior in PVA–P(AA-AMPS) blend membranes by GCMC and MD simulation
Chem. Eng. Sci.
Water vapor sorption and diffusion properties of sulfonated polyimide membranes
J. Memb. Sci.
Mixed water vapor/gas transport through the rubbery polymer PEBAX® 1074
J. Memb. Sci.
Sorption, diffusion, and perm-selectivity of toluene vapor/nitrogen mixtures through polydimethylsiloxane membranes with two cross-linker densities
J. Memb. Sci.
Effect of diamine composition on the gas transport properties in 6FDA-durene/3,3′-diaminodiphenyl sulfone copolyimides
J. Memb. Sci.
Hydrogen separation and purification in membranes of miscible polymer blends with interpenetration networks
Polymer
The thickness dependence of matrimid films in water vapor permeation
Chem. Eng. J.
Cited by (67)
Highly permeable and selective polymer inclusion membrane for Li<sup>+</sup> recovery and underlying enhanced mechanism
2024, Journal of Membrane ScienceRegulation of orientation birefringence for cellulose acetate film: The role of crystallization and orientation
2022, Carbohydrate PolymersCitation Excerpt :To characterize the structural evolution of films during heating quantitatively, we calculated the crystallinity (χc) and the crystalline size (L110). χc in this study is in good agreement with that obtained by DSC and XRD methods (Chen, Kanehashi, Doherty, Hill, & Kentish, 2015; Puleo, Paul, & Kelley, 1989). The maximum χc of CDA (DS = 2.45) and CTA (DS = 2.84) film were about 37 % and 52 % utilizing DSC measurements (Puleo et al., 1989).
Polyethylenimine grafted ZIF-8@cellulose acetate membrane for enhanced gas separation
2022, Journal of Membrane ScienceCryogenic technology progress for CO<inf>2</inf> capture under carbon neutrality goals: A review
2022, Separation and Purification Technology