Elsevier

Journal of Membrane Science

Volume 618, 15 January 2021, 118675
Journal of Membrane Science

Blended perfluoropolymer membranes for carbon dioxide separation by miscible and immiscible morphologies

https://doi.org/10.1016/j.memsci.2020.118675Get rights and content

Highlights

  • Blended perfluoropolymers can fabricate both miscible and immiscible membranes.

  • Miscibility dependent on different solvability in the solvent and evaporation rate.

  • Miscible blended membranes have improved CO2/CH4 selectivity.

  • Immiscible blended membranes have enhanced CO2 permeability.

  • Rigorous dual-sorption theory for blended membranes required to model permeability.

Abstract

Blending is a strategy to improve the permselectivity of gas separation membranes by combining the advantages of two or more polymers. The blending of polymers results in miscible or immiscible morphologies, which have distinctly different properties and performance benefits. Perfluoropolymers, specifically Teflon AF1600 and Hyflon AD60, have unique chemical properties that make them attractive as membranes for natural gas sweetening. Here, blended membranes of these two perfluoropolymers are generated, with miscible and immiscible morphologies achieved by varying the evaporation rate of the solvent. This ability to achieve two different blended morphologies is associated with the different solvability of the two perfluoropolymers in the solvent, not the miscibility of the polymers with each other. The miscible blended membranes had higher density than the pure polymers and CO2 sorption is bounded by that observed for pure Teflon AF1600 and Hyflon AD60 membranes, while CH4 sorption is depressed in the miscible blended membranes. Correspondingly, the CO2/CH4 selectivity of the miscible blended membranes was improved relative to the two pure polymer membranes. In contrast, the immiscible blended membranes had lower density and a more open morphology, especially the Teflon AF1600 discrete domains. This increased CO2 and CH4 permeability, compared to the pure polymer membranes, with comparable CO2/CH4 selectivity. To model the resulting performance, blended membrane mixing theories were applied; standard approximations based on the permeability of the pure polymer membranes and more rigorous modelling based on dual-sorption theory of the blended membranes sorption and diffusivity. The standard approximation models were unable to correlate with the experimental data for these blended membranes, while the dual-sorption based model was significantly more accurate. Therefore, to predict permselectivity of blended membranes performance it is more appropriate to employ the rigorous dual-sorption model.

Introduction

Amorphous perfluoropolymers are an important class of polymer because of their unique chemical properties, being insoluble in many organic solvents, high thermal and chemical stability as well as having a low tendency for swelling and plasticization in the presence of vapours [1,2]. Membranes based on these perfluoropolymers retain these properties which result in unique gas and vapour permeances, which arise from the very low surface energy of the polymer chain [3]. Teflon AF and Hyflon AD are two perfluoropolymers well reported in the literature for gas separation membranes [[4], [5], [6], [7]], where their amorphous nature is the product of bulky substituted dioxole moieties which disrupt polymer chain packing, limiting crystallinity, and generate high fractional free volumes [8]. Teflon AF based membranes have a higher CO2 permeability than Hyflon AD and Cytop based membranes, given the higher fractional free volume, while Hyflon AD and Cytop based perfluoropolymers have comparable CO2/CH4 selectivity [9]. However, all three perfluoropolymers are below the Robeson's upper bound for the CO2/CH4 gas pair and hence, it is their unique hydrocarbon and water vapour properties that make them attractive as gas separation membranes [10,11]. Particularly, hexane and toluene vapour had negligible impact on Teflon AF1600 and Hyflon AD60 based membranes in the separation of CO2/CH4 [12]. Furthermore, both membranes had extremely low water vapour permeance; which under certain conditions results in Teflon AF1600 having reverse selectivity for CO2 over water, a phenomenon not observed in any other polymeric membranes [5]. This means that perfluoropolymer based membranes can handle vapour rich natural gas feeds better than conventional membranes, reducing the pretreatment requirements. To improve the CO2/CH4 permselectivity performance of perfluoropolymer membranes, strategies are required that increase target gas permeance, while retaining or improving their selectivity. Conventional approaches to improve gas permeance, such as mixed matrix membranes, are challenging for perfluoropolymers because of their unique solvation and low surface energy prevent good compatibility between the additive and the polymer [13]. Alternatively, blending of polymers to fabricate the non-porous layer is an effective approach to improve permselectivity, as this enables synergy of the respective polymers' properties [14,15]. The blended nature of the membranes fabricated from multiple polymers is also attractive due to their ease of fabrication compared to alternative approaches such as novel copolymer synthesis and mixed matrix membranes [16,17]. Blended polymers are characterised by the miscibility of the polymers and the possibility of phase separation. Miscible blends represent a homogenous phase membrane, while phase separation immiscible blends result in membranes having distinct or intermingled phases present as well as interfaces between those phases. Both approaches have advantages in terms of membrane permselectivity and are applicable to perfluoropolymers given their unique solvation properties can influence miscibility of the blends.

Here, blended polymeric membranes based on the perfluoropolymers Teflon AF1600 and Hyflon AD60 are reported for their improved gas and vapour permselectivity. Importantly, the unique solubility properties of the perfluoropolymers were used to generate both miscible and immiscible morphologies which critically influence the gas separation properties. The gas permeability properties of the miscible and immiscible blends were further investigated through developments to dual-sorption theory for blended membranes, quantifying the changing morphology of the respective polymeric phases. These blended membrane morphologies highlight the advantages of perfluoropolymers, demonstrating that the polymers can be adapted into developing better performing membranes for natural gas processing.

Section snippets

Theory

The permeability (P) of gas A within the non-porous polymeric membrane can be described as a function of the diffusivity (D) and concentration of the gas/vapour (C) as a function of the change in fugacity (Δf) across a thickness (l) [18]:PA=D(CA)dCAdxlΔfA

The concentration gradient, dCA/dx, is assumed constant, due to steady-state operation, and if the fugacity of the permeate is zero, then the permeability is generally simplified to:PA=DA.SA=DA.(CAfA)where S is the solubility of gas A in the

Experimental

Amorphous Teflon AF1600 (copolymer of 65 mol% 2,2-bis-trifluoromethyl-4,5-difluoro-1,3-dioxole and 35 mol% tetrafluoroethylene) was purchased from DuPont (USA), while Hyflon AD60 (copolymer of 60 mol% 2,2-bis(trifluoromethyl)-4-fluoro-5-trifluoromethoxy-1,3-dioxole with 40 mol% tetrafluoroethylene) was purchased from Solvay (Japan). Chemical structures for the two polymers are provided in Fig. 1. Both polymers were used as supplied. Dense films were prepared for both perfluorinated polymers

Membrane morphology

The morphology of the blended perfluoropolymer membranes was strongly dependent on the solvent evaporation rate during casting, which are depicted in Fig. 2. Rapid solvent evaporation fabricated a uniform morphology with little structure and no evidence of phase separation, which is consistent with a blended membrane. In contrast, the slower evaporation clearly fabricated a morphology with two distinct domains which is consistent with phase separation and an immiscible blend. Hence, the rate of

Conclusion

The perfluoropolymers Teflon AF1600 and Hyflon AD60 were able to be fabricated as blended membranes for gas separation, particularly CO2 separation from CH4. Interestingly, the morphology of the blended membranes was strongly dependent on solvent evaporation rate, with a rapid rate producing a blended morphology while a slower evaporation enabled phase separation of the polymers, based on solubility, lead to an immiscible blend. The gas sorption characteristics of the blended membranes were

CRediT authorship contribution statement

Colin A. Scholes: responsible for all aspects of this manuscript, Conceptualization, Data curation, Formal analysis, Methodology, Project administration, Writing - original draft, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The author acknowledges the support of the Peter Cook Centre for Carbon Capture and Storage.

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