Fluid purification is an obligatory step for several industrial processes. For example, gas purification typically involves removal of water, carbon dioxide, or other unwanted gases that may interfere with the end use of the purified gas. Industrial gases that need to be purified before use include air, nitrogen, helium, argon, hydrogen, oxygen, and hydrocarbons.
Industrial gases also require careful purification before being released into the atmosphere. The most common contaminants present in these industrial gases are carbon dioxide, sulfur dioxide and trioxide, nitrogen oxides, hydrogen sulfide and small organic molecules. Removal of these impurities is important to reduce environmental pollution and minimize climate change. The most commonly used processes to purify gases on an industrial scale are liquid scrubbers (where a basic or acidic solution is used to absorb an acidic or basic gas, respectively), exchange resins (where immobilized bases or acids are used to absorb an acidic or basic gas, respectively), or membranes (which separate gases based on competitive adsorption, differences in diffusion rates, molecular discrimination, and/or sieving).
High purity solvents such as alcohols are commonly used in the food and pharmaceutical industry and other industries however, the energy consumption of traditional solvent purification techniques and stringent industrial requirements offsets the environmental benefits of recycling them. Polymer membranes are low carbon footprint solutions but their unstable and often insufficient separation performance has generally precluded them from solvent purification.
Separation membranes are likely to play an increasingly important role in reducing the environmental impact and the costs of industrial processes, because their use generates minimal amount of byproducts and has low energy footprint (Baker, 2002, Ind. & Eng. Chem. Res. 41 (6): 1393-141 1; oros, 2004, AlChE J. 50(10):2326-2334; Noble & Agrawal, 2005, Ind. & Eng. Chem. Res. 44(9):2887-2892). Commercially important gas separations include H2 purification from light gases related to coal gasification, and CO2 removal from hydrocarbons in natural gas processing. Dense membranes can separate gas mixtures based on competitive adsorption and/or differences in diffusion rates, whereas porous membranes can separate gas mixtures via molecular discrimination or sieving (Wijmans & Baker, 1995, J. Membr. Sci. 107(1-2): 1-21; Gin et al., 2008, Macromol. Rapid Comm. 29(5):367-389).
Certain organic polymers have been found to be particularly suitable for producing separation membranes on an industrial scale. Gas permeability in such polymer membranes is dominated by the diffusivity of the gas species throughout the polymer network. As the diffusivity is related to the mobility of gas molecules within the polymer, the differential transportation of gas species throughout a polymer membrane is believed to be dictated by two key parameters. These are (1) the accessible “free volume” of the polymer, and (2) the particular configuration of the pores and channels contributing to that free volume throughout the polymer mass, i.e. the “free-volume distribution”.
A material with a high glass transition temperature (Tg), high melting point, and high crystallinity is preferred for most gas separations. Glassy polymers (i.e., polymers below their Tg) have stiffer polymer backbones than other polymers and therefore allow smaller molecules such as hydrogen and helium to permeate the membrane more quickly and larger molecules such as hydrocarbons permeate the membrane more slowly. Super-glassy polymers have a rigid structure, high glass transition temperatures, typically above 100° C.degree. C., 200° C. or higher, and have unusually high free volume within the polymer material. These materials have been found to exhibit anomalous behaviour for glassy polymers, in that they preferentially permeate larger, more condensable, organic molecules over smaller inorganic or less condensable organic molecules. Use of such polymers to separate condensable components from lower-boiling, less condensable components is described in U.S. Pat. No. 5,281,255, for example.
While glassy polymers are initially very porous, and ultra-permeable, they quickly pack into a denser phase becoming less porous and permeable. Polymer physical aging is a well-studied process where the convergence of glassy polymer chains collapses the inter-chain free volume, also known as fractional free volume (FFV) content, required for molecular transport via diffusion and adsorption. Other than physical aging, separation performances of polymer membranes can also deteriorate due to membrane compaction, where high pressures compress polymer chains and collapse molecular transportation pathways. In a commercial setting, membranes on polymeric substrates are generally allowed to age and compact to achieve steady-state permeance for continuous operation. This approach sacrifices the initial tantalising membrane performance for stability.
On the other hand, the free-volume distribution relates to how the free volume is arranged spatially within the polymer, by way of interconnected porosity and channels. It is the free volume distribution that is of interest in understanding the mechanisms underlying the separation of fluid mixtures, since its configuration will dictate which molecules filters through the polymer and which molecules may remain adsorbed on the surface of the free volume pockets. While two polymers may have the same total free volume, they may have vastly differing transport properties based upon a different free volume distribution.
From a thermodynamic point of view, the molecular arrangement of polymer chains giving rise to a detectable free volume is one of non-equilibrium. As a result, such polymers tend to evolve into lower and more stable energy states over time. Consequently, the corresponding free volume tends to correspondingly collapse and diminish. This process is commonly referred to as “relaxation” or “physical ageing” of the polymer. In the context of separation membranes, this phenomenon can dramatically affect the available free volume and free volume distribution for gas separation purposes. Indeed, a common problem affecting the performance of separation membranes is their reduced capability to maintain their permeability characteristics over time due to such physical ageing effects causing a dramatic reduction of the available free volume.
This degradation in properties hampers the use of glassy polymers in industrial applications. Several approaches have been explored to increase or stabilize the initially high gas permeabilities of glassy polymers such as PTMSP. These include using physical blends, polymer cross-linking, copolymer synthesis and functionalization. International Patent Publication WO 2014/078914 and Lau et al. (Angew Chem 2014, 126, 5426-5430) describes the addition of PAF-1 to super glassy polymers to reduce or eliminate the effects of aging. While the use of PAFs is effective, their preparation and use significantly increases the complexity and cost of the microporous composition.
An opportunity therefore remains to develop new polymer compositions suitable for use as separation membranes that exhibit improved permeability properties such as an extended period of time over which permeability is maintained (i.e. membranes that show reduced or no aging effects). Membranes prepared with such compositions should be useful for separation processes, including but not limited to gas-phase separations and gas-liquid separations.