Effects of casting and post casting annealing on xylene isomer transport properties of Torlon® 4000T films
Graphical abstract
Introduction
Xylene isomers (Fig. 1) are important chemical intermediates, but manufacturing pure isomers is difficult due to their similarity in physical properties [1], [2], [3], [4], [5], [6]. Paraxylene is particularly important as its derivatives are precursors for polyester fibers, films, and solid-state packaging resins. As a result, global demand for p-xylene is projected to increase by an average of 6% annually, driven mainly by teraphthalic acid (TPA) and polyethylene terephthalate (PET) expansions in China, other Asian countries, and the Middle East [7]. Traditionally, xylene isomer purification has required energy intensive methods such as cryogenic crystallization or pressure swing adsorption to obtain sufficiently pure streams of the desired component [4]. Increase in demands for both xylene production and its energy efficient purification method has attracted attention to polymeric membranes for many industrial separations ranging from natural gas purification to pervaporation of organic liquids, including xylene isomers [8].
Membranes for pervaporation separation operate in a more complex manner as compared to gas separation membranes [9]. Membrane materials for organic liquids separation can be classified into two areas; organic polymeric and inorganic materials. Xylene separation has been studied with polyurethane [10], [11], polyvinyl alcohol (PVA) [10], [12], and polyimides [10], [13], [14], but none of them have produced substantial selectivies at reasonable permeabilities. For this reason, researchers began exploring the notion of blending bulk polymer with additives. Most of this work has focused on organic materials such as cyclodextrin (CD) [12]. Although CD blending shows a significant para/ortho selectivity of 2.96, experiments were performed at low temperature (25 °C). Polyvinyl alcohol (PVA) has been the most commonly investigated matrix, and since PVA has a low glass transition temperature (∼77 °C), the membrane is not useful under a realistic industrial temperature above 150 °C. Further, with CD being comprised of several adjoined sugar rings, it is unclear as to the attainable thermal stability of the additive itself, and is unlikely to withstand more aggressive feed streams at higher temperature while maintaining membrane integrity. For these reasons, and the concern of thermal stability, these materials have questionable industrial utility.
With these concerns in mind, this work investigates the transport properties of xylene isomers in both casting and post casting annealed Torlon® 4000T membranes, which are robust and can be used up to 200 °C for extended period. The preferred membrane preparation technique was developed to ensure consistent xylene transport properties. Pervaporation results of xylene isomers are reported for both casting and post casting annealed Torlon® 4000T membranes.
Section snippets
Materials
This research focuses on an established polyamide-imide, Torlon® from Solvay Advanced Polymers© in Alpharetta, Georgia. The chemical structure is shown in Fig. 2, as provided by Robertson et al. [15] and Table 1 shows the basic properties of Torlon® 4000T as published by Solvay Advanced Polymers ® [16]. Torlon® is designed to be chemically resistant, thermally stable, and mechanically durable. Also, it is believed that a “densely packed” polymer structure (one with a lower free volume) is
Methods for Torlon® membrane optimization
Several authors have described methods to form solution processable films for gas separation [23]. Torlon® 4000T was processed in a similar manner to these conventional approaches. It was draw-cast into dense film membrane form, and vacuum dried at 200 °C over approximately 18hrs. The results shown in Table 2, indicate a significant variance in transport properties but suggest Torlon® is a more attractive candidate than previously published materials [10], [11], [12], [13], [14], [24], [25], [26]
Conclusions
Torlon® polyamide-imide has unique properties that are useful as a pervaporation membrane for para-xylene purification at high temperatures. As a result of this work, the following conclusions about Torlon® 4000T can be made.
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The resulting permeability and selectivity over the ortho and meta isomers for a Torlon® 4000T dense film are 0.25 Barrer, ∼3.0, and ∼2.0, respectively.
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During formation of Torlon® membranes, an annealing step to 300 °C was introduced that removes essentially all of the
Acknowledgement
The authors would like to acknowledge financial support from BP, The Coca Cola Company, and Award No. KUS-I1-011-21 made by King Abdullah University of Science and Technology (KAUST). Also, we would like to convey special thanks to Chuck Hoppin for his assistance in making the viscometry measurements.
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