Gas Separation by Polyimide Membranes

Separation of a gas from the gas mixture is a key issue in the various industrial fields hydrogen recovery in petroleum refinery process, oxygen removal to prevent flame, air dehumidification to prevent moisture absorption, dehydration of fine chemical products, for example. Although there are various methods to perform gas separation [membrane separation, pressure swing adsorption (PSA) separation, cryogenic separation], the method using membranes attracts much attention. 

The gas separation membranes are comprised of either inorganic materials or organic materials (polymers). This chapter discusses the polymeric membranes, and the inorganic membranes are detailed elsewhere. 

The gas separation membrane was first commercialized in 1980 in order to recover hydrogen gas produced in the oil refining process. This membrane was a hollow-fiber membrane comprised of polysulfone. The shell of the hollow fiber exhibited an asymmetric cross-sectional structure, which showed the porous structure with pore size varying from one surface to the other surface of the membrane. The asymmetric structure is most preferred for the gas separation membranes due to several reasons described later. A certain number of the hollow-fiber membranes were integrated into a bundle, and the bundle of the hollow-fiber membrane was installed into the piping unit (module) and shipped to the user. 

Many types of polymers (polyolefins, polyimides, polysulfones, cellulosics, polycarbonates, etc.) have been explored for fabricating the practical gas separation membranes. In these polymeric membranes, polyimide membranes are the most fascinating due to their excellent properties of  

Advanced Membrane Technology and Applications. W. S. Winston Ho, and T. Matsuura Copyright 2008 John Wiley Sons, Inc. 

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During the last years increasing interest on noble gas separation by membranes is being noticed [170-172]. Hollow fiber membranes from polyimide [171], flat sheet membranes from PET or oriented polypropylene [172] were applied in the tests. 

Kharitonov et al. [59] have shown that direct fluorination of the polyimide Matrimid is possible, hence the resulting membrane should have a nice potential for use in harsh environment. Perfluorinated materials were also studied by Hagg [60] for chlorine gas purification, and were shown to be exceptionally stable in these harsh environments. The selectivity was however too low. In a later publication on chlorine purification [31] it was suggested to use perfluorinated monomers as surface-modifying compounds for pore tailoring of glass membranes for chlorine gas separation. 

During the past three decades, since the commercialization of Kapton polyimide, an impressive variety of polyimides have been synthesized [258, 259]. Polyimides possess outstanding key properties, such as thermooxidative stability [260], high mechanical strength [261], high modules, excellent electrical [262] and optical properties [263, 264], and superior chemical resistance [265]. Recently polyimides have also been applied as membranes for gas separation [266, 267]. Approximately 15 years ago the direct structuring or laser ablation of polyimides by excimer lasers was first described [73, 130]. 

Du Pom, a leader in reverse osmosis technology built aronnd a unique class of tailored aromeik polyamides, was also an early leeder in the gas separation field.27,1 14,16 Molecuiariy engineered arometic polyimides were found by Du Pont to provide extraordinarily good flux and selectivity properties For hydrogen separations.27 Posttreataiem processes for these membranes were not reported. 

Porous membranes can be made of polymers (polysulfones, polyacrylonitrile, polypropylene, silicones, perfluoropolymers, polyimides, polyamides, etc.), ceramics (alumina, silica, titania, zirconia, zeolites, etc.) or microporous carbons. Dense organic membranes are commonly used for molecular-scale separations involving gas and vapor mixtures, whereas the mean pore sizes of porous membranes is chosen considering the size of the species to be separated. Current membrane processes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), gas and vapor separation (GS), and pervaporation (PV). Figure 1 indicates the types and sizes of species typically separated by these different separation processes. 

Cellulose acetate, polysulfone and polyimides are by far the most important polymers for gas-separation membranes. When we look at the volume streams... 

Already in 1989, Spillman reported a first comparison of three different separation technologies in H2 recovery from refinery off-gas, by considering polyimide membranes (Table 19.6). This comparison is here provided by the... 

Plasticization behaviour induced by condensable gases and vapours (e.g. carbon dioxide, hydrocarbons and other organic vapours) in polymer membranes is stiQ a painful problem in polymeric membrane-based gas separation applications [27,28]. Recently, novel hyperbranched polyimides were prepared from telechelic polyimides and an attempt was made to improve its gas separation performance and physical stability by obtaining plasticization-resistant materials [29-33] (see e.g. Chapters 4, 6 and 7 of this book). 

Figure 11.15 The effect of an addition of hexane to a mixture of methane (90%) and carbon dioxide (10%) at a feed pressure of 1025kPag. (a) CO2 permeability, (b) CH4 permeability, (c) CO2/CH4 selectivity. Reprinted with permission from Ind Eng Chem Res., The effect of hydrocarbons on the separation of carbon dioxide from methane through a polyimide gas separation membrane by Hasan, R., C. A. Scholes, C. W. Stevens and S. E. Kentish, 48, 5415-5419, Copyright (2009) American Chemical Society...

Table 14.5 summarizes the data for H2 recovery from refinery off-gas by means of three different separation technologies, as reported by Spillman [80], considering polyimide membranes. A comparison among them is provided by the calculation of the relative three indicators. A lower investment cost than pressure swing adsorption (PSA) or cryogenic separation was estimated for the H2 recovery from refinery off-gas by polymeric membranes. This comparison is from 1989 however, since then polymeric membrane capital prices have dropped. 

In EP07708077A3 (Dabou et al. 1996), gas separation polymer membranes were prepared from mixtures of a polysulfone, Udel P-1700 and an aromatic polyimide, Matrimid 5218. The two polymers were proven to be completely miscible as confirmed by optical microscopy, glass transition temperature values and spectroscopy analysis of the prepared mixtures. This complete miscibility allowed for the preparation of both symmetric and asymmetric blend membranes in any proportion from 1 to 99 wt% of polysulfone and polyimide. The blend membranes showed significant permeability improvements, compared to the pure polyimides, with a minor change in the selectivity. Blend membranes were also considerably more resistant to plasticization compared with pure polyimides. This work showed the use of polysulfone-polyimide polymer blends for the preparation of gas separation membranes for applications in the separation of industrial gases. 

Polyimides are the most studied polymeric materials for membrane-based gas separation application. Although aromatic polyimides have received much attention as gas separation membrane materials, the polyimide family encompasses a large number of structural variants, and many studies on polyimide membranes indicate that separation properties can be tailored by using different dianhydride and diamine monomers. It was reported in the late 1980s that... 

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