Les techniques de séparation de gaz par membranes Gas Separation Techniques by Membranes

• Main Authors: Avrillon R., Deschamps A., Driancourt A., Mileo J. C., Robert E.
• Format: Article
• Language: English
• Published: EDP Sciences 2006-11-01
• Series: Oil & Gas Science and Technology
• Online Access: http://dx.doi.org/10.2516/ogst:1990032

Deux types de membranes peuvent être utilisés pour la séparation des gaz. Les unes sont poreuses et séparent les gaz sur la base de leur masse moléculaire selon un mécanisme de diffusion de Knudsen dans des micropores. Les facteurs de séparation obtenus sont généralement trop faibles pour présenter un intérêt industriel. Les autres, appelées membranes non poreuses, mettent en oeuvre un mécanisme de dissolution-diffusion des gaz dans une fine couche de polymère dénuée de toute porosité. C'est de ces dernières que nous allons parler. Commencé il y a une vingtaine d'années, le développement de ce type de membranes a conduit aux premières applications industrielles en 1979 avec l'introduction par Monsanto des séparateurs PRISM pour la récupération d'hydrogène à partir de différents gaz de raffinerie et de pétrochimie. Après des débuts modestes par suite de la compétition avec des technologies éprouvées comme la cryogénie, l'adsorption et l'absorption, la perméation gazeuse est en train de conquérir sa place parmi les techniques de séparation de gaz. Elle est aujourd'hui l'une des techniques membranaires présentant le plus fort taux de progression (30 % par an). Après un rappel des principes de base de la perméation gazeuse on abordera successivement les points suivants : - le choix du polymère constituant la couche séparatrice, - l'élaboration des membranes à structure asymétrique présentant une peau dense et fine, - les principales applications industrielles. Cela permettra de mettre en évidence les avantages et les limitations actuelles de la perméation gazeuse et de souligner les différents domaines où les efforts de R et D peuvent amener des progrès significatifs. <br> Principle of Gas permeation - Gas permeation is a technique for fractionating gas mixtures by using nonporous polymer membranes having a selective permeability to gas according to a dissolution-diffusion mechanism. Gas is made to pass through the membrane by applying a pressure difference on either side of the membrane. This pressure difference causes a difference in dissolved gas concentration between the two faces of the membrane and hence a diffusional gas flow through the membrane. Choice of Polymer - The gas sorption capacity of the polymer depends on its free volume and its physical affinity for the gas. For a gas mixture, differences in affinity are selectivity factors. The mobility of sorbed molecules depends on the free volume of the polymer and on the degree of rigidity of the chains. Vitreous polymers are more selective in this respect than rubbery polymers. This is why they are generally chosen for manufacturing gas permeation membranes. Permeability and selectivity are somewhat antinomic properties, especially when the selectivity is of a diffusional type, i. e. when the polymer acts as a molecular sieve. The definition of new polymers providing a better compromise between permeability and selectivity thus goes via research on the relationships between structure and the permeability to the gas to be separated. In polymers in a vitreous state, the chains are fixed overall, but small local movements, for example such as the rotation of an aromatic nucleus around bonds in the para position, remain possible. It seems that such small movements are beneficial for permeability, while not detracting very much from the selectivity. Structures of chains that are unfavorable for compact piling increase permeability in general to the detriment of selectivity. This is true for chains having non-coplanar aromatic nuclei or ones having bulky groups. Tables 2, 3 and 4 give some structural and permeability data for various polyimides. Figures 4 and 5 show the performances for hydrogen/ methane and carbon-dioxide/methane separations of various polyimides synthesized in our laboratories. Asymmetric MembranesThe membranes used in practice have a particular structure that is called asymmetric, which combines high permeability and good mechanical strength. This structure has a thin dense and selective skin (0. 1 to 1 µm thick) supported by a thick microporous substructure (50 to 200 µm). Such membranes come either in a flat shape or in the form of hollow fibers with their skin outside. The asymmetric structure is obtained by the so-called phase inversiontechnique, which consists in transforming a homogeneous polymer solution into a two-phase medium made up of a polymer-rich phase and a polymer-poor phase. The continuous rich phase prefigures the pore walls of the substructure. Once the poor phase becomes continuous, it will make up a network of communicating pores. Phase inversion can be caused in several ways: (a) solvent departure by evaporation (dry process) (b) introduction of a nonsolvent (wet process) (c) dry-wet process (d) temperature reduction (thermal process). The dense skin is formed on the side where evaporation takes place or where contact is made with the nonsolvent, or again on the cooled side (with the other side being in contact with the support for a flat membrane and a more or less coagulating liquid with a hollow fiber). A dense skin is formed by the superficial polymer overconcentration resulting from solvent evaporation or from its extraction by the nonsolvent before phase inversion. Figure 6 shows a ternary polymer-solvent-nonsolvent isothermal phase diagram on which arrows indicate how the homogeneous polymer solution (I) evolves toward a liquidliquid phase separation (II) or toward a gel structure (III). Industrial Development -Industrial permeators have large membrane surface areas in a compact form. These areas can be up to 500 m²/m³ for the flat version and up to 8000 m²/m³ for the hollow-fiber version. This compactness is obtained by the spiral winding of flat membranes or by the grouping of hollow fibers in bundles. Fig. 7 shows both types of permeators. The advantages of gas permeation lie in the small investment required, low energy consumption, great flexibility of implementation because of the modular nature and the possibility of the great automation of permeators, and the moderate size and weight of the installation. Limitations are linked to the difficulty in obtaining both good purity and high yield for a product with a membrane surface area that is economically acceptable. Likewise, the pressure drop undergone by the permeate may be a disadvantage. Gas permeation can be associated with other separation techniques such as pressure swing adsorption, freeze drying or absorption. The principal applications of gas permeation at present are as follows: (a) Hydrogen recovery from drain effluents from ammonia synthesis units in which hydrogen has to be separated from nitrogen, methane and argon. (b) Hydrogen recovery from refinery gas in which the hydrogen is mixed with hydrocarbons, hydrogen sulfide and water vapor. (c) Adjustment of the hydrogen/carbon-monoxide ratio of alcohol synthesis gas. (d) Air fractionating from the production of blanketing nitrogen. (e) Natural-gas dehydration and sweetening with a view to its transportation by pipeline, especially in offshore production. Table 5 lists the leading membrane suppliers and their fields of activity.