Reverse selective NH 3 /CO 2 permeation in fluorinated polymers using membrane gas separation Camel Makhloufi, Denis Roizard, Eric Favre n LRGP-CNRS Nancy Université, 1, rue Grandville, 54001 Nancy, France article info Article history: Received 2 January 2013 Received in revised form 2 March 2013 Accepted 22 March 2013 Available online 4 April 2013 Keywords: Polymers Permeability NH 3 CO 2 Selectivity Membranes abstract The analysis of the process of gas and vapour permeation through dense polymers is of primary importance for packaging, controlled release and membrane separation processes. A significant amount of polymer permeability data has already been reported, especially for permanent gases, such as N 2 ,O 2 , H 2 , CH 4 and CO 2 , although a very small amount of data is available for ammonia (NH 3 ). In this case, the experimental results show the faster permeation of NH 3 in comparison to CO 2 , which is in agreement with the solution diffusion model predictions. NH 3 is smaller and more condensable than CO 2 . In this study, the solubility, diffusion coefficient and permeability of NH 3 , CO 2 and N 2 in ten different polymers were investigated between 5 and 50 1C. A reverse NH 3 /CO 2 permeation selectivity is occasionally observed for dense fluorinated polymers (PTFE, FEP, Hyflon AD and Teflon AF). This unusual behaviour is interpreted in the light of the diffusion and sorption coefficients obtained from time lag experiments which tend surprisingly to show in fluorinated polymers: a NH 3 /CO 2 solubility selectivity systematically lower than expected by the usual correlations and, in some cases, lower diffusion coefficients for NH 3 than for CO 2 . This peculiar result is interpreted from the differences between NH 3 and CO 2 interactions with fluorine atoms, similarly to the differential solubility phenomena of these two species which have previously been clearly established in fluorinated liquids. & 2013 Elsevier B.V. All rights reserved. 1. Introduction The analysis of the process of gas, vapour and liquid permea- tion in dense polymeric materials is a key issue for numerous industrial interests, such as packaging, controlled release or membrane separation processes [1]. A significant amount of data on permeation through various polymers already has been reported [2], and their interpretation can be classically achieved using the solution-diffusion model [3]. The solution-diffusion framework suggests that the overall permeability P of a given compound through a polymer results from the product of two main contributions, expressed thus: P ¼ S:D ð1Þ i) S is a thermodynamic contribution and corresponds to the solubility coefficient of the penetrant in the matrix. Generally, S increases when the condensation of the permeating species is favoured (i.e., for a higher boiling temperature). ii) D is a kinetic term, i.e., the diffusion coefficient of the compound in the polymeric matrix. D depends on the size of the penetrant. A smaller kinetic diameter of the species yields a higher diffusion coefficient. The prediction of the permeability of a given compound with respect to a defined polymeric matrix remains a challenge; never- theless, significant progress has been made in this direction, especially for gases such as He, H 2 ,N 2 , CH 4 and CO 2 . In some cases, the structure–permeation relationships are precisely under- stood for these compounds [4,5,6]. A combination of experimental data, theoretical approaches of S and D prediction [7] and/or molecular dynamics computations [8] is often essential to achieve that purpose. Surprisingly, the previous situation does not prevail for the classical gaseous compound ammonia [9] (NH 3 ), which corre- sponds to one of the largest tonnages of chemicals produced in the world. The experimental permeability data of NH 3 in polymers are lacking [2]. Additionally, few publications have addressed the theoretical problem of the mechanism of NH 3 permeation into dense polymeric matrices. A tentative inventory of the perme- ability data of NH 3 in polymers, as reported to date, is summarised in Table 1. Compared to the classical gases, for which an extensive amount of data is available, the experimental data are very limited for NH 3 . Nevertheless, the main types of dense polymers are covered, such as polyolefins, cellulosics, and rubbery and super glassy matrices. From a quantitative point of view, NH 3 is a typical Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science 0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.03.048 n Corresponding author. Tel.: +33 383 17 53 90; fax: +33 383 37 99 77. E-mail address: Eric.Favre@ensic.inpl-nancy.fr (E. Favre). Journal of Membrane Science 441 (2013) 63–72