Desalination 199 (2006) 401–402
Study of polymer-carbon mixed matrix membranes for CO2 separation from flue gas Samuel Bertelle, Tarakranjan Gupta, Denis Roizard*, Cécile Vallières, Eric Favre Laboratoire des Sciences du Génie Chimique (UPR 6811), Groupe ENSIC 1, rue Grandville - 54001 NANCY Cedex - France email: [email protected]
Received 27 October 2005; accepted 3 March 2006
2. Theory and experimental
There is a great concern about green house gases (GHG) impact on the global climate mitigation and numerous studies aim at the reduction of GHG emissions. Among targeted gases (VOC, CH4), carbon dioxide remains actually the main objective because its emission is strongly associated with the industrial development and with the energy production from fossil sources which, up to now, cannot be quickly substituted by any other sources except by nuclear energy. If they are many types of stream containing CO2, one can easily classify them into two main categories according to their pressure and temperature ranges that can be either high (P > 50 bar, t° > 200°C) or low (P < 2 bar, t° < 150°C); generally speaking these cases are known as pre-combustion or post-combustion. The work reported in this study is dedicated to the postcombustion case and polymer based membranes able to improve CO2/N2 capture from flue gas were studied from readily available glassy polymers.
Gas permeation through dense membranes is known to be due to the combined sorption (S) and diffusion (D) phenomena. Thus the permeation selectivity arises from the different abilities of gas molecules to more or less penetrate a given polymer network and diffuse through the statistical available free volume . Hence composite membranes made out of selective polymers and selected fillers should be able to lead to new membranes materials with enhanced properties. Indeed it is known from literature that fillers such as zeolites can improve the separation properties of polymer materials, provided that the correct zeolite and polymer are chosen. However, carbon fillers in a polymer matrix have not been widely studied. Thus the preparation of composite membranes from glassy polymers (cellulose derivatives) and carbon fillers were investigated to elaborate mixed matrix materials in the field of gas separation. Indeed there is a strong interest to improve the properties of known glassy polymers whose intrinsic performances remained still limited for the separation of gases having close kinetic diameters like N2, CO2, CH4 (resp. 3.64, 3.3, 3.81 Å).
Presented at EUROMEMBRANE 2006, 24–28 September 2006, Giardini Naxos, Italy. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.desal.2006.03.207
S. Bertelle et al. / Desalination 199 (2006) 401–402
To improve membrane separation properties, one can follow two distinct strategies, i.e. either to synthesize new polymers with specific chemical architectures  or to modify existing polymers with the incorporation of fillers . This paper reports the results obtained according to this last route with cellulose derivatives (CA: cell. acetate, CP: cell propionate) and various types of carbon fillers, going from graphite ones (SFG, non permeable particles) to molecular sieves (CMS5A) and active charcoal (Maxsorb, Norit). 3. Results and discussion Cellulose derivatives were selected as organic matrix because they combine very good film forming properties with fairly good ideal selectivity for CO2/N2 separation (a » 20), two features which are important to prepare selective defect-free mixed matrix membranes. The different types of carbon fillers were retained because they are supposed to be able to improve gas selectivity through distinct mechanisms in relation with their structure. For instance, graphite fillers are non permeable fillers, so they should increase the mixed matrix tortuous pattern and thus favour decrease the diffusion of the larger molecules. On the other hand, carbon molecular sieves are porous structures having well defined doors leading to gas separation by size sieving effect, while active charcoals are endowed with extremely high surface area which may induce surface flow selectivity . SEM characterizations of the fillers and composite flat membranes were recorded. Time-lag experiments were carried out with N2 and CO2 at 25°C to determine the ideal selectivity and understand the role of the filler. The results showed that the resulting permeabilities of the composite membranes varied a lot with
experimental parameters such as the type of filler, the size of particles used, and the filler weight content introduced into the polymer. However after optimization of the above parameters, it was possible to improve the separation properties especially with CMS and active charcoal. The permeation results were examined in the light of the Maxwell model . 4. Conclusions Several types of composite membranes were prepared out of cellulose derivatives and carbon fillers and it was found that the preparation of improved composite materials was strongly related to the polymer-filler contact and adhesion. After careful selection and conditioning of the fillers, mixed matrix membranes having filler contents varying from 10 to 60 wt% were successfully obtained. According to the type of filler used, different selectivity enhancements were recorded. Thanks to the Maxwell equation, it was shown from these results that the interphase between the polymer and the filler played a determinant role in composite membrane permeability. References  
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