Improvement of CO2/CH4 separation characteristics of polyimides by chemical crosslinking
Introduction
Gas separation by polymer membranes is a proven technology that has found a wide range of applications. Flexible design, the compactness and the efficiency of membrane units compared to conventional gas separation methods like cryogenic distillation or absorption are particulary attractive 1, 2, 3. In the area of CO2/CH4 separation with membranes, the removal of CO2 in landfill gas recovery processes and CO2 removal from fractured wells as well as the removal of CO2 in enhanced oil recovery applications (EOR) are of interest. In some of these separation applications, the membranes are exposed to high CO2 concentrations in the feed stream. The resulting strong interactions between the CO2 and the polymer material often affects flux and permselectivity properties. In extreme cases, the gaseous feed stream can be composed of over 50% CO2 at an elevated temperature and up to a feed pressure of 60 atm. These extreme operation conditions result in the swelling and plasticization of most membrane materials by the CO2 present in the feed stream. For the application of membrane based gas separation in EOR processes, it is important to develop new membrane materials with reduced plasticization effects due to high CO2 partial pressure. This study was undertaken to determine if swelling and plasticization of the polymer structure can be achieved by crosslinking.
One method for crosslinking membrane materials is the UV-irradiation of benzophenone-containing polyimides [4]or bisphenol A based polyacrylates [5]. This leads, especially in the separation of gas pairs with a large difference in molecular size, to a significant improvement of permselectivity. But simultaneously the permeability is decreased by crosslinking due to the strongly reduced chain mobility and to the increased packing density of the polymer chains. A disadvantage of photochemical crosslinked membrane materials is that the degree of crosslinking depends strongly on experimental conditions, i.e. irradiation time or the type of mercury lamp. Moreover, under the experimental conditions of UV-irradiation crosslinking reaction as well as photo-fries rearrangements are possible [5].
For this study simultaneous inhibition of chain mobility and intrasegmental packing by changes in backbone structure was of interest. Therefore, polyimides with strong polar associating functional groups as well as chemical crosslinked polyimides and copolyimides were synthesized. Permselectivity and permeability properties for the CO2/CH4 separation were investigated. 6FDA-based polyimides were used, because it is well known that introduction of –C(CF3)2– linkages restricts the torsional motion of neighboring phenyl rings and tends to enhance the permselectivity for the CO2/CH4 system 6, 7, 8, 9, 10. With different 6FDA-based polyimides and copolyimides the influence of strong polar carboxylic acid groups in polyimides was investigated. The effects of such carboxylic acid groups, expected to lead to pseudo-crosslinked polymers due to hydrogen bondings, was investigated for CO2/CH4 separation. The permeation and separation properties of pseudo-crosslinked and truly chemical crosslinked polyimides, obtained by treatment of carboxylic acids containing polyimides with ethylene glycol, were compared. Also the influence of crosslinking degree on swelling and plasticization effects due to CO2 was studied up to 41 atm pure CO2 pressure. For all synthesized polyimides and copolyimides, permeability and permselectivity were investigated for the common gas pairs O2/N2 and CO2/CH4 using pure gas feeds. Mixed gas experiments were also performed using 50:50 CO2/CH4 feed mixtures.
Section snippets
Background and theory
In one-dimensional diffusion through a flat membrane, the local flux N of a penetrant can be described by Fick's law as shown in the following equation:where the diffusion coefficient, D, may be a function of local concentration, C [11]. The permeability coefficient P is generally defined as the flux N normalized by pressure and membrane thickness
Thereby, p2 and p1 are upstream and downstream pressures of the penetrants and δ is the membrane thickness. Permeation is a
Materials
Fig. 1 shows the chemical structures of the 6FDA-based crosslinked and uncrosslinked polyimides and copolyimides synthesized. It was also possible to synthesize the 6FDA–DABA polyimide with free carboxylic acid groups in every polyimide unit, but the obtained membrane films were too brittle for the investigation of gas permeation properties. By crosslinking of the 6FDA–DABA with ethylene glycol, better mechanical strength was reached, so that permeation experiments were possible.
Characterization
The structures
Pure gas permeation experiments
Table 2 shows the zeolite sieving diameter for CO2, CH4, O2 and N2 [14]. Table 3 shows the pure gas permeabilities and ideal CO2/CH4 and O2/N2 selectivities for the reference substance 6FDA–mPD, the crosslinked and uncrosslinked 6FDA–mPD/DABA 9:1 and the crosslinked 6FDA–DABA at 3.74 atm and 308 K. For the 6FDA–mPD polyimide used here as a reference substance, ideal CO2/CH4 selectivity calculated from pure gas permeability measurements at 308 K and 3.74 atm feed pressure is 58 with a CO2
Conclusion
Incorporation of polar groups (–COOH) in polyimide structures leads to a strongly reduced permeability. Plasticization due to CO2 can be reduced, but not eliminated in the presence of such “virtual” hydrogen bonded crosslinks.
By true chemical crosslinking of –COOH groups with ethylene glycol polyimides or copolyimides were formed that showed no plasticization up to 35 atm pure CO2 pressure. Even with a 10% degree of crosslinking, a 20% increased selectivity was seen compared to the reference
Acknowledgements
This research has been supported by the German Research Foundation (DFG) at Bonn, Germany, the US Department of Energy and the Separation Research Program at the University of Texas at Austin.
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