CO2/N2 separation

Hydrogel membranes are particularly attractive because of high permeability and separation factor [300], and good stability for CO2/N2 separation [299], PVDF hollow fiber membrane modified by alkali was coated by PYA hydrogel on its surface and PVDF-PVA hydrogel membranes show better hydrophilic performance. For carbonate hydrogel (sodium carbonate concentration of 3.7 %) membrane, C02, concentration from 1.3 % to 0.6 % in feed gas could be decreased to 0.9-0.4 % at the outlet at 25 °C. 

The chemical composihons of the zeolites such as Si/Al ratio and the type of cation can significantly affect the performance of the zeolite/polymer mixed-matrix membranes. MiUer and coworkers discovered that low silica-to-alumina molar ratio non-zeolitic smaU-pore molecular sieves could be properly dispersed within a continuous polymer phase to form a mixed-matrix membrane without defects. The resulting mixed-matrix membranes exhibited more than 10% increase in selectivity relative to the corresponding pure polymer membranes for CO2/CH4, O2/N2 and CO2/N2 separations [48]. Recently, Li and coworkers proposed a new ion exchange treatment approach to change the physical and chemical adsorption properties of the penetrants in the zeolites that are used as the dispersed phase in the mixed-matrix membranes [56]. It was demonstrated that mixed-matrix membranes prepared from the AgA or CuA zeolite and polyethersulfone showed increased CO2/CH4 selectivity compared to the neat polyethersulfone membrane. They proposed that the selectivity enhancement is due to the reversible reaction between CO2 and the noble metal ions in zeolite A and the formation of a 7i-bonded complex. 

Figure 7.11 Dual-membrane separation utilizing a highly selective metal or inorganic membrane for H2 purification and a conventional polymer membrane for the CO2/N2 separation.

This paper describes pure N2 permeation and CO2-N2 separation characteristics of ILM-s of pure water and aqueous 302 wt/wt K2CO3 solutions Immobilized in Celgard X-10 hollow fibers. Measurements were carried out over a wide range of applied pressure differences. The CO2 partial pressure difference was varied from about 40 cm Hg to 140 cm Hg while the N2 partial pressure difference was Increased from about 125 cm Hg to 425 cm Hg. The total applied pressure difference was varied between 140 to 550 cm Hg. Facilitated transport membranes of aqueous 3UZ wt/wt K2CO3 solution for the separation of CO2 from were utilized and the separation behaviors... 

The effect of different counter-ions (K, Rb and Cs) in the supported NaY zeolite membranes on CO2/N2 separations has also been studied. For an equimolar mixture the selectivity is increased in the order Rb > K > Cs > Na due to an increase of CO2/N2 sorption selectivity. In Table 17.6 the sorption, diffusion and overall permeation selectivity are reported for ion exchanged membranes.The sorption selectivity always increased for single and binary systems, however, the increase is very pronounced with the binary system and a zeolite membrane loaded with Rb ions shows the highest selectivity. 

MFl has been extensively studied in zeolite membranes preparation due to its pore size suitable for several industrially important separations." " Using MFl supported membranes it was demonstrated that the CO2/N2 separation factor increases with CO2 feed composition because of the higher CO2 adsorption on the zeolite wall, which consequently limits the N2 transport in zeolitic channels." The selectivity of this gas species reaehes the value of 20 at 180 °C when the carbon dioxide composition is higher than 60% in the feed. Other researchers using membranes with the same topology found the same effect of the CO2 feed concentration on its separation from nitrogen." ... 

Figure 17.7 Gas permeance and CO2/N2 separation factor as a function of the temperature for equimolar CO2-N2 mixture in dry and moist conditions. Reprinted from X. Gu, J. Dong and T. M. Nenoff, Synthesis of defect free FAU-type zeolite membranes and separation for dry and moist CO2/ N2 mixtures, Industrial and Engineering Chemistry Research, 44, 937-944, 2005, with permission from ACS.

Figure 17.12 Revised Robeson upper bound for CO2/N2. Reprinted from The upper bound revisited , Journal of Membrane Science, 320, 390-400,2008, with permission from Elsevier. Data point of SAPO-34 membrane is also shown for comparison. Reprinted from S. Li and C. Q. Fan, High flux SAPO-34 membrane for CO2/N2 separation. Industrial and Engineering Chemistry Research, 49, 4399-4404, 2010, with permission from ACS.

In summary, porous carbon-based materials for CO2 capture have experienced rapid development in the last several decades and will continue to blossom. The requirements of CO2 captures vary a lot depending on different processes, namely post-combustion (low pressure, predominantly CO2/N2 separation), pre-combustion (high pressure, predominantly CO2/H2 separation) capture and natural gas sweetening (predominantly CO2/CH4 separation). Thus, various kinds of new carbon materials with defined textural properties as well as tailored surface chemistry have been synthesized for a specific CO2 capture process. Another advantage lies... 

Until now, the reports of adsorptive separation of CO2 and other gases in a mixture by using MOFs, conducted by experimental separation process, are limited. Among various characterization methods in separation, the breakthrough experiment and gas chromatographic separation are simple and straightforward in the evaluation of the separation performance of a MOF toward a gas mixture. On the other hand, as previously mentioned, reported CO2 separation in MOFs mainly includes CO2/N2 separation for post-combustion capture, CO2/H2 from synthesis gas for pre-combustion capture, and O2/N2 and CO2/CO separation for oxy-combustion capture, which will be detailed as following. 

Functionalized Inorganic Membranes for High-Temperature CO2/N2 Separation... 

Amines have long been used as reactive liquid solvents in CO2 absorption columns [3, 22], The goal of the study presented in this chapter is to synthesize surface-modified inorganic membranes using an amine compound, 3-aminopro-pyltriethoxysilane (APTS), for CO2/N2 separation. 

Figures 7.13 and 7.14 show the data for the CO2/N2 separation factor and the CO2 permeance for the ALD-APTS-modified manbrane along with results for APTS membrane (without ALD the modification was done on a 5 nm Vycor substrate). As can be seen from these Figs, there is no hysteresis between the three conditions with respect to the separation factor. As far as the permeance is concerned, there is a bit of hysteresis between condition (a) and conditions (b) and (c). Also overaU, the separation characteristics of the ALD-APTS manbrane are better by about 30 % than those of APTS membrane, which is probably due to the smaller starting pore size of the substrate. Thus, we can conclude that a reduction in the pore size of the starting material can gready influence the separation characteristics.

Ostwal M et al (2011) 3-Aminopropyltriethoxysiltme functionalized inorganic membranes for high temperature CO2/N2 separation. J Membr Sci 369(1-2) 139-147... 

Poly(phenylene oxide) (Kumazawa and Yoshida 2000), PMMA (Yamamoto et al. 2003), and PES (Iwa et al. 2004) membranes were treated with NH3 plasma and the gas separation properties of the modified membranes were examined. The treatment resulted in an increase in the CO2/N2 separation factor as well as the permeability to CO2. 

Xiao, S., Feng, X. and Huang, R.Y.M. 2007. Trimesoyl chloride crosslinked chitosan membranes for CO2/N2 separation and pervaporation dehydration of isopropanol,... 

Temperature has a pronounced effect on the performance of SILMs, as demonstrated by the studies of Jindaratsamee et al. in 2011 [78]. The authors investigated several ILs impregnated onto microporous PVDF for CO2/N2 separation between 30 and 70 °C. As shown in Figure 21.15, aU SILM materials exhibited the highest selectivity at the lowest temperature. 

Figure 7.15 CO2/N2 separation performance for PEBAX 2533-zeolite imidazolium frameworks (ZIF)-8 mixed matrix membrane for pure and mixed gases (CO2 10 vol%, N2 90vol%, p=2.6bar, T=25°C).

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