CO2/N2 separation factors

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.

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.

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. 

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. 

Other promising separation data. Porous BaTiOs membranes have been prepared on alpha-alumina supports. A CO2 to N2 separation factor of 1.2 at 500 C which is higher than the Knudsen diffusion prediction of 0.8 has been obtained. If there had not been pinholes on the order of 100 nm in the membranes, the separation factor would have been higher [Kusakabe and Morooka, 1994]. The maximum CO2 permeance through the BaTiOs membranes is very high at 1.1x10 cm (STP)/s-cm -cm Hg. 

Small-pore DDR membranes have been used [81] showing CO2/CH4 separation factors as high as 220 at 301 K. Again, DDR membranes have pores (0.36 x 0.44 mn) that are similar in size to CH4, and therefore, a configurational impediment to the flux of methane takes place in addition to the preferential adsorption of CO2. DD3R zeolite manbranes have also excellent separation performance for CO2/CH4 mixtures (selectivity 100-3000) and exhibit good selectivity for N2/CH4 (20-45), CO2 and NO/air (20 100), and air/BCr (5-10) and only a modest selectivity for O2/N2 (similar to 2) separation [120]. 

Silica membranes prepared by the sol-gel process, the pore size of which can be controlled by the colloidal size, have been applied for CO2/N2 separations [58] and olefin/paraffin separations [59]. For example, silica microporous membranes showed a separation factor of propylene over propane as high as 75 with an approximate permeance of 0.2 x 10 m m s kPa at 35°C. However, it was pointed out that silica membranes needs to be improved with respect to their stability in humid air, because microporous silica becomes densified, resulting in a decrease in permeance [56]. 

The authors compared separation factors they obtained with more established polymer separation membranes (cf. Table 25-11. They noted for instance that the best CO2/CH4 separation factor, for a fluorinated polyimide, of ca. 60, could not match that obtained with the P(ANi) membranes, ca. 336. Similarly, cellulose nitrate yields a separation factor for O2/N2 of ca. 16, die best known, as compared to 27 obtained with P(ANi). Similarly again, He/N2 and H2/N2 selectivities of 2200 and 313 for poly(trifluorochloroethylene) could not match those of P(ANi), respectively 4075 and 3590. These authors however did not appear to seriously address an important issue regarding free-standing CP membranes, viz. their physical and chemical durability and ease of handling, as compared to well established membranes such as those of the fluorinated polymers. 

The concept of mixed-matrix membranes has been demonstrated at UOP ° in the mid-1980s using silicalite-cellulose acetate mixed-matrix membranes for CO2/H2 separation. In the demonstration, a feed mixture of 50/50 CO2/H2 with a differential pressure of 50 psi was used. The calculated separation factor for CO2/H2 was found to be 5.15 + 2.2. In contrast, a CO2/H2 separation factor of 0.77 + 0.06 was found for cellulose acetate membrane. This indicates that silicalite in the membrane phase reversed the selectivity from H2 to CO2. Experimental results and modeling predictions indicate that mixed-matrix membranes with the incorporation of fillers within polymeric substrates provide potential possibilities to achieve enhanced membrane performance, which will open up new opportunities for the separation and purification processes. Highlighted applications for mixed-matrix membranes include separation and purification of gas mixtures such as separation of N2 from CO2 removal from natural and separation... 

The values of permeability coefficients for He, O2, N2, CO2, and CH4 in a variety of dense (isotropic) polymer membranes and the overall selectivities (ideal separation factors) of these membranes to the gas pairs He/N2,02/N2, and CO2/CH4 at 35°C have been tabulated in numerous reviews (Koros and Heliums, 1989 Koros, Fleming, and Jordan et al., 1988 Koros, Coleman, and Walker, 1992). Moreover, several useful predictive methods exist to allow estimation of gas permeation through polymers, based on their structural repeat units. The values of the permeability coefficients for a given gas in different polymers can vary by several orders of magnitude, depending on the nature of the gas. Thevalues oftheoverall selectivities vary by much less. Particularly noteworthy is the fact that the selectivity decreases with increasing permeability. This is the well-known inverse selectivity/permeability relationship of polymer membranes, which complicates the development of effective membranes for gas separations. 

Fig. 3. (a) permeate flux and separation factor in H2/CO2 mixture/MTES silica composite membrane system at 293K, (b) permeate rate of N2 in MTES silica composite membrane at 293K (c) predicted pressure profile of CO2 on TPABr silica membrane at 4atm... 

As for CO2, VOCs can also be removed by using immobilized liquid membranes. Obuskovic et al. [35] immobilized a thin layer of silicone oil in the microporous of the hollow fiber polypropylene membrane beneath the dense-coated skin. The performance of the system has been proved for toluene, methanol, and acetone removal from N2. With respect to the simple hollow fiber, the presence of the oil layer led to a 2-5 VOC more enriched permeate (due to the reduction of nitrogen flux) with a separation factor of 5-20 times higher (depending on the VOC and the feed gas flowrate). The membrane was stable for 2 years. 

The effect of the pressure, temperature and pore radius on the separation factor is investigated also by Eichinann and Werner [19] using Eq. (9.34) with a constant and experimentally determined value of P for eiU gas membrane combinations, in contrast to Wu et al. who fitted the value of p for each gas membrane combination. Figure 9.14 shows the effect of the pressme ratio Pj. for different mean pressure levels P (assuming a linear pressure drop in the membrane) on the separation factor of a N2/CO2 mixture (ideal separation factor equals 1.25) in a membrane with pore radius Rp = 0.03 pm. 

Fig. 9.14. Influence of pressure ratio P, on the separation factor of N2/CO2 mixtures. Pore radius is 0.03 (im. After Eickmann and Werner [18].

Fig. 9.15. Influence of pore radius r on Ihe separation factor of N2/CO2 mixture at a pressure of 2 bar.

Fig. 9.16. Influence of pressure level on the separation factor of a N2/CO2 mixture. Pore radius is 2.5...

The effect of applied CO2 partial pressure difference on the separation factor was studied by varying the values of average AP in the range of about 43 cm Hg to about 133 cm Hg. The results 2 of these experiments are reported in Table III (Set A). It is evident that the value of OCO2-N2 lowest (/ A3 cm Hg) used... 

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