CO2 permeability

Some small-pore zeolite and molecular sieve membranes, such as zeolite T (0.41 nm pore diameter), DDR (0.36 x 0.44nm) and SAPO-34 (0.38nm), have been prepared recenhy [15-21]. These membranes possess pores that are similar in size to CH4 but larger than CO2 and have high CO2/CH4 selechvihes due to a molecular sieving mechanism. For example, a DDR-type zeolite membrane shows much higher CO2 permeability and CO2/CH4 selechvity compared to polymer membranes [15-17]. SAPO-34 molecular sieve membranes show improved selechvity for separation of certain gas mixtures, including mixtures of CO2 and CH4 [18-21]. 

Zeolite/polymer mixed-matrix membranes prepared from crosslinked polymers and surface-modified zeolite particles offered both outstanding separation properties and swelling resistance for some gas and vapor separations such as purification of natural gas. Hillock and coworkers reported that crosslinked mixed-matrix membranes prepared from modified SSZ-13 zeolite and 1,3-propane diol crosslinked polyimide (6FDA-DAM-DABA) synthesized from 2,2 -feis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, p-dimethylaminobenzylamine-and 3,5-diaminobenzoic acid displayed high CO2/CH4 selectivities of up to 47 Barrer and CO2 permeabilities of up to 89 Barrer under mixed gas testing conditions [71]. Additionally, these crosslinked mixed-matrix membranes were resistant to CO2 plasticization up to 450 psia (3100kPa). 

New developments for PET beer containers are overcoming problems with O2 and CO2 permeability. 

FIGURE 8 Literature data for CO2/CH4 separation factor vs CO2 permeability. 

Figure 9.1 Trade-off curve between the ideal separation factor of CO2/CH4 and the CO2 permeability [Koros and Fleming, 1993]...

Figure 18 Pressure dependence of the CO2 permeability through a microporous glass membrane. Experimental results are compared to theory of combined gaseous and surface flow. (I Barrer = 3.35 X 10" mol m m Pa -sec". ) (Adapted from Ref. 38.)...

Robeson [4] showed that there exists a trade-off relationship between selectivity and permeability for dense polymer membranes. This plot was later updated by Singh and Koros [9] (see Figure 4.1). Molecular transport of light gases in such membranes typically occurs by a solution diffusion mechanism (as discussed in Section 4.2.1). For a polymer membrane to be commercially considered for the removal of CO2 from H2, CH4, or air, both the CO2 permeability and selectivity must be competitively high. Since the gases in the mixture with CO2 often are smaller (H2) or about the same size as CO2, they may diffuse more rapidly through the polymers, and it follows that the diffusion selectivity will be <1. The only way... 

The second generation of nonporous membranes was silicon based which displayed increased CO2 permeabilities. In 1965, Bramson et al. commercialized the first nonporous membrane BO [18]. Since the diffusion coefficient of oxygen and carbon dioxide in air is about four orders of magnitude higher than in blood, the gas side mass-transfer resistance was negligible. The major resistance to respiratory gas transfer was due to the membrane and the liquid side concentration boundary layer [19]. Though nonporous membrane BOs reduced blood damage, up to 5.5 m membrane surface area was often required to ensure adequate gas transfer rates. 

Figure 15.22. CO2 permeability of cured polyimide films containing CH dCH-, )j-NlL montmorillonite vs. volume fraction of filler. [Data from Lan T, Kaviratna D, Pinnavaia T J, Chem. of Mat., 6, No.5, 1994, 573-5. ...

Examination of Figures 2 and 3 supports the previous observation that low permeability and high selectivity are generally related. For example, the CO2 permeability of poly(phenylene oxide) (PPO) is over twelve times higher than that of polysulfone, but its CO2/CH4 selectivity is less than half that of polysulfone. Kapton, on the other hand, is over twenty-five times less permeable to CO2 than polysulfone. yet its CO2/CH4 selectivity is more than twice as high as that of polysulfone. 

The apparent solubilities and average diffusivities, S and D, for CO2 and CH in a number of glassy polymers at 30"C and 20 atm are shown in Table I. The values reported for cellulose acetate were estimated from various sources in the literature (., 6,37). In the case of cellulose acetate where C02 plasticization is apparently significant, it was assumed that the CH4 permeability in CO2/CH4 mixtures will increase by at least the same percentage as the CO2 permeability. This assumption seems reasonable since the plasticized matrix becomes more rubber-like and less discriminating for penetrants of different sizes and shapes (see Figure 1). 

If blending could raise the solubility selectivity from two to four without a serious loss in CO2 permeability, the resultant material would be quite attractive as a membrane for CO2/CH4 separations. 

Polymer membranes have also been used as a "sandwich". In this configuration, the liquid film is supported between two polymer membranes. Ward (18) used two silicone rubber membranes to contain a solution of ferrous ions in formamide. Ward noted that Bernard convection cells could be maintained if the complex were formed at the upper surface. Ward (19) used this same system and membrane configuration to study electrically-induced, facilitated gas transport. The silicone rubber membranes provided the mechanical support so the electrodes could be placed next to each liquid surface. Otto and Quinn (20) immobilized the liquid film in a horizontal layer between two polymer films. The polymer was described as an experimental silicone copolymer having high CO2 permeability as well as excellent mechanical properties. They were studying CO2 facilitated transport in bicarbonate solutions with and without carbonic anhydrase. 

SILMs have been also employed for the separation of CO2/N2 and CO/ CH4 mixtures. Scovazzo et al. [54] used common ionic liquids such as imidazohum [emim] + and different water stable anions such as bis(trifluor-omethanesulfonyl)amide [T N] , trifluoromethanesulfone [CFsSOj] , and dicianamide dca in polyethersulfone as polymer substrate. It was observed an interesting increase in CO2 permeability (up to 50%) decreasing the gas phase pressure from 19 to 1.5 kPa with an ideal selectivity (CO2/N2) ranging from 15 (for CI ) up to 61 (for [dca] ) and permeabilities of 350 barrer (for CI ) to 1000 barrer (for [T N] ). In the case of CO2 separation from CH4, the ideal selectivity was ranging from 4 (for [Cl ) to 20 (for dca ) which are above the upper-bound for the CO2/CH4 Robson plot. 

The prepared membranes SILMs showed high CO2 permeability (744 barrer) and CO2/He selectivity of 8.6. Furthermore, the stabihty of the membranes at higher temperature (125 °C) approached the range of interest in the capture of CO2 from coal gasification plants. However, higher temperatures could not be reached mainly due to the support failure rather than any effect on the ionic liquid. 

In the case of CO2-N2 mixture, the reduction in the CO2 permeability in the presence of salt is less prominent due to CO2 facilitation discussed above, whereas the N, permeability is substantially lowered ("salting-out" effect in the aqueous 30% wt/wt K2CO, solution. 

Reports are also available on CO2 selective membrane reactors for WGS reaction. Zou et al. [40] first time synthesized polymeric C02-selective membrane by incorporating fixed and mobile carriers in cross-linked poly vinyl alcohol. Micro-porous Teflon was used as support. They used Cu0/Zn0/Al203 catalyst for low temperature WGS reaction. They investigated the effect of water content on the CO2 selectivity and CO2/H2 selectivity. As the water concentration in the sweep gas increased, both CO2 permeability and CO2/H2 selectivity increased significantly. Figure 6.18 shows the influence of temperature on CO2 permeability and CO2/H2 selectivity. Both CO2 permeability and CO2/ H2 selectivity decrease with increasing reactimi temperature. After the catalyst activation, the synthesis gas feed containing 1% CO, 17% CO2, 45% H2 and 37% N2 was pumped into the membrane reactor. They are able to achieve almost 100% CO conversion. They also developed a one-dimensional non-isothermal model to simulate the simultaneous reaction and transport process and verified the model experimentally under an isothermal condition. 

FIGURE 7.1 Selectivity for the gas pair CO2/CH4 as a function of CO2 permeability and the Robeson plots (squares for CMS membranes, solid circles for TR polymers, triangles for FSC membranes, and solid diamonds for PIMs). (Robeson plots from Ref. [5] data from Refs. [8-13].)... 

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