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Figure 17. However, these microporous inorganic membranes become essentially non perm-selective for CO2/N2 separation at high temperatures (>300 oC). Gas/vapor p ermeation through microporous inorganic m embranes i s de termined by adsorption a nd diffusion of the permeating species in the membrane pores [92]. At low temperatures the CO2/N2 perm- selectivity for these microporous inorganic membranes is mainly determined by the adsorption properties of the membranes for CO2 and N2. Their pore sizes are still too large to show any significant diffusion- controlled s electivity f or C O2 over N 2 [97, 98] . F or M FI or F AU t ype zeolite membranes, w ith z eolite pore di ameter r espectively of 0.55 nm and 0.7 nm, the r atio of t he molecular size to por e s ize, λ, i s smaller than 0.75 for CO2 and N2. If one considers possible microporous defects, the average pore size of the membranes would be even larger than the zeolite pores, yielding a much lower value of λ. With λ in this range, these two zeolite membranes will offer essentially no diffusion-controlled selectivity for CO2 over N2. At high temperatures, adsorption diminishes and therefore the adsorption-controlled selectivity disappears for these microporous membranes. It is unlikely for other microporous membranes with a pore size larger than FAU type zeolite, such as metal organic framework materials, to exhibit better selectivity and permeance for CO2/N2 than those summarized above. To improve the diffusion controlled CO2/N2 selectivity requires further reduction in the membrane pore size. Amorphous silica membranes obtained by the sol-gel method from acid catalyzed polymeric silica sol or by chemical vapor deposition at the intermediate temperature have an ultramicropore structure with pore diameter in the range of 0.3-0.4 nm. These membranes might offer high CO2/N2 selectivity at high temperatures if their thermal and hydrothermal stability can be improved. Efforts have been reported to improve the stability of the microporous silica membranes through surface modification or doping of a second m etal to t he s ilica f ramework. R ecent w ork sh owed t hat thermally st able m icroporous s ilica membranes with a pore diameter of around 0.3 nm can be prepared by a high temperature chemical vapor deposition m ethod [ 99]. H owever, t he C O2 permeance o f t he m embrane i s too l ow (about 2x1 0-10 mol/m2·s·Pa) which is expected from the membrane pore size. Crystalline z eolite m embranes w ith sm all p ore s ize o ffer b etter ch emical stability a nd more c ontrolled more structure than the amorphous ultramicroporous silica membranes discussed above. Two 8-member- ring zeolites, CHA type (e.g. SAPO-34) and DDR type zeolites have recently attracted much interest as membrane m aterials for g as s eparation i nvolving C O2 [100]. SAPO-34 a nd D DR z eolites have a po re diameter of about 0.38 and 0.36x0.44 nm, respectively, slightly larger than the kinetic diameter of CO2 and N2. SAPO-34 membranes exhibit good separation properties for CO2/CH4 mixture separation [100]. However, presence of water in the gas stream has a negative impact on SAPO-34 membrane performance due to the hydrophilic nature of the SAPO-34 framework. DDR zeolite contains pure silica, and, similar to pure silica 10- or 12-member-ring MFI type silicalite and FAU-type dealuminized-Y zeolites, is highly chemically an d thermally st able. A 5 μm t hick D DR z eolite m embrane, p ossibly co ntaining so me microporous intercrystalline defects [94], has CO2 permeance of about 3x10-7 mol/m2·s·Pa and CO2/N2 selectivity of a bout 30 a t 25 oC [ 101]. The se lectivity an d p ermeance o f t hese m embranes can b e improved i f t he m embrane t hickness i s f urther de creased a nd t he i ntercrystalline por es of m embrane eliminated. For m icroporous inorganic membranes there i s a limit to im prove the C O2/N2 selectivity w hile maintaining h igh p ermeance t hrough p ore s ize r eduction. D ense, n onporous ceramic m embranes ar e known for their infinitely large selectivity for O2 over N2, and high O2 permeance at temperatures above 700oC. R esearch ef forts on sy nthesis o f d ense L i2ZrO3 and L i4SiO4 membranes f or hi gh t emperature separation of CO2 were reported but these membranes e xhibit a CO2/N2 selectivity of about 5 a nd CO2 permeance o f 1 0-8 mol/ m 2·s·Pa at 5 25 oC [ 102]. It i s k nown t hat m olten c arbonate, such as Li2CO3/K2CO3, can conduct CO32- at a very high rate at high temperatures. A metal-carbonate dual-phase membrane was prepared and shown to be able to separate CO2 from mixture of N2, CO2 and O2 [101]. Carbon Capture Factual Document 35PDF Image | 2020 Carbon Capture
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