Permeance-permeability plots

The manufacture of DPMs based on infiltrated molten carbonates in the porosity of perovskite membranes constitutes the most recent and innovative application of perovskites for CO2 capture. Since the first studies reported by Wade and coworkers [33] and Lin and coworkers [34], many efforts have been made to develop stable high-flux membranes for CO2 separation. Table 39.5 collects the most remarkable results, whereas Figvue 39.12 plots the permeance versus permeability plots for dual-phase perovskite-carbonate materials compared with low-temperature CO2 separating membranes (zeolites and MOFs/ZIFs). 

Figure 8.11 Robeson plot of CO2/CH4 selectivity versus membrane permeability and permeance [12]. The points shown are based on low-pressure, pure-gas measurements. The performance of commercial membranes when used to separate carbon dioxide from high-pressure natural gas is shown on the same figure for comparison.

The problem with use of polymeric membranes in this application is plasticization, leading to much lower selectivities with gas mixtures than the simple ratio of pure-gas permeabilities would suggest. For this type of separation, a Robeson plot based on the ratio of pure-gas permeabilities has no predictive value. Although membranes with pure-gas propylene/propane selectivities of 20 or more have been reported [43, 44], only a handful of membranes have been able to achieve selectivities of 5 to 10 under realistic operating conditions, and these membranes have low permeances of 10 gpu or less for the fast component (propylene). This may be one of the few gas-separation applications where ceramic or carbon membranes have an industrial future. 

Sofer et al. ° is one of the earliest works in this topic. These membranes are important due to the improved trade-off upper limit between permeabiUty and selectivity compared with their polymer precursor membranes. Singh-Ghosal and Koros reported the Robeson s plot (Oj/Nj selectivity versus O2 permeability) for some carbon membranes and corresponding polymer membranes (Fig. 10.2). It is obvious that the performance of carbon membranes is much better than that of corresponding polymer membranes. Moreover, the permeance of the carbon membranes depends on the surface characteristics and the interactions between pores and gas molecules rather than on the bulk properties as for the polymer membranes. When carbon membranes separate molecules based on the molecular-sieving mechanism, the molecules have to overcome an energetic barrier created by the differences between pore dimension and gas molecules. 

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