Carbon permeance

Note that Q(23)=e1JQ + a. A measurement on an individual specimen can be expected to deviate from this estimate because of measurement error,and because of the specimen effect embodied in the S. Values of Q(23) and 3 obtained from the analysis of the permeance and the time-lag data are given in Tables I and II respectively. Since the time-lag for carbon dioxide depends upon the upstream pressure it is necessary to multiply the estimates obtained from equation 8 by a term of the form ... 

The observed decrease of the p values for the time-lag of carbon dioxide with increasing pressure is consistent with published data that have been interpreted using a partial immobilization model (11,12). It is somewhat surprising that we do not observe a concomitant decrease in the permeance perhaps the pressures used are too low or the pressure range is not broad enough. 

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. 

Carbon Dioxide Permeability, B, and Permeance, II, in the Studied Membranes... 

Until now, flat silica membranes were only tested at relatively low temperatures (up to 300°C) because of limitations in thermal stability of the polymer sealing rings [2], However, with the use of dense alumina rings together with carbon sealing it is possible now to measure membrane properties (e.g. permeance) at much higher temperatures (up to 600°C), which will be described below. 

The tubular supports were measured in a membrane reactor, which could also serve for steamreforming experiments when applicable (Figure 6). The tubes were sealed with carbon sealing at the enamelled ends of the tubes. Permeance measurements were performed at 500°C. 

FIGURE 4.13 Permeance of propane as a function of pressure and temperature through a CMS membrane. (From Lie J.A., S)mthesis, performance and regeneration of carbon membranes. Doctoral thesis NTNU 2005 152, Trondheim, Norway, 2005. With permission.)... 

This group is mainly formed by silica or carbon membranes. For silica in the small pore and intermediate pore region very good combinations of selectivity and fluxes are reported. The porosity of the membrane seems to be too low however (note good measurement methods for supported microporous membranes do not exist). Porosities of at least 20% of theoretical density should give considerable improvement in the permeance. A strategy to overcome this is. 

As has already been discussed, controlled formation of very thin high porosity microporous plugs within the pore entrance of the supporting system should be very interesting. Trials with silica, carbon and SiC deposits have been reported however without giving high selectivities and permeances. 

Defect-free zeolite membranes have so far only been produced for membranes of the MFI (silicalite type) with thicknesses of about 50 im on stainless steel supports and 3-10 pm on alumina and carbon supports. They are produced by in situ methods of zeolite crystals grown directly on the support system. There are some reports of formation of defective membranes with, e.g., zeolite A. Much more research is needed to widen the range of available zeolite membrane types especially small and wide pore systems. The permeance values of the defect-free membranes is lower than that of the amorphous membranes (see Chapter 6) and to improve this the layer thickness must be decreased together with improving the crystal quality (no impurities, no surface layers, high crystallinity, crystal orientation) and microstructure (grain boundary engineering). 

The MSC membranes are produced by carbonization of polyacrylonitrile, polymide, and phenolic resins [30]. They contain nanopores (typically <5 A in diameter) that allow some of the molecules of a feed gas mixture to enter the pores at the high-pressure side, adsorb, and then diffuse to the low-pressure side where they desorb into the gas phase. The other molecules of the feed gas are excluded from entering the pores and they are enriched in the high-pressure side. Thus the separation is based on the differences in the molecular sizes and shapes of the feed gas components. The smaller molecules preferentially diffuse through the membrane as schematically depicted by Fig. 22.7(a). Table 22.7 gives the permeance and the permselectivity of the smaller species (component 1) of several binary gas mixtures by the MSC membrane [25, 26, 30]. 

Recently, Bao et al. [68] compared the efficiency of facilitated transport of CO2 across a liquid membrane by different carriers (diethanolamine (DEA) and carbonic anhydrase (CA) + bicarbonate (NaHCO3) in a polypropylene HFCLM configuration. The hollow fibers used are made of polypropylene, pore size 0.04 pm. In all the experiments, the measured CO2 permeance and selectivity (CO2/O2) using CA bicarbonate as carrier was higher than in the case of DEA. The separation factor (CO2/O2) using DEA was about 152 which are 65% lower than the selectivity calculated with CA bicarbonate. 

In another work [51], glycerol carbonate was studied as a new physical solvent with and without carriers (poly(amidoamine) dendrimer and Na-glycinate) for carbon dioxide separation from CO2/N2 mixtures. The performance of pure glycerol carbonate appears to be independent of the CO2 partial pressure difference and the selectivity remains constant (80-100) for any value of the feed side moisture. Addition of the carriers significantly helps CO2 facilitation at low CO2 partial pressures. In particular, at 0.66 kPa the presence of the dendrimer and Na-glycinate increased the selectivity (CO2/N2) to 1000 and 480, respectively. It was also proved that the decrease of membrane thickness did not affect the selectivity (90-100), which was similar either for 25 and 250 pm thickness membranes, but slightly increased the CO2 permeance. 

Certainly the most promising application of MFI membranes is in hydrocarbon separations. Adsorption increases with the carbon number for linear compounds and steric effect allows to separate /2-butane from t-butane. Consequently at low temperature the permeance decreases when the carbon number increases and at high temperature, differences in diffusion factors... 

Li et al. [30] synthesized membrane reactor with three Pd tubes by varying the thickness from 5.6 to 6.1 pm. They evaluated membrane reactor for WGS reaction for 30 days. The H2 permeance results at two pressures are presented in Figure 6.16. It can be seen that the pure H2 permeance remained stable during the reaction test of 27 days, showing a good chemical and mechanical stability of the Pd membranes used in this study. It is implied that there was no degradation of the membrane performance due to, e.g., carbon formation on the... 

Noble RD. Achieving a 10,000 GPU permeance for post-combustion carbon capture with RTIL-based membranes, DOE NETL CO2 Capture Technology Meeting, Pittsburg, PA, 2011. 

Poisoning of coke both hydrogen permeance and perm-selectivity of a thin palladium membrane decrease after it is brought in contact with coke at elevated temperature [49]. This phenomenon can be addressed for the fact that carbon atoms penetrate into the palladium lattice and cause the failiue of the membrane owing to the expansion of the palladium lattice. 

Figure 17.18 H2 permeance (A) and CO permeance (B). Reprinted from F. J. Varela-Gandia, A. B. Murcia, D. L. Castello and D. C. Amoros, Hydrogen purification for PEM fuel cells using membranes prepared by ion-exchange of Na-LTA/carbon membranes, Journal of Membrane Science, 351, 123-130, 2010, with permission from Elsevier.

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