Selective surface diffusion membrane

Rao and Sircar [5-7] introduced nanoporous supported carbon membranes which were prepared by pyrolysis of PVDC layer coated on a macroporous graphite disk support. The diameter of the macropores of the dried polymer film was reduced to the order of nanometer as a result of a heat treatment at 1,000°C for 3 h. These membranes with mesopores could be used to separate hydrogen-hydrocarbon mixtures by the surface diffusion mechanism, in which gas molecules were selectively adsorbed on the pore wall. This transport mechanism is different from the molecular sieving mechanism. Therefore, these membranes were named as selective sitrface flow (SSF ) membranes. It consists of a thin (2-5 pm) layer of nanoporous carbon (effective pore diameter in the range of 5-6 A) supported on a mesoporous inert support such as graphite or alumina (effective pore diameter in the range of 0.3-1.0 pm). The procedures for making the selective surface flow membranes were described in [5, 7]. In particular, the requirements to produce a surface diffusion membrane were shown clearly in [7]. 

Rao, Sircar, and co-workers at Air Products and Chemicals, Inc., have pioneered the production of a specialty type of selective surface diffusion (SSD) membrane, which they refer to as SSF carbon membranes. The development and performance of these membranes have been described in a series of patents and articles (Anand et al., 1995, 1997 Paranjape et al., 1998 Rao and Sircar, 1993a,b, 1996 Rao et al., 1992, 1995a,b Sircar et al., 1999 Zhou et al., 2003). These membranes usually consist of a thin nanoporous carbon layer (5-7 A) on the surface of a macroporous support. The mechanism of gas transport through SSF membranes is illustrated in Figure 23.5. The more adsorbable component in a feed stream on the high-pressure side of the membrane selectively adsorbs onto the surface, diffuses across the surface of the membrane to the low-pressure side, and... 

A final application that can be envisaged for permselective IMRs concerns the enhancement of reaction selectivity toward intermediate products of consecutive reaction pathways. Such a goal could be attained by developing a membrane capable of separating the intermediate product from the reaction mixture [39,40]. The most critical point in this regard is that intermediate product molecules (e.g., partially oxidized hydrocarbons) are often larger in size than the complete reaction products (e.g., CO2) or the reactants themselves (e.g., O2). This seriously complicates the separation process, limiting the number of selective transport mechanisms that can be utilized for the purpose of capillary condensation, surface diffusion, or multilayer diffusion (described later in this chapter). 

Selective surface flow is, as Knudsen diffusion, associated with transport through microporous membranes, usually inorganic materials. The mechanism of surface diffusion is disputed and several different approaches have been proposed in the literature. 

Although Knudsen diffusion, shape selectivity, and molecular sieving play an important role in the separation of mixtures, the mechanisms which control the majority of the multicomponent separations in zeolite membranes are surface diffusion, and sometimes, capillary condensation. In addition, molecular simulations and modeling of M-S diffusion in zeolites [69,70] show that the slower moving molecules are also sped up in some mixtures [71,72] in the presence of fast-diffusing molecules and other times, slower molecules inhibit diffusion of faster molecules because molecules have difficulty passing one another in zeolite pores [73]. 

The free aperture of the main 100 channels in Y-type zeolite is 0.74 nm [7] and is much larger than the diameter of CO2 and N2 molecules. If the concentrations of CO2 and N2 in the micropores of the Y-type zeolite membrane are equal to those in the outside gas phase, these molecules permeate through the membrane at a low CO2/N2 selectivity. However, this was not the case. Carbon dioxide molecules adsorbed on the outside of the membrane migrate into micropores by surface diffusion. Nitrogen molecules, which are not adsorptive, penetrate into micropores by translation-collision mechanism from the outside gas phase. 

The adsorption-surface diffusion-desorption mechanism of transport through the SSF membrane can simultaneously provide high separation selectivity between H2 and the impurities of the PSA waste gas and high flux for the impurities even when the gas pressure in the high-pressure side of the membrane is low to moderate (3-5 atm). 

The gas is applied as a mixture to the retentate (high pressure) side of the membrane, the components of the mixture diffuse with different rates through the membrane under the action of a total pressure gradient and are removed at the permeate side by a sweep gas or by vacuum suction. Because the only segregative mechanisms in mesopores are Knudsen diffusion and surface diffusion/capillary condensation (see Table 9.1), viscous flow and continuum (bulk gas) diffusion should be absent in the separation layer. Only the transition state between Knudsen diffusion and continuum diffusion is allowed to some extent, but is not preferred because the selectivity is decreased. Nevertheless, continuum diffusion and viscous flow usually occur in the macroscopic pores of the support of the separation layer in asymmetric systems (see Fig. 9.2) and this can affect the separation factor. Furthermore the experimental set-up as shown in Fig. 9.11 can be used vmder isobaric conditions (only partial pressure differences are present) for the measurement of diffusivities in gas mixtures in so-called Wicke-Callenbach types of measurement. 

Four types of diffusion mechanisms can be utilized to effect separation in porous membranes. In some cases, molecules can move through the membrane by more than one mechanism. These mechanisms are described below. Knudsen diffusion gives relatively low separation selectivities compared to surface diffusion and capillary condensation. Shape selective separation can yield high selectivities. The separation factor for these mechanisms depends strongly on pore-size distribution, temperature, pressure, and interactions between the solute being separated and the membrane surfaces. 

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