Rubbery Polymeric Membranes

For illustration, rubbery polymeric membranes, whose polymeric network is sufficiently elastic and mobile to allow comparatively large organic compounds to diffuse through it (Table 3.6-2), are in general used for the recovery of organic compounds from aqueous solutions. Because of its small size, the bulk solvent, water, unfortunately diffuses through the membrane even better. This is why in organo-philic pervaporation the selectivity is mainly achieved and determined by the ratio of the solubility coefficients (sorption selectivity. Table 3.6-2). Membrane selectivity, as defined in Eq. (7), is an intrinsic parameter and can differ from the overall process selectivity, as wiU be shown later. 

Sorption Theory for Multiple Gas Components 11.2.1 Rubbery Polymeric Membranes... 

Hence, the solubihty of gas A within a rubbery polymeric membrane can be determined from the Flory-Huggins parameter. 

Where is the pressure of gas B, and K- the Henry s law constant. Hence, gas B may enhance or decrease the solubility of gas A within the rubbery polymeric membrane, depending on the relative strengths of the Flory-Huggins parameters for the gases between themselves and for the polymers. 

Scholes, C. A., Bacus, J., Chen, G. Q., Tao, W. X., Li, G., Qader, A., et al. (2012). Pilot plant performance of rubbery polymeric membranes for carbon dioxide separation from syngas. 

Note the difference between this permeability expression and that for Q,m for rubbery polymeric membranes (= SimDiD with Did = Am)- As the feed pressure increases, the permeability coefficient in the glassy membrane decreases. The permeability coefficient has the highest value in the limit of p,y(= P/) 0. The integrated flux... 

As long as there is no chemical reaction of the dissolved gas species with the liquid, the gas species flux expression through a thin liquid layer acting as a membrane is identical to that through a rubbery polymeric membrane, as discussed earlier. The major difference comes about in the magnitude of the permeability coefficients. The diffiision coefficient of a gas species in a liquid membrane will, in... 

In liquid mixtures of type (2), the solutions of primary interest are azeotropic and other mixtures containing variable amounts of water in organics dehydration of organic solvents containing very small amounts of water. Removal of water from azeotropic mixtures of ethanol-water, isopropanol-water, etc., is extensively practiced using polymeric membranes (of crosslinked polyvinyl alcohol) that are highly polar and selective for water. On the other hand, the membranes that are used to remove VOCs selectively from aqueous solutions are usually highly nonpolar rubbery polymeric membranes, e.g. dimethyl siloxane (silicone rubber). 

Recovery of C3-I- condensable hydrocarbons from natural gas using rubbery polymeric membranes. 

The preceding structural characteristics dictate the state of polymer (rubbery vs. glassy vs. semicrystalline) which will strongly affect mechanical strength, thermal stability, chemical resistance and transport properties [6]. In most polymeric membranes, the polymer is in an amorphous state. However, some polymers, especially those with flexible chains of regular chemical structure (e.g., polyethylene/PE/, polypropylene/PP/or poly(vinylidene fluoride)/PVDF/), tend to form crystalline... 

The basic transport mechanism through a polymeric membrane is the solution diffusion as explained in Section 4.2.1. As noted, there is a fundamental difference in the sorption process of a rubbery polymer and a glassy polymer. Whereas sorption in a mbbery polymer follows Henry s law and is similar to penetrant sorption in low molecular weight liquids, the sorption in glassy polymers may be described by complex sorption isotherms related to unrelaxed volume locked into these materials when they are quenched below the glass transition temperature, Tg. The various sorption isotherms are illustrated in Figure 4.6 [47]. 

To surpass Robeson s upper bound, materials are emerging that rely on transport mechanisms other than solution-diffusion through glassy or rubbery polymeric materials. In particular, a number of materials have been developed that possess fixed microporosity (2 nm or less) in contrast to the activated, transient molecular gaps that give rise to diffusion in most polymers. These materials include amorphous and crystalline (zeolite) ceramics [68-69], molecular sieve carbons [70], polymers that possess intrinsic microporosity [71-72], and carbon nanotube membranes [73-76]. Transport in such materials is determined primarily by the average size and size distribution of the microporosity - the porosity can be tuned to allow discrimination between species that differ by less than one Angstrom in size. However, surface... 

Sikdar et al. (2000) developed adsorbent-filled PV membranes for removing VOCs from waste water. These membranes were prepared by dispersing at least one hydrophobic adsorbent uniformly into a polymer matrix. Polymeric membrane was made of rubbery polymer selected from the group consisting of PDMSs, PTMSP, PUs, polycarbonates (PCs), PE-block-polyamides, silicon PCs, styrene butadiene rubber, nitrile butadiene rubber, and ethane-propene terpolymer. The hydrophobic adsorbent was selected from the group consisting of hydrophobic zeolites, hydrophobic molecular sieves, activated carbon, hydrophobic polymer resin adsorbents, and mixtures thereof. 

Sulfur dioxide has also been reported to plasticize polymeric membranes, which produces a more rubbery material and increases the diffusivity of penetrant gases [26-28]. Plasticization also reduces the mechanical integrity of the membrane, meaning it is more likely the membrane will rupture. However, plasticization is a strongly pressure dependent phenomenon, for example it has been reported in polyvinylidene membranes to occur at SO2 pressures greater than 10 psi [29]. For many of the processes in carbon capture, such high partial pressures of SO2 are not observed (Table 11.1), and therefore only minor plasticization by SO2 is likely to occur. 

The recovery of organic vapors from waste gas streams using polymeric membranes is a well established process (7). Typically, composite membranes are used for this process. These membranes consist of a diin, selective rubbery layer coated onto a microporous support material. The selectivities of these membranes for organic vapors over nitrogen are typically about 10-100. Currently, commercial vapor separation membrane applications include small systems (10-100 scfin) to recover fluorinated hydrocarbons (Freons) and other high-value solvent vapors from process vent streams to large systems (100-1,000 scfin) for recovery of hydrocarbon vapors in the petrochemical industry (7). 

Rubbery polymer membrane materials have the ability to permeate preferentially condensable vapors. Rubbery membranes were found to be very profitable for the recovery of high-value monomer in petrochemical plants purges gases (Table 6.12). ° ° In the case of large polymerization facilities, the value of purge monomers can reach amounts up to 2 million USD per year. 

Since temperatures are moderate at the outlet of the FGD unit (compare Table 7.1), polymeric membranes appear to be the most suitable ones for post-combusion CO2 capture. In the recent past, particularly, PEO-based block copolymers received considerable attention. The high polarity of PEO leads to a high CO2/N2 gas separation factor in combination with a high CO2 permeance. Due to the rubbery nature of PEO-based block copolymers, the presence of water hardly influences the transport of CO2 and other gases. The chemical and mechanical resistance of the polymer to flue gas conditions are currently evaluated in the NanoGLOWA project. Table 7.4 summarizes the rather favorable separation characteristics of PEO, which will be further used to investigate the economics of membrane gas separation processes of post-combusion CO2 capture. 

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