Hydrocarbon facilitated transport membranes

Uddin MW, Hagg MB. Natural gas sweetening—The effect on CO2-CH4 separation after exposing a facilitated transport membrane to hydrogen sulfide and higher hydrocarbons. J Membr Sci 2012 423-124 143-149. 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

Liquid membranes are versatile reagents that can be used to perform separatitms by a variety of mechanisms. Three of these are shown in Fig. 19.3-1. The simplest of these is that of selective permeation (Fig. 19.3-la) wherein mixtures can be separated by taking advantage of their different rates of diffusion through the liquid membrane. This type of mechanism has been used largely for the separation of hydrocarbons. More complicated separations can be achieved by using one or more of the facilitated transport mechanisms that can be built in a iiquid-membrane system. These mechanisms are ... 

Facilitated Transport of Unsaturated Hydrocarbons in Perfluorosulfonic Acid (Nafion) Membranes... 

Due to their extensive use in the polymer industry and as solvents, there is a continuing need for better separation processes for alkenes and other unsaturated organic compoimds from alkanes. Perfluorosulfonic acid (PFSA) membranes, such as Nafion (1), that have been ion-exchanged with silver(I) ion exhibit large transport selectivities for many unsaturated hydrocarbons with respect to saturates with similar physical properties. These selectivities are the result of reversible complexation reactions between the unsaturated molecules and Ag+ (2-4), which results in facilitated transport through the membranes (5). 

Experiments involving Ag+-Nafion membranes clearly indicate that facilitated transport provides a mechanism for separating a wide variety of hydrocarbons. Although calculations indicate that membrane/distillation column hybrid processes could provide a potential market, less expensive membrane materials with better productivity and stability must be developed before commercialization of such processes will occur. Hopefully, the knowledge gained by studying facilitated transport of hydrocarbons in Ag+-Nafion membranes will provide the necessary direction to promote the discovery of such membrane materials. 

Biological membranes present a barrier to the free transport of cations, as the hydrophilic, hydrated cations cannot cross the central hydrophobic region of the membrane which is formed by the hydrocarbon tails of the lipids in the bilayer. Specific mechanisms thus have to be provided for the transport of cations, which therefore allow for the introduction of controls. Such translocation processes may involve the active transport of cations against the concentration gradient with expenditure of energy via the hydrolysis of ATP. These ion pumps involve enzyme activity. Alternatively, facilitated diffusion may occur in which the cation is assisted to cross the hydrophobic barrier. Such diffusion will follow the concentration gradient until concentrations either side of... 

SILP systems have proven to be interesting not only for catalysis but also in separation technologies [128]. In particular, the use of supported ionic liquids can facilitate selective transport of substrates across membranes. Supported liquid membranes (SLMs) have the advantage of liquid phase diffusivities, which are higher than those observed in polymers and grant proportionally higher permeabilities. The use of a supported ionic liquid, due to their stability and negligible vapor pressure, allow us to overcome the lack of stability caused by volatilization of the transport liquid. SLMs have been applied, for example, in the selective separation of aromatic hydrocarbons [129] and CO2 separation [130, 131]. 

The extraction of phenol has been used by Schlosser and Kossaczky and Halwachs et al. as a model for studying liquid-membrane transport. The former group compared the liquid-membrane method to a conventional liquid-liquid double extraction process. They concluded that liquid membranes have distinct economic advantages in cases where type I or type 2 facilitation methods can be used (e.g., phenol or metal ion extractions). However, where one must rely on simple diCteerUial permeation (hydrocarbon separations) the advantage is less clear-cut. 

Kozlowski et al. [18] obtained the p CD polymers, which were prepared by crosslinking of 3-CD with 2-(l-docosenyl)-succinic anhydride derivatives in anhydrous N,N-dimethylformamide in the presence of NaH. It was established that the elongation of the hydrocarbon chain in the obtained 3-CD polymer in the reaction with 2-(l-docosenyl)-succinic anhydride results in the selectivity for Pb(ll) ions in the ion transport with the use of this ion carrier. At room temperature the dimmer was obtained, while at 100°C the polymers of 34kD and 13.5 kD fractions were received. The transport kinetics investigation on dependence of the carrier and Pb(II) concentrations have shown that the transport by the dimmer proceeded by the facilitated mechanism, typical for liquid membranes. The polymer however, has shown a linear increase of the transport flux in dependence on metal concentration in the source phase, this fact indicating that the polymer form of 3-CD prefers probably the fixed site mechanism of transport. PIMs containing dimmer and polymer of CD, in the transport of Zn(II), Cu(II) and Pb(Il) showed selectivity orders Pb(Il) Cu(II), Zn([]), and Pb(II) Cu(II) > Zn(II), respectively. The high selectivity factor for Pb(II)/Cu(II) equal to 163 for the dimmer was achieved (Table 1). 

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