Fixed-site carriers membranes carrier mobility

Yahaya, G. O. (2001). Kinetic studies on organic acid extraction by a supported liquid membrane using functionalized polyorganosiloxanes as mobile and fixed-site carriers. Sep. Sci. Technol. 36, 3563. 

If, however, a carrier-mediated transport membrane containing charged species — in the form of either mobile ions or fixed sites — were placed between two electrolytic mixtures, significant Donnan effects could be expected. For example, consider a membrane in which the carrier is a counterion to the permeant. The permeant would be expected to be preferentially included in the membrane phase. If significant inclusion were to occur, the use of simple first-kind boundary conditions would be inappropriate and could lead to underestimation of flux. On the other hand, if the permeant and carrier were coions, the permeant could be excluded and failure to account for exclusion could lead to overprediction of flux. Further complications would arise if the complex were charged or if other charged species were present, since the net charge density inside the membrane defines Donnan equilibrium conditions. 

Second, other types of membrane separation processes can also be proposed for the CO2/N2 separation step, in order to play the same role as the absorption process. In that case, it is absolutely necessary to carefully estimate the operating costs (OPEX) and capital costs (CAPEX) of the operation, so that an overall CO2 capture cost (in euros or per ton of recovered CO2) can be precisely estimated. This data will obviously be of utmost importance in order to evaluate the chances of the membrane process to possibly compete with the standard capture process. A CO2/N2 separation efficiency is clearly needed in that case. Depending on the membrane material, this characteristic can be obtained based on a physical mechanism (solution-diffusion in a dense polymer, for instance, which corresponds to a classical membrane gas separation operation ), or a chemical reaction in a polymeric matrix based on fixed sites (so-called fixed site carrier membranes ) or with membranes making use of a mobile selective carrier (liquid membranes ). 

Owing to observations on certain competitive and reciprocal antiport relations between sugar and amino acid transport, Alvarado and Crane and Alvarado have recently postulated a new polyfunctional, mobile carrier system. involved in the uphill transport of sugars, neutral amino acids and basic amino acids in the small intestine that consists of a mosaic of fixed, specific membrane sites which acquire mobility as a result of deformations of the mobile membrane resulting in local, transient engagements of the two protein surfaces, thus allowing bound substrates to be alternately exposed to the extra- and intercellular fluids. ... 

At a more detailed level, it is still not certain how even a relatively simple selective membrane operates (e.g. PVC containing plasticizer and valino-mycin, selective to K" ). The idea that PVC plus plasticizer acts as an inert structureless membrane in which the sensor species (the neutral carrier valinomycin in this case) dissolves, has been abandoned. A more heterogeneous structure is proposed, in which free dissociated counter-ions contribute to the membrane s conductivity. The counter-ions are presumed to arise from the dissociation of impurities associated with the method of manufacture of the PVC. Both fixed and mobile charged sites have been identified, and may represent, via the process of dissociation, an important source of counter-ion charge carriers (9-11). It is known that water penetrates the PVC film, but the exact role of this water is a matter still open to debate. Even the method of entrance of the water molecules is not clear do they enter in a uniform manner,... 

Selective separation of lead(II) and cadmium(II) chloride complexes by both liquid membranes containing liquid anion-exchangers as mobile carriers and solid polymeric membranes with anion-exchange sites as fixed carriers is described by Hayashita in Chapter 21. A novel polymeric plasticizer membrane, which is composed of cellulose triacetate polymer as a membrane support, o-nitrophenyl octyl ether as a membrane plasticizer, and trioctylmethylammonium chloride as an anion-exchange carrier, provides enhanced permeation selectivity and efficiency. 

Based upon the above concept, three different types of membrane separation, i.e., (1) liquid membrane separation which utilizes lipophilic anion-exchangers as mobile carriers, (2) polymeric membrane separation in which the anion-exchange sites function as fixed carriers, and (3) polymeric plasticizer membrane separation in which the membrane is composed of polymeric support, membrane plasticizer, and lipophilic anion-exchangers as a novel membrane material are discussed in relation to their transport efficiency and selectivity for separation of heavy metal chloride complexes. 

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