Liquid membrane supports considerations

In this paper an overview of the developments in liquid membrane extraction of cephalosporin antibiotics has been presented. The principle of reactive extraction via the so-called liquid-liquid ion exchange extraction mechanism can be exploited to develop liquid membrane processes for extraction of cephalosporin antibiotics. The mathematical models that have been used to simulate experimental data have been discussed. Emulsion liquid membrane and supported liquid membrane could provide high extraction flux for cephalosporins, but stability problems need to be fully resolved for process application. Non-dispersive extraction in hollow fib er membrane is likely to offer an attractive alternative in this respect. The applicability of the liquid membrane process has been discussed from process engineering and design considerations. 

Following the work of Bloch and Vofsi, other methods of producing immobilized liquid films were introduced. In one approach, the liquid carrier phase was held by capillarity within the pores of a microporous substrate, as shown in Figure 11.3(a). This approach was first used by Miyauchi [7] and by Largman and Sifniades and others [8,9], The principal objective of this early work was to recover copper, uranium and other metals from hydrometallurgical solutions. Despite considerable effort on the laboratory scale, the first pilot plant was not installed until 1983 [10], The main problem was instability of the liquid carrier phase held in the microporous membrane support. 

Bardstu, K.F., Ho, T.S., Rasmussen, K.E., Pedersen-Bjergaard, S. and Jonsson, J.A. (2007) Supported liquid membranes in hollow fiber liquid-phase microextraction (LPME) — Practical considerations in the three-phase mode. Journal of Separation Science, 30, 1364. 

In SLM extraction, the most widely applied type of three-phase membrane extraction, the membrane consists of an organic solvent, which is held by capillary forces in the pores of a hydrophobic porous membrane supporting the membrane liquid. Such membrane support can be either flat porous PTFE or polypropylene membrane sheet or porous polypropylene hollow fibers. Typical solvents are long-chain hydrocarbons like n-undecane or kerosene and more polar compounds like dihexyl ether, dioctyl phosphate, and others. Various additives can increase the efficiency of extraction considerably. The stability of the membrane depends on the solubility and volatility of the organic liquids, and it is generally possible to obtain membrane preparations that are stable up to several weeks. 

Permeation is a general term for the concentration-driven membrane-based mass transport process. Application of a pressure difference, an electric held, or temperature considerably intensibes the process, but these special methods are beyond the scope of this overview. Three groups of liquid membranes are usually considered bulk liquid membrane (BLM), supported liquid... 

The ability of calixarenes to bind large metal ions with high kinetic stability is important in the search for complexants for radionuclides such as Cs (ti/2 = 30.2 yr) and Sr (ti/2 = 65 d) from the reprocessing of exhausted nuclear fuel. There has been considerable interest in caesium-complexed calix[4]-bis-crowns as selective Cs-carriers. Transport isotherms of trace level Cs through supported liquid membranes containing calix[4]-bis-crowns have been determined as a function of the ionic concentration of the aqueous feeder solutions, and l,3-calix[4]-bis-o-benzo-crown-6 appears to be much more efficient in decontamination than mixtures of crown ethers and acidic exchangers, especially in highly acidic media. " ... 

Selection of the ideal support for a liquid membrane requires careful consideration of the characteristics of the particular separation such as gas or liquid phase, pressure, temperature, and chemical nature of the phases in contact with the membrane. However, a few generalizations can be made. The ideal support should be thin (< 100 pm), have a high porosity (> 50%), have a mean pore size of less than 0.1 pm, have a narrow pore size distribution, and be available in geometries that will produce permeators with a high surface area /volume ratio. 

Liquid membranes can be of three types—bulk liquid membrane, immobilized on a solid supported hquid membrane, and liquid membrane as double emulsions. Of these three types, ELMs can achieve much higher mass transfer area than the other two membranes. ELMs were first used by Li [1] for separation of hydrocarbons. Since then, considerable work has been done to demonstrate qualitatively the feasibihty of performing separations with specific formulations. 

Separations of molecules by membranes play a crucial role in biological systems, and they are proving to have important industrial applications as well. Therefore, considerable attention, particularly by Reinhoudt and coworkers, is being devoted to the use of calixarenes as the carrier molecules in liquid membranes. An early demonstration showed, for example, that selective transport of can be achieved through a supported liquid membrane (SLM) containing the calix[4]arene-crown-5 (130a R = f-Bu n = 3). A more recent... 

The main application of this technology for metal extraction will probably be in the treatment of effluents and wastewaters as shown by the many research papers that have been published. This is particularly true of the supported liquid membrane because of the many modules required to treat significant volumes of feed solution. This then creates a large capital expenditure, aud, although lifetimes of polymeric membranes have now been increased considerably, the initial outlay will probably be too great for the value of any benefits or metal recovered. However, if this process can be used for high-value products, then the expenditure can be more easily justified— hence, the potential use of such systems for recovery of pharmaceutical compounds. 

In contrast to osmotic, dialysis, filtration or size-exclusion type membranes, PIMs as other liquid membranes (i.e., bulk liquid membranes, emulsion liquid membranes and supported liquid membranes) rely on the action of a chemical agent to extract the solute of interest from an aqueous phase (Kolev, 2005). The action of this chemical agent is the most important factor in the performance of any PIM and its behavior shares considerable similarities with SX apphcations (e.g., hydrometaUurgy). 

The attempt to show that surface tension phenomena were the cause of osmotic pressure was first made by Jager, and his theories were vigorously supported and developed by Traube, whose conclusions we shall state and examine briefly. He finds that the more a dissolved substance reduces the surface tension of water the greater is the velocity of osmosis of the solution. Hence he concludes that it is the difference in the surface tensions of solvent and solution which determines the direction and velocity of osmosis. The direction of flow Traube obtains by the following consideration let M (Fig. 7) be a membrane separating two liquids A and B. The molecules of each liquid are then drawn into its interior by the cohesion or intrinsic pressure. If the intrinsic... 

Apart from hydrocarbons and gasoline, other possible fuels include hydrazine, ammonia, and methanol, to mention just a few. Fuel cells powered by direct conversion of liquid methanol have promise as a possible alternative to batteries for portable electronic devices (cf. below). These considerations already indicate that fuel cells are not stand-alone devices, but need many supporting accessories, which consume current produced by the cell and thus lower the overall electrical efficiencies. The schematic of the major components of a so-called fuel cell system is shown in Figure 22. Fuel cell systems require sophisticated control systems to provide accurate metering of the fuel and air and to exhaust the reaction products. Important operational factors include stoichiometry of the reactants, pressure balance across the separator membrane, and freedom from impurities that shorten life (i.e., poison the catalysts). Depending on the application, a power-conditioning unit may be added to convert the direct current from the fuel cell into alternating current. 

Currently, most solution-coated composite membranes are prepared by the method first developed by Riley and others [45,56,57], In this technique, a polymer solution is cast directly onto the microporous support. The support must be clean, defect-free and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a liquid layer 50-100 xm thick, which after evaporation leaves a thin selective film 0.5-2 xm thick. A schematic drawing of the meniscuscoating technique is shown in Figure 3.25 [58], Obtaining defect-free films by this technique requires considerable attention to the preparation procedure and the coating solution. 

Considerable effort is being devoted to developing new polymeric membrane materials. A special type of oxygen-enrichment membrane has also been explored, which consists of a solvent immobilized within a microporous solid support (Fig. 7D). Dissolved in the liquid is a carrier... 

The resulting SILMs were stable under assayed conditions. These authors highlighted the importance of the consideration of two main possible effects on the performance and stability of SILMs in water mediums (a) the loss of ionic liquids phase from the supporting membrane to the adjacent aqueous solutions by dissolution/ emulsification and (b) the formation of water microenvironments inside the ionic liquids phase, which constitute new, non-selective environments for solute transport, leading to a deterioration of the SLM performance and selectivity. 

It is understood that the economical success of any membrane process depends primarily on the quality of the membrane, specifically on flux, selectivity and service lifetime. Consideration of only the transport mechanisms in membranes, however, will in general, lead to an overestimation of the specific permeation rates in membrane processes. Formation of a concentration boundary layer in front of the membrane surface or within the porous support structure reduces the permeation rate and, in most cases, the product quality as well. For reverse osmosis. Figure 6.1 shows how a concentration boundary layer (concentration polarization) forms as a result of membrane selectivity. At steady state conditions, the retained components must be transported back into the bulk of the liquid. As laminar flow is present near the membrane surface, this backflow is of diffusive nature, i.e., is based on a concentration gradient. At steady state conditions, the concentration profile is calculated from a mass balance as... 

Bardstu KF, Ho TS, Rasmussen KE, Pedersen-Bjergaard S, Jonsson jA. Supported hquid membranes in hollow fiber liquid-phase microextraction (LPME). Practical considerations in the 3-phase mode. J Sep Sci 2007 30 1364-1370. 

Kong, Lu, Yang, and Wang (2007) investigated a stainless-steel-supported zeolite silicalite-1 membrane reactor packed with an iron oxide catalyst for styrene production. Without a reduction in the styrene selectivity, the conversion in the membrane reactor was nearly 7% higher than the hxed-bed reactor at temperatures above 600 °C. Yua and Xu (2011) extracted H2 from ethylbenzene to the styrene reaction side by a palladium membrane. They reported that under the optimal reaction conditions (T = 570 °C, P = 1.4 atm, liquid hourly space velocity = 0.36/h), the styrene yield shows an 11.3% increase in membrane reactor with respect to the conventional one with no considerable change in styrene selectivity. 

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