Liquid membrane system transport mechanisms

Liquid membranes are versatile reagents that can be used to perform separations 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 liquid-membrane system. These mechanisms are ... 

Facilitated transport of penicilHn-G in a SLM system using tetrabutyl ammonium hydrogen sulfate and various amines as carriers and dichloromethane, butyl acetate, etc., as the solvents has been reported [57,58]. Tertiary and secondary amines were found to be more efficient carriers in view of their easy accessibility for back extraction, the extraction being faciUtated by co-transport of a proton. The effects of flow rates, carrier concentrations, initial penicilHn-G concentration, and pH of feed and stripping phases on transport rate of penicillin-G was investigated. Under optimized pH conditions, i. e., extraction at pH 6.0-6.5 and re-extraction at pH 7.0, no decomposition of peniciUin-G occurred. The same SLM system has been applied for selective separation of penicilHn-G from a mixture containing phenyl acetic acid with a maximum separation factor of 1.8 under a liquid membrane diffusion controlled mechanism [59]. Tsikas et al. [60] studied the combined extraction of peniciUin-G and enzymatic hydrolysis of 6-aminopenicillanic acid (6-APA) in a hollow fiber carrier (Amberlite LA-2) mediated SLM system. 

The mechanisms by which various components in a liquid or gaseous feed stream to the membrane system are transported through the membrane structure determine the sq>aiation properties of the membrane. These transport mechanisms are quite different in liquid and in gas or vapor phases. So are their effects on permeate flux (or permeability) and retention (or rejection) coefficient or separation factor in the case of gas separation. 

General properties of liquid membrane systems have been a subject of extensive theoretical studies. Six basic mechanisms of transport are schematically shown in Figure 13.2. In a simple transport (Figure 13.2a), solute passes through due to its solubihty... 

There have been several modified systems since the invention of liquid membranes, including a facilitated transport mechanism. One of them is to disperse the receiving solution in an organic membrane phase on one side of a porous hollow fiber.Two plants to treat contaminated groundwater were built and operated based on this revised liquid membrane system. More discussion about this application is given below in the application section. 

More complicated separations can be achieved by using one or more of the facilitated transport" mechanisms that can be built in a liquid-membrane system.1 2 These mechanisms ate ... 

The transport of cobalt(II), copper(II), nickel(II), and zinc(II) from aqueous sulfate solutions by novel di(p-alkylphenyl)phosphoric acid carriers in bulk and emulsion liquid membrane transport processes is reported by Walkowiak and Gega in Chapter 13. To probe the mechanism of the liquid membrane transport processes, interfacial tension measurements are conducted. A multistage emulsion liquid membrane system for separation of the transition metal cation mixtures is developed. 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

In renewable-based energy systems PEM electrolysis seems to have an advantage over alkaline in that the thin membrane and ion transport mechanism can react to nearly instantaneously with the rapidly changing energy output of renewable sources, especially wind. Stacks involving the circulation of a liquid electrolyte have inherently more inertia in the transport of ions in solution than the PEM systems. 

Polymer membranes have also been used as a "sandwich". In this configuration, the liquid film is supported between two polymer membranes. Ward (18) used two silicone rubber membranes to contain a solution of ferrous ions in formamide. Ward noted that Bernard convection cells could be maintained if the complex were formed at the upper surface. Ward (19) used this same system and membrane configuration to study electrically-induced, facilitated gas transport. The silicone rubber membranes provided the mechanical support so the electrodes could be placed next to each liquid surface. Otto and Quinn (20) immobilized the liquid film in a horizontal layer between two polymer films. The polymer was described as an experimental silicone copolymer having high CO2 permeability as well as excellent mechanical properties. They were studying CO2 facilitated transport in bicarbonate solutions with and without carbonic anhydrase. 

The mechanism of the competitive pertraction system (CPS) is presented schematically in Fig. 5.4 together with the compartmental model necessary for constructing the reaction-diffusion network. The simple flat-layered bulk liquid membrane of the thickness En and interface area S separates the two reservoirs (f, feed and s, stripping) containing transported divalent cations A2+ and B2+ (most frequently Zn2+ and Cu2+ or Ca2+ and Mg2+) and/or antiported univalent cations H+. At any time of pertraction t, their concentrations are [A]f, [B]f, and [H]f and [A]s, [Bj, and [H]s, for the feed and stripping solution, respectively. The hydrophobic liquid membrane contains a carrier of total concentration [C]. Its main property is the ability to react reversibly with cations at respective reaction zone and to diffuse throughout the liquid membrane phase. 

An idealized schematic diagram of alkali metal cation transport across a liquid surfactant (emulsion) membrane by an lonizable crown ether is shown in Figure 7. Thus a metal cation is transported from an external aqueous source phase across the liquid surfactant membrane which forms the outer surface of the emulsion droplet into an interior aqueous receiving phase. Metal ion transport is driven by a pH gradient and back transport of protons from the internal to the external aqueous solution according to the mechanism illustrated earlier in Figure 1. In this system, transport is rapid due to the thin organic membrane. 

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