Liquid membrane supports operating pressure

Way, Noble and Bateman (49) review the historical development of immobilized liquid membranes and propose a number of structural and chemical guidelines for the selection of support materials. Structural factors to be considered include membrane geometry (to maximize surface area per unit volume), membrane thickness (<100 pm), porosity (>50 volume Z), mean pore size (<0.1)jm), pore size distribution (narrow) and tortuosity. The amount of liquid membrane phase available for transport In a membrane module Is proportional to membrane porosity, thickness and geometry. The length of the diffusion path, and therefore membrane productivity, is directly related to membrane thickness and tortuosity. The maximum operating pressure Is directly related to the minimum pore size and the ability of the liquid phase to wet the polymeric support material. Chemically the support must be Inert to all of the liquids which It encounters. Of course, final support selection also depends on the physical state of the mixture to be separated (liquid or gas), the chemical nature of the components to be separated (inert, ionic, polar, dispersive, etc.) as well as the operating conditions of the separation process (temperature and pressure). The discussions in this chapter by Way, Noble and Bateman should be applicable the development of immobilized or supported gas membranes (50). 

The immobilization method was also found to have influence on the membrane stability. A comparative study of the preparation of SILMs by two different methods, under pressure and vacuum were reported by Hemandez-Femandez et al. [26]. They used the ionic liquids, [bmim+][Cl"], [bmim ][BF ], [bmim ][PF "] and [bmim llNTf ] as liquid phase supported on a nylon membrane. Small losses of ionic liquid were observed after 7 days of operation when the ionic liquid was immobilized under pressure in a diffusion cell using n-hexane/n-hexane as surrounding phases. However, the losses of IL were higher when immobilization was carried out under vacuum, especially with the most viscous ionic liquids ([bmim+] [PF ] and [bmim+][CT]). This behaviour was explained by the fact that the higher viscosity of ILs makes difflcult their penetration into the middle of the deeper pores of the membrane, and therefore, the ionic liquid was mainly immobilized on the most external layer of the membrane, and consequently, the immobilized ionic liquid is more easily removed during operation. 

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. 

The same principle of operation as described above is applicable also to liquid-liquid extraction where an aqueous liquid and an organic liquid contact each other inside the contactor for extraction of a solute selectively from one phase to another [6-8]. The critical breakthrough pressure for liquid-liquid system could be calculated by Equation 2.1, except that the term A would now be the interfacial tension between the two liquids. Further variation of membrane contacting technology is called gas membrane or gas-gap membrane where two different liquid phases flow on either side of the membrane, but the membrane pores remain gas filled [9-10]. In this situation two separate gas-hquid contact interfaces are supported on each side of a single membrane. 

Anisotropic cellulose ester fibers (useful for reverse osmosis) with a dense skin and porous substructure were employed as supports for Immobilizing aqueous AgN03 solutions for separating olefins from paraffins ( ). However, these authors did not operate under any significant AP conditions, primarily to reduce membrane liquid loss under positive applied pressure differential. 

When the operation of a solenoid valve makes one of the three pneumatic actuators on the microchip connected with the compressed air source, as shown in Fig. 4a, the pressure in the pneumatic actuator will rise up and the elastic membrane will deflect propelling liquids and consequently shutting down the flow in the liquid flow channel. When the operation of a solenoid valve makes a pneumatic actuator connected with the atmosphere, as shown in Fig. 4b, the pressure in the pneumatic actuator will equal to the atmospheric pressure. The elastic membrane will release and the actuator stops working. The time-phased deflection of three elastic membranes along the microchannel length can be realized by the sequence actuation of three external pneumatic solenoid valves. Since each solenoid valve in the supporting system is connected directly to one actuator on the microfluidic chip. 

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