Liquid immobilized membranes

Figure 11.24 Facilitated transport of hydrogen sulfide through an immobilized carbon-ate/bicarbonate solution [26]. Reprinted with permission from S.L. Matson, C.S. Herrick and W.J. Ward III, Progress on the Selective Removal of H2S from Gasified Coal Using an Immobilized Liquid Membrane, Ind. Eng. Chem., Prod. Res. Dev. 16, 370. Copyright 1977, American Chemical Society and American Pharmaceutical Association...

Supported liquid membrane (SLM) Aqueous solution An immiscible immobilized liquid membrane containing specific metal ion carriers Divalent metal ions Integrative field sampling, preconcentration of trace elements, mimicking biological membranes Days 74... 

FIGURE 37 Mechanisms of carrier-facilitated immobilized liquid membrane extraction, also referred to as coupled transport. The species, R, refers to the carrier component responsible for complexation. 

Shukla, J.P., Sonawane, J.V., Kumar, A., and Singh, R.K., Amine facihtated up-hill transport of plutonium(IV) cations across an immobilized liquid membrane. Ind. J. Chem. Tech., 1996, 3 145-148. 

As for CO2, VOCs can also be removed by using immobilized liquid membranes. Obuskovic et al. [35] immobilized a thin layer of silicone oil in the microporous of the hollow fiber polypropylene membrane beneath the dense-coated skin. The performance of the system has been proved for toluene, methanol, and acetone removal from N2. With respect to the simple hollow fiber, the presence of the oil layer led to a 2-5 VOC more enriched permeate (due to the reduction of nitrogen flux) with a separation factor of 5-20 times higher (depending on the VOC and the feed gas flowrate). The membrane was stable for 2 years. 

Quinn R, Appleby JB, and Pez GP. Hydrogen sulfide separation from gas streams using salt hydrate chemical absorbents and immobilized liquid membranes. Sep. Sci. Technol. 2002 37 627-638. 

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). 

Criteria for immobilized liquid membrane (ILM) support selection can be divided into two categories structural properties and chemical properties. Structural properties include geometry, support thickness, porosity, pore size distribution and tortuosity. Chemical criteria consist of support surface properties and reactivity of the polymer support toward fluids in contact with it. The support thickness and tortuosity determine the diffusional path length, which should be minimized. Porosity determines the volume of the liquid membrane and therefore the quantity of carrier required. The mean pore size determines the maximum pressure difference the liquid membrane can support. The support must be chemically inert toward all components in the feed phase, membrane phase, and sweep or receiving phase. 

By judicious choice of the membrane liquid, complexation agent and support, immobilized liquid membranes (ILM) can have both high selectivity and high permeant fluxes. Liquid membranes have the additional advantage that diffusion coefficients in liquids are several orders of magnitude larger than in polymeric membranes. Previously reported ILM research in the literature includes purification and recovery processes in both gas and liquid phases ( ). This variety of applications creates different requirements for supports for ILMs. This paper discusses criteria which influence selection of ILM support materials. 

Immobilized liquid membranes have traditionally been prepared using commercially available materials such as ultrafiltration or reverse... 

Hughes immobilized AgN03 solutions in cellulose acetate hollow fibers to prepare immobilized liquid membranes for ethylene and propylene transport. 

Figure 1. Cross Section of an Immobilized Liquid Membrane...

The pressure difference is directly proportional to the cosine of the contact angle. For a nonwetting fluid, 6 approaches 90°, and AP approaches zero. The implication of immobilizing a nonwetting or poorly wetting fluid through solvent-exchange or other methods is that the Immobilized liquid membrane would have little resistance to small transmembrane pressures. 

Several investigators have demonstrated the feasibility of immobilized liquid membrane gas separations in applications where large pressure differences are encountered such as acid gas removal from synthetic natural gas. The immobilized liquid membranes prepared by Kimura et al. (16) using 100 pm cellulose acetate supports withstood CO2 partial pressure differences of up to 6.89 10 N/m. Matson et al. (15) used mlcroporous cellulose acetate and polyethersulfone films of 25-75 pm thickness to successfully immobilize potassium carbonate solutions for H2S transport at pressure differences of up to 2.07 10 N/m. The ILMs were supported by macroporous non-wetting polymer films such as polypropylene and polytetrafluoro-ethylene to increase the resistance to high transmembrane pressures. 

Bryjak, M., Wieczorek, P., Kafarski, P., Lejczak, B. (1988). Crown-ether mediated transport of amino acids through an immobilized liquid membrane. J. Membr. Sci., 37, 287-91. 

Liquid membranes can be prepared in two different configurations (see fig. 1). A liquid can be Impregnated in the pores of a porous solid for mechanical support. This form is commonly known as an immobilized liquid membrane (ILM). In the alternate configuration, the receiving phase is emulsified in an immiscible liquid membrane. This type of liquid membrane is known as a liquid surfactant, or emulsion liquid membrane (ELM). 

Immobilized Liquid Membranes. Facilitated transport liquid membranes for gas separations can be prepared In several configurations. The complexatlon agent solution can be held between two nonporous polymer films (2j1), Impregnated Into the pore structure of a micro-porous polymer film (25), or the carrier can be exchanged for the counterion In an Ion exchange membrane (it). 

However, Immobilized liquid membranes supported with porous substrates have two prlmeu y experimental problems loss of solvent and loss or deactivation of the carrier. Matson et al. ( ) prevented evaporative loss of liquid by maintaining the relative humidity of the gas streams in the range of 60 to 90%. Another problem may arise when humidification is used. If solvent condenses out of the feed gas stream onto the ILM and a pressure gradient between the feed and sweep gas stream exists, solvent may flow through the support pore structure leaching the carrier out of the membrane. 

Immobilized Liquid Membranes. Many of the early studies on liquid membranes dealt with Immobilized liquid membranes. Therefore, a large amount of modeling describes these systems. Also, many of the modeling efforts have focused on facilitated transport where a nonvolatile carrier Is present In the membrane. The reaction scheme most often used Is = AB where A is the solute to be separated, B Is the nonvolatile carrier, and AB is the carrier-solute complex. 

---