Liquid membrane system factors

Vicens and coworkers (Chapter 26) report the use of novel calix-fcw-crown ether compounds as carriers in a supported liquid membrane system for the removal of cesium from nuclear waste water. Decontamination factors of greater than 20 are obtained in the treatment of synthetic acidic radioactive wastes. Very good stability (over 50 days) and high decontamination yields are achieved. 

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. 

For radioactive effluent treatment, the relevant membrane processes are microfiltration, ulfrafiltration (UF), reverse osmosis, electrodialysis, diffusion, and Donnan dialysis and liquid membrane processes and they can be used either alone or in conjunction with any of the conventional processes. The actual process selected would depend on the physical, physicochemical, and radiochemical nature of the effluents. The basic factors which help in the design of an appropriate system are permeate quality, decontamination, and VRFs, disposal methods available for secondary wastes generated, and the permeate. 

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. 

Supported liquid membrane stability and lifetime limit the industrial application of this separation technique. Therefore, the stability of these membranes needs to be enhanced drastically. A proper choice of the operating and membrane composition factors might improve the lifetime of SLM systems. 

The instability of the system is a serious challenge in a gas-liquid membrane contactor, for instance, the wetting and bubbling problems occur when the pressure difference across a porous membrane is too high. Factors like pore size, pore size distribution, and hydrophobicity and hydrophilicity of the membrane will play a major role in determining the breakthrough of gas or liquid across the membrane [159]. 

Co(ll) concentration in the feed solution decreased, metal permeation was independent of the mass transfer coefficient and the aqueous diffusion film controlled the permeation process (aqueous mass transfer coefficient = 4.8 x 10 cm/s). On the other hand, separation of Co(II)/Li(I) with the leqnired purity (separation factor of Co/Li 25) is possible by this process under optimized conditions. The performance of the system is better when Acorga PT5050 or Cyanex 272 was used as carriers. Also, membrane diluents chosen in any liquid membrane process influences the membrane performance. In Figure 32.6, the effect of the diluent on Co(II) transport is represented, and... 

Cini et al. (1991b) proposed the use of a tubular Pd/AljOj mesoporous membrane for the hydrogenation of a-methylstyrene to cumene. A comparison between the tubular catalyst and a fully-wetted pellet revealed a rate increase by up to a factor of 20. From that study, several other theoretical (Torres et al, 1994) and experimental ones confirmed that a three-phase membrane reactor can improve the mass transfer rate of gas-liquid-solid systems. 

The two phases (phase 1 and phase 2) are generally aqueous solutions, while the liquid membrane phase is an organic phase which is immiscible with water. The solubility is a very important factor with respect to the stability of these system. This stability effect will be discussed below. 

Ideal separation factor Ideal solution Immersion precipitation Immobilised liquid membranes Inorganic membranes Integrally skinned membranes Imcrfaciai polymerisation Interaction parameter Interactive systems Ion-exchange Ionic membranes Ionic strength Isoelectric point Isotaciic polymers... 

The general types of liquid membranes are reviewed by Peterson and Lamb in Chapter 4, and factors which influence the effectiveness of a membrane separation system are summarized. These factors include the complexation/decomplexation kinetics, membrane thickness, complex diffusivity, anion type, solvent type, and the use of ionic additives. A novel membrane type, the polymeric inclusion membrane, is introduced. 

Liquid membrane (LM) transport is a novel separation technique whose potential for application in effluent treatment in the nuclear industry is being explored (7). LM transport offers several advantages over other separation techniques, e.g., lower capital cost and space requirements, low energy consumption, and low solvent inventory which allows the use of expensive carrier molecules. High feed/strip solution volume ratios, the possibility of utilizing feed solutions which contain suspended solids, and high separation factors are other merits of the LM transport method. A prime requirement for a successful LM system is the selection of a selective and efficient carrier for the species to be separated. 

Transport of a species through a liquid membrane is superior to solvent extraction (SX) since extraction and stripping are performed in a single unit operation. Also, liquid membrane transport is a non-equilibrium, steady state process which depends upon kinetic factors in contrast to SX which is an equilibrium process. Furthermore, even solvents with low distribution coefficients for Ae desired species may be utilized in LM processes. Although LM systems generally have slower rates than ion-exchange (DC) processes, the latter are particularly sensitive to the presence of suspended solids and other foulants and also must be operated in cycles. 

In another study, dicyclohexano-18-crown-6 (DC18C6) was shown to be an excellent carrier in liquid membranes due to its high selectivity for both U(VI) (77) and Pu (72) even in the presence of several other undesirable radiotoxic elements Table 1). Maximum transport through the macrocycle-based BLM and SLM systems (0.2 M DC18C6 in toluene as the membrane phase) was attained from a 3 M nitric acid feed solution for Pu(IV) and 7 M nitric acid for U(VI). The maximum fluxes for Pu(IV) and U(VI) were 1.6 X 10 and 2.9 X 10 mol/mVs, respectively (77,72). Enrichment factors of 6 or greater were readily achieved by proper manipulation of the feed solution/strip solution volume ratios (72). 

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