Aqueous liquid membranes

In the recent past separation of isomers has been attempted using aqueous liquid membranes based on p-cyclodextrin. Thus, separation of a mixture of o- and p-nitroaniline (in 80% i-octanol, 20% -heptane) has been studied, with the p-isomer showing a selectivity of 5 at 0.7 molar p-cyclodextrin. Even stereoisomers of stilbene cis and trans) were separated using a 0.02 to 0.2 M cyclodextrin solution, but the selectivity was less than 2 (Mandal et al, 1998). 

Interestingly, the didecylphosphate anion represents a fixed component of the non-aqueous liquid membrane. As the concentration of Ca+ ions present in the solutions on either side of the membrane varies hence, the concentration of didecylphosphate anion at every membrane surface would also vary accordinly, thereby causing a potential that may be expressed by the following equation ... 

Kislik V. Application of organic and aqueous liquid membranes in separa tion of metals and acids Comparative analysis. In Proceedings of the Abstracts on International Conference Advanced Membrane Technology, Italy, October 2001 214. 

Table 6.11 Separation of isomers by hollow-fiber contained liquid membrane permeator (HFCLMP) using fS-cyclodextrin (P-CD) as a carrier in aqueous liquid membrane...

Emulsion liquid membranes have been formulated for the removal of species without chemical carriers. These systems rely on solubility differences between permeant species. The first work performed by Li (73) dealt with the separation of a bineiry mixture of aromatic eind paraffinic hydrocarbons. The hydrocarbons were encapsulated in an aqueous liquid membrane forming an emulsion which was distributed in a hydrocarbon solvent and mixed. The aromatic hydrocarbons preferentially permeated through the aqueous liquid membrane phase due to solubility differences. Cahn and Li (7 ) describe a liquid membrane formulation for phenol removal in which sodium hydroxide is encapsulated by an organic liquid membrane. The resulting membrane phase is mixed with a continuous aqueous phase containing phenol. 

Exchange Procedures for Immobilization of Aqueous Liquid Membrane In Hydrophobic Hollow Fibers... 

Discussion. The method and technique used for fabrication of the aqueous liquid membranes are very important in achieving good stability. Membranes filled by condensation methods are much more stable than those which are soaked and allowed to dxy. One possible reason for this may be that the condensation method avoids the problem of ovofilling and thus ensures good curvature at the pore opeiungs. 

Applications of liquid membrane technology, 110-122 Aqueous liquid membranes, preparation, 141,142/... 

The electrical potential difference would arise on account of difference in (CP) in the two compartments, which would be controlled by the influx of NHj in the form of NH4" ion and subsequent interaction at the aqueous/liquid membrane interface particularly in compartment B. Hence, there is a larger concentration of CF in compartment A as compared to that in compartment B. Consequently, compartment B (containing KCl) becomes negative while compartment A becomes positive. The positive current one Faraday of electricity will move form left to right which would result in the transfer of t+g-ion of cations from left to right (A->B) and transfer of CF in the opposite direction. Noting that there are different electrolytes in different compartments, the liquid-junction potential or diffusion potential A(j> would be given by... 

The first applications employed symmetric microporous hollow fibers in which the fiber materials were hydrophobic or hydrophilic. The membrane liquid may or may not wet the pores spontaneously. To separate a gas mixture in a permeator employing symmetric hydrophobic microporous fibers with gas-filled pores and nonwetting aqueous liquid membranes (Figure 2a), the liquid membrane pressure (PjJ is maintained higher than those of the feed gas (Ppo) and the sweep gas (Psg) to prevent their dispersion in the membrane liquid (4), However, the excess pressure of the liquid membrane over that of either gas stream must be less than the breakthrough pressure for the membrane liquid (which is a function of the membrane material, pore size, and the interfacial tension between the pore fluid (here, gas) and the membrane liquid). For other fibers and configurations. Figures 2b and 2c provide appropriate details. 

Gas Separation at Higher Pressures. In the basic gas separation technique using aqueous liquid membranes and symmetric microporous hydrophobic fibers, the excess pressure of the liquid membrane over the gaseous strip/sweep/permeate streams can at the most be around 100 psi for currently available hydrophobic microprous polypropylene membranes (minimum pore size 0.03 pm) without membrane liquid breakthrough especially on the sweep side. Smaller pore sizes can enhance this value. However, a smaller pore size is also generally accompanied by a reduction in porosity and an increase in tortuosity. This substantially increases the resistance to transport. 

In the above examples, both the feed and the permeate phases were gaseous and aqueous liquid membranes were used. Cahn and Li (1976) have demonstrated facilitated transport separation of the olefin 1-hexene from n-heptane through an aqueous membrane containing cuprous ammonium acetate the permeate phase was made up of n-octane. Kuo and Gregor (1983) have illustrated facilitated transport of acetic acid from an aqueous solution through an organic membrane of decaUn containing trioctylphosphine oxide (TOPO (0113(012)7)31 )), which complexed with acetic acid. 

The hicilitated transport separation of CO2 from a gas mixture through an aqueous liquid membrane of diethanolamine (DEA) occurs via the following reactions ... 

Reaction (2) is considerably faster than reaction (1) (Danckwerts and Sharma, 1966), and is therefore assumed to be in equilibrium (the equilibrium constant for this protonation reaction is Kp, whereas that for (1) is Assume a negligible H contribution to the electroneutrality relation. Develop two governing ordinary differential equations (one for CO2 and the other for the free amine R2NH) describing the facilitated transport of CO2 and write the boundary conditions. You are given that the total initial amine concentration in the aqueous liquid membrane is C nh- The governing ordinary differential equations should contain only the concentrations of free CO2 and free amine and C, nh-... 

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