Boundary layer, membrane extraction

A Effective membrane area C, Concentration of metal ion in bulk feed at time t Co Initial concentration of metal ion in bulk feed Membrane thickness Thickness of aqueous boundary layer Membrane diffusion coefficient Aqueous diffusion coefficient Aqueous feed film mass transfer coefficient Organic membrane phase mass transfer coefficient Overall mass transfer coefficient Distribution ratio Two-phase extraction constant Aqueous feed mass transfer coefficient Membrane mass transfer coefficient Aqueous strip mass transfer coefficient Length of the fiber Molecular weight of the complex Number of fibers Permeability coefficient (cm/s)... 

Guo, X. and S. Mitra. 1999. Enhancement of extraction efficiency and reduction of boundary layer effects in pulse introduction membrane extraction. Anal. Chem. 71 4407 -412. 

The greatest challenge in membrane extraction with a GC interface has been the slow permeation through the polymeric membrane and the aqueous boundary layer. The problem is much less in MIMS, where the vacuum in the mass spectrometer provides a high partial pressure gradient for mass transfer. The time required to complete permeation is referred to as lag time. In membrane extraction, the lag time can be significantly longer than the sample residence time in the membrane. An important reason is the bound-... 

Gas injection membrane extraction (GIME) of aqueous samples has been developed to address the issues of boundary layer effects and sample dispersion [66]. This is shown in Figure 4.20. An aqueous sample from the loop... 

Variations of the feed and strip flow rates have little effect on the cadmium transport performance the values of individual cadmium mass-transfer coefficients are similar at carrier or strip flow rates variations. Thus, diffusion of cadmium species through the feed and strip aqueous boundary layers does not control the transport rate. The ratecontrolling steps could act as resistances to diffusion of the cadmium species in the carrier solution layers, especially in the membrane pores or the interfacial backward-extraction reaction kinetics. 

The transport of the substances from the feed solution to the strip side can be divided into the foUowing steps diffusion of substance S across the boundary aqueous layer in the feed (donor) phase, extraction (sorption) of substance on the donor/membrane phase interface, diffusion across the boundary layer on the feed (donor) side, convection transport in the liquid membrane zone, diffusion across the boundary layer on the strip (acceptor) phase of LM, re-extraction (desorption) on the membrane/strip phase... 

Danesl and coworkers have developed a model for metal extraction using supported liquid membranes. Danesl et al. (74) included both Interfaclal reaction and boundary layers In their analysis. As they demonstrate, both effects can be Important. Recently, Danesl (75) developed a simplified model of metal extraction In hollow fiber membranes based on the model above. Danesl and Relchley-Ylnger (76) have expanded this model to Include deviations from a first order rate law. 

In membrane extraction of metals, the mass transport of solute from one phase to another occurs by diffusion. It is controlled by phase equilibrium and the resistances of boundary layers in two phases and the membrane material. Both types of materials are used for membrane extraction and stripping, hydrophilic and hydrophobic, and composite hydro-philic/hydrophobic barriers are also developed to avoid the membrane solubilization [122,123]. To enhance separation, the reactive liquids that induce chemical reaction with one of the separated species can be used. In membrane SX of metals, extracting agents, such as tri- -octylphosphine oxide (TOPO), di(2-ethylhexyl)phosphoric acid (D2EHPA), and n-octyl(phenyl)-A,A-diisobutylcarbamoylmethylphosphine oxide (CMPO), and commercial reagents like CYANEX 301, CYANEX 923, LIX622, and LIX622N are applied. 

Step 2 At the aqueous-organic interface, the solute of the aqueous phase reacts with the carrier present in the organic phase in the membrane pore to form the solute-extractant complex species. The ions released to the aqueous phase boundary layer by the chemical reaction with the carrier diffuse to the bulk of the aqueous phase. 

The transport process of As through the SLM using Cyanex 921 as extractant takes place in five steps in series (1) As(V) diffusion from feed bulk to membrane surface in the non-stirred boundary layer at the feed-membrane interface (2) complexation As(V) at membrane interface feed side, thus forming the As(V)-Cyanex 921 complex (3) diffusion of the As(V)-Cyanex 921 complex through the membrane thickness (4) stripping of As(V) from the As(V)-Cyanex 921 complex at membrane interface strip side (5) As(V) diffusion from membrane surface (strip side) to strip bulk, through the non-stirred boundary layer at the strip-membrane interface. 

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