Countercurrent liquid membrane separation

A most comprehensive description of the emulsion liquid membrane is available in Ho and Sirkar (2001, 

Phenoi reaction inside dropiets phenoi + NaOH 

Externai continuous phase, feed Membrane phase 

In this particular example, the extractant used was a sulfur-containing derivative of di-2-ethyUiexylphosphoric 

Due to the presence of a strong concentration of H2SO4 in the internal droplet phase, its osmotic pressure is far 

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As described above, the application of classical liquid- liquid extractions often results in extreme flow ratios. To avoid this, a completely symmetrical system has been developed at Akzo Nobel in the early 1990s [64, 65]. In this system, a supported liquid-membrane separates two miscible chiral liquids containing opposite chiral selectors (Fig. 5-13). When the two liquids flow countercurrently, any desired degree of separation can be achieved. As a result of the system being symmetrical, the racemic mixture to be separated must be added in the middle. Due to the fact that enantioselectivity usually is more pronounced in a nonaqueous environment, organic liquids are used as the chiral liquids and the membrane liquid is aqueous. In this case the chiral selector molecules are lipophilic in order to avoid transport across the liquid membrane. 

In the following part of this section, we provide simple mathematical descriptions of a few common features of two-phase/two-region countercurrent devices, specifically some general considerations on equations of change, operating lines and multicomponent separation capability. Sections 8.1.2, 8.1.3, 8.1.4, 8.1.5 and 8.1.6 cover two-phase systems of gas-Uquid absorption, distillation, solvent extraction, melt crystallization and adsorption/SMB. Sections 8.1.7, 8.1.8 and 8.1.9 consider the countercurrent membrane processes of dialysis (and electrodialysis), liquid membrane separation and gas permeation. Tbe subsequent sections cover very briefly the processes in gas centrifuge and thermal diffusion. 

Fig. 5-13. Schematic representation of the Akzo Nobel enantiomer separation process. Two liquids containing the opposing enantiomers of the chiral selector (FI and K) are flowing countercurrently through the column (4) and are kept separated by the liquid membrane (3). The racemic mixture to be separated is added to the middle of the system (1), and the separated enantiomers are recovered from the outflows of the column (2a and 2b) [64],...

In Section 5.4.4, we studied a variety of chemical reaction facilitated separation where the reaction was taking place in a thin liquid layer acting as the liquid membrane Figure 5.4.4 illustrated a variety of liquid membrane permeation mechnisms. Here we will identify first the structural configuration of the liquid membranes as they are used in separators with countercurrent flow pattern (as well as for the cocurrent flow pattern). There are three general classes of liquid membrane structures emulsion liquid membrane (ELM) supported liquid membrane (SLM) or immobilized liquid membrane (ILM) hollow fiber contained liquid membrane (HFCLM). Each will be described very briefly. 

Another type of gas exchange process, developed to the pilot plant stage, is separation of gaseous olefin/paraffin mixtures by absorption of the olefin into silver nitrate solution. This process is related to the separation of olefin/paraffin mixtures by facilitated transport membranes described in Chapter 11. A membrane contactor provides a gas-liquid interface for gas absorption to take place a flow schematic of the process is shown in Figure 13.11 [28,29], The olefin/paraffin gas mixture is circulated on the outside of a hollow fiber membrane contactor, while a 1-5 M silver nitrate solution is circulated countercurrently down the fiber bores. Hydrophilic hollow fiber membranes, which are wetted by the aqueous silver nitrate solution, are used. 

In mass transfer apparatus one of two processes can take place. Multicomponent mixtures can either be separated into their individual substances or in reverse can be produced from these individual components. This happens in mass transfer apparatus by bringing the components into contact with each other and using the different solubilities of the individual components in the phases to separate or bind them together. An example, which we have already discussed, was the transfer of a component from a liquid mixture into a gas by evaporation. In the following section we will limit ourselves to mass transfer devices in which physical processes take place. Apparatus where a chemical reaction also influences the mass transfer will be discussed in section 2.5. Mass will be transferred between two phases which are in direct contact with each other and are not separated by a membrane which is only permeable for certain components. The individual phases will mostly flow countercurrent to each other, in order to get the best mass transfer. The separation processes most frequently implemented are absorption, extraction and rectification. 

Another example of the use ofHF-SLM separation concerns the resolution of racemic ofloxacin [200]. This important drug, a fluoroquinolone antibiotic with one chiral center, was separated in chiral systems by hoUow-fiber liquid-supported membrane technology combining with countercurrent fractional extraction. The two chiral solutions contained L-dibenzoyltartaric acid and D-dibenzoyltartaric acid in 1-octanol, and flowed through the lumen side and the sheU side of fibers, respectively. The solution which Uowed through the lumen side of fibers also contained racemic ofloxacin. The waU of hoUow fibers was fiUed with an aqueous of 0.1 mol/l Na2[ ll O4/[ fd O4 buffer solution of pH 6.86 containing 2 mmol/1 of cetyltrimethylammonium bromide for 48 h. The obtained optical purity for ofloxacin enantiomers was up to 90% when 11 hoUow-fiber membrane modules of 22 cm in length in series were used. 

Separation and enrichment of metal ions, using reversible chemical reactions in liquid or gel-type membranes Using selective extragents, it is possible to obtain high separation and enrichment factors for metal ions (e.g., Cu, In, U, Pu), with a simultaneous extraction and stripping (pertraction). Driving force for this process is the countercurrent diffusion of... 

The principle and unique design of the separation column is shown in Fig. 1 and Figs. 2 and 3, respectively. In Fig. 1, a pair of separation channels is partitioned by a dialysis membrane. A concentrated (C) AS solution is introduced through the upper channel at a high flow rate (V) from the right, whereas water is fed into the lower channel from the left at a much lower rate (v). This countercurrent flow of the two liquids through the channels results in AS transfer from the upper channel to the lower channel at every point, as shown by arrows across the membrane. Because the AS transfer rate... 

In sum, distillation is a countercurrent vapor-liquid operation with the external reflux of a condensed liquid phase at the top and the external recycle or reflux of reboiled or vaporized vapor at the bottom. This reflux or recycle feature produces a sharp separation between the two key components of the feed mixture, and the same applies to membrane operations. The key components are the two components of a mixture between which the separation is to be made. 

System type (4) Two miscible phases flow countercurrently in two regions of the device separated by a membrane. One of the phases may be generated from one feed phase by the application of pressure energy. Examples include reverse osmosis, ultrafiltration, microflltration, gas permeation, pervaporation. Examples where the other phase is introduced from outside are electrodialysis, dialysis, sweep vapor/ liquid based system. 

Figure 8.1.4(c) introduces through the example of dialysis (see Section 4.3.1) a countercurrent flow configuration of a membrane device where two separate feed streams tu-e entering the separator as in the separation system type (2). In the electrodialysis (Section 3.4.2.S) process of selective transport of ions through an ion exchange membrane, the liquid solutions on two sides of any ion exchange membrane are sometimes in countercurrent flow. 

Figure 8.1.4. (a) Type (4) systems. Countercurrent flow of feed gas mixture and permeated gas mixture in a membrane device, (b) Continuous membrane column method of gas mixture separation, (c) Countercurrent dialyzer with the feed solution and the dialyzing liquid entering the device countercurrently on two sides of the membrane. 

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