BAHLM membrane

Therefore, all above-mentioned bulk LM processes with water-immiscible hquid membrane solutions may be unified under the term bulk organic hybrid hquid membrane (BOHLM) systems. Bulk LM processes with water-soluble carriers [22] are defined as bulk aqueous hybrid liquid membrane (BAHLM) systems. These new technologies have the necessary transport and selectivity characteristics to have potential for commercial apphcations and are considered in detail in the respective chapters. 

Theoretical models (analytical and numerical), developed for simulation of the BOHLM and BAHLM transport kinetics, are based on independent experimental measurements of (a) individual mass-transfer coefficients of the solutes in boundary layers and (b) facilitating parameters of the liquid membrane (LMF potential) and lEM potential in the case of ion-exchange membrane (lEM) application. Satisfactory correlation between experimental and simulated data is achieved. 

Selectivity parameters, needed for the BOHLM or BAHLM module design and their determination techniques, are analyzed. Selectivity can be controlled by adjusting the concentration, volume, and flow rate of the LM phase. Such control of the selectivity is one of the advantages of the bulk liquid membrane systems in comparison with other liquid membranes configurations and Donnan dialysis techniques. The idea of dynamic selectivity and determination techniques are presented and discussed. 

Applications of the BAHLM technology (Chapter 6) in metal ions, salts separation, biotechnological, and isomers separations are reviewed. Commercially available membrane modules and equipment may be used in the BOHLM and BAHLM. It should be noted that Chapters 5 and 6 may also contain relevant information for other fields. 

Exciting opportunities also exist in design of production cycles by combining various LM operations suitable for separation and other separation/ conversion units, thus reahzing highly integrated membrane processes. Examples include BOHLM and BAHLM processes where membrane... 

Bulk Aqueous Hybrid Liquid Membrane (BAHLM) Processes with Water-Soluble Carriers Application in Chemical AND Biochemical Separations... 

BAHLM) separation process [1-7] overcomes most of these problems. As can be seen from the scheme in Fig. 6.1, the technological concept of BAHLM transport is quite simple an aqueous solution of a carrier, E, flows between two membrane barriers, which separate the carrier from the feed, F, and strip, R, aqueous solutions. It can be seen that the BAHLM system is similar to the BOHLM except the liquid membrane... 

Figure 6.1 Schematic diagram of the three-aqueous phase (BAHLM) module F, E, and Ru are the compartments of the feed, LM, and receiving (strip) solutions, respectively Mi and AI2 are the ion-exchange membranes 1 and 2 are the inlet and outlet of the feed, LM, andstrip solutions, respectively. Gaskets, madeofVytone, were inserted between compartments and membranes. H is the width of the compartment. From Ref. [5] with permission.

Feasibility tests of the BAHLM system [3-5, 7, 14-17] show promising results relatively high transport rates of solutes at high selectivities, and a long lifetime of the membranes. 

Donnan dialysis The BAHLM systems with ion-exchange membranes, based on Donnan dialysis [18,19], will be considered below. Donnan dialysis is a continuously operating ion-exchange process. There are many theoretical models describing transport mechanisms and kinetics of DL) [18-26]. All transport kinetics models are based on Fick s or Nernst-Planck s equations for ion fluxes. In both cases, the authors introduce many assumptions and simplifications. 

The BAHLM differs from all bulk liquid membrane systems by application of polyelectrolyte aqueous solutions as carriers and charged (ion-exchange) membranes (lEM) as barriers. As can be seen in Fig. 6.2, the physicochemical aspects of the BAHLM processes are complicated. Transport of solutes or their complexes consist of the following steps (see Fig. 6.2A) ... 

For the BOHLM systems (see Chapter 5) with water-immiscible carriers, the concentration gradient-driven solute-solvent complexation/ decomplexation interactions are the dominant driving forces. For the BAHLM systems, Donnan membrane potential [18-26, 32-36], osmotic pressure gradient [27, 37], and possibly pressure gradient [38-40], have to be added as driving forces. Therefore, the theory should take into account both diffusive and convective transport. 

Based on Donnan s [18, 19] and Onsager s [41, 42] fundamental works, the theories for Donnan dialysis systems were developed [20-26, 32-36]. The BAHLM system could be considered as two DD systems, operating in consecutive order, continuously in one module (see Fig. 6.2) the first is composed of feed/LM and the second is composed of LM/strip compartments, separated by ion-exchange membranes. Therefore, the Kedem-Katchalsky equations [43, 44] can be applied to our case ... 

As described in the previous chapter for the BOHLM system, the BAHLM system is driven also by two main driving forces the external driving force, derived from the coupling effect ofthe BAHLM system, and the internal (carrier) driving force, derived from the extraction distribution ratio of solute between the liquid membrane phase and feed, and receiving phases. FCjp and Ehp. denoted as external driving force coefficients and Keff are denoted as internal (carrier) driving force coefficients. 

This is a set of model equations for simulation and preliminary optimization of the BAHLM parameters membrane -working area feed, carrier, strip solutions initial concentrations flow velocities, etc. 

Huge amounts of liquid byproduct and waste effluents in the fertilizer industry contain various heavy metals, some of which are highly toxic. Cd, Cu, and Zn are commonly encountered in these effluents and are selected for selective removal studies using liquid membrane systems [1-7, 14-17]. Below, experimental and calculated data, obtained for Cd, Cu, and Zn separation, are used for the BAHLM process design considerations. 

Internal (carrier) driving force coefficients, and K, <, or distribution coefficients, Ep and E, are determined by membrane-based extraction experiments. Membrane-based forward and backward extraction is carried out in two-compartment modules using the F and R compartments, separated by the same membranes as in the BAHLM tests. The experiments lasted up to equilibrium conditions, when the concentration of solutes in every compartment does not change with time. Examples of membrane-based extraction of copper, cadmium, and zinc from the concentrated phosphoric acid solution by PVSH and backward extraction by 2 M HCl are presented in Table 6.2 [7]. 

So, the rate-controlling step of the BAHLM transport of the solute to the strip phase is determined by the Kf/e overall mass-transfer coefficient. In this case, at designing the BAHLM module, the main attention has to be taken to the determination of optimal feed-side membrane area. 

Nondiffusible ionic species of the poly electrolyte present in the carrier membrane solution of the BAHLM should affect the anomalous osmosis in both sides in the feed-side and strip-side membranes. The disadvantage of conventional osmosis, dilution ot the product, may be obviated if not turned out to the advantage of enrichment of the product. 

Let us evaluate the BAHLM system for the removal of cadmium from industrial wet-process phosphoric acid (WPA) (for details, see Ref. [7]) containing 50 ppm Cd ([Cd],J and 20 ppm Cu ( Cu]ini, and reducing Cd to a final concentration of 1 ppm ([Cd] ). We would also like to evaluate the BAHLM system for the selective separation of cadmium and copper from WPA. Using the known data presented in Table 6.7, the effective feed-side membrane area, (Sp), can be evaluated by equation ... 

We can now design a preliminary pilot setup. We use a spiral-type, flowing liquid membrane module, developed by the Teramoto group [87, 88], in which the effective membrane area is about 40% of the total membrane area (the increase of the membrane area is mainly due to blocking of the membrane surface by spacers, and by the adhesive used to seal the sides of the module). For our system, the total feed-side membrane area is 570 m and the total strip-side membrane area is 763 m , in which 360 m is the area needed for the separation of the strip solution concentrated by copper. By designing standard, three-compartment spiral-type BAHLM modules, with 100 m of the membrane on each side (feed and strip), and two-compartment modules, with 200 m of the membrane, we will obtain a setup, of six standard three-compartment modules and one two-compartment module connected in consecutive order (see Fig. 6.7). After the fourth module, we will... 

The low mass-transfer rates of copper and cadmium on the strip side of the designed BAHLM system wiU lead to the accumulation of about 15% cadmium and copper in the liquid membrane phase after each cycle. Periodical regeneration of the liquid membrane solution wiU be needed to purify it from metal ions. Additional research for choosing poly electrolytes with more effective transport characteristics is needed. 

Copper-cadmium separation from chloride aqueous solutions [4, 5, 14]. The copper-cadmium separation tests were carried out in a BAHLM module comprising (I) aqueous solutions ofCuCl2 + CdCL, 0.1-0.2 mol/kg each as the feed solutions, (II) an aqueous 0.5 mol/kg PVSNa solution as the liquid membrane (see Table 6.5), and (III) aqueous 0.5—2.0 mol/kg NaCl solutions as the receiving solutions [5]. Cation-exchange membranes Neosepta CM-1 or CMS were used as barriers between the solutions (see Table 6.6). 

Transport of carboxylic acids into a strong basic strip solution. BAHLM systems were tested for the transport of carboxylic acids such as lactic (HLac), citric (H3Cit), and acetic (HAc) or their anions. In feed solutions containing a mixture of HLac and H-,Cit or HAc (or their anions), the initial molar ratio between the former and the other acid is 8/1. Aqueous 0.5 mol/kg BPEI solution was used as the carrier, anion-exchange membranes ACH-45T or AM-3 were used as the barrier between the phases, and 1 mol/kg sodium hydroxide was used as the receiving (strip) solution. 

Transport of carboxylic acid or its salt into a mineral acid-containing strip solution. The BAHLM system—contained 1 mol/kg NaLac feed solution, 0.5 mol/kg BPEI carrier solution, and 1.1 mol/kg HNO3 strip solutions [5]—was tested. AM-3 membranes were used as a barrier. Flux of lactate to the strip phase versus time is presented in Fig. 6.14. 

Figure 6.14 Transport rates of lactate anion into the strip solution in BAHLM system with 0.5 mol/kg BPEI aqueous solution as a carrier and Tokayama Soda AM-3 membranes. Initial compositions feed 1.0 mol/kg NaLac, strip 1.1 mol/kg HNO3. From Ref. [5] with permission.

In comparison with other hquid membrane systems, BAHLM technology has some decisive advantages, such as ... 

The BAHLM modules may be based on commercially available ion-exchange hollow-fiber or spiral membrane modules and equipment... 

Manipulation by the carrier concentration (and therefore, by the LMF potential), by the volume of the circulating bulk LM solution, or by both, enables to obtain selectivity that is as close to its highest level as necessary for the process developed. It is one of the biggest advantages of the BAHLM system in comparison with other liquid membrane technologies. 

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