BAHLM

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... 

As potential directions for the BAHLM systems development, drug separations from biochemical mixtures, fermentation, catalysis and separation with enrichment of valuable compounds (BAHLM bioreactors), desalination of wastewater, and sea water and some integrated water-soluble complexing/filtration techniques are considered. It is suggested that the proposed BAHLM techniques may successfully and effectively replace the presenting separation systems with lower capital and operational costs. 

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. 

In this chapter, the basic principles of the BAHLM processes—such as theoretical considerations in the mass transfer and the transport kinetics, considerations in the process development and module design, and selected applications in chemical and biochemical products separation—will be discussed. 

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 following theoretical analysis of the BAHLM transport mechanisms and kinetics is based on the approach ... 

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. 

Figure 6.2 Simulation of the BAHLM transport (A) concentration profiles in a regular scheme and (B) concentration profiles in a simplified scheme. Layers controlling the permeation rate are h(, feed-side boundary layer feed-side lEM filled with the feed solution hg., feed-side boundary layer of the LM solution strip-side boundary layer of the LM solution strip-side lEM filled with the strip solution /i strip-side bound-...

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 ... 

At the absence of pressure-driven transport, Ap = 0, 5 = 0. After some transformations, we obtain the basic equation for the BAHLM model ... 

The second component in Eq. (3), 2<,cCs( A7t), represents the solute volumetric flux, driven by osmotic pressure gradient. Osmotic mass-transfer coefficient corresponds to the osmotic overall mass-transfer coefficients fCrtF on the feed side and on the strip side of the BAHLM system which... 

The interactions of fixed ion charges with counterions inside the pores of the lEM and in the polyelectrolyte molecules of the LM are expressed through the counterion concentration gradients (see Fig. 6.2A) [Aexperimental measurement of some parameters. Numerical integration and a considerable amount of computation time are required for most of them. 

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. 

The overall mass-transfer coefficients of the BAHLM system were calculated by equations ... 

In the model for the BOHLM systems [46], only the solute concentration was stated as a time-dependent variable. In the BAHLM system, there are two time-dependent variables solute concentration and solution volume. Transforming the two variables into one, we obtain... 

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. 

It must be stressed that the individual mass-transfer coefficients, determined using Eqs (21)-(23), include both the diffusive and the osmotic transport components. Qf, and Qf, i, Qe, and Qe, i, Qr, and Qr,, represent the changes in the quantities of the solute, that is, in the concentrations and volumes between sampling time i, and i, i. Therefore, experimental determination of the osmotic pressure gradients. An, and calculation of osmotic mass-transfer coefficients, FQ, may be excluded. Overall mass-transfer coefficients of solute species through the BAHLM system are calculated using Eqs (8) and (9). 

This semiempirical model may be used to minimize experimental testing at the BAHLM processes design. 

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. 

According to the theoretical model for transport kinetics (see Section 2.2), most preliminary parameters needed for BAHLM process design and optimization may be obtained by a number of known or experimentally obtained data. 

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]. 

At low solute concentrations, there are discrepancies between the theoretical and experimental hues even after corrections. It may be explained by following. The values of distribution coefficients p and depend on many conditions including lEM and LMF capacities. At low metal concentrations, the Donnan exclusion is pronounced and lEM potential is much higher, so its contribution to the overall BAHLM transport kinetics is more considerable than at higher concentrations. But the metal concentrations data, used for calculation of the averaged sums of lEM and LMF potentials, are much higher. So, the calculating parameters obtained do not satisfy the real transport kinetics at low concentrations. 

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. 

Selectivity of the BAHLM system is an important parameter that should be used designing module. According to the transport model equations, the selectivity of two metal species (SM1/M2) is determined by relation ... 

Based on the principle of resistance additivity, the total overall mass-transfer coefficient, K, of every metal passing through the BAHLM is related to the overall mass-transfer coefficients on the feed and strip sides as follows ... 

For preliminary evaluation of the selectivity of two metal species separation by the BAHLM process, we can assume that in the same solution environment (water), the ditfusion coefficients of these metal ions with the same charge have similar values and the diffusion coefficients of the metal-carrier complexes have similar values. Thus, substituting Eqs (8) and (9) in Eq. (41), we can represent the separation factor as dependent only on the distribution coefficients ... 

As was shown in the BAHLM model for transport kinetics, the values of the overaU mass-transfer coefficients govern the location (i ax) and the maximum quantity (Ofemax) of the metal species in the LM phase (see Eqs (27) and (28)). At Ormax Qf, the BAHLM is working mainly as a Donnan dialysis system, in which the loaded carrier solution is a treated feed. In this... 

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