Liquid membranes, general discussion

The discussion of synthetic membranes can be structured in terms of the function or the structure of the membrane used in a particular application. For instance, one can consider whether a membrane is used to separate mixtures of gas molecules vs particles from liquids (function) vs whether the membrane structure is primarily microporous or dense (structure). In fact, function and stmcture are linked, but to facilitate the consideration of physical science issues related to membranes appropriate for this reference, emphasis on functional aspects are probably most appropriate. This approach reflects the fact that the use of a membrane generally involves one or more physical sci-... 

Their historical developments and various membrane preparation methods will be discussed in Chapters 2 and 3, respectively. Chapter 4 reviews the general separation and non-separation properties of the membranes and the methods by which they are measured. Chapter 5 presents commercial membrane elements and modules and their application features which are followed by discussions of liquid-phase separation applications in Chapter 6. Many of those applications are commercially practiced. Potential gas separation and other applications (such as sensors and supports for liquid membranes) will be discussed in Chapter 7. 

In this section, we describe recent developments in water purification membranes, beginning with RO membranes, which are the tightest and nK t permselective, and proceeding to the more open UF and MF membranes. We d e with a diort discussion of liquid membranes, which are generally not polymeric membranes but vdiich merit consideration here because they are highly permselective, they are potentially useful in water purification, and they process certain unique and interesting properties. 

The data in Table 19.3-1 suggest that liquid membranes should be as nonviscous as possible for optimum extraction rates. Up to a point this is true. However, membrane stability generally is reduced as the viscosity decreases and the tendency to rupture or leak the internal phase becomes greater, especially at long contact times. This effect is illustrated by the curves in Fig. 19.3-2. Initially, the extraction rate is more rapid with the less viscous membrane. However, at longer contact times the weaker formulation shows a tendency to leak the internal phase at a rate exceeding the membrane s ability to reabsorb it. Membrane stability is an important consideration which is discussed in more detail below. 

These types of separators consist of a solid matrix and a liquid phase, which is retained in the microporous structure by capillary forces. To be effective for batteries, the liquid in the microporous separator, which generally contains an organic phase, must be insoluble in the electrolyte, chemically stable, and still provide adequate ionic conductivity. Several types of polymers, such as polypropylene, polysulfone, polytetrafluoroethylene, and cellulose acetate, have been used for porous substrates for supported liquid membranes. The PVdF-coated polyolefin-based microporous membranes used in gel polymer lithium-ion battery fall into this category. Gel polymer electrolytes/membranes are only discussed briefly. 

The two phases (phase 1 and phase 2) are generally aqueous solutions, while the liquid membrane phase is an organic phase which is immiscible with water. The solubility is a very important factor with respect to the stability of these system. This stability effect will be discussed below. 

The designation membrane is used here in a very general sense, as many materials are used. Commercially available electrodes include liquid membrane units, solid-state electrodes, glass membrane electrodes, and plastic membrane electrodes. General classes of ion specific electrodes in addition to those cited are immobilized-liquid membrane electrodes, mixed-crystal membrane electrodes, enzyme electrodes, and antibiotic electrodes (Rechnitz, 1973). Certain of these membrane electrodes are now discussed in some detail. A generalized membrane electrode is illustrated in Figure 6.1a. 

Liquid membrane separation systems possess great potential for performing cation separations. Many factors influence the effectiveness of a membrane separation system including complexation/ decomplexation kinetics, membrane thickness, complex diffusivity, anion type, solvent type, and the use of ionic additives. The role that each of these factors plays in determining cation selectivity and flux is discussed. In an effort to arrive at a more rational approach to liquid membrane design, the effect of varying each of these parameters is established both empirically and with theoretical models. Finally, several general liquid membrane types are reviewed, and a novel membrane type, the polymeric inclusion membrane, is discussed. 

The general results of the 3-D models are more-or-less a superposition of the 2-D models discussed above. Furthermore, most of the 3-D models do not show significant changes in the 1-D sandwich in a local region. In other words, a pseudo-3-D approach would be valid in which the 1-D model is run at points in a 2-D mesh wherein both the channel and rib effects can easily be incorporated. Another pseudo-3-D approach is where the 2-D rib models are used and then moved along the channel, similar to the cases of the pseudo-2-D models described above. This latter approach is similar to that by Baker and Darling. In their model, they uncouple the different directions such that there is a 1-D model in the gas channel and multiple 2-D rib models. However, they neither treat the membrane nor have liquid water. In all, the use of CFD means that it is not significantly more complicated to run a complete 3-D model in all domains. 

In many cases, these polymer chains take on a rod-like (calamitic LCPs) or even disc-like (discotic LCPs) conformation, but this does not affect the overall structural classification scheme. There are many organic compounds, though not polymeric in nature, that exhibit liquid crystallinity and play important roles in biological processes. For example, arteriosclerosis is possibly caused by the formation of a cholesterol containing liquid crystal in the arteries of the heart. Similarly, cell wall membranes are generally considered to have liquid crystalline properties. As interesting as these examples of liquid crystallinity in small, organic compounds are, we must limit the current discussion to polymers only. 

The separation schemes vary with the state of the products. For example, intracellular products must first be released by disrupting the cells, while those products bound to cell membranes must be solubilized. As the concentrations of products secreted into the fermentation media are generally very low, the recovery and concentration of such products from dilute media represent the most important steps in downstream processing. In this chapter, several cell-liquid separation methods and cell disruption techniques are discussed. 

Product Recovery. Comparison of the electrochemical cell to a chemical reactor shows the electrochemical cell to have two general features that impact product recovery. Cell product is usually liquid, can be aqueous, and is likely to contain electrolyte. In addition, there is a second product from the counter electrode, even if this is only a gas. Electrolyte conservation and purity are usual requirements. Because product separation from the starting material may be difficult, use of reaction to completion is desirable cells would be run batch or plug flow. The water balance over the whole flow sheet needs to be considered, especially for divided cells where membranes transport a number of moles of water per Faraday. At the inception of a proposed electroorganic process, the product recovery and refining should be included in the evaluation to determine true viability. Thus early cell work needs to be carried out with the preferred electrolyte/solvent and conversion. The economic aspects of product recovery strategies have been discussed (89). Some process flow sheets are also available (61). 

The widespread interest in transport across membranes of living cells has led to studies of diffusion in lyotropic liquid crystals. Biological membranes are generally thought to contain single bimolecular leaflets of phospholipid material, leaflets which are like the large, flat micelles of lamellar liquid crystals. No effort is made here to review the literature on transport either across actual cell membranes or across single bimolecular leaflets (black lipid films) which have often been used recently as model systems for membrane studies. Instead, experiments where lamellar liquid crystals have been used as model systems are discussed. 

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

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