Liquid-filled membrane

A large fraction of the hazardous waste generated in industry is in the form of dilute aqueous solutions. The special challenges of separation in highly dilute solutions may be met by the development of new, possibly liquid-filled, membranes by processes involving selective concentration of toxic chemicals on the surfaces of particles or by the use of reversed micelles. 

This very simple and established method has become a standard technique used by suppliers to measure the largest active pores (as well as cracks or pinholes) in a membrane. The principle is to measure the pressure needed to force air through a liquid-filled membrane. The bottom of the filter is in contact with air and, as the air pressure is gradually increased, air bubbles penetrate through the membrane at a certain pressure. The pressure and pore radius are related by the Laplace equation... 

During the quenching process the homogeneous polymer solution separates into two phases a polymer-rich solid phase, which forms the membrane structure, and a solvent-rich liquid phase, which forms the liquid-filled membrane pores. Generally, the pores at the film surface, where precipitation occurs first and most rapidly, are much smaller than those in the interior or the bottom side of the film, which leads to the asymmetric membrane structure. There are different variations to this general preparation procedure described in the literature e.g., Loeb and Sourirajan used an evaporation step to increase the polymer concentration in the surface of the cast polymer solution and an annealing step during which the precipitated polymer film is exposed for a certain time period to hot water of 70° to 80°C.28... 

The ideal relationship of two straight lines crossing one another can never arise because of certain unavoidable phenomena that occur during the test. First, there must always be gas flow at pressures below the bubble point this is a result of diffusion of gas through the liquid-filled membrane (see below). Diffusional flow is proportional to the applied upstream gas pressure. The second factor is associated with the distribution of pore sizes in commercial membranes. As with... 

The bubble-point point method provides a simple means of characterising the maximum pore size in a given membrane. The method was used by Bechold even in the early years of this century. A schematic drawing of the test apparatus is given in figure IV - 7. The method essentially measures the pressure needed to blow air through a liquid-filled membrane. The top of the filter is placed in contact with a liquid (e.g. water) which fills all the pores when the membrane is wetted. The bottom of the filter is in contact with air and... 

A multiphase reactor design, very similar to the trickle-bed reactor, is the tubular multiphase hollow membrane wall reactor sketched in Eigure 13.2h. In a regular trickle-bed reactor, the liquid flows over a partially wetted pellet as a thin film and supplies the liquid phase reactant to the catalyst pores. This action, however, has the effect of hindering pore access to the gas, thus lowering the reaction rate. On the other hand, in the multiphase membrane reactor, the liquid-filled membrane is directly accessible to the gas flowing in the inside tube. Thus, mass transfer in this reactor is considerably more efficient than in the conventional trickle-bed reactor. 

Solute diffusion through porous liquid-filled membrane We consider here solute diffusion through a porous liquid-filled membrane under the condition of no convection (Figure 3.4.5(c)). Examples of separation pro-cesses/techniques where such a situation is encountered are isotonic dialysis, membrane based nondispersive gas absorption/stripping or solvent extraction, supported or... 

Figure 33.1 (a) Cross section of a liquid-filled membrane cell showing a UTDR transducer and... 

Measures the minimum pressure required to force air to flow through a liquid-filled membrane... 

In the physical model, there are two separate structures for the membrane depending on whether the water at the boundary is vapor or liquid these are termed the vapor- or liquid-equilibrated membrane, respectively. The main difference between the two is that, in the vapor-equilibrated membrane, panel c, the channels are collapsed, while, in the liquid-equilibrated case, panel d, they are expanded and filled with water. These two structures form the basis for the two types of macroscopic models of the membrane. 

In opposition to the single-phase treatment of the membrane system mentioned above are the models that assume the membrane system is two phases. This type of model corresponds to the liquid-equilibrated membrane shown in panel d of Figure 6. In this structure, the membrane is treated as having pores that are filled with liquid water. Thus, the two phases are water and membrane. 

Fig. 4a, b. Supported liquid membranes a membrane liquid fills the open pores of a solid support b membrane liquid is supported on both sides by two porous solid membranes... 

Transport in a microporous biomedical membrane is described in Fig. 11. Membranes consist of cylindrical liquid-filled pores of length l and radius rp with spherical solute molecules of radius rs diffusing through the pores. The solute... 

Membranes are made of glass or a glass of a particular compound and certain insoluble polymers filled with certain liquids. Each membrane electrode is designed for the specific measurement of a particular ion. The fluoride (F-) ISE, for example, contains an Ag/AgCl electrode immersed in a solution of NaCl and NaF and a membrane glass made from LaF3. ISEs have also been designed for the determination of Cl-, Br, I-, CN-, SCN-, N03, S2-, Ag+, Cd2+, Ca2+, Pb2+, and so on. Table 12.1... 

None of these indirect techniques relate the breakage of an individual particle to its mechanical properties. To achieve this, direct techniques are required. These are described later, and their capabilities and limitations discussed. Direct techniques also allow more sophisticated mathematical modelling to be undertaken, which is particularly valuable when the particles are not homogeneous, for example, cells with walls and membranes surrounding cytoplasm, or a liquid-filled microcapsule. 

FIG. 9 Measurement setup. Note that by measuring differentially with respect to a second ISFET at which no coulometric titration is carried out. a conventional liquid-filled reference electrode is superfluous. The ISFET is provided with a membrane containing incubated protein. 

Membrane bioreactor Asymmetric poly-sulfone membrane Diffusion of carbohydrates in liquid-filled pores Enzymatic hydrolysis of lactose Whole cells of Sutfolobus solfataricus entrapped in the tube wall 65... 

When hydrophobic membranes are used (olefins are preferred because of their low cost), the aqueous absorbent cannot penetrate through the pores and the membrane is gas filled whereas if hydrophilic membranes are employed, the membrane is liquid filled (Figures 38.1 and 38.2). Latter situation is preferred only if the reaction between the gaseous species and the absorbent solution is fast or instantaneous if not, it is better to work with a gas-filled membrane, to reduce mass-transfer resistances. The module design and flow configuration also play an important role in defining the membrane contactors efficiency. This aspect is discussed in detail in Section 38.5. 

The resistances to the mass transport that a species encounters when is transferred from the gas to the hquid phase are reported in Figure 38.3. Gas and liquid phases contribute to the overall resistance because of the formation of boundary layers close to the membrane surface. This imphes that the concentration of a generic species i in the bulk of the two phases is different from its concentration at the membrane surfaces. The resistance offered by the membrane with gas-filled pores will be different (generally lower) than that with liquid-filled pores, due to the different effective diffusion coefficients. The overall mass-transport coefficient is given by... 

Dense membranes are made from solid layers of metals (e.g. Pd alloys) for hydrogen separation, or of mixed (electronic, ionic) conducting oxides for oxygen separation. A special form are the LIMs (liquid immobilised membranes) which consist of a porous support filled with a liquid or molten salt which is semipermeable. 

There is one complication in the bubble-point test referred to as "diffusional-flow." A small amount of gas-flow can result even through a pore is filled with liquid. The gas dissolves in the liquid in the pores at high pressure, diffuses across the liquid-filled pore in solution, and comes out of solution on the low-pressure side of the membrane. In practice, "diffusional-flow" is not even detected when small membrane areas are involved. Even for large areas, it is easily distinguished from the much larger gas-flow at the bubble point. 

The coupled transport process is illustrated in Figure 9.1 for Cu2+ and H ions and a complexing agent denoted by RH. The water-immiscible agent fills the pores of a microporous membrane, thus forming a liquid organic membrane, The equilibrium that exists at the two membrane-solution interfaces is ... 

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