Permselectivity, membranes discussion

Polyamides and their analogue are also effective for the selective membranes and there have been developed many kinds of permselective membranes. In early 1960 s, du Pont started to investigate the membranes for demineralization of water by reverse osmosis. After screening polymers, aromatic polyamides and polyhydrazides were shown to have superior properties9-11. In the present review various polyamides and their analogue are in focus as barrier materials for membranes, and their permeative characteristics will be discussed from the view point of their chemical structures. 

The discussion given in this section shows that non-homogeneity of membrane electrical properties is widespread and markedly influences ionic sorption and diffusion behaviour. Proper understanding of these effects is, therefore, important and may be expected to contribute materially to the design of more highly permselective membranes. 

Membrane extraction encompasses a class of liquid-phase separations where the primary driving force for transport stems from the concentration difference between the feed and extractant liquids rather than a pressure gradient, as is the case with most of the other processes discussed above. A microporous membrane placed between the feed and the extractant liquids functions primarily as a phase separator. The degree of separation achievable is determined by the relative partition coefficients among individual solutes. This operationx is known as membrane solvent extraction. If a nonporous, permselective membrane is used instead, however, the selectivity of the membrane would be superimposed on the partitioning selectivity in this case the process may be referred to as perstraction. These process concepts are illustrated in Fig. 34. 

The mass-, heat- and momentum-transfer equations and their corresponding boundary conditions discussed so far are obviously very complex and their solutions are not trivial to obtain. Moreover, the thickness, diffusivities and conductivity of each layer in the membrane element are difficult to measure. It is, therefore, convenient and reasonable to consider the permselective membrane layer and the support layer(s) as an integral region with effective thickness, diffusivities and conductivity for the composite region. And it is also desirable to search for simpler models which are capable of providing the... 

Using a developed plug-flow membrane reactor model with the catalyst packed on the tube side, Mohan and Govind [1986] studied cyclohexane dehydrogenation. They concluded that, for a fixed length of the membrane reactor, the maximum conversion occurs at an optimum ratio of the permeation rate to the reaction rate. This effect will be discussed in more detail in Chapter 11. They also found that, as expected, a membrane with a highly permselective membrane for the product(s) over the reactant(s) results in a high conversion. 

The availability of good, reliable membranes will not, of course, eliminate the need for optimal reactor design and process analysis, necessary to determine the t) e of membrane to be used and the optimal operating conditions. As was discussed previously, some reactions do not need permselective membranes. Other process parameters like the reactor configuration or the amount of sweep gas utilized can affect dramatically the observed performance. 

There are a number of membrane reactor systems, which have been studied experimentally, that fall outside the scope of this model, however, including reactors utilizing macroporous non-permselective membranes, multi-layer asymmetric membranes, etc. Models that have been developed to describe such reactors will be discussed throughout this chapter. In the membrane bioreactor literature, in particular, but also for some of the proposed large-scale catalytic membrane reactor systems (e.g., synthesis gas production) the experimental systems utilized are often very complex, in terms of their configuration, geometry, and, of course, reaction and transport characteristics. Completely effective models to describe these reactors have yet to be published, and the development of such models still remains an important technical challenge. 

Uragami et al. (1998) synthesized benzoyl chitosan as a membrane material for the separation of benzene-CYH mixtures via PV. When the benzoyl chitosan membrane was applied for the separation of benzene-CYH mixtures in PV, both the permeation rate and benzene concentration in the permeate increased with increasing benzene concentration in the feed and thus this membrane showed benzene permselectivity. It was reported that the benzene permselectivity was dependent on both the sorption and diffusion selectivity but was significantly governed by the latter. A tentative model for benzene permselectivity was discussed. 

The separation efficiency (e.g. permselectivity and permeability) of inorganic membranes depends, to a large extent, on the microstructural features of the membrane/support composites such as pore size and its distribution, pore shape, porosity and tortuosity. The microstructures (as a result of the various preparation methods and the processing conditions discussed in Chapter 2) and the membrane/support geometry will be described in some detail, particularly for commercial inorganic membranes. Other material-related membrane properties will be taken into consideration for specific separation applications. For example, the issues of chemical resistance and surface interaction of the membrane material and the physical nature of the module packing materials in relation to the membranes will be addressed. 

It is obvious from the above discussion that porous and dense membranes form two different cases, each with its own advantages and disadvantages. Dense membranes, (permeable only to one component) operating at optimum conditions, can be used to obtain complete conversions. However, because the permeation rate is low, the reaction rate has also to be kept low. Porous membranes (permeable to all components but at different permselectivities) are limited under optimum conditions to a maximum conversion (which is not 100%) due to the permeation of all the components. The permeation rates through porous membranes are, however, much higher than those through dense membranes and consequently higher reaction rates or smaller reactor volumes are possible. 

It is at present still difficult to correlate the absolute intensity of the SHG with the number of cationic complexes at the membrane surface. Therefore, a quantitative discussion, showing how the permselective uptake of primary cations forming SHG active complexes into the membrane side of the phase boundary corresponds to the increase in the membrane potential, is not possible yet. Lipophilic derivatives of photoswitchable azobis(benzo-15-crown-5) were recently shown as a molecular probe to determine photoinduced changes in the amount of the primary cation uptake into the membrane phase boundary in relation to the photoinduced EMF changes under otherwise identical conditions. 

As the permeance and permselectivity measurements show, it is possible to prepare high-quality doped silica membranes with excellent properties. Moreover it was possible to perform permeance and permselectivity measurements at temperatures up to 600°C on flat membranes. To the author s knowledge these are the first reliable measurements ever performed on flat membranes at such a high temperature. A more detailed discussion of the permeance and permselectivity results follows. It must however be noted that the relatively low hydrogen permeances obtained for the described membranes were at least partly due to the used AKP-30 supports, which had a bare-support hydrogen permeance of-8 10 mol/m sPa. 

The author wishes to express his thanks to Dr. H. Weyten (VITO) for performing the high temperature measurements, Dr. R. Bredesen (SINTEF) for performing SASRA treatments, XRD-measurements and specific surface area measurements, Dr. J.A. Dalmon (IRC) for performing permeance and permselectivity measurements on the tubular membranes and Dr. A. Vredenburg (University of Utrecht) for performing the RBS measurements. These people are also kindly thanked for fruitful discussions on their respective measurments. 

V. M. Gryaznov and his co-workers (e.g. IGryaznov, 1986]) have extensively explored the permselective properties of palladium and its alloys as dense membranes and membrane reactors. While their studies will be discussed in later chapters, it suffices to say that the palladium-based membranes have reached the verge of a commercialization potential for the process industry. 

Possible transport mechanisms in a fluid system through the membrane pores are multiple. They vary to a great extent with the membrane pore size and, to a less extent, with chemical interaction between the transported species and the membrane material. Under the driving force of a pressure gra nt, permeants (whether in the form of solvents, solutes or gases) can transport across a membrane by one or more of the mechanisms to be discussed below. The degree by which they affect permeability and permselectivity depends on the operating conditions, membrane characteristics and membrane-permeating species interactions in the application environment. 

The generic permselectivity of a membrane can be described by the retention coefficient for liquid phase or the separation factor for gas phase. Separation factor will be defined and discussed in Chapter 7. In the case of liquid-phase membrane separation, the retention coefficient of the membrane can be characterized by some commonly used model molecules such as polyethylene glycol (PEG) polymers which have linear chains and arc more flexible or dextians which arc slightly branched. The choice of these model molecules is due to their relatively low cost. They are quite deviated from the generally... 

Thermal stability. Thermal stability of several common ceramic and metallic membrane materials has been briefly reviewed in Chapter 4. The materials include alumina, glass, silica, zirconia, titania and palladium. As the reactor temperature increases, phase transition of the membrane material may occur. Even if the temperature has not reached but is approaching the phase transition temperature, the membrane may still undergo some structural change which could result in corresponding permeability and permselectivity changes. These issues for the more common ceramic membranes will be further discussed here. 

When combining the separator and the reactor functions into one compact physical unit, factors related to catalysis need to be considered in addition to those related to selective separation discussed in previous chapters. The selection of catalyst material, dispersion and heat treatment and the strategic placement of catalyst in the membrane reactor all can have profound impacts on the reactor performance. The choice of membrane material and its microstructure may also affect the catalytic aspects of the membrane reactor. Furthermore, when imparting catalytic activity to inorganic membranes, it is important to understand any effects the underlying treatments may have on the permeability and permselectivity of the membranes. 

The membrane reactors and their models discussed so far utilize the permselective properties of the membranes. The membranes which can be catalytic or inert with respect to the reactions of interest benefit the reactor performances mostly by selectively removing a product or products to effect the equilibrium displacement. 

The above discussion pertains to dense membrane reactors. For the case of a semipermeable membrane reactor which has finite permselectivities for the various reaction components, isothermal operations favor the plug flow membrane reactor over the perfect mixing membrane reactor for both endothermic and exothermic reactions. In the case of exothermic reactions, the difference between the two flow patterns is rather small for low feed temperatures [Mohan and Govind, 1988b]. 

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. 

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