Nanofiltration membranes structure

Membrane Porosity Separation membranes run a gamut of porosity (see Fig. 22-48). Polymeric and metallic gas separation membranes, electrodialysis membranes, pervaporation membranes, and reverse osmosis membranes are nonporous, although there is hnger-ing controversy over the nonporosity of the latter. Porous membranes are used for microfiltration and ultrafiltratiou. Nanofiltration membranes are probably charged porous structures. 

Metal oxides, used for manufacturing of ceramic nanofiltration membranes, are intrinsically hydrophilic. This limits the use of these membranes to polar solvents filtration of nonpolar solvents (n-hexane, toluene, cyclohexane) usually yields zero fluxes. Attempts have been made to modify the pore structure by adding hydrophobic groups, for example, in a silane coupling reaction [38, 43]. This approach is similar to modifications of ultrafiltration and microfiltration membranes... 

Due to recent advances in membrane development, nanofiltration membranes are nowadays increasingly used for applications in organic solvents [27, 58]. This narrows the gap between pervaporation and nanofiltration. It is even possible that the requirements for membrane structures completely overlap for the two processes whereas membrane stability becomes more important for nanofiltration membranes, the performance of pervaporation membranes could be improved by using an optimized (thinner) structure for the top layers. It might even be possible to use the same membranes in both applications. At this moment it is not possible to define which membrane structure is necessary for nanofiltration or for pervaporation, and which membrane is expected to have a good performance in nanofiltration, in pervaporation or in both. Whereas pervaporation membranes are dense, nanofiltration membranes... 

Polymeric membranes with a less porous structure, pervaporation membranes as well as nanofiltration membranes, can be described by a solution-diffusion mecha-... 

Nanofiltration (NF) and RO are closely related in that both share the same composite membrane structure and are generally used to remove ions from solution. However, NF membranes use both size and charge of the ion to remove it from solution whereas RO membranes rely only on "solution-diffusion" transport to affect a separation (see Chapters 16.2 and 4.1, respectively). Nanofiltration membranes have pore sizes ranging from about 0.001 to 0.01 microns, and therefore,... 

Besides the use of homogeneously soluble polymethacrylates or poylstyrene, as for the examples described above, other soluble supports may be used in order to yield a catalyst which can be retained by ultra- or nanofiltration membranes. Several groups have introduced catalysts (chiral and nonchiral) coupled to dendrimers and dendrimer-like structures [54, 59-76]. Compared with catalysts coupled to polymers, such complexes offer the advantage of a more defined structure. Thus, the number of active sites can be controlled more accurately. As these will be present at the surface of a globular structure they will be easily accessible. 

The main characteristics of nanofiltration membranes made of oxide ceramics is that they exhibit a microporous structure with charged pore walls depending on pH and ionic strength of feed solutions. Three main cases are distinguished in the discussion of mechanisms involved in permeation and separation processes using microporous ceramic nanofilters ... 

As a general conclusion to this part dedicated to nanofiltration with ceramic membranes one can assume that the general behaviour of these membranes can be assimilated to the behaviour of electrically charged organic nanofiltration membranes. However some specificities exist with ceramic nanofilters due to a sintered metal oxide grains derived porous structure and an amphoteric character... 

The use of nanofiltration membranes as supporting membranes have been also reported [28]. In this case, direct filtration of ionic liquids through the nanofiltration membrane was not possible at a gas pressure up to 7 bars. The ionic liquids with cations associated with straight or branched hydrocarbon chains were easily absorbed into the polymeric membrane allowing the nanoporous structure saturated with the ionic liquids. 

W ang X.-L., Tsuru T., Togoh M., Nakao S.-i., Kimura S. (1995a), Evaluation of pore structure and electrical properties of nanofiltration membranes. Journal of Chemical Engineering of Japan, 28, 2,186-192. 

All the aforementioned SILMs were prepared using microflltration membranes and operated with low varying pressure differential (<2 bar). To avoid the pitfaU of liquid instability associated with microporous membranes, nanofiltration membranes were used in SILMs, which greatly reduce the instability problem only at the expense of increased gas transport resistance provided by the nanofiltration (NF) membranes [87]. Experimental stability tests demonstrated that the impregnated ILs did not discharge from the NF membrane structure even under a high transmembrane... 

In contrast to the polymeric materials for reverse osmosis and nanofiltration membranes, for which the macromolecular structures have much to do with permeation properties such as salt rejection characteristics, the choice of membrane material for ultrafiltration does not depend on the material s influence on the permeation properties. 

Hollow fiber membranes with a positively charged nanofiltration selective layer have been fabricated by using asymmetric microporous hollow fibers made from a Torlon PAI type as the porous substrate followed by a post-treatment with poly(ethyleneimine) [79]. The membrane structure and the surface properties can be tailored by adjusting the polymer dope composition, spinning conditions, and the posttreatment parameters. 

Rajesh S, Fauzi Ismail A, Mohan DR. Structure-property interplay of poly(amide-imide) and T1O2 nanoparticles impregnated poly(ether-sulfone) asymmetric nanofiltration membranes. RSC Adv 2012 2(17) 6854-70. 

It should be noted for simplicity reasons that a two-layer model has been assumed for the HR95 reverse osmosis membrane, but a more complex structure, including an intermediate layer with gradual changes in the pore radii/porosity from one layer to another (three-layer model), could be more realistic (Zholkovskij 1995). In this context, the compaction or partial inclusion of the intermediate layer due to membrane aging determined by IS measurements for nanofiltration membranes shows the utility of this technique for membrane modification characterization (Benavente and Vazquez 2004). 

Ismail, A.F. and Hassan, A.R. 2004. The deduction of fine structural details of asymmetric nanofiltration membranes using theoretical models. / Memb. Sd. 231 25-36. 

Ismail, A.F. and Lau, W.J. 2009a. Theoretical studies on structural and electrical properties of PES/SPEEK blend nanofiltration membrane. 55 2081-2093. 

Robinson J.P., Tarleton E.S., Millington C.R. and Nijmeijer A., 2004. Evidence for swelling-induced pore structure in dense PDMS nanofiltration membranes. Filtration, 4(1), 50-56. Robinson J.P., Tarleton E.S., Ebert K., Millington C.R. and Nijmeijer A, 2005. Influence of cross-linking and process parameters on the separation performance of poly(dimethyl-siloxane) nanofiltration membranes, Ind. Eng. Chem. Res., 44(9), 3238-3248. 

Various pressure-driven membrane processes can be used to concentrate or purify a dilute (aqueous or non-aqueous) solution. The characteristic of these processes is that the solvent is the cominueous phase and that the concentration of the solute is relatively low. The particle or molecular size and chemical properties of the solute determine the structure, i.e. pore size and pore size distribution, necessary for the membrane employed. Various processes can be distinguished related to the panicle size of the solute and consequently to membrane structure. These processes are microfiJtration, ultrafiltration, nanofiltration and reverse osmosis. The principle of the four processes is illustrated in figure VI - 2. 

Composite membranes constitute the second type of structure frequently used in reverse osmosis while most of the nanofiltration membranes are in fact composite membranes. In such membranes the toplayer and sublayer are composed of different polymeric materials so that each layer can be optimised separately. The first stage in manufacturing a composite membrane is the preparation of the porous sublayer. Important criteria for this sublayer are surface porosity and pore size distribution and asymmetric ultrafiltration membranes are often used. Different methods have been employed for placing a thin dense layer on top of this sublayer ... 

These various methods have been discussed in chapter III. Since reverse osmosis membranes may be considered as intermediate between porous ultrafiltration membranes and very dense nonporous pervaporation/gas separation membranes, it is not necessary that their structure to be as dense as for pervaporation/gas separatipn. Most composite reverse osmosis and nanofiltration membranes are prepared by interfacial polymerisation (see chapter in. 6) in which two very reactive bifunctional monomers (e.g. a di-acid chloride and a di-amine) or triiunctional monomers (e.g. trimesoyicbloride) are allowed to react with each other at a water/organic solvent interface and a typical rietwork structure is obtained. Another example of monomers used for interfacial polymerisation are given in table VI.6 (see also table m.1). 

Oliveria, E.E.M., Barbosa, C.C.R. Afonso, J.C. (2012) Selectivity and structural integrity of a nanofiltration membrane for treatment of liquid waste containing uranium. The Membrane Water Treatment, 3,... 

Nanofiltration membranes consist of an active layer and a support, which determine the separation properties and mechanical strength, respectively. The active layer may be integrally connected to the support structure, such as membranes prepared via an immersion precipitation process (Bowen et al, 2001 He et al, 2002). This type of membrane has distinct pores in the nanometer range at the skin layer. The active layer can also be an extra coating layer on to a tailor-made support structure via interfacial polymerization or dip-coating. 

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