Nanofiltration, membrane separation

Kosutic, K., Furac, L., Sipos, L. and Kunst, B. (2005) Removal of arsenic and pesticides from drinking water by nanofiltration membranes. Separation and Purification Technology, 42(2), 137-44. 

Schaep, J. et al.. Modeling the retention of ionic compounds for different nanofiltration membranes, Separ. Purif. Technol, 22-23,169,... 

Fig. 2 Schematic diagram of the nanofiltration membrane separation system...

Labbez, C., Fievet, P., Szymezyk, A., Vidonne, A., Foissy, A., and Pagetti, J., Retention of mineral salts by a polyamide nanofiltration membrane, Separ. Purif. Technol., 30, 47-55, 2003. 

J. Liu, Z. Xu, X. Li, Y. Zhang, Y. Zhou, Z. Wang, X. Wang, An improved process to prepare high separation performance PA/PVDF hollow fiber composite nanofiltration membranes. Separation arulPurification Technology, 58 (2007) 53-60. 

N. Stafle, D.F. Stamatialis, M. Wessling, Effect ofPDMS cross-linking degree on the permeation performance of PAN/PDMS composite nanofiltration membranes. Separation and Purification Technology, 45 (2005) 220-231. 

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. 

The separation of homogeneous catalysts by means of membrane filtration has been pioneered by Wandrey and Kragl. Based on the enzyme-membrane-reactor (EMR),[3,4] that Wandrey developed and Degussa nowadays applies for the production of amino acids, they started to use polymer-bound ligands for homogeneous catalysis in a chemical membrane reactor (CMR).[5] For large enzymes, concentration polarization is less of an issue, as the dimension of an enzyme is well above the pore-size of a nanofiltration membrane. 

The first SRS unit was built as a demonstration plant and has been in operation since September 1997. The basic principle of operation is that a solution of sodium chloride and sodium sulphate in contact with a nanofiltration membrane at high pressure, will separate into a sulphate-lean permeate stream and a sulphate-rich concentrate stream. 

The globular shape of dendritic macromolecules with a persistent nanosize and radius should allow easy separation or retention by ultra- or nanofiltration membranes. This concept of separating the catalysts from the product/substrate... 

The idea of using membranes to filter molecules on the basis of size is not without precedent. Dialysis is used routinely to separate low molecular weight species from macromolecules [105]. In addition, nanofiltration membranes are known for certain small molecule separations (such as water purification), but such membranes typically combine both size and chemical transport selectivity and are particularly designed for the separation involved. Hence, in spite of the importance of the concept, synthetic membranes that contain a collection of monodisperse, molecule-sized pores that can be used as molecular filters to separate small molecules on the basis of size are currently not available. 

Membrane reactors have been investigated since the 1970s 11). Although membranes can have several functions in a reactor, the most obvious is the separation of reaction components. Initially, the focus has been mainly on polymeric membranes applied in enzymatic reactions, and ultrafiltration of enzymes is commercially applied on a large scale for the synthesis of fine chemicals (e.g., L-methionine) 12). Membrane materials have been improved significantly over those applied initially, and nanofiltration membranes suitable to retain relatively small compounds are now available commercially (e.g., mass cut-off of 400—750 Da). 

Nanofiltration membranes usually have good rejections of organic compounds having molecular weights above 200—500 (114,115). NF provides the possibility of selective separation of certain organics from concentrated monovalent salt solutions such as NaCl. The most important nanofiltration membranes are composite membranes made by interfacial polymerization. Polyamides made from piperazine and aromatic acyl chlorides are examples of widely used nanofiltration membrane. Nanofiltration has been used in several commercial applications, among which are demineralization, oiganic removal, heavy-metal removal, and color removal (116). 

Nanofiltration membranes are commercially available, e.g. with a retention capacity of 400 Da. The dalton unit serves as a measure of the separation ability... 

J. Cadotte, R. Forester, M. Kim, R. Petersen and T. Stocker, Nanofiltration Membranes Broaden the Use of Membrane Separation Technology, p. 77, Copyright 1988, with permission from Elsevier... 

A difficult problem that prevented the use of nanofiltration in organic solvents for a long time was the limited solvent stability of polymeric nanofiltration membranes, and the lack of ceramic nanofiltration membranes. For polymeric membranes, different problems occurred zero flux due to membrane collapse [54], infinite nonselective flux due to membrane swelling [54], membrane deterioration [55], poor separation quality [ 5 6], etc. I n an early study of four membranes thought to be solvent stable (N30F, NF-PES-10, MPF 44 and MPF 50), it was observed that three of these showed visible defects after ten days exposure to one or more organic solvents, and the characteristics of all four membranes changed notably after exposure to the solvents [15]. This implies that these membranes should be denoted as semi-solvent-stable instead of solvent stable. 

Ultrafiltration and microfiltration membranes produce high porosities and pore sizes in the range of 30-100 nanometers (UF) and higher (MF), which enable the passage of larger dissolved particles and even some suspended particles. The separation-filtration mechanism is based on molecule/particle sizes. The nanofiltration membrane lies between the UF and RO membranes, combining the properties of both so that the two mechanisms coexist. In addition, the NF membrane may be... 

In reactions with polymer-bound catalysts, a mass-transfer limitation often results in slowing down the rate of the reaction. To avoid this disadvantage, homogenous organic-soluble polymers have been utilized as catalyst supports. Oxazaborolidine 5, supported on linear polystyrene, was used as a soluble immobilized catalyst for the hydroboration of aromatic ketones in THF to afford chiral alcohols with an ee of up to 99% [40]. The catalyst was separated from the products with a nanofiltration membrane and then was used repeatedly. The total turnover number of the catalyst reached as high as 560. An intramolecularly cross-linked polymer molecule (microgel) was also applicable as a soluble support [41]. 

Nanofiltration (NF) is a pressure-driven membrane separation technology used to separate ions from solution. Nanofiltration membranes were widely available beginning in the 1980 s. This technology uses microporous membranes with pore sizes ranging from about 0.001 to 0.01 microns. Nanofiltration is closely related to RO in that both technologies are used to separate ions from solution. Both NF and RO primarily use thin-film composite, polyamide membranes with a thin polyamide skin atop a polysulfone support (see Chapter 4.2.2). 

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,... 

Immunoisolation Molecular sieves and channels Nanofiltration membranes Nanopores Separations Biological research Nanobiology... 

Membrane degumming. Membrane separation has also been evaluated as an alternative process to conventional oil refining processing. Ultrafiltration (UF) and nanofiltration (NF) membranes separate phospholipids almost completely, and FFAs, pigments, and other components can also be removed with the phospholipids to a certain extent. Less effort is required in the later processing steps. 

To make supercritical extraction processes more economic, separation of solute and solvent can be performed thanks to a membrane system. Sartorelli and Brunner [19] demonstrated that a membrane separation process can be proposed instead of the typical supercritical fluid cycle in the case of supercritical extraction to drastically reduce the energy losses. In fact, a stream of low volatile compounds (LVC) extracted by SC CO2 can be discharged of 80%-90% of LVC using a nanofiltration membrane with a drop of pressure equal to 2 MPa instead of about 20 MPa in the typical supercritical fluid cycle. 

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