Section 4.21 Membranes Nanofiltration

Under carefully adjusted experimental conditions unmodified catalysts can be used in nanofiltration coupled homogeneous catalysis. Also non-dendritic but nanosized rigid catalytic systems can be retained by nanofiltration membranes. In this section, unmodified catalysts and rigid non-dendritic systems applied in continuous catalysis will be discussed. 

Figure 4 Schematic cross section of a thin-film-composite nanofiltration membrane...

Several researchers have shown that the application of an electrostatic field over the cross section of a membrane leads to a significant reduction in the energy cost per unit of permeate for microfiltration [38 0], ultrafiltration [41 4], and nanofiltration systems [45]. 

For some of them, the use of membrane reactors for their recovery or application in continuously operated reactors has been demonstrated. Examples include the use of dendrimer-bound nickel catalysts for the Kharasch addition [54, 59] and dendritic palladium catalysts for an allylic substitution [73, 60]. The membrane reactor concept has also been transferred to reactions at higher pressure, as shown for the hydrovinylation of styrene (cf. Section 3.3.3) [75]. Modem ultra-and nanofiltration membranes allow an effective recovery of the homogeneously soluble catalyst. However, in some cases the long-term stability of the catalyst under operating conditions has to be improved. 

The second section refers to polyelectrolyte membranes prepared by alternating electrostatic layer-by-layer assembly of cationic and anionic polyelectrolytes on porous supports. Mass transport across ultrathin polyelectrolyte multilayer membranes is described. The permeation of gas molecules, liquid mixtures, and ions in aqueous solution has been investigated. The studies indicate that the membranes are excellently suited for separation of alcohol/water mixtures under pervaporation conditions and for ion separation, e.g. under nanofiltration conditions. 

When, to satisfy ultrafiltration, nanofiltration, or gas separation requirements, the required pore size is under 0.1 pm, the ceramic powder approach is no longer viable. Indeed, individual particles yielding pore diameters smaller than 0.1 pm cannot be handled by powder processing. In fact, particles of this type enter the category of colloids and must be maintained as a stable suspension during the process. As indicated in the introduction, the sol-gel method is a very suitable way to produce mesoporous and nanoporous membranes. The latter is elaborated in the next section. 

For any other membrane (i.e., nanofiltration, reverse-osmosis, and so on), manipulation of the appropriate flux model should replace the right-hand side of Equation 9.9. The subscript ref denotes a chosen reference component. In Sections 9.5 and 9.6, component 2 is the reference component, and the following relative permeabilities have been assumed throughout ajj = 3, ot = 1, and Qt = 1.5. Similarly to previous chapters, these permeabilities can be arranged in a vector format such that = [3, 1, 1-5]. This is known as a... 

Focusing on the recent advances and updates, this section addresses new development in chemical and pharmaceutical industries and in the conservation of natural resources. Included in this edition are newer practices and technologies and their applications or trends for future applications with relevant references that have appeared in the literature since the first edition was published. Several new chapters on emerging areas such as membrane separation in petrochemical oil refinery, chitosan as new material for membrane preparation, new membrane material for ultrafiltration (UF) and nanofiltration (NF), and potential application of reverse osmosis (RO) in chemical industry have been added in the second edition. 

A range of membrane processes are used to separate fine particles and colloids, macromolecules such as proteins, low-molecular-weight organics, and dissolved salts. These processes include the pressure-driven liquid-phase processes, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), and the thermal processes, pervaporation (PV) and membrane distillation (MD), all of which operate with solvent (usually water) transmission. Processes that are solute transport are electrodialysis (ED) and dialysis (D), as well as applications of PV where the trace species is transmitted. In all of these applications, the conditions in the liquid boundary layer have a strong influence on membrane performance. For example, for the pressure-driven processes, the separation of solutes takes place at the membrane surface where the solvent passes through the membrane and the retained solutes cause the local concentration to increase. Membrane performance is usually compromised by concentration polarization and fouling. This section discusses the process limitations caused by the concentration polarization and the strategies available to limit their impact. 

Section 4.15 describes membranes and introduces a range of membrane separation options. Molecular geometry is exploited in separations of gases via gas permeation. Section 4.16. Dialysis and electrodialysis are considered in Sections 4.17 and 4.18 respectively. Other methods to separate species in liquids are given in Section 4.19, pervaporation Section 4.20, reverse osmosis Section 4.21, for nanofiltration Section 4.22, for ultrafiltration Section 4.23, for microfil-tration and Section 4.24 for chromatographic separations. Separations of larger sized species are considered heterogeneous systems and are considered in Chapter 5. 

Nanofiltration. The nanofiltration process is, along with baclq)ulse filtration (Section 7.5.4.3), another example of the new possibilities in processing that are due to recent developments in the field of membrane technology. At conditions intermediate between those used in ultrafiltration and RO, this technology can selectively reject multivalent ions such as 804 while passing monovalent ions [147-149]. The process therefore can remove sulfate and other multivalent anions selectively from alkali chloride brines. Since the equivalent amount of alkali metal ions is held back by electrostatic forces, the net effect is the removal of M2SO4. 

Membrane separation processes have been applied to many industrial production systems for the purpose of clarification, concentration, desalting, waste treatment, or product recovery. Broadly speaking, membrane filtration can be classified as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and dialysis or electrodialysis. In this section, the discussion will only cover microfiltration and ultrafiltration, both of which are pressure-driven membrane processes. 

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