Nanofiltration, membrane process

Uddin, M.T., Mozumder, M.S.I., Islam, M.A. et al. (2007a) Nanofiltration membrane process for the removal of arsenic from drinking water. Chemical Engineering and Technology, 30(9), 1248-54. 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

Process Description Reverse osmosis (RO) and nanofiltration (NF) processes utilize a membrane that selectively restricts flow of solutes while permitting flow of the solvent. The processes are closely related, and NF is sometimes called loose RO. They are kinetic processes, not equilibrium processes. The solvent is almost always water. 

The second major membrane type is a composite. Starting with a loose asymmetric membrane, usually a UF membrane, a coating is applied which is polymerized in situ to become the salt rejecflng membrane. This process is used for most high-performance flat-sheet RO membranes, as well as for many commercial nanofiltration membranes. The chemistry of the leading RO membranes is known, but... 

When ionic liquids are used as replacements for organic solvents in processes with nonvolatile products, downstream processing may become complicated. This may apply to many biotransformations in which the better selectivity of the biocatalyst is used to transform more complex molecules. In such cases, product isolation can be achieved by, for example, extraction with supercritical CO2 [50]. Recently, membrane processes such as pervaporation and nanofiltration have been used. The use of pervaporation for less volatile compounds such as phenylethanol has been reported by Crespo and co-workers [51]. We have developed a separation process based on nanofiltration [52, 53] which is especially well suited for isolation of nonvolatile compounds such as carbohydrates or charged compounds. It may also be used for easy recovery and/or purification of ionic liquids. 

In the field of membrane filtration, a distinction is made based upon the size of the particles, which are retained by the membrane. That is micro-, ultra-, nanofiltration and reverse osmosis. Figure 4.8 shows a schematic picture of the classification of membrane processes. The areas of importance for application with homogeneous catalysts are ultra- and nanofiltration, depicted in gray. 

Reverse osmosis membrane process, 27 637 Reverse osmosis membrane cleaning citric acid application, 6 647 Reverse-osmosis membranes, 75 811, 825 development of, 75 797 Reverse osmosis models, 27 638-639 Reverse osmosis permeators, 76 19 Reverse osmosis seawater desalination process, 26 85 Reverse osmosis systems blending in, 26 80-81 brackish and nanofiltration, 26 80-83 Reverse osmosis technology... 

A bottleneck in all membrane processes, applied in practice, is fouling and scaling of the membranes. These processes cause a decrease in water flux through the membrane and a decrease in retention. Much attention is paid, especially in case of nanofiltration and hyperfiltration, to prevent fouling of the membrane by an intensive pretreatment and the regular removal of fouling and scaling layers by means of mechanical, physical or chemical treatment. 

Two types of continuous membrane reactors have been applied for oligomer- or polymer-bound homogeneous catalytic conversions and recycling of the catalysts. In the so-called dead-end-filtration reactor the catalyst is compartmentalized in the reactor and is retained by the horizontally situated nanofiltration membrane. Reactants are continuously pumped into the reactor, whereas products and unreacted materials cross the membrane for further processing [57]. 

Researchers at Degussa AG focused on an alternative means towards commercial application of the Julia-Colonna epoxidation [41]. Successful development was based on design of a continuous process in a chemzyme membrane reactor (CMR reactor). In this the epoxide and unconverted chalcone and oxidation reagent pass through the membrane whereas the polymer-enlarged organocatalyst is retained in the reactor by means of a nanofiltration membrane. The equipment used for this type of continuous epoxidation reaction is shown in Scheme 14.5 [41]. The chemzyme membrane reactor is based on the same continuous process concept as the efficient enzyme membrane reactor, which is already used for enzymatic a-amino acid resolution on an industrial scale at a production level of hundreds of tons per year [42]. 

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... 

Solvent-resistant nanofiltration and pervaporation are undoubtedly the membrane processes needed for a totally new approach in the chemical process industry, the pharmaceutical industry and similar industrial activities. This is generally referred to as process intensification and should allow energy savings, safer production, improved cost efficiency, and allow new separations to be carried out. 

Problems to be solved are related to membrane stability (of polymeric membranes, but also the development of hydrophobic ceramic nanofiltration membranes and pervaporation membranes resistant to extreme conditions), to a lack of fundamental knowledge on transport mechanisms and models, and to the need for simulation tools to be able to predict the performance of solvent-resistant nanofiltration and pervaporation in a process environment. This will require an investment in basic and applied research, but will generate a breakthrough in important societal issues such as energy consumption, global warming and the development of a sustainable chemical industry. 

Hassan, A.M., Al-Sofi, M.A.K., Al-Amoudi, A., Jamaluddin, T.M., Dalvi, A.G.I., Kitner, N.M., Mustafa, G.M. and Al-Tisan, I.A. (1998) A new approach to membranes and thermal seawater desalination processes using nanofiltration membranes. International Desalination and Water Reuse Part 1 May 1998, Part 2 Aug. 

These compounds are commonly present in complex fermentation media, or even in natural raw materials and subsidiary streams resulting from the processing of these materials. Their recovery is usually difficult due to their low concentration, often vestigiary, and the complexity of the original matrix where they have to be recovered from. This chapter discusses, and illustrates with recent applications, the use of different membrane processes able to deal with the recovery of small biologically active molecules (see Figure 11.2) electrodialysis, pervaporation, and nanofiltration. 

In general, two types of CFMRs are applied in homogeneous catalysis the dead-end-filtration reactor (Fig. 3B) and the loop reactor (Fig. 3C) [19]. In the dead-end-filtration reactor the nanosized catalyst is compartmentalized in the reactor and is retained by nanofiltration membranes. Reactants are continuously pumped into the reactor, whereas small molecules (products and substrates) cross the perpendicularly positioned membrane due to the pressure exerted. Unreacted materials can be processed by adding them back into the reactor in this set-up. Concentration polarization of the catalyst near to the membrane surface can occur using this technique. In contrast, when a loop reactor is used, such behavior is prevented, since the solution is continuously circulated through the reactor and no pressure is exerted in the direction of the parallel-positioned membrane, so small particles cross the membrane laterally. 

It is produced in recombinant CHO cells cultured in a medium free of animal-derived components. The BeneFix production process involves an ultrafiltration/diafiltration step, followed by four chromatographic steps ion exchange (Q resin), pseudo-affinity (Cellufine sulfate resin), hydroxyapatite, and affinity (immobilized Cu2+ ions). After these chromatographic steps, there are membrane processes (nanofiltration for viral clearance and diafiltration for solvent exchange), after which the purified protein is formulated (Edwards and Kirby, 1999). 

The work described in this chapter is especially concerned with three of the most widely used pressure driven membrane processes microfiltration, ultrafiltration and nanofiltration. These are usually classified in terms of the size of materials which they separate, with ranges typically given as 10.0-0.1 xm for microfiltration, 0.1 p.m-5 nm for ultrafiltration, and 1 nm for nanofiltration. The membranes used have pore sizes in these ranges. Such pores are best visualised by means of atomic force microscopy (AFM) [3]. Figure 14.1 shows an example of a single pore in each of these three types of membrane. An industrial membrane process may use several hundred square meters of membrane area containing billions of such pores. 

The aim of the present chapter is to provide an overview of recent work on the development of quantitative predictive methods for membrane processes at the Centre for Complex Fluids Processing, University of Wales Swansea. The main aim of such work is the development of predictive methods that require no adjustable parameters—that is methods that are truely ab initio. Three theoretical aspects will be illustrated the prediction of rates of ultrafiltration, the prediction of rejection in microfiltration and ultrafiltration, and the prediction of the rejection in nanofiltration. These examples show the importance of bridging the gap between physical chemistry and process engineering in the development of new process technologies. Finally, the use of AFM in the quantification of the adhesion of... 

An example of the excellent agreement that can be found between theoretical prediction and process performance is shown in Figure 14.7. This is for the diafiltration of a charged red dye. Such dyes are manufactured in solutions of high salt content but sold as products of low salt content. Removal of the salt (NaCl) can be achieved by extensive washing through a nanofiltration membrane—the dye is retained by the membrane but the salt can pass through. However, as the salt concentration decreases,... 

Taylor, JamesS., and Ed. P. Jacobs, "Reverse Osmosis and Nanofiltration," in Water Treatment Membrane Processes, Joel Mallevialle, Peter E. Odendaal, and Mark R. Wiesner, eds., McGraw-Hill Publishers, New York, New York, 1996. 

Membrane pretreatment includes microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF). Microfiltration and UF membrane processes can remove microbes and algae. However, the pores of MF and UF membranes are too large to remove the smaller, low-molecular weight organics that provide nutrients for microbes. As a result, MF and UF can remove microbes in the source water, but any microbes that are introduced downstream of these membranes will have nutrients to metabolize. Therefore, chlorination along with MF and UF is often recommended to minimize the potential for microbial fouling of RO membranes. The MF or UF membranes used should be chlorine resistant to tolerate chlorine treatment. It is suggested that chlorine be fed prior to the MF or UF membrane and then after the membrane (into the clearwell), with dechlorination just prior to the RO membranes. See Chapter 16.1 for additional discussion about MF and UF membranes for RO pretreatment. 

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