Loose reverse osmosis

The goal of most of the early work on reverse osmosis was to produce desalination membranes with sodium chloride rejections greater than 98 %. More recently membranes with lower sodium chloride rejections but much higher water permeabilities have been produced. These membranes, which fall into a transition region between pure reverse osmosis membranes and pure ultrafiltration membranes, are called loose reverse osmosis, low-pressure reverse osmosis, or more commonly, nanofiltration membranes. Typically, nanofiltration membranes have sodium chloride rejections between 20 and 80 % and molecular weight cutoffs for dissolved organic solutes of 200-1000 dalton. These properties are intermediate between reverse osmosis membranes with a salt rejection of more than 90 % and molecular weight cut-off of less than 50 and ultrafiltration membranes with a salt rejection of less than 5 %. 

Membranes having effective pore sizes between 0.001 and 0.01 pm are used in nanofiltration. NF is placed between reverse osmosis and ultrafiltration, and because of that it is sometimes considered as loose reverse osmosis. Typical operating pressures for NF are 0.3-1.4 MPa. The process allows to separate monovalent ions from multivalent ions, which are retained by NF membrane. The process can be used for separation of organic compounds of moderate molecular weight from the solution of monovalent salts. The very well-known application in nuclear industry is boric acid recovery from contaminated cooling water in nuclear reactor. There are some examples of nanofiltration applications and studies done with the aim of implementation in nuclear centers described in literature. Some of them are listed in the Table 30.4. 

Nanofiltration is a rapidly advancing membrane separation technique for concentration/separation of important fine chemicals as well as treatment of effluents in pharmaceutical industry due to its unique charge-based repulsion property [5]. Nanofiltration, also termed as loose reverse osmosis, is capable of solving a wide variety of separation problems associated with bulk drug industry. It is a pressure-driven membrane process and indicates a specific domain of membrane technology that hes between ultrafiltration and reverse osmosis [6]. The process uses a membrane that selectively restricts flow of solutes while permitting flow of the solvent. It is closely related to reverse osmosis and is called loose RO as the pores in NF are more open than those in RO and compounds with molecular weight 150-300 Da are rejected. NF is a kinetic process and not equilibrium driven [7]. 

T. Tsuru, M. Urairi, S. Nakao and S. Kimura, Negative rejection of anions in the loose reverse osmosis separation of mono- and divalent ion mixtures. Desalination, 81 (1991) 219. 

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 tubular reverse osmosis device is shown in Figure 4.10. The tube serves as the pressure vessel and the membrane is installed inside the tube. Tubes with inside diameters of % and 1 inch have been used. Uniformly porous fiberglass reinforced plastic tubes have been used and nonporous but perforated copper, stainless steel and fiberglass tubes have also been successfully used. The membrane can be bonded to the tube in which case it is cast in situ or the membrane can be loose. The loose membrane is cast in sheet form and a cylindrical section is formed and placed in the tube. Packing densities for the Vi-inch diameter tube are about 100 square feet per cubic foot and about 50 square feet per cubic foot for the 1-inch diameter tube. 

The second type of membrane module, which is the hardest to visualize, is the spiral-wound element. This module essentially consists of a large membrane envelope loosely rolled like a jelly roll. The feed stays outside the envelope and products are harvested from the inside via a central tube. In some more sophisticated designs, many envelopes may come out from the central tube, so that a cross-section of the module would look like a daisy with petals twisted in a circular direction. This type of module has become the dominant geometry for reverse osmosis. While it has less membrane area per volume than a hollow-fiber module, it plugs less easily. However, even if only part of the membrane fails, the entire module must be discarded. 

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