Reverse osmosis membrane length

The ability of a reverse osmosis membrane to withstand chlorine attack without showing significant loss in rejection is measured in ppm h. This is the product of chlorine exposure expressed in ppm and the length of exposure expressed in hours. Thus, 1000 ppm - h is 1 ppm chlorine for 1000 h or 10 ppm chlorine for 100 h or 1000 ppm chlorine for 1 h, and so on. 

Reverse osmosis membrane is produced in sheet form-up to 60 inches wide and lengths up to 1,500 feet-and as a hollow fine fiber. The asymmetric cellulose acetate was originally produced as a sheet and later as a hollow fine fiber. The asymmetric aromatic polyamide was originally produced as a hollow fine fiber and later in sheet form. The composite membranes with polyamide or polyurea membrane barrier layers are produced in sheet form as of the end of 1987, but research has been and will continue to be done to produce the composite reverse osmosis membranes as a hollow fine fiber. 

Figure 19.5. The Permasep hollow fiber module for reverse osmosis, (a) Cutaway of a DuPont Permasep hollow fiber membrane module for reverse osmosis a unit 1 ft dia and 7 ft active length contains 15-30 million fibers with a surface area of 50,000-80,000 sqft fibers are 25-250 pm outside dia with wall thickness of 5-50pm (DuPont Co.), (b) The countercurrent flow pattern of a Permasep module.

A concentration boundary layer theory clearly is needed to relate C to C, so that membrane properties such as L, a, and P can be correlated with R, at various operating conditions. Slso, since ir in Equations 1 and 5 is an independently determined function of C, a boundary ayer theory could correlate the observed filtrate velocity, J (averaged along the fiber length), with average applied pressure AP. For sufficiently high axial flow velocities, C == C, and a major theoretical barrier to data analysis is removeS. Some early work in reverse osmosis ( ) was done with flat-sheet membranes and large feed stream velocities. 

Macrovoid (MV) pores are observed in both the dry-cast and wet-cast processes. MVs are large-scale defects with a characteristic length that can approach the membrane thickness. MVs are generally undesirable because they decrease the permselective performance of membranes in applications such as microfiltration, ultrafiltration and reverse osmosis. 

Mass transfer in the entry region is far from a rare event Transport in the larger blood vessels lies entirely in the entry region, and so does mass transfer in commercial reverse osmosis desalination plants and other membrane processes (see Chapter 8). Entry lengths of many meters are not uncommon, particularly for low-solute dilfusivities. The only requirement is that flow must be in the laminar regime (Re < 2(XX)). 

Chai et al. (2001) used UTDR to smdy calcium-sulfate fouling from aqueous solutions in a 2521 Koch spiral-wound reverse osmosis (RO) membrane module. Only the reflections from the outer wrapping and the second and third membrane layers denoted by a, (3, and 7 in Figure 33.2 were smdied. The UTDR system consisted of a 3.5-MHz transducer having a focal length of 7 cm (Research Institute of Acoustics, Chinese Academy of Science), pulsar receiver (Panametrics 5052 PRX), and digital oscilloscope (Nicolet Pro 50). 

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