Hollow configuration

RO membrane performance in the utility industry is a function of two major factors the membrane material and the configuration of the membrane module. Most utility applications use either spiral-wound or hollow-fiber elements. Hollow-fiber elements are particularly prone to fouling and, once fouled, are hard to clean. Thus, applications that employ these fibers require a great deal of pretreatment to remove all suspended and colloidal material in the feed stream. Spiral-wound modules (refer to Figure 50), due to their relative resistance to fouling, have a broader range of applications. A major advantage of the hollow-fiber modules, however, is the fact that they can pack 5000 ft of surface area in a 1 ft volume, while a spiral wound module can only contain 300 ftVff. 

The hollow fine fiber configuration (refer to Figure 51) consists of a bundle of porous hollow fine fibers. These fibers are externally coated with the actual membrane and form the support structure for it. Both ends of each fiber are set in a single epoxy tube sheet, which includes an 0-ring seal to match the inside diameter of the pressure vessel. 

Membrane systems consist of membrane elements or modules. For potable water treatment, NF and RO membrane modules are commonly fabricated in a spiral configuration. An important consideration of spiral elements is the design of the feed spacer, which promotes turbulence to reduce fouling. MF and UF membranes often use a hollow fiber geometry. This geometry does not require extensive pretreatment because the fibers can be periodically backwashed. Flow in these hollow fiber systems can be either from the inner lumen of the membrane fiber to the outside (inside-out flow) or from the outside to the inside of the fibers (outside-in flow). Tubular NF membranes are now just entering the marketplace. 

Let us consider the following case of removing an inorganic salt from an aqueous stream. It is desired to reduce the salt content of a 26 m /hr water stream (Qf) whose feed concentration, Cp, of 0.035 kmol/m (approximately 2,000 ppm). The feed osmotic pressure (rrp) is 1.57 atm. A 30 atm (Pp) booster pump is used to pressurize the feed. Sixteen hollow fiber modules are to be employed for separation. The modules are configured in parallel with the feed distributed equally among the units. The following properties are available for the HFRO modules ... 

There are at least two ways to make the struts. They could be solid members, or they could be hollow members. You could well imagine that, under certain circumstances, solid members would be perhaps preferable to hollow members, and for other reasons just the opposite would be true. It is up to you to figure out which of those two choices is best for this application. Look at both choices, or you can, alternatively, give a compelling argument as to which configuration, solid cross section, or hollow cross section, you use, with an emphasis on why. Then, you will not have to do the other calculations. 

The initial configuration is set up by building the field 0(r) for a unit cell first on a small cubic lattice, A = 3 or 5, analogously to a two-component, AB, molecular crystal. The value of the field 0(r) = at the point r = (f, 7, k)h on the lattice is set to 1 if, in the molecular crystal, an atom A is in this place if there is an atom B, 0, is set to —1 if there is an empty place, j is set to 0. Fig. 2 shows the initial configuration used to build the field 0(r) for the simple cubic-phase unit cell. Filled black circles represent atoms of type A and hollow circles represent atoms of type B. In this case all sites are occupied by atoms A or B. 

Nonselective membranes can assist enantioselective processes, providing essential nonchiral separation characteristics and thus making a chiral separation based on enantioselectivity outside the membrane technically and economically feasible. For this purpose several configurations can be applied (i) liquid-liquid extraction based on hollow-fiber membrane fractionation (ii) liquid- membrane fractionation and (iii) micellar-enhanced ultrafiltration (MEUF). 

The volume of the equipment for a given area requirement depends on the chosen membrane configuration. For example, spiral wound membranes have a typical packing density of around 800 m2-m 3, whereas the packing density for hollow fiber membranes is much higher, at around 6000 m2-m 3. 

Configurations used include tubes, plate-and-frame arrangements and spiral wound modules. Spiral wound modules should be treated to remove particles down to 20 to 50. im, while hollow fiber modules require particles down to 5 im to be removed. If necessary, pH should be adjusted to avoid extremes of pH. Also, oxidizing agents such as free chlorine must be removed. Because of these restrictions, reverse osmosis is only useful if the wastewater to be treated is free of heavy contamination. The concentrated waste material produced by membrane processes should be recycled if possible but might require further treatment or disposal. 

At a pressure of 1 x 10 5 Torr 1,4-cyclohexadiene, the surface shows ordered domains of hexagonal rings approximately 18 A in diameter made up of six adsorbed molecules with an intermolecular distance of approximately 10 A (Figure 7.22a). A ( 43 x y/43)i 7.6° structure was proposed as schematically shown in Figure 7.22b. 1,4-Cyclohexadiene adsorbs in a boat configuration on bridge or hollow sites on the Pt (1 1 1) surface with comparable adsorption energies of 145.6 and 141.6kj mol-1, respectively [39]. 

Figure 5.4 Structure and manner of assembly of a simple virus, tobacco mosaic virus, (a) Electron micrograph at high resolution of a portion of the virus particle, (b) Assembly of the tobacco mosaic virion. The RNA assumes a helical configuration surrounded by the protein capsomeres. The center of the particle is hollow.

An interesting application of these principles is the prediction of CO dissociation routes on the closed-packed (111) surface of rhodium (see Fig. A.17). Two factors determine how the dissociation of a single CO molecule proceeds. First, the geometry of the final situation must be energetically more favorable than that of the initial one. This condition excludes final configurations with the C and the O atom on adjacent Rh atoms, because this would lead to serious repulsion between the C and O atoms. A favorable situation is the one sketched in Fig. A.17, where initially CO occupies a threefold hollow site, and after dissociation C and O are in opposite threefold sites. The second requirement for rupture of the CO molecule is that the C-0 bond is effectively weakened by the interaction with the metal. This is achieved when the C-O bond stretches across the central Rh atom. In this case there is optimum overlap between the d-electrons of Rh in orbitals, which extend vertically above the surface, and the empty antibonding orbitals of the CO molecule. Hence, the dissociation of CO requires a so-called catalytic ensemble of at least 5 Rh atoms [8,21,22]. 

Figure 1 Diagrams showing the essential electron-optical configurations used for various imaging modes in CTEM and STEM as seen by two points A and B on the sample, (a) CTEM axial bright field, (b) CTEM tilted dark field, (c) CTEM hollow cone dark field, and (d) STEM with bright field and annular dark field detectors.

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