Hollow fiber reverse osmosis module

Immobilized Liquid Membranes. A pilot plant study of the recovery of ethylene and propylene from a polypropylene reactor off-gas stream was presented by Hughes et al. (23). Aqueous solutions of Ag Ion were Immobilized In the pore structure of commercial reverse osmosis hollow fiber modules. The pilot plant operated at feed pressures of 414-827 kPa, feed flow rates of 1.42-4.25 m /h at STP, and sweep flow rates of 3.79 10" - 0.114 m /h hexane. Permeate streams with propylene concentrations In excess of 98 mole % were observed in pilot plant operation with modules containing 22.3 to 37.2 m membrane area. 

Research effort at Albany International Research Co. has developed unit processes necessary for pilot scale production of several species of reverse osmosis hollow fiber composite membranes. These processes include spin-dope preparation, a proprietary apparatus for dry-jet wet-spinning of microporous polysul-fone hollow fibers, coating of these fibers with a variety of permselective materials, bundle winding using multifilament yarns and module assembly. Modules of the membrane identified as Quantro II are in field trial against brackish and seawater feeds. Brackish water rejections of 94+% at a flux of 5-7 gfd at 400 psi have been measured. Seawater rejections of 99+% at 1-2 gfd at 1000 psi have been measured. Membrane use requires sealing of some portion of the fiber bundle for installation in a pressure shell. Much effort has been devoted to identification of potting materials which exhibit satisfactory adhesion to the fiber while... 

Spiral-wound membrane channel in reverse osmosis" Hollow fiber membrane module Laminar tube-side flow Sh = = 0.065 Be Da Re = vdh/o) Schock and Miquel (1987) (3.1.170)... 

Fig. 23. Two types of hollow-fiber modules used for gas separation, reverse osmosis, and ultrafiltration applications, (a) Shell-side feed modules are generally used for high pressure appHcations up to - 7 MPa (1000 psig). Fouling on the feed side of the membrane can be a problem with this design, and pretreatment of the feed stream to remove particulates is required, (b) Bore-side feed modules are generally used for medium pressure feed streams up to - 1 MPa (150 psig), where good flow control to minimise fouling and concentration polarization on the feed side of the membrane is desired.

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. 

Albany International Research Co. has developed an advanced hollow fiber composite reverse osmosis membrane and module under the name of Quantro II . This composite membrane is comprised of a porous hollow fiber substrate on which has been deposited a rejection barrier capable of fluxes of commercial importance at high rejection of dissolved salts at elevated temperatures. Resistance to active chlorine has been demonstrated. Proprietary processes have been developed for spinning of the fiber, establishment of the rejection barrier and processing of the fiber to prepare modules of commercial size. Prototype modules are currently in field trials against brackish and seawater feed solutions. Applications under consideration for this membrane include brackish and seawater desalination as well as selected industrial concentration processes. 

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.

Figure 3.47 A cross-flow hollow fiber module used to obtain better flow distribution and reduce concentration polarization (the Tyobo Hollosep reverse osmosis module). Feed enters through the perforated central pipe and flows towards the module shell...

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

A simplified flow scheme for a brackish water reverse osmosis plant is shown in Figure 5.24. In this example, it is assumed that the brackish water is heavily contaminated with suspended solids, so flocculation followed by a sand filter and a cartridge filter is used to remove particulates. The pH of the feed solution might be adjusted, followed by chlorination to sterilize the water to prevent bacterial growth on the membranes and addition of an anti-sealant to inhibit precipitation of multivalent salts on the membrane. Finally, if chlorine-sensitive interfacial composite membranes are used, sodium sulfite is added to remove excess chlorine before the water contacts the membrane. Generally, more pretreatment is required in plants using hollow fiber modules than in plants using spiral-wound modules. This is one reason why hollow fiber modules have been displaced by spiral-wound systems for most brackish water installations. 

Spiral-wound and hollow-fiber modules developed for reverse osmosis... 

Figure 10. Effect of Feed Concentration on the Reverse-Osmosis Performance of NS-lOO Hollow-Fiber Modules (250 psi, ambient temperature)...

The Monsanto hollow-fiber module Tor gas separation appears to be a compromise between the two preceding reverse osmosis module designs/ As shown in Fig, 20. [-4, the hollow fibers are closed at one and with an epoxy plug or similar devioe, and Teed enters at the side at one end, The feed then flows axially along Ibe fibers, and (he nor permeant exits at the opposite end from which the feed eaters. Permeant diffusing through the hollow-fiber wall into the bora moves con me (currently to the shell-side flow and unimeieiy is collected in the chamber where all the open ends of (he fibers terminate,... 

According to Figure 6.10, an optimal fiber length exists for a given fiber diameter. Figure 6.8 clearly indicates that hollow fiber modules for reverse osmosis should be made as short as possible. It should be noticed that a simple correlation between yield of the single fiber and yield of the bundle can be derived in any case only if the concentration and the pressure in the shell (outside of the bundle) are considered to be constant In principle, the concentration of the feed increases between entrance and outlet while the pressure of the feed... 

In the case of reverse osmosis, the relative flow configuration does not affect the performance to any large extent. As already indicated, the situation is quite different for gas permeation. While countercurrent flow improves the separation efficiency of hollow fiber modules in reverse osmosis only slightly, as far... 

The most important applications of hollow-fiber modules are in hemodialysis (Figure 5.70), reverse osmosis, and gas separation units. Modules up to 50 cm diameter containing hundreds of thousands of fibers are used in gas separation. 

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. 

A second factor determining module selection is resistance to fouling. Membrane fouling is a particularly important problem in Hquid separations such as reverse osmosis and ultrafiltration. In gas separation appHcations, fouling is more easily controlled. Hollow-fine fibers are notoriously prone to fouling and can only be used in reverse osmosis appHcations if extensive, costiy feed-solution pretreatment is used to remove ah. particulates. These fibers caimot be used in ultrafiltration appHcations at ah. 

---