Fiber Membranes and Modules

Capillary membrane modules are not as inexpensive or compact as hollow fine fiber modules, but are still very economical. Their principal drawback is the limited pressure differential the fibers can support, typically not more than 10 to 15 bar. This limitation means capillary modules cannot be used at the high pressures needed for hydrogen or natural-gas processing applications. However, capillary modules are ideally suited to lower-pressure separations, such as nitrogen from air or air dehydration. In these applications, capillary modules have essentially the entire market. 

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This chapter provides a state-of-the-art review of HFMST including a general review of hollow fiber membrane contactors, operating principles, design consideration, commercial availability of hollow fiber membrane, and module for scale-up and large-scale studies. Application of HFMST in pharmaceutical, biotechnological, gas absorption and stripping, wastewater treatment, and few latest studies of metal ion extraction are described in detail. 

Li and Sirkar (2004, 2005) reported on novel hollow-fiber membranes and modules for use in both DCMD and VMD configurations. The presented new types of membranes were commercial porous PP hollow fibers (Accurel MEMBRANA, Wuppertal, Germany) of different dimensions and thicknesses coated with a variety of ultrathin microporous silicone-fluoropolymer layer on their external surface by plasma polymerization at Applied Membrane Technology (AMT) Inc. (Minnetonka, MN). The fibers were arranged in a rectangular cross-flow module design for the hot feed to flow over the outside surface of the fibers and to reduce the temperature polarization effect. Both the DCMD and VMD... 

Because membranes appHcable to diverse separation problems are often made by the same general techniques, classification by end use appHcation or preparation method is difficult. The first part of this section is, therefore, organized by membrane stmcture preparation methods are described for symmetrical membranes, asymmetric membranes, ceramic and metal membranes, and Hquid membranes. The production of hollow-fine fiber membranes and membrane modules is then covered. Symmetrical membranes have a uniform stmcture throughout such membranes can be either dense films or microporous. 

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. 

Design of the membrane module system involves selection of the membrane material the module geometry, eg, spiral-wound or hollow-fiber product flow rate and concentration solvent recovery operating pressure and the minimum tolerable flux (9,11). The effects of these variables can be obtained from laboratory or pilot experiments using different membranes and modules. The membrane module as well as the solvent recovery can be chosen to minimize fouling. Spiral-wound modules are widely used because these offer both high surface area as well as a lower fouling potential. 

Figure 13.19 A hollow fiber membrane reactor. Nutrients (S) diffuse to the microbial cells on the shell side of the reactor and undergo reaction to form products (P) such as monoclonal antibodies [31]. Reprinted from J. Membr. Sci. 39, K. Schneider, W. Holz, R. Wollbeck and S. Ripperger, Membranes and Modules for Transmembrane Distillation,...

Improvements made over the last few years in MF and UF membranes and modules, including the development of a new generation of hollow-fiber (HF) membranes and modules for industrial applications has led to wider application of these membrane separation technologies.2 The new generation HF membranes are characterized by high porosity, strength, and flexibility, all important characteristics for MF and UF applications. 

To avoid major fouling and clogging problems, the nature of the feed to be treated has to be considered when hollow fiber modules are used. In the case of lumen to shell filtration, the inside diameter of the fiber is supposed to be at least 10 times the diameter of the largest species present in the feed. However, when the permeate flows from shell fo lumen, concentration and viscosity of the feed and the density of hollow fiber membrane per module may be critical parameters in the design process. Specific aeration or mixing requirements are necessary to keep the feed particles in suspension, and to avoid the clogging of the membrane module. 

Ceramic membranes (a) hollow-fiber elements and modules and (b) ultrafUtration unit for fruit juice clarification. (Courtesy Mempro Ceramics Corporation.)... 

In all these endeavors, rigorous or approximate, transport parameters play a key role and must be addressed first. They are dependent on a number of variables, including flow conditions, membrane and module geometry, and the specific membrane process being addressed. Thus, the flow can be laminar or turbulent, involve gases or liquids, or take place in hollow-fiber or spiral-wound geometries. We listed some of these features and the resulting transport parameters in Table 8.8. 

To overcome the problems of cellulose acetate membranes, many synthetic polymeric materials for reverse osmosis were proposed, but except for one material, none of them proved successful. The only one material, which could remain on the market, was the linear aromatic polyamide with pendant sulfonic acid groups, as shown in Figure 1.2. This material was proposed by DuPont, which fabricated very fine hollow fiber membranes the modules of this membrane were designated B-9 and B-10. They have a high rejection performance, which can be used for single-stage seawater desalination. They were widely used for mainly seawater or brackish water desalination and recovery of valuable materials such as electric deposition paints, until DuPont withdrew them from the market in 2001. 

However, we have succeeded in the development of ozone-resistant filtration modules by developing PVDF hollow-fiber membranes and potting materials having a high resistance to ozone, and assembling technology (Mori et al., 1998). 

Module Structure Figure 5.5 shows an ozone-resistant module. FUtrafion takes place from outside to inside of the hoUow-fiber membrane. The module is designed to feed raw water under high Unear velocity, and air can be fed for the air scmbbing. This module is constructed by hoUow-fiber membranes, potting material, and housing, and ozone resistance is required for aU of these materials. 

Figure 6.4 Profile of a hollow-fiber-type membrane and modules (Pall, X-flow, Inge AG, GEwater, Hydracap, Lenntech, Koch).

Spiral-wound modules are much more commonly used in low pressure or vacuum gas separation appHcations, such as the production of oxygen-enriched air, or the separation of organic vapors from air. In these appHcations, the feed gas is at close to ambient pressure, and a vacuum is drawn on the permeate side of the membrane. Parasitic pressure drops on the permeate side of the membrane and the difficulty in making high performance hollow-fine fiber membranes from the mbbery polymers used to make these membranes both work against hollow-fine fiber modules for this appHcation. 

The fluxes in hoUow-fiber membranes used in seawater desalination are 20—30-fold smaller, but the overall RO system size does not increase because the hoUow-fiber membranes have a much larger surface area per module unit volume. In use with seawater, their RR is about 12—17.5% and the salt rejection ratio is up to 99.5%. 

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