Hollow fiber membrane module permeation

Polymer-Assisted Ultrafiltration of Boric Acid. The Quickstand (AGT, Needham, MA) filtration apparatus is pictured schematically in Figure 3. The hollow fiber membrane module contained approximately 30 fibers with 0.5 mm internal diameter and had a nominal molecular weight cut-off of 10,000 and a surface area of 0.015 m2. A pinch clamp in the retentate recycle line was used to supply back pressure to the system. In a typical run, the transmembrane pressure was maintained at 25 psig and the retentate and permeate flow rates were 25 ml/min and 3 ml/min, respectively. Permeate flux remained constant throughout the experiments. 

Hollow fine fiber membranes are extremely fine polymeric tubes 50-200 micrometers in diameter. The selective layer is on the outside surface of the fibers, facing the high-pressure gas. A hollow-fiber membrane module will normally contain tens of thousands of parallel fibers potted at both ends in epoxy tube sheets. Depending on the module design, both tube sheets can be open, or as shown in Figure 8.1, one fiber end can be blocked and one open. The high-pressure feed gas flows past the membrane surface. A portion of the feed gas permeates the membrane and enters the bore of the fiber and is removed from the open end of the tube sheet. Fiber diameters are small because the fibers must support very large pressure differences feed-to-permeate (shell-to-bore). 

A commercial nitrogen enrichment system is illustrated in Fig. 17. Hollow-fiber membrane modules are connected to a compressed air feed at 70-150 psi. The feed in usually to the bore side of the hollow fibers. Oxygen (and water vapor that may be present) permeate out of the fiber into the shell and exit at low pressure. Dry, nitrogen-enriched air... 

Membrane technology is also offered by other licensors an example is the Polysep Membrane System of UOP [970], In addition to the systems based on hollow fibers, membrane modules have been developed in which the membrane is in the form of a sheet wrapped around a perforated center tube using spacers to separate the layers. The raw gas flows in axial direction in the high pressure spacer and the permeate is withdrawn in the low pressure spacer. Such a module, for example, is marketed under the name Separex [971], [972],... 

A hollow-fiber membrane module similar to the one described in Example 2.14 is used for water desalinization. The feed water flows on the shell side at a superficial velocity of 5 cm/s, 298 K, 70 bar, and 2 wt% NaCl. The permeate flows in the fibers lumen at a pressure of 3 bar and a salt content of 0.05 wt%. For this particular membrane, a water permeance of 1.1 x 10-5 g/cm2-s-bar, and a salt rejection of 97% have been measured. 

Raghuraman and Wiencek [11] developed a hybrid technique where an emulsion is fed into a hollow fiber contactor on the tube side. Since the solid membrane support is hydrophobic, the continuous phase of the water-in-oil emulsion easily wets the pores of the tube wall and permeates to the shell side. On the shell side of the hollow fiber, the aqueous feed phase is exposed and held at an elevated pressure that prevents the permeating liquid membrane phase from exiting the pores. Thus, extraction occurs on the shell side, and stripping on the tube side of the hollow fiber membrane module. This methodology is closely related to SLMs, but the key difference is the presence of the emulsion on the tube side, which allows for long-term stability because the membrane liquid is continuously replenished to make up for any loss by solubility. 

A hollow fiber membrane module is the most commonly used membrane bioreactor (Figure 7.7). The hollow fiber membrane module contains hundreds to thousands of hollow fibers in an assembly similar to a shell and tube heat exchanger. The feed can be applied to either the fiber (lumen) side or to the shell side. However, the feed is usually applied to the shell side because the flow path from the lumen side to shell side is too narrow, which inaeases the risk for membrane clogging. The permeate stream from the membrane module usually contains the product and the retentate contains the concentrated feed stream. 

D. Chang, Perturbation solution of hollow-fiber membrane module for pure gas permeation, J. Membr. Sci., 1998, 143, 53-64. 

The operation of hollow fiber membrane modules is influenced by various parameters on different scales. In order to design a membrane module the flow patterns (module scale) as well as the site of the inlet and the active membrane layer (fiber scale) have to be specified. These specifications affect the module performance due to an influence on the driving force of permeation and on the membrane area. 

Inside-out, Outside-in Filtration in Hollow-Fiber Membranes Hollow-fiber membrane modules can be operated in two different flow modes— inside-out and outside-in —based on the direction of filtration flow. In the inside-out configuration, pressurized feed water flows through the bore of a hollow fiber, and permeate is collected on the outside of the membrane fibres. In the outside-in configuration, the pressurized feed stream flows from the outside of a hollow fiber, and permeate is collected inside the bore of the hollow fiber. 

PERMEATION AND SEPARATION MODEL IN HOLLOW-FIBER MEMBRANE MODULE 481... 

Table 23. Single gas permeation data of SPPOBr (20% brominated with an I. E. C. value of 1.8 meq./g of dry polymer) coated TFC hollow fiber membrane modules (small modules)...

Salts rejected by the membrane stay in the concentrating stream but are continuously disposed from the membrane module by fresh feed to maintain the separation. Continuous removal of the permeate product enables the production of freshwater. RO membrane-building materials are usually polymers, such as cellulose acetates, polyamides or polyimides. The membranes are semipermeable, made of thin 30-200 nanometer thick layers adhering to a thicker porous support layer. Several types exist, such as symmetric, asymmetric, and thin-film composite membranes, depending on the membrane structure. They are usually built as envelopes made of pairs of long sheets separated by spacers, and are spirally wound around the product tube. In some cases, tubular, capillary, and even hollow-fiber membranes are used. 

The use of membrane introduction mass spectrometry (MIMS) was first reported in 1963 by Hoch and Kok for measuring oxygen and carbon dioxide in the kinetic studies of photosynthesis [46], The membrane module used in this work was a flat membrane fitted on the tip of a probe and was operated in the MIS mode. The permeated anaytes were drawn by the vacuum in the MS through a long transfer line. Similar devices were later used for the analysis of organic compounds in blood [47], Memory effects and poor reproducibility plagued these earlier systems. In 1974, the use of hollow-fiber membranes in MIMS was reported, which was also operated in the MIS mode [48], Lower detection limits were achieved thanks to the larger surface area provided by hollow fibers. However, memory effects caused by analyte condensation on the wall of the vacuum transfer line remained a problem. 

The hollow fiber membranes are the optimum choice for gas separation modules due to their very high packing density (up to 30,000 m /m may be attained [1]). Figure 4.21 shows alternative configurations for such modules [108]. Modifications of this configuration exist, where possibility for introduction of sweep gas on permeate side is included, or fibers may be arranged transversal to the flow in order to minimize concentration polarization [109,110]. The hollow fiber membranes are usually asymmetric polymers, but composites also exist. Carbon molecular sieve membranes may easily be prepared as hollow fibers by pyrolysis. 

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. 

The low ethanol rejection and the instability of the hollow-fiber NS-100 membranes preclude the use of this membrane for practical ethanol enrichment. Nevertheless, for the purpose of demonstrating the concept of CCRO using hollow-fiber membranes, CCRO experiments were conducted at the reduced ethanol concentration of 10 vol%. The permeate fluxes of NS-100 modules were measured at 250 psi in the absence and presence of recirculation with a 10-volX ethanol solution. The results were varied recirculation brought about flux increases ranging from 5% to about 20%. The limited flux increase may again be explained in terms of the formation of a polyamine gel during NS-100 membrane fabrication. Nevertheless, the flux increase shows that the hollow-fiber geometry is a viable one for CCRO operation. 

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