Capillary fiber modules

Figure 6.14 Backflushing of membrane modules by closing the permeate port. This technique is particularly apphcable to capillary fiber modules...

The second type of hoUow-fiber module is the bore-side feed type illustrated in Figure 23b. The fibers in this type of unit are open at both ends, and the feed fluid is usually circulated through the bore of the fibers. To minimize pressure drops inside the fibers, the fibers often have larger diameters than the very fine fibers used in the shell-side feed system and are generally made by solution spinning. These so-called capillary fibers are used in ultrafiltration, pervaporation, and in some low to medium pressure gas appHcations. Feed pressures are usually limited to less than 1 MPa (150 psig) in this type of module. 

Another factor is the ease with which various membrane materials can be fabricated into a particular module design. Almost all membranes can be formed into plate-and-frame, spiral-wound and tubular modules, but many membrane materials cannot be fabricated into hollow fine fibers or capillary fibers. Finally, the suitability of the module design for high-pressure operation and the relative... 

Figure 7.19 Photograph of a 25 million gal/day capillary hollow fiber module plant to produce potable water from a well, installed by Norit (X-Flow) in Keldgate, UK. Courtesy of Norit Membrane Technology BY...

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. 

Each membrane/module type has advantages and disadvantages [2,7]. Hollow fine fibers are generally the cheapest on a per-square-meter basis, but it is harder to make very thin selective membrane layers in hollow-fiber form than in flat-sheet form. This means the permeances of hollow fibers are usually lower than flat-sheet membranes made from the same material. Also, hollow fine fiber modules require more pretreatment of the feed to remove particulates, oil mist and other fouling components than is usually required by capillary or spiral-wound modules. These factors offset some of the cost advantage of the hollow fine fiber design. 

The investment in time and equipment to develop a new membrane material in a high-performance hollow fine fiber or capillary form is far larger than that required to develop flat-sheet membranes, and many materials cannot be formed into fiber modules at all. For this reason, flat-sheet membranes, formed into spiral-wound modules, are used in many niche applications which cannot support the development costs associated with fiber modules. Spiral-wound modules are also competitive in the natural-gas processing area, where their general robustness is an asset. 

A hollow fiber module is conceptually similar to the capillary module, but differs in dimensions. In this case the diameter of the tubular membrane varies between 50 and 100 pm and several thousand of fibers can be placed in the vessel. The hollow fiber module is the configuration with the highest packing density (with values up to 30,000 m m ). 

While the previously described three membrane modules required flat sheet membrane material for their preparation, special membrane configurations are needed for the preparation of the tubular, capillary, and hollow fiber modules. The tubular membrane module consists of membrane tubes placed into porous stainless steel or fiber glass reinforced plastic pipes. The pressurized feed solution flows down the tube bore and the permeate is collected on the outer side of the porous support pipe, as indicated in Figure 1.33 (d). The diameters of tubular membranes are typically between 1-2.5 cm. In some modules, the membranes are cast directly on the porous pipes and in others they are prepared separately as tubes and then installed into the support pipes. 

Figure 1.33 Schematic diagram showing membrane modules presently used in industrial separation processes (a) pleated membrane filter cartridge (b) plate-and-frame membrane module (c) spiral wound membrane module (d) tubular membrane module (e) capillary membrane module (f) hollow fiber membrane module.

The second advance that made industrial membrane-separation processes possible was the development of methods to incorporate large membrane areas into economical membrane packets or modules. Even with the best anisotropic membranes, most industrial processes require several hundred, sometimes several thousand, square meters of membrane to perform the separation. The three most important configurations, shown schematically in Fig. 7.1, are hollow-fme-fiber, capillary-fiber and spiral-wound modules. Tubular and plate-and-frame... 

Cylindrical—utilized in tubular and capillary, or hollow fiber, modules... 

A number of module designs are possible and all are based on two types of membrane configuration i) flat and ii) tubular. Plate-and-frame and spiral-wound modules involve flat membranes whereas tubular, capillary and hollow fiber modules are based on mbular membrane configurations. The difference between the latter types of module arises mainly from the dimensions of the tubes employed, as is shown in table Vin. I. 

Figure VIII 11. Schemadc.drawing of a capillary module/hollow fiber module.(Ieft) inside-out or tube-side, feed outside-in or shell-side feed (right)...

A number of module designs are possible and all are based on two types of membrane geometry (i) flat sheet membranes and (ii) capillary fibers. Typical dimensions are shown in Figure 5.2. 

Verberk (2005) showed that combined water and airflow resulted in a much more intensive cleaning than a forward flush performed at a water velocity equal to the sum of the individual air and water velocities. Optimal values for the water and air velocities obtained in the smdy were 0.2 and 0.3 m/s, respectively. Equal distribution of air and water over the cross-sectional area of a membrane module is important to have the same cleaning conditions in every membrane fiber in a module. Verberk (2005) found that water was well distributed over the cross section of the module when a forward flush was performed. When a water-air flush was performed, there was an even distribution of water and air in the capillary membrane module membranes. However, this was not the case for the tubular modules, as stagnant water levels were observed. Intermittent application of water-air flush was recommended as a probable solution to this maldistribution problem. Chemical consumption is likely to be low, and stable operation was achieved by making use of the AirFlush cleaning method (Verberk, 2005). 

Depending on their dimensions, it is possible to distinguish hollow-fiber membranes (diameter < 0.5 mm), capillary membranes (0.5 mm < diameter < 5 mm), and tubular membranes (diameter > 5 mm) [34]. The preparation of hollow-fiber membranes through PI is more complex, with respect to flat sheet, due to the higher number of parameters involved. However, hollow-fiber modules are usually preferred, because they ensure space savings, more productivity, and reduction of costs, which is also connected to maintenance, as these modules can be backflushed [35]. 

For ultrafiltration appHcations, hollow-fine fibers have never been seriously considered because of their susceptibiUty to fouling. If the feed solution is extremely fouling, tubular or plate-and-frame systems ate still used. Recentiy, however, spiral-wound modules with improved resistance to fouling have been developed, and these modules are increasingly displacing the more expensive plate-and-frame and tubular systems. Capillary systems are also used in some ultrafiltration appHcations. 

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