Hollow fiber packing density

Figure 19.3 shows a typical hemodialyzer. Device properties such as the fiber length, membrane surface area, number of fibers, hollow fiber packing density, and header design aU affect solute clearances. 

Hollow-Fiber Packing Density Fiber packing densities are optimized to provide uniform dialysate distribution. Typical dialyzers employ packing densities of roughly 50-75%. 

Aromatic polyamide (aramid) membranes are a copolymer of 1-3 diaminobenzene with 1-3 and 1-4 benzenedicarboxylic acid chlorides. They are usually made into fine hollow fibers, 93 [Lm outer diameter by 43 [Lm inner diameter. Some flat sheet is made for spirals. These membranes are widely used for seawater desalination and to some extent for other process applications. The hollow fibers are capable of veiy high-pressure operation and have considerably greater hydrolytic resistance than does CA. Their packing density in hoUow-fiber form makes them veiy susceptible to colloidal fouling (a permeator 8 inches in diameter contains 3 M fibers), and they have essentially no resistance to chlorine. 

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. 

Fig. I. A. hollow-fiber spool B. Imllow-liher cartridge employed in hennxfial-ysis C. cam idee idcntic.il to nem B demonstrating high packing density. D. hollow-fiber assembly employed for tissue cell growth L. hotlow-liher bundle polled at its ends to be inserted into a cartridge or employed in a situation that requires mechanical llexihiliiy.

As the name suggests, flat-sheet membranes are flat, like a sheet of paper, and can be made as thin as less than 1 pm. However, they need special holders to hold them in place. Hollow-fiber membranes are shaped like tubes (200 to 500 pm ID), allowing fluids to flow inside as well as on the outside. Hollow fibers are self-supported and offer the advantage of larger surface area per unit volume and high packing density. A large number of parallel fibers can be packed into a small volume. 

Two main types of membrane modules are normally employed for animal cell separation the plate-and-frame and the hollow-fiber modules. The latter type has as an important feature, a high packing density, which results in a high permeation area for a compact module. Both module types may be used in cell separation in batch processes, and also in perfusion processes. The main characteristics of these two module types are described in Table 11.1. 

First commercial hollow-fiber membrane module developed by DuPont. This module configuration further increased the packing density of membrane modules. 

Hollow Fiber Small footprint Low capital cost Can be backwashed Can easily be integrity tested High membrane area per unit volume (high packing density) Plugging of fiber (inside-out feed) Bridging of fiber bundle (outside-in feed) Difficult to clean... 

Polymeric membrane elements and modules which consist of elements come in different sh4>es. The shape strongly determines the packing density of the element or module which is indicative of the available membrane filtration area per unit volume of the element or module the packing density, in turn, can affect the capital and operating costs of the membranes. The packing density is often balanced by other factors such as ease and cost of maintenance and replacement, energy requirements, etc. Most of the polymeric membranes are fabricated into the following forms tube, tubes-in-shell, plate-and-frame, hollow-fiber, and spiral-wound. 

Membranes that arc catalytically active or impregnated with catalyst do not suffer from any potential catalyst loss or attrition as much as other membrane reactor configurations. This and the above advantage have the implication that the former requires a lower catalyst concentration per unit volume than the latter. It should be mentioned that the catalyst concentration per unit volume can be further increased by selecting a high "packing density" (surface area per unit volume) membrane element such as a honeycomb monolith or hollow fiber shape. 

There is some indication that a high "packing density" membrane shape may result in a greater conversion than, for example, membrane tubes. In a study of using an a-alumina hollow fiber (1.6 mm in diameter) coated with an y-alumina membrane as a membrane reactor for dehydrogenation of cyclohexane, Okubo et al. [1991] found that a given... 

A still higher membrane packing density can be accomplished with some geometries available to certain commercial organic membranes. One such attempt is hollow fibers [Baker et al., 1978 Beaver, 1986]. Sintered monolithic hollow fibers of metals or metal oxides have been developed [Dobo and Graham, 1979] but their burst strengths appear to be critical. Further developments are needed before commercialization. 

As mentioned earlier, one of the membrane element shapes with the highest packing density is hollow fibers. Typically several fibers are bundled to provide higher strength. In a packed-bed membrane reactor of this type, catalyst particles arc packed around the bundles. 

Current commercial inorganic membranes come in a limited number of shapes disk, tube and monolithic honeycomb. Compared to other shapes such as spiral-wound and hollow-fiber that are available to commercial organic membranes, these types of membrane elements have lower packing densities and, therefore, lower throughput per unit volume of membrane element or system. 

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. 

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 ). 

Hollow fiber BOs use hydrophobic membranes. In most designs, the blood flows outside and across the fibers. Consequently, the fibers help dismpt the blood side concentration boundary layer. Hollow fiber and flat sheet BOs have almost equivalent hemocompatibility and gas exchange performance. However, the gross air handling is different due to differences in the packing density (total available membrane surface area per unit volume). For flat sheet BOs, the packing... 

Hollow fibers have the highest packing density of the existing modules but fluxes are low. They are mostly used in MBR in the outside-in mode (MF and UF membranes) in the pulp and paper industry at close to subcritical fluxes to avoid fouling. In addition, their cleaning is improved by backflushing techniques. Aeration is used to minimize particle adhesion and concentration polarization. 

The choice between the two concepts is mainly based on some parameters such as operation pressure, pressure drop, or type of membrane available. The fiber wall has a structure of the asymmetric membrane, and the active skin layer being placed to the feed side. The hollow-fiber module is featured by a very high packing density, which can reach values of 30,000 vtPlm . 

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