The Hollow Fiber Module

Depending on the process application, one or the other of these requirements is of primary importance and, for this reason, a number of different modules have been designed. The most important designs are the plate and frame, the spiral wound and the hollow fiber module. Optimization procedure and some of its results will be discussed for only one module configuration, the hollow fiber module-and for only one application-RO. 

Hollow fiber modules contain very fine fibers forming asymmetric or symmetric membranes and capable of withstanding pressure differences (high-pres- 

In theory, larger fiber diameters should increase the flux, i.e., the performance of a single fiber. However, at the same time, the membrane area per unit volume decreases. For some simplifying assumptions, it can be easily discussed how fiber diameter and fiber length have to be chosen in order to maximize the yield per unit volume of the bundle. 

Neglecting (1) the radial pressure losses in the feed and (2) the influence of osmotic pressure on flux, the specific yield of the fiber bundle is given by  

K0 has to be varied according to the fiber arrangement. K0 - 30 is valid for fibers in parallel. 

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RO membrane performance in the utility industry is a function of two major factors the membrane material and the configuration of the membrane module. Most utility applications use either spiral-wound or hollow-fiber elements. Hollow-fiber elements are particularly prone to fouling and, once fouled, are hard to clean. Thus, applications that employ these fibers require a great deal of pretreatment to remove all suspended and colloidal material in the feed stream. Spiral-wound modules (refer to Figure 50), due to their relative resistance to fouling, have a broader range of applications. A major advantage of the hollow-fiber modules, however, is the fact that they can pack 5000 ft of surface area in a 1 ft volume, while a spiral wound module can only contain 300 ftVff. 

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. 

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 ). 

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 . 

The hollow-fiber module is often used when the feed stream is relatively clean, such as in gas separation and pervaporation. It has also been used in the case of seawater desalination, but pretreatment is needed. The module construction given in Fig. 15 A is a typical RO module, where a central pipe is used to uniformly distribute the feed solution throughout the module. This is to avoid the problem of channelling in outside-in model, which means the feed has a tendency to flow along a fixed path, thus reducing the effective membrane surface area. In gas separation, as shown in Fig. 15B, the outside-in model is used to avoid high pressure losses inside the fiber and to attain a high membrane area (13). 

This reactor can be operated in two ways. When the substrate is fed to the shell side of the hollow fiber module ("back flush mode"), the substrate comes in contact with the enzyme in the fiber wall and product passes into the lumen of the fiber from which it exits the module. When the substrate is fed to the lumen of the fibers (with all permeate ports closed), it will pass from the lumen to the shell side where it contacts the enzyme and products will recycle back to the lumen ("recycle mode") (see Figure 3.72). The "recycle mode" has the advantage over the "back flush mode" in that the substrate does not have to be free of suspended matter. In the "back flush mode", particles in the substrate would plug the sponge wall. 

To design the deaerator, we must first decide on the geometry for the hollow-fiber module. We will use commercially available microporous polypropylene hollow fibers in a module similar to that shown in Figure 2.4. Water at 298 K will flow through the shell side, parallel to the fibers, at a superficial velocity of 10 cm/s. Pure nitrogen at 298 K and 1 atm at the rate of 40 L/min will be used as a sweep gas in countercurrent flow through the lumen. The outside diameter of the available fibers is 290 pm, the packing factor is 40%, and the surface area per unit volume is a - 46.84 cm-1 (Prasad and Sirkar, 1988). 

The membrane shapes described are usually incorporated into compact commercial modules and cartridges. The four more common types of modules are (1) plate-and-frame, (2) spiral-wound, (3) tubular, and (4) hollow-fiber. Table 9.2 is a comparison of the characteristics of these four types of modules. The packing density refers to the surface area per unit volume of module, for which the hollow-fiber modules are clearly superior. However, hollow-fiber modules are highly susceptible to fouling and very difficult to clean. The spiral-wound module is very popular for most applications because of its low cost and reasonable resistance to fouling. 

Aqueous 40% vol/vol ethanol stored In reservoir (A) was pumped Into the shell and tube side of the hollow fiber module at a sufficiently high flow rate (>200 cm /mln). This solution completely wetted the fibers. Thus a 40% aqueous ethanol solution was incorporated In the pores of the Celgard hollow fibers. 

The experimental procedure for permeation studies with hollow fiber modules was Identical to that described by Bhave and Slrkar (12) for flat Celgard films except that the flat film test cell was replaced by the hollow fiber module (see Figure 1 in reference (12)). 

By the use of hollow-fiber membranes, the ideal module comprises short fibers with a wide bore to avoid a high pressure drop in the flow direction thereby disturbing the uniform radial flow pattern and creating channeling. The membranes should also possess thick porous walls with small pore size and a high ligand density. The hollow-fiber modules can be operated in cross-flow mode that makes them especially suitable in the treatment of solutions containing particulate material. 

The choice of module design for CMS membranes will typically be the hollow fiber module with counter-current flow. Membrane module construction is, however, seldom referred in open literature as details on this will typically be confidential information for a company producing membrane modules. To date, only tubular and hollow fiber laboratory scale modules have been reported for carbon membranes. The potential industrial use of these... 

By solving the partial differential equations, the solution of C as a function of r and z will be obtained. The mixing-cup concentration that defines the average concentration of a flowing stream can be used to obtain the outlet concentration of solute from the hollow fiber module ... 

In what follows the methodology for the selection of the operating conditions of a nondispersive solvent extraction process will be developed. As an example the removal and recovery of Cr(VI) from an indnstrial effluent of a surface treatment plant will be considered. The kinetic modeling including the extraction reactions. Equation (6.17) and Equation (6.22), and the mass balances of chromium compounds to the three fluid phases and considering the hollow fiber modules and the homoge-neization stirred tanks. Equation (6.30) through Equation (6.50) were described in Sections 6.3 and 6.4. 

Cai and pi in the hollow fiber module. Calculate the sodium chloride concentration in the permeate from the module. Calculate the volumetric flow rate of the permeate. 

Figure 8.6. Comparative NDSX data of U (1 g fr- ) and Th (200 g fr- ) through the hollow fiber module.

The influence of various numbers of runs through the hollow fiber module on the concentration of As ions in the outlet solutions was studied by using 35% (v/v) Aliquat 336 and 0.5 M NaOH as the stripping solution. The percentages of extraction and recovery for As ions increased when... 

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