Bore-side feed

Fig. 23. Two types of hollow-fiber modules used for gas separation, reverse osmosis, and ultrafiltration applications, (a) Shell-side feed modules are generally used for high pressure appHcations up to - 7 MPa (1000 psig). Fouling on the feed side of the membrane can be a problem with this design, and pretreatment of the feed stream to remove particulates is required, (b) Bore-side feed modules are generally used for medium pressure feed streams up to - 1 MPa (150 psig), where good flow control to minimise fouling and concentration polarization on the feed side of the membrane is desired.

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. 

In bore-side feed modules, it is important to ensure that all of the fibers have identical fiber diameters and permeances. Even fiber variation as small as 10 % from the average fiber can lead to large variations in module performance [116,117], The flow of fluid through the fiber bore is proportional to the fiber... 

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

The largest current application of gas-separation membranes is separation of nitrogen (N2) from air. Capillary modules formed into bore-side feed modules are used almost exclusively in this application [10, 11]. The feed air is compressed to 6-10 bar and pumped through the membrane capillaries. Oxygen (02) permeates the membrane preferentially, leaving an oxygen-depleted, nitrogen-rich residue stream. The first membranes used for this application were based on poly(4-methyl-l-pentene) and ethyl cellulose, and had 02/N2 selectivities of about 4. Because of the modest... 

Hollow-fiber reverse-osmosis modules (a) sheU-side feed and (b) bore-side feed. 

Figure 5.8 Schematic of a hollow fiber module with the feed on the inside of the fiber (bore side feed) and the feed on the outside of the fibers (shell side feed).

Bore side feed Pretreatment required from air Air Products... 

Generally, there are two types of modules distinguished by the gas flow direction The first type feeds the gas mixture into the inside of the hollow fibers, and thereby the permeated gas is collected outside of the fiber bundle (bore-side feed) the second type feeds the gas mixture outside of the fiber bundle, and thereby die permeated gas is collected inside of the hollow fibers (shell-side feed). The suitable type of the module (bore-side feed or shell-side feed) is decided on the basis of feed gas pressure, pressure drop in the module, the composition of the feed gas, and the gas permeability of the membrane. 

The hollow-fiber membranes are arranged in a module so that the gas concentration gradient along the gas flow direction is constant and the short pass is inhibited. The membrane module of the bore-side feed case is schematically shown in Figure 22.3. 

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