Shell-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. 

SelF-siipported cylindrical membranes For liquid separations are made From 250 jlrn up to 6 rnrn, but there is no obvious limit to Future oFFerings. Membrane devices For liquids are almost alvvws tube-side Feed, with tw o major exceptions at the extremes oF porositv. High-pressure RO is almost always shell-side Feed, and one supplier oF verv lovv -pressiire MF also rims with shell-side Feed. 

Hollow-fiber membranes may be run with shell-side or tube-side feed, cocurrent, countercurrent or in the case of shell-side feed and two end permeate collection, co- and countercurrent. Not shown is the scheme for feed inside the fiber, common practice in lower-pressure separations such as air. 

FIG. 22-75 Air fractionation by membrane. O2 in retentate as a function of feed fraction passed tbrougb tbe membrane (stage cut) showing tbe different result with changing process paths. Process has shell-side feed at 690 kPa (abs) and 298 K. Module comprised of hollow fibers, diameter 370 im od X 145 im id X 1500 mm long. Membrane properties (X = 5.7 (O2/N2), permeance for O2 = 3.75 X 10 Barrer/cm. Coutiesy Innovative Membrane Systems/ Fraxair)... 

Shell-side feed hollow-fiber modules,... 

Shell-side flow. The hot shell-side flow enters the exchanger, as shown in Fig. 19.1, through the top inlet nozzle. Not shown on this sketch is the impingement plate, which is simply a square piece of metal, somewhat larger than the inlet nozzle. Its function is to protect the tubes from the erosive velocity of the shell-side feed. The plate lies across the upper row of tubes. 

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. 

Spiral-wound module) (Shell side feed)... 

The mass balances for all the species involved, for both the tube and shell side of the system shown in Figure 13.2 (shell-side feed configuration, system with cylindrical symmetry) can be written as ... 

Gas separation membranes have diameters as small as 135 pm onter diameter by 95 pm iimer diameter. For low-pressure applications such as air, they may run with tube-side feed. Gas membranes operating at high pressure (above 1.5 MPa) are almost always run with shell-side feed. The outer diameter for gas membranes may be as high as 500 pm. 

Self-supported cylindrical membranes for liquid separations are made from 250 pm up to 6 mm, but there is no obvious limit to future offerings. Membrane devices for liquids are almost always tube-side feed, with two major exceptions at the extremes of porosity. High-pressnre RO is almost always shell-side feed, and one supplier of very low-pressure MF also runs with shell-side feed. 

With respect to gas separation hollow fiber modules, the chemical compatibility and the mechanical stability of the fibers and potting are paramount. High pressure modules use shell-side feed while low pressure modules... 

Flowrates 1.5 gal/min, shell side, feed 1.0 gal/tnin, lumen side, strip. 

Introduction of feed gas on the shell side of the module can reduce concentration polarization. Figure 10 shows three shell side feed configurations [13]. Notice that in each configuration, one tube sheet is open to the bore of the hollow fibers and the opposite end is closed. Concentration... 

Introducing the high pressure feed into the fiber lumens (lumen feed) is recommended when the stage strips a more permeable component present at low concentration in the feed (less than -25%). Shell-side feed is recommended when the more permeable component is present at higher concentrations (greater than -80%). 

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