Membrane Module Constructions

Fig 7.3 Membrane module construction. Courtesy of the American Chemical Society. 

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... 

A membrane fouling simulator (MFS) provides a similar visualization method to DOTM and DVO. This technique features microscopic visualization of the surface of a spiral wound membrane and a crossflow membrane module constructed from stainless steel plate. TMP during filtration is also measured, while fouling deposition of biomass on spacer is visualized simultaneously. MFS is easy to handle, simple, robust and small, with a flow capacity of 15-25 Lh [70, 71]. 

Figure 19.3. Tubular and plate-and-frame membrane modules for reverse osmosis and ultrafiltration, (a) Construction and flow pattern of a single 1 in. dia tube with membrane coating on the inside in Table 19.4, the Ultracor model has seven tubes in a shell and the Supercor has 19 [Koch Membrane Systems (Abcor)]. (b) Assembly of a plate-and-frame ultrafiltration module (Danish Sugar Co.), (c) Flow in a plate-and-frame ultrafiltration module.

The membrane module and design will obviously depend on the type of membrane used. The flat-sheet membranes are commonly constructed in a plate-and-frame configuration or as spiral-wound (SW) modules. F1F/CT/MTmembrane types are commonly manufactured into bundles that are installed in housing units or designed to be unconfined in the fluid, that is, immersed units. The membranes are... 

Challenges in this field include the need for higher productivity, membranes and modules specifically designed for the emulsification process, modules construction standardization, and design of innovative intensified processes. 

Figure 4.15 shows the cross section of the spiral wound module.24 The spiral construction starts with two sheets of membrane placed back to back with a nylon tricot mesh spacer material in between. This tricot spacer provides the permeate channel for the membranes. These sheets of membrane and spacer are glued on 3 sides so that the permeate can only exit the spacer on one side. This set of membranes and spacer is called a "leaf." Leaves are then placed together with a low density polypropelene mesh spacer to provide the feed/reject channel for the membranes. The thickness of the mesh feed spacer can be adjusted from 28 mils to 34 mils to accommodate higher solids influent water (thicker feed spacers are more forgiving with respect to fouling with suspended solids than thinner spacers—see Chapter 4.4.2.3). The entire collection of leaves and mesh feed spacers are then wrapped around a perforated permeate collection tube so that the open side of the leaf is toward the perforated permeate tube (see Figure 4.16). Note that an 8-inch diameter membrane module has about 16 leaves, and each leaf is about 50 inches in length.

High influent temperature—this is to protect the membrane module materials of construction that can "melt" at high temperatures. 

Shims are installed at the feed end of the module/pressure vessel assembly (refer to figures 4.18 and 4.19). Because pressure vessels are constructed with slight variations in length (known as "freeboard"), membrane modules can slide during pressurization and depressurization. Shims are installed between the face of the lead module and the adapter hub to prevent this motion. Membrane modules should be pushed completely against the thrust ring prior to installation of the shims. Shims are washer-like plastic rings that may be purchased from the pressure vessel manufacturer or fashioned out of PVC (must be free of burrs and be cut parallel to work properly). 

E.R. Geus, W.J.W. Bakker, J.A. Moulijn, and H. van Bekkum, High-temperature stainless steel supponed zeolite (MFI) membranes Preparation, module construction and permeation experiments, Microporous Mater. 7 131 (1993). 

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). 

Because polymeric membranes are operated at near-ambient temperatures, matching the coefficient of thermal expansion (CTE) of materials used to construct the membrane module is not so critical. However, ceramic and metal membranes that will be operated at several hundred degrees may experience unacceptable strain, leading to failure, due to mismatched CTE. A simple example is a stainless steel... 

As a result of tlie construction of the membrane module, the interface area for mass transfer per volume is very high compared to extraction towers and is not influenced by flow volumes. The flow volumes are only restricted by the phase breakthrough into the other phase caused by the pressure loss along the contactor. HTU values are lower compared to other extraction units therefore the membrane contactor has higher extraction efficiency. HTU values increase with higher loading of the membrane module. To reach a higher theoretical plate munber, comparable to extraction towers, more module units have to be set in series. 

Figure A. Construction of a Hollow-Fiber Membrane Module...

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