Spiral wound membrane modules cross section

Figure 33.2 Cross section of a spiral-wound membrane module having 12 membrane layers and an outer wrapping that is encased in a housing onto which a UTDR transducer is attached schematic shows reflections fi om the outer wrapping and second and third membrane layers.

Figure 3.42 Exploded view and cross-section drawings of a spiral-wound module. Feed solution passes across the membrane surface. A portion passes through the membrane and enters the membrane envelope where it spirals inward to the central perforated collection pipe. One solution enters the module (the feed) and two solutions leave (the residue and the permeate). Spiral-wound modules are the most common module design for reverse osmosis and ultrafiltration as well as for high-pressure gas separation applications in the natural gas industry...

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.

Applying UTDR to a spiral-wound module is complicated by multiple reflections that occur from each of the interfaces associated with the wrapping material and concentric membrane, membrane support, and channel-spacer layers as shown in the schematic cross section in Figure 33.2. Two methods that have been developed to address this... 

The second type of membrane module, which is the hardest to visualize, is the spiral-wound element. This module essentially consists of a large membrane envelope loosely rolled like a jelly roll. The feed stays outside the envelope and products are harvested from the inside via a central tube. In some more sophisticated designs, many envelopes may come out from the central tube, so that a cross-section of the module would look like a daisy with petals twisted in a circular direction. This type of module has become the dominant geometry for reverse osmosis. While it has less membrane area per volume than a hollow-fiber module, it plugs less easily. However, even if only part of the membrane fails, the entire module must be discarded. 

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