Spiral wound membrane modules 8-inch diameter

Figure 4.14 Eight-inch diameter spiral wound membrane module. Courtesy of Dow Water and Process Solutions.

Flat-sheet membranes are made in continuous rolls 500-5000 m long. Sheets of membrane 1-2 m long are cut and folded and then packaged as spiral-wound module envelopes. A single module may contain as many as thirty membrane envelopes. Currently, the industry standard spiral-wound module is 8 inches (1 inch = 2.54 cm) in diameter and about 35-40 inches long it contains 20-40 m2 of membrane. 

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.

A large-scale natural gas producer made available a test skid that would allow for membrane prodncts to be demonstrated under real field conditions. Commercial CA membranes from both Grace and another supplier and the developmental membrane were delivered to the field as full-scale 20 cm (8 inch) diameter spiral wound modules. 

For new system builds the same strategy can be done with spiral wound modules designed for natural gas treatment. Grace has constructed a 30-cm (12-inch) diameter module versus the standard 20-cm (8-inch) diameter module. Since membrane area is a squared function of diameter, the ratio of 144 to 64 means a 225% increase in membrane area per module. Although the larger diameter modules and pressure housings each cost more there is potential for reduction in total system cost. 

A tubular membrane of 4 inches diameter and 9 feet length was also manufactured using a polypropylene porous substrate tube. A larger coater head was used for this purpose. It is recorded that the composite film was soaked in a hot (140 - 150 °F) aqueous solution of 3.5 g/L sodium dodecyl benzene sulfonate to increase the flux. When this tubular membrane was opened, a flat sheet with about 12-inch width could be obtained. These flat sheets were used for making spiral wound modules. 

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