Pressure vessels spiral-wound modules

Spira.1- Wound Modules. Spiral-wound modules were used originally for artificial kidneys, but were fuUy developed for reverse osmosis systems. This work, carried out by UOP under sponsorship of the Office of Saline Water (later the Office of Water Research and Technology) resulted in a number of spiral-wound designs (63—65). The design shown in Figure 21 is the simplest and most common, and consists of a membrane envelope wound around a perforated central coUection tube. The wound module is placed inside a tubular pressure vessel, and feed gas is circulated axiaUy down the module across the membrane envelope. A portion of the feed permeates into the membrane envelope, where it spirals toward the center and exits through the coUection tube. 

Figure 3.44 Schematic of a spiral-wound module [115] installed in a multimodule pressure vessel. Typically four to six modules are installed in a single pressure vessel. Reprinted from Reverse Osmosis Technology, B.S. Parekh (ed.), Marcel Dekker, New York (1988), p. 81, by courtesy of Marcel Dekker, Inc.

Figure 4.26 Cut-away of a pressure vessel with a spiral-wound modules inside.

Figure 4.27 Spiral wound module in pressure vessel without pressure vessel end caps installed. The O-ring is used to seat the end cap in place. Courtesy ofNako-Cmssbow LLC.

Figure 431 ROCHEM ST module, a modified spiral wound module, showing a) a cross-section and b) the module within a pressure vessel housing. Courtesy ROCHEM Ultrafiltrations Systems GmbH.

Figure 6.8 Pressure vessel containing six spiral-wound modules housed in series.

Usually, a number of spiral-wound modules are assembled in one pressure vessel and are connected in series via the central permeate tubes. Some industrial-scale spiral-wound modules are shown in Fig. 11. 

The module assembly is placed within a case that possesses manifolds to direct feed water along the membrane surface, withdraw product water from the collection tube, and remove the rejected water. A rectangular case might be used but a more compact unit is obtained by rolling the envelope and feed spacer around the permeate collection tube the feed spacer defines a channel for feed water flow between successive layers of the envelope. A cylindrical pressure vessel is used to hold the spiral wound module thus produced. 

The membrane modules, which are commonly used in organic vapor separation, are spiral-wound modules or the envelope-type GKSS GS modules. Gapillary or hollow-fiber modules are only used in small-scale laboratory applications. The spiral-wound module and the envelope module are based on flat-sheet membranes. Spiral-wound modules are compact and cheaper in comparison to installed membrane area, but there are limitations in mass transfer on both sides of the membrane. The packing density - the ratio of installed membrane area over pressure vessel housing volume - of a spiral-wound module varies from approx. 300 to 1000 m /m (Fig. 1.3). 

Figure 2.25 A typical RO skid showing a 4 2 two-stage membrane array, high-pressure pump, instruments and control panels. The high-pressure RO pump is a horizontal, multi-staged submersible type. Each pressure vessel contains six spiral-wound modules, 20 cm dia. x 100 cm long. Source USFilter.

Figure VUI - 7. Schematic drawing of a pressure vessel containing three spiral-wound modules arranged in series.

Recovery through individual membrane modules changes, based on the position of the module in the pressure vessel. Most spiral wound membrane modules operate with individual module recoveries ranging from 10% to 15%, with an average of 11% to achieve 50% recovery in a single, 6-module pressure vessel stage. The module at the feed end of the pressure vessel typically exhibits the lowest recovery of all modules in the pressure... 

In the above examples, the fractional water recovery is around 0.15. If one increases APy the fractional water recovery will increase, and so will the energy cost via the cost of pumping. On the other hand, usually a higher APyis used with a higher feed salt concentration therefore water recovery may not increase. To increase the water recovery, a number of spiral-wound modules are connected in series (as shown in Figure 7.2.4(a)) inside the pressure vessel. There is a brine seal between the module and the pressure vessel so that the brine is forced to go through the channels of the module. The permeate tubes are connected in series. Concentrated brine from one module enters the next module as feed and so on. The fractional water recovery is ultimately limited by the difference between the concentrated feed pressure and the osmotic pressure of the concentrate and the level of acceptable flux. 

Figure 7.2.4. (a) Schematic of a numiter of spiral-wound modules in series in a module housing which is a pressure vessel, (b) Configuration for multiple modules in series or multi-tube system for reverse osmosis operation. 

Plate-and-frame as well as spiral-wound modules house flat membranes. Capillary and hollow fibers are packed in larger bundles in a pressure vessel. Commercially relevant are flat membranes and hollow fibers. However, new developments in perovskite membranes for oxygen transport are based on tubular membranes as well. ... 

Spiral-wound modules are used where pressure drop has to be considered and when counter eurrent flow is not needed to maximize separation efficiency. Higher pressure applieations involving eostly pressure vessels and piping make the hollow fiber modules more favorable because this reduces the component costs of the system by as mueh as a faetor of ten in some cases. An overview of the features and applications of the different module types is given in Table 5.2. 

Standardization Unlike the high-pressure processes that use spiral-wound modules as a standard, the low-pressure systems are not standardized. The benefit of standardization comes at membrane replacement time when alternative suppliers can be used. However, in the contained hoUow-fiber market, there is some attempt to use standard pressure vessels, and in the submerged membrane market it would be feasible (but possibly difficult) to retrofit an alternative design. 

Four to six spiral-wound membrane modules are normally connected in series inside a single pressure vessel (tube). A typical 8-in.-diameter tube containing six modules has 100-200 m2 of membrane area. An exploded view of a membrane tube containing two modules is shown in Figure 3.44 [115]. The end of each module is fitted with an anti-telescoping device (ATD) which is designed to... 

---