Pressure vessel membrane modules

Figure 8.12 Block diagram and photograph of a contained in the horizontal pressure vessels, membrane fuel-gas conditioning unit (FGCU) The unit produces 0.5-1.0 MMscfd of clean gas. used for a field gas compressor engine (the unit Reproduced with permission from Ind. Eng. uses silicone rubber membranes in spiral-wound Chem. Res. 2008, 47(7), 2109-2121. Copyright modules). The membrane modules are 2008 American Chemical Society [17].

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. 

Hollow fiber modules, or permeators, are precisely machined units containing bundles of fine hollow fibers, positioned parallel to and around a perforated center FW tube, with only one or two bundles per pressure vessel. They are widely used for brackish and seawater supply applications. Hollow fiber modules exhibit a low flux rate (permeate flow per unit membrane per unit time) and foul easily, but... 

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... 

Hollow fine fiber modules made from cellulose triacetate or aromatic polyamides were produced in the past for seawater desalination. These modules incorporated the membrane around a central tube, and feed solution flowed rapidly outward to the shell. Because the fibers were extremely tightly packed inside the pressure vessel, flow of the feed solution was quite slow. As much as 40-50 % of the feed could be removed as permeate in a single pass through the module. However, the low flow and many constrictions meant that extremely good pretreatment of the feed solution was required to prevent membrane fouling from scale or particulates. A schematic illustration of such a hollow fiber module is shown in Figure 3.47. 

Figure 5.24 Flow schematic of a typical brackish water reverse osmosis plant. The plant contains seven pressure vessels each containing six membrane modules. The pressure vessels are in a Christmas tree array to maintain a high feed velocity through the modules...

A comprehensive presentation of all membrane types, modules and geometries is beyond the scope of this chapter, reference available membrane books for details [12,17, 55, 60, 71, 77,90]. The examples in Figure 16.2 are an illustration of a typical membrane module and installation. The most widespread FS membrane system is mounted as a spiral-wound (SW) unit. In the SW example the actual membrane module is shown together with how they are mounted inside a pressure vessel. A typical installation is shown where several pressure vessels are subsequently mounted in a stack. Pressurized HF units are typically operated as a crossflow system. In the example shown the HF modules are mounted vertically and arranged in a skid. Several variations of the theme can be found depending on the type of module and the manufacturer, where Figure 16.2 is not specific to a particular item. 

Focusing on spiral wound membrane modules as the most common type of membrane modules used in industry today, an RO array or "skid" or "train" consists of a number of pressure vessels arranged in specific patterns. Figure 5.1 shows an array of 3 pressure vessels. The pressure vessels are arranged into 2 sets, with 2 pressure vessels in parallel followed by 1 single pressure vessel. The 2 sets of pressure vessels are in series. Each set of pressure vessels in parallel (even if there is only 1 vessel) is called a STAGE. 

An RO skid includes the pressure vessels in which the membrane modules are contained (see Chapters 43.3 and 6.3 for detailed discussions about pressure vessels). Skids also commonly include cartridge filters in a housing or housings and an RO feed pump, although combinations exist with just pressure vessels or pressure vessels with cartridge filters. Finally, there are included on the skid instrumentation and controls for the system. Figure 6.1 shows an RO skid with these components. 

Pumps should be started slowly to prevent water hammer (a surge resulting from a sudden change in liquid velocity). Water hammer can cause cracks in the outer shell of the membrane modules as well as compaction of the membrane itself (compaction results in lower flux through the membrane at constant pressure). Also, water hammer causes the membrane modules to move in the vessel, which can cause wear to the O-rings used on standard interconnectors and lead to leaks of feed water into the permeate (see Chapter 4.3.3). An increase in pressure of no more than 10 psi per second is recommended.3 Some motors may be equipped with a "soft start" that regulates the speed with which they start up. Other considerations to minimize water hammer include ... 

A pressure vessel is the pressure housing for the membrane modules and contains the pressurized feed water. Various pressure ratings are available depending on the application ... 

Pressure vessels are made to specifically to accommodate whatever diameter of membrane module being used, be it a 2.5-inch diameter tap water membrane module up to 18-inch diameter industrial membrane module. The length of the pressure vessel can be as short a one membrane module in length up to seven membrane modules in series (see Figure 6.8). 

Proper installation of membrane modules into a pressure vessel is critical. The membrane modules are guided into the pressure vessel in series. Membranes should be loaded into pressure vessel in the direction of flow. That is, the concentrate end of the module (the end without the brine seal) is inserted first into the pressure vessel. The brine seal and O-rings on the module inter-connectors can be... 

Modules should also be removed from pressure vessel in the same direction as the flow. Hence, the first module into the vessel, which is the last one in the series, is the first module out. Figures 6.10 a, b, and c show the removal sequence for a FilmTec iLEC membrane module (see Chapter 4.3.3). 

Thrust rings/cones and shims are used in conjunction with pressure vessel end caps to minimize longitudinal movement of membrane modules within the pressure vessel. Movement of the membrane modules can cause the O-rings to wear as well as cause telescoping of the membranes and spacers during pressurization. Thrust rings/cones also serve to distribute the axial pressure load to the full end cap. 

Figure 6.10 Sequence of module removal for Dow Water and Process Solutions-FilmTec iLEC membrane modules, a) module removal device, b) pulling module out of pressure vessel, c) disconnecting 2 modules. Courtesy of Nalco-Crossbow Water UC.

Shims Shims are used to prevent modules from moving back and forth during pressurization and depressurization. Such movement could wear on the internal O-ring seals. Shims are plastic spacer rings similar to washers. They are typically 0.20-inches thick, and can be purchased from the manufacturer of the pressure vessel or fashioned from polyvinylchloride (PVC) pipe. Shims fashioned from PVC pipe must be cut parallel and free of burrs to work correctly. They are installed between the face of the lead membrane module and the adapter hub (see Figure 6.16) after all the... 

Figure 9.12 Individual membrane module recovery and rejection as a function of position in a 2-stage RO system with 6 modules per pressure vessel.

The system array is determined at this point. The designer enters the number of pressure vessel per stage and the number of membrane elements or modules per pressure vessel. The program then calculates the number of elements in the stage. 

Figure 11.5 Axial pressure load on 8-inch diameter membrane modules operating at 200 psig. Assumes six, 8-inch modules per pressure vessel.

Figure 11.6 Comparison of axial pressure load on end module with recommend 3-4 psig pressure drop per module and a maximum recommended pressure drop of 8.3 psig per membrane module. Assume six, 8-inch diameter modules per pressure vessel.

Recirculate the solution. The solution should be recirculated at about 35 gpm per pressure vessel. In other words, for an 8-pressure vessel stage with 8-inch diameter membrane modules, the recirculation rate should be 8 times 35 gpm or 280 gpm. Recirculation should be conducted using as little pressure as possible, thereby minimizing the formation of permeate. If permeate is generated, it increases the likelihood of re-deposition of removed species on the membrane.1 If the cleaning solution comes out dark, it should be discarded and a new batch should be prepared. Temperature and pH should be monitored and adjusted during the recirculation as needed. Recirculation should continue for about 45 to 60 minutes. 

If the normalized salt rejection is low or the normalized permeate flow is high, the integrity of the membrane may be in question. The vacuum decay test is a direct test for the integrity of a spiral wound RO membrane module. The test is best used to identify leaks within the membrane modules rather than leaks due to chemical attack. The test requires the isolation of an individual membrane module or the entire pressure vessel. A vacuum is then pulled on the membrane(s) and the rate of decay in pressure is observed. A decay of greater than 100 millibar per minute is indicative of a leaky membrane. Refer to ASTM Standards D39235 and D69086 for a more detailed review of the technique. 

Off-site membrane cleaning involves removing membrane modules from the pressure vessels and shipping them off site for cleaning... 

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