Direct membrane module

Spiral-wound modules were used in a number of early artificial kidney designs, but were fully developed for industrial membrane separations by Gulf General Atomic (a predecessor of Fluid Systems, Inc.). This work, directed at reverse osmosis membrane modules, was carried out under the sponsorship of the Office of Saline Water [112-114], The design shown in Figure 3.42 is the simplest, consisting of a membrane envelope of spacers and membrane wound around a perforated central collection tube the module is placed inside a tubular pressure vessel. Feed passes axially down the module across the membrane envelope. A portion of the feed permeates into the membrane envelope, where it spirals towards the center and exits through the collection tube. 

Figure 4.19(a) shows the concentration of water vapor on the feed and permeate sides of the membrane module in the case of a simple counter-flow module. On the high-pressure side of the module, the water vapor concentration in the feed gas drops from 1000 ppm to about 310 ppm halfway through the module and to 100 ppm at the residue end. The graph directly below the module drawing shows the theoretical maximum concentration of water vapor on the permeate side of the membrane. This maximum is determined by the feed-to-permeate pressure ratio of 20 as described in the footnote to page 186. The actual calculated permeate-side concentration is also shown. The difference between these two lines is a measure of the driving force for water vapor transport across the membrane. At the feed end of the module, this difference is about 1000 ppm, but at the permeate end the difference is only about 100 ppm. 

Another new trend is called membrane distillation. This is based on open hydrophobic membranes that enable the passage of water vapor only. The product quality is expected to be better than RO since only water vapor may pass through the membrane. Vapor condensation is allowed on colder surfaces adjacent to the membranes or outside the membrane module, where vapors are pumped out. Another way is to condense the vapor in direct contact with a cold-water stream. The main problem using this technique is the need to evaporate the water. The energy demand for this is around 650kWh/m3. This enormous amount of energy may be reduced when energy reuse is possible, in a similar way to the multieffect distillation... 

In the submerged MBR the driving force is achieved by pressurizing the bioreactor or creating negative pressure on the permeate side. A diffuser is usually placed directly beneath the membrane module to facilitate scouring on the filtration surface. Aeration and mixing are also achieved by the same unit. 

One of the major advantages of membrane chromatography, similarly to EBA, is the possibility for direct processing cell suspensions, enabling an integration of the clarification and primary purification steps. However, this is only possible by the use of membrane modules with suitable hydrodynamics, capable of effectively avoiding membrane fouling (Bel-ford, 1988 Castilho and Anspach, 2003). 

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). 

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. 

Membrane modules should always be stored in a cool, dark place out of direct sunlight and kept from freezing. Wet-tested membranes should be stored at no lower than about 5°C to prevent freezing of the sodium metabisulfite preservative solution (FilmTec membranes can go to -4°C).6 Dry membranes will not be affected by freezing temperatures. (Note that once wetted, membranes should not be allowed to dry out, as irreversible loss of flux may occur.)... 

There are also techniques involving the use of nonporous, solid or liquid membranes that separate the donor phase from the receiving phase by an evident phase boundary. Most often used are three-phase systems (donor phase, membrane, and acceptor phase) or two-phase systems, in which one of the surrounding phases is the same as the membrane. Solid membranes are made of chemically resistant, hydrophobic polymers (PTFE, PVDF, PS, PP, silicates), metals (Pd alloys), or ceramic materials. Channels of membrane modules have a volume ranging from 10 to 1000 pL and, according to their geometry, can be classified as planar or fibrous. For setting up a membrane system, two modes can be used the membrane can be immersed in a sample (membrane in sample, MIS) or the sample can be introduced into a membrane (sample in membrane, SIM). In both systems, only a small amount of sample is in direct contact with membrane, because ratio of the membrane surface area to the sample volume is small. 

Membrane technology is also offered by other licensors an example is the Polysep Membrane System of UOP [970], In addition to the systems based on hollow fibers, membrane modules have been developed in which the membrane is in the form of a sheet wrapped around a perforated center tube using spacers to separate the layers. The raw gas flows in axial direction in the high pressure spacer and the permeate is withdrawn in the low pressure spacer. Such a module, for example, is marketed under the name Separex [971], [972],... 

Wang KL and Cussler EL, Baffled membrane modules made with hollow fiber fabric. Journal of Membrane Science 1993, 85, 265-278. Prasad R, Runkle CJ, and Shuey HF, Method of making spiral-wound hollow fiber membrane fabric cartridges and modules having flow-directing baffles, US Patent 5,264,171, 1993. 

Fig. 22 shows a reaction scheme enhanced by continuously removing water directly from the reactor. In this case, the water is removed from the vapor phase. A vapor stream is sparged from the reactor and circulated through a vapor permeation membrane module, where water is selectively permeated through the membranes. The membrane unit is sized, such that all the reaction water can be removed with the water/ alcohol ratio just below the azeotropic composition. 

Way, Noble and Bateman (49) review the historical development of immobilized liquid membranes and propose a number of structural and chemical guidelines for the selection of support materials. Structural factors to be considered include membrane geometry (to maximize surface area per unit volume), membrane thickness (<100 pm), porosity (>50 volume Z), mean pore size (<0.1)jm), pore size distribution (narrow) and tortuosity. The amount of liquid membrane phase available for transport In a membrane module Is proportional to membrane porosity, thickness and geometry. The length of the diffusion path, and therefore membrane productivity, is directly related to membrane thickness and tortuosity. The maximum operating pressure Is directly related to the minimum pore size and the ability of the liquid phase to wet the polymeric support material. Chemically the support must be Inert to all of the liquids which It encounters. Of course, final support selection also depends on the physical state of the mixture to be separated (liquid or gas), the chemical nature of the components to be separated (inert, ionic, polar, dispersive, etc.) as well as the operating conditions of the separation process (temperature and pressure). The discussions in this chapter by Way, Noble and Bateman should be applicable the development of immobilized or supported gas membranes (50). 

Membrane research and development started in Du Pont in 1962 and culminated in the introduction of the first B-9 Permasep permeator for desalination of brackish water by reverse osmosis (RO) in 1969. The membrane in this B-9 Permasep module consisted of aramid hollow fibers. In 1969, proponents of RO technology had ambitious dreams and hopes. Today, RO is a major desalination process used worldwide to provide potable water from brackish and seawater feeds. Du Font s membrane modules for RO are sold under the trademark Permasep permeators. The RO business is a virtually autonomous profit center that resides in the Polymer Products Department. The growth and success of the Permasep products business is a direct result of Du Font s sustained research and development commitment to polyamides, a commitment that dates back to the 1930 s and the classic polymer researches of Wallace H. Carothers. Since 1969, improved and new Permasep permeators have been introduced six times, as shown in Table I. 

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