Membrane element types

Selection of membrane element type (SW, BW, LPRO, etc.), element age, associated fouling factor (if not using DS2 default values), annual salt passage increases (as a%), etc. 

Cassettes Cassette is a term used to describe two different cross-flow membrane devices. The less-common design is a usually large stack of membrane separated by a spacer, with flow moving in parallel across the membrane sheets. This variant is sometimes referred to as a flat spiral, since there is some similarity in the way feed and permeate are handled. The more common cassette has long been popular in the pharmaceutical and biotechnical field. It too is a stack of flat-sheet membranes, but the membrane is usually connected so that the feed flows across the membrane elements in series to achieve higher conversion per pass. Their popularity stems from easy direct sc e-up from laboratoiy to plant-scale equipment. Their hmitation is that fluid management is inherently veiy hmited and inefficient. Both types of cassette are veiy compact and capable of automated manufacture. 

In order for membranes to be used in a commercial separation system they must be packaged in a manner that supports the membrane and facilitates handling of the two product gas streams. These packages are generally referred to as elements or bundles. The most common types of membrane elements in use today include the spiral-wound, hollow fiber, tubular, and plate and frame configurations. The systems currently being marketed for gas separation are of the spiral-wound type, such as the SEPAREX and Delsep processes, and the hollow-fiber type such as the Prism separator and the Cynara Company process. 

However, despite these obvious differences, various types of anion-selective sensor are being increasingly reported within the literature, with new aspects of development covering the transducer, receptor and membrane elements being described. Frant and Ross reported the mechanism of the first anion-... 

The membrane elements are selected here. Double clicking on the input box will open up a window containing all Toray membrane elements and specifications. The designer can select a membrane or use the "AutoSelect" button and the program will select an appropriate model. Each stage can contain a different type of membrane, if required (see Chapter 17.2.20). 

The overall membrane element shape comes in different types sheet, single tube, hollow fiber, and multi-channel monolith. Photographs of some commercial membrane elements are shown in Figure 5.1. The use of disks (or sheets) has been confined to medical, pharmaceutical and laboratory applications, while tubes and monoliths are employed in larger-scale applications ranging from removal of bacteria from wine and beer fermentation to oil-water separation to waste water ueatmenL... 

Two types of connection (and therefore sealing) are involved in assembling membrane modules. The first type connects tubular or monolithic membrane elements in bundles using header plates at the ends and, in some cases, in the middle of the module length. The second type provides sealing between the plates and the module housing. 

As mentioned earlier, one of the membrane element shapes with the highest packing density is hollow fibers. Typically several fibers are bundled to provide higher strength. In a packed-bed membrane reactor of this type, catalyst particles arc packed around the bundles. 

Horizontal versus vertical membrane tubes or modules. Two general types of fluidized-bed membrane reactors have been tested. The first type places the membrane elements or modules perpendicular to the general direction of the fluidizing reaction gases (see Figures 10.14a and 10.14b). In the second type of FBMR, the membrane elements or modules are essentially parallel to the fluid flow direction inside the reactor, as schematically shown in Figure 11.50. It appears that the vertical type exhibits more advantages for practical implementation. 

Current commercial inorganic membranes come in a limited number of shapes disk, tube and monolithic honeycomb. Compared to other shapes such as spiral-wound and hollow-fiber that are available to commercial organic membranes, these types of membrane elements have lower packing densities and, therefore, lower throughput per unit volume of membrane element or system. 

The term membrane element refers to the basic form in which a membrane is prepared. There are three types of membrane elements flat sheets, hollow hbers, and tubular membranes. The device within which the membrane element is housed is referred to as the membrane module. The design of the membrane module largely depends on the type of membrane element, as well as on additional requirements such as the need for cleaning and disassembling, the required transmembrane pressure (TMP), and the required hydrodynamic conditions. Some of the different modules types are (see Figures 18.3 through 18.7) ... 

The key to the successful use of the RO/UF membrane is in the packaging of the membrane. Until economical, easily fabricated and stable packages were developed, membranes were of little commercial value. We use the spiral wound type of membrane element for all of our equipment. Other packages such as the tube, the hollow fiber with bore flow and the hollow fiber with outside flow are all valuable and useful packages. However, the spiral package is by far the most universal in its ability to... 

FIGURE 9.2 Evolution of the geometry of ceramic membrane elements, (a) Conventional cylindrical-shaped channels, (b) Flower-like designed channels, (c) Honeycomb-type structure. 

Seals required on the membrane elements vary with the membrane type and the module geometry. Many flat-fihn designs employ thermal and ultrasonic welding techniques and thereby avoid the introduction or minimize the use of dissimilar seating materials. 

The choice of membrane and membrane element is usually determined by the composition of feed water (e.g., fouHng potential) and product water quahty. Membranes are synthesised with rejection tailored to the type of appHcation for example, in the case of SWRO membrane rejection is typically >99.6%, whereas in the case of BWRO,... 

A typical RO skid is shown in Figure 2.25. It is a single-pass, two-stage (4 2 array) unit with pressure vessels containing six spiral wound membrane elements (20 cm X 100 cm) in series in each vessel. There is room on the backside of the skid to double the number of vessels to make it into an 8 4 array with permeate flow rates approaching 70 m /h at 75%. The RO high-pressure pump is multi-stage, horizontal, submersible type. The skid... 

In general, the adsorbed coatings are relatively simple to apply and the process can be performed in commercial membrane elements. In addition, the type of coating can be tailored to the specific application of interest. However, despite the flexibility of the coating and the adsorption methods to change the hydrophilicity, smoothness, and charge of the membrane surface, their main drawback is the limited stability of the modified layer over time because of the possible desorption of the coated/ adsorbed polymers from the membrane surface into the bulk of the feed solutions. 

RO membrane module types, most utility applications use either spiral-wound or hollow-fiber elements. Hollow-fiber elements are particularly prone to fouling and, once fouled, are hard to clean. Thus, applications that employ these fibers require a great deal of pretreatment to remove all suspended and colloidal material in the feed stream. Spiral-wound modules, due to their relative resistance to fouling, have a broader range of aH)lications. A major advantage of the hollow-fiber modules, however, is the fact that they can pack 5000 ft of surface area in a 1 ft volume, while a spiral wound module can only contain 300 ff/fl . 

The construction of ISEs used in clinical measurements is of the membrane electrode type, i.e., the ion-sensitive membrane separates the sample from an internal reference electrolyte, which is the site of the internal reference element, usually a silver wire covered by silver chloride. The membrane can be shaped to different forms such as flat, convex, tubular, etc. Sodium sensitive membranes are made from special composition glass, the other ion-sensitive membranes from a polymer matrix such as plasticized polyvinylchloride (PVC) or silicon rubber. The particular selectivity of polymer membranes is first of all due to a small percentage of active material, e.g., valinomycin, dissolved in the polymer. Important secondary effects have been attributed to the type and permittivity of the polymer. The useful lifetime of the sensors also depends on the polymer. The time response [13] may again depend on membrane composition. 

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