Membrane flat

Hat membranes are used in plate-and-frame and spiral-wound systems whereas tubular membranes are used in hollow fiber, capillary and tubular systems. These module designs are described in greater detail in chapter Vni. The same flat membranes can be used for both flat membrane configurations (plate-and-frame and spiral wound). The preparation of flat membranes on a semi-technical or technical scale is shown schematically in figure in -5. 

Other preparation parameters are polymer concentration, evaporation time, humidity, temperature, and the composition of the casting solution (e.g. additives). These parameters are mainly determining the ultimate membrane perfonnance (flux and selectivity) and hence for its application. The relation between these parameters and membrane structure will be described in greater detail in section IH - 6. The membranes obtained after precipitation can be used directly or a post treatment (e.g. heat treatment) can be applied. 

Free flat membranes can be obtained by casting the po mer solution upon a metal or polymer bell. After coagulation (and thorough washing ) the free flat-sheet can be collected. 

Since flat membranes are relatively simple to prepare, they are very useful for testing on a laboratory scale. For very small membrane surface areas (less than 1000 cm ), the membranes are cast mostly by hand or semi-automadcally, not on a non-woven polyester but often on a glass plate (other materials can also be used, e. g. metals, and polymers such as polytetrafluoroethylene, polymethylmethacrylate etc.). The same procedure is followed as that depicted in figure HI - 5. 

Membrane morphology characterization ias carried out using several microscopy techniques (SEM, CSLM, AFM, etc.). Two case studies are presented and discussed next. 

---

Spiral Wound. A spiral-wound cartridge has two flat membrane sheets (skin side out) separated by a flexible, porous permeate drainage material. The membrane sandwich is adhesively sealed on three sides. The fourth side of one or more sandwiches is separately sealed to a porous or perforated permeate withdrawal tube. An open-mesh spacer is placed on top of the membrane, and both the mesh and the membrane are wrapped spirally around the tube (Fig. 16). 

The earhest reverse osmosis and ultrafiltration units were based on flat membrane sheets ia arrangements similar to that of a plate and frame filter press. Siace then, mote efficient membrane configurations, ie, tubular, spiral wound, and hoUow fiber, have emerged (96—98). 

Spiral-wound modules consist of several flat membranes separated by turbulence-promoting mesh separators and formed into a Swiss roll (Figure 16.18). The edges of the membranes are sealed to each other and to a central perforated tube. This produces a cylindrical module which can be installed within a pressure tube. The process feed enters at one end of the pressure tube and encounters a number of narrow, parallel feed channels formed between adjacent sheets of membrane. Permeate spirals roward the perforated central tube for collection. A standard size spiral-wound module has a diameter of about 0.1m, a length of about 0.9 m and contains about 5 m2 of membrane area. Up to six such modules may be installed in series in a single pressure tube. These modules make better use of space than tubular or flat sheet types, but they are rather prone to fouling and difficult to clean. 

Whereas the liquid-solid filtration processes described so far can separate particles down to a size of around 10 xm, for smaller particles that need to be separated, a porous polymer membrane can be used. This process, known as microfiltration, retains particles down to a size of around 0.05. im. A pressure difference across the membrane of 0.5 to 4 bar is used. The two most common practical arrangements are spiral wound and hollow fiber. In the spiral wound arrangement, flat membrane sheets separated by spacers for the flow of feed and filtrate are wound into a spiral and inserted in a pressure vessel. Hollow... 

The photosynthetic apparatus in green plants and algae is located in the chloroplast, which is a flattened, double-membraned structure about 150-200 A thick/4,5 The two flat membranes lie one above the other and are united at their peripheries. These double-membraned structures have been termed thylakoids (from the Greek sacklike )/ Each membrane of the thylakoid consists of a water-insoluble lipoprotein complex which contains the light-absorbing chlorophyll and other pigments utilized in photosynthesis. 

Step 3 Biocompatibility. The biocompatibility of selected polymers, identified in Steps 1 and 2, were evaluated by implanting flat membranes into a C57/B16 mouse (Jackson Labs, Bar Harbor, ME). The membranes and capsules were implanted at various internal sites or in the back tissue under the skin. The results of these tests are not reported herein and will be discussed in a subsequent publication. They do, however, have important implications as to the ultimate selection of a polymeric system. 

This model for the modulated state of tubules leads to an interesting speculation on the kinetic evolution of flat membranes or large spherical vesicles into tubules.68,132 In this scenario, when a flat membrane or large spherical... 

Figure 5.51 Scenario for kinetic evolution of flat membranes into tubules.68,132 (a) When membrane is cooled into tilted phase, it develops stripes in tilt direction and then breaks up along domain walls to form ribbons, (b) Each ribbon twists in solution to form helix, (c) Helical ribbon may remain stable or may grow wider to form tubule. Reprinted with permission from Ref. 139. Copyright 2001 by the American Chemical Society.

Flat membranes from these polymers were tested for desalination and found to be of low salt rejecting type. Hov/ever, the copolymer was found to possess more than 90 per cent rejection for 1 per cent dextran solution with 10.0 gfd water flux at 200 psi thus indicating the possibility of application of these membranes in ultrafiltration and hemodialysis. 

Keywords. Bioartificial liver, cell culture, hollow fiber bioreactor, flat membrane bioreactor, spheroids... 

Bader et al. [35] and De Bartolo et al. [36] developed the flat membrane bioreactor which consists of a multitude of stackable flat membrane modules as shown in Fig. 5. Each module has an oxygenating surface area of 1150 cm. Up to 50 modules can presently be run in parallel mode. Isolated hepatocytes are co-cultured with non-parenchymal cells. Liver cells are located of a distance of 20 pm of extracellular matrix from a supported polytetrafluorethylene (PTFE) film. Medium and cells in the modules are oxygenated in the incubator by molecular diffusion of air across the non-porous PTFE membrane. The design of the bioreactor is also the basis for its proven potential for cryostorage with fully differentiated adult primary human liver cells. 

The method of impregnating liquid membranes has become more and more popular. By impregnating fine-pore polymer films with a suitable membrane liquid, relatively stable heterogeneous solid-liquid membranes are obtained. These membranes are shaped as thin, flat barriers or hollow fibers. Usually they are manufactured from oleophilic polymers, wettable by membrane liquid. The two interfaces, F/M and M/R, have equal or close areas which can be made very large by employing modules of spirally wounded flat membrane or bundles of hollow fibers. 

For a flat membrane, the mass transfer fluxes through the two hquid films on the membrane surfaces and through the membrane should be equal to... 

A number of flat membranes are stacked with appropriate supporters (spacers) between the membranes, making alternate channels for the feed (retentate) and the permeate. Meshes, corrugated spacers, porous plates, grooved plates, and so on, can be used as supporters. Ihe channels for feed distribution and permeate collection are built into the device. Rectangular or square membrane sheets are common, but some modules use round membrane sheets. 

A buffer solution containing urea flows along one side of a flat membrane and the same buffer solution without urea flows along the other side of the membrane, at an equal flow rate. At different flow rates the overall mass transfer coefficients were obtained as shown in Table P8.1. When the liquid film mass transfer coefficients of both sides increase by one-third power ofthe averaged flow rate, estimate the diffusive membrane permeability. 

In the situation where the effect of filtration - that is, water movement across the membrane due to the difference in hydrostatic pressure and/or osmolarity - can be neglected, the overall resistance for mass transfer in hemodialyzers with flat membranes is given as... 

Spiral Wound. A spiral-wound cartridge has two flat membrane sheets (skin side out) separated by a flexible, porous permeate drainage material,... 

One should take into account the specific features of gas diffusion in porous solids when measuring effective diffusion coefficients in the pores of catalysts. The measurements are usually carried out with a flat membrane of the porous material. The membrane is washed on one side by one gas and on the other side by another gas, the pressure on both sides being kept... 

Several commercial flat membrane supports have been used including polyethylene [235], polyamide [230, 231], cellulose [236, 237], PVDF [229, 238], PTFE [222, 239], polyurethane [235] and porous alumina [240]. Fibres and hollow fibres of glass [241], polypropylene [233, 234] or PVDF [242] were also employed as support. 

Fig. 16. a Schematic diagram of the pressure chamber for characterisation of flat membranes by means of a pneumatic SFM. b Height image of a nuclear pore membrane of polycarbonate recorded by the pneumatic SFM at a pressure difference of 0.4 bar. The elevations are resulted from the nitrogen streams through the pores. Reproduced from [167]... 

---