Pervaporation plates modules

Hollow fibers or capillary modules have not yet found an industrial application in pervaporation or vapor-permeation processes. A few data have been reported where organic capillary structures with an outside diameter of 0.5 to 1 mm have been coated with silicon and used in organophilic separation. With the flow on the shell side permeate pressure losses inside the bore of the fiber control the process. For specific organophilic applications, these pressure losses may be tolerable. For hydrophilic processes, however, the useful length of a module would be of the order of 20 to 30 cm only, even at an inner diameter of the capillary of 1 mm. Such a module, including housing and connection in any industrial application, is more costly than a plate module. So far no potting material is available that combines the necessary chemical and mechanical stability at the operation temperature and pressure of a dehydration plant. 

Yeom, C. K., Kazi, M., and Baig, F. (2002). Simulation and process design of pervaporation plate-and- rame modules for dehydration of organics solvents. Membr. J. 12, 226-239. 

Pervaporation operates under constraints similar to low pressure gas-separation. Pressure drops on the permeate side of the membrane must be small, and many prevaporation membrane materials are mbbery. For this reason, spiral-wound modules and plate-and-frame systems ate both in use. 

Pervaporation Most pervaporation systems are small so plate-and-frame systems were used in the first systems. Spiral-wound and capillary modules are being introduced. 

Modules Every module design used in other membrane operations has been tried in pervaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is very low (0.1-1 Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for heat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construction was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capillary bundles. The capillary device with the feed on the inside of the tube has many advantages in principle, such as good vapor-side mass transfer and economical construction, but it is still limited by the availability of membrane in capillary form. 

Plate-and-frame units have been developed for some small-scale applications, but these units are expensive compared to the alternatives, and leaks through the gaskets required for each plate are a serious problem. Plate-and-frame modules are now only used in electrodialysis and pervaporation systems and in a limited number of reverse osmosis and ultrafiltration applications with highly fouling feeds. An example of one of these reverse osmosis units is shown in Figure 3.39 [111],... 

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

Figure 9.11 Photograph of a 50-m2 GFT plate-and-frame module and an ethanol dehydration system fitted with this type of module. The module is contained in the large vacuum chamber on the left-hand side of the pervaporation system [44]...

Plate-and-frame modules were the earliest version of membranes modules and are today still used in ultra-filtration and pervaporation processes. There is only one plate-and-frame configuration used in solution-diffusion membranes. This design may be relevant in the future for flat perovskite membranes. " " The most important elements of the module construction are "... 

The plate-and-frame module is highly effective in pervaporation applications, but is much less popular than spiral wound and hollow fiber modules for gas... 

Pervaporation membranes can be made in flat sheet (Fig. 17.4) or hollow fiber (Figs. 17.5 and 17.6). So far, flat-sheet membranes in plate-and-firame type of modules were the only type of membranes available for very few applications. There has been significant failure in spiral-wound modules and hollow-fiber modules due to membrane failure as well as lack of compatibility of epoxy and glue. Even though flat-sheet pervaporation membranes showed good performance in ethanol applicalions, there have been numerous failures in other applications. 

Recent developments have changed the future of pervaporation. For the first time in the history of pervaporation, AZEO SEP pervaporation solvent-resistant membranes are now available in hoUow-fiber configuration as well as flat sheets in plate-and-firame modules. Significant work has been done on the solvent-resistant epoxy and module design to use hollow-fiber pervaporation membrane configuration. 

The third type of membrane module is the plate-and-frame assembly, which closely parallels that used in filtration. Such a module has a high capital cost but is resistant to fouling. Moreover, if the membrane in one of the plates fails, it can be individually replaced the entire module does not have to be discarded. This geometry is often used for ultrafiltration and has been used for pervaporation. I would expect its use for physical separations to be sustained but its use for diffusion-based separations to wane. 

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