Positive ribs

SEPARATOR STRUCTURE DESIGN AND FABRICATION 4.4.1 Positive Ribs... 

With the positive ribs facing the positive electrode, added oxidation protection is provided to the separator substrate or back-web thickness. In a typical automotive... 

A special attribute of these processes is the abiHty to pre-position reinforcement, inserts, and core materials for stiffening ribs. Gel coatings can be apphed to the mold surface to eliminate post-mold finishing. Because both surfaces of the part are formed in a mold to close tolerances, accurate assemblies are possible, which is a requirement for many automotive or tmck body appHcations. 

The filter structure consists of a stack of plates attached to a hollow shaft which are mounted inside a pressure vessel with each plate covered with a suitable filter medium. The slurry is fed under pressure into the vessel and the cake, which is retained by the filter medium, forms on the top of each plate whilst the filtrate passes through the hollow shaft further to the process. Filter sizes may vary but generally the maximum is 60 m area and designed for a 6 bar operating pressure. Each circular plate in the stack is constructed with radial ribs that are welded to the bottom and support a horizontal coarse mesh screen which is covered with a finer woven metal screen or filter cloth to retain the cake. The bottom of the plate slopes towards the hollow central shaft which lets the filtrate flow freely through circumferential holes and further down the shaft to the filtrate outlet. The clearance between the plates is maintained by special spacers with "o" rings to positively seal between the slurry that surrounds the plates and the shaft that collects the filtrate. The height of the spacers determine the clearance for cake build-up and may be replaced to meet various process conditions. 

Leaf of Eucalyptus corymbosa, Sm.—This veuatiou is indicative of the presence of pinene in the oil. Note the close parallel lateral veins, the thick mid-rib, and the position of the marainal vein close to the edge of the leaf. The yield of oil from leaves showing this venation is small, there not lieing room between the lateral veins for the formation of many oil glands. 

A mixture of powdered poly(vinyl chloride), cyclohexanone as solvent, silica, and water is extruded and rolled in a calender into a profiled separator material. The solvent is extracted by hot water, which is evaporated in an oven, and a semiflexible, microporous sheet of very high porosity ( 70 percent) is formed [19]. Further developments up to the 75 percent porosity have been reported [85,86], but these materials suffer increasingly from brittleness. The high porosity results in excellent values for acid displacement and electrical resistance. For profiles, the usual vertical or diagonal ribs on the positive side, and as an option low ribs on the negative side, are available [86],... 

Needless to say, uniform concentration distribution of an electrolyte over a membrane surface is the most important factor to maintain good membrane performance. Sufficient internal circulation results in good electrolyte mixing inside the chamber. As shown in Fig. 19.2,12 ribs installed in both anode and cathode chambers work as downcomers in the same manner as in B-l. The horizontal cross-sectional area of the downcomer in the Improved B-l has been approximately doubled. All electrolyte concentrations measured at various points over the whole electrolysis area are maintained at specification, even at 6 kA m-2 through to 8 kA m-2, as is shown in Fig. 19.3 (with the relevant data at the downcomer positions 1-4 given in Table 19.1). 

Figure 19. Liquid saturation and current density of the cathode as a function of position for the case of dry air fed at 60 °C. (a) Liquid saturation in the gas-diffusion layer where the channel goes from x = 0 to 0.05 cm and the rib is the rest the total cathode overpotential is —0.5 V. (b) Current-density distributions for different channel/rib arrangements. (Reproduced with permission from ref 56. Copyright 2001 The Electrochemical Society, Inc.)...

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