Resistance compression permeability cell

The constants a0, k, and n are determined most simply in compression-permeability cells as explained in Section 11.4, but those found from filtration data may be more appropriate because the mode of formation of a cake also affects its resistivity. Equations (11.14) and (11.2) together become... 

The compressibility of cakes, i.e. the increasing resistance of cakes with pressure, can be tested in various ways. As was briefly mentioned in section 9.2.2, one way of testing the relationship a =/(Apc) is in the compression-permeability cell. The solids compression is created by the mechanical action of the piston—an obvious assumption made is that hydraulic pressime can be simulated by mechanical compression. 

Figure 11.4. Data of compressibilities and porosities of filter cakes, (a) Parameters of the correlation a = a0(hP)n for resistivity of CaSi03 filter cakes at two rates and two concentrations (Rushton and Katsoulas, 1984). (b) Resistivity as a function of pressure measured in a compressibility-permeability (CP) cell [Grace, Chem. Eng. Prog. 49, 303, 367, 427 (1953)]. (c) Porosity as a function of pressure for the same six materials (Grace, loc. tit.).

The compression force distribution can affect the GDL durability significantly. Severe compression may result in substrate fibers puncture into the membrane, and light compression may result in high contact resistance between the cell components and poor sealing effect. The optimized compression is a compromise between durability and performance. The compression force also changes the GDL properties under the land (e.g., porosity, permeability, and effective thermal and electron conductivities). The... 

It is important to operate the fuel cell at different compression pressures in order to determine the correct compression pressure for a DL material. If the applied compression pressures are too high, the DLs may deform, both the porosity and permeability of the DL decrease, and the probability of failure modes increases significantly. On the other hand, if the pressures are too low, then gas leaks and serious contact resistance between the components of the cell may be present. Various studies have been presented in which the compression pressure of the fuel cell is varied in order to observe how the cell s performance is affected [25,183,252]. In general, there is an optimal compression pressure range in which the cell s performance is the highest however, this depends on the DL material and on the MPL thickness (see Figure 4.21). 

Such equipment consists of a hollow cylinder fitted with a permeable bottom and a permeable piston under controlled pressure. Slurry is charged to the CP cell, cake is formed with gentle suction, and the piston is lowered to the cake level. The rate of flow of filtrate at low head through the compressed cake is measured at a series of pressures on the piston. From the results the resistivity of the cake becomes known as a function of pressure. The data of Figures 11.4(b) and (c) were obtained this way those of Figure 11.4(a) by filtration tests. 

It is beyond the scope of this entry to review the basic principles governing filtration. However, it is interesting to note that filtration produces a more concentrated and dewatered cell sludge (20-35% w/v) or cell solids (>40% w/v) than settling. A variety of filter media, membranes, and equipment are commercially available. In the case where the deposited cake is compressible with low permeability and thereby adds more resistance to filtration, filter aids or precoats often alleviate the problem. Two of the most widely used filter aids are diatomaceous earth and perlite. 

---