Pressure compression

The hydrostatic pressure varies from a maximum at the point where suspension enters the cake, to zero where liquid is expelled from the medium consequently, at any point in the cake the two are complementary. That is, the sum of the hydrostatic and compression pressures on the solids always equals the total hydrostatic pressure at the face of the cake. Thus, the compression pressure acting on the solids varies from zero at the face of the cake to a maximum at the filter medium. 

SFC (see Figure 7.6) occurs when both the critical temperature and critical pressure of the mobile phase are exceeded. (The locus of critical points is indicated in Figure 7.2 by the dashed line over the top of the two-phase region. It is also visible or partly visible in Figures 7.3-7.8). Compressibility, pressure tunability, and diffusion rates are higher in SFC than in SubFC and EFLC, and are much higher than in LC. 

The nominal compression ratio (which is usually specified) is the displacement volume plus the clearance volume divided by the clearance volume. Because of the mechanics of intake value closing, the actual compression ratio r is less than the nominal. Thus, the compression pressure p (psia) may be estimated by... 

Condensation occurs in all compressors, and the effects are most prominent where cooling takes place - in intercoolers and air-receivers, which therefore have to be drained at frequent intervals. Normally the amount of moisture present in a compression chamber is not sufficient to affect lubrication, but relatively large quantities can have a serious effect on the lubrication of a compressor. Very wet conditions are likely to occur when the atmosphere is excessively humid, compression pressures are high, or the compressor is being overcooled. 

Fig. 18 Effect of compression pressure on dissolution rate. See the text for an explanation. (From Ref. 17.).

Sound is a small-amplitude compression pressure wave, and the speed of sound is the velocity at which this wave will travel through a medium. An expression for the speed of sound can be derived as follows. With reference... 

Consider the case of pure methanol for which the values of Cp and a are known. Using a specific volume of 0.00127 m3/kg, a temperature of 25°C (298 K), and a compression pressure of 1000 bar, Equation 13.2 predicts the eluent temperature will increase approximately 15°C assuming adiabatic conditions. In actual practice, the increase in eluent temperature entering the column will be lower than this upper limit due to thermal losses in the pump, connecting tubing, and injection system, as well as entropic changes (AS A 0). 

The thickness of the DL material without any compression pressure can be measured by using a micrometer. However, depending on the material, the thickness can change substantially when there is compression. Thus, another method is to exert a specific compression pressure (which must be reported) on the sample while measuring the thickness [9]. In addition, the thickness measurements should be performed at various points over the sample and at multiple times, in order to be as precise as possible. Chang et al. [183] designed a test stand that can measure the thickness of the sample material with a thickness gage at different compression pressures. 

As sfafed previously, fhe capillary pressure dafa, fhe overall pore disfribu-tion (when ocfane is fhe working fluid), and fhe hydrophilic pore distribution (when water is the working fluid) can be obfained through this technique. In addition, these measurements can be used with different compression pressures of fhe sample DL and wifh a wude range of temperatures inside the system [200,201]. For more information regarding this technique, please refer to fhe paper by Volfkovich ef al. [198]. 

Fairweather et al. [204] developed a microfluidic device and method to measure the capillary pressure as a function of fhe liquid water saturation for porous media wifh heferogeneous wetting properties during liquid and gas intrusions. In addition to being able to produce plots of capillary pressure as a function of liquid wafer safuration, their technique also allowed them to investigate both hydrophilic and hydrophobic pore volumes. This method is still in its early stages because the compression pressure and the temperatures were not controlled however, it can become a potential characterization technique that would permit further understanding of mass transport within the DL. 

Gostick et al. [212] designed an apparatus in which the in-plane permeability was measured as a function of the DL thickness with different compression pressures (see Figure 4.25). The DL specimen was compressed between plates, which had spacers of different thicknesses in order to control the total thickness in each test. The sample was located between two grooves or channels, one of which corresponded to the inlet of the air and the other to the air outlet. Therefore, the air had to flow in plane through the sample in order to... 

For each thickness, at least 10 different flow rate measurements were obtained in order to cover the range of flow rates that a DL experiences during normal fuel cell operation. To obfain fhe corresponding permeabilify, fhe pressure drop resulfs were ploffed as a function of the mass flow rate. After this, the Forchheimer equation was fitted to the plotted data to determine the viscous and inertial permeabilities. As expected, the in-plane permeabilities of each sample DL maferial decreased when the compression pressure was increased. It is also important to mention that these tests were performed in two perpendicular directions for each sample in order to determine whether any anisotropy existed due to fiber orienfation. 

Feser et al. [214] used a radial flow apparatus to determine the viscous in-plane permeability of differenf DLs af various levels of compression (see Figure 4.26). A stack of round-shaped samples, wifh each layer of material separated with a brass shim, was placed inside two plates. Thicker shim stock was also used in order to control the total thickness of the stack of samples. Compressed air entered fhe apparafus fhrough the upper plate and was forced through the samples in the in-plane direction. After this, the air left the system and flowed through a pressure gage and a rotameter in order to measure the pressure drop and the air flow rate. The whole apparatus was compressed using a hydraulic press for each compression pressure, 10 different flow rates were used. 

In order to determine the viscous and inert through-plane gas permeabilities of diffusion layers at varied compression pressures, Gostick et al. [212] designed a simple method in which a circular specimen was sandwiched between two plates that have orifices in the middle, aligned with the location of the material. Pressurized air entered the upper plate, flowed through the DL, and exited the lower plate. The pressure drop between the inlet and the outlet was recorded for at least ten different flow rates for each sample. The inert and viscous permeabilities were then determined by fitting the Forchheimer equation to the pressure drop versus flow rate data as explained earlier. 

Through assumptions and the use of values for known resistances of the materials used in the apparatus, the actual bulk resistance of the DL material could be calculated. This resistance was then used so that the electrical conductivity could be solved. Nitta and colleagues noted that the in-plane conductivities of the DL materials were a linear fxmction of the compressed thickness (i.e., the conductivity increased when the thickness decreased with increased compression pressure). This resulted from a decrease in thickness that led to a loss of porosity in the DL materials and higher contact between fibers. 

The most typical way of measuring this parameter is to place a sample material between two plates (compressed at a defined pressure) and apply a direct current through the DL. The voltage drop between the plates is measured to determine the resistance [9,100,247]. Through this system, the resistance can be measured at different compression pressures. To minimize the contact resistance between... 

As mentioned by Mathias et al. [9], reliable methods to measure the thermal conductivity of diffusion layers as a function of compression pressures are very scarce in the open literature. Khandelwal and Mench [112] designed an ex situ method to measure accurately the thermal conductivities of different components used in a fuel cell. In their apparatus, the sample materials were placed between two cylindrical rods made out of aluminum bronze (see Figure 4.28). Three thermocouples were located equidistantly in each of the upper and lower cylinders to monitor the temperatures along these components. Two plates located at each end compressed both cylinders together. The temperatures of each plate were maintained by flowing coolant fluids at a high flow rate through channels located inside each of the plates. A load cell was located between two plates at one end so that the compression pressure could be measured. 

These tests were performed on materials with the same characteristics but with different thicknesses thus, the intrinsic thermal conductivity could be resolved at different temperatures and compression pressures. Through these tests, the thermal conductivity of TGP-H carbon fiber papers was measured and achieved the same value as that reported by the manufacturer. In addition, it was observed that the thermal conductivity of the CFPs decreased from 1.80 + 0.27 W m i K i (af 26°C) to 1.24 + 0.19 W m-i K i (at 73°C). This result was suggested to be due to the presence of carbonized thermosetting resin on the CFPs. The thermal conductivity of fhe resin, which is a thermosetting polymer and acts as a binder, decreases with increasing temperature. For carbon cloth (without any resin), no significant changes in thermal conductivity were noted when the temperature was increased. 

The membrane and catalyst layers in a fuel cell are thin and delicate components that require mechanical support in order to prevent rupture or substantial bending when a compression pressure is applied to the whole cell. Therefore, the diffusion layers must provide the necessary mechanical support to those components without affecting the other parameters discussed previously. 

The compressive behavior of a DL is a very important mechanical property. Therefore, to study the mechanical properties of various diffusion materials (carbon cloths, carbon fiber papers, and carbon felts), Escribano et al. [251] used a compression cell. The sample diffusion materials were placed between the two plates of the cell, and the thickness and deflection of each sample were measured as a function of the compression pressure. These researchers... 

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). 

FIG. 4. The effect of compression pressure on the axial and radial disintegrating pressures of compacts made with AcDiSol (5%) and (A) DiTab or (B) lactose. (Data from Ref 27.)... 

Compression Pressure (MPa) n Axial Pressure M, Radial Pressure... 

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