Hydrogen permeability improvement

The improved process reduced the cycle time from 3 min to less than 10 sec and showed excellent capability and controllability with the value of Cp] at 2.33. The ex situ testing of the plate exhibited an encouraging performance, which included high electrical conductivity (600 800 S/cm), high flexural strength (600 - 7,000 psi), and low hydrogen permeability (smaller than 2 x 10 cm /sec)—all of which exceeded or reached DoE 2010 fargefs (see Table 5.1)... 

Pure Pd membranes become brittle upon thermal cycling in a H2 atmosphere due to a phase transition between the different Pd hydride phases (a and P) with distinctly different crystal lattice parameters (see Fig. 11.3). Allojting with various other metals (for example Ag, Ru, Rh) tends to lower the phase transition temperature and under some circumstances improves the hydrogen permeability. The phase transition problem is not technically insurmountable, if good care is exercised during start-up (when the membrane is first exposed to hydrogen)... 

Krueger CW. Method of improving and optimizing the hydrogen permeability of a palladium-copper membrane and novel membranes manufactured thereby. US Patent 6372363, 2002. 

Figure 28 shows the relationships between the amount of inorganics in the clay and the gas permeability coefficient. The gas permeabiUty coefficient decreased as the amount of added clay increased. The gas barrier performance of PPCN-5 increased by 1.7 times. It has been reported that the barrier performance of the nylon-clay nanocomposites and polymer-clay nanocomposites was improved. This barrier effect is explained as being attributed to the geometrical detour effect of the dispersed nanosized silicates. The barrier effect of PPCN, however, was smaller than that of the nylon-clay nanocomposites. hi the case of the nylon-clay hybrid, the addition of 1.8 wt % of mont-morillonite caused its hydrogen permeability to decrease to 70%. In the case of PPCN, about 3 wt % of montmorillonite must be added to obtain the same... 

Table 2.2 Improvement in hydrogen permeability of various binary and tertiary palladium alloys at 350°C [57]...

Moreover, the palladium alloys improve chemical resistance of the membranes. For example, Pd-Cu and Pd-Au increase the resistance to H2S [59] as well as palladium-coated amorphous Zr-M-Ni (M = Ti, Hf) alloy membranes are resistant enough in a hydrogen atmosphere and have stable hydrogen permeability in the range of 200-300" C [60]. 

As shown in the previous section, the conventional ammonia production plant requires several gas separation units. The attempt to minimise the use of those units contributes to the idea of catalytic membrane reactor. The catalytic membrane is a combination of catalyst bed and gas separation membrane. The membrane reactor improves the ammonia formation rate by product removal or the control of reactant concentration. However, only a few catalytic membrane reactors for ammonia production are reported. Itoh, Machida, and Adachi (2000) and Itoh, Saito, Tajima, and Machida (2007) reported ammonia production in a catalytic membrane reactor consisting of Ru catalysts loaded on a Ag-Pd hydrogen permeable membrane. It was found that the presence of high-reactivity atomic hydrogen from the Ag—Pd membrane enhances the ammonia formation rates. 

The introduction of other metals to form palladium based alloys has had promising results. In particular doping of the palladium with silver has been shown to improve the stability of the film and increase the solubility of hydrogen. Further, the temperature above which the a palladium hydride occurred was lowered with increasing silver content (Uemiya et al.,1991 Kikuchi Uemiya, 1991). The hydrogen permeability was optimized when the silver content of the alloy was aroimd 23 wt%. Silver occupies interstitial sites in the palladium lattice and so moderates the lattice expansion and contraction due to hydrogen absorption/desorption. 

Attention has therefore been drawn to solid oxides, though not as efficient as metallic membranes but significantly cheaper [73]. Until now, no solid oxide has shown high enough hydrogen permeability to allow for commercial utilization and improved membrane materials are thus needed [74]. 

TABLE 25.1 Improvement in Hydrogen Permeability of Varioirs Binary Pd Alloys at 350°C... 

As discussed before, several metals other than Pd exhibit excellent characteristics in terms of hydrogen permeability. Alloys are commonly used to improve the characteristics of pure metals. Some of the advantages of a metal alloy compared to pure metal are the following ... 

The double bond present in the diene part of the elastomer is generally more susceptible to thermal and oxidative degradation. The selective hydrogenation of olefmic unsaturation in NBR imparts significant improvements in resistance to degradation and other properties, such as permeability, resistance to ozone and chemicals, and property retention at high temperature. 

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