Pressure palladium membranes

A. S. Augustine, Y. H. Ma, N. K. Kazantzis, High pressure palladium membrane reactor for the high temperature wateregas shift reaction, Int. J. Hydrogen Energy 36 (2011) 5359-5360. 

Augustine, A.S., Ma, Y.H. and Kazantzis, N.K. (2011) High pressure palladium membrane reactor for the high temperature water-gas shift reaction. Internatkmal Journal of Hydrogen Energy, 36, 5350-5360. 

Palladium Diffusion. Palladium is very permeable to hydrogen but not permeable to other gases. As a result, it is a useful hydrogen purifier. A palladium membrane, heated to 400 °C, purifies hydrogen to <10 ppb but requires a high pressure differential for net diffusion to take place at reasonable rates of hydrogen supply. 

Membrane processes are based on the selective transmission characteristics of the membrane material for different molecules, whereby the most effective membranes are usually also the most expensive. For example, the purest hydrogen can be captured by palladium membranes with suitable additives, but their low permeability make it necessary to use large membrane surfaces and high pressures, which result in high costs. 

Franz et al. [93] developed a palladium membrane micro reactor for hydrogen separation based on MEMS technology, which incorporated integrated devices for heating and temperature measurement. The reactor consisted of two channels separated by the membrane, which was composed of three layers. Two of them, which were made of silicon nitride introduced by low-pressure chemical vapor deposition (0.3 pm thick) and silicon oxide by temperature treatment (0.2 pm thick), served as perforated supports for the palladium membrane. Both layers were deposited on a silicon wafer and subsequently removed from one side completely... 

Figure 9.10 Hydrogen pressure drop due to depletion, concentration polarization, surface effects, transport in the palladium membrane and porous support, compared to the total hydrogen partial pressure drop, (a) H2 N2 = 50 50 (b) H2 N2 C02 = 50 25 25 ...

US Patent 6,183,542 was issued in 2001 for a palladium membrane process. This process provides an apparatus that can handle high flow rates of gas, per unit area of membrane, while using a minimal amount of hydrogen-permeable material. This is accomplished by using stainless steel mesh elements to reinforce the thin-walled, palladium or palladium alloy membranes. This process also provides the ability to withstand large pressure gradients in opposite directions and thus will make it easier to clean membranes that have been clogged with contaminants. 

Measurement of palladium membrane permeability. The permeation rate of hydrogen gas through the palladium membrane, Q , was assumed to obey the half-power pressure law(20). The permeation flux of hydrogen through the membrane is proportional to the difference between the souare roots of the hydrogen partial pressure on the high and low pressure sides of membrane. 

The permeation flux of oxygen through a mixed oxide membrane described above depends on the oxygen partial pressures across the membrane, membrane thickness and temperature. The dependence, however, is embedded in a complicated implicit equation [Lin et al., 1994]. Only in special cases the permeation Oux shows a pressure dependence similar to that for palladium membranes as given in Eq. (4-10). For example, when electronic conductivity predominates, the value of the exponent, n, is equal to 0.5 for thin membranes and 0.25 [Dou et al., 1985 Itoh et al., 1993] for thick oxide films. If the oxide membrane is essentially an ionic conductor and the surface reaction is the rate-limiting step, n takes on a value of 0.5. 

Not only the permeability, permselectivity and mechanical properties, but also the catalytic properties are affected by the two hydride forms that can exist in palladium. The a phase corresponds to solid solutions with a H/Pd ratio of about 0.1 and the P phase with a H/Pd ratio of about 0.6. The phase change is associated with a large change in lattice constant that often leads to microcracks and distortion in the palladium membrane. As a result, the mechanical properties are reduced. The transformation depends on the operating conditions such as temperature and hydrogen partial pressure. Repeated thermal cycles, for example, between 100 and 250 C under 1 atm of hydrogen pressure can make a 0.1 mm thick Pd foil expand to become 30 times thicker [Armor, 1992]. 

For membranes implemented in the second reactor only the results of micro-porous membranes will be discussed in detail, because palladium membranes gave almost the same results and the performance is better than that of Knudsen diffusion membranes. The yield and selectivities have been calculated at different permeate pressures and are plotted in Fig. 14.11. The results for the conventional reactor are obtained without a membrane implemented in the process. 

Membrane-based separation processes to capture either H2 or CO2 from the gasifier are new and less studied methods of CO2 separation and capture. Membranes separate the desired gas component without requiring phase changes or chemical or physical sorption. The cost of membrane separation is generally dictated by the overall pressure drop. Membranes made of various types of materials such as polymers, metals, and rubber composites have been investigated. Palladium and molecular sieves are currently under study. ° ... 

Although membrane cost may ultimately be low for non-palladium membrane purification, compression costs off-set the benefits this provides compared with PSA purification, except for use with low-pressure ATRs. 

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