High-Temperature Structural Degradation of Chemical Nature

13 High-Temperature Structural Degradation of Chemical Nature 

The combination of high temperature and chemical exposure poses a very challenging material problem that is quite common in high-temperature membrane reactor applications. The consequence of structural degradation as a result of such a combination not only affects the permeability and permselectivity but also leads to physical integrity or mechanical properties. These issues apply to both metal and ceramic membranes. 

Membrane embrittlement. StrucUiral changes or degradations in mechanical propeities of the membranes in response to the application environments can be a significant issue. Pd alloys are preferred over pure Pd as the material of choice for dense membranes in that pure Pd can suffer from embrittlement after repeated cycles of hydrogen sorption and desorption particularly at a temperature higher than approximately lOO C. It is believed that there are two hydride phases of pure palladium a phase at lower hydrogen partial pressures and P phase at higher pressures. The two phases can coexist under 

Corrosive reaction streams. In some application environments, the reactive or corrosive nature of one or more of the reaction components in a membrane reactor can pose a great technical challenge to the selection as well as the design of the membrane element Feed streams often contain some Impurities that may significantly affect the performance of the membrane. Therefore, attention should also be paid to the response of the selected membrane material to certain impurities in the reactant or product streams. Care should be taken to pretreat the feed streams to remove the key contaminants as far as the membrane is concerned in these cases. For example, palladium alloy membranes can not withstand sulfur- or carbon-containing compounds at a temperature higher than, say, 500 C [Kamcyama et al., 1981]. Even at lOO C, the rate of hydrogen absorption (and, therefore, permeation) in a pure palladium disk is 

Exposures of some metal oxide membranes, both dense and porous, to extreme pH conditions (e.g., pH less than 2 or greater than 12) can cause structural degradations, particularly with extended contact time. The extent of degradation depends on the specific phase of the material, porosity, and temperature. Steam can also be deleterious to some metal oxide and Vycor glass membranes. For example, as mentioned earlier, porous glass membranes undergo slow structural changes upon exposure to water due to partial dissolution of silica. 

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