Ageing phenomena

Transient and Aging Phenomena in Anodic Alumina Films... 

The Swiss painter Cuno Amiet (1868 1961) carried out experiments with binding media from at least 1902 onwards. Today, many of his paintings show several ageing phenomena that probably are due to the nature of the applied painting technique. On the... 

Unwanted degradation and oxidation processes can be avoided or at least suppressed for some time either by structural modiflcation of the polymer or by special additives. In practice, the addition of so-called antioxidants is particularly effective. Chemical substances that slow down oxidations and the following aging phenomena serve for this purpose. Antioxidants are sufficiently effective even in concentrations below 1 wt% and are added as early as possible to the polymer to be protected, e.g., already during the drying of powdery polymeric materials or during the preparation of granulates. Some of the most important so-called primary antioxidants are sterically hindered phenols and secondary aromatic amines secondary antioxidants are thioethers as well as phosphites and phosphonites. 

Two types of experimental current scans were performed. After introducing the photoanode into solution, successive current vs. potential (i vs. ) scans were performed at slow sweep rates (2mV/S). All of the photoanode materials studied exhibited an initial drift of the i vs. scans to more anodic bias. This occurred for both uninterrupted scans as well as intermittent scanning. The origin of this aging phenomena is not understood. However, it was observed that after approximately thirty minutes the i vs. scans achieved a profile which was reproduced during further cyclic sweeps. This result was taken as a criterion for proper electrode pre-conditioning. 

Although the design of a reactor system for the MTG process involves classical chemical engineering principles, the unique catalyst and reaction mechanisms impose important design constraints. These include the highly exothermic nature of the reaction, the need for essentially complete methanol conversion, steam deactivation of the catalyst, the "band-aging phenomena, and durene formation. 

In Figure 6a, a multibed reaction system is shown for which only partial conversion is taken across each bed the effluent is cooled before entering each succeeding bed such that the effluent from each bed does not exceed a specified maximum temperature. The band-aging phenomena previously discussed would complicate the development of such an MTG process since the catalyst appears to age from the front to the back. Thus, it might be difficult, especially for the first bed, to maintain partial conversion at a given level for a reasonable amount of time. 

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