Catalyst accelerated aging

The life time of a catalyst is an important criterion for its commercial application. The reforming reactions in most of the cases have been studied only for hours of time on-stream, but not for days or weeks. Superior catalysts need to be evaluated by operating continuously for thousands of hours. Simple accelerated aging test that allows assessment of catalyst life in hours or days rather than the usual priod of months will be helpful. 

In an automobile s catalytic converter, CO and hydrocarbons present in the exhaust gases are oxidized. Unfortunately the effectiveness of these units decreases with use. The phenomenon was studied by Summers and Hegedus in /. Catalysis, 51, 185 (1978) by means of an accelerated aging test on a palladium impregnated porous pellet packed bed converter. From the reported data on hydrocarbon conversion shown below, develop an expression to represent the deactivation rate of this catalyst. 

Accelerated aging tests have been commonly used in facilities ranging from academic bench scale to industrial pilot-plant scale. In such tests, the addition of an accelerant increases the deactivating agent or its precursor, so that a certain Level of deactivation (however measured) is reached at lower run times. In this manner, the performance of a catalyst at a high deactivation level can, in principle, be measured without expending the time and effort required under normal conditions to bring the catalyst to this level of deactivation,... 

While Barrow solved the main problem, there are other modes of degradation which must be considered. As an organic material, cellulose and paper can be easily oxidized. Very small amounts of the transition metals, compounds of iron, copper, and cobalt, under humid conditions can accelerate oxidation and embrittlement of paper. This type of degradation, as is shown later, does not show up in the dry-oven accelerated aging which Barrow used. Thus his alkaline papers, if they contained the oxidation catalysts, may not always have been permanent. 

As shown in Table I, complete deactivation for these three catalysts occurs around 0.6 to 0.8 wt% sulfur, based on the active site content. These values are typical for complete deactivation in a commercial reactor. The metal surface area measured by hydrogen chemisorption is almost three times the active site concentration determined from the fit of the model to the accelerated aging data. Some of this difference may be due to a poor separation of the product kg C" into the individual constants. How-... 

Char acts as a vapour cracking catalyst so rapid and effective separation from the pyrolysis product vapours is essential. Cyclones are the usual method of char removal and two are usually provided - the first to remove the bulk of the material and the second to remove as much of the residual fines as possible. However, some fines always pass through the cyclones and collect in the liquid product where they accelerate aging and exacerbate the instability problem, which is described below. 

Excess iron can lead to diabetes mellitus, faulty liver functions, and endocrine disturbance. Iron is a catalyst for oxidative damage leading to lipid peroxidation. The latest hypotheses link peroxidation to heart disease, cancer, and accelerated aging. Iron is involved in the Fenton Reaction, which catalyzes the formation of free radicals that cause excessive damage to cells and their components. 

A typical lifetime of commercial catalysts is 1-10 years. Thus, for a new catalyst product, the final confirmation of the product performance in industry cannot take place until many years after the product has been produced even if accelerated ageing tests have been carried out. 

Equation 12.2 reflects a base case of 9 mg/mile with a 0.29 m2 surface area, ratio of airflow to vehicle speed at 40%, and conversion of base case to 80% from the UAM. The deactivation factor (DF) is DF = (aged ozone conversion/ fresh ozone conversion). To determine the deactivation factor, on-road aging of radiators coated with catalyst was run for 150,000 miles and DFs at that point were determined. Freshly coated radiators were exposed to an accelerated aging test and DFs were calculated and compared for accuracy. Figure 12.4 shows the calculation of deactivation factors for two different radiators with different core geometries. 

Aging of spirits involves oxidation. It is this reaction which one attempts to hasten by the processes devised for accelerated aging. Methods for aging spirits artificially fall into four main classes as follows (1) treatment with air, oxygen, or ozone (2) exposure to actinic rays (3) electrolytic treatment and (4) use of catalysts. Combinations of these methods are likewise employed. 

Accelerated ageing studies on the titania based catalyst confirm that the catalyst is expected to have a long life even in the presence of acidic atmospheres. 

Accelerated ageing of the catalysts on line was performed by heating to 800°C for 120 h. This treatment caused the catalyst activities to drastically decrease. The best catalyst was still the chromium-promoted system but conversion decreased to 40% at 450°C. The performance of the vanadium catalyst after ageing was poor and the authors attribute this to the melting point of vanadium (690°C), which caused loss of the promoter from the catalyst. It is also evident that the BET surface area of the vanadium aged catalyst decreased by 84% from the initial value. However, these studies indicate that the incorporation of other metals with platinum may have a beneficial effect for complete oxidation activity. 

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