TECHNOLOGY Simple tests determine catalyst regeneration temperature

Laboratory results from thermogravimetric analysis (TGA) and derivative thermal analysis (DTA) were used to determine the temperature range for regeneration of a commercial hydrodesulfurization catalyst.

Further testing of this catalyst after three cycles of use and regeneration shows good activity.

The technique and commercial results were described by Stephen Murff of Eurecat U.S. Inc., Pasadena, Tex., at Oil & Gas Journal's International Catalyst Conference & Exhibition, Feb. 1-2, Houston.

Laboratory testing

A spent sample of CoMo hydrodesulfurization catalyst was thermally analyzed using the TGA and DTA techniques (Fig. 1 [25934 bytes]). The experiment was performed at a heating rate of 10^o C./min.

The TGA curve shows three distinct reaction steps:

The evaporation of hydrocarbons, shown by a moderate exotherm at less than 200^o C.
The oxidation of metal sulfides, which starts at about 250^o C.
The strongly exothermic carbon burn-off, which, along with oxidation of the remaining sulfides, takes place between 350^o C. and 500^o C.

A sample of the same catalyst was then regenerated in a laboratory muffle furnace equipped with air flow. The results show that both carbon and sulfur removal begin at less than 500^o C. At 800^o C., however, residual carbon is less than 0.1 wt %, while about twice as much sulfur remains on the catalyst.

Murff says a dynamic oxygen chemisorption (DOC) test is useful for indicating the number of active sites present on the catalyst. Eurecat uses DOC, along with the more traditional methods of evaluating catalyst surface area, such as the Brunauer, Emmett, and Teller (BET) method. Carbon and sulfur removal rates also are evaluated.

Based on all test results (TGA, DTA, DOC, and BET), a temperature zone around 525^o C. was selected for regeneration of this catalyst.

Commercial results

Commercial regeneration was performed using the optimum temperature range determined by the laboratory studies. Table 1 [23112 bytes] shows catalyst characteristics over three cycles of use and regeneration at this temperature.

Plotting the DOC and surface area results from Table 1 as a function of regeneration cycle, the decline in DOC is shown to be slightly more rapid than that of surface area.

Fig. 2 [23226 bytes] compares DOC and surface area for another commercial catalyst regenerated over five cycles.

"Some variation in the trends is seen," says Murff, "primarily due to the addition of supplemental fresh catalyst after Regenerations 3 and 4. This suggests that surface area evaluation alone may not provide a complete picture of the condition of the catalyst surface."