Regeneration of Hydrotreating Catalysts

Hydrotreating catalysts lose tiieir activity in several ways  

The deactivation usually occurs in three steps initial rapid deactivation, intermediate slow but steady deactivation and rapid deactivation at the end of the cycle. Commercial processes are operated at constant conversion. This constant conversion is achieved by gradually heating the reactor to higher temperatures to compensate for the slow but steady catalyst deactivation. 

The initial rapid deactivation phase is believed to be due to rapid coking on active sites having very high acidity. Deposited carbon can be characterized by temperature programmed combustion and Raman 

More important to the design of the catalyst is to improve the hydrogenation activity in the vicinity of acidic sites. This could hydrogenate hydrogenate the coke precursors by cracking them or making them soluble in the matrix. 

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Advantages of In-situ Catalyst Regeneration. Replacement of hydrotreating catalysts is an expensive operation in refineries. 

The quality of the regenerated catalyst was studied by means of Surface Area (SA) and Dynamic Oxygen Chemisorption (DOC). DOC has been proven previously to be an elegant technique for the evaluation of hydrotreating catalysts. Hydrodesulfurization activity has been correlated with the amount of oxygen chemisorbed at low temperature (5, 6). 

Bogdanort, J. M., and Rase, H. F., Characteristics of a Commercially Aged Ni-Mo/AL203 Hydrotreating Catalyst Component Distribution, Coke Characteristics, and Effects of Regeneration. Ind. Eng. Chem. Prod. Res. Dev, 1986. 25 pp. 220-230. 

Decoking and Regeneration of a Hydrotreating Catalyst by Supercritical Extraction... 

Hydrotreating catalysts are usually alumina supported molybdenum based catalysts with cobalt or nickel promotors. By 1990, the demand for hydrotreating catalysts is expected to reach 80,000,000 pounds annually (1). The increased demand for these catalysts and the limitations on the availability and supply of the active metals increase the urgency to develop effective catalyst regeneration techniques. 

Solubilities of heavy hydrocarbons in supercritical fluids depend on the type of solvent (6). Moradinia and Teja (7) showed that the solubilities of solid n-alkanes (n-C2g, n-C Q, n-C ) are about ten times higher in supercritical ethane than in carbon dioxide. Therefore, it is reasonable to search and find an appropriate solvent which can disintegrate and dissolve the carbonaceous deposits from hydrotreating catalysts, resulting in their decoking and regeneration. 

In this project, the feasibility of catalyst regeneration by supercritical fluid extraction was studied. A spent catalyst from an industrial naphtha hydrotreater was extracted with tetrahydrofuran, pyridine, carbon dioxide, and sulfur dioxide under subcritical and supercritical conditions. The coke reduction and changes in the catalyst pore characteristics were measured and to a limited extent the catalyst activity was evaluated. It is shown that by supercritical extraction, the coke content of spent hydrotreating catalysts can be reduced and the catalyst pore volume and surface area can be increased. 

The hydrotreated base naphtha contained less than 0.5 ppm sulfur. The naphthas were dried over molecular sieve and stored under an inert gas (Ar) prior to use in the pilot unit. Using Karl-Fischer analysis the water content of the dried naphthas was measured to be 5-8 wt ppm. In order to compensate for the effect of the remaining water on the chloride content of the catalyst, 0.8 wt ppm chloride as 1,1,2-trichloroethane was added to the naphthas. Hj (99.995%, Norsk Hydro), supplied from gas cylinders, was passed over a deoxo catalyst (BASF R3-11) at 70°C and a 4A molecular sieve to remove traces of oxygen and water, respectively. The deoxo catalyst as well as the molecular sieve were regenerated between each test run. 

A spent commercial deactivated NiMo/Al203 hydrotreating catalyst, whose chemical analysis data are presented in Table 1, was used in this study. It was considered as a model of a spent industrial catalyst that is commonly discarded after its lifetime. The catalyst was employed in its sulfided form in hydrotreating of diesel fractions for about 3 years in a brazilian refinery. It was not regenerated during its lifetime and was kept in its original form (cylinder extrudates 5mm). 

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