Bauxite-based catalysts in heavy crude oil hydrotreating

Abstract

In hydrotreating heavy crudes, shale oils and coal liquefaction products, the cost of the catalyst is expected to be a critical factor in the process economy. This is due to non-volatile metal components in the feedstocks which cause high catalyst consumption. In a systematic study of the use of natural bauxite as carrier for low cost hydrotreating catalysts, the catalytic performance of bauxite-based catalysts in hydrotreating a 60% reduced crude from the Adriatic Sea region has been determined in a trickle-bed reactor. The following systems have been considered: MoO 3/bauxite, WO 3/bauxite, CoO-MoO 3/bauxite, NiO-MoO 3bauxite, NiO-WO 3/bauxite. Data on a commercial catalyst,containing 4 and 12 wt % of CoO and MoO 3 respectively, are included for comparison. For maximum surface dispersion of the active components, Mo(VI) and W(VI) were incorporated on bauxite by anionic exchange. The Mo(VI) and W(VI) contents of the catalysts (6.6 wt % MoO 3 and 11.6 wt % WO 3) correspond to the chemisorption capacity of the natural material (after thermal activation). The catalytic properties of the bauxite-based catalysts resemble those of alumina-based catalysts. The synergistic effect of cobalt and nickel on molybdenum and tungsten in hydrodesulfurization (HDS) is evident. Tungsten-based catalysts are less active than molybdenum-based systems, in accordance with literature data. The lower activity of all bauxite-based catalysts, compared with the commercial CoO-MoO 3-a1umina system, is attributed to differences in composition and surface properties. All of the catalysts examined promoted the production of a low boiling fraction in similar amounts, i.e., all catalytic systems show similar cracking activity. However, hydrogen uptake, as measured by the increase of the H/C ratio of the product, is substantially different and suggests large differences in the hydrogen-ating activity of the catalysts. Activity in hydrodenitrogenation (HDN) is lower than in HDS. Also the nitrogen removal properties of bauxite-based catalysts are comparable with the alumina-based systems. Differences in the kinetic steps involved in the HDS and HDN processes have been proposed; the experimental data indicate that the nitrogen removal process takes place through a number of kinetically important steps and this is consistent with the reaction network proposed for the HDN of model compounds. In metal removal, bauxite-based catalysts exhibit about the same activity as the commercial catalyst; this result is attributable to the relevant role played in this reaction by the pore structure of the support. These data show that, by the use of appropriate evaluation criteria, natural minerals, such as bauxites, can be selected to prepare efficient hydrotreating catalysts and that the activity and selectivity of these systems can be, to a certain extent, adjusted by a proper choice of chemical composition.