Hydrodesulfurization Reactions of Residual Oils (Part 5)

Abstract

Gel permeation chromatography (GPC) was used to study the sizes of sulfur and metal compounds in residualfeed oils and hydrodesulfurized oils. In this paper, the molecular weight distributions and distributions of sulfur, metal and asphaltene were investigated, and the results of application of the GPC method to desulfurization catalyst were discussed. The GPC column was a stainless steel tube, 22mm i.d. by 1, 200mm, packed with crosslinked polystyrene gel (102-103Å). About 1g of residue dissolved in 5ml chloroform was charged to the column. The separated fractions were collected with a siphon, solvent was evaporated, and samples were weighed and analysed. Molecular weights were determined by vaporphase osmometry using benzene as the solvent.Molecular weights of the major portion of the various atmospheric residues were below 700, and the molecules smaller than GPC extended chain length 50Å occupied the major portion (about 80-90%) of these residues (Fig. 3). Though the distribution of sulfur roughly corresponded to the molecular size distribution of each residue, sulfur was somewhat concentrated in the region of larger molecular sizes (Fig. 4). By comparing the sizes of sulfur containing molecules of feed oils with those of products in hydrodesulfurization, it was found that desulfurization reaction of smaller molecules proceeded preferentially compared with larger ones (Fig. 5).Though vanadium and nickel were more concentrated in the region of larger moleculars compared with sulfur (metal containing molecules larger than GPC extended chain length 100Å occupied 40-70% of the total metal containing molecules), they were present not only in asphaltene but also in the region of smaller molecules (Figs. 6, 7). Sizes of asphaltene molecules were in the range of 100-500Å, and their molecular weights were in the range of 3, 000-5, 000 (Fig. 9). Considering the result about the size of asphaltene, it is supposed that the inhibiting effect of asphaltene on desulfurization reaction12) is attributed to the steric hindrance near the inlet of the catalyst pores.