Strategies for heavy oil desulfurization were evaluated by reviewing desulfurization literature and critically assessing the viability of the various methods for heavy oil. The desulfurization methods including variations thereon that are discussed include hydrodesulfurization, extractive desulfurization, oxidative desulfurization, biodesulfurization and desulfurization through alkylation, chlorinolysis, and by using supercritical water. Few of these methods are viable and/or efficient for the desulfurization of heavy oil. This is mainly due to the properties of the heavy oil, such as high sulfur content, high viscosity, high boiling point, and refractory nature of the sulfur compounds. The approach with the best chance of leading to a breakthrough in desulfurization of heavy oil is autoxidation followed by thermal decomposition of the oxidized heavy oil. There is also scope for synergistically employing autoxidation in combination with biodesulfurization and hydrodesulfurization.
The selective removal of nitrogen-containing compounds from oil and oil fractions is of interest because of the potential deleterious impact of such compounds on products and processes. Problems caused by nitrogen-containing compounds include gum formation, acid catalyst inhibition and deactivation, acid−base pair-related corrosion, and metal complexation. A brief overview of the classes of nitrogen compounds found in oil is provided. The review of processes to remove nitrogen from oil emphasizes studies that investigated denitrogenation of industrial feedstocks, such as refinery fractions, heavy oils, and bitumens. The main topics covered are hydrotreating, liquid−liquid phase partitioning, solvent deasphalting, adsorption, chemical conversion followed by separation, and microbial conversion. Chemical conversion processes include oxidative denitrogenation, N-alkylation, complexation with metal salts, and conversion in high-temperature water. There are many processes for denitrogenation by separation of the nitrogen-rich products from oil without removing the nitrogen group from the nitrogencontaining compounds. As a consequence, most of these processes are viable mainly for removal of nitrogen from low-nitrogencontent oils, typically with <0.1 wt % N. At present, hydrodenitrogenation appears to be the only industrially viable process for nitrogen removal from oils with high nitrogen content.
On a molecular level Fischer-Tropsch syncrude is significantly different from crude oil. When syncrude is treated as if it is a crude oil, its refining becomes inefficient. Refining technologies developed for crude oil can be employed to refine Fischer-Tropsch syncrude, but in order to conform to green chemistry principles (preventing waste; maximising atom economy; increasing energy efficiency) the technology selection must be compatible with the syncrude composition. The composition of Fischer-Tropsch syncrude is discussed in relation to the molecular requirements of transportation fuels and the refining gap that needs to be bridged. Conversion technologies are evaluated in terms of their refining objective, chemistry, catalysts, environmental issues, feed requirements, and compatibility to Fischer-Tropsch syncrude, in order to suggest appropriate technologies for efficient refining of Fischer-Tropsch products. The conversion technologies considered are: double bond
Oligomerization of hexene and heavier olefins is important for synthetic fuel producers to reduce refinery complexity, as well as to produce good quality middle distillates and lubricating oils. Conversion of such olefins with homogeneous catalysts is much easier than with solid acid catalysts, which are prone to deactivation and are limited to middle distillate production. Solid phosphoric acid, amorphous silica-alumina, sulfated zirconia, MCM-41, ZSM-5, zeolite-Y, and zeolite-omega heterogeneous catalysts were evaluated in fixed-bed reactors with 1-hexene and 1-octene to determine the key catalyst properties required. The influence of added chromium and nickel was also evaluated. It was found that solid acids with average pore size of more than 10 nm had consistently better catalyst lifetime, because heavy oligomers were less readily trapped in the catalyst. It was also found that in large-pore catalysts chromium improved selectivity to lubricating oil significantly by changing the oligomerization mechanism.
Oligomerization of Fischer-Tropsch-derived olefins over a solid phosphoric acid catalyst was studied to improve the quality of the hydrogenated motor-gasoline. It was shown that, with a narrow-cut Fischer-Tropsch butene mixture, rich in 1-butene and low in isobutene, a hydrogenated motor-gasoline with an RON and an MON of 86-88 could be produced at 80% olefin conversion. The influence of propene and pentenes was investigated, and a reactivity sequence for C 3 -C 5 mixtures could be established: isobutene > methylbutenes > n-butenes ) propene > n-pentenes. It was noted that 1-butene was closer in behavior to isobutene than to 2-butene and had to be treated as such to yield the highest-quality product. It was advantageous to oligomerize 1-butene-rich feed at 160°C or less and to exclude propene and n-pentenes from the reaction mixture. These effects could be explained.
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