This article describes improvements in thermal conversion processes for heavy oil cuts (visbreaking and hydrovisbreaking) in the presence of additives capable of increasing both the quality and degree of conversion by limiting coke formation. A great many laboratory tests in a discontinuous reactor of different kinds of residues in the presence of various diluents and additives are used to describe the thermal behavior of fossil organic matter. Hence an increase in processing temperature or in residence time leads to an increase in Safaniya vacuum residue conversion into light products (50%) as well as in coke yield (8.5%). The use of hydrogen donor diluents (tetrahydronaphtalene, dihydroanthracene) leads to a substantial reduction of the coke yield (close to zero) due to condensation and dehydrogenation reactions control, but also to a lower conversion level (30%) from partial inhibition of the chain fragmentation reactions. The use of sulfur-containing compounds (thiols, disulfides or sulfoxides) as hydrogen transfer agents or radical initiators, however, leads to an activation of organic matter conversion at lower temperature, although to the detriment of the quality of the liquid produced. At the same time, the use of a sulfur-containing compound, such as DMDS, associated with a hydrogen donor compound (tetrahydronaphtalene), results in high conversion into satisfactory quality liquid fractions (50%) associated with minimum coke and gas production. An effect of synergism between these two types of additives is thus obtained, i.e. an appreciable decrease in the asphaltene fraction linked to effective inhibition of coupling and dehydrogenation reactions as well as activation of craking reactions by the RS./RSH pair and hydrogen transfer reactions by the H-2/RSH (H2S) pair. Molecular hydrogen no longer seems to be required for conversion with limited coke production during operating in the presence of hydrogen donor additives. This specific effect is clearly demonstrated for pyrene, which under these conditions operates more like a solubilizing agent for coke precursors. These findings have been confirmed by research on the thermal behavior of other heavy feeds (Nigerian and Kirkuk vacuum residues, Rospomare atmospheric residue and ex Safaniya hydrotreated vacuum residue). However, the increase in performance brought about by using additives in residues depends greatly on their chemical nature. A series of tests performed in a dynamic micropilot reactor with the same diluents and model additives as in the presence of industrial aromatics cuts (LCO, HCO) has confirmed the above findings, on the hole. For example, conversions of 50% into 500-degrees-C- products were obtained with zero coke. At lower concentrations and under operating conditions closer to the ones encountered in conventional hydrovisbreaking, research in an industrial pilot plant has revealed a slight influence of aromatic diluents (LCO, HCO) and sulfur-containing additives (DMDS) on the quality, from the standpoint of stability, of base stocks for commercial fuel oils produced from Safaniya vacuum residue. In particular, the use of LCO, as opposed to HCO, tends to reduce the amount of fuel oil available (40 cSt at 100-degrees-C) to the benefit of lighter products. The adding of DMDS to the feed, however, tends to decrease the effects exerted by such industrial aromatic diluents. A set of physical methods for analysis and structural investigation can be used to characterize thermal processing effluents or some of their fractions more finely. The oxidizing pyroanalysis method makes it possible to follow the evolution of the heavier fraction closely by determining the H/C and S/C ratios as well as the residual carbon content. Liquid chromatography (SARA analysis) and gel permeation chromatography help visualize the effect of additives on heavy products content and on the evolution of the molecular weights of the thermal cracking products and of the asphaltenes. NMR H-1 and C-13 techniques and X-ray diffraction reveal the structural transformation of the matter during heat treatments, especially the dealkylation of the heavy fraction. Lastly, the print of such treatments is visualized by optical and electron microscopy observation of the microtexture, after a supplementary thermal concentration of the liquid at 370-degrees-C. From the standpoint of the quality, analytical findings confirm the advantage of associating a sulfur-containing compound with a hydrogen donor in the heat treatment of heavy oil cuts.
