Influence Of Residue Composition On Hydrotreating Reaction Performance And Properties Of Catalyst

With the increasingly reducing of the petroleum reserves and the degrading of crude oils, coupled with the more stringent environmental regulations, the demand for high quality petroleum product (such as middle distillate, gasoline) is increasing. How to make full use of petroleum resources and diminish environmental pollutions are the most important questions for many countries around the world. Residue, characterized by large molecular weight, high boiling point, high viscosity and maximums polarity, is the most complex fraction in crude. Residue includes most of the sulfides, nitrides, oxides, resin and all of the asphaltene and heavy metals of crude. How to convert residue to light oils is an important and tough problem for petroleum processing. Hydrotreating of residue is a chief method which can improve the properties of residue significantly and then use it effectively. It is a really realized zero-discharge technology of petroleum processing. This paper provided a theoretical basis for determination of operation conditions and development of high efficiency catalyst by research and understanding of the reaction chemistry of residue hydrotreating.Comparative experimental studies of hydrotreating were made by Suizhong 36-1AR (Atmospheric Residue), Tahe AR, Fushun VR (Vacuum Residue) and Shengli VR in an autoclave under selected reaction conditions. The experimental results demonstrated that the residues of high density and asphaltene content have better reaction performance whether it is AR or VR. The residues of poor quality had a higher residue conversion, higher light oil yield, lower residue yield and slightly higher coke yield. In order to study the influence of asphaltene contents on residue hydrotreating, feedstocks which were blended with deasphalted oil (DAO) and different content of deoiled asphaltene of Tahe AR were hydrotreated in an autoclave. The results showed that with the increase of asphaltene content, the conversion of residue, the yield of gas, coke and light oil increased at different degree, but the yield of VGO and>500?residue decreased, and the ratios of asphaltene in hydrotreated residue plus coke to asphaltene in feedstock decreased. The conversion of residue improved by 11.41%, the yields of gasoline and diesel oil had an increase of 6.62% and 3.82% respectively when asphaltene contents increased from 2% to 12%.Asphaltene in hydrotreated residue was separated by classical chromatography method and then analyzed by elemental analysis, vapor pressure osmometry,1H NMR, and so on. The structure parameters of asphaltene were calculated by Browen- Landner method based on the analysis data above. The results showed that the molecular weight, H/C, HAU/CA,?, H?,H?, H?and substituted aromatic-carbon ratio of asphaltene decreased after hydrotreating; but aromatic-carbon ratio, HA, protonic aromatic-carbon ratio increased. All the changes show that the condensation degree of hydrotreated asphaltene was increased and asphaltene was dealkylated in residue hydrotreating. The sulfur contents, nitrogen contents, numbers of aromatic ring and naphthene ring of hydrotreated asphaltene increased with the increase of asphaltene content in feedstocks. The aromatic ring numbers of hydrotreated asphaltene even more than that of original asphaltene. Asphaltene react in the forms of structural units in residue hydrotreating. The reaction mechanism of asphaltene in residue hydrotreating is not only free radical reaction, but also carbonium ion reaction.Compared with the original asphaltene, the molecular weight, numbers of aromatic ring and naphthene ring of hydotreated asphaltene unit were larger. Asphaltene units can condense to larger asphaltene units which has higher condensation degree and more ring numbers by dehydrogenation in residue hydrotreating. The alkyl side chains in hydrotreated asphaltene units include 2-3 carbons which is coincident with thermal cracking reaction through a free radical mechanism.A new characterization index, the key parameter for residue hydrotreating (KRH) which can be expressed in terms of H/C, density, aromatic-carbon molar ratio and average molecular weight of the feedstock, was put forward based on the previous research. KRH can reflect chemical composition and hydrotreating reaction performance of residue very well. It correlated well with nitrogen removal rate, sulfur removal rate, residue conversion and coke yield for residue hydrotreating. KRH can be used as a satisfactory index to predict the reaction performance and guide residue hydrotreating operation. Residue hydrotreating performance increased with increasing of KRH. KRH larger than 5 has a better hydrotreating performance and hence a high quality feedstock for residue hydrotreating.14 spent catalysts which derive from commercial residue fixed-bed hydrotreating unit have been analyzed by CHNS/O elemental analyzer, XRF, BET, SEM, TEM. The results showed that the coke contents, metal contents and surface properties of spent catalysts have not any change law along the catalyst bed. The existing forms of metal on catalyst are metal sulfides. Catalysts of high metals with lower coke and higher H/C proved that metal deposited on catalyst can enhance the hydrotreating reactions and residue hydrotreating is an autocatalytic reaction. Metal and coke deposition mainly decreased pore size, pore volume and specific surface area of catalyst. But, sometimes when coke deposited on the catalyst, new meso and micro pores may formed by pore inducing, new construction forming or soft-coke hydrotreating under the action of deposited metals, which greatly increased the specific surface area of spent catalyst. The contents of coke and metals on catalyst, surface properties of catalysts were affected by the local reaction conditions, reaction state in reactor and deposition state of coke and metals.The nitrogen to carbon plus hydrogen ratio of the deposition deposited on catalyst ranged from 0.015 to 0.020 in commercial residue fixed-bed hydrotreating unit. It is proved that the coke deposited on catalyst was mainly from nitrogenous condensed nucleus which was difficult to hydrotreat in asphaltene.