Hydrogen Solubility In Residues And Mechanism Of Residue Thermal Processing Under Hydrogen

With the decreasing of conventional crude oil reserves day by day,attention to the exploitation and utilization of heavy oils has been focused gradually by the petroleum engineers and researchers all over the world.Comparing with the conventional crude oils,heavy oils possess the properties of higher viscosity,more content of heteroatoms and metals.The fact that crude oils become inferior brings great difficulties to transport and processing of the oils,thus,the heavy oils should be upgraded previously.Visbreaking under hydrogen,in which heavy oils are processed in the atmosphere of hydrogen without catalysts,is one of important thermal processings of heavy oils.Hydrogen acts as the hydrogen source which can provide hydrogen radicals to the reaction system.Hydrogen radicals then capture the free radicals formed from cracking of the high molecular components in the heavy oils.This reduces the probability of recombination of free radicals and the formation of higher molecular species with low hydrogen content,which are coke precursors.During visbreaking of heavy oils under hydrogen,solubility and activation of hydrogen are the important factors that impact the upgrading effect of heavy oils.However,detailed descriptions about the two factors can be rarely found in the literatures.Thus,hydrogen solubility in different heavy oils and hydrogen activation possibilities during upgrading of heavy oils were investigated in this paper.This study can provide theoretical principles for optimization of upgrading techniques of heavy oils under hydrogen.Firstly,an apparatus for determining hydrogen solubility in liquids at high temperature and pressure was built to measure the hydrogen solubility in four heavy oils of which the properties are in big differences.Results show that hydrogen solubility increases as the temperature and pressure increasing,and at the same conditions,hydrogen solubility in the four heavy oils increases in the order of Venezuela atmospheric residue(VNAR)< Liaohe atmospheric residue(LHAR)< Karamay atmospheric residue(KRAR)< Venezuela heavy coking gas oil(HCGO).Based on the hydrogen solubility data determined previously and the properties of the feedstocks,molecular simulation method was applied to analyze the relationship between hydrogen solubility and the molecular structures by using model components.Results show that the main factors that have impacts on hydrogen solubility include the non-bonding interactions between solvent molecules and hydrogen molecules,dispersity of solvent molecules in hydrogen atmosphere and the H/C of the solvents.For the paraffinic hydrocarbons,the non-bonding interactions between solvent molecules and hydrogen molecules strengthen as the length of alkyl chains growing,and at the same dispersity,paraffinic hydrocarbons with longer chains have the bigger hydrogen solubility.For the aromatic hydrocarbons,at the same dispersity,hydrogen solubility directly relates to the H/C of the solvents.Solvents with higher H/C have the greater hydrogen solubility.Moreover,the non-bonding interactions among the aromatic hydrocarbon molecules are much greater than the ones among the paraffin hydrocarbon molecules and the ones between the aromatic hydrocarbon molecules and hydrogen molecules.The aromatic hydrocarbon molecules tend to associate to each other.This fact decreases the hydrogen adsorption sites of the aromatic hydrocarbon molecules,making the hydrogen solubility of the aromatic hydrocarbons lower than paraffin hydrocarbons.For the heavy oils which have the similar boiling range,hydrogen solubility can be compared by comparing the H/C of the oils.Anthrancene was taken as the probe to react with different distillates,SARA fractions and different residues.The activation possibilities or reaction paths of hydrogen were discussed indirectly through analyzing the conversions of anthranene.Results show that thermal activation of hydrogen plays an important role in thermal processing.The hydrogen donating ability of hydrogen activated by heat is greater than that of oil samples at the same conditions.Hydrogen sulfide produced by desulfidation during thermal processing may acts as “hydrogen shuttle” to transfer hydrogen radicals.Metals in the oils can slightly improve the activation of hydrogen in the presence of hydrogen sulfide.The effect caused by Ni is better than V.Moreover,oil molecules containing condensed aromatic rings can be hydrogenated partly in the first stage to form naphthenic aromatic structures which have high donating ability,and then donate the hydrogen out at the later stage.This process can transfer or activate hydrogen by “hydrogen shuttle”.During the process,effects of thermal activation and “hydrogen shuttle” of the oil occupy the dominant position.Otherwise,superposition method was applied to analyze the interaction between hydrogen and oil samples during thermal processing.Results show that the hydrogen donating ability of oils under hydrogen is greater than the sum of hydrogen donating ability of oils and hydrogen.This indicates that synergism between hydrogen and oil samples exists in thermal processing,which improves the hydrogen transfer ability.Finally,different residues were upgrading under hydrogen and nitrogen respectively.Results show that for the same residue,upgrading effect under hydrogen is better than that under nitrogen which is proved by characterizing viscosity reduction rate,molecular weight,heteroatom content,boiling range and stability of the upgrading products.Hydrogen improves the upgrading effect of the residues.For the different residues,the upgrading effect decreases in the order of VNAR>LHAR>KRAR.The more inferior residue has the better upgrading effect.However,VNAR has the lowest hydrogen solubility among the three residues,thus,the upgrading effect of heavy oils is not determined completely by hydrogen solubility when the conditions are the same.The hydrogen activation and synergism with oil play the more important role during upgrading.Refinaries can use the oils which have the better synergism with hydrogen as the cycle oils to improve the upgrading effects.