Study On The Mechanism Of Desulfurization And Coking During The Heavy Oil Upgrading With Supercritical Water

Non-traditional heavy oil resources are of great significance for maintaining the current energy supply.Using supercritical water as a solvent to upgrade heavy oil is a good choice.Although this can improve the quality of oil,there are some difficulties that have not been overcome,such as the low removal efficiency of sulfides,coking,etc.At this stage,a thorough understanding of the specific causes and mechanisms of heteroatom detachment and coking is a key step to further improve the upgrading effect.The task of this subject is to explore the essential issues affecting the upgrading effect such as the decomposition mechanism of sulfide and the formation mechanism of coke on the basis of simulation calculations and related experiments.In the desulfurization mechanism,we first explored the rate-determining steps of thiophene compounds from three aspects: the electronic structure of the thiophene ring,the transition state structure of addition rection,and the steric hindrance of the substituents.The results showed that under the influence of the substituents,the delocalization energy of thiophene ring will be weakened and the steric hindrance will be increased at the same time.Based on the above conclusions,the easy decomposition sequence is trimethylthiophene>thiophene>dibenzothiophene.The desulfurization rate measured by the experiment verifies the above conclusion.In the subsequent study,the reaction path of the above compounds was calculated in detail,and the alkane component in heavy oil was simulated with n-tetradecane,and the influence of other components of heavy oil on the decomposition of thiophene was discussed.Among all the pathways,the free radicals generated by the cracking of alkanes participate in the lowest energy reaction.So free radicals accelerate the decomposition of thiophene compounds.Subsequent experiments on the reaction intermediate product and final product analysis verified the above reaction path,and further verified the role of water in the decomposition process of thiophene compounds.The average molecular model of each component is determined by the physical parameters of the heavy oil,and the heavy oil model is constructed.The validity of the model is tested by the density of the heavy oil at each temperature.The results show that the model can reflect the properties of the heavy oil well,and the error between the calculated value and the experimental value is less than 5%.The model shows that the degree of solubility of components of heavy oil in water depends on the size of the aromatic part of the component molecules.In the subsequent simulation of the established model,the saturated part quickly dissolves in supercritical water,and the aromatic part quickly accumulates into a dense supramolecular structure,and most of it is a flat-layered structure,which can not dissolve in toluene.Therefore,it can be considered that the supramolecular structure formed in the simulation is the precursor of coke formation.The main factors for the formation of this structure are the accumulation of the aromatic part.