Thermal Cracking Behavior And Reaction Kinetics Of Heavy Oil In Supercritical Water

Heavy oil characterized by the high content of heteroatoms,high viscosity and high specific gravity makes up the largest fraction of oil reserves in the world.With the increasing demands for fuel oil but the decreasing reverses of light crude oil,a comprehensive utilization of heavy oil becomes vital to global energy security.Pyrolysis following "carbon rejection"mechanism is an important and sometimes the only feasible way for the processing of unconventional inferior heavy oil.Owning to the special properties of water in its supercritical region,the upgrading of heavy oil in the environmentally benign solvent of supercritical water(SCW,Tc=647.3 K,Pc=22.1 MPa)has attracted increasing attention in the field of oil processing.Asphaltenes are the direct precursor of coking,and the intermediates of the condensation of maltenes to coke.By this condiseration,the upgrading of heavy oil through pyrolysis in SCW was investigated,experimentally and theroretically.With reaction kinetic analysis,molecular dynamics(MD)simulation,and physical characterization,the reaction behavior of asphaltenes in SCW was examed with due amphasis.Based on the results obtained,the feasibility of the removal of carbon residue of heavy oil through pyrolysis in SCW was further discussed.At any reaction temperature applied,the pyrolysis under SCW environments is faster than that under N2 environments.Meanwhile,at lower temperatures the pyrolysis under both environments is accelerated by the introduction of coke into the feedstock.On the basis of a first-order four-lump reaction network consisting of the sequential condensation of maltenes and asphaltenes,the pyrolysis in whichever medium can be preferably described either by the lumped reaction kinetic model modified with autocatalysis and pseudo-equilibrium or by the model modified solely with pseudo-equilibrium.Benefited from the reduced limitation of diffusion to reaction kinetics,the pyrolysis in the SCW phase is more sensitive to the increase in reaction temperature than that in the oil phase,disengaging readily from the dependence on autocatalysis at a lower temperature.The MD simulation results indicate that the repulsive van der Waals interaction has a primary influence on the nature and the magnitude of the interaction between asphaltenes and SCW.A cavity short-range solvent structure surrounding asphaltenes thus is formed,and the solvation free energy(Gsol)of asphaltenes in SCW has a positive sign in most cases.The scale of cavities and the value of Gsol both are determined mainly by the bulk density of water.By the repulsive asphaltenes/SCW interaction and the attractive π-π aromatic interaction,the aggregation of asphaltenes in SCW occurs spontaneously.The asphaltene clusters formed,containing asphaltene monomers and nanoaggregates,present a coke-like supramolecular structure.Benefiting from excellent diffusivity in SCW,the aggregation of asphaltenes in SCW can be accomplished rapidly.Under hydrothermal environments,the pyrolysis of asphaltenes consisting mainly of condensation to coke and decomposition to maltenes is significantly faster than that under N2 environment.The H-donors introduced,decalin or maltenes,may provide non-aromatic H atoms,capping the carbon radicals essential to pyrolysis.Accordingly,the apparent activation energies of the condensation and the decomposition of asphaltenes both increase to varying degrees.The pyrolysis of asphaltenes in the presence of a small quantity of decalin is seriously retarded in subcritical water,but recovers rapidly in supercritical water owning to the promoted initiation efficiency at high temperature.Accompanied by a large amount of maltenes,the decomposition to maltenes involved in the original pyrolysis network of asphaltenes can be neglected.Driven by the π-π attractive interaction between aromatic sheets and the superb diffusivity in SCW,the coke-like self-assembly of heavy aromatics occurs spontaneously and rapidly.The self-assembly behavior of aromatics in SCW depends not only on the thermodynamic state of SCW but also on the average scale of aromatics.With the aid of self-assembly in dense SCW,the condensation of heavy aromatics,distributed mainly in vacuum residue,to coke is significantly accelerated,by which the rate of the removal of carbon residue is improved simultaneously.Owning to the preferential self-assembly of heavy aromatics,an increasing yield of liquid products can also be obtained under optimized SCW environment.