Wax Molecular Diffusion And Deposition Mechanism Research Based Onheat-Mass Synergy In Pipeline Process Of Crude Oil

The crude oil produced in our country mainly has the characteristics of high viscosity and high pour point. Waxy crude oil is a complicated mixture system including aromatic hydrocarbons, paraffin wax, asphaltene, resin, light hydrocarbons and other components. In the process of such pipeline transportation, wax deposition occurs inevitably with the decreasing temperature. As a result of wax deposition, the effective flow area as well as the throughput of pipeline decreases, and the pigging frequency increases. And plugging accident could happen when wax deposition become more serious.Based on the previous experimental results and theoretical results, wax deposition process of waxy crude oil in pipeline is regarded as an irreversible process in this paper. From the perspective of non-equilibrium thermodynamics and heat and mass transfer, the entropy production rate equation is deduced according to the basic field balance equations. On the basis of entropy production rate equation, thermodynamic “force” and “flow” are chosen according to the Curie law, Onsager reciprocal principle and linear law. The linear phenomenological equations of wax molecular diffusion is derived. According to the mass and energy conservation law, heat-mass synergy differential equations are established. And the wax molecular diffusion rate is obtained through Henry transformation. By using the knowledge of crystallography, physical chemistry and interfacial chemistry, the physical and chemical processes of wax deposition are studied, and the stress of the wax crystal adhesion process is analyzed. Then the mathematical model of wax deposition rate is established by introducing the attachment probability equation. At last, by taking a pipeline of Daqing as an example, the molecular diffusion coefficient, wax deposition rate and net wax deposition rate are calculated. The wax deposition rate of oil sample is tested by laboratory circuit experiment. The coupling calculation results are compared with the experimental results to show the accuracy and suitability of heat-mass synergy mechanism analysis, which can provide theoretical basis for the further study of wax deposition issues in waxy crude oil pipeline process.