Analysis And Application Study Of Irreversible Entropy Generation Characteristics In Waxy Crude Oil Pipe-transportation Process

This paper was financially supported by National Natural Science Foundation of China (No.51174042, No.51106020) and PetroChina Innovation Foundation (2012D-5006-0610)Most crude oil produced in our country has the characteristics of high viscosity, low solidifying point and multi-wax, which requires heat transport processes. It makes continuous pipeline to consume substantial energy. As the domestic demand of crude oil growing, safe and economic operation of long distance pipelines will face greater challenges. From the thermodynamics point of view, crude oil transportation process involves various irreversible phenomena such as diffusion, heat transfer and fluid flow, etc. They overlap and interact with each other. It is necessary to use relevant theories and methods of non-equilibrium thermodynamics to research the irreversible entropy production characteristics of waxy crude oil and provide theoretical and technical support for the safe and economic operation of pipelines.The main contents of this paper include:(1) Taking a pipeline of Daqing oil field as an example, a detailed research that includes the influence from viscosity-temperature characteristics, rheological properties, wax precipitation parameters of purified oil and temperature on the amount of wax precipitation, the effect from thermal history and shear history on purified oil flow properties is going on. The essential reasons of wax deposition issue during pipeline transportation are analyzed on the perspective of non-equilibrium thermodynamics.(2) The entropy production rate equation suit for pipeline oil is deduced with basic conservation laws and Gibbs equation. The entropy production rate for irreversible processes of diffusion, heat transfer and fluid flow are analyzed respectively, and the trend that total entropy production rate changes with temperature as well. The effect of operation parameters such as pipe diameter, throughput and insulation thickness on entropy production rate is studied, and it gets comparison with the result of thermal economic analysis.(3) According to the "force" and "flow" gotten from entropy production rate equation of pipeline transportation process, the phenomenological coefficients of heat transfer, diffusion and viscous phenomenon are deduced. They represent the intensity of "force" on "flow" in irreversible thermodynamic phenomena, and the trend between them and temperature is analyzed. Based on the coefficients, diffusion rate of wax molecules is obtained, and compared with test results as well.(4) On the basis of energy quality concept of thermodynamics, various kinds of exergy flow in pipeline process are deduced and calculated, especially the driving exergy that drives crude oil from starting point to terminal point. The variation rule of driving exergy with pipe diameter, throughput and insulation thickness is analyzed, and the conclusions are obtained that the value of driving exergy loss is proportional to the diameter and throughput, and inversely proportional to the insulation thickness.(5) Based on exergy flow classification, exergy economic analysis of oil transportation process is carried out. According to heat exergy consumption and pressure exergy consumption, the cost of exergy consumption and remainder exergy consumption are determined. The variation rule between exergy cost and parameters such as pipe diameter, throughput and insulation thickness is studied. Then, the optimal parameters that fit to minimum exergy loss are obtained.