Research On Phase Change Heat Transfer Characteristics Of Removing Wax Deposition By Thermal Washing In The Gathering Pipeline

Most of the crude oil produced in China contains significant quantities of wax.The wax deposition has been a frequent problem during the production and transportation of crude oil,which directly affects the safe and efficient production of oil fields.The wax deposition gives rise to many problems to the petroleum industry such as reduce of flow area and capacity of the pipeline,increase in frictional resistance,and possibly damage equipment,which in return causing a significant economic loss.Therefore,exploring the problem of wax removal in the pipeline is of great significance for maintaining regular production and reducing operating energy consumption in the oil field.At present,the thermal washing has become a common method due to its simple operation and low cost.Therefore,this paper uses experimental and numerical simulation methods to carry out relevant research on the process of thermal washing,reveals the phase change process of wax,and studies the influence of different parameters on the process.The main work and conclusions are as follows:(1)A dynamic cleaning/wax device was built to conduct an experimental study on the process of hot washing.The influence of water temperature,wall temperature,rotation speed and carbon number distribution on the wax melting process was investigated.The experimental results show that increasing the water temperature and wall temperature can change the temperature field distribution of the wax and accelerate the melting process.When the flow rate is low(n<500 r/min),the weight of wax removal mainly depends on the melting effect of hot water;When the rotating speed is between 500 and 1000 r/min,the weight depends on the scouring and melting effect of hot water;When the flow rate is high(1000<n<2000 r/min),the weight mainly depends on the scouring effect of hot water.As the wax type in the oil sample increases,the thickness of the wax deposit layer becomes smaller,but the wax crystal structure is dense,resulting in an increase in the wax content,which is not conducive to melting the wax deposit layer.(2)A cross-section of the pipeline was selected as the research object,a two-dimensional model of the contact between hot water and wax was established.The melting process of the wax was simulated by the enthalpy-porosity and VOF models in Fluent software.The influence of Ste,GL and X on water temperature,initial temperature and thickness of wax layer on the melting process were analyzed.The simulation results show that due to the effect of natural convection,the wax melts unevenly.The wax in the lower part of the pipeline melts first,followed by the left and right sides,while the wax in the upper part is finally melt.Theliquid fraction increases with the increase of Ste and X,and the melting process can be accelerated by decreasing the GL.(3)A unit length of the pipeline was selected as the research object,a three-dimensional model was established,the process of wax phase change and flow were numerically simulated.The melting process of wax in the pipeline under forced convection was investigated,and the effects of water temperature,flow rate,initial temperature and wax layer thickness on the melting process were analyzed.The results show that the wax at the lower part of the pipeline melts fastest,while the wax at the upper part of the pipeline melts very slowly,and the melting time takes a large proportion of the total time.Increasing the water temperature and flow rate can accelerate the melting process,but the effect gradually decreases.Compared with water temperature and flow rate,the initial temperature of the wax layer has little influence on the melting process.