Experimental Study On Wax Deposition Law Of Thermal Insulation Crude Oil Pipeline

Wax deposits in thermal insulation pipelines can reduce the effective inner diameter of pipelines,increase the resistance of pipelines,and may cause pipeline blockage accidents in serious cases,which poses a great threat to the safe transportation of crude oil.It is of great practical significance to study the wax deposition law of thermal insulation crude oil pipeline for scientific formulation of wax pigging scheme and safe operation of pipeline.Based on the actual situation of transporting the mixed wax-containing crude oil through a newly built pipeline with the insulation layer in Northeast China,a cold finger experimental device with stirring function and an indoor thermal insulation loop experimental device were built.The wax deposition law and the influencing factors of mixed waxy crude oil under different experimental conditions were investigated.The calculation formula of wax molecular diffusion coefficient suitable for mixing waxy crude oil system was established.The calculation model of wax deposition thickness and wax content of insulated crude oil pipeline based on molecular diffusion,aging and shear energy principle was established.The results of the loop experiment and the model calculation results were compared and analyzed.The research results are as follows:(1)The effects of deposition time,hot and cold bath temperature and stirring conditions on the wax deposition law were studied using the self-made cold finger experimental device.The experimental results show that: as the experimental time increases,the deposition quality and the sediment hardness increases,the deposition rate decreases.When the temperature of the hot bath is constant,the deposition quality and rate increase with the decrease of the temperature of the cold bath,and the sediment is soft.When the temperature of the cold bath is constant,as the temperature of the hot bath increases,the hardness of the deposition increases.When there is stirring,the molecular diffusion increases,resulting in an increase in the mass of the deposition,while the low-temperature gelation without agitation causes the mass of the deposition to decrease first and then increase,the temperature corresponding to the maximum wax deposition is the starting point of the peak of wax deposition at this time.Under the same temperature difference,the wax deposition and deposition rate in the higher temperature range are smaller,but the precipitates in the high temperature range are dominated by large carbon number wax molecules which causes larger wax content.The sediment mass,deposition rate and wax content under agitation conditions were larger than those without agitation.Within a certain speed range,as the rotational speed increases,the wax deposition and deposit rate decrease,and the hardness becomes larger.(2)The Hayduk-Minhas empirical formula was improved,and the calculation formula of wax molecular diffusion coefficient suitable for mixing waxy crude oil system was established.The relative error between the calculated value and the experimental value is less than 22%,the average value is only 8.83%,and the calculation accuracy is greatly improved.(3)The calculation results of the temperature field distribution of the loop indicate: The temperature gradient of the tube wall at the entrance of the test tube is large,and then rapidly decays and the rate decreases along the length of the tube tends to be gentle.When there are sediment appeared on the wall,the thermal resistance of the oil to the wall increases,and the heat flux density of the heat dissipation decreases,which causes the temperature gradient of the inner surface of the sediment layer to decrease from the initial time,and the decreasing amplitude gradually decreases as the distance from the entrance increases.The greater the difference between the oil temperature in the test tube and the temperature of the cooling water in the casing,the greater the average temperature gradient of the wall.The average temperature gradient of the wall increases as the crude oil flow rate in the test tube increases under the same environmental conditions.(4)Considering the influence of molecular diffusion,aging and shearing on the wax deposition process,a dynamic prediction model of wax deposition dynamics of waxy crude oil pipeline was established.A modified Euler method was proposed to solve the differential equations of mass transfer and heat transfer.A series of loop experiments were designed and conducted to investigate the effects of operating time,axial position,flow rate and ambient temperature on wax deposition thickness and wax content.The experimental results show that when the ambient temperature is constant,the larger the flow rate,the smaller the thickness of the wax deposit layer and the higher the wax content.When the flow rate is constant,the higher the ambient temperature,the thinner the thickness of the layer and the higher the wax content.The results obtained by using the model for simulation prediction are in good agreement with the experimental values.(5)For on-site pipelines,the thickness of wax deposition increases first and then decreases with the distribution along the insulated crude oil pipeline.From the starting point of the pipeline to the maximum thickness of the deposition layer,the temperature of the oil drops rapidly.However,due to the low temperature of the wall,there is a large temperature gradient between the center position of the pipeline and the interface of the deposition layer.When the axial position exceeds the maximum thickness of the deposition layer,the temperature of the center of the oil flow drops to a lower temperature,and the temperature gradient at the interface between the center of the oil flow and the deposition layer decreases,resulting in a gradual decrease in the thickness of the wax deposition layer.The thickness of the wax deposition along the insulated pipelines with different running times in different seasons was predicted.The prediction results provide a scientific basis for the development of pipeline wax pigging cycle.