| The LNG light hydrocarbon separation process is of great significance in the chemical industry.The hydrocarbon components in LNG will affect the calorific value of natural gas,and the obtained ethane and other products can bring great economic benefits,so it has been used in the chemical industry.But the lack of rectification and separation efficiency of rectification devices such as demethanizers and deethanizers in the process leads to insufficient methane content in pipeline gas,low recovery rate of by-products,and high power consumption in the process.Therefore,starting from the thermodynamic mechanism such as thermodynamic coupling mechanism and field synergy principle,this dissertation analyzes the heat and mass transfer process in the separation process of LNG light hydrocarbons,reveals the factors that promote the phase change process of rectification in the process from a new perspective,and provides a theoretical basis for process optimization.And an optimized LNG light hydrocarbon separation process is proposed by using the principle of cold energy cascade utilization and heat integration technology.The specific work is as follows:Firstly,five existing classic LNG light hydrocarbon separation processes were simulated by Aspen Hysys software,and the simulation results of their logistics and power consumption were obtained.The simulation results are analyzed for different processes,and the shortcomings of the existing processes are mainly focused on the following points:(1)only a demethanizer is installed,and without further deethane tower for further separation of ethane and LPG and other light hydrocarbon products,resulting in great economic waste;(2)Only setting up the single-level heat exchange leads to a low process heat exchange rate(3)The separation efficiency of light hydrocarbons in the process needs to be improved,mainly because the efficiency of the rectification phase change process in the rectification equipment such as the demethanizer needs to be improved;(4)The power consumption is relatively large,Power-consuming equipment is mainly concentrated in rectification equipment such as compressors and demethanizer reboilers;(5)The methane-rich natural gas obtained in the process still has a relatively high amount of unused cold energy,resulting in a low utilization rate of cold energy in the process.Secondly,for the rectification phase change process in the rectifying tower in the LNG light hydrocarbon separation process,starting from the divergence theory and transport theorem,and combining the field synergy theory to establish and obtain the same liquid side material of the rectification phase change system The relationship equation between the variation of liquid energy with time and the divergence of heat flow,which is mutually supportive,proves the correctness of the divergence equation.Applying the chain rule and dimensionless processing to the obtained divergence equation,the relationship between the enhancement of the liquid-side convection source term and the magnitude of the velocity gradient and temperature gradient and the angle between them in the process of rectification phase transition can be guided.Optimize the structure of the rectification equipment and the way the material enters the rectification equipment to strengthen the convective heat source items on the gas side and the liquid side of the rectification phase change process,and then strengthen the convective heat transfer to strengthen the mass transfer process of the phase change process and promote the phase change efficiency.In order to improve the separation efficiency of LNG light hydrocarbon components.Furthermore,combined with the thermodynamic coupling and field synergy theory,the coupled analysis of heat and mass transfer in the rectification phase transition process was carried out to explore the internal mechanism of the enhanced phase transition process.The analysis shows that the liquid-side evaporation process in the rectification phase transition process is a spontaneous mass transfer process with an entropy yield greater than zero,the gas-side condensation phase transition process is a non-spontaneous mass transfer process,and the liquid-side heat transfer process is a non-spontaneous evaporation process.The endothermic process,the gas-side heat transfer process is a spontaneous condensation and exothermic process,and the heat-mass process on both sides of the gas-liquid conforms to the thermodynamic coupling mechanism.Further analysis shows that the angle between the gas-side mass flow and the heat flow during the rectification phase change process is less than 90 °,and when the angle is smaller,the gas-side heat transfer in the rectification phase change system will be more enhanced,which will lead to the heat transfer in the boundary layer being reduced.Strengthening;the angle between the mass flow on the liquid side and the heat flow is greater than 90°,and when the angle is larger,the heat transfer on the gas side of the rectification phase change system will be strengthened.It is instructive to increase the phase change efficiency of rectification,and the structural parameters of the equipment can be optimized to make the heat transfer enhancement effect stronger,thereby promoting the phase change process and improving the separation efficiency of LNG light hydrocarbons.Finally,after analyzing the main rectification phase transition process in the separation process of LNG light hydrocarbons,the principle of cascade utilization of cold energy and heat integration technology are applied from the large process,and the methane recovery rate and separator feed rate under different pressures of the LNG booster pump are used.The relationship between the feed temperature and the relationship between the system power consumption and the feed temperature of the separator is used to determine the optimal parameters,so as to propose an optimized LNG light hydrocarbon separation process,and carry out exergy analysis and economic analysis.By comparison,it is concluded that its advantages are obvious. |
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