| Natural gas occupies an important position in the low-carbon energy structure improvement due to its high-quality,high-efficiency,and clean characteristics,and its demand continues to increase.Liquefied natural gas（LNG）is essentially the liquid form of natural gas,which has been widely used in industries,civil applications and peak shaving.It is one of the main ways to natural gas storage and transportation.As a new generation of natural gas treatment equipment,supersonic separator equipment has good refrigeration effect.It is planned to apply supersonic cyclone sepa ration technology to the natural gas liquefaction process and develop natural gas liquefaction technology and equipment.Taking methane gas,the main component of natural gas,as the research object,the micro characteristics and supersonic flow characterristics of methane gas condensation,and the micro characteristics and supersonic flow characterristics of methane and heavy hydrocarbon heterogeneous condensation were studied by combining theoretical research,computational fluid dynamics（CFD）and molecular dynamics（MD）numerical simulation and experimental research.This article provides a theoretical basis for the development of supersonic separation natural gas liquefaction technology and equipment.The main work and results are as follows:（1）The Laval nozzle is designed,and a supersonic and no condensation flowing model of methane gas is established.Based on the no condensation CFD model,the reasonable range of pressure and temperature parameters of methane gas at the Laval nozzle inlet is determined:temperature>170K,1MPa<pressure<3MPa.（2）According to the principle and algorithm of MD simulation technology,a reasonable MD simulation system was constructed for the condensation process of methane gas.Through MD simulation of the condensation process of methane gas,its microscopic characteristics are obtained:the nucleation process of methane gas is the collision of methane molecules,causing a large number of small clusters to repeatedly form and break.As the small clusters grow,a few larger clusters are formed and stable existence.The larger clusters enter the droplet growth process,which is the process of clusters continued growth and collision and coalescence.Changing the simulation conditions and confirm that higher initial pressure and lower cooling temperature are beneficial to the process of methane gas condensation.The threshold method is used to analyze and summarize the nucleation rate of MD simulation,and compare it with the results of CNT and ICCT nucleation theories,confirming that ICCT nucleation theory has higher accuracy in predicting the nucleation rate.（3）The CFD mathematical model of supersonic condensation flowing of methane gas is established.The accuracy of the CFD mathematical model is verified by the relevant experimental results.The similarities and differences between supersonic condensation flow and no condensation supersonic flow are compared.The simulation results show that the nucleation rate increases sharply from 0 to 2.482×1021/（m3·s）in a short distance in the nucleation stage after the condensation of methane gas occurs.And then due to the release of latent heat,the undercooling and supersaturation of methane gas both can not meet the nucleation requirements so the nucleation process ends.In the droplet growth stage,a large number of gas molecules gather on the surface of the condensation core and continue to grow.The droplet radius continues to increase,but the growth rate gradually slows down.Finally,the liquefaction rate of methane gas at the outlet of the Laval nozzle reached 19.72%.（4）In order to study the promotion mechanism of heavy hydrocarbon core on methane condensation,an MD simulation system of methane-heavy hydrocarbon mixed heterogeneous condensation was established.The MD simulation results of homogeneous condensation and heterogeneous condensation of methane gas quenched to the same low temperature under the same initial pressure and temperature are compared.The results show that the hexane molecules in the heterogeneous condensation system are directly as the condensation cores,it slows down the rapid changes in the nucleation process of methane molecules in the system and reduces the latent heat of condensation released by methane gas,which is conducive to the progress of the methane condensation process.Analysis of the changes in methane potential energy in the two simulation systems shows that the presence of heavy hydrocarbon molecules is reduced the nucleation barrier of methane gas and is conducive to methane gas condensation;the final methane liquefaction rate of heterogeneous condensation system is 10.45%higher than that of homogeneous condensation.And change the simulation conditions,under the condition that the homogeneous condensation of methane gas cannot occur.It indicates that the presence of heavy hydrocarbon molecules reduces the requirement of methane gas condensation on pressure and temperature conditions.（5）In order to apply the promotion effect of heavy hydrocarbon core on methane condensation to supersonic natural gas liquefaction technology,a CFD simulation mathematical model of methane-heavy hydrocarbon mixed heterogeneous condensation was established.Add heavy hydrocarbon condensation/crystallization core to the CFD model for simulation.The simulation results show that the condensation/crystallization core radius formed by heavy hydrocarbon components is generally 10-8～10-10m.Under a reasonable core concentration,the core radius is within this range.The change of radius basically does not affect the occurrence of heterogeneous condensation in the nozzle.Therefore,it is considered that the radius of the condensation/crystallization core formed by heavy hydrocarbon components is within a reasonable range.The higher the concentration of the condensation/crystallization core,the higher the degree of heterogeneous condensation,and when the core concentration increases to a certain value,only heterogeneous condensation occurs in the Laval nozzle.After that,with the further increase of the core concentration,the total liquid phase mass fraction at the Laval nozzle outlet increases obviously,thus achieving the purpose of improving the liquefaction efficiency of methane gas flowing through the Laval nozzle. |
