Calculation And Simulation Study On The Preparation Of Acetylene By Partial Oxidation Of Natural Gas

Partial oxidation of natural gas is the main process technology for the industrial production of acetylene（C₂H₂）.Typical downstream processes using acetylene as feedstock include the production of 1,4-butanediol,acetaldehyde,acrylic acid,acrylonitrile,and polyvinyl chloride,etc.This process can co-produce syngas（CO and H₂）while preparing acetylene.Syngas can further use in the synthesis of other high-value chemicals.At the same time,because no catalyst is needed,the partial oxidation process is very advantageous in long-time running and low-cost maintenance.In recent years,this process technology becomes increasingly important with the rapid development in the exploitation of shale gas and other types of non-conventional natural gas.At present,in actual production,the yield of acetylene is lower than the expected value,which results in low raw material utilization,high energy consumption,and high production cost.These will restrict the development of its industry.Therefore,to achieve green development and technological upgrading,it is urgent to carry out technological innovation study to increase the utilization rate of raw materials,enhance the yield of acetylene,and reduce energy consumption.However,due to the severe conditions used in the process of acetylene generation,the vast quantity of free radicals and other species involved in this reaction system,it is very difficult to get detailed data by experiments.Therefore,calculation and simulation methods are widely used in the study of this process.In this paper,the detailed reaction process of natural gas to acetylene is studied by using density functional theory,thermodynamic analysis and reactive force field molecular dynamics.The main conclusions are included below:（1）Theoretical study on pyrolysis of natural gas to acetylene at high temperatures.High temperature pyrolysis is a commercial process to convert natural gas（the main component is methane）to acetylene.The process of methane pyrolysis to acetylene at high temperatures was investigated using thermodynamic analysis,density functional theory,and reactive force field（Reax FF）molecular dynamics method.Thermodynamic calculation analyses show that the pyrolysis of methane to acetylene is advantageous when the temperature is greater than 1500 K.The high temperatures and low pressures are beneficial to the conversion of methane to acetylene.The preferred pyrolysis temperature is about 1800 K and the initial pressure is about 70～80 k Pa.Reax FF molecular dynamics show that the pre-exponential factor and the activation energy are1.16×10¹³s^-1 and 234.30 k J/mol respectively at temperatures from 2750 K to 3750 K for methane pyrolysis.The mechanism analyses through the trajectories find that the main formation pathways of C₂H₆ originate from the dimerization of CH₃ and the collision of CH₄ and CH₃.The reactions of C₂H₆ homolysis to CH₃ and its dehydrogenation to C₂H₅are the two main consumption pathways of C₂H₆.For C₂H₄ produced by pyrolysis,the main formation pathways are the dehydrogenation of C₂H₅ and the hydrogenation of C₂H₃.Its main consumption pathways are its dehydrogenation to C₂H₃and the hydrogenation to C₂H₅.For the product C₂H₂,there are many formation pathways.The dominant reactions are the cleavage of C₂H₃ and the C-C fission of C₃H₅.The hydrogenation of C₂H₂ and the reactions with CH₂ or CH₃ are the main consumption pathways for C₂H₂.It can be found that the accumulation of C₂H₂ quantity is closely related to the reactions of C₂H₃ free radicals.The analysis of the high-temperature pyrolysis reactions of pure methane will help to understand the reaction process in an oxygen-containing environment.（2）Calculations and simulations of the partial oxidation of methane to produce acetyleneThe Reax FF molecular dynamics simulation method is used to study the process of partial oxidation of methane to acetylene under different parameter conditions(system density,heating time,preheating temperature,isothermal temperature,and the ratio of n（O₂）/n（CH₄）.The calculation results show that the process is mainly divided into two stages.The first is the oxidation reaction stage,which mainly generates many free radicals and releases a lot of heat.The second stage is mainly pyrolysis and reforming reactions,which will promote the accumulation of C₂H₂ amount and the production of other species.For C₂H₂,it is formed in both stages.In the oxidation stage,its precursors are mainly C₂H_nO（n=1,2,3）and C₂H₃ free radicals.In the pyrolysis stage,C₂H₃ is the mainly precursor.C₂H_nO（n=1,2,3）originates from CH₃CHO and its isomers CH₂=CH-OH.CH₂=CH-OH and C₂H₃ can be mutually converted in the process of partial oxidation.This shows that C₂H₃ is still very important for the formation of C₂H₂ in the partial oxidation process.For CH₂O,it is not only an important intermediate of methane oxidation,but also critical to the formation of acetylene precursor C₂H_nO（n=1,2,3）.The simulation results show that the maximum yield of acetylene under different parameters and its appearance time are mainly determined by the species produced during the heating stage.If the amount of C₂H₂,C₂H_nO（n=1,2,3）and C₂H₃ radicals accumulated more in heating stage,the higher yield of acetylene will finally be obtained in a shorter time.The maximum acetylene yield and selectivity are 10.5%,and 35.2%,respectively under the conditions that the ratio of n（O₂）/n（CH₄）is 0.56,the system density is 0.20 g/cm³,the preheating temperature is 873 K,the heating time is500 ps,and the continuous temperature is 3900 K.These results are evidently higher than the yield and selectivity in actual process,which provide a reference for the optimization of process parameters.