Simulation And Optimization Of Methane-ethane Separation Process

As a kind of clean fossil fuel,natural gas is economical and safe,and plays a more and more important role in the energy structure of the world.Methane is the main component of natural gas,and its mole fraction can reach 90%.In addition,natural gas also contains ethane,propane and other light hydrocarbons.These substances are high-quality chemical raw materials.Therefore,it is necessary to recover the light hydrocarbons in natural gas,especially the ethane with a higher content,to realize the full utilization of the natural gas.In this thesis,the traditional cryogenic separation process of methane and ethane is studied firstly.An energy integrated process of Liquefied natural gas（LNG）light hydrocarbon recovery and air separation is proposed,and technical and economic analysis and parameter optimization are carried out.LNG is the main form of long-distance transportation of natural gas,which contains not only light hydrocarbons,but also much cold energy produced by the process of regasification.Based on the idea of cold energy cascade utilization,this thesis combines two kinds of LNG cold energy utilization processes:light hydrocarbon recovery and air separation,which are most widely used in LNG terminals in China.The energy integrated process can not only improve the utilization ratio of LNG cold energy,but also realize the recovery and utilization of ethane and other light hydrocarbons.The utilization ratio of LNG cold energy,the ethane recovery ratio and the partial annual cost（PAC）are the optimization objectives of the process.The optimal process conditions are determined by sensitivity analysis and multi-objective optimization.Results show that utilization ratio of LNG cold energy is 67.05%,ethane recovery ratio is 99.76%,and PAC is 3.144×10⁷ USD/year.After that,a new separation process of methane and ethane,Zeolitic Imidazolate Framework-8（ZIF-8）slurry absorption-adsorption separation process is studied.Based on the experimental data,a single equilibrium stage model for the separation process is established.Since the absorption amount of gas in slurry is very little at 273.15 K,only the adsorption process of ZIF-8 is considered,so that the dissolution of methane and ethane in ZIF-8 slurry is regarded as a quasi adsorption process.The improved Langmuir isotherm adsorption equation is used to express the phase equilibrium relationship between mixture gas and ZIF-8 slurry.The parameters of the equation are fitted according to the phase experiment data of pure component gas and mixture gas respectively.There are two steps in the process of model construction.First,only phase equilibrium equation,material balance equation and fraction summation equation are considered;then,the heat balance equation is included to form the complete MESH equations.The simulation deviation of single equilibrium stage model with different MESH equations is slightly different.On the whole,the simulation deviation AARDy₁₂ is about 5%,and AARDx₁₂ is about 2%to 4%,which shows that the simulation results are accurate.Finally,on the basis of single equilibrium stage model,the whole process model of ZIF-8 slurry adsorption-absorption separation of methane and ethane is established,which mainly includes two parts:one is absorption-adsorption column and flash tank model,the other is desorber model.The improved Langmuir equation is used to describe the phase equilibrium relationship in the whole process model.Because the parameterλof the equation is related to the pressure,the adsorption experiment data of the mixture gas at 0.1 MPa and 0.5 MPa are measured to fit the parameterλ.The sensitivity analysis is used to determine suitable operation conditions.Results show that the whole process works at 273.15 K,the absorption-adsorption column includes 6 stages,its operation pressure is 1 MPa,the feed gas is fed on the 5^th stage,the gas slurry volume ratio of the feed system is 70,the operation pressure of the flash tank is 0.1 MPa,and the operation pressure of the desorber is 0.01 MPa.The ethane recovery ratio reaches 90.97%,the ethane fraction of ethane product gas is 94.38 mol%,and the energy consumption per unit volume of ethane product is 0.7999 k W·h/Nm³.The whole process model demonstates the feasibility of recovering ethane from natural gas by ZIF-8 slurry method,and guide the design and operation of pilot plant and scale-up of the industrial process.