Research On Anti-sublimation Characteristics Of Carbon Dioxide In Natural Gas

Liquefied natural gas （LNG） technology used for the recovery ofoffshore natural gas （including associated gas） resources has become aresearch hotspot at home and abroad in recent years. However, the highcost of offshore platforms limits the practical application of LNG FloatingProduction Storage&Offloading plants （LNG-FPSO）. Accordingly,reducing the area of liquefaction plants will be the key of the maritimeLNG production.In conventional natural gas liquefaction process, CO₂in feed gasneed to be removed to about50ppm. The area of CO₂removing deviceoccupies a large part of whole LNG plant. The solubility of CO₂in LNGwill increase to more than0.5%when the liquefaction temperature rises to145K（corresponding to pressure about0.8MPa）. Therefore, PLNGtechnology will make it possible to cancel the CO₂removing device andenhance the economics of offshore natural gas liquefaction plant.Obviously, the application of PLNG process depends on CO₂anti-sublimation data, which are still very lacking. In this work, the authorputs focus on the anti-sublimation of CO₂in LNG from both experimentaltest and theoretical calculation, and the following work has been done:1. The calculation of CO₂frosting temperature in CH₄-CO₂binarysystem based on gas-solid phase equilibrium model is conducted by PRequation of state （EOS）. The predicted value of PREOS is compared withexperimental data. By comparison, the results show that three methods arein good agreement with experimental data.2. Taking the idea that removes CO₂by anti-sublimation in apressurized liquefied natural gas （PLNG） process as the background, this paper analyzes CO₂anti-sublimation heat transfer in natural gas flow in atube heat exchanger. Natural gas is simplified as CH₄+CO₂binary mixture.A mathematical model of this heat transfer process is built, and thefourth-order Runge-Kutta method is used to solve the differentialequations. Some affects such as the flow velocity of natural gas and N2、the inlet CO₂concentration and the temperature of inlet nitrogen on theCO₂anti-sublimation heat transfer process are analyzed under the first、second and third class boundary condition.3. A heat transfer experimental apparatus has been designed and built,which wasused to analyze CO₂anti-sublimation heat transfer in natural gasflow. To give the confidence of the anti-sublimation heat transfer data, theuncertainty for the measurements of temperature, pressure andcomposition were analyzed.