Experimental And Theoretical Study Of Carbon Dioxide Solubility In Liquefied 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, CO₂ in feed gas need tobe removed to about 50ppm. The area of CO₂ removing device occupies alarge part of whole LNG plant. The solubility of CO₂ in LNG will increaseto more than 0.5% when the liquefaction temperature rises to145K(corresponding to pressure about 0.8MPa). Therefore, PLNGtechnology will make it possible to cancel the CO₂ removing device andenhance the economics of offshore natural gas liquefaction plant.Obviously, the application of PLNG process depends on accurate CO₂solubility data, which are still very lacking. In this work, the author putsfocus on the solubility of CO₂ in LNG from both experimental test andtheoretical calculation, and the following work has been done:1. A solid-liquid equilibrium (SLE) experimental apparatus has beendesigned and built, which is used to measure the solubility of CO₂ in LNG.To give the confidence of the SLE data, the uncertainty for themeasurements of temperature, pressure and composition were analyzed. 2. The SLE data of the CH₄+CO₂ binary system were measured in PLNGtemperature region (112-170K) and they had a high degree of agreementwith the data from Davis et al. The average absolute deviation (AAD) isonly 4.14%. In addition, measurements of ternary system CH₄+N₂+CO₂and CH₄+C2H6+CO₂ were completed. Results show that the solubility ofCO₂ in CH₄+N₂ mixtures increases with the addition of nitrogen contentwhen T<140K; with the temperature increases, the solubility of CO₂decreases with the increase of nitrogen content. While in liquid CH₄+C2H6mixtures, it increases with the increase of ethane content in the wholetemperature region.3. The solution method and the equations-of-state (EOS) were selected tocalculate the solubility of carbon dioxide in saturated liquid methane in thispaper. By comparison with experimental data, the results from EOSmethod were closer to the experimental data than those from solutionmethod, and the EOS method was recommended for the solubilitycalculations. Additionally, PR EOS and SRK EOS were selected tocalculate the solubility of carbon dioxide in CH₄+N₂ and CH₄+C2H6mixtures and the theoretical results are close to the experimental data.4. In order to establish more accurate theoretical model, the binaryinteraction coefficient kij as the functions of the temperature were obtainedfrom experimental data. And the results calculated by this new interactioncoefficient were closer to the experimental data than the former results.Therefore, a more suitable SLE theoretical calculation model for binarysystem CH₄ + CO₂ in 112-170K temperature range was obtained. Moreover,the new kij can also improve the calculation accuracy of ternary systems.