Study On Separate Carbon Dioxide From Natural Gas Based On Clathrate Hydrate Technology

Gas hydrates （clathrate hydrates） are crystalline solid structures consisting of water andsmall gas molecules such as CH₄, C2H6, CO₂, N₂, H₂, H₂S, et al. which are formed underconditions of low temperature and specified （generally high） pressure. A clathrate is astructure composed of a molecule or molecules of one or several components （guestmolecules） which are enclosed within a cage built from molecules of anther component （hostmolecules）. The majotity of gas hydrates are known to form three typical hydrate crystalstructures: structure Ⅰ （sⅠ）, structure Ⅱ （sⅡ）, and structure H （s H）. The temperature andpressure, gas formation hydrate, are different for special gas，which is theoretical foundationof separating mixture gas with hydrate.Separation mixture gas using hydrate is a new type separation method. The phaseequilibrium conditions of CH₄and CO₂forming hydrate are different, CO₂can form hydrateeasier than CH₄, so CO₂/CH₄mixture gas can be separated by formation hydrate.Hydrate-based separation method has the characteristics of low energy consumption, easyoperation technology and environment friendly, et al. At present, hydrate-based separationmethod was still at the initial stage. Many problems, such as the high pressure of operation,the high rate of forming hydrate, the low separate efficiency, et al, need to be solved. Thiswork measured the phase equilibrium conditions of CO₂/CH₄mixture gas forming hydrate,and the change of phase equilibrium condition in the presence of tetra-n-butyl ammoniumbromide （TBAB）, tetra-n-butyl ammonium chloride （TBAC）, and tetra-n-butyl ammoniumfluoride （TBAF）. The process of hydrate forming was studied. The separate efficiency wasalso discussed, as well as the influence of TBAB.The phase equilibrium conditions of CO₂（33.00mol%）/CH₄mixture gas were measured inan isochoric reactor. The phase equilibrium conditions in the presence of TBAB, TBAC andTBAF were also measured. According to the data, when the concentration of TBAB, TBACor TBAF is0.293mol%, the phase equilibrium conditions can be reduced （increasetemperature and reduce pressure）, TBAF was the best promoter among the3additives forCO₂/CH₄mixture gas forming hydrate.The properties of CO₂/CH₄mixture gas hydrate-based separation were studied, and the system adding TBAB as promoter was compared. The experiments were operated at thecondition, which the CH₄concentration in initial mixture gas was67.00mol%and the stirringspeed was580RPM. The effects of the system temperature, the feed pressure, and gas liquidvolume ratio on mixture gas hydrate-base separation were studied.The process of CO₂/CH₄mixture gas forming hydrate was studied, when the temperaturewas4.7⁵.0℃, and in the presence of0.293mol%TBAB TBAC and TBAF. The parameters（such as system temperature, mixture gas pressure, the concentration of mixture gas and thespace velocity of reaction） were analysed. Respectively, at the same reaction conditions, theamount of temperature rised was2.0℃,1.1℃and0.9℃for TBAB TBAC and TBAF solution.The final CH₄concentration was also located at80.60-81.10mol%. The highest point ofspace velocity of three systems:132h^-1for TBAB systems,48h^-1for TBAC system and87h^-1for TABF system.