By using grand canonical ensemble Monte Carlo method, machanism for the adsorption of methane in single wall carbon nanotubes(SWNTs) at 74.05 K has been investigated. It is found that the adsorption of methane in smaller SWNTs, whose diameters are 1.22nm and 1.632nm, exhibits "pore filling" behavior. In contrast, for the SWNTs of diameter larger than 2.04nm, capillary condesation takes place. Simulation shows that the required condition for the capillary condensation is that two layers of molecules can be accommodated in SWNTs. In addition, a relationship between the quantity of methane adsorbed and the diameter of SWNT for the capillary condensation is developed. A comparison of the isotherms for the SWNT diameters of 2. 04 nm and 4.077nm at ambient temperature, T=300 K, indicates that proper adsorption pressure range is 5.0～6.0 MPa. In this case, the weight fraction of adsorbate is about 16%～19%.

Key words: METHANE, ADSORPTION, COMPUTERIZED SIMULATION, MONTECARLO SIMULATIONS