Abstract. We report experimental measurements of pure gas diffusivity of light alkanes in open-ended carbon nanotubes by the ZLC technique using helium as purge gas. The measurements extend a large range of temperature and pressure conditions covering both the low and high ranges of pore filling. Gas composition is monitored on-line using mass spectrometry (MS Dymaxion DM100 from Ametek) through continuous sampling by a silica capillary directly connected to the MS chamber. The whole setup is computer-controlled using a software package developed in LabView. The ZLC cell was built in such a way that its length is variable. In order to operate a more flexible and versatile cell, the small SS vessel (0.82 cm length x 0.48 cm diameter, maximum volume 0.15 cm3) is prepared to adjust the axial length depending on the adsorbent sample weight considered. The setup is composed by a 1/8'' OD SS Swagelok tubing system prepared with two auxiliary vacuum or MS analyzer lines at both feed and exit ends. Dead volumes were minimized in all the apparatus. An absolute pressure transducer is coupled to the unit for pressure measurements. Two mass flow meters/controllers (MFC) Teledyne Hastings HFC-202 (1% of FS error) are coupled to the feed line: one for the pure gas and other for the inert gas. The ZLC cell is immersed in a water-circulating bath Selecta Ultratherm and the rest of the line is insulated with a heating cable for temperature control of the gas. A Pt100 temperature probe measures the temperature and sends the signal to a digital controller Ero Electronic that actuates in the cable. This ensures temperature control over the ZLC cell surrounding and allows the adsorbent regeneration in situ.

Zero length column (ZLC) technique is a powerful and well-known technique for diffusivity measurement of pure gases on porous solids (Ruthven et al., 1988, 2005, Rodrigues et al., 1996, Brandani et al., 2003, 2005). The procedure to measure the diffusivities of pure gases on the CNTs follows the standard technique. The sample (1-10 mg) is first saturated with the mixture of the adsorbate until equilibration, and secondly, the carrier stream was fed at constant flow rate to the ZLC cell to clean the adsorbent. Simultaneously, the composition of the effluent is monitored using an on-line quadrupole MS detector. From the desorption stage, the pure gas diffusivity parameters are extracted. The results are analyzed using single-file diffusion and Maxwell-Stefan formulations.