427521 Assessment of the O2/N2 Separation Ability of Carbon Molecular Sieves By Spontaneous Liquid-Gas Imbibition

Monday, November 9, 2015: 3:40 PM
255D (Salt Palace Convention Center)
Xinyu Jia, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian, China

Assessment of the O2/N2 separation ability of carbon molecular sieves by spontaneous liquid-gas imbibition

Xinyu Jia,Yuanyuan Cong,Shaoping Xu* 

State Key Laboratory of Fine Chemicals, Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, P.R.China

(*Corresponding Author's E-mail: huizixu@hotmail.com)

ABSTRACT

Objective:

The kinetics of spontaneous water-oxygen or nitrogen imbibition at 303.2K on carbon molecular sieves (CMSs) were carried out to establish an assessment method for the O2/N2 separation performance of the CMSs in pressure swing adsorption (PSA).

Method:

The spontaneous water-gas imbibition was conducted this way: in the sample holder with a constant volume, CMS particles saturated with gas were immersed into water at a given temperature, and the pressure of the gas with time on stream, which is a measure of the volume of the gas recovered by water, is recorded. The results of the spontaneous water-oxygen imbibition on eight CMSs are shown in Fig.1.

Fig. 1. Kinetics of the spontaneous water-oxygen imbibition at 303.2K on CMS-1CMS-8.

Results and discussion:

The spontaneous water-gas imbibition on CMSs involves the surface adsorption of water, the diffusion of the gas along the micropores, and the diffusion of the gas through the barrier at the micropore entrance of the CMS. To elucidate the rate-controlling step of the process, the pseudo-second-order (PSO) kinetic model and the linear driving force (LDF) model has been used to describe the spontaneous water-gas imbibition.

For CMS-1CMS-6, the kinetics of the water-oxygen imbibition obeys the PSO model, indicating the surface adsorption is the rate-controlling step. On the contrary, for CMS-8 the diffusion of oxygen through the barrier at the micropore mouth is the rate-controlling step with the kinetics following the LDF model. For CMS-7, up to 200s, the dynamics obeys the LDF model, and over 200s it follows the PSO model. Hence, the size distribution of the micropore mouths for CMS-7 is far less uniform than that of other CMSs. The Kinetic parameters of the two models demonstrate that the average sizes of the micropore mouths are in the order of CMS-1>CMS-2>CMS-3>CMS-7>CMS-4>CMS-5> CMS-6>CMS-8.

The kinetics of the water-nitrogen imbibition on the CMSs indicate that CMS-1 and CMS-2 fit with the PSD model and the physical adsorption process is the rate-controlling step. For CMS-3CMS-5, the rate-controlling step is mainly the diffusion of nitrogen through the micropore mouth. For CMS-6 and CMS-8, their micropore sizes are small enough to make the kinetics of the process controlled by not only the barrier resistance at the micropore mouth but also the diffusion in the micropore interior. And for CMS-7, the kinetics is controlled by all three steps mentioned above. In addition, the same order of the average sizes of the micropore mouths of the CMSs are  obtained from the kinetics of the water-nitrogen imbibition as that of the water-oxygen imbibition.

An assessment method for the O2/N2 separation ability of CMSs has been established, which has been verified by the PSA of air:

The micropore mouths of CMSs such as CMS-1 are too large to separate O2/N2 sufficiently on which the rate-controlling steps of the water-O2 or N2 imbibition are both the surface adsorption.

The micropore mouths of CMSs are too small to adsorb O2 within a short time in SPA cycles leading to lower selectivity of O2/N2. Since the diffusion in the micropore interior is one of the rate-controlling steps in the water-nitrogen imbibition on them.

CMSs whose micropore mouths dimensions are not uniform as CMS-7 are unsuitable for the kinetic separation of O2/N2.

CMSs owning proper and uniform micropore mouths dimensions such as CMS-3CMS-5 are favorite adsorbents for the O2/N2 separation by PSA.


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