Preparation and modification of hollow fiber type carbon molecular sieve membranes for gas separation

ガス分離用中空糸型分子ふるい炭素膜の作製と改質

Preparation and modification of hollow fiber type carbon molecular sieve membranes for gas separation

Degree Grantors Yamaguchi University

Rapid population growth and economic progress over the past decades have triggered a sharp increase in the global demand for fossil fuels thereby resulting in an energy crisis. The problem may be alleviated by upgrading and producing gaseous energy, but one of the major challenges associated with gaseous energy is to separate it effectively from other less desirable gases. Thus, energy-saving and high-efficiency separation technology is needed. In the past three decades, gas separation membranes, including polymeric membranes and inorganic membranes, have attracted much attention due to their advantages in terms of energy efficiency, operational simplicity, cost competitiveness, and small footprint. Although polymeric membranes have been utilized in practical gas separation, their separation performance is not sufficient for widespread practical application. Carbon molecular sieve (CMS) membranes, one of the inorganic porous membranes, can be prepared by pyrolyzing polymeric precursors.Their pore structures provide molecular sieving ability and possess good thermal and chemical resistance. Especially, the separation properties of the CMS membranes for a variety of gas pairs have exceeded the upper bound of polymeric membranes. These characteristics have made them attractive candidates for gas separation.
The pore structures, separation properties, and transport mechanism of the CMS membranes depend critically on the type of the polymeric precursors, pyrolysis conditions and pre- and post-treatments. Thus, in this thesis, I prepared CMS membranes derived from different polymeric precursors and investigated the effect of pyrolysis conditions and post-treatment on the gas permeation properties.
In Chapter 2, toluene vapor addition was performed for the first time during the pyrolysis process to prepare highly selective CMS membranes. Adding toluene vapor in the pyrolysis process was a simple method to improve the selectivity compared with the traditional chemical vapor deposition post-treatment technique. Additionally, the use of toluene can avoid the high transportation costs of gaseous hydrocarbons. The results indicated that toluene vapor addition increased selectivities of the H2-related gas pairs compared with CMS membranes without toluene vapor addition. This could not be realized simply by increasing the pyrolysis temperature without toluene vapor addition. The CMS membrane with toluene vapor addition also showed higher permeance with a moderate selectivity compared with the CMS membrane with gaseous hydrocarbon addition reported in the literature. Furthermore, the gas permeance and selectivity could be readily controlled by adjusting the pyrolysis temperature and duration of the addition. The optimal preparation conditions of the CMS membrane with toluene vapor addition depend on the targeted gas pair to be separated. This study indicated that adding liquid hydrocarbon vapor in the pyrolysis process can be a simple and effective method for preparing highly selective CMS membranes.
In Chapter 3, the mechanism of achieving high selectivity for the CMS membranes prepared by adding toluene vapor was further investigated. The physical and chemical properties of CMS membrane with adding toluene vapor were characterized using some sophisticated characterization techniques. It was found that toluene vapor addition formed carbon deposition on the outer surface region of the CMS membrane, which agreed with the previous report. The gas adsorption experiment suggested that toluene vapor addition also resulted in the loss and the narrowing of ultramicroporosity. Additionally, I preliminary analyzed the mechanism of achieving high selectivity for the CMS membrane with toluene vapor addition.
In Chapter 4, a novel porous carbon fiber (PCF) was investigated to prepare supported CMS membranes derived from wood tar solution, as the development of wood tar-derived CMS membranes has been limited by the availability of porous supports in recent years. Moreover, the CMS membranes supported on commercially available porous ceramic tubes were also prepared under the same conditions for comparison purposes. The PCF consisted of interconnected pore structures, which provide additional paths and channels for gas transport, whereas the porous structure of the ceramic support consisted of voids between the alumina particles. It was found that for both supports, 70 wt% wood tar solution was the optimal solution for preparation of CMS membranes. The PCF-supported CMS membranes exhibited higher gas permeance and selectivity than the NA3-supported membranes. Furthermore, a series of PCF-supported CMS membranes from 70 wt% wood tar solution were prepared at different pyrolysis temperatures, the membrane pyrolyzed at 600 °C exhibited the highest H2 selectivity. This study demonstrated that PCF can be used for supported CMS membranes derived from wood tar solution. Additionally, PCF is also a promising support for the supported CMS membranes derived from other polymeric precursors.
Finally, Chapter 5 summarized the main contents of this thesis.

Creators NIE JING