ReviewA review on the latest development of carbon membranes for gas separation
Introduction
The development of porous inorganic membranes dates from before 1945, long before the development of today’s synthetic organic membranes. Not much publicity was given to the early development of inorganic membranes because the first porous inorganic membranes were developed for separation of uranium isotopes, therefore, they were mainly used for military purposes or nuclear applications [1]. Non-nuclear applications of inorganic membranes started at the beginning of the 1980s with Membralox produced by Ceraver (now SCT), Carbosep produced by SFEC (now TECHSEP) and Ceraflo produced by Norton (now by SCT) [2]. The potential of inorganic membranes was not widely recognized until high quality porous ceramic membranes were produced for industrial usage on a large scale [3]. Nowadays, inorganic membranes are used primarily for civilian energy-related applications. They have become important tools for beverage production, water purification and the separation of dairy products [1]. In addition, they play a significant role in the gas separation processes of industrial sector. Numerous European, American and Japanese companies are now competing to produce inorganic membranes [2].
Hsieh has provided a technical overview of inorganic membranes in his paper [3]. He divided the inorganic membranes into two major categories based on its structure: porous inorganic membranes and dense (non-porous) inorganic membranes as shown in Fig. 1. Besides that, porous inorganic membranes have two different structures: asymmetric and symmetric. Porous inorganic membranes with pores more than 0.3 nm usually work as sieves for large molecules and particles. Glass, metal, alumina, zirconia, zeolite and carbon membranes are commercially used as porous inorganic membranes. Other inorganic materials, such as cordierite, silicon carbide, silicon nitride, titania, mullite, tin oxide and mica also have been used to produce porous inorganic membranes. These membranes vary greatly in pore size, support material and configuration. On the other hand, dense membranes made of palladium and its alloys, silver, nickel and stabilized zirconia have been used or evaluated mostly for separating gaseous components. Application of dense membranes is primarily for highly selective separation of hydrogen and oxygen; transport occurs via charged particles. However, the dense membranes have limited industrial application due to their low permeability compared to porous inorganic membranes. Therefore, today’s commercial inorganic membranes market is dominated by porous membrane [1], [2], [3].
Although inorganic membranes are more expensive than organic polymeric membranes, they possess advantages of: temperature and wear resistance, well-defined stable pore structure, and chemically inertness. These advantageous characteristics encouraged many researchers in the early 1980s to investigate the gas separation properties of these membranes, especially porous inorganic membranes. Furthermore, many studies regarding applications of inorganic membrane reactors have been carried out [1].
At present, interest in the development of porous inorganic membranes providing better selectivity, thermal stability and chemical stability than polymeric membranes has grown. The attention has focused on materials that exhibit molecular sieving properties, such as silica, zeolites and carbon [4], which appear to be promising in separation of gas as shown in Fig. 2. Silica-based inorganic membranes selectively separate hydrogen from other gases but permselectivity between similar-sized molecules, such as oxygen and nitrogen is not sufficient [5]. Zeolites can separate isomers, but it is difficult to obtain a large, crack-free zeolite membrane. Hence, it is more feasible to form carbon molecular sieve membranes [5], [6]. Therefore, the purpose of this paper is to give an overview regarding development of carbon molecular sieve membranes in the past 30 years. This paper also looks towards the future direction of carbon membranes development in the new millennium.
Section snippets
Transport mechanism of carbon membranes
Mass transfer of gas through a porous plug can involve several processes, depending on the nature of the pore structure and the solid [8]. There are four different mechanisms for separation of a gas mixture through a porous membrane: Knudsen diffusion, partial condensation/capillary condensation, surface diffusion/selective adsorption and molecular sieving [3], [9]. The predominant transport mechanism of most carbon membranes is molecular sieving as shown in Fig. 3. The carbon membranes contain
Carbon membranes
The concept of carbon membrane or film for gas separation can be found in the early 1970. Ash et al. compressed non-porous graphited carbon into a plug, called as carbon membrane [18]. Bird and Trimm used polyfurfuryl alcohol (PFA) to prepare unsupported and supported carbon molecular sieve membranes. During carbonization, they met shrinkage problem, which lead to cracking and deformation of the membrane. Hence, they failed to obtain continuous membranes [8].
Carbon molecular sieves produced
Configurations of carbon membranes
Carbon membranes can be divided into two categories: unsupported and supported carbon membranes [4]. Unsupported membranes have three different configurations: flat (film), hollow fiber, and capillary while supported membranes consisted of two configurations: flat and tube. Fig. 6 shows the configurations of carbon membranes.
Application of carbon membranes
The most important large application of carbon membrane is in the production of low cost and high purity nitrogen from air. Other separations are hydrogen from gasification gas and purification of methane [36]. In addition, carbon membranes are used to recover a valuable chemical (H2) from a waste gas without further compression of the feed gas while rejecting a substantial portion of the hydrocarbons [9].
Carbon membranes are promising candidates for the separation of light alkenes/alkanes
Advantages of carbon membrane compare with polymeric membrane
- 1.
Carbon membranes display superior permeabilities-selectivity combination than polymeric membranes [11], [13], [14], [35], [58].
- 2.
Carbon membranes are mechanically much stronger and can withstand higher pressure differences for a given wall thickness [11]. Carbon membranes have higher elastic modulus and lower breaking elongation than the polyimide membranes [58].
- 3.
The permeation properties of carbon membranes are hardly affected by the feed pressure [12], [58] because carbon membranes do not
Disadvantages of carbon membranes
Carbon membrane is very brittle and fragile. Therefore, it requires more careful handling [11], [35], [65]. This may be avoided to a certain degree by optimizing precursors and preparation methods [35]. Therefore, it is difficult to process and expensive to fabricate carbon membranes [65].
Carbon membranes require a pre-purifier for removing traces of strongly adsorbing vapors, which can clog up the pores due to the transport is through a pore system rather than through the bulk system. This is
Current research and future direction
At this century, membrane systems process more than 4000 million m3 of gas annually [64]. Increasing interest in gas separation by organic membranes had lead to exploitation of inorganic membranes for high temperature or corrosive gas separation applications [3]. Nowadays, inorganic membrane producers are generally in the start-up and technology push stage. Meanwhile, the end-user industry has exhibited a “wait-and-see” attitude when it comes to adopting advanced inorganic membrane
Conclusions
Increasing research of carbon membrane technology indicates that carbon membranes definitely will become another alternative for industry separation process. They consist of four major configurations: flat, supported on tube, capillary and hollow fiber. Carbon membranes have great potential to be used widely in the gas separation processes, especially carbon hollow fiber membranes. Nowadays, hollow fibers are the most used membrane geometry due to their high surface area per unit volume of
Acknowledgements
The authors would like to express sincere gratitude to professor William J. Koros for his insightful comments and valuable contributions towards the completion of this paper. The authors gratefully acknowledge the financial support from the Ministry of Science, Technology and Environment, Malaysia.
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