A review on the latest development of carbon membranes for gas separation

Abstract

Inorganic membranes have been developed before 1945. The earlier application of inorganic membranes was primarily concentrate on military purpose. Carbon membrane is one type of porous inorganic membrane. Although the concept of carbon membrane for gas separation has been found in the early 1970, the interest to develop carbon membrane only increased, since Koresh and Soffer successfully prepared apparently crack-free molecular sieving hollow fiber carbon membranes. Nowadays, plenty of researchers have used different polymeric materials; including polyimides, to prepare carbon membranes by using pyrolysis. In general, carbon membranes can be divided into four major configurations: flat sheet, membrane supported on tube, capillary, and hollow fiber. Permeation properties of carbon membranes have been improved greatly in these 20 years. Carbon membranes offer advantages over polymeric membranes especially in terms of selectivity as well as thermal and chemical stability. More attention will be paid to carbon membranes in this century. This paper will review the development of carbon membranes in the last 30 years and give a clear future direction in research for carbon membrane.

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.