Gas Separations with Inorganic Membranes

In this last section some recent developments are mentioned in relation to gas separations with inorganic membranes. In porous membranes, the trend is towards smaller pores in order to obtain better selectivities. Lee and Khang (1987) made microporous, hollow silicon-based fibers. The selectivity for Hj over Nj was 5 at room temperature and low pressures, with permeability being 2.6 x 10 Barrer. Hammel et al. 1987 also produced silica-rich fibers with mean pore diameter 0.5-3.0nm (see Chapter 2). The selectivity for helium over methane was excellent (500-1000), but permeabilities were low (of the order of 1-10 Barrer). 

Ulhom, R. J. R., and Burggraaf, A. J., Gas separation with inorganic membranes, in Inorganic Membranes (R. R. Bhave, ed.), Van Nostrand-Reinhold, New York,... 

Fain DE (1991) Technical and economic aspects and prospects for gas separation with inorganic membranes. Key Eng Materials 61/62 327-336... 

Microfiltration with uniform transmembrane pressure Commercial and developing liquid phase applications Gas separations using inorganic membranes... 

Although the literature of gas separation with microporous membranes is dominated by inorganic materials, polymer membranes have also been tried with some success. The polymers used are substituted polyacetylenes, which can have an extraordinarily high free volume, on the order of 25 vol %. The free volume is so high that the free volume elements in these polymers are probably interconnected. Membranes made from these polymers appear to function as finely microporous materials with pores in the 5 to 15 A diameter range [71,72], The two most... 

This chapter will only deal with the possible gas transport mechanisms and their relevance for separation of gas mixtures. Beside the transport mechanisms, process parameters also have a marked influence on the separation efficiency. Effects like backdiffusion and concentration polarization are determined by the operating downstream and upstream pressure, the flow regime, etc. This can decrease the separation efficiency considerably. Since these effects are to some extent treated in literature (Hsieh, Bhave and Fleming 1988, Keizer et al. 1988), they will not be considered here, save for one example at the end of Section 6.2.1. It seemed more important to describe the possibilities of inorganic membranes for gas separation than to deal with optimization of the process. Therefore, this chapter will only describe the possibilities of the several transport mechanisms in inorganic membranes for selective gas separation with high permeability at variable temperature and pressure. 

Research and development activities associated with inorganic membranes as gas separation media have flourished in recent years. Both material and engineering fronts have advanced. There are, however, several technical issues that need to be addressed before the full potentials of some inorganic membranes can be realized. Clearly, a more systematic and coordinated approach will faciliute the material and application development. This will be discussed below. 

Molecular sieving and the interactions of gas molecules with the membrane are possible alternatives. As discussed in Chapter 4, if surface diffusion is operative on a gas but not the other, it can enhance the separation factor. Although surface diffusion contribution decreases with increasing temperature, it becomes more important as the pore diameter becomes smaller. Therefore, it is possible that as inorganic membranes with smaUer pore sizes become available their separation performance may increase not only due to molecular sieving effects but also surface diffusion or other transport mechanisms. 

Javaid A and Ford DM. Solubility-based gas separation with oligomer-modified inorganic membranes. Part II Mixed gas permeation of 5 nm alumina membranes modified with octadecyltrichlorosilane. J. Membr. Sci. 2003 215 157-168. 

Javaid A, Krapchetov DA, Ford DM (2005) Solubility-based gas separation with oligomer-modified inorganic membranes—part III. Effects of synthesis conditions. J Membr Sd 246(2) 181-191... 

Some of the available membrane separation processes can already be applied on an industrial scale. Hence, inorganic ceramic membranes (zeolites and their derivatives, e.g. silico aluminophosphates), organic polymer membranes and facilitated transport membranes, which rely on a carrier molecule with high CO2 affinity to achieve selective CO2 transport (such as metallic ions or liquid amines), have been used in separating CO2 from flue gas in post-combustion. As single-stage separation with these membranes is still difficult, new membrane materials are being developed [1]. Typically, the initial separation of carbon dioxide accounfs for 60-80% of the total cost of CO2 sequestration [24,25]. 

In this regard, there is an excellent review article on MMMs for gas separation, with a detailed discussion on the morphology of the interface between the inorganic particles and the polymer matrix (Chung et al. 2007). Unlike many other articles, this deals with asymmetric membranes for both flat sheets and hollow fibers aimed at the formation of an ultrathin defect-free mixed-matrix skin layer. 

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