THE SEPARATION OF GASES BY MEMBRANES

The study of gas transport in membranes has been actively pursued for over 100 years. This extensive research resulted in the development of good theories on single gas transport in polymers and other membranes. The practical use of membranes to separate gas mixtures is, however, much more recent. One well-known application has been the separation of uranium isotopes for nuclear weapon production. With few exceptions, no new, large scale applications were introduced until the late 1970 s when polymer membranes were developed of sufficient permeability and selectivity to enable their economical industrial use. Since this development is so recent, gas separations by membranes are still less well-known and their use less widespread than other membrane applications such as reverse osmosis, ultrafiltration and microfiltration. In excellent reviews on gas transport in polymers as recent as 1983, no mention was made of the important developments of the last few years. For this reason, this chapter will concentrate on the more recent aspects of gas separation by membranes. Naturally, many of the examples cited will be from our own experience, but the general underlying principles are applicable to many membrane based gas separating systems. 

Membranes for the separation of gas mixtures are of two very different kinds one a microporous membrane, the other nonporous. Microporous membranes were the first to be studied and the basic law governing their selectivity was discovered by Graham.1 When pore size of a microporous membrane is small compared to the mean-free-path of the gas molecules, permeate will be enriched in the gas of the lower molecular weight. Since molecular weight ratios of most gases are not very large and since the selectivity is proportional to the square 

Since 1983, newer systems for hydrogen separations have appeared. Ube Industries, Ltd. announced the development of a polyimide resin in hollow fiber membrane form for H2 /Ci separations.11 12 Commercial production of this membrane and the engineering of skid mounted units were to begin in the fall of 1985.13 

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Stern, S.A., The separation of gases by selective permeation, in Membrane... 

The separation of gases by microporous membranes depends on the ratio of pore size to the mean free path which is given by ... 

S. A. Stem, The Separation of Gases by Selective Permeation, in P. Meats (Ed.), Membrane Separation Processes, Oiiqi. 8, Elsevier Scientific, New York, 1976. 

The different mechanisms that operate in the separation of gases have been previously described in Section 10.4.1. In pervaporation, the transport mechanism can be described by an adsorption-diffusion mechanism [74,114] similar to one for polymeric membranes [115]. However, it is necessary to consider that the specific interactions between the permeating component and the zeolitic material are different in zeolites. Moreover, the diffusion through the ordered zeolite nanopores is different than in the dense organic matrix. 

AU the techniques used to increase the stabihty of the SLM, such as the geUed SLM techniques [10, 11] (Fig. 7.5B) and the addition of thin top-layer by interfacial polymerization reaction on the SLM (Fig. 7.5C) [12], are essentiaUy applied in the removal of (metal) ions from solution. The stability of liquid membranes used for the separation of gases is more comphcated. Here, the addition of a top-layer on the macroporous support can negatively influence the permeabihty of gases through the membrane. Therefore, a careful choice of the layer material is important because it has to be impermeable to the solvent and should posses a high permeabihty for the gas molecules considered. In addition, the thickness of the top-layer as weU as that of the whole liquid membrane has to be minimized. 

The SAPO-34 structure is three-dimensional, with a pore diameter of 0.38 nm, and H—SAPO-34 has mild-to-moderate acidity. The separation of gases, such as CO2 and CH4, can be achieved by using H—SAPO-34 zeolite membranes modified with Li+, Na+, K+, NH4+, and Cu + as ion exchangers [6]. It was verified that the presence of the cations on the zeolite increases the CO2—CH4 separation selectivities up to 60%. 

It is theoretically possible to accomplish a complete separation of a pair of gases by membrane separation, either by very highly selective membranes combined with very low partial pressure of the permeating component on the permeate side of the membrane, or by multistage cascades. However, it usually is not economical to accomplish high purity and high recovery of both components of a binaiy by membrane separation alone. Therefore, it usually b necessary to couple membrane gas separation widi other processing steps when neatly complete separation te required. 

The advantages of PPO as a gas separation material has therefore been well appreciated. The search for new and improved membranes with highly efficient performance characteristics has however led many researchers to chemically modify PPO. Most chemical modifications of PPO reviewed in this chapter are for improvement of CO2/CH4 or O2/N2 permselectivities. Modifications have been made to improve the solubility selectivity of PPO without sacrificing its high intrinsic permeability. Many researchers have also tried to improve the solubility of PPO in polar solvents by structure tailoring the polymer. It may be noted that PPO can be easily modified via chemical reactions. The following section will describe the advances that have been made in the past few decades in the use of modified PPO as membrane materials for the separation of gases. 

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