Air Separation Membranes

Most solution-cast composite membranes are prepared by a technique pioneered at UOP (35). In this technique, a polymer solution is cast directly onto the microporous support film. The support film must be clean, defect-free, and very finely microporous, to prevent penetration of the coating solution into the pores. If these conditions are met, the support can be coated with a Hquid layer 50—100 p.m thick, which after evaporation leaves a thin permselective film, 0.5—2 pm thick. This technique was used to form the Monsanto Prism gas separation membranes (6) and at Membrane Technology and Research to form pervaporation and organic vapor—air separation membranes (36,37) (Fig. 16). 

Oxygen-Nitrogen Because of higher solubility, in many polymers, O2 is faster than N2 by a factor of 5. Water is much faster still. Since simple industrial single-stage air compressors provide sufficient pressure to drive an air-separating membrane, moderate purity N2 (95-99.5%) may be produced in low to moderate quantities quite... 

Oxygen-enriched air will be produced on the low pressure permeate side of the air-separating membranes. The oxygen-enriched permeate stream is usually vented, but there is an increasing interest in using this gas for combustion. High-purity oxygen... 

Wang, H. Huang, L. Holmberg, B.A. Yan, Y. Nanostructured zeolite 4A molecular sieving air separation membranes. Chem. Comm. 2002, 1708-1709. 

Example 26A An air-separation membrane has an O2/N2 selectivity of 5.0, and the O2 permeability is 0.2 scf/ft -h-atm. (a) For a counterflow separator operating with a residue containing 95 percent N2, what is the permeate composition and the fraction of the feed obtained as permeate if the feed pressure is 150 Ib /in. absolute and the permeate pressure 15 Ib /in. absolute (i) What membrane area is needed for a feed rate of 300 scfm ... 

A lanthanum calcium ferrite (LCF) perovskite system (LaxCai-xFeOs) was joined to itself to demonstrate the feasibility of the TLP method. LCFs can be used as a component of air separation membranes, so the assemblies need to operate at high temperatures. Therefore the joint interface needs to resist creep, avoid problems with thermal cycling, and be chemically stable in the high-temperature environment. Ideally, the interface in such systems is the same as the parts in the assembly. 

Abstract This chapter is devoted to the state of the art of the most important aspects of high temperature ceramic air separation membranes for oxygen production and oxidation processes. Aiternative technologies, operational principle, fields of application, energy efficiency and cost aspects, materials science, module design, and sealing will be discussed. 

Sundkvist SG, Klang A, Thorshaug NP. AZEP - development of an integrated air separation membrane - gas turbine. 2001. 

A leading choice of transition metal for the B-site of the perovskite is cobalt. Because the requirements for ionic conductivity are so stringent, it is often desired that the perovskite material selected contain cobalt on the B-site. The materials are also usually heavily doped to create the required oxygen vacancies in the high oxygen partial pressure environment of an air separation membrane. 

The final section explores emerging applications in which membranes may play a role in the next few decades [4], The discussion will focus on air separation membranes for advanced combustion cycles and membrane solutions for the capture of CO2 emissions. 

Air separation membranes are typically dense ceramic (typically perovskite) membranes, which selectively permeate oxygen in ionic form. Over the past two decades. Air Products (ITMs) and Praxair (oxygen transport membranes [OTMs]) have worked towards the commercial scale-up of these membranes for applications in power generation, gasification, and gas to liquid conversion [94]. Air Products has focused on a planar configuration, whereas Praxair on tubular membranes. 

A commonly used production unit consists of a compressor and an air separation membrane. The air (typically ambient air) is pressurized by the compressor and run through an air separation membrane, which separates it into one component rich in oxygen and one rich in nitrogen. 

K. Meier, M. Langsam, H. C. Klotz, Selectivity enhancement via photooxidative surface modification of polyimide air separation membranes, J. Membr. Sci., 94, 195-212 (1994). 

Boiler size and SO2 scrubber are reduced The technology is not yet considered mature Polymeric air separation membranes can be used for the separation of oxygen from nitrogen (commercialized, for example, by DuPont, Air Liquid, etc.). 

The most common gas separation hybrid process is the membrane/ adsorption hybrid system which can be very cost effective if each of the separation unit is operated for what it is most suited. For example, for air separation, membranes are best suited for bulk separation and adsorption is preferred for the removal of low concentration of the contaminats, an adsorption unit followed by a membrane can offer a economical solution to produce high putrity nitrogen and oxygen. The hybrid processes are generally preferred for large size plants because of the capital and operation expense of two separate processes. 

Air separation membrane units can be operated in either pressure or vacuum mode, as shown in Figure 24.17. In the pressure mode, feed air is typically pressurized to several bars, while permeate is maintained near atmospheric pressure ( 1 bar). The differential pressures are higher than those of the vacuum mode, leading to reduced membrane area requirements. In the vacuum mode, feed air is pressurized slightly above atmospheric pressure, and a vacuum is maintained on the permeate side of the membrane. The retentate is vented at atmospheric pressure. The vacuum mode is typically more energy efficient than the pressure mode because a vacuum is applied only to the permeate stream. 

---