OCR Text |
Show Traditional air separation processes for generating OEA, such as on-site pressure swing adsorption (PSA) and cryogenics are both energy and capital intensive, while delivered liquid oxygen is also costly for the small volume user. The expense of either oxygen generation or delivery has severely limited industry implementation of OEA combustion. Membrane gas separation technology has recently emerged as a new option for on-site OEA generation. Membrane processes, based on differential gas permeability and solubility in polymeric materials, have a number of characteristics which make them well-suited to OEA generation for combustion: Simple, straightforward system design; Continuous operation; Minimum startup time; High reliability; Energy efficient; Minimal operator attention; Modular nature; and, Attractive capital and operating costs. The simplicity of membrane systems is one of their overriding advantages versus competing technologies. The design, construction, operation and maintenance of membrane air separation systems are all straightforward, particularly in a single-stage unit. The modular nature of membrane systems make them suitable for a wide range of capacities from several liters/minute to tonnage quantitites and allow system production capacity to be easily augmented by add-on modular assemblies. Furthermore, membrane systems are compact, particularly when the hollow fiber membrane geometry is used. There are several factors which affect the economics of OEA generation via membranes. These factors range from the intrinsic properties of the membranes to the operating conditions under which the membrane modules are employed. The key factors governing the economics of membrane OEA generation are: Membrane gas selectivity; Membrane effective perm eab il i t y; OEA system mode of operation; OEA concentration; and, OEA system capacity. Each of these factors will be considered below over a range of alternatives compatible with OEA generation and discussed with regard to their relative affect on membrane system energy requirements and economics. BACKGROUND INTRINSIC PROPERTIES - Gas separations in polymeric membranes, in the absence of any bulk reaction. occur due to different levels of solubility 180 in, and diffusion through, the separating layer. (4). Solubility being a thermodynamic property whIle diffusion is a kinetic quantity. The permeability, P , of a given gas is an intrinsic property of the mrembrane. It is closely associated with Henry's Law for simple and non-interacting gases, and for low concentration levels is given by where, P . n P. n x D. I (1) Intrinsic Perm¥bility of gas ~ in the membrane, cm (STP) cm/cm sec cm-Hg. Sol~bility of :¥as i in the membrane, cm (STP)/cm cm-Hg. Dif~uSivity of gas i in the membrane, cm /sec. The mass transport of gas through a membrane is given by Fick' sLaw: N. I where, P. n A P N. Flowrate of gas i, <pt3(STP)/sec Al Membrane area, cm (2) , Membrane separating barrier thickness, cm .6P Transmembrane partial pressure difference of gas i, cm -Hg For a given membrane system, the degree of separation between gases will depend upon the relative permeabilities of the gases to be separated. This ratio of gas permeabilites is termed the separation factor, or membrane selectivity, and the larger the value of this factor, the better the separation. For the separation to be economically feasible the flowrate, N, through the membrane must be sufficiently high. By convention, the oxygen enriched air stream passing through the membrane is termed the permeant, while the oxygen depleted (nitrogen enriched) air stream remaining is termed the retentate. It follows from Eq.(2) that for a given membrane area the intrinsic permeability, P ., is not the only parameter contributing to the ~~s flowrate through the membrane. Equally important is the membrane thickness and the driving force across the membrane ( 6 Pl. For economical and energy conservation reasons, the driving force must be kept as low as possible. Thus, polymers which have high intrinsic permeability values and good separation factors are not necessarily suitable for commercial membrane systems if they cannot be rendered into sufficiently thin layers. What must be taken into account is |