Practical oxygen separation process

The amount of work used in a practical oxygen separation process is much greater than this minimum because of irreversibility. Figure 1 illustrates the process flow for one type of oxygen production cycle. The process quantities refer to separation of... 

The separation of oxygen from air is extremely important in connection with medical treatments, combustion processes, etc. Relatively thin effective membrane layers are required for practical oxygen separation systems, because the permeability coefficients of oxygen of most polymers are lower than those of hydrogen and carbon dioxide. 

In an effort to explore this aspect further, a paper written by Gyftopoulos and Benedict concerning the maximum potential efficiency of an air separation plant provided some insight (4 ). Compressed air is separated by cryogenic distillation into oxygen and nitrogen. In a unique approach, the authors developed an idealized process wherein all thermodynamic inefficiencies which could be corrected by capital investment were eliminated. The losses in the distillation tower were not much affected by this approach. Their thermodynamic analysis for the practical and idealized processes are compared in Figure 7. 

The limiting expressions (Equation 8.15d and Equation 8.16) are invaluable in arriving at a first assessment of gas-separation processes. Suppose, for example, that a = 5, which is a typical membrane selectivity for the separation of nitrogen and oxygen. Then in any practical operation, the pressure ratio will be in excess of a and the operation will consequently take place in the selectivity-limited region. The maximum enrichment of nitrogen attainable is then given by (Equation 8.16)... 

In the case of molten salts, the functional electrolytes are generally oxides or halides. As examples of the use of oxides, mention may be made of the electrowinning processes for aluminum, tantalum, molybdenum, tungsten, and some of the rare earth metals. The appropriate oxides, dissolved in halide melts, act as the sources of the respective metals intended to be deposited cathodically. Halides are used as functional electrolytes for almost all other metals. In principle, all halides can be used, but in practice only fluorides and chlorides are used. Bromides and iodides are thermally unstable and are relatively expensive. Fluorides are ideally suited because of their stability and low volatility, their drawbacks pertain to the difficulty in obtaining them in forms free from oxygenated ions, and to their poor solubility in water. It is a truism that aqueous solubility makes the post-electrolysis separation of the electrodeposit from the electrolyte easy because the electrolyte can be leached away. The drawback associated with fluorides due to their poor solubility can, to a large extent, be overcome by using double fluorides instead of simple fluorides. Chlorides are widely used in electrodeposition because they are readily available in a pure form and... 

Practical concerns, specifically mass transfer, Umit the recovery to values in the high 70% range. All this said, the vast majority of operating air separation units are the small capacity medical oxygen concentrators. These operate under either PSA or VSA or trans-atmospheric process cycles. The key objective for medical O2 is small unit size and power consumption prior to the push for portability was a secondary consideration. There are many such PSA air separation units that operate at recoveries as low as 35%. 

Despite these problems, PSA is often used for oxygen and hydrogen purification, and a recent runner-up Kirkpatrick Award for Union Carbide s Polybed hydrogen process (T ) attests to the fact that it is now both practical and economical to perform bulk separations on feed streams in excess of one million cubic feet per hour. 

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