Ideal oxygen separation process

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... 

Benefits of on-line SPE include elimination of eluate collection, evaporation, and processing in a completely closed system with protection from contact with hazardous solvents, light and oxygen. On-line SPE offers ruggedness, excellent precision and high sample throughput, and is ideally suited to perform a fast matrix-analyte separation prior to e.g. LC-MS or direct MS analysis. 

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 larger the selectivity, the easier the separation of component i from component j by adsorption. Zeolites with a selectivity as high as 10 for nitrogen relative to oxygen are used in pressure-swing adsorption processes" to produce oxygen from air. The specific amount of each component adsorbed for an ideal solution is given by... 

Adsorption is a physical process in which a substance, in this case a gas, is attracted to and held on the surface of another material. Neither material is changed in the process, and it can be easily separated again under the right conditions. In the VS A cycle, adsorption takes place on both the outer and inner surfaces of each structure. Adsorption is a temperature- and pressure-sensitive phenomenon. Both of these variables must be kept in an ideal range for ef cient oxygen production to occur. 

Thermochemical production of hydrogen involves the separation of water into hydrogen and oxygen through chemical reactions at high temperatures. Ideally, water can be separated directly (thermolysis) however this process requires temperatures in excess of 2 500°C. 

This paper presents an availability analysis of one type of oxygen production cycle centering around separation of oxygen and nitrogen in a fractionating tower. The plant is driven by work inputs to compressors and blowers. The analysis shows the irreversible entropy production in the various units and, in turn, the added work inputs required as a consequence thereof. Furthermore, a comparison is made with an ideal process of the same type, wherein all irreversibilities are reduced to the minimum possible, subject to the constraints imposed by (a) the use of a tower, and (b) the properties of the flowing streams. 

Future applications of adsorption are limited by the availability of new and better sorbents. Ideally, the sorbent shonld be tailored with specific attributes to meet the needs of each specific application. Development of better sorbents can also improve the performance of the cnrrent commercial processes. A good example is the invention of the LiX (Si/Al = 1) zeolite (Chao, 1989). Air separation has been performed by pressnre swing adsorption, and the generic sorbents 13X (i.e., NaX) and 5A (i.e., CaA) zeolites were nsed prior to this invention. By switching from NaX to LiX (Si/Al = 1), the prodnctivity of oxygen increased instantly by 1.4-2.7 times and the power consnmption reduced by 21-27% depending on the operating conditions nsed (Leavitt, 1995). 

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