Pre-enrichment in the Air Separator

The pre-enriched product (d) typically contains 85% of the krypton, which is fed to the air separation unit via the process air and about 95% of the xenon. The krypton yield is lower than the xenon yield, because Kr is more volatile than Xe (see Table 3.2). 

The Kr/Xe concentration in the sump of the enrichment column is controlled by the amount of withdrawn sump product (d). The smaller the flow, the higher will be the concentration. Thus the Kr/Xe-concentration could be theoretically increased up to the solubility limit of krypton (30%) and xenon (2%). The maximal admissible concentration is however determined by the solubility and the explosion limit of the hydrocarbon components, which accumulate in the sump product as well. The most prominent component is CH4, which is contained in ambient air with up to 5 ppm. Owing to its low volatility (Table 3.2), CH4 accumulates together with Kr/Xe in the liquid bath. The methane concentration in the Kr/Xe pre-product must remain well below its lower explosion limit of about 4.4%. 

In order to reduce the CH4 content in the Kr/Xe-pre-product, the enrichment column (4) is divided into an upper and lower section. This allows the discharge of a part of the methane, which enters the enrichment column via stream (a), over the top of the column together with the oxygen product. This is achieved by suitable adjustment of the liquid and vapour flow L und V in the two sections. 

A component is driven upwards in the column, whenever the vapour flow V is so large, that for the separation factor S the inequality 

The component equilibrium factor K = y /x of a component describes the mass transfer equilibrium between the liquid phase with the concentration and the vapor phase with the concentration y (see Table 2.8, Section 2.2.5.S). 

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