Separation of Oxygen and Nitrogen

Membrane material (pore dia. in nm) Temp( TMP C) (bar) S.F. P (barrer) Reference Note 

A recent development of a ceramic membrane appears to be promising for selectively removing oxygen from air. Multi-channel membrane elements have been fabricated for that purpose [Anonymous, 1995]. The membrane has the potential for reducing the cost of converting natural gas to synthesis gas. 

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Rein, H. T., Miville, M. E., Fainberg, A. H. Separation of oxygen and nitrogen by packed column chromatography at room temperature. Anal. Chem. 35, 1536 (1963). 

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. 

Poly[l-(trimethylsilyl)-l-propyne] (PMSP) is a typical glassy polymer at room temperature that was first syndiesized by Masuda and Higashimura in the 1980 s (1). Recently, membranologists have studied their gas permeation properties. The PMSP membrane has the highest gas permeability of all polymeric membranes. Therefore, this polymer is expected to have potential utiliQ in industrial applications such as the separation of oxygen and nitrogen from air. 

In the early 1990s Robeson (1993) found an upper limit to the performance of polymer membranes in the commercially important separation of oxygen and nitrogen from air. On a log-log plot of selectivity versus oxygen permeability (a Robeson plot), the upper bound plots as a straight line (see Problem 17.D17 for more details). Although theoretical reasons for this limit have not been found, very few new membranes have been developed that are able to perform better than Robeson s limit. Membrane research has focused on ways to do better than Robeson s upper limit. 

The increase in separation for the different flow patterns can be conpared for separation of oxygen and nitrogen. Geankoplis (20Q2) presents a problem with a feed that is 20.9% oxygen, Pj. = 190 and Pp... 

The cross-flow calculation requires no trial-and-error and is very fast even with n = 20,000. The spreadsheet iiputs the required values and records results. All calculations of the equations in Section 17.7.2 are done in the Visual Basic prograra The spreadsheet is shown with input values for the separation of oxygen and nitrogen in Example 17-11 and the final results. 

Fig. 5.2. Separation of oxygen and nitrogen on molecular sieve column 25-m PLOT.

Niwa, M. Yamazaki, K., and Murakami, Y., Separation of oxygen and nitrogen due to the controlled poreopening size of chemical vapor deposited zeolite-A, Ind. Eng. Chem. Res., 30( I), 38-42 (1991). 

The separation of oxygen and nitrogen is difficult because the size and shape (and hence the diffusivity) of the molecules are quite similar. In addition, the solubility and diffusivity of the faster gas (oxygen) are generally quite low, resulting in a low rate of permeation and the requirement for a large membrane area. However, because of the industrial importance of this separation and the scarcity of simple alternatives, it has been the subject of extensive re.search and development work. In addition, the feedstock is "free. Therefore, high recovery is not a requirement for this separation. Low recovery operation can be used to improve the separation efficiency (i.e., the permeate purity). 

In low-temperature gas separation processes, in particular the separation of oxygen and nitrogen from air, which involve liquefaction and distillation of the mixture to be separated, the distillation step is clearly of prime importance. 

The spaces between the natural layers can be enlarged to form pillared interlayered clays. This is carried out by ion exchanging the charge compensation cations with polynuclear metal ion hydro-complexes which are formed in hydrolysed solutions of polyvalent metal ions such as Al(III) or Zr(IV). The polynuclear cations dehydrate on calcination to create metal oxide clusters which act as pillars between the clay layers and create spaces of molecular dimensions. Example separations with pillared clays include the separation of oxygen and nitrogen, and the separation of isomers. 

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