Linde double-column system

The impurity problem noted in the previous paragraph was solved by the introduc tion of the Linde double-column system shown in Fig. 11-118. Two rectification columns are placed one on top of the other (hence the name double-column system). 

This problem was solved by the introduction of the Linde double-column system. Two rectification columns are placed one on top of the other (hence the name double-column system). In this system, liquid air is introduced at an intermediate point in the lower column. A condenser-evaporator at the top of the lower column provides the reflux needed for the rectification process to obtain essentially pure nitrogen at this point. In order for the column to also deliver pure oxygen, the oxygen-rich liquid (—45% oxygen), from the boiler in the lower column is introduced at an intermediate level in the upper column. The reflux and the rectification process in the upper column produce pure oxygen at the bottom and... 

The nitrogen purity from the Linde double column system is limited to about 5 ppm oxygen. In order to produce a higher purity nitrogen product, additional trays in the low pressure distillation column and some additional complexity is required in the process. 

The Linde double-column system was introduced in 1910 to solve the problem of oxygen losses in the nitrogen stream of the Linde single-column system. As noted earlier, the maximum purity of the top product in the single column is approximately 94 mol % nitrogen. If this purity had been attained in Example 6.12, nearly 25% of the oxygen in the feed would have been removed in the top product. 

Figure 6.28 shows a double-column gas-separation system presently used for the production of gaseous oxygen. Such a column has both theoretical and practical advantages over the Linde double-column system shown in Fig. 6.26. For example, a Second-Law analysis for the two columns shows that the contemporary double column has fewer irreversibilities than are present in a Linde double column. This results in lower power requirements. From a practical standpoint, only two pressure levels are needed in the contemporary column instead of the three required with the Linde double column. The net result of this modification is a lower air pressure requirement with an accompanying lower compressor power input. A further advantage of the contemporary column is that it does not require a reboiler in the bottom of the lower column. Thus, a smaller amount of heat transfer is required to provide the needed vapor flow in the lower column.

The Linde-Frankl system using patented Frankl regenerators was developed in the 1930s to meet the huge demands for oxygen and nitrogen by the steel and chemical industries. The liquefaction part of this system is very similar to an ammonia-precooled dual-pressure Claude liquefaction system and thus operates with about one-half of the power consumed by the Linde double-column system. 

In 1910, Linde found the answer the double distillation column (Figure 5). The double-column system improved the production of high-purity oxygen by dramatically increasing the fraction of oxygen produced from the feed air (22). If one invention can be said to have created an industry, this one created the air separation industry. 

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