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... 

Equations for the double column system Separation of the binary mixture... 

The distillation of air is usually carried out in a double column system consisting of a high pressure column and a low pressure column. The two are stacked with the low pressure column on top of the high pressure unit. 

The entire assembly is enclosed in a highly insulated cold box to conserve energy. The columns must be made as compact as possible to minimize capital investment as well as reduce heat leak. Up to 150 distillation trays are used in a double column system and tray spacings are kept small at 10 to 20 cm. The trays are typically multipass sieve trays with small diameter perforations. Because the distillation is an extremely clean service, perforations are typically as small as 1 mm. Many tray geometries are used, including multipass cross-flow, split cross-flow (parallel) and circular flow (race track) trays. Each has certain attributes which are used to optimize the column design for different design conditions. 

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. 

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. 

The double-column process shown in Fig. 2.3A produces oxygen and nitrogen. There are industrial applications requiring exclusively nitrogen. For these, numerous modifications of the double-column system have been developed, two of which are introduced here ... 

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. 

The double-column system works like the single-column system except for the addition of the rectification section. In the double-column system, entering air is introduced in the middle of the lower column instead of at the top. Part of the liquid nitrogen product stream from the lower column is throttled to the operating pressure of the upper column and sent to the top of the upper column as reflux. The enriched liquid air from the lower reboiler is also throttled and introduced as feed into the middle of the upper column. Depending on the number of plates used, any practical purity level of either or both components may be obtained. When extremely high-purity products are desired, the argon present in the air must be considered as a third component of the mixture and removed in a draw-off stream from the upper column." The operation of such a column can best be shown with the aid of an example. 

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. 

---