The heat requirement of a single column can also be minimized by heat exchange between the warm products and the cold feed as illustrated in Figure 7.8. In addition, other process streams or steam condensate can be used. If a process has several sources and sinks of heat, before one decides to implement feed preheating, pinch analysis should be considered to anticipate any potential pinch violations resulting from sub-optimal match of cold and hot process streams (see Chapter 6).
In the case of a vapor feed, the latent heat is used only in the rectifying section of the column. Therefore, a vapor feed can be condensed in a second reboiler before it is introduced into the column. In this way, latent heat is used in both rectifying and stripping section of the column, which can lead to a lower external heat-input requirement .
The exergy describes the ability of heat to be converted into work, where the temperature difference of the heat compared with the ambient temperature is relevant. In a distillation column, a specified amount of heat is required at the bottom at a certain temperature and often an almost equivalent amount of heat is removed at the top at lower temperature level. The temperature difference of a distillation column is low for close-boiling mixtures, so exergy losses are also low . However, the required heat is usually supplied in the form of steam at much higher temperature compared with the bottom temperature. In addition, the energy is removed by cooling water with a temperature much lower than the condensation temperature. In this way, larger amounts of exergy are lost during separation than necessary .
Often, the condensation heat of a distillation column is not used when the condensation temperature is too low. The implementation of a vapor recompression can avoid this waste of energy. After the oil crisis in 1973 vapor recompression
Q>) Condenser s
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