Figure 7.9 Distillation column with vapor recompression.
was introduced in chemical and petrochemical industries. As shown in Figure 7.9, the overhead vapor is compressed to the level that enables the use of the condensation heat in the reboiler. The vapor recompression separation scheme brings substantial energy savings for fractionating mixtures characterizing in close boiling points, where small temperature differences between the top and bottom of the column, small compression ratios and consequently small compressor duties are required [2, 3].
A single stage compressor with a compression factor up to three is applicable only for mixtures with a low boiling point difference. Mixtures with a high boiling point difference often require multistage compressors, which are in most cases not economical. Therefore, vapor recompression is widely used in the petrochemical industry for the separation of hydrocarbons, and in the food industry . In the chemical industry, where mixtures with a high boiling point difference are often separated, vapor recompression is rarely applied. In addition, high flow rates (approx. > 100000m3h_1) or vacuum operation (approx. < lOOmbar) often make vapor recompression uneconomical.
Thermal vapor recompression, beside mechanical vapor recompression, is another way of increasing energy efficiency of distillation. A common application of a thermal vapor recompression can be found at distillation or stripping columns where the heating steam is injected directly into the bottom. An example is waste water stripping. Usually, waste water strippers have a lower operating pressure
Steam 1.1 bar
Steam Jet Vacuum Pump
Steam 5 bar
Vapor 500 mbar
_2z Flash Vessel 500 mbar
Figure 7.10 Waste water stripping column with vapor recompression.
compared with the lowest steam pressure level. By the introduction of a flash vessel with a steam jet vacuum pump on top, as illustrated in Figure 7.10, the heating steam is used to generate a certain amount of low pressure steam. By doing so, the total steam consumption can be decreased. Further benefit is that the flash vessel acts as an additional stage of the stripping column.
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