Prefractionator-reverse heat integrated column

Figure 6.24 Heat-integrated columns, (a i Feed split binary) b) light-split reverse [binary) (e) prefractionator reverse (ternary).

In the forward heat integration scheme (FHI) the top vapour of the prefractionator is used to heat the reboiler of the main column, and after condensation split in reflux and secondary feed for the main column (Fig. 11.19). The pressure in the prefractionator has to be high enough to ensure a temperature difference of minimum 10 K in the reboiler of the main column. In the reverse heat integration scheme (RHI) displayed in Fig. 11.20, the top vapour from the main column is used to drive the reboiler of the prefractionator. Note that this vapour is practically the pure component A. After condensation this stream is split in top product and reflux of the main column. 

Prefractionator reverse (ternary). Figure 6.24c shows a third alternative flowsheet that combines heat integration with a complex configuration. This system can be used to separate a ternary mixture. In the system shown, the sidestream column is run at high pressure and the prefractionator at low pressure. 

To illustrate the very large energy savings that are possible with this complex/heat-integrated system, consider the separation of a benzene, toluene, and xylene mixture. A conventional two-column light-out-first separation flowsheet with no heat integration uses twice the energy7 that the prefractionator-reverse flowsheet uses. 

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