Heat Integration of Distillation Columns

The dominant heating and cooling duties associated with a distillation column are the reboiler and condenser duties. In general, however, there will be other duties associated with heating and cooling of feed and product streams. These sensible heat duties usually will be small in comparison with the latent heat changes in reboilers and condensers. 

Both the reboiling and condensing processes normally take place over a range of temperature. Practical considerations, however, usually dictate that the heat to the reboiler must be supplied at a temperature above the dew point of the vapor leaving the reboiler and that the heat removed in the condenser must be removed at a temperature lower than the bubble point of the liquid. Hence, in preliminary design at least, both reboiling and condensing can be assumed to take place at constant temperatures.  

The consequences of placing distillation columns in different locations relative to the pinch will now be explored. There are two possible ways in which the distillation column can be integrated. The reboiler and condenser can be integrated either across the pinch or not across the pinch. 

Distillation across the pinch. This arrangement is shown in 

Rgure 14.1 The appropriate placement of distillation columns. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.) 

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LinnhoflF, B., Dunford, H., and Smith, R., Heat Integration of Distillation Columns into Overall Processes, Chem. Eng. Sci., 38 1175, 1983. 

Heat integration of distillation columns can lead to significant energy saving. However, this should not penalise operability. In this section we will show how simultaneous design and control can solve conveniently this problem (Bildea and Dimian, 1999). 

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