Several distillation steps are required for the complete separation of a multicom-ponent mixture into individual components. For complete fractionation of multi-component mixtures a large number of different separating sequences are possible. The selected separating path determines the investment and operating costs. Operating costs are determined primarily by energy costs.
In processes with several distillation columns the coupling of columns is advantageous. Columns are directly coupled by means of their product streams in direct coupling. By indirect or thermal coupling the waste heat from one column is used to heat a second column .
In indirect coupling the main goal is to use the condensation enthalpy for other purposes in the process. In addition, a further aim is to replace the required evaporation duty at the reboiler with 'waste heat' from other parts of the process. Heat integration by thermal or indirect coupling of columns is a very effective way of reducing the heat requirement in distillation processes and is used more often than direct coupling  . The heat integration methodology by pinch analysis is recommended to determine the best network of indirect coupling (see Chapter 6).
A number of examples have proven that for a multicomponent separation a direct coupling of distillation columns is particularly useful and always leads to substantial reduction of the heat requirement . In addition the investment costs are reduced, because significantly fewer columns and heat exchangers are required to perform separation. On the other hand, the operation and control of directly coupled distillation columns is more difficult.
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