For many mixtures, the relative volatility increases with decreasing pressure [2, 3]. As a consequence, lower pressure operation can be used to achieve the same product purity at lower reflux ratios and with lower energy requirements. However, condensation of the overhead vapors with cooling water or air at approx. 35 °C is a limit for the operating pressure decrease. Alternatively other cooling media like chilled water or refrigerants can be used for the condensation, but these are usually more expensive than cooling water. Additionally, the heat transfer conditions of the reboiler and the condenser will change. A pressure decrease will also cause the volume flow of the vapor phase in the column to increase, influencing the capacity of the column. Consequently, detailed considerations and calculations are usually required for a large decrease of the operating pressure. Therefore, for existing columns typically smaller pressure reductions are realized.
On the other hand, operation at increased pressure can enable a more effective recovery of waste heat due to the higher condensation temperature at the condenser, although the column may require more energy.
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