Vacuum Rectification Design

In the conceptual design stage of a rectification unit, the initial objective is to determine the reflux number and the quantity of theoretical stages required to achieve the desired fractionation split at minimum costs. As a result of this procedure, the heat and mass balances of the column including the internal liquid and vapour flows get known, which are essential for the column design as well as for the selection of the type of internal mass transfer equipment best suited for the specific separation task. 

Separation trays and structured and random packings are the prevailing mass transfer internals at choice for rectification columns and, in first deciding between them, a comparative performance design needs to be prepared including examinations on capacity limitations, pressure drop and separation performance. 

Such examinations require a sound knowledge about the following characteristic parameters and physical quantities to permit process engineering calculations and column design. 

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Process vapours from the esterification reactors and EG from the EG-vapour jet, as well as from the vacuum stages of the spray condensers, are purified in the distillation unit. The distillation unit commonly consists of two or three columns and is designed for continuous operation. The purified EG is condensed at the top of the third vacuum rectification column and returned to the process via a buffer tank. Gaseous acetaldehyde and other non-condensables are vented or burned and high-boiling residues from the bottom of the third column are discharged or also burned. 

Due to the increased difficulty of separating THF and ethylacetate in the presence s of water, it was necessary to dewater the THF/ ethylacetate-mixture before the rectification. The ethylacetate/ THF rectification column (119) was designed for 60 theoretical stages and a reflux ratio of 15. The column was operated at 0.5 bar since the VLE-data are slightly more favorable under vacuum.3 A further reduction of the rectification pressure was not possible since water was used for condensing the distillate. 

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