Column Operation

The extractive distillation unit of this cyclohexane production plant consists of two distillation columns (Figure 10), which we can easily classify as direct sequence columns. The first column acts as an extractive column where the solvent is introduced at the second stage of the column, so that it will be present throughout the column and exits with the bottoms. As were stated above, the solvent alters the natural volatility of the binary mixture by forming hydrogen bonds with benzene and allowing it to be recovered as the bottom product.

The bottom product of the first column will then fed to the second column, i.e. the solvent recovery column, to undergo the normal distillation to separate both the components for further usage, i.e. benzene being recycled to the reactor for further conversion while solvent to the first column for reuse. The main operation parameter of the distillation unit is shown in Table 5.

Unit Operationand StreamDescriptionOperating Parameters
Distillation Column
T-20First column (extractive column)Operating pressure: 150 kPaNumber of trays: 45Solvent (str. 27) feed tray: 2Feed (str. 47) tray = 28
T-21Second column (solvent recovery column)Operating pressure: 105 kPaNumber of trays: 20Feed stream: 10
Heat Exchanger
X-22Cool down the solvent for recyclingOutlet temperature: 80 °C
P-23Pump the solvent for recyclingOutlet pressure: 150 kPa
Stream 27Solvent stream of 1,2-propanediolMolar flow rate: 3600 kgmol/h
Stream 47Feed streamMolar flow rate: 355.43 kgmol/h
Stream 28Product stream of Column T-20 distillate(cyclohexane)Molar flow rate: 158.75 kgmol/hwith a purity of 99.3%
Stream 29Benzene solvent stream of bottom productfrom column T-20, fed to solvent recoverycolumn T-21Molar flow rate: 3795.5 kgmol/hwith a purity of 94.8%
Stream 30Product stream of column T-21 distillate(benzene) for recycleMolar flow rate: 223.91 kgmol/hwith a purity of 84.96% benzene
Stream 31Solvent stream of bottom product fromcolumn T-21 (solvent) for recycleMolar flow rate: 3571.6 kgmol/hwith a purity of 99.87% solvent


Description: extractive-distillation-16
Figure 10: Extractive Distillation Unit for Cyclohexane Production Plant



Description: extractive-distillation-17
Figure 11: Distillation Ternary Diagram for the Extractive Distillation Unit

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