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Published byBrett Challis Modified about 1 year ago

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Structural Design and Separation Characteristic of Divided Wall Column by the Shortcut Method ● What is DWC ? -The DWC consists of one column in which the prefractionator is separated from the main column by a divided wall. No remixing Reducing feed mismatch Thermal efficiency benefit Only one column Reduced energy Capital saving Improved yield/quality Smaller plot area A B C Prefractionator Main Column A B C Prefractionator Main Column A C B A B C -The reflux flow of 1 st column = The internal liquid flow fed to prefractionator. - The boil-up flow of 1 st column = The internal vapor flow fed to prefractionator. * The simple column configuration is not thermally coupled. And total energy consumption is likely to be different from DWC’s. However, the relative location of each major section is not much affected by this structural difference between the DWC and the equivalent conventional column configuration. ● DWC Structure Design Fenske-Underwood equation (Shortcut Method) is used for the structure design of DWC by applying it on three conventional simple column configuration. ● Optimal Design of Internal Flow Distribution A B C A C B I II III IV A B C A B C I II III IV 1st column2nd column3rd column Ext. Reflux Ratio Total Stage No.1814 Feed Stage No.9510 Rectify Vap/ Liq (kmol/h)41.87/ / /31.85 Stripping Vap /Liq (kmol/h)41.87/ / /54.79 Main structure of the equivalent conventional column ● Conclusion This study emphasizes a shortcut method using the Fenske- Underwood equation for the design of DWC. The method utilizes the three conventional column configuration equivalent to the DWC to find the proper DWC structure in simple manner. The proposed method shows little difference from that through optimization study in terms of energy consumption. Extensive simulation study illustrates the proposed method is suitable to DWC structure design. DWC (1st Column) DWC (case study) Two sequential column Rectify Liq Flow/ Internal Liq Flow 19.81kmol/h16.00kmol/h Stripping Vap Flow/ Internal Vap Flow 41.87kmol/h38.00kmol/h Total Reb Duty (kJ/h)1.790 ☓ ☓ ☓ 10 6 Internal flow & energy consumption of the DWC based on the shortcut method and optimization. ● DWC Simulation F1F2F3 Feed Mixture A: n-Pentane A : 0.40 B : 0.20 C : 0.40 A : 0.33 B : 0.33 C : 0.33 A : 0.15 B : 0.70 C : 0.15 B: n-Hexane C: n-Heptane * 1 atm, 45kmol/h HYSYS TM simulation program is used with the Fenske- Underwood equation for the design of column structure. Feed condition of ternary mixture Shortcut ColumnDWC2TOP Rectify Liq Flow Stripping Vap Flow Liq Internal flow Vap Internal flow F F F Reboiler Duty (10 6 kJ/h) F1 : 1.79 F2 : 2.02 F3 : 2.19 F1 : 1.76 F2 : 1.92 F3 : 2.05 F1 : 2.63 F2 : 2.85 F3 : 3.33 Internal flow & energy consumption of the DWC based on the shortcut method and optimization for difference mixtures. The existence of the optimal internal flow distribution can be explained by the two competing effects of internal flow. The energy consumption by internal flow distribution - The top, side and bottom product purity => 98/ 98.5/ 98% - The composition of light key component (n-Pentane) at the bottom and heavy key component (n-Heptane) at the top in the 1 st column is set to less than 0.1mol%. → This is based on the fact that the separation efficiency is highest when the low (A) and high (B) boiling component are completely separated in the prefractionator section. In the proposed method, how to select the product purity of the 1 st column is very important not only to the optimal structure but also to the optimal internal flow distribution. Although there is some difference in resulting internal flow, the difference in energy consumption is insignificant just as 1.7% Improvement of energy efficiency is most significant when the composition of middle boiling component is high. This result seem to be authentic because the energy efficiency by the DWC comes from the prevention of remixing. The difference of energy consumption between the DWC based on the proposed shortcut method and that by the optimization method is only 1.7~6.3%. It indicates the proposed method can be properly used to design the DWC.

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