and the reboiler). The benzene and cyclohexane mixture is fed to the twenty-first ideal stage. Two models are developed, one with ideal stages and the other with actual trays estimated to correspond to 70% OTE. These two UniSim models are otherwise identical, but distillate rate needs to be adjusted by about 0.9% to make the actual tray model converge. In this example, the differences in the maximum vapour- to-tray flow rates in the three sections of the columns are respectively less than 0.14%, 0.02%, and 0.01%. The dif- ferences in the maximum liquid from tray flow rates in the same three sections are respectively 11.6%, 0.27%, and 0.6%. As such, the ideal stage and the actual trays with OTE options give reasonable agreement on maximum vapour-to-tray volumetric flow rates. However, the actual trays with the OTE option result in an 11.6% higher maximum liquid from tray rate in the top section of the column compared to that from the ideal stage option. The liquid flow data affects tray downcomer design but may or may not have direct impacts on the column diameter. This example shows liquid flow rates from trays could vary by more than 10% when switching from ideal stage simulation to actual trays with OTE. Conclusion With the commonly recommended property packages for oil refining crude columns, the example crude column sim - ulation model shows reasonable agreement of vapour-to- tray flow rates simulated using these packages when the model uses the ideal stages. Property estimating options in
each package may need to be properly selected to provide good agreements on liquid from tray flow rates. Tray efficiency estimates needed for converting the ideal stages to the actual number of trays have been discussed. Compared to tray vapour loading data from the example crude column model using ideal stages, data from the same model using the actual number of trays and the typical OTE show good agreement. However, using point efficiency instead of OTE could result in significant differences in tray vapour loading data. Simulation models for a C₃ splitter and an extractive distil - lation column, both involving pure components, show good agreement on the vapour-to-tray flow rates simulated with ideal stages or actual number of trays with OTEs. However, the liquid flow rates simulated from the extractive distil - lation model show significant difference when switching from ideal stages to actual trays with OTE.
References 1 Fenske M, Ind, Eng. Chem , 24, 482, 1932.
2 O’Connel H E, Plate Efficiency of Fractionation Columns and Absorbers, Transactions of the American Institute of Chemical Engineers, 42, pp.741-775, 1946. 3 PRO/IITM Process Engineering Reference Manual, 2018. Tek Sutikno is a Process Engineering Manager with Fluor and a Professional Engineer registered in 11 US states with more than 35 years of experience in the process industries. He holds BSc, MSc, and DEngr degrees in chemical engineering and a MBA degree, all from the University of Kansas. Email: tek.sutikno@fluor.com
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