The diameter of a distillation column is determined by the maximum vapor velocity. If this velocity is exceeded, the column liquid and vapor hydraulics will fail and the column will flood. Reliable correlations are available to determine this maximum vapor velocity.
Since the vapor flowrates change from tray to tray in a nonequimolal overflow system, the tray with the highest vapor velocity will set the minimum column diameter. If the vapor mass flowrate and the vapor density are known, the volumetric flowrate of the vapor can be calculated. Then, if the maximum allowable velocity is known, the cross-sectional area of the column can be calculated.
Aspen Plus has an easy-to-use tray sizing capability. Click the subitem Tray Sizing under the C1 block, and then click New and OK for the identification number. A window will open, where the column sections to be sized and the type of tray can be entered. Figure 3.53a shows the parameter values used in the example. The stages run from stage 2 (the top tray) to stage 31 (the bottom tray). Sieve trays are specified.
The simulation must be run by clicking the N button. Then the page tab Results is clicked (see Fig. 3.53b), and the column diameter is seen to be 7.75 m. This is a very large distillation column, and therefore a single liquid pass would produce very large liquid gradients across the tray and liquid heights over the weir. A column this large would use at least 2-pass trays. Changing the number of passes to 2 on the Specifications page tab produces a large change in the calculated diameter, dropping it from 7.73 to 5.91 m.
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