Note that no derating was used here. The previous flood calculation was derated because of the foaming tendency at high pressure. This is not taken into account in the interpolation, which is based on data at lower pressure. On the other hand, the pressure drop criterion used applies specifically for high-pressure distillations, and should therefore contain any needed derating.
The flood-point and the maximum pressure drop criteria gave comparable tower diameters. The more conservative of the two criteria gives diameters of 5.50 and 6.12 for the top and bottom section of the tower, respectively. As the diameters for the top and bottom sections are not much different, it is attractive to use uniform column diameter. The decision of whether to make the top and bottom section diameters the same is based on the same criterion as for tray columns (Sec. 6.5.3). The preliminary column diameter is the larger for the two column sections, i.e., 6.12 ft. This diameter is normally rounded to the next nearest half foot, but in this example it is rounded only to the next nearest quarter foot. A diameter of 6.12 is far closer to 6 ft them to 6.5 ft. Column diameter is relatively small, and three excessive inches substantially increase the costs. The column is operated at high pressure, and high-pressure shells are expensive. Therefore, the preliminary column diameter is 6 ft 3 in.
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