Turndown Checks Based on Third Trial

These checks will use all the parameters in the layout summary in Sec. 6.5.3 (Table 6.11, third trial). The minimum throughput calculations will be performed for stages 8 and 9 in the depropanizer, where the loads are lowest. Since the example requires column turndown to 60 percent of the expected design loads, the vapor and liquid loads shown in Table 6.10 for stages 8 and 9 are multiplied by 0.6 for the turndown checks. 

Weep point check. If the trays operate above their weep points at turndown conditions, checks of the fractional weep and dump point may not be needed. The weep point check will be performed using Fair s correlation (Sec. 6.2.12). 

Dump point check. This check will be performed using the Prince and Chan correlation (Sec. 6.2.14). 

Weep fraction calculation. For high-pressure systems ( 165 psia), the Hsieh and McNulty correlation was recommended (Sec. 6.2.12) and will therefore be used here. The Hsieh and McNulty correlation requires a calculation of dear liquid height using Colwell s correlation 

Comment. The calculation converged on the first trial to within reasonable accuracy. The calculation showed that the weep fraction is small ( 0.07), As per Sec. 6.2.12, a weep fraction of up to 0.1 can be tolerated. This small weep fraction appears inconsistent with what would be expected based on the proximity of the operating point to the predicted dump point and on the large gap between the operating point and the predicted weep point. 

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