Capacity, trays lower limit

FIGURE 12.29 Design chart for estimating maximum vapor capacity of crossflow trays (both bubble cap and sieve trays). The lower values at the left indicate that liquid capacity prevails over vapor capacity as the limiting parameter. 

The condenser pressure controls the tower pressure and thus the feed tray pressure. There is a pressure valve in the overhead, which can be used to control tower pressure. The lower limit of the tower pressure is defined by the column overhead condensing duty, net gas compressor capacity, and column flood condition. During extended turndown periods, reducing pressure up against an equipment limit can avoid dumping. Many of the new APC systems have pressure control implemented. 

The deethanizer in a naphtha-fed olefins plant also is a column with a smaller diameter upper section than lower section. This column operates at a pressure between 340 and 410 psia with a top temperature of 0° to 20°F. A trayed column typically contains 45 to 70 actual trays. Again the liquid surface tension is very low, and the density difference between the liquid phase and the vapor phase is small. Therefore, the capacity of a trayed column usually is limited by the downcomers. In a trayed tower, the cross-sectional area is rather large compared to vapor flow rate. Replacing trays with an IMTP system increases the capacity by about 35% above the design rate for the trays. 

K is reduced from 0.6 to 0.3 by using the rounded downcomer outlet. To obtain this feature from the tray vendor, you need to specify a "Full Radius Downcomer Outlet." This will lower the liquid level in the downcomer by an inch or so. If you re limited by downcomer backup, this would provide about an extra 5 percent of capacity. No great improvement. But it doesn t cost very much either. 

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