Capacity, trays upper limit

Figure 6.6 is a typical tray stability diagram. The area of satisfactory operation (shaded) is bound by the tray stability limits. These limits are discussed in the following sections. The upper capacity limit is the onset of flooding. At moderate and high liquid flow rates, the entrainment (jet) flooding limit is normally reached when vapor flow is raised, while the downcomer flooding limit is normally reached when liquid flow is raised. When flows are raised while the column operates at constant LIV (i.e., constant reflux ratio), either limit can be reached. At very low liquid rates, as vapor rate is raised, the limit of excessive entrainment is often reached. 

Flooding is by far the most common upper capacity limit of a distillation tray. Column diameter is set to ensure the column can achieve the required throughput without flooding. Towers are usually designed to operate at 80 to 90 percent of the flood limit. 

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. 

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