Tray Capacity Limits

Sieve tray capacity limited by coalesced layer flood... 

Capacity Limitations. The fluid flow capacity of a bubble tray may be limited by any of three principal factors. 

Tray active panels installed with valves under downcomer of tray above. Limited capacity. High and unstable pressure drop at moderate rates. Installation and inspection error. 

Trays operate within a hydraulic envelope. At excessively high vapor rates, liquid is carried upward from one tray to the next (essentially back mixing the liquid phase in the tower). For valve trays and sieve trays,. i capacity limit can be reached at low vapor rates when liquid falls through the rray floor rather than being forced across the active area into tlic downcomers. Because the liquid does not flow across the trays, it rass.scs contact with the vapor, and the separation efficiency drops dramatically. ... 

Figure 8-123 illustrates a typical sieve tray capacity chart. Entrainment by jet flooding or limitation by downcomer flooding are two of the main capacity limiting factors. The liquid backup in the downcomer must balance the pressure drop across the tray, with the process balance [209]. 

Kunesh [126] presents tm overview of the basis for selecting rsuidom packing for a column application. In first deciding between a trayed tower or a packed one, a comparative performance design and its mechanical interpretation should be completed, considering pressure drop, capacity limitations, performance efficiencies (HETP), material/heat balances for each alternate. For one example relating to differences in liquid distribution performance, see Reference 126. 

Hole Sizes Small holes slightly enhance tray capacity when limited by entrainment flood. Reducing sieve hole diameters from 13 to 5 mm ( to in) at a fixed hole area typically enhances capacity by 3 to 8 percent, more at low liquid loads. Small holes are effective for reducing entrainment and enhancing capacity in the spray regime (Ql < 20 m3/hm of weir). Hole diameter has only a small effect on pressure drop, tray efficiency, and turndown. 

Baffle Trays Baffle trays ( shed decks, shower decks ) (Fig. 14-28 ) are solid half-circle plates, sloped slightly in the direction of outlet flow, with weirs at the end. Gas contacts the liquid as it showers from the plate. This contact is inefficient, typically giving 30 to 40 percent of the efficiency of conventional trays. This limits their application mainly to heat-transfer and scrubbing services. The capacity is high and pressure drop is low due to the high open area (typically 50 percent of the tower cross-sectional area). Since there is not much... 

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 only devices capable of debottlenecking a tray system-limit device are those that introduce a new force that helps disentrain the vapor space. Devices that use centrifugal force (see Centrifugal Force Deentrainment ) are beginning to make inroads into commercial distillation and have achieved capacities as high as 25 percent above the system limit. Even the horizontal vapor push (see "Truncated Downcomers/Forward-Push Trays ) can help settle the entrained drops, but to a much lesser extent. It is unknown whether the horizontal push alone can achieve capacities exceeding the system 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. 

In contrast with vapor-liquid columns, tray efficiencies are very low (5 to 30%) in liquid-liquid systems. The trays do limit continuous phase backmixing as well as provide drop redispersion, but at the expense of reduced capacity. 

The downflowing liquid is transported from a tray to the tray below by means of conduits called downcomers, and it is evident that if the downcomer is not sufficiently large to handle the required liquid load, the pressure drop associated with liquid flow will serve as a constriction and a point of flow rate limitation. In fact, downcomers usually serve to bottleneck operations of high-pressure fractionators and absorbers. They must be sized such that they do not All completely under the highest flow rates expected for the column. As will be shown, the vapor flow rate contributes toward the liquid capacity limitation. 

There are two capacity limits related to liquid loading, which are the downcomer baekup limit and downcomer veloeity limit. The downcomer backup limit is set at 80% of liquid settling height based on the froth level. The downcomer velocity limit is 75% of maximum velocity allowed to avoid downeomer choke. The number of tray passes is the most important parameter affeeting the downcomer loading and thus these two downcomer limits. 

The following example is designed with the purpose of enhancing the understanding of column tray design, operation window and the capacity limits. 

Step 1 Determine capacity limits for spray, tray flooding, downcomer backup, downcomer velocity, weeping, liquid rates, and so on. 

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