Tray liquid accumulation

As illustrated, liquid accumulates on the low side of this tray. Vapor, taking the path of least resistance, preferentially bubbles up through the high side of the tray deck. To prevent liquid from leaking through the low side of the tray, the dry tray pressure drop must equal or exceed the sum of the weight of the aerated liquid retained on the tray by the weir plus the crest height of liquid over the weir plus the 2-in out-of-levelness of the tray deck. 

Flooding is an excessive accumulation of liquid inside a column. Flood symptoms include a rapid rise in pressure drop (the accumulating liquid increases the liquid head on the trays), liquid carryover from the column top, reduction in bottom flow rate (the accumulating liquid does not reach the tower bottom), and instability (accumulation is non-steady-state). This liquid accumulation is generally induced by one of the following mechanisms. 

Entrainment (Jet) Flooding Froth or spray height rises with gas velocity. As the froth or spray approaches the tray above, some of the liquid is aspirated into the tray above as entrainment. Upon a further increase in gas flow rate, massive entrainment of the froth or spray begins, causing liquid accumulation and flood on the tray above. 

Downcomer Backup Flooding Aerated liquid backs up in the downcomer because of tray pressure drop, liquid height on the tray, and frictional losses in the downcomer apron (Fig. 14-32). All these increase with increasing liquid rate. Tray pressure drop also increases as the gas rate rises. When the backup of aerated liquid exceeds the tray spacing, liquid accumulates on the tray above, causing downcomer backup flooding. 

Downcomer Choke Flooding This is also called downcomer entrance flood or downcomer velocity flood. A downcomer must be sufficiently large to transport all the liquid downflow. Excessive friction losses in the downcomer entrance, and/or excessive flow rate of gas venting from the downcomer in counterflow, will impede liquid downflow, initiating liquid accumulation (termed downcomer choke flooding) on the tray above. The prime design parameter is the downcomer top area. Further down the downcomer, gas disengages from the liquid and the volumes of aerated liquid downflow and vented gas... 

Fractionation, by definition, is simply the mass transfer between a liquid phase and a gas phase in contact with each other. A fractionation column is simply a tall, vertical, cylindrical pressure vessel that contains numerous flat internal metal plates called trays. Each tray allows liquid to flow over it, so the liquid flows from tray to tray by the force of gravity. The liquid thus enters the top tray. The liquid portion not vaporized in the column s trays is taken out in the column s bottom liquid accumulation. Gas enters the column s bottom section and flows through each tray to the top section. Entering vapor pressure is its driving force. Gas not absorbed by the liquid exits the column s top section. 

Spray entrainment flooding (Fig. 6.7a). At low liquid flow rates, trays operate in the spray regime, where most of the liquid on the tray is in the form of liquid drops (Figs, 6,25c and 6.276), As vapor velocity is raised, a condition is reached where the bulk of these drops are entrained into the tray above. The liquid accumulates on the tray above instead of flowing to the tray below,... 

Froth entrainment flooding (Fig. 14.1b). At higher liquid rates, the dispersion on the tray is in the form of a froth (Fig. 6.1a). When vapor flow rate is raised, froth height increases. When tray spacing is small, the froth envelope approaches the tray above. As this surface approaches the tray above, entrainment rapidly increases, causing liquid accumulation on the tray above. 

Downcomer choke flooding (Fig. 6.7d). As liquid flow rate increases, so does the velocity of 2ierated liquid in the downcomer. When this velocity era eeds a certain Umit, M on losses in the downcomer and downcomer entrance become excessive, and the frothy mixture cannot be transported to the tray below. This catises liquid accumulation on the tray above. 

If the liquid velocity becomes too large, the liquid accumulates in the downcomer, and if the velocity is reduced too much we get inequalities of the flow on the tray. Then it becomes difficult to hold enough liquid on the tray to enable sufficient gas-liquid contact, and the efficiency decreases. 

Coalesced liquid on the trays The depth h of coalesced dispersed liquid accumulating on each tray is determined by the pressure drop required for counterflow of the liquids [9] ... 

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

Here, we refer to small amounts of water rather than large slugs that could damage the trays. Often the water will boil overhead and be drawn off in the overhead accumulator bootleg (water drawoff pot). However, if the column top temperature is too low, the water is prevented from coming overhead. This plus too hot a bottom temperature for water to remain a liquid will trap and accumulate water within the column. The water can often make the tower appear to be in flood. 

The maximum capacity of a tray column is usually limited by the onset of flooding, which occurs when liquid excessively accumulates inside the column. Flooding is... 

---