Weep wells

Figure 13 shows two pipe distributors, one in a branched and one in a ring configuration. These distributors minimize weeping, have good turndown, may requite the lowest pressure drop, and avoid the need for a plenum chamber. They are also well suited to multiple-level fluid injection. The disadvantages of these distributors are that there are defluidized soHds beneath the distributor and the mechanical design is more complex. 

Many of the above factors that affect column operation are due to vapor flow conditions being either excessive or too low. Vapor flow velocity is dependent on column diameter. Weeping determines the minimum vapor flow required while flooding determines the maximum vapor flow allowed, hence column capacity. Thus, if the column diameter is not sized properly, the column will not perform well. Not only will operational problems occur, the desired separation duties may not be achieved. 

Reading the intersection of 3.45 vs. 2.08 shows that for either weep point curve, the weep point is well below the ralues for operation, so this design not near the weep point... 

The lower limit of the operating range occurs when liquid leakage through the plate holes becomes excessive. This is known as the weep point. The vapour velocity at the weep point is the minimum value for stable operation. The hole area must be chosen so that at the lowest operating rate the vapour flow velocity is still well above the weep point. 

So minimum operating rate will be well above weep point. 

At last I was too weary to weep any longer, but half-lay against the cushions on the settle. Mai sat with her arms about me, staring into the fire. When my woman entered with comfits and sweet wine, I could command myself well enough to eat and drink, then asked for the children. Mai had never seen Bridget, and knew the others scarcely more, and she had brought presents for them all. 

From the bubble size and the average flow rate, a first approximation of frequency (thereby total cycle time), as well as the limits between which the chamber pressure oscillates, is determined. The weeping time is calculated from the wave-form equations of chamber pressure, which are ... 

Dumping As gas velocity is lowered below the weep point, the fraction of liquid weeping increases until all the liquid fed to the tray weeps through the holes and none reaches the downcomer. This is the dump point, or the seal point. The dump point is well below the range of acceptable operation of distillation trays. Below the dump point, tray efficiency is slashed, and mass transfer is extremely poor. Operation below the dump point can be accompanied by severe hydraulic instability due to unsealing of downcomers. 

Vapor-Liquid Loads and Reflux Ratio Vapor and liquid loads, as well as the reflux ratio, have a small effect on tray efficiency (Fig. 14-43) as long as no capacity or hydraulic limits (flood, weep, channeling, etc.) are violated. 

Alessa shrugged. Fairly well, considering how different they were. He could never replace his martyred brothers, of course. And the age gap was so great. Enrico is... She mused. Enrico is hard to describe. He shows the world a cold outside, like crystal plate mail. Yet he is passionate. I assure you, heis passionate I have known him to weep with happiness after making love, spilling tears on my breasts. He... 

Figure IW Weep point prewure balance for sieve and valve trays, (a) Sieve tray (6] well-designed valve tray, (c) valve tray with too many velvet or with valves that are too light (d) valve tray with too many valves, but fewer than in C (a) well-designed valve trey with two valve weights. (From w. h. Haifa, Chem. Eng. Png., 72 (B), p 43 (September 1876), reprinted courtesy of the American Institute of Chemical Engineers.) 305... 

Several experiences of severe weeping from valve trays have been reported (1,71,75). A well-designed valve tray is unlikely to have too many valves, but trays with light valves are common in an effort to reduce pressure drop. To avoid the turndown problems, manufacturers often specify a valve tray with two valve weights (Fig. 6.19e). When the light valves open, the heavy ones are still shut, which reduces the ratio of slot to active area and avoids weeping. This practice is discussed in detail elsewhere (1,71). 

Comment. Figure 6.20 predicts that at turndown, neither top nor bottom section will dump. However, both sections will operate close to the dump point, and excessive weeping is likely. This is consistent with the prediction from Fair s weep correlation (above), which indicates that at turndown both sections operate well below the weep point. 

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