Dumping trays

Trays suffer from lost tray efficiency as a result of both flooding and dumping. Trays have some entrained droplets of liquid lifted by the flowing vapors to the trays above. This tends to blow butane up into the lighter propane product. Perforated trays always have some leakage of liquid through the tray deck to the trays below. This tends to drip propane down into the heavier butane product. 

The chemist now gives her knuckles a crack and begins. An appropriate sized flask or PP container is placed in a tray of water on the stirplate. Into the flask is dumped 800mL acetone, 25mL... 

Fractionators and Other Towers - An equivalent "tower dumped" level is calculated by adding the liquid holdup on the trays to the liquid at normal tower bottom (high liquid level). The surface that is wetted by this equivalent level and which is within 7.5 m of grade is used. 

Dumping A condition caused by low vapor rates where all of the liquid falls through some holes (rather than over the weir) to the tray below, and vapor rises through the restnaining holes. 

These trays will dump liquid excessively through the perforations giving exceeding low efficiencies [47] unless a minimum vapor rate is maintained for a given liquid capacity. The smaller the holes the lower the dump point (vapor velocity). 

It is recommended that trays be designed for a minimum of 10% above the lower plate activation values. Below these values the tray will dump liquid and become inoperable. 

Tray efficiency is as high as for bubble caps and almost as high as sieve trays. It is higher than bubble caps in some systems. Performance indicates a close similarity to sieve trays, since the mechanism of bubble formation is almost identical. The real point of concern is that the efficiency falls off quickly as the flow rate of vapor through the holes is reduced close to the minimum values represented by the dump point, or point of plate initial activation. Efficiency increases as the tray spacing increases for a given throughput. 

Note that Figure 8-147 indicates the operating liquid minimum range is quite stable in the region of design for these trays. The vapor rate must never fall below the above values or instability will immediately set in and dumping will result. 

If there is the possibility of apor and liquid rates being reduced to 50% of the indicated values, this would place the trays as selected above at the dumping point, or activation point, which is not a good operating condition. In this situation the number of holes should be reduced in order to maintain a velocity of vapor through the holes greater by at least 15% than the activation velocity. 

Greater capacity and efficiency than fractionation trays and other dumped packings. 

Jell-O shots, one of those rare, raucous cocktails you do, not drink, is enjoying improbable popularity this summer in New York, and it s gotten kind of fancy on itself, too. This is not the tilt, slip and slurp finger-food shot dumped from an ice cube tray and set quivering on a paper napkin on the bar, ready to rock. 

The vapor flowing between trays was at its dew point. A sudden increase in tower pressure caused a rapid condensation of this vapor and a loss in vapor velocity through the tray deck holes. The resulting loss in vapor flow caused the tray decks to dump. 

Either way, erratic tower pressure results in alternating flooding and dumping, and therefore reduced tray efficiency. While gradual swings in pressure are quite acceptable, no tower can be expected to make a decent split with a rapidly fluctuating pressure. 

As the weir height of the trays is 3 in, it is a safe assumption that the low tray efficiency is due to tray deck dumping, rather than flooding. As shown in Fig. 3.3, this column has no reflux. This is a typical design for strippers when feed is introduced on the top tray, there is no need for reflux. 

The net effect of reducing the stripper pressure was to greatly reduce the amount of isobutane in the heavier normal butane bottoms product. Undoubtedly, most of the improvement in fractionation was due to enhanced tray efficiency, which resulted from suppressing tray deck leaking, or dumping. But there was a secondary benefit of reducing tower pressure increased relative volatility. 

The first two factors help make fractionation better, the last factor makes fractionation worse. How can an operator select the optimum tower pressure, to maximize the benefits of enhanced relative volatility, and reduced tray deck dumping, without unduly promoting jet flooding due to entrainment ... 

The problem we have just discussed—poor fractionation efficiency due to inadequate vapor and liquid initial distribution—is rather similar to tray deck dumping in trayed fractionators. And, just like trays, packed towers are also subject to flooding. 

As discussed in Chap. 2, tray deck dumping also greatly reduces tray efficiency. Unfortunately, steam strippers can have widely varying vapor rates, between the top and bottom trays of a column. 

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. 

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