Tower flooding result

But there s a nasty potential problem with inlet weirs. If downcomer A bulges out (see Fig. 5.5), then the gap shown as distance X between the inlet weir, which seals the downcomer, and the downcomer itself will be reduced. As I ve seen before, when this happens, tower flooding results. So maintaining the gap shown as distance X in Fig. 5.5 is very important. 

Another important consideration in tower design is tray downcomers size. At high ratios of liquid flow to vapor flow a proportionally greater area on the tray must be allotted to the downcomer channel opening. Downcomers are designed from basic hydraulic calculations. If the downcomer is inadequately sized and becomes filled with liquid, liquid level will build on the tray above. This unstable situation will propagate its way up to the tower and result in a flooded tower condition. Excessive entrainment can also lead to this same condition and, in fact, is usually the cause of flooding. 

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 a result, the tower flooded above the holddown plate. To fix this problem, the packing holddown was dropped 15 in below the chimney, orifice plate, distributor. As a result, the tower fractionated properly. 

Whether the overhead vapor line contains liquid can be a good indicator of tower flooding. For example, a propane-isobutane splitter had a high C4 content in the overhead vapor line, and high C3 content in the bottoms. Both reflux and feed were running at rates that normally resulted in good fractionation. 

Normally, a change in reflux rate should improve fractionation. If certain changes in overall tower operations do not occur as a result of changing the reflux rale, tower flooding can be suspected. 

Foam-induced flooding has been considered a problem occurring on tray decks or inside packed beds. Certainly this is correct. For many columns, however, it is a high foam level formed in the bottom of the tower which causes premature flooding. When this foam level rises to cover the reboiler vapor return nozzle, flooding results. 

Consider a distillation column whose heat input is being controlled in ratio to the feed rate. Throughout the normal operating range, this ratio would be maintained. But even if the feed should drop to zero, heat input must not, because it could cause loss of liquid in the trays. An excessively high heat input is also to be avoided, because flooding of the tower could result. To avoid the possibility of these accidents, high and low limiters can be used, as illustrated in Fig. 6.18. 

This is like unsealing a tray s downcomer in a distillation tower. If the bottom edge of the downcomer from a tray is above the top edge of the outlet weir on the tray below, then vapor can blow up through the unsealed downcomer. This will prevent the internal reflux ft om draining down the column. Tower flooding and loss of product separation efficiency will result. This is called liquid flooding or excessive downcomer backup due to loss of the dovmcomer liquid seal. 

Kaiser [140] presents a correlation analysis for flooding in packed towers that is more analytical in the performance approach. It is based on single phase hydraulics. It would have been helpful for the article to present a comparison of results tvith the other more conventional techniques. 

Up to the present time, work has been done which allows prediction of the onset of large waves (H2), and of formation of other types of waves (VI, HI), but only on flat uniform liquid surfaces. The extent to which these results can be applied to pipe line flow is uncertain. Apparently, Gazley s papers are still the only basic reports of stratified and wave flow in horizontal pipe incidentally they also show a parallel between liquid instability in pipe flow as evidenced by wave formation, and that evidenced in packed towers by flooding. 

Figure 1.10 illustrates this point, from plant test data obtained in a Texas refinery. Point A is called the incipient flood point, that point in the towers operation at which either an increase or a decrease in the reflux rate results in a loss of separation efficiency. You might call this the optimum reflux rate, that would be an alternate description of the incipient flood point, neglecting the energy cost of the reboiler steam. 

It is a characteristic of process equipment, that the best operation is reached, at neither a very high nor a very low loading. The intermediate equipment load that results in the most efficient operation is called the the best efficiency point. For distillation trays, the incipient flood point corresponds to the best efficiency point. We have correlated this best efficiency point, for valve and sieve trays, as compared to the measured pressure drops in many chemical plant and refinery distillation towers. We have derived the following formula ... 

The liquid on the tray deck was at its bubble, or boiling, point. A sudden decrease in the tower pressure caused the liquid to boil violently. The resulting surge in vapor flow promoted jet entrainment, or flooding. 

Remember, though, that the increased tower-bottom liquid level will not be reflected on the indicated bottom level seen in the control room, which is actually the level at the end of the kettle reboiler. This is a constant source of confusion to many operators, who have towers that flood, as a result of high liquid levels, yet their indicated liquid level remains normal. 

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