Trays tower pressure drop/flooding

The most conservative definition of vapor flooding capacity is the load for which tower pressure drop exhibits a sharp increase, signifying liquid buildup at some tray. It is, however, possible to operate the tower at somewhat higher load, perhaps 10% more. As the load is increased, reboiler pressure rises, allowing a semistable operation, albeit with reduced separating... 

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 ... 

Should the liquid level in the bottom of the tower rise to the reboiler vapor return nozzle, the tower will certainly flood, but the reboiler heat duty will continue. Unfortunately, reboiler shell-side fouling may also lead to tray flooding. This happens because the fouling can cause a pressure-drop buildup on the shell side of the reboiler. 

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. 

The flood-point and the maximum pressure drop criteria gave comparable tower diameters. The more conservative of the two criteria gives diameters of 5.50 and 6.12 for the top and bottom section of the tower, respectively. As the diameters for the top and bottom sections are not much different, it is attractive to use uniform column diameter. The decision of whether to make the top and bottom section diameters the same is based on the same criterion as for tray columns (Sec. 6.5.3). The preliminary column diameter is the larger for the two column sections, i.e., 6.12 ft. This diameter is normally rounded to the next nearest half foot, but in this example it is rounded only to the next nearest quarter foot. A diameter of 6.12 is far closer to 6 ft than to 6.5 ft. Column diameter is relatively small, and three excessive inches substantially increase the costs. The column is operated at high pressure, and high-pressure shells are expensive. Therefore, the preliminary column diameter is 6 ft 3 in. 

Operation of bubble cap trays at high vapor rate in effect may be limited by massive entrainment of a liquid into the vapor stream. This causes high-pressure drop for vapor flow, which in turn prevents liquid from flowing down the tower. Thus, while the term vapor flood is often applied to conditions of high entrainment, it is also true that flooding refers to a failure to handle liquid flow. 

Ultimately, purely mechanical difficulties arise. High pressure drop may lead directly to flooding. With a large pressure difference in the space between trays, the level of liquid leaving a tray at relatively low pressure and entering one of high pressure must necessarily assume an elevated position in the downspouts, as shown in Figure 4.6. As the pressure difference increases due to an increased rate of flow of the gas, the level in the downspout will rise further to permit the liquid to enter the lower tray. Ultimately, the liquid level in the downcomer may reach that on the tray above and the liquid will fill the entire space between the trays. The tower is then flooded, the tray efficiency falls to a very low value, the flow of gas is erratic, and liquid may be forced out of the gas exit pipe at the top of the tower. 

Because of their simplicity and low cost, sieve (perforated) trays are now the most important of tray devices. In the design of sieve trays, the diameter of the tower must be chosen to accommodate the flow rates, the details of the tray layout must be selected, estimates must be made of the gas-pressure drop and approach to flooding, and assurance against excessive weeping and entrainment must be established. 

If the liquid is unable to flow down the downcomer fast enough, the liquid level will increase, and if it keeps increasing until it reaches the top of the weir of the tray above, the tower will flood. This downcomer flooding must be prevented. Downcomers are designed on the basis of pressure drop and liquid residence time, and their cost is relatively small. Thus, downcomer design is done only in the final equipment sizing. 

In both downcomer back up and choke cases, downcomer liquid inventory increases and downcomer liquid backs up until the downcomer froth level reaches the tray above H > Hs). This phenomenon is called downcomer flood. When downcomer flood occurs to any tray, the whole tower will be flooded very quickly. A tower under downcomer flood provides virtually no distillation. In contrast, under tray flood, liquid can still leave the tower and the tower could stiU operate if the control system allows it although distillation efficiency suffers. Downcomer flood can be prevented in design by providing adequate downcomer area and clearance underneath the downcomer and minimizing tray pressure drop. Reducing reflux rate in operation could be effective in avoiding downcomer flood in operation. 

A common example of foam formation in the bottom of a fractionator inducing flooding occurs in a crude preflash tower. In this case, stable foam accumulates in the bottom of the column as a consequence of the "flow improver" chemicals added to crude oil. These chemicals reduce pressure drop in the crude pipelines. Once the foam level rises to the feed inlet nozzle, the trays flood and black distillate is produced. Please see Chapter 18 (Preflash Towers). 

Flooding was being initiated from the bottom of the tower. I knew this because 79 in. of water-pressure drop represented a AP per tray of about 7 in. of water. Therefore, each of the trays below the LCO product draw tray was flooding. Put another way, the apparent fluid specific gravity on a flooded tray deck in hydrocarbon service is typically 0.3. For trays spaced 24 in. apart, the observed pressure drop corresponding to a fully flooded tray is then ... 

When I inspected the tower, I found an absolutely positive indication of floodings a vent line on the tower overhead line emitted liquid when cracked open. This observation, coupled with the low measured pressure drop per tray, indicated that only the top tray was flooding. When we opened the tower for inspection, vve found the deck of the top tray encrusted with corrosion products. These deposits had caused a high pressure drop on only the lop tray and had caused the downcomer on only the top tray to back-up. 

As the reflux rate was increased, the differential pressure gauge reading across the bottom trays would slowly rise. At 48,000 B/SD reflux, the pressure drop would start increasing exponentially. This meant that liquid was accumulating on the tray decks and backing up the downcomers. The tower was starting to flood. 

A tower in need of washing will not always exhibit overt signs of flooding (except for an increase in tray pressure drop.) The alkylation depropanizer would gradually lose fractionating ability. That is, with 5% propane in the bottoms product and feed and reflux rates held the same, the butane content in the tower overhead would increase. When the butane content of the propane product exceeded LPC specs, it was time to wash the tower. This was a simple procedure. The trick was to dry the tower out prior to start-up. 

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