Tower-bottom liquid level

Tower-bottom liquid level covering the reboiler vapor return nozzle. 

Increasing the tower bottoms liquid level. However, should this level reach the reboiler return nozzle, thermosyphon flow will be restricted, or even stop. Then, the reboiler heat duty will be reduced, and the tower pressure will drop. 

Note that in a kettle reboiler, the bottoms product level control valve does not control the level in the tower it controls the level on the product side of the reboiler only. The liquid level on the boiling or heat-exchanger side of the kettle is controlled by the internal overflow baffle. But what controls the tower-bottom liquid level ... 

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. 

However, the tower flooding was being induced by a high tower bottoms liquid level. As the reboiler duty was increased, the reboiler pressure also increased. This raised the level in the tower bottoms. When this level reached the reboiler vapor return nozzle, the tower flooded due to entrainment of liquid from the bottom of the tower up to the bottom tray. The operators had noticed that the liquid level rose when the reboiler duty was increased, but they thought that the bottoms liquid level would not cause tray flooding until this level reached the bottom tray. This point deserves emphasis ... 

Most often, tower packing supports are dislodged, or distillation trays upset, when a tower is operated with an excessive bottom liquid level. Forcing heat into a tower when the liquid level is several feet above the bottom tray deck often results in dislodging the bottom few trays. Occasionally, trays are mis-assembled during a turnaround. The results of either of these misadventures is diminished fractionation efficiency. 

My field experience has shown that the most common cause of flooding in mechanically intact lowers is fouling. A close second is high bottoms liquid levels. For towers served by forced circulation reboilers, a high liquid level will cause the tower to flood. 

Partial plugging of the combination tower bottoms screen. This screen keeps large pieces of coke out of the suction of the coking heater charge pump. Figure 2-2 shows the effect of a carry-over on the screen. A minor foamover will plug its bottom section. A symptom of this is the need to raise the combination tower bottom s level to secure a steady flow of liquid to the healer s charge pump. 

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. 

The kettle unit used in the reboiling service usually has an internal weir to maintain a fixed liquid level and tube coverage. The bottoms draw-off is from the weir section. The reboiling handled in horizontal thermosiphon units omits the disengaging space because the liquid-vapor mixture should enter the distillation tower where disengaging takes place. The chiller often keeps the kettle design but does not use the weir because no liquid bottoms draw off when a refrigerant is vaporized. 

For once-through natural circulation reboilers, the liquid backup height is calculated from the pressure balance equation. If this height, plus an allowance for froth, reaches the bottom tray level, flooding of the tower will occur. 

The great advantage of forced circulation is that careful calculation of the pressure drop through the reboiler and associated piping is not critical. But, as we can see in Fig. 5.6, the operator now has two tower-bottom levels to control. Further, if the hot-side liquid level rises above... 

Note that it is the elevation, or the static head pressure, in the tower that drives the kettle reboiler. That is why we call it a gravity-fed reboiler. Also, the pressure in the kettle will always be higher than the pressure in the tower. This means that an increase in the reboiler heat duty results in an increase of liquid level in the bottom of the tower. 

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. 

In our calculation above, we had 4 ft of liquid in the glass and 5 ft of liquid in the tower. But what happens if the distance between the two taps is 4 ft 6 in I have drawn a picture of the observed result in Fig. 6.2. Liquid circulates through the glass, pouring through the top tap, and draining through the bottom tap. The apparent liquid level would then be somewhere between 4 ft 0 in and 4 ft 6 in let s say 4 ft 2 in. The indicated liquid level on the control room chart would then be 92 percent (i.e., 4 ft 2 in-i-4 ft 6 in). As the liquid level in the tower increases from 5 ft to 1000 ft, the indicated liquid level would remain at 92%. 

Once the actual liquid level inside the tower bottom rises above the top-level tap, no further increase in level can be observed in the gauge glass. 

The particular problem I encountered is illustrated in Fig. 10.4. The jet fuel product was steam-stripped to remove a lighter naphtha contaminant. But much naphtha was left in the jet fuel. Apparently, the packing in the stripper tower was not working properly. However, a discussion with the unit operator indicated that they were using very little stripping steam. Introduction of a normal amount of steam resulted in a loss of liquid level in the bottom of the stripper. 

The problem is to find how the heat transfer rate can vary when the other quantities change. U is an experimental value that is known only to a certain accuracy. AT may be uncertain because of possible fluctuations in regulated steam and tower pressures. A, the effective area, may be uncertain because the submergence is affected by the liquid level controller at the bottom of the column. Accordingly,... 

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