Bottom sump

Arrangements such as bottom sumps, chimney trays, side-draw drums, and surge drums are usually designed for avoiding the presence of vapor in the liquid outlet. On the other hand, downcomer trapouts are usually designed to allow for the presence of vapor in the outlet liquid. 

Providing adequate residence time is a primary consideration in arrangements that avoid vapor in liquid outlets. Such arrangements t )-ically include bottom sumps, chimney trays, external side-drawoff drums, or surge drums. Sufficient residence time must be provided in the liquid-drawoff sump for one or more reasons ... 

When the controlling consideration is satisfactory vapor disen-trainment, the relevant volume is from the top of the liquid outlet nozzle to the normal liquid level. The relevant liquid flow rate is the total liquid flow rate leaving the sump. For example, if the bottom sump is not separated by a baffle from the reboiler compart-... 

When the controlling consideration is surge volume to buffer the column or downstream equipment from upsets, the relevant liquid volume is from the low to the normal (or normal to high, depending on the nature of upsets expected) liquid level. The relevant liquid flow rate is only the liquid stream going to the next unit. In the above bottom sump example, the relevant liquid flow rate for buffering from upsets is the bottom product flow. 

Guidelines for sump residence time are scarce in the published literature. Those given by Wheeler (420) for chimney trays are the only detailed list available (Table 4.1). Wheeler (420) did not state the residence time definition on which his guidelines are based. The author believes that their application with the above definition is reasonable for chimney trays, bottom sumps, and side-draw drums. Caution is needed when these guidelines are applied, particularly when the main consideration is to buffer upsets. 

Unbaffled arrangement (Fig. 4.4a) Here both bottom product and reboiler liquid are withdrawn from a common bottom sump. 

Once-through reboiler arrangement Here reboiler liquid is withdrawn from the bottom downcomer or from a chimney tray located above the bottom sump. Typical arrangements are shown in Fig. 4.7. 

In services where the feed consists mainly of lights and a small amount of residue, in order to avoid residue accumulation in the reboiler loop and thickening of material due to excessive residence times in the bottom sump. 

Figure 4.6 Preferential-baffle arrangements for bottom sumps. (Henry Z. Kister. Excerpted by special permission from Chemical Engineering, July 28,1980, copyright by McGraw-Hill, Inc., New York, NY 10020.)...

They maximize bottom sump residence time when vapor disengagement is the main consideration. 

Column bottom sumps must provide sufficient residence time for vapor disentrainment (see guideline 4 for the only exception to this rule). Inadequate vapor disentrainment may lead to cavitation of the bottom pump or choking of the bottom line. 

When the bottom sump does not supply liquid to a reboiler (e.g., when a reboiler trapout pan is used, or when the column has a bottom feed and no reboiler), the design in Fig. 4.86 can be used with thermally unstable materials. This arrangement eliminates the need for a surge drum and self-venting lines, and immediately quenches liquid reaching the column base. 

When reboiler circulation is much larger than bottom flow rate, baflles can cause unstable bottom flow. Small variations in the reboiler heat input or column flow rates show up as large changes in reboiler sump overflow, and therefore in bottom sump level. Bottom level fluctuations cause bottom flow fluctuations, which may be troublesome if the bottom flow is the feed to another column or is used to preheat column feed. This problem is illustrated elsewhere (258) and is most severe when the bottom sump compartment is relatively small. Providing additional residence time in the bottom sump can minimize this problem. 

Additional guidelines for differential-pressure level transmitters on column-bottoms sumps have been presented by Buckley (67). 

To enable access from the bottom sump (where the manhole is often located) to the bottom tray, rungs are often installed at the column shell. Internal ladders are also sometimes used for the above purpose, but these may corrode and become unsafe. Corrosion-resistant materials are recommended if ladders are to be used. 

Using side downcomers below the bottom tray is especially recommended when a preferential baffle (Sec. 4.5) divides the bottom sump into a reboiler compartment and a bottom compartment. Here, an overflow weir arrangement (Fig. 4.6cD is commonly used to direct the liquid from the bottom downcomer(s) into the reboiler compartment. This arrangement works far better with side downcomers than with center downcomers. The author is familiar with an experience where a column with a center downcomer below the bottom tray failed to achieve its separation efficiency because liquid from the bottom tray overflowed the higher weirs (Fig. 4.6d) into the bottom compartment. 

As much liquid as possible should be drained out of a hot column prior to and during the initial cooling period. This is particularly important if the liquid solidifies or becomes highly viscous as it cools. The author experienced a case where overlooking this guideline caused a hard, solid mass to form at the bottom sump. The mass was extremely difficult to break up. 

In another case (150a), a subcooled feed entered the bottom sump of a packed rectifier. Preheating this feed consumed 40 percent of the boilup the balance entered the rectifier s packed section. When the feed was shut off, the heat sink was eliminated, causing excessive vapor flow and consequent flooding in the packing. The problem was solved by installing an override controller that would cut back boilup upon excessive packing pressure drop. 

If the liquid level in the bottom sump of the column rises above the reboiler return nozzle (or, alternatively, the bottom vapor inlet nozzle), vapor from the reboiler has to travel upward through a layer of liquid. If this layer is shallow, the vapor can bubble through it or atomize the liquid and carry over liquid as a mist into the first tray from the bottom. This may lead to premature flooding (71, 145, 192, 207, 237, 238) and possibly some wave action that would interfere with level control. 

Low liquid levels can be as troublesome as high liquid levels. When bottom level is lost, vapor can flow out of the column bottom. In one incident (210), such vapor flow ruptured the bottom product storage tank. Low bottom levels can also cause cavitation and overhehting of bottom pumps. In some services, a low bottom level can excessively cottCentrate some chemicals, inducing an undesirable reaction. If these chemicals are unstable (e.g., peroxides, acetylenic compounds), an explosion may result. Some reported accidents (97, 275) were initiated by low liquid levels at the bottom sump. 

When a low level can cause such hazards, it is a good practice to install low-level alarms or trips. The trips usually cut the bottom flow rate, or introduce a diluent into the bottom sump. 

Figure 13.8 A faulty level sensor can cause overheating, (a) A nudfunctioning level sensor mistakenly senses a high level. It opens bottom valve. (6) Level drops and uncovers temperature sensor. If vapor temperature is lower, the controller will 1 for more heat, (c) Level is low, and bottom sump liquid is overheating, hut instruments indicate normal level and normal temperature.

Liquid flows from the bottom sump of the column to the reboiler base, where it is distributed to the tubes. The feed leg contains resis-... 

Excessive circulation. Excessive circulation occurs when reboiler sump level is too high and cannot be lowered (e.g., when the level is set by the top of a preferential baffle in the bottom sump). This may restrict heat transfer rate as described in the previous section. This problem is uncommon in pressure systems, but more widespread in vacuum and atmospheric reboilers. 

Insufficient circulation is usually caused by plugging, a leaking reboiler preferential baffle or draw pan, or by insufficient liquid head (alternatively, excessive pressure drop in the reboiler loop). Leakage across the preferential baffle is implied when the bottom sump level influences reboiler heat transfer rate despite the presence of a baffle. 

Installing a valved dump line connecting the column bottom outlet line with the reboiler inlet line (Fig. 15.4a). This technique is only needed when the column reboiler sump is separated from the column bottom sump by a baffle or when the reboiler liquid comes from a trapout pan. The valve remains shut during normal operation, but is opened during startup to lower the level and inspire thermosiphon action during startup. One case where this technique was successfully used has been described (237) the author has had several similar experiences. 

Bottom product surge. The liquid draw compartment of kettle reboilers is much smaller than most column bottom sumps, and usually provides less liquid residence time and product surge. It is often impractical to incorporate the desired residence time (Sec. 4.4) in this draw compartment, and one needs to either live with the lower residence time or add a surge drum downstream of the reboiler. 

In an MB control scheme, product composition is controlled by manipulating the flow of material into and out of the column. This concept can be illustrated by examining the action of one of the common MB control schemes (Fig. 16.4a). Suppose the concentration of lights rises in the column feed. This will be sensed by a temperature drop, and the temperature controller will increase boilup. This will raise column pressure, and the pressure controller will step up condensation. Accumulator level will rise, and the level controller will increase distillate rate. Meanwhile, the increased boilup mentioned above will reduce the amount of liquid reaching the bottom sump, and the level controller will lower bottom product rate. 

Figure 17.7e shows column pressure control by adjusting column boilup. This method is complex, but it has worked smoothly in some instances (234). Either a flooded or a nonflooded reflux drum can be used in the latter case, reflux drum level can regulate the rate of condenstaion. Bottom flow is regulated by the bottom sump level. This method may be beneficial in some stripping columns receiving subcooled feeds, where feed temperature variations can affect column pressure to a larger extent than overhead condenser action. 

Sump level. Usually, the bottom level manipulates the bottom flow or boilup. If the bottom sump is unbaffled, it is commonly used as a liquid source for both the reboiler and bottom product. In this case, for certain reboiler types (e.g., thermosiphon), the level must be controlled within a narrow range in order to supply a constant head to the reboiler. Level fluctuations may lead to unsteady reboiler action and column upsets (Sec. 15.3). If a baffle is installed, tight level control is normally not required, particularly if bottom product goes to storage. 

Insufficient surge volume between columns. This usually compels level controller tuning for a fast response (in the bottom sump or reflux drum), causing flow fluctuations. 

A preferential baffle in the bottom compartment (Sec. 4.5) increases the tendency of a bottom sump level and, therefore, bottoms flow, to fluctuate. 

---