Flooded reflux drum

Flooded reflux drums. These are used as part of flooded condenser control schemes (Sec. 17.2.2). A flooded drum is only suitable when substantial fluctuations in product rate can be tolerated (e.g., product goes to storage). A flooded drum provides no surge for controlling and smoothing out product fluctuations, but it maintains surge for the reflux pump and reflux circuit. 

Figure 17.5 Pressure control by condenser flooding, (a) Control valve in condenser outlet ib) flooded reflux drum (c) flooded reflux drum with automatic noncondensables venting [d) hot vapor bypass (c) a poorly pip hot vapor bypass if) control valve in condenser inlet. (Part c from "Unusual Operating Histories of Gas Processing and Olefins Plant Columns, H. Z. Kister and T. C. Hower, Jr., Plant/Operations Progi a, vol. 6. no. 3, p. 153 (July 1987). Reproduced by permis-...

Figure 17.56 illustrates the flooded reflux drum method. Here the level control of the reflux drum is eliminated. The reflux drum runs full of liquid, and sometimes the reflux drum itself can be omitted. The pressure controller directly controls distillate flow. Due to the tight pressure control usually required, distillate flow controlled by this method is likely to fluctuate. These fluctuations may cause instability in downstream units. Distillate control by this method should be avoided (77, 301, 362) unless the product goes to storage. 

Inerts accumulation in flooded reflux drum caused unflooding of the drum and poor control Manual venting could not solve problem because plant was not continuously attended. 

The function of the reflux drum in a flooded condenser design is to... 

Sometimes we see tower pressure control based on feeding a small amount of inert or natural gas into the reflux drum. This is bad. The natural gas dissolves in the overhead liquid product and typically flashes out of the product storage tanks. The correct way to control tower pressure in the absence of noncondensable vapors is to employ flooded condenser pressure control. If, for some external reason, a variable level in the reflux drum is required, then the correct design for tower pressure control is a hot-vapor bypass. 

A fourth degree of freedom is consumed to control column pressure. The valves available are condenser cooling (by far the most commonly7 used), reboiler heat input, and feed (if the feed is partially vapor). If a flooded condenser is used, the cooling water valve is wide open and an additional valve, typically located between the condenser and the reflux drum, is used to cover or expose heat-transfer area in the condenser. 

For example, the cost of a distillation column can be assembled from the cost of elements vertical cylindrical vessel, plus internals (trays or packing), reboiler, condenser, and reflux drum. The height of the shell can be determined from the number of trays and inter-stage height. The column diameter can be found by hydraulic calculations based on the flooding point. In this way, the volume of the cylindrical part can be easily evaluated. The volume of auxiliary vessels, as drum and reboiler, can be estimated from the residence time, typically of 10 minutes. 

Flooding initiated by a deep liquid layer can be particularly severe. In one incident (237), a 150-ft absorber filled with liquid. The liquid overflowed into the reflux drum, and then via a vapor vent into the fuel system, spilling out of burners and initiating several fires. 

In some systems, the reflux drum is omitted or run flooded (e.g.. Fig. 17.56). This eliminates one variable (accumulator level). This also eliminates a manipulated stream by making the condensation rate a slave to the top product rate. Similarly, when an internal condenser is employed, the accumulator level is eliminated, together with one manipulated stream (reflux becomes a slave to the condensation rate). In either situation, the rest of the variables are paired in the normal manner. The discussions below therefore apply to that situation too. 

If the condenser is flooded (Fig. 16.2d), the unpaired variables and streams are the same as with a top liquid product (Fig. 16.2a), except that the reflux drum level and the top product stream are out of the pairing considerations. In the case of the internal condenser in Fig. 16.2c, the pairing is the same as for a vapor top product (Fig. 16.26), except that reflux drum level and the reflux rate are out of the pairing considerations. 

Figure 17.5/ shows a flooded condenser scheme similar to that of Fig. 17.5a, but with the control valve located at the condenser inlet. This method is inferior compared to Fig. 17.5a (77). It requires a larger control valve, is more difficult to understand, and it affects condensation at a lower temperature. The condenser outlet line must enter the reflux drum well below the liquid level. A pressure-equalizing line as in the method shown in Fig. 17.5a is also required.

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. 

Flooded condenser schemes shown in Fig. 17.5a to c and f can be used instead of coolant flow variations. In such cases, the inerts normally leave finm the top of the condenser instead of the reflux drum (Fig. 17.8e). If the reflux drum is not flooded, a pressure balance line must be included otherwise, a stable pressure will be impossible to keep in the reflux drum. An overflow line should also be included in this arrangement. 

Column pressure was controlled using a hot vapor bypass scheme (partially flooded condenser). Severe pressure and reflux drum level upsets occurred whenever the reflux drum surface was inadvertently agitated. 

Condenser controlled using a hot vsqwr bypass (partially flooded condenser). bcooM liquid entered the reflux drum vsqwr qiace (presumably due to unflooding the liquid inlet), and contacted drum vapor that was 100°F hotter. The rapid condensation sucked the liquid leg between the condenser and drum in seconds. 

The manipulated boilup scheme is quite easy to start up after some liquid is accumulated in the reflux drum. However, this scheme does not shed environmental disturbances very well. When a sudden rain storm hits the distillation tower, the temperature disturbance is shed by the temperature controller increasing the steam flow rate to the reboiler. This can be a problem if the distillation tower needs to be running at its maximum capacity near to flooding. 

The plugged top tray will prevent the reflux from cascading down to the lower trays. The liquid reflux will just overflow into the condensers and circulate back to the reflux drum. The tip-off to this problem is that neither the reboiler duty nor the bottoms temperature is affected in the normal way by raising reflux. The tower s heat balance appears as if the reflux rate had never been increased. This is not much different from the signs of normal tower flooding, except that the AP on all but the top tray is not excessive. 

The extra 7,000 B/SD of reflux did not flow down through the tower because the tray decks were already flooding at the 9,000 B/SD reflux rate. The extra reflux overflowed the tower and recycled back to the reflux drum via the overhead condensers. 

The purpose of the hot vapor bypass controller is to pump heat into the reflux drum. Obviously, if one is limited by condensing capacity, introduction of extra heat to the reflux drum aggravates the limitation. Usually, the rubber type seat in the hot vapor bypass butterfly control valve dries out with age and needs to be renewed. My experience is to eliminate the hot vapor bypass control scheme entirely and convert the tower to flooded condenser-type pressure control. 

We must also calculate the size of the reflux drum and the column base. These provide liquid surge capacity, which helps to filter disturbances in both flow and composition to downstream units. They also permit the column to ride through large disturbances without upsetting the column to the point where liquid or vapor hydraulic limitations are encountered (flooding or weeping), which can result in the loss of separation and the production of off-specification products. 

Hi Norman. We have a distillation tower that floods. Delta P on trays below feed point is stable delta P above feed (trays 16-22), increase from 9 to 19 KPA. Condenser and reflux drum is internal in tower, and we cannot measure the reflux rate. Yesterday, bypassed steam trap on reboiler outlet, and flooding stopped. Conclude that flooding tower due to defective steam trap. What s your opinion Note tower fractionation also improved after trap bypassed. 

AiK>ther configuration, as shown in Figure 3.16B, has the condenser mounted above the reflux drum. As suggested by Chin the hot-gas line around the condenser has no valve in it. A valve on the liquid from the condense floods the condenser to hold column [X essure. The liquid in the reflux drum is subcooled, so there is condensation of vapor at the liquid—gas interface in the drum. A vertical reflux drum is recommended to reduce this interfacial area. The liquid line from the condenser should extend down into the liquid in the drum so that the cold liquid is introduced near the bottom of the drum. 

In addition to one or more product compositions, other process variables need to be controlled. Consider the separation of a binary mixture and the conventional tray-distillation column shown in Fig. 13.2. Assume that the chief control objective is to control both product compositions, Xjy and jc. However, the liquid levels in the reflux drum, and the column base (or sump), Hb, must be kept between upper and lower limits (Guideline 2). The column pressure, P, must also be controlled in order to avoid weeping or flooding in the column and to control the vapor inventory in the column. Thus, this column has a total of five CVs. 

Loss of reflux on the column should be considered as a total loss of cooling. It is assmned that with loss of reflux the liquid accumulates and floods the reflux drum, and then the cooler. The cooler loses the cooling duty and the column pressure will increase to its relieving condition. No credit should be taken for the normal outflow of uncondensed vapor. The stahc head of the flooded system should be allowed for in design pressure consideration as explained previously. 

There are several reasons for loss of reflux, including misoperation of isolation valves, reflux pump failure, failure of the control valve, etc. Reflux failure will result in accumulation of more liquid in the reflux drum, eventually flooding the drum and overhead condenser. This will result in loss of cooling. The column pressure will increase and the PRV will lift. 

Conversion of any existing scheme to flooded condenser is more than just a change in control configuration. It requires a full process design check and is likely to result in changes to relief valves and other safety-related systems. There are also a number of ways in which it can be configured. For example the drum can also be flooded thus avoiding the need to install a valve on its inlet. The pressure controller can then either manipulate either the reflux flow or the distillate flow directly. 

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