Level control reboilers

Finally, the control of recycle column is quite simple distillate is taken-offby level control, reboiler duty is used to control the losses of toluene in bottom, and a bleed stream is paired on the reboiler-level control for the removal of the heavy impurities. 

The column bottom level is sometimes controlled by bottoms draw. Varying reboiler heating medium is another possibility. For some cases, bottoms draw level control works better and for others, heating medium level control. BojnowskF gives a case where heating medium level control was desired for two columns in a plant. One... 

Our example system has a flow-controlled feed, and the reboiler heat is controlled by cascade from a stripping section tray temperature. Steam is the heating medium, with the condensate pumped to condensate recovery. Bottom product is pumped to storage on column level control overhead pressure is controlled by varying level in the overhead condenser the balancing line assures sufficient receiver pressure at all times overhead product is pumped to storage on receiver level control and reflux is on flow control. 

An alternate means of reboiler control is to remove the control valve from the steam line and provide a condensate level controller for the chest cascaded from the tray temperature. The alternate method uses A tube surface for control, with the condensate covering more or less tube surface to vary the area exposed to condensing stream. Condensing area is many times more effective for heat transfer than area covered by relatively stagnant condensate. The reboiler must have extra surface to allow part of its surface to be derated for control purposes. 

Ketde horizontal reboilers consist of either a U-bundle or a shell and tube bundle inserted into an enlarged shell. The enlarged shell provides disengaging space for the vapor outside and above the liquid, which is usually held by level control at the top level of the tube bundle or possibly a few in. below the top of the tubes. The heating medium is inside the tubes. 

The best designs provide for the percentage vaporization per pass to have been completed by the time the fluid mixture reaches the upper end of the tube and the mixture is leaving to enter the bottom chamber of the distillation column. In order to assist in accomplishing this, the initial reboiler elevation should be set to have the top tubesheet at the same level as the liquid in the column bottom section. A liquid-level control adjustment capability to raise or lower this bottoms level must exist to optimize the recirculation. Sometimes, the level in the bottom of the column may need to be 25-30% of the reboiler tube length above the elevation of the tubesheet. Therefore, the vapor nozzle return from the reboiler must enter at sufficient elevation to allow for this possibility. 

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 ... 

Figure 3.6. Some modes of control of liquid level, (a) Level control by regulation of the effluent flow rate. This mode is externally adjustable, (b) Level control with built in overflow weir. The weir may be adjustable, but usually only during shutdown of the equipment, (c) Overflow weir in a horizontal kettle reboiler. The weir setting usually is permanent.

Column base level (or reboiler level in a kettle reboiler) can be held by the flowrate of the bottoms, the vapor boilup, or the feed (if the feed is partially liquid and the stripping section does not contain too many trays). Since the typical hydraulic lag is 3 to 6 seconds per tray, a 20-tray stripping section introduces a deadtime of 1 to 2 minutes in the feed-to-base-level loop. Because of these hydraulic lags, reflux is only very7 rarely used to control base level. For this loop to work successfully, the column must be relatively short (less than 30 trays) and the holdup in the base must be large (more than 10 minutes). 

A rectification zone in which the dissolved bntenes fraction is displaced from the solvent in countercurrent flow with butadiene t apdr, which is obtain by controlled reboiling of the extract. These butenes, which contain butadiene, are returned to the absorption zone. A sidestream is drawn off at a level where the olefin content is practically nil, the acenlene content low, and the 1,3-butadiene content a maximum. Separation takes place in a column with about 45 travs operatins at the bottom around 75 C at 0.7. 10 Pa. 

Four alternative control schemes are commonly used for distillation column control, as shown in Figure 3.15 through Figure 3.18, respectively. Scheme 1 directly adjusts the material balance by manipulation of the distillate flow. If the distillate flow is increased, then the reflux accumulator level controller decreases the reflux flow. As less liquid proceeds to flow down to the sump, the sump level controller decreases the bottoms flow a like amount. The separation is held constant by manually setting the reboiler steam flow to maintain a constant energy per unit feed. 

Note that the column may have additional controllers such as condenser and reboiler level controls. The levels, however, are not independent variables since they must be maintained within dehned bounds. The liquid levels, which have no effect on the separation, may be controlled by manipulating the distillate and bottoms product rates. 

For example, assume that you want to perform tests on the plant, represented by Figure 15.74. The plant is a simple distillation column with overhead accumulator pressure controlled by moving the hot vapor bypass, bottoms level maintained by bottoms product draw rate, and the overhead accumulator level controlled by adjusting the overhead product draw rate. Reflux is on flow control, and the reboiler is on temperature control. Typical move sizes for this plant are shown in Table 15.12. 

In the first approach, we may consider a classical standalone control structures, as displayed in Fig. 13.5. Reactor feed is on flow control (PI), and outlet stream on level control (PI). For the distillation column a classical inventory control is column pressure with condenser cooling (PI), base level with bottom product (P), and reflux drum level with distillate (P). Quality control loops are top composition with reflux and bottom composition with reboiler duty, both as PI controllers. 

Let s take a look at the control loops. Now the make-up stream of reactant B is fed on level controller LCf of the buffer tank. The flow rate of the exit stream (Recycle) is set at a constant value by a simple specification on the stream s script. There are also two level controllers LCt and LCb for the top and bottom inventories of the distillation column, which manipulate the distillate and bottom products, respectively. Besides, the top column pressure is kept constant by means of the condenser duty. Quality control is implemented only for the bottom product, the reflux being fixed. We considered a composition measure with a first-order lag transmitted to a controller that manipulates the reboiler duty. 

Control structure CSl shown in Fig. 13.15 makes use of the feed in recycles of both reactants. Recycles flow rates are also fixed on flow control. Note that the make-up of A and B may be done directly in the reflux drum and in the reboiler sump, respectively. The reactor outlet is put on level control. Single-point composition control and fixed reflux are used for distillation columns. Composition controllers can be co-ordinated by the composition measurement for the end product. This structure works well, but has the disadvantage of an indirect setting of production. 

Bottom feed and reboiler return nozzles should not be too close to the maximum liquid level. The space between the bottom of the reboiler return (or bottom feed) nozzle and the maximum liquid level (Fig. 4.16) should be at least 12 in (143, 192, 207, 208, 237, 375). Failure to do this will promote turbulence on the liquid surface, erratic level control, and liquid entrainment into the rising vapor. 

Liquid flows through a thermosiphon reboiler without a level controller, to maintain a level in the sump. 1... 

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. 

Readings such as levels, control valve positions, and times at which readings are taken may not appear important but often turn out to be extremely valuable in analyzing test results. For instance, a reboiler control valve setting can provide useful information for determining if the steam side is flooded with condensate. These should be included on the data collection sheets. 

If overheating is not a concern (e.g., in refrigerated services), a reboiler level controller can replace the overflow weir, and used to control the boiling rate by flooding or unflooding tubes. In some cases, bottom product is drawn from the bottom of the column, and... 

Rgure 17.1 Continued) Reboiler steam control, (e) inlet control, with condensate pot, no level in reboiler if) inlet control, with condensate pot, level kept in reboiler g) inlet control with a pressure regulator ih) inlet control with a seal loop. 

To overcome this problem, a submerged condensate pot is often installed instead of the steam trap (Fig. 17.le) as described earlier (item 5 above). An alternative remedy is replacing the steam trap by a level condensate pot (Fig. 17.1/). By varying the level control set point, the surface in the reboiler can be adjusted so that the reboiler operates at a pressure high enough to ensure condensate removal at all times without a pump. Note that the bottom of this drum is located below the bottom of the condensing side of the reboiler (189) otherwise, "dry reboiler operation at high rates will not be possible, and reboiler capacity will be reduced. 

---