Control bottom level

Since the boiling liquid forms froth, which may vary in density, controlling bottom level is difficult. This makes these reboilers even less attractive, particularly in foaming and vacuum services. Applications where these reboilers are sometimes used are... 

When a steady bottom flow is desired, scheme 16.4e is at a major advantage and is usually preferred (258, 259, 309, 332, 362) unless inverse response is likely. When inverse flow is troublesome, an unconventional direct MB control scheme (Fig. 16.6a) can provide the cure. Buckley et al. (65, 66) reported satisfactory performance of this type of scheme in a column where scheme 16.4e failed due to inverse response. In that column, bottom flow was too small to effectively control bottom level, and column base holdup was to be kept at a minimum to avoid material degradation (i.e., schemes 16.4a, b, and d were unsuitable). 

Control of bottom level by manipulating boilup was unstable due to inverse response. Stqipingup reboiler steam displaced tray liquid into the column base so that IxAtom level rose instead of falling. Problem was solved by manipulating reflux flow to control bottom level. 

If bottoms flow is chosen to be manipulated for composition control, bottoms level must be controlled from steam flow. This is not as simple a process as in most liquid-level loops, because of the reboilcr between the manipulated and the controlled variables. Since smooth control over boilup is mandatory, this loop must, be heavily damped. Consequently, a wide proportional band is necessary, and reset is relied upon t o maintain level. Most of the vapor is eventually returned to the reboiler as liquid some time later, again affceding liquid level. This results in a natural period of several minutes. But because bottoms flow in this instance is small, relative to the rate of boiling, little coupling exists, and upsets are few. 

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... 

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. 

The second method could be automatic intermittent blowdown. With this, a timer-controlled valve is installed at the bottom of the boiler prior to the main blowdown valve. A program is then designed to operate this valve in short bursts, which disperses any sludge and controls the levels of solids. This method is preferred for boilers having internal treatment. 

The stripper bottoms contain Cj s, C4 s, and gasoline the debutanizer separates the Cj s and C4 s from the gasoline. In some units, the hot stripper bottoms can be further preheated before entering the debutanizer. In a number of units, the stripper bottoms is sent directly to the debutanizer. The feed enters about midway in the tower. Debutanizer feed is always partially vaporized because the debutanizer operates at a lower pressure than the stripper. A control valve that regulates stripper bottoms level is the means of this pressure drop. As a result of this drop, part of the feed is vaporized across the valve. 

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 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. 

The task of the lights column is to remove the light components from the recycled EDC, with chloroprene and tri-chloroethylene being the most important impurities. Therefore, a concentration-cascade scheme was implemented, with chloroprene concentration and reboiler duty as controlled and manipulated variables, respectively. The distillate to feed ratio was kept constant using feedforward control. This ratio can be used to adjust the level of tri-chloroethylene in the bottom product. The level in the condenser drum was controlled by the reflux. Note that fixing the reflux and controlling the level by distillate does not work, because the distillate rate is very small. 

Control of the heavies column is simpler. A fixed fraction of the feed is taken as bottom product, in a feedforward manner. The reboiler duty controls the level in the column sump. Note that this arrangement, which is required because of the small bottoms stream, cannot be implemented if a kettle reboiler is used. The column is operated at constant reflux, while the distillate rate controls the condenser level. 

American investigators, who have always viewed the drag traffic from the bottom levels upward, never "cracked" the controlling financial relationship that Hong Kong exercises over the traffic. 

Control structure CS4 (Fig. 2.13a) controls reactor effluent flow, brings fresh A in to hold reactor composition zAy and brings fresh B in to control reactor level. In both columns, the base levels are controlled by manipulating bottoms flowrates and the refhtx drum levels are controlled by manipulating distillate flowrates. 

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). 

R-V Reflux flow controls distillate composition. Heat input controls bottoms composition. By default, the inventory controls use distillate flowrate to hold reflux drum level and bottoms flowrate to control base level. This control structure (in its single-end control version) is probably the most widely used. The liquid and vapor flowrates in the column are what really affect product compositions, so direct manipulation of these variables makes sense. One of the strengths of this system is that it usually handles feed composition changes quite well. It also permits the two products to be sent to downstream processes on proportional-only level control so that plantwide flow smoothing can be achieved. 

R-B When the boilup ratio is high bottoms flow should be used to control bottoms composition and heat input should control base level. However, in some columns potential inverse response may create problems in controlling base level with boilup. 

Figure 6.96 shows a column that is separating a mixture with a low relative volatility, so the column has a large number of trays and operates with a high reflux ratio. This type of column is called a superfractionator. Because of the high reflux ratio, reflux should be used to control reflux drum level. For the same reason, vapor boilup should be used to control base level. Therefore the two manipulators left to control composition are distillate and bottoms flowrates. Obviously these two... 

The flowTate of the purge stream from the base of the purge column is quite small, so it would not do a good job in controlling base level. This is especially true when the large steam flow has been selected to control the reflux drum level. Base level in the purge column can. however, be controlled by manipulating the bottoms flowrate from the DIB column. 

Seven liquid levels are in the process separator and two (base and overhead receiver) in each column. The most direct way to control separator level is with the liquid flow to the stabilizer column. Then stabilizer column overhead receiver level is controlled with cooling water flow and base level is controlled with bottoms flow. In the product column, distillate flow controls overhead receiver level and bottoms flow controls base level. 

After these choices, we must now decide about level control in the recycle column. Contrary to the other columns, here the boilup ratio is large since the bottoms diphenyl flow is quite small compared with the toluene recycle rate. For this case, we choose to control base level with the steam flow because it has a much larger effect. 

Step 7. Methane is purged from the gas recycle loop to prevent it from accumulating, and its composition can be controlled with purge flow. Diphenyl is removed in the bottoms stream from the recycle column, where steam flow controls base level. Here we control composition (or temperature with the bottoms flow. The inventory of benzene is accounted for via temperature and overhead receiver level control in the product column. Toluene inventory is accounted for via level control in the recycle column overhead receiver. Purge flow and gas-loop pressure control account for hydrogen inventory. 

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. 

A pilot-scale distillation column located at the University of Sydney, Australia is used as the case study [60]. The 12-tray distillation column separates a 36% mixture of ethanol and water. The following process variables are monitored temperatures at trays 12, 10, 8, 6, 4, and the reflux stream, bottom and top levels (condenser), and the flow rates of bottoms, feed, steam, distillate and reflux streams. The column is operated at atmospheric pressure using feedback control. Three variables are controlled during the operation top product temperature, condenser level, and bottom level. Temperature at tray 8 is considered as the inferential variable for top product composition. To maintain a desired product composition, PI controllers cascaded on flow were used to manipulate the reflux, top product and bottom product streams. 

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. 

A sufficient liquid level in a reboiler used to supply steam to the column is important from a performance and safety standpoint. The reboiler is under a slight pressure and therefore must be a pressure vessel. It usually has a gasketed and bolted lid. Welded steam outlet and stillage return lines connect the reboiler to the column. The liquid return enters the reboiler below the desired liquid level. The outflow of bottoms can be controlled by regulating the back-pressure in the bottoms line, in turn controlling the level of liquid in the pot. The pot must be designed so it can be completely drained to ease clean-out. 

Long time response (2) long time response and increasing complexity (3) V/F control is not desirable in general (4) column bottom level control by steam flow is desirable. (Comments are due toT. Umeda). b Direct M.B. = direct material balance control Indirect M.B. indirect material balance control V/F = vapor to feed ratio control... 

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