Reboilers 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. 

Reboiler control systems are selected as part of the general control system for the column and are discussed in Section 5.8.7. 

Water Hammer and Condensate Backup Steam-Side Reboiler Control... 

The second column supplies benzene of high purity. One-point composition control is more robust in a recycle system. In inferential mode the heat input in reboiler controls a sensitive temperature in the column. If the sampling point is placed in the stripping section, this control loop ensures good composition control of the bottom product. The reflux is set in ratio with the feed flow rate. For moderate disturbances, this allows good purity of distillate simultaneously with high recovery. The levels in the flash drum and in the base are hold by manipulating distillate rate and bottom product, respectively. 

Manual bypasses. A manual bypass around a control valve may affect the column relief load. For instance, an open b3rpass around the reboiler control valve can increase the heat input into the column dvm-ing reboiler controller failure (Fig. 9.2a). 

Controller cycling This is usually caused by inadequate tuning. Switching the controller to manual should stop the cycling if the controller is the culprit. Special caution is required with cycling of reboiler controllers these can cause surging (98). 

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. 

Reboiler control should not only provide good response to column disturbances, but should also isolate the column from disturbances occurring in the heating medium. The best variable for regulating column boilup is selected when the column overall control philosophy is devised (Chap. 16). In the majority of cases, boilup is regulated either to achieve a desired product purity, normally in the bottom section, or at a constant rate. WTien boilup is kept at a constant rate, the reboiler control valve is usually manipulated by a heating medium flow con-... 

The following discussion generally applies regardless of the variable that regulates boilup and the directness of control. Perhaps the only exception is that the need to avoid sluggish and oscillating boilup manipulation is emphasized when boilup is manipulated to achieve a desired product purity. For simplicity only, most of the examples discussed assume that the heating medium flow rate manipulates the reboiler control valve. 

The location of the reboiler control valve often has a major bearing on the performance, operation, and heat efficiency of the entire column. In an excellent paper, Mathur (280) reviewed pros and cons and several control variations for each location. This review is supplemented and expanded below. The following considerations are important ... 

A fired reboiler control S3rstem using the Wobbe index is shown in Fig. 17.3. The density meter should be installed directly in the fuel gas hne, downstream of the knockout drum and any points where fiiel gas streams are added, and where it would be unaffected by vibrations (370). 

Figure 17.7 Miscellaneous pressure control methods, (a) Recirculation temperature control (b) Column overhead control (c) inerts pressure control (d) drained condenser bypass (e) pressure-reboil control (/) floating pressure control, combined with coolant flow manipulation.

Both the reflux and reboiler control valves on a distillation tower are sized for design flows. At reduced rates, these valves may be operating in a range that makes tower control difficult. Also, the dynamics of distillation control are such that overfractionation improves the stability of a tower s operation. Both of these factors encourage shift operators to overreflux towers and waste reboiler energy. 

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