Reboiler inlet line

Reboiler oscillations can often be dampened by increasing flow resistance in the reboiler inlet lines or reducing flow resistance in the reboiler outlet lines. This shifts flow resistance from the outlet to the inlet lines. This is analogous to dampening U-tube oscillations by increasing friction in the bottom of the tube while easing the restriction at the open end of the tube. Design practices for reboiler inlet and outlet lines were discussed in Sec. 15.2. 

Installing a throttling valve in the reboiler inlet line (Fig. 15.4a). Throttling this valve reduces the length of the preheat zone. This technique is mainly effective in vacuum reboilers. 

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. 

Typical examples are steam reboilers and refrigeration vapor reboilers. The control valve may be located either in the reboiler inlet line (Fig. 17.1c) or in the reboiler condensate outlet line (Fig. 17.16). 

Fluctuating reboiler duty will have the same effect on a distillation tower as pumping the gas pedal on a car (that is, unpleasant changes in acceleration). A typical thermosiphon reboiler configuration is shown in Figure 18-12. The density difference between the liquid-filled reboiler inlet line and the vapor-liquid mixture in the outlet line drives the process-side fluid through the reboiler. 

Inlet Line. Unstable circulation can result if the inlet line to a vertical theimosyphon reboiler is too large. The tubes of a vertical thermosyphon reboiler fire individually. The tubes can backfire excessively if the liquid inlet line is too large. They don t have to backfire all the way into the tower to cause problems, just to the inlet tubesheet. It is common to put flanges in the inlet liquid line so an orifice can be added later, if required, to provide proper dampening effect. 

The thermosiphon reboiler has inherent instabilities. A valve or other flow restriction in the inlet line helps overcome these instabilities. Adjustment possibilities of a valve also compensate for variations in reboiler duty as imposed by changes in operation of the fractionator. 

Thermosyphon reboilers can suffer from flow instabilities if too high a heat flux is used. The liquid and vapour flow in the tubes is not smooth but tends to pulsate, and at high heat fluxes the pulsations can become large enough to cause vapour locking. A good practice is to install a flow restriction in the inlet line, a valve or orifice plate, so that the flow resistance can be adjusted should vapour locking occur in operation. 

If these frictional losses are less than the 4.7 psig given above, then the inlet line does not run liquid full. If the frictional losses are more than the 4.7 psig, the reboiler draw-off pan overflows, and flow to the reboiler is reduced until such time as the frictional losses drop to the available thermosyphon driving force. 

The flow of steam to a reboiler can be controlled by using a control valve on either (1) the steam inlet line or (2) the condensate outlet line. 

Figure 8.4 shows a control valve on the steam inlet line. The rate of steam flow to the reboiler is not really controlled directly, however, by this control valve. The actual rate of steam flow to the reboiler is controlled by the rate of condensation of the steam inside the tubes. The faster the steam condenses, the faster it flows into the channel head. The function of the control valve is to reduce the steam pressure in the channel head of the reboiler. For example, in case 1 ... 

Lines interconnecting exchangers with other process equipment can run just above the required headroom or about on the same level as the yard piping. Reboiler line elevations are determined by the draw-off and return line nozzles on the tower. Symmetrical reboiler piping arrangements — between the tower draw-off and reboiler inlet nozzles and between reboiler outlets and return... 

Reboiler return and bottom feed lines must be correctly sized. Undersized reboiler return lines can lead to liquid level backup to the reboiler outlet nozzle and may result in premature flooding. The author is familiar with an incident where a column flooded at 50 percent of its design rates because of an undersized reboiler line. Excessive inlet velocities may also induce turbulence and entrainment and have also been reported (98) to cause tray vibration, loosening of tray fasteners, and tray failure. 

A detailed sizing procedure for reboiler return inlets is discussed elsewhere (113). For bottom feeds, the criterion for sizing mixed and vapor feeds (Sec. 2.3, guideline 4) can be used (355). For reboiler return lines, it has been recommended (82) that the velocity in feet per second not exceed V4000/, where... 

A dump line connecting the bottom of the column with the reboiler liquid inlet line should be provided (Fig. 4.7a to d). Without it, liquid will weep through the tray at startup (there is no vapor flow until the reboiler starts up), and no liquid will be available to commission the reboiler and initiate vapor generation. 

Lines for introducing the feed at the reboiler inlet sump These lines are useful for avoiding excessive temperatures at the column bottom. When column heating is performed solely by reboil action, they can also speed up column heating (i.e., they eliminate the condensing action of a cold feed in the bottom section). 

Inlet line This line is usually sized for a cross-sectional area of 25 to 50 percent of the total tube cross-sectional area of the reboiler (124, 253, 358). A relatively high pressure drop in the inlet line is important for preventing excessive circulation, dampening oscillations, and for minimizing preheat zone length. 

Distance from bottom of reboiler, % liquid inlet line... 

In an existing installation, or in a service where oscillations are anticipated, an oscillation problem can be overcome by installing a restriction orifice or a throttling valve (see earlier in this section Fig. 15.4a) in the inlet line to the reboiler (150c, 313,358,360). Figure 15.7 (360) demonstrates the effectiveness of a throttling valve in dampening reboiler oscillations. 

A careful pressure drop survey indicated that there was an inexplicably high pressure drop in the liquid inlet line. When the tower was opened for inspection, the carcass of a dead rat was discovered lodged in the reboiler liquid inlet nozzle. Inadvertently, this rat caused the tower to flood as the reboiler rate was increased. If the rat had expired in the vapor outlet line, the effect would have been the same. 

Sometimes, thermosiphon reboilers seem to die at low loads. Instead of a gradual reduction in heat output, the reboiler suddenly seems to stop working. This is because the low-percent vapor in the reboiler return line does not result in very much density difference between the reboiler inlet and outlet lines. Without the density difference driving force, thermosiphon circulation stops and heat transfer is reduced. 

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