Bottom Feed and Reboiler Return Inlets

The space between the bottom tray or packing support plate and the liquid level is a prime potential troublespot in distillation and absorption columns. Numerous mishaps originating in this section have been reported (12, 49, 71, 107, 203, 231, 238, 255, 296, 375, 440). It is estimated (143, 207) that 50 percent of the problems in the lower part of the column are initiated in this space. Many of these problems are associated with the reboiler return (or bottom feed) inlet. Therefore, it is important to follow these guidelines. 

Bottom feed and reboiler return inlets should never be submerged below the liquid level (see the Sec. 4.2 for the only acceptable ex- 

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. 

Bottom feed and reboiler return flows should not impinge on the bottom liquid surface. It is best to enter these flows in parallel to the liquid level. Impingement can occur when an internal pipe is elled down, or if the column inlet pipe slopes downward at the entrance. Arrangements to be avoided are shown in Fig. 4.2. Impinge- 

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. 

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