Here the reboiler is fed with a wide boiling range mixture. For the case where the column-base liquid contains only a small amount of low boilers and a large amount of material with a much higher boiling point, perhaps approaching or exceeding maximum steam temperature, thermosyphon action and heat transfer may fall off as liquid level in the column base falls much below the elevation of the top tube sheet. What often happens is that either base level or low boiler concentration gets too low, thermosyphon action ceases, low boilers are completely stripped out in the tubes, and boiling stops. The column contents then drop into the column base. The increase in level and low boilers then permits boilup to resume. The alternate cessation and resumption are violent enough to merit the term burping. In view of what is often a rather narrow range of acceptable operating conditions, it is questionable whether a thermosyphon reboiler should be used for such applications. A forced-circulation reboiler would be better. If, however, a thermosyphon is used, one may maintain constant head on it by the overflow and external tank design of Figure 4.17.
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