In a once-through reboiler, the liquid flow coming out of the reboiler is limited to the bottoms product. In a circulating reboiler, the liquid flow coming out of the reboiler can be extremely high. If the reboiler return nozzle is located too close to the bottom tray of the tower, the greater volume of liquid leaving the nozzle can splash against the bottom tray. This alone can cause the entire column to flood. The best way to stop this flooding is to lower the tower bottom level.
Sometimes higher rates of thermosyphon circulation are good. They help prevent fouling and plugging of the reboiler due to low velocity and dirt in the bottoms product and especially high vaporization rates. If the percentage of vaporization in a once-through reboiler is above 60 percent and dirt in the bottoms product is expected, then a circulating reboiler would be the better choice.