Figure 7.6 shows a once-through forced-circulation reboiler. Such a reboiler differs from a thermosyphon reboiler in that it has a pump to force circulation, rather than relying on natural or thermosyphon circulation. This extra pump seems rather wasteful—and it is.
The great advantage of forced circulation is that careful calculation of the pressure drop through the reboiler and associated piping is not critical. But as we can see in Fig. 7.6, the operator now has two tower bottom levels to control. Further, if the hot-side liquid level rises above the reboiler return nozzle, the force of the vapor and liquid rushing back into the column will cause the trays to flood, but the reboiler heat input will not be affected.
Most often, forced circulation is used with fired reboilers. If flow is lost to such a reboiler, furnace tube damage is likely to result. Hopefully, this is less likely to occur with a forced-circulation reboiler. Also, the higher pressure drop of a furnace may force the designer to use a pump. Sometimes we also see a forced-circulation reboiler system if the reboiler heat is to be recovered from a number of dispersed heat sources that are far away from the tower and hence a lot of pressure drop has to be overcome.