In thermosiphon reboilers, flow through the reboiler is achieved naturally via the density difference between,
- the 2-phase reboiler outlet, and,
- the liquid feed from the column bottoms.
The circulation flowrate is rarely measured. Assuming a constant flowrate (e.g. design flowrate) is not appropriate since the circulation rate varies with reboiler heat duty as required to operate the column. Often, a better assumption is to assume a fixed vapor fraction.
Determining an appropriate value for the fraction of vaporization is often problematic. If there is a return temperature measurement, it may be possible to adjust/tune the vaporization fraction to match the return temperature. The success of this approach depends on how sensitive the temperature is to the vaporization fraction. For example, if the bottoms stream is relatively pure. In that case, the return vapor fraction should be fixed at a typical value.