There are a number of ways to switch reflux to finite reflux mode:
1) Adjust the bottoms, distillate and reflux flow rate controllers as necessary to maintain steady levels in the reflux drum and the reboiler, and to maintain a target reflux ratio R D = L/D.
2) At some point during refluxing, the level of solids in the reflux drum will reach a threshold that triggers an automatic switch to finite reflux mode. To determine when this occurs, it is useful to monitor solids concentration (by volumetric or specific gravity measurements) and temperature (using an infrared thermometer). If these parameters start to trend in a particular direction (i.e., markers are reaching higher or lower thresholds), it may be suggestive that refluxing has converged into a more finłxed state, and a switch to finite reflux mode may be appropriate. In general, it is best to