The bottom of the column has large enough capacity so as to run the ATFE at constant feed rate. Constant means that the feed rate is independently decided along with heating temperature so that the bottom remains flowable with reasonable recovery. If the feed rate is decided by the level in bottom, you can expect large variations in feed rate which can lead to loss in recovery or thickening of residue. 
After maintaining the constant feed rate if the level in bottom is found to increasing or decreasing, the feed rate may be changed with change in set point of feed controller. Also the heating temperature may also be manipulated to maintain the recovery and the residue flowability to previous values.
Because the bottom volume is large, the change in level will be slow enough for manual intervention of set point of the feed controller.
