A feasible separation region is bounded by the product composition multitude (i.e., the distillate and bottoms composition points at total reflux and infinite number of stages) and by the distillation limit, which is defined for total condenser, partial reboiler and saturated liquid feed only. The present work addresses the dependence of the distillation limit on feed quality and column equipment and is based on material balances for a stripper or rectifier (with the envelope going through the pinch zone). Simple relationships for the distillation limit (in cases of saturated liquid/vapor feed and total/partial condenser or reboiler) were obtained. The application of generalized distillation limits to process synthesis may yield new, more efficient distillation systems. Furthermore, suitable selection of column equipment type, and thermodynamic state of the feed, may lead to extended possibilities for crossing distillation boundaries. In certain cases, the location of the distillation limit allows a composition profile to cross a pair of distillation boundaries: the total-reflux boundary and the pitchfork distillation boundary. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.