Cost of separation can be reduced by utilizing all available energy streams at various temperature levels. In the simplest case a waste energy heat stream can be used to partially vaporize a liquid feed stream. A more beneficial process involves an entire evaporation of a portion of the feed and introducing it into a column below the liquid portion of the feed. One can also use the waste energy stream as a heating medium in an intermediate reboiler in the column. There is, however, a limit to the amount of the waste energy that can be utilized in each case, beyond which this approach is no longer beneficial. Detailed analysis of the waste heat utilization enables us to determine this limit and compare each of these flowsheet options.