2-FEED DISTILLATION - SAME-COMPOSITION FEEDS WITH DIFFERENT ENTHALPIES

Additional separation can be achieved in flash distillation by separating the liquid feed into two parts, vaporizing only one part, and feeding these (now) two feeds to the top and bottom of a column. The driving force for the additional separation is the difference in chemical potential between liquid and vapor feeds with the same composition. This idea of using two feeds with the same composition but different enthalpies (herein called two-enthalpy feed) is applied to stripping and enriching columns and fractional distillation. Two-enthalpy-feed distillation, a new method for using waste heat effectively, should be useful in heat-integrated plants. When an ordinary distillation column has a two-phase feed, the use of two-enthalpy feed increases separation (same N and L/D), or decreases the number of stages (same L/D, x(D), and x(B)), or decreases the reflux ratio (same N, x(D), and x(B)). The two-enthalpy-feed system has a lower minimum reflux ratio than ordinary distillation with a two-phase feed. For other types of feed, two-enthalpy-feed distillation requires either less energy or energy at a less extreme temperature (i.e., lower temperature for reboilers or higher temperature for condensers) than ordinary distillation. Examples are presented for constant relative volatilities, hydrocarbon systems, and ethanol-water.