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

Additional separation can be achieved in flash distillation by separat ing the liquid feed into two parts, vaporizing only one part, and feed ing these (now) two feeds to the top and bottom of a column. The drivi ng force for the additional separation is the difference in chemical p otential between liquid and vapor feeds with the same composition. Thi s idea of using two feeds with the same composition but different enth alpies (herein called two-enthalpy feed) is applied to stripping and e nriching columns and fractional distillation. Two-enthalpy-feed distil lation, a new method for using waste heat effectively, should be usefu l in heat-integrated plants. When an ordinary distillation column has a two-phase feed, the use of two-enthalpy feed increases separation (s ame 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 tw o-enthalpy-feed system has a lower minimum reflux ratio than ordinary distillation with a two-phase feed. For other types of feed, two-entha lpy-feed distillation requires either less energy or energy at a less extreme temperature (i.e., lower temperature for reboilers or higher t emperature for condensers) than ordinary distillation. Examples are pr esented for constant relative volatilities, hydrocarbon systems, and e thanol-water.