The total annual cost is compared for standard vapor feed distillation and two-enthalpy feed distillation in which part of the feed is condensed and fed above the vapor feed. Total annual costs for new equipment are typically slightly, but not significantly, lower for two-enthalpy feed. Retrofitting from vapor feed to two-enthalpy feed increases capacity at lower total annual cost than building another vapor feed column. The two-enthalpy feed system has better turndown properties than vapor feed columns. If a less expensive coolant can be used for condensing feed than for condensing distillate, two-enthalpy feed distillation has significantly lower total annual costs than vapor feed for both new equipment and retrofits. The systems studied were binary separations of methanol and water, ternary separation of methanol, ethanol, and water, and separation of two five component mixtures of methane, ethane, propane, n-butane, and n-pentane.
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