Methods are developed to reduce column diameter for distillation when the calculated diameter is largest at the bottom of the column. This diameter can often be reduced by generating vapor above the reboiler with an intermediate reboiler or with two-enthalpy feed. For a saturated liquid feed that is 50% n-butane and 50% n-pentane the two-enthalpy feed system reduced column volume by 8.2% with no change in energy use. For distillation of dilute acetic acid (5%)-water (95%) mixtures the best system (an intermediate reboiler) resulted in decreases in column volume as large as 53.9% with no increase in energy use. For a 50% acetic acid mixture the two-enthalpy feed system resulted in up to 32.5% reduction in column volume, while vaporizing the entire feed resulted in 31.5% volume reduction. Vapor bypass was effective in concentrated absorbers.