• Distillation Operation:

    Distillation at an essentially constant pressure (isobaric distillation) is by far the most common approach because it lends itself to the common distillation-column configuration where the vapor phase travels upward under the sole impetus of the pressure drop from plate to plate. Though there is a pressure drop in the direction of vapor flow (which is relatively a fraction of the input pressure), it is assumed that the whole column is at the same pressure, and calculations are done for the vapor liquid equilibrium (VLE).

    If the temperature were to be maintained constant from stage to stage (isothermal distillation), it would be necessary for the pressure to increase from stage to stage in the direction of vapor flow. As a result, there would have to be compressors between all stages to move the vapor to a higher pressure.