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Shortcut Distillation Calculation


Shortcut distillation calculations are based on the standard Fenske, Underwood, Gilliland and Kirkbride equations to provide an initial estimate of the number of theoretical trays, reflux ratio and optimal feed tray location. These estimates are most effective when generated before performing full simulation calculations to set up initial values for those simulation calculations. The Fenske method estimates the minimum number of theoretical stages (at total reflux) assuming constant relative volatility of the components. The Underwood method estimates minimum reflux (for an infinite number of theoretical stages) assuming constant molar flow through the column, an optimal feed tray location and saturated reflux. The Gilliland method estimates the number of theoretical trays required for a given split with the reflux at a fixed multiplier of the minimum reflux ratio. The Kirkbride method estimates an optimal feed tray location (numbered from top of the column).


The shortcut distillation calculations presented here allow for some subcooling or superheating of the feed and up to eight components can be tracked. Non-distributed components can be lumped together into respective light and heavy components. Note that the light and heavy key components do not have to be, but usually are, adjacent in volatility. However, significant levels of components intermediate between the light and heavy keys can build up in the middle of the column in full simulation and actual operations. This situation is often alleviated by withdrawing these middle boilers through an intermediate side draw on the column.


Relative volatilities will be renomalized by the program. As a consequence you, the user, do not have to perform this work and you can just as easily input vapor pressures for the components (in consistent pressure units) when ideal liquid solution thermodynamics are appropriate. The Fenske, Underwood, Gilliland and Kirkbride equations are best suited for ideal liquid solution. Typical areas of application for these shortcut distillation calculations include refining, gas processing and LNG. These calculations are not applicable for separations of major components with widely divergent molar enthalpies of vaporization (violates the assumption of constant molar flow) or mixtures with large deviations from ideal solution behavior, including azeotropic mixtures (violates the assumption of constant relative volatility).


References:

  • Carl Branan, Process Engineer's Pocket Handbook (Houston, Texas: Gulf Publishing Co., 1981), p. 72-74.
  • J. D. Seader, PhD., Perry's Chemical Engineer's Handbook 6th Ed. (New York, New York: McGraw-Hill Book Co., 1984), p. 13-37 - 13-39.
  • Matthew Van Winkle, Distillation (New York, New York: McGraw-Hill Book Co., 1967), p. 236-241.
  • Egon Eckert, "Global Approach to the Computation of Separations by Approximate Methods," Chemical Engineering Science (No.3 1982) p. 425-432.



    Shortcut Distillation Calculation
    Copyrighted 2002, 2003
    Last updated 04May03