Feed preconditioning targets for distillation through invariant rectifying-stripping curves

Abstract

Optimal thermal condition of the feed contributes significantly in increasing the thermodynamic efficiency of a distillation process. In this work, a methodology is proposed to target the optimal thermal condition of the feed and the feed preconditioning efficiency through novel graphical representations based on invariant rectifying-stripping (IRS) curves. The IRS curves are independent of feed location, feed condition, and operating reflux of the distillation column. The amount of heat required for feed preconditioning (to change the feed stage temperature by a certain degree) is directly targeted in terms of the enthalpy gap between the translated IRS curves. The resulting change in the reboiler and condenser duties, because of the change in the thermal condition of the feed, may be targeted through the utility partitioning curve (UPC) introduced in this paper. The optimal feed preconditioning, in terms of utility savings, may then be identified through the load distribution diagram (LDD) obtained from the translated IRS curves along with the UPC. In fact, IRS curves simultaneously target feed location, feed preconditioning, and minimum energy (for reboiler and condenser) ahead of configuring the column. The proposed targeting procedures reduce the tedium of repeated simulations and are illustrated through two examples. For the binary distillation example, the IRS curves are truly invariant and provide rigorous targets. For the multicomponent distillation example (where the pseudo-binary concept of light and heavy keys is used), the IRS curves are near-invariant and provide practically useful targets.