Estimation of relief load and realistic relieving temperature for heavy-end fractionating columns

摘要

Relief calculation is one of the most discussed aspects of chemical engineering design.Licensors, contractors, industry literature and the American Petroleum Institute(API)specify the broad boundaries of”dos”and”don'ts”for relief system analysis and sizing.Still, much is left for engineering judgement to define the optimum safe design.This article examines the purview of relief load estimation and a realistic relieving temperature calculation method for distillation columns handling heavier cuts in a refinery.The conventional approach of tower relief load calculation, especially for grassroots units, is to balance the unbalanced heat across the tower during an upset scenario.Although the unbalanced heat method has its limitations, it is one of the most trusted methods for relief load estimation for a distillation column.One of the basic assumptions for the method is an unlimited supply of liquid to the top tray, and the liquid is considered to vaporize from the top tray during a relieving scenario.This results in a conservative(high)relief load owing to the low latent heat of vaporization of top-tray liquid.