Control of a Train of Distillation Columns for the Separation of Natural Gas Liquid

Abstract

The recovery of valuable heavier hydrocarbons from natural gas is accomplished in a series of five distillation columns. The first and second columns are cryogenic high-pressure columns in which the methane and ethane are sequentially taken overhead. The third column produces a propane distillate product. The fourth column removes isobutane and normal butane overhead for separation in the fifth column. The bottoms product from the fourth column is a mixture of hydrocarbons that are heavier than the C4 components.This paper studies the dynamic control of this train of distillation columns. Effective control structures are developed and tested on all columns using conventional PI controllers. The product–quality loops in three of the columns use tray temperature control, but the first and last columns require composition controllers to effectively handle disturbances in feed composition. The difficult separation in the large, high reflux ratio de-isobutanizer is well-controlled by an unusual control structu...