Real Time Calculation and Closed-Loop Control of Dewpoint and Salt Deposition Temperature at a CDU, Using an Online Analyzer

Corrosion of a CDU column overhead system can have dramatic consequences for refinery operation. Presence of water, chlorides, ammonium, amines, etc… creates a corrosive environment requiring overhead system monitoring. Since there is a significant economic penalty associated with increasing the overhead temperature to minimize the deposition of salts, refiners are typically trying to limit this temperature increase to control the presence of some key chemical species, and measure their remaining concentrations. This presentation addresses some of the challenges associated with computing water dewpoint and ammonium chloride salt deposition temperature in real time, and shows how TOTAL has successfully achieved closed-loop control of these temperatures using a NALCO online chlorides and ammonium analyzer coupled to an online calculation software and a multivariable predictive controller.