(34d) Rigorous Troubleshooting Identified Crude and Vacuum Distillation Column Constraints | AIChE

(34d) Rigorous Troubleshooting Identified Crude and Vacuum Distillation Column Constraints


Valero Ardmore refinery operates a 90 MBD Crude and Vacuum distillation unit, charging primarily mid-continent crude through the Cushing terminal. The heavy products from this unit are routed directly a gas oil hydrotreater (CFHT) or a blowstill tower for asphalt sales. Crude charge capacity became severely constrained 2-years into the operating cycle between turnarounds. Rigorous troubleshooting, including extensive isotope scanning, identified three separate and unrelated areas of limitation in the crude column and one area in the vacuum tower.

The Ardmore refinery crude tower is a typical atmospheric operation (630degF, 40 psig flash zone) with 42 trays, a packed gas oil wash section, and a shrouded stripping section. Overhead light naphtha is routed to a gas plant and atmospheric tower bottoms are routed partially to at CFHT, with the balance to the vacuum tower. Five side draw products from the crude tower are further processed within the refinery. Severe instability and tray flooding was noted throughout the tower, with specific constraints initiating in the top reflux trays, kerosene pumparound trays, and AGO wash bed. Because the limitations were extended through a very large area of the column, dissecting the data and identifying the true limits was a complicated task. The root cause analyses ultimately pointed to contaminants within the crude feed (phosphorous), inherent design issues with the tower internals, as well as operating practices.

The vacuum tower is operated dry, with a moderate flash zone severity (C-factor <0.4fps). The wash bed reliability is a typical run-limiting constraint to the unit, with wash oil controlled to maintain a slop wax draw rate (metered and pumped to CFHT) indicative of adequate overflash. Loss of slop wax draw occurred at a similar time to the atmospheric constraints. The troubleshooting identified an obstruction on the tray that created residence time and ultimately coke.

In this paper, the authors will describe the troubleshooting methodology, discuss an unconventional hot tap bypass that allowed the unit to remain in operation until a planned turnaround and outline design changes that were implemented during the outage. The authors will also present turnaround findings that prove the conclusions drawn from operational and troubleshooting data analysis were correct.