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Maximizing diesel recovery in your columns

Increased demand for automotive diesel is challenging refiners to adjust operating strategies to maximize middle distillate production. While some refiners choose to alter catalyst formulations or build new cracking units to increase diesel yields, optimizing the separation between gas oil and distillate in existing columns can be an excellent opportunity with little or no investment.

Areas of Opportunity to Optimize Diesel Cuts
Diesel streams are drawn from the primary fractionators in the Atmospheric Crude, Vacuum, FCC, Coker, and Hydrocracker units. These cuts can be further treated and blended into the refinery diesel pool.

Product overlaps between the middle distillate and gas oil cuts in these columns can contribute to a significant portion of refinery gas oil volumes. Shrinking these overlaps is a major target for refinery operations.

Targeting the Maximum Distillate Recovery
Increasing distillate recovery and improving the separation between
products can be accomplished through several strategies,
as follows:
- Increase vaporization (higher flashing temp)
- Reduce over-flash rate
- Improve bottoms stripping
- Increasing the number of theoretical stages

The balance of heat and pumparound duties in the column required to achieve those conditions most often requires higher hydraulic capacity and / or improved stage fractionation efficiency. Increasing hydraulic capacity and fractionation efficiency in diesel fractionation and bottoms stripping sections is possible with the help of the Sulzer design engineers and the installation of high performance mass transfer internals such as Sulzer VGPlusTM High Capacity Trays or Sulzer MellapakTM or Mellapak- PlusTM Structured Packing.

A Shrinking Gap Example
In one refinery’s Crude Atmospheric column, the capacity of the wash section and the AGO/Diesel fractionation section were limiting the boardman’s ability to raise fired heater outlet temperatures. The design of this wash section with basic two-pass moveable valve trays also limited the separation efficiency. The refinery noted that the Vacuum unit feed heater was hydraulically limited and the vacuum overhead distillate yield was high.

Sulzer proposed two options to help improve diesel recovery:
1. Replace the conventional trays in the wash section with a higher number of Sulzer VGPlus trays with a shorter tray spacing.
2. Replace the conventional trays in the wash section with Sulzer Mellapak structured packing.

The wash section modifications were coupled with similar recommendations for the diesel–heavy gas oil fractionation section and an optimized design of the bottoms stripping section with Sulzer VGAFTM high capacity trays with anti-fouling properties. These proposed options promised a decrease of in the dieselgas oil overlap and the loss of light product to the Vacuum unit, with an overall improvement in diesel recovery of over 1.0% of crude feed.

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