Diesel Recovery Optimized into Diesel and Gas Oil Maximization

Objective:
A client approached Ascent with a project to add a diesel recovery section to the top of their crude unit’s vacuum tower. The client’s goal was to maximize distillate recovery by recovering diesel being left in their vacuum gas oil. The gas oil is routed to the cat feed hydrotreater then to the cat cracker (FCC), and thus recovering diesel would shift refinery yield from gasoline to diesel.

Summary:
The initial project objective was to recover diesel at the vacuum unit and route that additional diesel to the diesel hydrotreaters. Ascent was asked to design the addition of a diesel recovery section to the top of the vacuum tower.

• The client’s diesel hydrotreaters were already at capacity and would require expensive debottlenecking to accept the additional diesel recovered in the vacuum tower. Not only would the additional diesel require additional hydrotreating capacity, but the incremental diesel recovered would be heavier with more difficult to treat sulfur compounds, further taxing the limited, low pressure diesel hydrotreaters.
• In addition to the positive economics of recovering diesel from FCC feed, Ascent identified an even larger economic benefit by increasing vacuum gas oil recovery from vacuum residue. Ascent recommended modifications to increase the vacuum tower flash zone temperature by 35°F which would result in an asphalt yield reduction of 2.4% on crude.
• The client’s existing high pressure, low space velocity gas oil/cat feed hydrotreater was already treating the vacuum tower diesel commingled with the vacuum gas oil and had spare capacity to accept the additional vacuum gas oil generated by the proposed vacuum tower yield improvements.
• Ascent identified changes to the crude unit atmospheric tower operation which would shift approximately 2% of the crude yield from naphtha to distillate, allowing the refinery to increase the distillate yield even more.

Based on this information, Ascent proposed that better opportunities existed than a new vacuum tower diesel recovery section. Ascent instead proposed that the client make the vacuum tower gas oil yield improvements, continue to route all vacuum tower gas oil products to the cat feed hydrotreater, and then install a new vacuum tower on the back end of the cat feed hydrotreater to recover diesel from the treated gas oil/cat feed. This arrangement would achieve the original project objective of recovering diesel from cat feed, but it also resulted in hydrotreating that additional diesel to ULSD specifications without an expensive debottlenecking project.

The client accepted this new plan, and Ascent was authorized to proceed with the design for a new diesel recovery tower located at the cat feed hydrotreater along with vacuum unit and minor hydrotreater modifications.

During this new design effort Ascent was able to offer additional enhancements that reduced project costs and/or improved operation. For example, the diesel recovery tower was originally proposed to be located in the FCC. While technically feasible, this location would have required a new fired heater. Ascent recommended the tower be located in the cat feed hydrotreater unit where process heat from the existing product fractionator would provide the energy required with no new heater needed.

The new diesel recovery tower was designed to operate under vacuum, which lowered the required operating temperatures and eliminated the need for a new fired heater. This configuration also produced a dry diesel product and allowed drying the existing CFH main fractionator diesel product. This eliminated the need for diesel product salt driers and further reduced the project cost.

The final project was predicted to reduce the FCC charge rate by 30% and the coker charge rate by 13%. The diesel recovery tower design allows even more diesel to be recovered from cat feed in the future; the recovery is currently limited by the minimum FCC charge rate.

Bottom Line:
Ascent’s outside the box approach helped uncover project opportunities outside of the initial project scope. Identification of the HVGO recovery from vacuum residue, using the cat feed hydrotreater to make ULSD, and other innovations maximizing the existing asset value resulted in a much higher payout than the original project scope.

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