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Oct-2009

Flexible solutions for increased 
diesel production

Distillation changes and optimised hydroprocessing units represent a broad range of technical and economic options for increasing production of diesel

Robert Karlin and Aris Macris, Shell Global Solutions (US) Inc
Raul Adarme and Kathy Wu, Criterion Catalyst & Technologies LP

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Article Summary

Since 1998, diesel demand in 
the US has increased by approximately 40% and is expected to continue increasing as distillate margins outperform gasoline in the long term. As a result, refiners have been, and will continue to be, increasingly compelled to evaluate their processing options to expand diesel production, while at the same time complying with non-road ultra-low sulphur diesel (ULSD) regulations and also competing with diesel imports and the consequences of the reduction of bunker/fuel oil. 
These trends continue as feedstock 
quality declines, while governments 
and consumers demand increasingly cleaner fuels, and as crude prices fluctuate widely within short periods of time.

Despite market variations, the higher margin for diesel over gasoline has remained, although the gap is decreasing (see Figure 1). These trends have forced refiners to develop flexible plans in taking advantage of price fluctuations in market conditions.

Although Table 1 shows the wider range of issues that impact refinery diesel production, this article will not cover topics such as hydrocarbon stream reconfiguration, hydrogen management, advance process control, best practices for unit reliability or turnaround management. Instead, it will focus on diesel gains via distillation changes along with optimisation of FCC and hydro-processing units.1 The right solution will depend on the individual refinery’s configuration and marketing position. The best solution should develop out of deliberate evaluation of the application of technologies and their integration within the refinery to improve return on investment as a result of improving the refiner’s ability to control the gasoline-to-diesel ratio.

Distillation
An easy way to increase diesel production in refineries is through distillation, since adjusting product cut points in the crude tower or in conversion units can favour diesel production over gasoline. Other opportunities for increasing diesel through distillation can include:
•  Crude towers (atmospheric and vacuum) Maximise diesel while keeping flash point and 90% or 95% boiling point constant or expanding crude tower capacity.

•  Fractionation towers serving:
    •    FCCU: include heavy naphtha and increase light cycle oil (LCO) production without introducing a tail end
    •    Delayed coking unit: revamp to increase capacity and recovery
    •    Hydrocrackers: adjust product cutpoints (eg, recycle oil 5% boiling point) or revamps to increase capacity or match changing yield slate
    •    Hydrotreaters: Revamps to allow for diesel production capacity increase.

These distillation-related revamps typically require feasibility studies to identify and define options for increasing diesel production, while considering the requirements for implementation and capital expenditure. These studies also consider how technologies such as Shell GS tower internals add value, particularly when more severe operation is required. As reference, Table 2 provides the advantages of Shell GS’s several vessel and tower internals that could contribute to distillation revamps for increasing diesel production.

Internal advantages comments
To illustrate crude unit revamps for increasing diesel recovery in a generic crude unit (atmospheric tower and vacuum tower), Table 3 summarises the options concerning tower internals, pumparound location and steam usage, while maintaining the same product specifications. The type of crude unit revamp represented in Table 3 would provide a potential increase of 3 vol% to 4500 bpd more diesel in a 150 000 bpd refinery. This additional diesel will affect the downstream ULSD hydrotreater operation, which would need to be considered in a final economic evaluation.
Similar approaches for increasing distillate production have been used to debottleneck conversion unit fractionation sections while maintain-ing diesel product specifications. Projects have ranged from small to large Capex, with small Capex projects using tower internals and/or changing tower pressure using pumparounds and steam, which allows for minimum changes to the existing fractionation tower, to handle the increased distillates production. In contrast, large Capex projects have involved more significant changes, such as the addition of a tower to handle additional product fractionation — discussed here in the context of hydrocracking and catalytic feed hydrotreating.

As an alternative to cutpoint changes, the list of crudes the refinery can run could be expanded to change the distillates-to-naphtha ratio in the crude tower and, at the same time, to increase utilisation of the diesel-producing units within the refinery. This effort is simplified by a crude database and the associated tools available to evaluate the compatibility and profitability of new crudes. Once the right crude or crude blend is selected, crude tower operations should be optimised, which may require small Capex debottlenecking.

Hydrocracking
Most of the conversion capacity of hydrocrackers operating in North America is geared to the gasoline market. Nevertheless, many hydro-crackers have some flexibility for switching from gasoline to distillate mode by adjusting process parameters, such as cracking conversion, liquid recycle rate and product cut points, without requiring Capex. Further increases in diesel production and quality from existing assets can be achieved through changes in catalyst and investment in small or large revamp projects.

Clearly, each hydrocracker is unique and requires a detailed analysis of feed diet, operating constraints and desired yield to adjust and optimise operations. Maintaining flexibility to adjust to market conditions and building in the capability to handle a variety of feeds will allow for maximum profitability from the hydrocracker.

The remainder of this section discusses options for increasing diesel in existing hydrocracking units and the development of grassroots units for current and future crude sources.

Increasing diesel from hydrocrackers
Figure 2 illustrates the potential increase in distillate yield based on a single- or two-stage hydrocracker, for which the main fractionator bottoms are used in diesel pool blending. Feed quality, especially the final boiling point, needs close monitoring in these units to meet diesel specifications.


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