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GTL blending could increase overall US refinery efficiency by improving diesel efficiency

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A team from Sasol Synfuels, Jacobs Consultancy and Argonne National Laboratory has used results from a US industry-wide linear programming (LP) modeling study of individual US refineries to examine the impacts of a number of significant and looming changes—such as shifts in refinery crude slates; regional and seasonal variation; gasoline/diesel (G/D) production ratio; and GTL diesel blending—on US refinery, unit, and product efficiencies. (LP is the the primary tool for analysis and optimization in the refining industry.)

Results of their study, which appear in the ACS journal Environmental Science & Technology, suggest that refinery and product-specific efficiency values are sensitive to crude quality; seasonal and regional factors; and refinery configuration and complexity—which in turn are determined by final fuel specification requirements and regulations. Additional processing of domestically sourced tight light oil could marginally increase refinery efficiency, but these benefits could be offset by crude rebalancing, they found.
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To respond to future demand requirements, refiners will need to reduce the G/D production ratio, which will likely result in greater volumes of diesel being produced through less efficient pathways resulting in reduced efficiency, particularly on the marginal barrel of diesel, they found.

Diesel efficiency appears to be intimately related to the number and severity of units required to produce diesel. Tighter diesel fuel specifications such as CARB diesel result in a lower diesel efficiency. In the future, if refineries produce incremental diesel through relatively inefficient pathways, diesel efficiency within a refinery would be expected to decrease.

The decline in diesel efficiency could be offset by blending of Gas-to-Liquids (GTL) diesel, which could allow refiners to uplift intermediate fuel streams into more efficient diesel production pathways, thereby allowing for the efficient production of incremental barrels of diesel without added capital investment for the refiner.

The study. While earlier studies have investigated product-specific energy and GHG emissions intensities, these were based on generic or average refinery models that do not accurately represent the various complexities and operational flexibilities in actual operations, the team noted. The new study aimed to provide a broad view of the current US refining industry with a focus on current operations of large refineries; the work was based on the analysis of 43 US refineries that have an individual capacity of greater than 100,000 bbl/day. This data set represents approximately 70% of total US refining capacity in 2012.

The researchers refined and calibrated the LP model to highly refined fuels such as reformulated and CARB gasoline and low-sulfur diesel fuels. For each refinery to be analyzed, process submodels represented the existing refinery configuration.

The product slate was developed based on knowledge of the markets being served by each refinery. Models were constructed to produce on-specification fuels, including low-sulfur (30 ppm) gasoline; low-benzene (0.62 vol %) gasoline; ethanol 10% (E10) gasoline; Ultra-Low Sulfur Diesel (ULSD, 10 ppm); Texas Low Emissions Diesel (LED); and CARB products for California.

The researchers optimized the refineries in the models for profit margin, not for emissions—i.e., they represent as accurately as possible current US refinery operations. Price set is thus a significant driver for the LP model and an important element of the study.

They calculated refinery, unit, and product-specific efficiencies.

  • Refinery efficiency is a roll-up of the energy in all refinery end products divided by energy in all refinery inputs. Refinery inputs include crude oil,
    natural gas, hydrogen, electricity, blendstocks, and unfinished
    oils.
  • Unit efficiency is calculated by considering each unit in a
    refinery. Output streams can be end products, intermediates, and/or utilities; input streams include refinery inputs, intermediates, and
    utilities.
  • Product-specific efficiency is the efficiency of producing an end product: energy in an end product divided by energy associated with the
    production of the end product.

The results showed significant variation in refinery and product-specific efficiency depending on a number of seasonal and regional factors. Based ont the varied results, the researchers suggested that average refinery carbon intensity values should not be relied upon for policy implementation.

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Among their findings was that the processing of incremental amounts of domestic tight oil by US refiners is likely to have a neutral to slightly positive impact on refinery efficiency. Refining of additional amounts of domestic tight light oil could increase crude API gravity, reduce sulfur content, reduce heavy products, and reduce complexity.

However, a rebalancing of crude dieet based on the financial realities associated with optimal unit operation (e.g., fundamental changes in refinery configuration to accommodate incremental domestic tight oil) could offset the positive impact.

To conclude, currently US refineries are operated to optimize profit margin. In the context of possible future legislative requirements to reduce CO2 emissions from refining and to satisfy changes in demand and quality constraints, US refiners may be also be required in the future to optimize for efficiency. The dynamic relationship between efficiency and key parameters such as crude API gravity, sulfur content, heavy products, and refinery complexity are key to understanding any future changes in US refinery efficiency.