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Oct-2011

Gasoline and diesel imbalances in the Atlantic Basin: part II

Options to re-orientate a refinery’s production towards middle distillates. European refineries currently produce a surplus of gasoline and insufficient diesel.

Sébastien Fraysse and Sébastien Huchette, Axens
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Article Summary
This series of articles examines the prospects for evolution and the technological solutions available, to help re-orientate the production to better fit demand. This is the second and final in the series, following an article discussing the market outlook published in PTQ Q2 2011.1

The first article presented various scenarios for refinery output and demand in 2020. The evolution of the current situation was envisaged in light of different influences, including passenger car sales, incorporation of biofuels and reduction in refining capacity. Differences in gasoline and diesel supply and demand will persist in Europe, regardless of which scenario is envisaged, as these imbalances are structural. Moreover, the emergence of a similar but less marked supply/demand mismatch in North America is probable.

Technology and catalysts can play a major role in adapting production to market demand. Axens’ solutions to address these challenges in the European context are presented in this article.

Many refiners the world over are confronted with an increasing shift in demand towards distillate fuels and away from gasoline. Several of the options available to refiners to shift their fuels balance towards more diesel have been compared in a case study examining technologies around the FCC unit. This article describes the outcome of this study, with several of the technical options considered (see Figure 1). The options are positioned according to the magnitude of their impact on balancing the gasoline/distillate output and their investment cost.

The lowest-cost solutions, involving minor changes in the FCC operation to maximise the production of light cycle oil (LCO), are described first. Intermediate-cost options that add technologies around the FCC unit, with a medium impact on the mix of gasoline/distillate output, are considered next. Finally, the addition of a high-conversion hydrocracking unit is described as the most effective option for resolving imbalances in gasoline/distillate supply and demand, but also as the most costly option.

Tuning FCC operation
Currently, refineries equipped with a FCC unit that wish to produce more middle distillates and less gasoline can fine-tune the operation of the unit as a first step towards optimising the production of distillate and gasoline. Various solutions exist to increase and maximise the production of LCO from the FCC unit.

After selecting the most appropriate catalyst to maximise LCO, a first approach is to operate the FCC unit in diesel mode by decreasing the severity (see Figure 2) with a reactor temperature of about 500°C. For the feed considered in the study, the LCO yield will increase by 7% from a baseline of 15% at normal gasoline operation to up to 22% at reduced severity. Subsequently, the gasoline cut point temperature can be reduced from 220°C to 150°C, to directly reduce the gasoline yield and increase the distillate. This will increase the distillate yield by about 10% to 32%.
Another solution that can improve upon the 32% distillate yield is to widen the LCO cut further by heavy cycle oil (HCO) management. The 360°C+ fuel oil is often separated into HCO and LCO cuts. The LCO cut point can be increased to about 390°C to recover another 3% yield of light HCO fraction as distillate (LCO).

An additional option is to recycle the remaining heavy HCO fraction to the FCC reaction section; from a typical yield of 6% heavy HCO, about 4% can be converted to distillate, with 2% as fuel oil.

Using these low-cost solutions, a significant impact can be made on refinery output. Experience demonstrates that distillate yield can be improved from 15% to around 40% and gasoline yield decreased from around 50% to 35%.

Adding a process unit: Polynaphtha oligomerisation
To further respond to the gasoline/distillate mix demand, an interesting solution is the Polynaphtha oligomerisation process. When gasoline is no longer the main process objective, co-produced C4 olefins normally used for alkylate production and light gasoline olefins can be converted via oligomerisation into distillate products. The net result is an increase in distillate production at the expense of light gasoline and butenes.

The Polynaphtha oligomerisation process, originally developed to convert light LPG olefins to motor fuels, has been extended to the C5/C6 FCC olefins to maximise refinery profitability by reducing gasoline production. The LPG (C4 separately or mixed C3/C4) and C5 olefins produced in the FCC unit are sent to the oligomerisation unit, where they are converted into olefinic C6+ oligomers boiling at up to 350°C. The Polynaphtha unit utilises a fully regenerable solid acid catalyst that is environmentally friendly with no waste effluents and a long on-stream life. An important feature designed into the process is the ability to alter the selectivity to C6+ oligomers by adjusting the operating conditions, thereby changing the gasoline/distillates balance as required. To counter the imbalances in the gasoline/distillates mix, the maxi-distillate mode is chosen.

The olefinic gasoline produced by the Polynaphtha unit is often partially hydrotreated to moderate the olefins in the gasoline pool. Distillates produced are fully hydrotreated, resulting in excellent blending properties for light distillate products. When used in conjunction with the FlexEne solution, this can even bring about a bonus by increasing propylene in the FCC unit (see Figure 3).
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