Jul-2024

Hybrid model of gasoline blend

High-fidelity gasoline blend models are essential to reduce octane number giveaway. Using machine learning can enhance a well-known non-linear analytic blend model.

Gadi Briskman, Ariel Kigel and Tom Rosenwasser
Modcon-Systems Ltd

Article Summary

Gasoline blending is the process of combining various components to produce a final gasoline product that meets specific quality and performance standards. The goal of blending is to create a fuel that optimises engine performance, improves fuel efficiency, and meets regulatory requirements. The components used in gasoline blending can include:
· Base gasoline
· Oxygenates
· Octane enhancers
· Stabilisers and antioxidants
· Detergents and deposit control additives
· Corrosion inhibitors.

Base gasoline is typically a refinery-produced gasoline that serves as the primary component of the final blend. Base gasoline can vary in properties such as octane rating, volatility, and sulphur content.

Oxygenates are compounds that contain oxygen and are added to gasoline to improve combustion efficiency and reduce emissions. Ethanol and methanol are common oxygenates used in gasoline blending. Oxygenates can also enhance the octane rating of gasoline.

Octane enhancers are additives or components that increase gasoline octane ratings. Higher octane ratings help prevent engine knocking and improve engine performance. Common octane enhancers include alkylates, ethers (for example, methyl tertiary-butyl ether, MTBE, or ethyl tert-butyl ether, ETBE), and aromatics (such as benzene, toluene, and xylene).

Stabilisers and antioxidant additives are used to improve the stability of the gasoline and prevent degradation over time, which can lead to the formation of gum and varnish deposits in fuel systems. Detergent and deposit control additives help keep engine components clean by preventing the build-up of deposits on fuel injectors, intake valves, and combustion chambers. Clean engine components can improve fuel efficiency and reduce emissions. Corrosion inhibitor additives protect fuel system components from corrosion caused by moisture and other contaminants present in the fuel.

Gasoline blending is a carefully controlled process that takes into account factors such as regional fuel specifications, environmental regulations, and market demands. Blenders must balance considerations such as cost, performance, and environmental impact to produce a final gasoline product that meets both regulatory requirements and consumer expectations.
Profitability improvement

Achieving an improvement in the profitability of gasoline blending, in its essence, drills down to engaging the least costly available blend components while fulfilling the requirements on the quantity of the produced gasoline batch and complying with the specifications on the properties of the blended product. Ensuring the latter has traditionally led refiners to exceed the minimal specification – the phenomenon known in the industry as a quality giveaway.

Rather than a mistake, the giveaway results from the refinery’s conscious decision to incur extra costs in blend components rather than risking the gasoline batch not meeting the specification, hence requiring additional costly processing. Reducing giveaways is an obvious direction to increase refinery operation marginality. However, its implementation cannot be reduced to one measure and requires a concerted implementation of several process and equipment improvements.

The octane number stands out as a gasoline property with significant economic ramifications, particularly regarding its impact on refinery profitability. With an average loss estimated at approximately $0.7 per barrel and considering the substantial annual gasoline production volume in the US, which surpasses 3.22 billion barrels, the potential losses stemming from octane number giveaway alone could conservatively amount to a staggering $2.1 per year for US refineries. This substantial financial implication arises from the intrinsic connection between gasoline grade market prices and the octane number specification associated with each grade.

The market dynamics dictate that gasoline blends possessing higher octane numbers command greater value within the refinery sector. Consequently, high-octane components of gasoline blends become prized assets for refineries, as they contribute significantly to the overall value of the final product. Given this economic reality, discussions surrounding reducing giveaway losses invariably pivot around the octane number.

Reducing octane number giveaway
Efforts to mitigate octane number giveaway losses are thus at the forefront of practical discourse within the industry, as refineries seek to optimise their blending processes and minimise financial losses while ensuring compliance with regulatory standards and meeting consumer demand for high-quality gasoline products. By addressing the octane number giveaway issue, refineries stand to enhance their competitive edge and bolster their bottom line in a dynamic and fiercely competitive market landscape.

The giveaway reduction requires improving confidence in the expected blend properties at the batch planning and execution steps. Finding the optimal recipe (the ratios of the involved components) is equivalent to solving a constrained optimisation problem. The total cost of the involved blended components is minimised under several constraints. The component costs are most often the subjective shadow prices of the components determined by the refinery.

The constraints include the desired blend volume, the allowed specification ranges of blend properties, the available amounts of the blending components, and the quantity and the property of the blend remaining from the previous blending batch (the heel).

Perhaps the most challenging constraint to satisfy is the dependencies between the properties of the blended streams and the blend. This dependency is the blend model. Its fidelity is crucial to ensure that the blended gasoline’s actual measured properties coincide with its properties according to the model.

The dependency between the octane numbers of the blending components and the blended gasoline is non- linear. There is no analytic formula fully describing this dependency. Over time, various approaches to approximating this dependency have been developed. One of the developed approaches is a polynomic approximation:

Where:
xi is the concentration of the i-th blend component.

Oi is the octane number of the i-th blend component.

αi,j and αi,j,k are interaction coefficients.

Its second-degree version has become a commonly used approximation method introduced by the DuPont Company in 1975.