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Residue upgrading using fixed-bed hydroconversion

Dealing more effectively with the bottom of the barrel is high on the refining industry’s agenda for a several reasons. Crude oil feedstocks are becoming heavier, which is making lighter crudes more expensive. Environmental pressures are undermining the market for heavy fuel oil. In addition, the demand for clean transportation fuels is growing, with countries such as China and India taking the lead.

There are several processes available to refiners seeking an upgrading solution. One of them is fixed-bed residue hydroconversion. It has many advantages over thermal technologies such as delayed coking, but getting the most out of the process is not easy. Effective use of catalysts is the key to success. This is an area where Criterion Catalysts & Technologies has much to offer.

CHALLENGE FOR THE OPERATOR
Most fixed-bed residue upgrading units are used to make fluidised catalytic cracking feedstock; the rest produce low-sulphur fuel oil. Conversion to lighter fractions in these units is generally on the low side (20–30%, 520°C+). Efforts to raise conversion levels, especially with heavier oils, create all kinds of problems, particularly instability in the unconverted material, which leads to sediment-induced fouling. Catalysts also deactivate more rapidly, product quality deteriorates, pressure drop becomes a bigger problem and the end-of-run temperature is reached more quickly. All of these factors combine to reduce the achievable cycle length to uneconomic levels.

THE SARA MATRIX
Sediment falls into two classes: Type I inorganics and coke, and Type II, arguably the worse culprit, precipitated asphaltenes. High in sulphur, nitrogen, oxygen and metals, and relatively insoluble, asphaltenes are the most complex and least characterised of all the heavy oil macromolecules. They are colloidally dispersed in the oil and sit at the heart of the so-called SARA matrix (saturates, aromatics, resins and asphaltenes) that characterises all heavy oils. The colloidal instability index, defined as (asphaltenes + saturates)/(resins + aromatics) is particularly important, as it largely dictates the extent of sedimentation and also the maximum level of conversion that can be achieved before fouling limits operations. Understanding the SARA matrix, which is different for each heavy oil, and its behaviour is crucial to designing effective catalysts for successful residue upgrading.

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