Oct-2006

Evaluating opportunity crude processing

Is processing opportunity crudes for every refiner? Technical challenges and economic benefits from processing high-acid crudes and bottom-of-the-barrel feedstocks are identified

Thomas W Yeung
Hydrocarbon Publishing Company

Article Summary

The current buzzword in the refining industry is opportunity crudes, as benchmark crudes surpassed $77 a barrel in mid-July. However, is processing opportunity crudes advantageous for refiners throughout the world? Yes and no (Figure 1). Yes, because a refiner must be a low-cost producer in the refining business, which is known to be cyclical and at the mercy of many uncontrollable factors, such as oil prices, economic conditions and governmental regulations. Benefiting from the least expensive crudes in the market such as high-acid crudes (HACs), heavy-sour crudes and extra-heavy oil produced from oil sands can keep refining margins high and make refiners stay ahead of the competition. On the other hand, the answer is no, since not every refiner can handle crudes with a 
high corrosion propensity and a disproportionate amount of bottom-of-the-barrel (BOB) fractions. In order to join the “can do” camp, a refiner must evaluate the obstacles and find cost-effective ways to handle HACs and BOB.

Increasing supply of opportunity crudes
There are basically four types of crude available to refiners around the world. They are light-sweet (30–40 °API, <0.5 wt% S), light-sour (30–40 °API, 0.5–1.5 wt% S), heavy-sour (15–30 °API, 1.5–3.1 wt% S) and extra-heavy (<15 °API and ≥ 3 wt% S). HACs have a neutralisation number or total acid number (TAN) exceeding 0.5mg KOH.

HACs represent the fastest-growing segment of global oil production. California, Brazil, North Sea, Russia, China, India and West Africa are known to supply HACs. Also, oil sands-derived syncrudes are deemed highly acidic. Over half of the world’s oil supply is heavy and sour. The trend of crude becoming heavier and more sour will continue in the longer term despite new discoveries of light-sweet crudes in West Africa and the Caspian Sea. Due to the lack of HAC and BOB processing capacity by many refineries and the need to produce light products, high-quality crudes are often in high demand, resulting in abundant supply and significant discounts for opportunity crudes.

As of late 2005, HACs were offered at a discount of $10/bbl. According to the US EIA, the spread between relatively light-sweet Brent and heavier sour Mexican Maya had widened from $5/bbl in May 2002 to $15/bbl in May 2005 as the global production of light-sweet crude declined between 2000 and 2004, and fuel sulphur specifications  became increasingly stringent.

Processing HACs 
and BOB
HACs, which have a high naphthenic acid concentration, can adversely impact refinery reliability and operations with corrosion, desalter problems, fouling, catalyst poisoning, product degradation and environmental discharges. Additionally, diesel produced from HACs has a low cetane number. There are various ways of managing naphthenic acid corrosion, including:
—    Predicting the corrosion behaviour of crudes and their relative risk levels
—    Inspection and monitoring methods to identify specific areas that are at risk of naphthenic acid corrosion and to verify that control methods are being used effectively
—    Applying corrosion control methods.

There are also alloys that appear to be even more corrosion resistant than existing favourites (for example, 316 SS and 317 SS). In the future, modifying or removing naphthenic acids will be accomplished via neutralisation, decarboxylation, hydrotreating and extraction according to recent research and development works.

A major concern with processing heavy crudes is blending and mixtures, since it is common for compatible crudes, such as Louisiana Light-Sweet, West Texas Intermediate and Alaskan North Slope, from multiple fields to be mixed in pipeline systems, since a lot of refineries are short on crude tankage. These crudes have different compositions and there are economic penalties for that variance. Since there are incompatibilities between the high-asphaltene crudes and the 
high-paraffinic crudes, Asian refiners who often process waxy crudes may have problems accepting heavy crudes from Canada and Venezuela. Aromatics/heavy crudes are often separated from light-paraffinic crudes to avoid asphaltene precipitation.

Crude oil incompatibility and the precipitation of asphaltenes on the blending of crudes can lead to catastrophic fouling and coking in the preheat train, resulting in net economic loss. Common problems are stable water emulsions in the desalter, fouled preheat exchangers and/or coking in the pipestill furnace tubes. Therefore, it is important to use the oil compatibility model and tests to predict the proportions and order of blending of oils that will prevent incompatibility prior to the purchase and scheduling 
of crudes.

No matter which resid conversion is selected, coke and sediment formation are often major concerns. Furthermore, byproducts of a heavier carbonaceous nature will be produced and refiners have to find outlets for these materials as part of their overall economic considerations. Most BOB upgrading can be undertaken using combinations of separations (distillation or deasphalting), thermal processing (coking or visbreaking), fluid catalytic cracking and hydroprocessing.

Crude distillation
When crude blends get heavier, atmospheric and vacuum tower distillate cut points tend to suffer due to the increasing difficulty of vapourisation. Therefore, changes can be made, such as increasing the temperature and residue stripping efficiency, lowering the pressure and flash zone oil partial pressure, and modifying the pumparounds. For the atmospheric unit, other key areas include the oil preheat train and charge furnace, column internals and the metallurgy of the unit exposed to higher sulphur and high TAN. For the vacuum towers, evaluations should be made regarding furnace sizing and outlet temperature, decoking capability, wash-zone capacity and steam requirement if it is a wet column. Deep-cut vacuum distillation via a revamp of the unit to cut deeper into the resid, to make additional FCC or hydrocracker feed, is one of the first and most attractive options a refiner should consider.