Jan-2010

Processing high TAN crude: part I

Huizhou, which is the first refinery for processing 100% high TAN crude, started up in 2009. This article describes the challenges of an opportunity crude operation

Wu Qing
CNOOC

Article Summary

The trend in crude oil supplies after 150 years of petroleum processing is towards heavier, lower quality feedstocks. According to ENI Group of Italy, the ratio of heavy crude oil (API 10–26) in the total crude oil slate rose from 11% in 1995 to 14% in 2005, and the trend in recent years has accelerated. So the oil refining industry must increasingly face the challenges presented by heavy, low-quality crudes, cleaner production standards, and the demand for cleaner fuels and high-value petrochemical products.

Opportunity crude oil generally refers to crude oils with a relatively high metals and sulphur content, and a high total acid number (TAN) and density. High TAN or acid content heavy crude oil is the typical opportunity crude oil. Usually, high TAN crude has the following properties: a high acid value, fewer light components, a high density and viscosity, a high gel-asphalt content, and high salts and a heavy metals content, which give rise to equipment corrosion and severe problems with product quality and environmental protection.

The price of opportunity crude oil is about 80% of the price of conventional crude oil. The extra cost of processing high TAN crude is in the range $1.15–10.73/bbl, but the savings compared to conventional crude processing are $43.54–62.7/bbl. The cost of crude accounts for about 90–95% of the total running costs of refineries, so it is very attractive for refineries to process opportunity crude, especially high TAN crude.

Huizou refinery
With an annual capacity of 12 million tonnes (240 000 b/d) CNOOC’s Huizhou refinery is the first large-scale refinery designed for the full conversion of high TAN crude. The refinery has done a tremendous amount of work to address the potential risks of refining opportunity crude oil and to achieve successful production start-up and smooth operation. The grassroots refinery started operations in April 2009 and currently all units are running smoothly, with excellent product quality achieved from a slate of around a dozen high-acid crude oils.

The first part of this article deals with specific corrosion types and environmental issues arising from high TAN crude processing. Part two will address operational issues at Huizhou.

High TAN crude: resources,
properties and acid distribution
Usually, we use acid number to distinguish the acidity of crude oil. Acid crude oil means that the acid number is >0.5mg KOH/g, and high TAN crude means that the acid number is >1.0mg KOH/g. For high TAN crude, there are two main types, according to sulphur content: one is high TAN, low-sulphur heavy crude, and the other is high TAN, high-sulphur heavy crude. The latter are produced in Venezuela and California, while high TAN, low-sulphur heavy crudes are more common; there are five main oil fields in four continents producing this type of crude. In Europe, it is mainly represented in the North Sea and processed by refineries in Europe and the Gulf of Mexico. In the Americas, it is mainly represented by Brazilian Marlim crude oil and is processed in several continents. In Africa, it is mainly represented by West African Kuito and Sultan crudes and is processed in refineries in the US and Asia. In Asia, it is mainly represented by Bohai sea crudes (for instance, Penglai crude) and is processed mainly in China. Table 1 shows the output of high TAN crudes in 1998–2004. Table 2 shows the main properties of high TAN crude, and the acid distribution of a typical high TAN crude is shown in Tables 3 and 4.

In general, the features of high TAN crude are:
• Higher acid number (for instance, Penglai 19-3’s acid value is 3.57 mgKOH/g, but most have a comparatively low sulphur content)
• Lower API number (or higher density, most of them higher than 930 kg/m3) with higher viscosity and higher asphalt and gel content
• Low solidification point
• Higher nitrogen content
• Higher content of heavy metals
• Lower yield of light oil distillates.
 
Problems of high TAN crude processing
Table 5 lists the processing challenges posed by high TAN properties and solutions to these challenges.

Potential problems in processing high TAN crude are set out in Table 6. Apart from the huge risks associated with high-temperature corrosion, Huizhou refinery’s crude distillation unit has other problems to contend with; for instance, increased amounts of sump oil, difficulties with wastewater treatment, and constraining issues in sole crude and mixed refining. Figure 1 shows the locations of these problems, including the crude oil tanks, electric-desalting tank heat exchanger system, overhead system and wastewater treatment system.

High-temperature naphthenic acid corrosion
High-temperature naphthenic acid corrosion mainly occurs at temperatures above 200°C, in particular above 220°C, affecting equipment that has the closest contact with naphthenic acid. The most seriously affected component is the vacuum distillation column system of the atmospheric and vacuum distillation system, including the vacuum heater, transfer line, vacuum draw three, vacuum draw four, vacuum tower feeding line and internal structures. 

High-temperature naphthenic acid corrosion includes four main steps:
• Naphthenic acid molecules are transferred to a metal surface
• The molecules become adsorbed onto the metal surface
• The molecules react with surface active centres
• Corroded materials are desorbed.

Corrosion-influencing factors: Temperature
Naphthenic acid corrosion is heavily influenced by temperature. Corrosion at low temperatures is not significant but, in the boiling state — especially in a high-temperature, anhydrous environment — corrosion is most significant. Most high-temperature naphthenic acid corrosion occurs in the liquid phase, but if naphthenic acid is condensed in the gas phase then gas-phase corrosion may occur and the extent of corrosion will be influenced by acid value.