Jan-2011

Processing high TAN crude: part II

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

High total acid number (TAN) crude has the following properties: high acid value, fewer light components, high density and viscosity, high gel asphalt content, and high salts and 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.

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 dealt with specific corrosion types and environmental issues arising from high TAN crude processing. This second part deals with operational solutions to those issues.

Design and operation of crude distillation
We have co-operated with domestic and international research and design institutes, and have carried out crude oil assessments, demulsifier selection and electric desalting operation condition analysis, anti-corrosion mechanism and material selection, corrosion process analysis, detection and monitoring system analysis, and scientific assessment and risk identification for all potential problems relating to high acid crude oil processing. To guarantee safe, environmentally sound and extended operations, we have formulated a technical document that includes solutions to supplement the original designs.

Measures taken to deal with the characteristics of high TAN crude oil are listed in Table 1, while problems and solutions for processing high TAN crude oils in the electric desalting unit are listed in Table 2. 

Equipment material selection (metallurgy)
The atmospheric and vacuum distillation unit of Huizhou refinery is designed for crude oil with an acid value of 3.57 mgKOH/g and a sulphur content of 0.28–0.3%. However, 14 different oils with acid numbers of 0.3–3.57 mgKOH/g from all over the world have been processed by the refinery. In order to supply qualified and approved materials to downstream units, and guarantee fouryears of operation, we have formulated a guide for high acid crude oil equipment material selection. When it comes to the selection of pipeline material, the following recommendations are made:
• For 220°C or lower, use carbon steel
• For 220–288°C, use 1Cr5Mo, 1Cr9Mo, 0Cr19Ni10Ti or 00Cr17-Ni14Mo2 (SS316L material in this plant is required for higher molybdenum content)
• For media of 288°C and above, 00Cr17Ni14Mo2 austenitic stainless steel is recommended
• For temperatures of ≥220°C and a flow velocity of ≥30 m/s, 00Cr17Ni14Mo2 austenitic stainless steel is recommended
• For large-diameter pipelines, a composite of carbon steel and 00Cr17Ni14Mo2 stainless steel is recommended
• Where flow velocities are highest, large-radius elbows and slot-branching tee joints should be selected. Monitoring methods should include both:
• Corrosion probe monitoring: there are 17 probes installed in different positions on the atmospheric and vacuum distillation unit (see Table 3)
• Test analysis and monitoring: thickness measuring and coupons are fixed at specific points.

Chemical agents injection
An integrated, multiple-treatment system at Huizhou refinery applies the following chemical agents: crude oil dewatering demulsifier EC2045A, electric desalting demulsifier EC2506A, reverse demulsifier EC2345A, electric desalting tank demetalisation agent EC2047A, low-temperature corrosion inhibitor EC1021A, high-temperature inhibitor EC1023N and scale inhibitor EC3238A. Table 4 shows a sample of the design for the chemical agents injection system.

Environmental protection
Since its start-up, Huizhou refinery has kept wastewater from the plant treated to the approved level, including wastewater from electric desalting, which is drained into deep seawater via pipeline after approved treatment. Any oilcontaminated wastewater is recycled after approved treatment.

Periodic back flush water from the bottom of the electric desalting tank cannot be processed as is by the wastewater treatment system because of its high salts content. For this reason, during back flushing, this waste stream is first fed to a tank, where its concentration can be adjusted. The water is then blended to adjust its concentration and treated gradually. We are now trying to use a continuous, low-level flush to replace periodic (once-perweek) flushing. Other possible approaches to treating this highly polluted wastewater are under development, such as adding acidic organic substances extracted from naphtha.

In view of the process technology chosen for the subsequent level 2 and 3 processing units, all units are operating smoothly and all products meet the approved quality standards. However, because the atmospheric and vacuum distillation unit is designed for operation with Penglai 19-3 crude oil, with a light oil yield of only 4.8%, when this crude is blended with other crude oil types overhead load restriction on the atmospheric distillation tower is a concern. Of course, the sulphur content in blend crude oils especially for the refining blended crude oils and sulphur recycle capability are the main restrictions in the selection of crude oil type and amount for refining blending.

Another factor to be considered with respect to appropriate crude oil blending ratios and oil types is the change in the quality of waste water from atmospheric and vacuum distillation electric desalting after crude oil blending, and the acceptance level of the wastewater treatment plant.