Expanding horizons

Oil companies in China are expanding their facilities in a drive to increase the country’s oil processing capacity. One such expansion is the PetroChina Dushanzi Petrochemical Company (Dushanzi) oil refining and ethylene project in the Xinjiang Uyghur Autonomous Region.

Dushanzi expects the project to increase its crude oil refining capacity from 6 to16 million tonnes a year and its ethylene production capacity from 0.22 to 1 million tonnes a year. This will make it one of the largest integrated oil refineries in China. Keen to enhance the profitability of the project, Dushanzi aims to improve margins by recovering more waxy distillates from the crude slate and, to this end, has incorporated a high-vacuum unit (HVU).

The HVU was designed by Shell Global Solutions and uses Shell deep-flash technology. This technology aims to recover the highest possible vacuum gas oil yields from atmospheric residue while maintaining the lowest possible contaminant levels in the vacuum distillate feed to the downstream conversion units.

Deep-flash technology incorporates several important features:

• a furnace design that applies suppressed vaporisation, which lowers the fouling and coking tendency and enables a higher run length;
• a Schoepentoeter^® inlet device, which is a very low pressure drop device designed to maximise vaporisation of feed at the column inlet while providing sufficient separation between vapour and liquid, and uniform vapour distribution;
• a wash-oil section design, which has been developed from Shell’s experience of operating and designing more than 40 vacuum units, that removes contaminants from waxy distillates while maintaining a low pressure drop;
• insulated, low-pressure-drop draw-off trays that minimise the condensation of heavy vacuum gas oil (HVGO) on the underside of the wash-oil draw-off tray to avoid the downgrading of waxy distillates; and
• direct-contact condensing sections that help to maintain the pressure drop of the HVU at the minimum level for maximum recovery of distillates.

The HVU successfully started up in September 2009. However, laboratory analysis revealed that the HVGO fraction had a high carbon content. This was because Dushanzi was using a different crude blend from the design specification. So, to reduce the HVGO’s carbon content to within the limit, the company was operating the HVU at a lower-than-design severity.

During a performance test run, Shell Global Solutions worked closely with a site team comprising technology, operations and maintenance staff to adjust the operating parameters and bring the unit nearer to the optimum operating window. These included changing the column pressure to the design value, gradually increasing the furnace outlet temperature and altering the pumparound duties and other column operating conditions, including the wash-oil rate.

According to Dushanzi, after these adjustments, the HVU operated at full design capacity and met the performance guarantees. The unit no longer had a problem with carbon residue in the HVGO, and all the products met the required specifications. The company attributes this success to the close collaboration between the dedicated site staff and the Shell Global Solutions team.

“Our work with Shell Global Solutions has been a success, and we hope to continue strengthening our co-operation and technical exchange,” said Cui Zheng Jun, Operations Director, PetroChina Dushanzi Petrochemical Company. “We are extremely satisfied with the technical support provided during the various phases of the project.”