Deep-Cut Vacuum Unit Provides Thai Oil With Increased Distillate Recovery

In today’s challenging economic climate, refining businesses are keener than ever on enhancing their profitability. One way refineries can improve their margins is by recovering more waxy distillates from the residue of the crude distillation unit. Thai Oil Public Company Ltd (Thaioil) asked Shell Global Solutions to perform a feasibility study on the best way to revamp its existing high vacuum unit (HVU) to achieve this goal.

Shell Global Solutions developed several options: the best of which was installing a new deep-flash HVU in series with the existing HVU. This option would best meet Thaioil’s objectives of raising distillate yield and lowering fuel production while meeting the required product specifications.

The short residue from the existing HVU contains valuable waxy distillates, so the aim was to design a deep-cut HVU that used this residue as feedstock and recovered the waxy distillates by deep flashing. Thaioil then asked the organisation to produce the basic design package for the new HVU. This posed several challenges, including the technical complexity of the unit and the tight 12-week time frame for delivering the final package. The new unit also needed to function completely independently of the existing one.

The new design incorporates some innovative features in a standard Shell deep-flash HVU design. One is a steam stripping section in the bottom of the deep-flash vacuum column. In a typical design, the HVU feed contains about 5–10% light material, which gives a lifting effect that minimises the requirement for stripping steam.

In the new design, the lifting effect is lost, as the feed comes from the bottom of the existing vacuum column from where most of the light material has already been recovered. In this case, using stripping steam recovers significant amounts of light material while limiting the contaminant levels in the waxy distillates.

The feed inlet device, a Shell Schoepentoeter® Plus, is also innovative. During operation, vapour travels up the column and liquid short residue sinks to the bottom. However, separation is imperfect, so some residue rises with the vapour and causes coking in the wash bed of the column. The Schoepentoeter Plus device offers better liquid–vapour separation than the original Schoepentoeter device; consequently, the rising vapour contains less entrained residue. This helps to minimise coking and improve the run length of the wash bed.

The design also addresses the furnace’s operability range. Because Thaioil’s crude slate ranges from light to heavy crude, the unit’s turndown ratio is about 40%, taking into consideration the complete operating range for the different crude diets. The design makes sure that the furnace and the column function well during turndown conditions.

Despite the tight timeline, Shell Global Solutions delivered the basic design package a week ahead of schedule. Holding virtual meetings online and videoconferencing facilitated this. “It worked well because Shell Global Solutions prepared for the meetings by providing diagrams and replying to our comments in advance,” said Sander van Schalkwijk, Area-C Production Unit Technology Manager, Thai Oil Public Company Ltd.

Thaioil was also pleased with the frankness of Shell Global Solutions. “They were open about what they could and could not do, and what they believed was in our best interests. Their service was excellent, and we were impressed with the technical content of the basic design package,” concludes van Schalkwijk.

For more details on Shell Global Solutions’ distillation and separation technologies.

For more information contact Prabhakar Satbhai

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