Nov-2023

Unicracking unit flow scheme optimisation and integration

How an optimisation and integration approach can assist in maximising refinery margin for a residue upgrading project.

Vinod Ramaseshan, Stephen Gajadhar, Abdulaziz A Ghabbani & Hassan A. Abdulal
Saudi Aramco
Neeraj Tiwari, Mahendra Balodia, Massimo Sangalli & Simmi Sood
Honeywell UOP

Article Summary

Integration of solvent deasphalting (SDA) and hydrocracking upgrading complex provides a favorable techno-economic return for residue upgradation at relatively lower CAPEX than other alternatives for a brownfield project.  With a diminishing demand for high sulfur fuel oil and the consequent reduction in market price, Saudi Aramco decided to upgrade bottom-of-the-barrel material to higher value products.

This paper outlines the efforts made by Saudi Aramco, along with the technology provider Honeywell UOP, in optimising the residue upgrading section of the complex to maximise the overall return on investment (ROI) of the project.

It also examines the various optimisation and rationalisation steps done with the technology provider through the development of the design of the units. The design currently being engineered ensures safety, operability and reliability while leveraging the asset margins available within the existing refinery complex.

Background
Saudi Aramco operates a semi-conversion refinery at Ras Tanura (RTR) on the east coast of Saudi Arabia. Since its inception in the 1940s, RTR has gone through multiple phases of expansion and upgrades to meet the kingdom’s demand for transportation fuel and to generate value for Saudi Aramco’s downstream business. RTR has a nominal capacity of 550,000 million barrels per day (MBD) processing a crude mixture of Arabian Extra Light (AXL), Arabia Light (AL), Arabian Heavy (AH) and Condensate (Khuff). The refinery has multiple conversion units, which include naphtha hydrotreating (NHT), reformers (both semi-regeneration [SR] and continuous regeneration [CCR]), a light naphtha isomerisation unit (Isom), a two-stage hydrocracking unit (VGO HCK), a distillate hydrotreating unit (DHT), a visbreaking unit (VBU), asphalt oxidation Unit  along with auxiliary process units, such as multiple sulfur recovery blocks, Merox units, a hydrogen generation unit  along with utilities and offsites.

The refinery is a semi-conversion complex producing between 50-75 MBD of high sulfur fuel oil. As fuel oil (FO) has limited marketability and consequently low value, the refinery economics is negatively impacted over the long run. Consequently, in order to improve the refinery profitability and ensure robust economics going forward, the residue upgrading project (RUP) was conceived with the objective of upgrading the residue to optimal white oil products (distillates and naphtha) while meeting Saudi demand for asphalt.

Multiple flow schemes involving hydrogen addition and carbon rejection were initially screened.  However, based on the target product slate requirements, market demand, limitations in plot space, the existing refinery configuration and blending component availability, a solvent deasphalting with fixed bed high conversion hydrocracking flow scheme was  chosen for the residue upgrade which gives higher Net Present Value (NPV).

Figure 1 provides the high-level flow scheme along with Table 1, which provides the current product slate and the expected product slate post the RUP.

In order to ensure the targeted production rate from the refinery, the major conversion units for the RUP are as follows:
• 75 MBD two-stage hydrocracking (Unicracking Process) unit (HCU) with a target conversion of > 97%
• 32 MBD high lift two product cut UOP/FW Solvent Deasphalting (SDA) unit
• 30 MBD kerosene treating (Merox Process) unit

All the above process units were licensed from Honeywell UOP (UOP). In addition to the conversion complex, additional auxiliary units such as the hydrogen production unit (HPU) with a sulfur recovery block have been proposed.

The feed being processed in the refinery is a mix of Arabian Crudes (AL, AH, AXL and Khuff Condensate). Based on the targeted product slate, the optimised crude diet was initially considered as a mix of AL, AXL, and Khuff Condensate.

Based on the crude slate, it was necessary to upgrade the available vacuum residue (VR), atmospheric gas oil (AGO), excess vacuum gas oil (VGO) and distillate range material within the refinery. As the objective of the project was to minimise fuel oil while meeting the asphalt demand, it was imperative that the SDA unit chosen was operated at high lift (high deasphalted oil [DAO] yield). The lift needed to be optimised considering the quality of the pitch produced in the SDA to be blended with VR to make asphalt, while at the same time the DAO quality was not compromised to be processed within an HCU.

Moreover, the limitation imposed by the overall asphaltene level in a feed to a hydrocracking unit would necessitate fractionation of the bottoms from the condensate processing section (when processing a mix of condensate and crude). The mix properties of the intermediates formed (DAO, excess VGO, distillate) must be such that it can be processed in a high conversion HCU with the objective of maximising distillate products and minimising bleed. A closed-couple operation of the HCU with the SDA would thus be necessary.  Based on the above, the feed qualities optimised for the SDA and HCU were developed and are provided in Table 2.

Conversion Complex (SDA-HCU)
The heavy crude characteristics that distinguish them from conventional crudes includes unusually high concentrations of contaminants such as nitrogen, sulphur, Conradson Carbon Residue (CCR) and metals. The concentration of sulphur, nitrogen and CCR in the VGO fraction increases as the crude becomes heavier and requires a careful selection of graded bed catalyst system to protect active catalysts in the unit. The complexity of the composition of these materials creates new challenges for catalyst and process design.