Dec-2020

Revamping an integrated hydrocracker and lube base oil unit

Process and catalytic advances enabled large-scale expansion of an integrated unit to increase fuels and base oil production.

SANJEEV GANGURDE and SUBHASIS BHATTACHARYA
Chevron Lummus Global and Advanced Refining Technologies

Article Summary

A refinery located in South East Asia embarked on a challenging expansion plan in early 2019. This comparatively complex refinery processes local crude along with a large diet of Middle Eastern crudes. After the crude is fractionated into various boiling cut points, a network of complex process units is employed to make valuable fuels, chemicals, and lubricant base oil products.

At the heart of its relatively large capacity, the refinery operates a high conversion hydrocracker integrated with a lubricant base oil unit utilising Isocracking, Isodewaxing, and Isofinishing technologies from Chevron Lummus Global (CLG). The hydrocracking unit operates in two stage recycle (TSREC) configuration to achieve upwards of 80% conversion of straight-run vacuum gasoil to produce Euro-VI quality distillate products. The unconverted oil (UCO) mainly feeds the base oil unit to produce API Group II and III lubricant oil base stocks (LOBS), and the balance is used as feed to the FCC unit.

Over its 15 years of operation, the hydrocracking unit has seen minor modifications, mainly to accommodate feed blend changes. In 2019, the mandate from the refinery was to revamp the integrated HCU-LOBS unit to process 150% fresh VGO feed to enhance lube base oil throughput, along with additional UCO production to be used as a petrochemical feedstock. CLG carried out a revamp design to achieve this objective with minimum changes to the existing assets, keeping in mind to implement the modification recommendations within a regular plant turnaround period.

The continued safety and reliability of the unit guided this revamp design. An extensive audit was carried out to evaluate the suitability of materials of construction for the unit under revamp conditions by materials experts from the owner and technology provider. The capacity of the LOBS unit increased to 170% of the original design. This revamp project design included a new food grade white oil unit. An innovative process and latest generation catalyst technology were employed to achieve the revamp discussed in this article.

Background
Over the years, the refinery’s capacity has been augmented to process about 250000 b/d of crude oil through progressive revamps. Notable among these was the implementation of the hydrocracking project at the turn of this century to further boost the versatility of the refinery to process a wide range of crude oils while making high quality fuel products. After a rigorous technology evaluation process, the TSREC process licensed by Chevron Lummus Global was selected for a 40000 b/d hydrocracking unit. Figure 1 shows a simple schematic of the process.

The first stage reactor is designed for deep hydrotreating to eliminate most of the heteroatomic contaminants and to achieve an optimum level of conversion with some aromatic saturation. The first stage products are distilled off in intermediate distillation, and a certain fraction of the unconverted oil is recycled to a clean second stage for further conversion to desired high quality distillate products. This process separates catalyst functions in the first stage and the second stage for maximum effectiveness. The clean second stage operates in recycle mode at a much lower temperature to achieve target per-pass conversion. As a result, the TSREC process requires an overall lower reactor or catalyst volume, resulting in the lowest cost hydrocracker for relatively high capacity plants (see Figure 2).

During the detailed engineering phase of project implementation of the hydrocracking unit, the refiner decided to build a LOBS manufacturing unit utilising Isodewaxing and Isofinishing technology. This integrated unit was designed to process unconverted oil from the hydrocracking unit as feedstock to produce premium base oil grades.

CLG has licensed both grassroots designs and revamps of several commercial integrated hydrocracking and Isodewaxing/Isofinishing units. In these integrated units, the services of some of the high pressure equipment, like the make-up hydrogen compressor, recycle gas compressor, and associated equipment in the hydrocracking unit, are combined with those required in the Isodewaxing/Isofinishing unit. As a result, the overall investment cost of the integrated unit is reduced, with lower combined energy consumption when compared to two standalone units.

Figure 3 shows a schematic of such an integrated unit. The unconverted oil is fractionated in the feed preparation unit (FPU) into various waxy cuts based on the desired viscosity grades of final base oil products, namely API group II 65N, 100N, 150N, and 500N. These waxy cuts are stored and then processed in the LOBS unit in block mode of operation. This unit has the capability to produce API Group III 250N base oil products as well.

The latest revamp
One factor that remained constant for this refinery, especially with the integrated hydroprocessing unit at the centre, was change, be it with optimum utilisation of hydraulic capacity by processing incremental quantities of straight-run distillates, or routing several secondary streams from the refinery through this unit. However, the innate strength of this TSREC hydrocracking configuration was sustained with the production of premium middle distillate and lube base oil grades. A few significant revamps were undertaken when, as a result of compliance projects to build dedicated distillate hydrotreating units, additional VGO processing potential emerged in the hydrocracking unit. Current operational best practices from the refiner and CLG were applied, but none of these improvements over the years would compare to the magnitude of the next revamp project.

Salient features of the latest revamp project were:
- Debottleneck the hydrocracking unit to process 50% additional fresh VGO feed
- Generate more UCO while maintaining the distillate production at last revamp level
- 170% expansion of the LOBS unit’s UCO processing capability
- 20000 b/d of additional UCO generated as feedstock for a new petrochemical complex while continuing to cater to the FCC unit’s feed requirement
- A new 4500 b/d nominal capacity white oil unit

Hydrocracking unit revamp design was carried out for a total of 64 000 b/d of fresh VGO feed that was 50% higher than the design fresh feed rate to the hydrocracker in TSREC mode. As Figure 4 shows, CLG’s catalyst system and process technology were employed in this revamp design to process fresh VGO feed in both the first stage and second stage of the hydrocracker. The hydraulic capacities of the reactor stages were fully utilised at an optimum overall conversion to continue to make Euro-VI quality distillate fuel products and upgraded UCO as premium base oil feedstock. The refiner decided to revamp the FPU and the LOBS unit to process additional UCO. Most of the medium and higher viscosity draws from the FPU are utilised as feedstock for the desired base oil grades at the revamped 170% feed capacity of the LOBS unit, and the balance is split between FCC unit feed and feedstock for a new petrochemical complex.

After collecting the required waxy lube base oil cuts, a blend of various waxy side-cuts that are lighter than waxy 500N draw but heavier than diesel is prepared. Based on the specific gravity, distillation, and other characteristics, this blended stream is considered an excellent feedstock for the steam cracker. A quick estimate using the SRT-VII heater from Lummus Technology indicates an ethylene yield of 32 wt% and a propylene yield of 14 wt% for this feed, with less than 9 wt% pyrolysis fuel oil. The balance UCO fulfills the FCC unit’s feed requirement.