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Once through hydrocracker revamp (TIA)

Chevron Lummus Global (CLG), a joint venture between Chevron USA Inc. and McDermott, has safely and successfully executed a once through hydrocracker unit (OHCU) revamp project with Chennai Petroleum Corporation (CPCL), a group company of Indian Oil Corporation Limited (IOCL), located in Chennai, India with a refinery installed capacity of 215 000 b/d.

Gokul Bhuvaragun
Chevron Lummus Global
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Article Summary
In March 2004, CPCL commissioned a single-stage once through (SSOT) hydrocracker unit using CLG’s Isocracking process with an installed capacity of 37611 b/d. The original feed design basis for the OHCU was 90% vacuum gasoil and 10% visbreaker gasoil, and the unit was designed for 60% conversion of 360°C+ material to produce high quality middle distillates. The unconverted oil is fed to CPCL’s FCC unit.

Subsequently in 2009, CPCL decided to install a delayed coker unit (DCU) as a part of a resid upgradation project, so there was a need to process a new 7969 b/d heavy coker gasoil (HCGO) stream in the existing OHCU. CLG evaluated two options for the OHCU revamp:
• Option 1: Processing additional HCGO in the existing SSOT
• Option 2: Processing 100% HCGO in a parallel HCGO HDT reactor section and integrating with the existing hydrocracker (OHCU).

Option 1 was not attractive because HCGO processing was resulting in higher severity operation (more challenging feed and higher capacity), leading to lower run length and extensive modifications in the HP section, so Option 2 was chosen with tie-ins from the new HCGO train to the existing train HP recycle gas section.

To address CPCL’s future processing needs, CLG proposed a unique and innovative design of processing all HCGO in a new hydrotreating reactor that was integrated with the existing high pressure gas loop. While feed rate to the existing hydrocracking reactors remained unchanged at 37 611 b/d, it was tougher feed due to the addition of small quantities of deasphalted oil (DAO) and other streams from Gr I Base oil unit. Process modelling was done with existing and new HCGO trains as one block and the existing OHCU high pressure section equipment was retained by suitably distributing process loads.  

A new HCGO hydrotreating reaction section was fully integrated into the existing recycle gas loop, with a five-bed reactor that included CLG’s Isomix high performance reactor internals (see Figure 1) and new separator vessels to process the additional 7969 b/d HCGO. The design included flexibility to operate the new hydrotreating section with HCGO, diesel or VGO.

One of the design basis constraints was to keep the unconverted oil production rate at the same level prior to the revamp since the FCC unit capacity was unchanged. The new HCGO reactor, with all hydrotreating reactions, necessitated increased conversion across the existing hydrocracker from 60% to 67%. For the existing hydrocracking reactors, CLG provided a new generation catalyst system that included a high middle distillate selective, high activity ICR series catalyst to achieve these objectives. For the new HCGO reactor, CLG provided a high performance hydrotreating catalyst system that included a high performance grading system for pressure drop control as well as catalysts for demetallation and deep desulphurisation and denitrification performance.

Despite the high exotherms associated with processing 100% HCGO in the new HCGO reactor, the new reactor bed axial temperature increases have been kept under 35°C in bed 1 and under 22°C in the subsequent four beds. This was possible due to careful design of the reactor bed system with respect to bed heights and diameter as well as the choice of the right catalyst from CLG’s portfolio of ICR catalysts.

CLG worked closely with CPCL to come up with an ‘out of the box’ solution that was acceptable in terms of project cost as well as the ability to be implemented within a 35-day turnaround. CLG provided an innovative process design and delivered a revamp design package that minimised modifications and met CPCL’s processing objectives. Process modelling was done to ensure the existing recycle gas compressor (RGC) and reactor effluent air cooler (REAC) were adequate for the revamp conditions with the new integrated section.

To minimise turnaround time for revamp modifications on the low pressure section, CPCL decided to construct a new parallel fractionator furnace, as existing furnace modifications would require higher downtime. The product stripper was replaced with a bigger column and a few fractionator trays were also modified to accommodate revamp flows and to provide some operational flexibility. CLG also included an amine absorber in LPG service to meet LPG mercaptans specifications.

Advantages of the HCGO reactor integration are:
•  HCGO hydrotreating section flexibility to process VGO, HCGO and diesel
• Flexibility to maintain the overall conversion at SSOT
• No separate recycle gas compressor required
• No separate make-up hydrogen compressor required
• High capex saving with common light ends recovery (LER) and LPG gas treating section
• Minimal turnaround duration through use of tie-ins.

The integrated HCGO train was commissioned in March 2017 after performing a critical hot hydrogen integrity test (HHIT) to ensure the safe mechanical integrity of the new unit. Since the catalyst in the HCR reactors was in service already, a selective sulphiding procedure was followed for catalyst in the HCGO reactor. The HCGO section was operated initially feeding VGO and then, upon commissioning the delayed coker unit, began processing >70% HCGO. Currently, the unit operation is stable and meets all process guarantees. The catalyst and reactor internals performance graphs shown in Figures 2 and 3 indicate this stable operation.

This short case study originally appeared in PTQ's Technology In Action feature - Q3 2018 issue.

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