Revamps 2026 Issue

Yield and energy improvement in atmospheric and vacuum unit Impact of BS III diesel specifications on legacy AVU configuration called for optimising fractionation performance through improved mass transfer efficiency

Rahul Srivastava, Manish Rohit, and Harshit Budholiya Indian Oil Corporation Ltd Giuseppe Mosca Sulzer Italy Srl Unmesh Bhagwat, Mukesh Gupta, and Amar Patil Sulzer India Private Limited

I ndia’s transition from Bharat Stage (BS) II to BS III fuel specifications marked a significant tightening of die - sel-quality norms, driven by national emission-reduction objectives. Under BS III, the sulphur limit in high-speed die- sel (HSD) was reduced from 500 ppm to 350 ppm, with phased implementation that began in 13 cities in April 2005 and extended nationwide by April 2010. Alongside sulphur reduction, a less visible but equally impactful change was introduced in the diesel distillation specification, wherein the T95 recovery temperature (ASTM D86) was lowered from 370°C to 360°C to limit heavy-end hydrocarbons and associated particulate emissions. While these regulatory changes were environmentally progressive, they created a structural mismatch with the design intent and hydraulic capability of several legacy crude distillation units (CDUs) in India, including the atmos- pheric and vacuum unit (AVU) at the Indian Oil Corporation Ltd Mathura refinery. Legacy design constraints The Mathura refinery AVU was commissioned in 1982 with a design capacity of 6 MMTPA and was later expanded to 8 MMTPA. The CDU fractionation scheme was designed under pre-BS-III conditions, when diesel T95 limits were more relaxed, enabling recovery of heavier gasoil compo- nents directly into the straight-run diesel pool. Although a pre-fractionator column was added post- Preheat-II to manage increased vapour traffic following throughput augmentation, the CDU main column internals, particularly in the light gasoil and heavy gasoil (LGO-HGO) section, were last revamped in 2004, prior to BS III imple- mentation. The packing hydraulic design in this section was not intended for sharp LGO-HGO separation under stricter diesel end-point control. Regulatory impact on fractionation strategy With BS III implementation, the diesel T95 constraint of 360°C had to be strictly maintained at the CDU draw itself, especially given Mathura refinery’s single CDU configura - tion and absence of crude segregation or parallel process- ing flexibility. This eliminated the earlier operational latitude of routing the HGO cut, drawn just above reduced crude oil (RCO), into the diesel pool.

Consequently, the HGO stream became a swing cut, dynamically routed to either the HSD or VGO pool depend- ing on available specification headroom and product mar - gin. However, the HGO stream contained more than 50% diesel-range material, rendering it unsuitable for consistent routing to the VGO pool. This was due to significant overlap between LGO and HGO, driven by: • Existing packing and internals limitations. • Low liquid-to-vapour (L/V) ratios caused by high vapour traffic. Limitation imposed on secondary units Slippage of HSD material into the VGO pool resulted in avoidable loading of high-severity secondary conversion units, namely the fluid catalytic cracking (FCC) unit and once-through hydrocracker unit (OHCU), both of which are energy-intensive units with inherently lower liquid product efficiency for diesel-range feeds. When diesel-range material is routed to the hydrocracker, only about 65-70% is converted into lighter products such as HSD and kerosene, while the balance remains as uncon - verted oil (UCO). This UCO is subsequently routed to the FCC unit for further processing. In the FCC, processing of this UCO leads to intrinsic yield losses, with approximately ~5 wt% of feed converted to coke and ~4-5 wt% converted to fuel gas. These losses reduce overall liquid yield and increase energy consump- tion and catalyst stress. Consequently, routing diesel-range material through the hydrocracker to FCC pathway results in a compounded conversion penalty, higher hydrogen and fuel usage, and increased coke generation, representing a clear economic loss when compared to direct treatment routes. In contrast, processing the same diesel-range material in hydrotreating units such as diesel hydrodesulphurisation (DHDS) or diesel hydrotreating (DHDT) offers approxi- mately 99% (~99%) liquid yield conversion to on-spec- ification HSD or naphtha, with significantly lower energy intensity and minimal yield loss. Fast-track remedy To overcome these bespoke issues, the refinery imple - mented a fast-track remedy: routing the HGO from the

Revamps 2026