The Digboi Refinery is located in Assam, India (Credit: Indian Oil Corporation at
The refinery is owned by Indian Oil. (Credit: Talpa from Pixabay)
The refinery is developed by Indian Oil Corporation. (Credit: Sanjivkumarsharma at English Wikipedia)

The Digboi Refinery Expansion Project is a proposed capacity augmentation project of Digboi Refinery from existing 0.65 million metric tons per annum (mmtpa) to 1mmtpa. It is located in Digboi in Tinsukia District of Assam.

Owned and Operated by M/s Indian Oil Corporation Limited (IOCL) (Assam Oil Division), a public sector unit of the Government of India, the proposed expansion will contribute to the northeast region vision of the Government of India by receiving additional crude oil produced by ONGC or OIL and other private players thereby, improving the profitability of the refinery.

An Environmental Impact Assessment (EIA) and Rapid Risk Assessment Studies for the project has been prepared and published in January 2023.

Location and Site Details of the Digboi Refinery

The Digboi Refinery is located in Margherita Taluk of Tinsukia District in northeast Assam.

The refinery site is 0.19km away from the nearest Digboi Town and 0.19km from Digboi Railway Station.

Dibrugarh Airport is the nearest airport at a distance of 70km and NH 38/SH 24 is the nearest highway to the project site.

The new units proposed under the expansion part will be located within plant area adjacent to the existing units of Digboi Refinery.

The refinery covers an area of 150.33 acres (6,08,364 km2) of land at Borjan Forest Village but an additional 8.9 acres of non-forest land is being acquired for offsite facilities.

About Digboi Refinery

The Digboi Refinery was initially commissioned in 1901 and underwent rebuilding in 1923. It had an installed capacity of 0.5mmtpa which expanded to 0.65mmtpa. The modernisation of the refinery under the Digboi Refinery Modernisation Project (DRMP) in 1981 after nationalisation and merger with IOCL.

The Digboi Refinery processes crude in wax form from oil fields near Digboi. The refinery underwent modernisation by the commissioning of a Crude and Vacuum Distillation Unit (CDU), a Delayed Coking Unit (DCU), and a Catalytic Reforming Unit (CRU) at the end of the 1990s.

New units of Solvent Deoiling/Dewaxing Unit (SDU), Wax Hydro Finishing Unit (WHFU), Hydrotreater Unit (HDTU), Hydrogen Generation Unit (HGU), Sulphur Recovery Unit (SRU), and Captive Power Plant were commissioned in early 21st century.

In 2010, the commissioning of the Motor Spirit Quality Upgradation Unit (MSQU) was completed. MSQU consists of 130tmtpa Naphtha Splitter Unit (NSU), Naphtha Hydrotreater Unit (NHT), and Isom-Hydrotreater Unit (ISOM) (46.7 thousand metric tonnes per annum (tmtpa)) units.

Since 2018, the refinery has been processing crude condensate and has commissioned a Crumb Rubber Modified Bitumen (CRMB+) unit based on IOCL-Research and Development technology. The Wax Moulding Unit and Wax Pelletisation Unit are also present in the refinery. These units produce finished wax in slab and pellet forms.

Digboi Refinery Existing facilities and Utilities

The existing facilities at Digboi Refinery include CDU which processes a crude oil blend of Duliajan, Digboi, and Kharsang crude oils.

The three units of CRU receive naphtha cut (IBP-160oC TBP) from CDU and process it to produce feedstock for the reformer unit. The reformer unit reforms the feedstock producing light naphtha (IBM-90oC), stabilised reformate or gasoline, and hydrogen-rich gas. The light naphtha and stabilised naphtha undergo blending and form motor spirit and the hydrogen-rich gas is sent to the wax hydro finishing unit and fuel gas system.

The DCU has a blend feed throughput of 1,70,000 tons per year and consists of heavy waxy distillate (480-530oC), short residue excluding vacuum distillation unit (above 530oC) and foots oil excluding SDU. It can process long residue (above 370oC).

The HDTU receives feed from DCU and processes it to produce a product stream (Off gas and Coker off gas) stored in the product storage tank. From here, it enters the Amine Treatment Section where it is treated with amine to reduce hydrogen sulphide levels. The produced sour acid gas from this unit is sent to SRU.

The SRU has a capacity of 3 tons per day (tpd) (about 96% sulphur production).

The processing in SDU involves feed chilling, filtration and deoiling, recovery of wax, foots oil recovery, and recovery of solvent.

The WHFU treats waxy distillate produced in SDU by improving its properties through deoiling in sweating after undergoing dewaxing in wax extraction unit (later SDU). Nickel-Molybdenum catalyst is used in the process.

The MSQU unit splits naphtha, hydrotreats, and isomerise the light naphtha feed before blending into gasoline pool via its three units.

Currently, Digboi Refinery produces Liquefied Petroleum Gas (LPG), Fuel Oil, Raw Petroleum Coke (RPC), Wax, Sulphur, BS-VI-compliant Motor Spirit (MS) and High-Speed Diesel (HSD).

The existing utilities of the refinery include a 45.5MW captive power plant (3*8.5 and 1*20MW gas turbines), a 210 total petroleum hydrocarbon (tph) steam generation system (3*35 and 1*105), a 6,600m3/h FS cooling tower, a 4,400m3/h WS cooling tower, a 137m3/h*3 chain DM water system, a 1,550nm3/h (gaseous) nitrogen unit, and air compressors (two 3,650nm3/h and two 4,170nm3/h).

Digboi Refinery Expansion Project Details

The Digboi Refinery is proposed to undergo expansion of its refining capacity. The facilities proposed under the refining capacity expansion are mainly revamping of the Atmospheric Vacuum Distillation (AVU) unit from 650 kilo tonnes per annum (ktpa) to 1,000ktpa, modification of the Delayed Coking Unit (DCU) from 170ktpa to 220ktpa, and skid based modular HGU (2ktpa).

The major offsite facilities will include new crude tank at DR (1*10,000 kilo litre (kl), MS tank at DR (1*4,000kl), foots oil tank at DR (1*4,000kl), reformate or alkylate unloading tanks near Digboi terminal (2*4,000kl), MS or naphtha tank near Digboi terminal (1*4,000kl), foots oil or gas oil tank near Digboi terminal (2*4,000kl), fire water tank near terminal (2*4,000kl), and associated transfer pumps.

Rake loading Piping modifications at Digboi terminal will be included under major offsite facilities.

According to EIA, the estimated capital cost for the project is INR 740.2 crores ($1.08bn).

Contractor involved in the Digboi Refinery Expansion Project

M/s Engineers India Limited (EIL) has been selected by IOCL for preparing the environment impact assessment study and environment management plan for different environmental components of the project.