Abstract

Hydrogen Sulphide is a hazardous gas on the surface as it is extremely toxic to human and even animal life, and is extremely corrosive to most metals. Its control in an environmental friendly manner has been a challenge for oil & gas industry.

At Uran Plant Hydrogen Sulphide (H2S) is received along with oil & gas through Mumbai-Uran Trunk line (MUT line) and Heera-Uran Trunk line (HUT line) from western offshore and satellite fields. Crude oil at Uran is stabilized in Crude Stabilization Unit (CSU) for removal of water and dissolved gases. The Natural gas is treated in Gas Sweetening Units (GSU) to remove acid gases viz CO2 and H2S prior to extraction of value added products. Gases liberated from Crude Stabilization Unit and Condensate Fractionation Unit, are also mixed with natural gas stream prior to gas sweetening.

An increasing trend of H2S level in oil & gas lines has been observed over a period of time. However a steep increase has been noticed recently in associated gas of HUT oil line where H2S level has increased from 400-500 ppm to 2500-3500 ppm attributed to the production of highly sour crude oil in B&S asset. As a result H2S level in CSU off gas has also increased from 600-700 ppm to the range of 1500 2000 ppm. Attempts to use spent amine solution from GSU as H2S Scavenger increased organic load and oxygen demand (COD/BOD) of the effluent water.

Lab studies at Uran revealed that sodium hypochlorite and Hydrogen Peroxide (H2O2) are suitable for reducing Chemical Oxygen Demand (COD) in treated effluent water, however H2O2 is found more effective. Accordingly a field trial run was carried out at Uran using Spent Amine and H2O2 during March 23-28, 2015.

The present work includes assessment of impact of simultaneous dosing of two chemicals i.e.

spent amine at CSU and H2O2 at ETP to allow usage of Spent Amine solution as H2S scavenger without affecting environment. Effect of H2O2 on critical parameters covered under State Pollution Control Board consent as well as other vital parameters e.g. Dissolved Oxygen and COD.

The results reveal that Spent Amine solution @ 500 ppm can be used as H2S Scavenger to reduce H2S level in CSU off gas to the range of 1000-1300 ppm from the existing level of 1500-2000 ppm. The reduction in COD of effluent water from Clarifier to Guard Pond Outlet (GPO) during Pre-dosing period was only 0.9%, and during dosing period there was about 47% reduction in COD values at GPO, which clearly indicate the level of impact of dosing H2O2 @300 ppm at inlet of Guard Pond of Effluent Treatment Plant (ETP).

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