Gas Sweetening is the most important operation of any Natural Gas Processing Plant. At ONGC, Uran Plant, gas received from offshore is combined with the crude stabilization off gas (CSU off gas) and condensate fractionating off gas and then passed through Gas Sweetening Units (GSU) to remove CO2 and H2S from the gas before further processing in LPG and EPRU for extraction of value added products.
GSU plant was originally designed for and operated with gas sweetening solvent system consisting of DIPA (Di-Isopropanol Amine) & Sulfolane. Contamination of solvent by amine degradation products and comingled composition of solvent system became a bottleneck for increasing gas processing capacity and to enhance plant capacity, comingled solvent system was replaced with a third generation solvent system consisting of MDEA, Sulfolane & Piperazine.
After change-over, approximately 500 MT of comingled amine solution having a mixture of DIPA, MDEA, Sulfolane, Piperazine and Oxazolidone (an amine degradation product), was available as a hazardous waste. Owing to the complexity of the mixture having five different chemicals, reclaiming of useful solvent from the comingled solution was ruled out and the only option left was its disposal by incineration.
Keeping in view the large quantity of spent solution & huge cost of disposal, study was undertaken to explore possibility of use of spent solution for some other in house applications.
Extensive lab studies carried out to evaluate anti-corrosive, anti-microbial and H2S scavenging properties of spent solution & found that amine solution has adequate H2S Scavenging & corrosion inhibition efficiency at 400–500 ppm and bactericidal efficiency at 30 ppm of dosage.
The subsequent field trial at 400–500 ppm of doses in one of the Oil trunk line having three phase flow (oil, associated gas and water) for quite a period has shown substantial reduction of H2S content in the off gas.