Unexpected Formation of Elemental Sulphur in Critical Sour Plant

Nowadays, high critical sour facilities face tremendous challenges in acid gas injection processes. In facilities in which a Sulphur recovery unit is unavailable and high levels of CO2 and H2S are handled, such is the case in this critical sour plant, acid gas injection requires several stages of compression. Moreover, this acid gas must be injected at extremely high pressures. The main source of acid gas is from the Gas Sweetening units are used in sour gas facilities. In one of the highest sour gas facilities, the acid gas from the Gas Sweetening Units is compressed in LP compressors followed by HP compressor trains before feeding to the injection compressor. In the second year of plant operation, high differential pressure was observed across the compressor after cooler normally operating at 70 barg pressure and 50-60 deg C temperature. This differential pressure across the after cooler continued to rise day by day and led to a Compressor trip on high-high discharge pressure. After investigation, it was found that elemental Sulphur formation was the cause for high differential pressure across the compressor after cooler. Major sulfur deposition was witnessed predominantly in one of the HP compressor trains and specifically at the 3rd and 4th passes of the second stage air cooler where the tubes are almost completely blocked and minor layer of sulphur deposition on the discharge piping is found. Minor sulphur depositions were witnessed in the Anti Surge control valve (ASCV), line from downstream of the second stage cooler to the ASCV and inlet KO drum liquid outlet piping. No sign of deposition observed within the compressor package and even the first & second passes of the cooler were clean from any depositions. Signs of Sulphur appearance continued to be seen in downstream TEG dehydration unit equipment such as frequent choking of rich TEG filters, knockout drum internals started to get chocked, frequent scaling of TEG reboilers observed, and finally fouling of lean-rich TEG heat exchangers. All symptoms of Sulphur formation were confirmed as it was later revealed that approximately 4 to 5 tons of elemental Sulphur formed within the system in three to four months which represented a significant threat to plant asset, availability, and production. The removal of solid Sulphur from the equipment required extensive cleaning activities, maintenance cost, and significant time for unit shutdowns. Troubleshooting process began with reviewing available literature and identifying a clear mechanism of unexpected formation of Sulphur.