Change in Liquid Splitter Operation—Averted Environmental Flaring

Abstract Liquid Splitter (a tray type Stripper Column) is one of the key process units in the LNG/LPG plant which handles liquid hydrocarbons coming from Feed Gas Compressor inter-stage Separators. In the Liquid Splitter, lower fraction hydrocarbons and acid gases (H2S&CO2) are stripped off as Overhead product and recycled back to the Feed Gas Compressor. The bottom product, rich in C4 & C5 joins the Fractionation section for separation of Butane and Pentane plus products. An ultrasonic inspection was carried out under FFS (fitness for service) assessment, which indicated that the shell external of Liquid Splitter Column of Train-2 had extensive evidence of HIC (hydrogen induced cracking) type damage. The report concluded that the column was not fit to operate at the normal operating pressure of 15.6 Barg and had to be discontinued from service. This paper elaborates how efficiently the situation was handled when one of the key process units in the LNG/LPG plant was declared as "not fit for service" and how the huge environmental flaring was averted while continuing with plant production. The discontinuation of the liquid splitter operation would have severely affected both ADGAS operation & ADMA-OPCO oil production, ADGAS had two choices to operate without the liquid splitter. One was to operate Train 2 on high pressure gas mode which meant that the upstream producer ADMA would have to cut down their oil production by half. The other option was to divert the entire liquid hydrocarbon from the Feed Gas Compressor inter-stage Separators to the flare, which would not only have an impact on the environment but would have resulted in significant LPG production loss. In recognition of a potential need for ADGAS to continue operation of the vessel in the short term - without any compromise on the plant safety and equipment integrity-whilst replacement was being procured, ADGAS commenced investigations for the possibility of using the vessel under less severe conditions. The study showed that as an interim measure, the operation of the column at a reduced pressure would be safe with the probability of failure reduced below the acceptable minimum level. This however posed another problem. Even if the Liquid Splitter was to be operated at this reduced pressure, then the Overhead vapor from Liquid Splitter could not be recycled to Feed Gas Compressor due to stage pressure limitation and would have to be diverted to Flare, which would again result in significant environmental flaring. Needless to say the operation of Liquid Splitter at a reduced pressure was the only viable option, but an alternative means had to be identified for recovering the Overhead vapors to avoid the flaring. At the same time, any modifications proposed had to be reasonably simple to execute and capable of being carried out in a short period, without any adverse impact on Plant safety and integrity. This task became really challenging, various scenarios and options were considered and for each case, detailed process simulations were carried out along with impact assessment. Apart from the environmental aspect, it also warranted a detailed safety study including task risk assessments. Finally a scheme was proposed for operating Liquid Splitter at a reduced pressure of 10.5 barg (instead of the normal 15.6 barg) and to route the Overhead product to Fuel Gas system. Utilizing the window of short shutdown, modifications were completed in a timely and efficient manner with the appropriate changes in operating procedures. Through this modification and routing the Overhead product of Liquid Splitter to the Fuel Gas system, about 4745 Million Standard Cubic feet per Annum (MMSCFA) of gas flaring was TOTALLY averted.