Recovery of thermodynamic hydrate inhibitors methanol, ethanol and MEG with supersonic separators in offshore natural gas processing

Abstract

In offshore natural gas (NG) production, hydrate formation is a big concern that can impact the production and even stop NG flow. In this context, the injection of Thermodynamic Hydrate Inhibitors (THIs) in wellheads is widely employed in order to avoid these undesirable problems on subsea flowlines to gas processing rigs. However, in the main three-phase high-pressure separator in the gas rig, THI losses for gas phase are significant, particularly when the adopted THI is volatile like methanol and ethanol. This work discloses a new supersonic separator (SS) THI recovery process – SS-THI-Recovery – that treats the gas effluent from three-phase high-pressure separator achieving four simultaneous results: (i) gas water dew-point adjustment (WDPA); (ii) gas hydrocarbon dew-point adjustment (HCDPA); (iii) production of C3+ (propane and heavier) liquids as LPG; and (iv) recovery of almost all THI which would be lost in the gas otherwise. The proposition employs a supersonic separator (SS) battery followed by an anti-hydrates separator (LTX), a liquid-liquid THI extraction step and auxiliary THI distillation. SS-THI-Recovery was evaluated with HYSYS 8.8 simulator using methanol, ethanol and monoethylene glycol (MEG) as THIs. Supposing that the THI in the gas phase would be totally lost along with the exported gas otherwise, with SS-THI-Recovery the losses of methanol, ethanol and MEG were reduced by 91.9%, 79.3% and 99.2%, respectively, and such recovery factors could be further improved by increasing water flow rate in liquid-liquid THI extraction. Such high THI recovery entails reduction of THI costs with make-up, storage and transportation. Additionally, SS-THI-Recovery process is simple, with low footprint, and of easy implementation even for non-volatile THIs like MEG. Furthermore, the produced NG is ready for commercialization, dismissing additional treatment steps, not counting the commercial value of LPG. Therefore, new SS-THI-Recovery process configures an innovative and profitable alternative for gas treatment and THI recovery on gas processing offshore platforms.