Reducing Methane Emissions from Glycol Dehydrators

Abstract

Abstract Separator-condenser applications for BTEX emissions reduction can also recover more than 97% of the methane that might otherwise be released directly to the atmosphere through reboiler vents. Texaco has 26 separator-condenser applications in Louisiana, and now recovers 0.1 mmcf/d of methane that would otherwise be vented. Separator-condenser applications are located at unattended remote locations which often do not have electrical power. Separator-condenser applications operate safely and reliably, and the investment and operational cost are minimal. Methane emission rates, are almost directly related to the glycol circulation rate; hence efforts to reduce BTEX emissions by lowering glycol circulation can also result in methane emission reductions. Glycol circulation rate reduction can be an important method for reducing BTEX and methane emissions where it is not practical to recompress or use the recovered gas. The Methane Source The oil and gas industry dries natural gas to pipeline specifications in oil and gas fields with about 20,000 glycol dehydrators. These glycol dehydrators use triethylene glycol to remove water vapor from 600 to 1200 psig natural gas streams. Wet glycol is recycled by vaporizing water at low pressure and at elevated temperatures. Methane and other hydrocarbons are somewhat soluble in triethylene glycol particularly at high pressure and ambient temperature. These hydrocarbons are also vaporized from the glycol as the pressure is reduced and temperature is increased. Steam that is vented from the glycol dehydration operation often is contaminated with methane and other hydrocarbons. Field operating conditions drive the design of oil and gas field glycol dehydrators. (Figure 1) All glycol dehydrators have a vessel in which liquid glycol contacts the high-pressure natural gas. This is usually done in a trayed tower. The glycol of choice is usually triethylene glycol. All glycol dehydrators have a means of lowering the pressure of the wet glycol as it leaves the contactor. All glycol dehydrators have a means of heating the glycol to 250 to 350 F to dry the glycol, typically in a fired vessel called a reboiler. All glycol dehydrators cool and re-pressure the glycol before it is recycled to the gas contactor vessel. Triethylene glycol is usually dried to 1 to 2% water before it is recycled to the natural gas contactor, and the wet glycol is re-dried before it is loaded with more than 10% water. In practice, glycol circulation rates are frequently higher than the rates required to dry the gas to pipelines sales specifications because of production declines that have occurred since initial equipment installation. Gas driven Kimray pumps are frequently used for glycol re-pressuring. Power is recovered with this same pump as the wet glycol is de-pressured. Where gas recovery with glycol separators is unavailable, minor methane emission reductions could be realized by replacing gas with electrically driven pumps. However, electrical power is often not available at the field dehydration sites. In theory, the carry-over of the methane with the glycol into the reboiler facilitates stripping of water from the hot glycol. But in practice, the gas dehydrators can dry the gas to meet pipeline specifications without much difficulty. In fact, methane stripping gas to remove water from the wet glycol in the reboiler is usually not needed in oil and gas field applications. About 15% of the field glycol dehydrators were initially constructed with three phase separators on the glycol stream. They were placed downstream of the Kimray de-pressuring pump and upstream of the reboiler. Prior to the advent of BTEX (Benzene, Toluene, Ethyl Benzene, Xylene) emission regulations, the installation of the separator was primarily motivated by the economics of the gas recovery. Consequently, separators were only installed on the largest glycol dehydrators. Texaco and other companies learned that glycol flash separators are a key element for high BTEX and volatile organic compound (VOC) recovery with condensers. Texaco began to retrofit glycol dehydrators with separators in the 1990's to achieve high BTEX and VOC recovery with condensers. Texaco found that it was very practical to use wet glycol as a cooling medium. Low pressure glycol from the outlet of the Kimray pump flows through the outer jacket of a tube condenser. The low pressure glycol stream at this point is actually a viscous foam. The foam is a poor coolant in a condenser of standard design. However, improved condenser design features resolved this problem. In fact, the glycol temperature increases 20 to 50 F as a result of flowing through the condenser. P. 305^