Introduction There are approximately 250,000 tank batteries in the United States. These tanks store crude oil and condensate prior to transportation to sales via pipeline, barge or trucking. The sources of the vent gases from tanks are from flash, standing and working losses. There are approximately 8,000 to 10,000 mechanical vapor recovery units (VRUs) installed at oil production sites with four tanks typically connected to each VRU. Most of these tanks are fixed-roof storage tanks.
This project demonstrated the use of an alternative method for handling low-pressure natural gas that is typically vented to the atmosphere, flared or recovered with a mechanical compressor by oil and gas production facilities. This application used a venturi jet ejector as a device to compress natural gas from crude oil/water storage tanks to an intermediate pressure for use at the location or ultimate injection into the sales pipeline.
Upon injection of the oil into the storage tanks, lower molecular weight hydrocarbons dissolved in the crude oil come out of solution or "flashes" when the pressure drops as the oil enters the tank. The storage tanks are typically downstream of a low-pressure vessel such as a two or three phase separator or heater treater. The hydrocarbons that flash include methane, ethane, volatile organic compounds and hazardous air pollutants. These gases collect in the plenum of the fixed roof storage tanks that are typically used in oil and gas production operations.
Standing losses are due to diurnal and seasonal temperature changes. Working losses result from plenum vapors released during filling of the tank and agitation of tank contents when fresh oil mixes in the storage tanks. Fixed roof tanks typically have a pressure relief valve that vents gas when the tank pressure exceeds 0.25 psig.
Vapor recovery systems for crude oil storage tanks are installed for several reasons. Capturing vent gas for use in the system (e.g., fuel gas, gas lift) or injection into the sales pipeline results in improved efficiency, increased profits and conservation of natural resources. State and federal air quality regulatory requirements can require vapor recovery and other emission controls. These regulatory issues arise because tank vent gases contain volatile organic compounds (propane plus fraction) that are precursors to the formation of ozone. Also, regulatory requirements limit emissions of vent gases that contain high concentrations of hydrogen sulfide (H2S) gas and hazardous air pollutants such as n-hexane, benzene, toluene, ethylbenzene and xylenes. In addition, the greater concern over global warming has resulted in scrutiny of methane emissions, an identified greenhouse gas.
Statement of Theory According to Bernoulli's equation, if no work is done on or by a flowing frictionless fluid, its energy due to pressure and velocity remains constant at all points along the streamline. As a result an increase of velocity is always accompanied by a decrease in pressure. This principle can be used to collect a low pressure natural gas stream with a high pressure motive gas stream for entrainment and compression to an intermediate pressure.