Heater treaters can be significant sources of “flash gas” from oil and gas production facilities. Flash gas is natural gas that is released from a wellstream when the liquid undergoes a pressure drop or when the liquid’s temperature is increased. In the treater, the crude oil/condensate and produced water experience a pressure drop and a temperature increase. The greater the pressure drop and operating temperature, the greater the amount of gas generated per barrel of oil throughput.
Figure of Typical Vertical Heater Treater (Reference: TCEQ Upstream O&G Heaters and Boilers Final Report, August 30, 2013.
Heater treaters are heated, vertical or horizontal separators that are typically used for the following purposes:
- Break up emulsions to separate the oil from produced water and inorganic salts
- Solids (sediment) removal
- Stabilize the crude oil or condensate by separating volatile, lighter hydrocarbon fraction (C1-C4) from the heavy, less volatile fraction (C5+) for safety reasons
S&W (basic sediment and water) present in most crude oil can be an emulsion of oil, water and sediment. Most crude oil purchasers have a maximum specification for BS&W content, usually less than 1 percent, although BS&W up to 3 percent can be common for transported crude oil/condensate.
Typical pressure and temperature operating conditions which affect flash gas release include the following.
- Pressure: 20 to 80+ psig
- Temperature: 120 to 250°F
These parameters are dependent on many factors including pressure needed to send oil to storage tanks, emulsion type, API gravity of the oil, oil throughput and treatment needed (BS&W removal, stabilization).
The heat source used can include direct or indirect fired heaters, water baths and waste heat recovery systems.
Heater Treater Flash Gas
The gas generated in a heater treater will be high BTU gas ranging from 1500 to 2500+ BTU/SCF and so will have a very high VOC content (>60% by volume VOC). Also the gas will contain methane (CH4), a regulated greenhouse gas. This is important for compliance with air quality regulations and permits and reporting of greenhouse gases (GHG) under 40 CFR 98 Subpart W. Higher VOC content gas has greater regulatory control requirements.
Flash Gas Destination
Air permits and greenhouse gas reporting for production facilities require companies to account for all emissions from their sites. If the flash gas is vented to the atmosphere, then the emissions of natural gas (VOCs and GHGs) must be accounted for in the facility’s air permit and GHG reporting.
Recovery of this gas reduces VOC and greenhouse gas emissions.
Most companies route this gas to a compressor, vapor recovery unit (VRU) or to a combustion device (VCU, flare). This is done for economic, safety and air permitting reasons. Although, as a facility’s oil production rate declines, this may not be the case and the heater treater may be changed to vent to the atmosphere – resulting in a new emission source of VOCs. A good management of change (MOC) system can be used to ensure that the heater vent gas is recovered or controlled (VCU, flare).
Quantification of Heater Treater Flash
Methods used to quantify the amount of flash gas produced include:
- Direct measurement of gas with a flow meter (HY-BON/EDI IQR service)
- Simulation using equations of state models (e.g/, HYSYS, PROSIM, VGMSim)
The volume of gas and mass amount of VOCs and methane liberated from heater treaters can be at a very high rate and may exceed the amount of flash gas liberated from atmospheric storage tanks. This would be a function of the pressure drop occurring at the treater, operating temperature and throughput of oil.
Vent gas rates of 20,000 standard cubic feet per day (SCFD) or much higher can be generated in a treater depending on operating conditions and oil throughput.
Periodic measurement of heater vent gas can yield valuable information after changes in production (new wells, natural decline). This information can help with informed decisions regarding process changes that affect profits (amount of gas recovered) and to ensure air permit compliance.
Recovery and Controls
As oil production increases or declines with time, a facility should reassess the methods used for recovery and/or control of the heater vent gas.
Heater vent gas that is currently combusted in a flare or enclosed combustor may be economic for recovery using a VRU that can boost the pressure high enough to send it to the gas handling system (e.g., low pressure suction of compressor). Recovering the gas rather than flaring reduces the facility’s greenhouse gas emissions since flaring gas results in CO2 emissions.
NOTE: a VRU that recovers the gas is process equipment and not emission control device which is subject to more air quality regulations.
Gas that cannot be recovered with a VRU can be sent to a vapor combustion unit (VCU).
The high BTU content of heater vent gas makes this gas stream of considerable value that increases facility profits when recovered and sent to sales. Companies looking to increase profits and reduce emissions will take step to quantify and recover this gas stream (IQR).
HY-BON/EDI Products and Services
Let HY-BON/EDI assist your company stay in compliance with EPA and state vent gas regulations. Our company’s 60 plus years of experience can help with your vent gas management (VGM) needs. This `includes the following:
- Vent gas measurement and leak detection using our IQR services
- Vapor recovery units (VRU)
- Vapor recovery towers (VRT)
- Vapor combustion devices (VCU)
- Field Service (VRU/VCU maintenance, tank seal inspection/replacement, leak detection)