Thousands of small gas dehydrators are being operated in oil and gas fields. These dehydrators usually have the potential to vent benzene and other volatile organic compounds to the air. A small fraction of them have already been identified as significant emission sources and have been retrofitted with emission control devices. As regulation become more vigorous, more units will be retrofitted with emissions control devices.
Dehydrators contact production gas with triethylene glycol to absorb water from the gas. Wet triethylene glycol is drained from the contactor, heated at atmospheric pressure to drive off steam, and pumped back into the contactor. Some of the hydrocarbon that is absorbed by the glycol is liberated with steam. Triethylene glycol has an affinity for benzene, ethylbenzene, toluene and xylenes (BTEX); consequently the liberated hydrocarbons (VOC) are relatively rich in BTEX. Many units vent insignificant mass of VOC and BTEX, but some of the larger field units might vent more than 50 T/Y of VOC and 10 T/Y of benzene.
Gas dehydrators are placed adjacent to production wells, and at central oil, gas and water separation facilities. The operators want to dry the gas as far up-stream as possible to avoid hydrate and corrosion problems. Many sites do not have electrical power. These dehydrator are usually inspected by gas and oil field personnel from 1 to 3 times a day. The rest of the time they operate unattended. They are sized to dry from 300,000 to 100,000,000 cubic feet per day of wet gas, but actual through-put is frequently much less than rated capacity. Any process used to recover hydrocarbons from dehydrator vent gas must be very simple, inexpensive and robust.
Vapor from reboilers varies in composition and volume. The more important factors include the gas composition, temperature and volume, the glycol circulation rate, and the use of glycol flash separator up-stream of the reboiler. The reboiler vapor data shown in Table I is for a large field dehydrator.
Vapor from a typical field dehydrator is 50 to 90% water on a weight basis. The condensable hydrocarbons content might be from 5 to 50%. The non-condensable hydrocarbons content (methane, ethane, etc.) is minor when a glycol flash separator is up-stream of the reboiler. But the volume fraction becomes significant when a glycol flash separator is not used. Toluene and benzene are usually the most concentrated of the many condensable hydrocarbon components, but rarely does either one exceed 10% of the condensable hydrocarbon content. A field dehydrator typically vaporize from 0.1 to 1 barrel/day of hydrocarbon. Liquid hydrocarbon recovery provides a small economic incentive for not venting the hydrocarbon. However, safety and environmental concerns are providing the greater incentive for curtailing emissions.
The low gas volume, its low pressure and its high water content impose significant limitation on utilization of the vent gas as fuel. Because of its high temperature and high condensable component composition, liquids can easily be recovered from it with air, water or glycol cooled condensers. The remaining non-condensed gas can be burned as fuel, incinerated or vented.