Control of Aromatic Emissions From Glycol Dehydrators

Abstract

SPE Members Abstract Glycol dehydration units utilized in natural gas service have been identified as potential sources of aromatic hydrocarbon emissions into the atmosphere. Depending upon the concentration of aromatic content in the natural gas, large amounts of aromatics may be concentrated in the still overhead water vapor vent. Federal and state regulations have targeted glycol dehydration units for emission control on these carcinogenic compounds. In the near future, some states will levy annual fees to operators who exceed maximum daily discharge limits. Conoco has developed and applied for a patent on a novel addition to the standard glycol dehydration unit that eliminates the emission of almost all of the aromatics and other hydrocarbons. Hydrocarbons, that are normally vented with the water vapor, are recovered as a condensate product or as regenerator fuel. A prototype of the Aromatic Recovery Unit (ARU) was successfully placed in operation in November 1990, along with a new 30 MMSCFD glycol dehydration unit. Two additional units are now built and in operation. This paper describes the design and Conoco's experience with the prototype Aromatic Recovery Unit. Background Most natural gas sale contracts limit water content of the gas to 7.0 pounds or less per million standard cubic feet of gas (1 MMSCF). To achieve these low water contents, producers use various means of dehydrating the gas. Among the most popular is the use of Triethylene Glycol (TEG) as the desiccant. By contacting the gas with TEG, water vapor is absorbed and removed, yielding dehydrated gas for pipeline sale. TEG, in addition to having good water absorption characteristics, has a high affinity for aromatic hydrocarbons. When dehydrating natural gas containing aromatic compounds, TEG will readily absorb the aromatics along with the water vapor. This phenomenon tends to concentrate the aromatic compounds in the "rich" glycol stream. The "lean" TEG is regenerated with heat by vaporizing the water and hydrocarbons out of the "rich" TEG. In a standard glycol dehydration unit (GDU), the still overhead vapors are vented to the atmosphere. The vented stream, which is primarily water, contains varying amounts of hydrocarbons. These contaminants include free gas passing through the system and small amounts of heavier paraffinic and aromatic hydrocarbons. In many systems, some of the vapors are condensed in the still vent line through natural cooling. The condensate that reaches the ground contributes toward contamination of the soil and groundwater. The main aromatic hydrocarbons found in natural gas are the B-T-E-X components. Component Chemical Formula Benzene C H Toluene C H Ethyl Benzene C H Xylenes C H P. 655^