Benzene Emissions in Glycol Dehydrators
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| Summary based on “Control of Benzene Emissions from Glycol Dehydrators”, a Best Management Practices Guide by the Canadian Association of Petroleum Producers (June 2006) Over the last decade, there has been a trend to reduce benzene emissions from glycol dehydrators. The Alberta Energy and Utilities Board (AEUB) recently introduced a directive to further encourage emissions reductions; non-compliance with AEUB directives could potentially result in suspension of operations. Specific documentation and graphs are required which can potentially identify operational improvements that will result in lower benzene emissions. In addition, capital modifications and process alternatives may also potentially result in vastly reduced benzene emissions. The upstream oil and gas industry extensively uses glycol dehydrators to remove water from natural gas streams in order to enhance the properties of the raw natural gas as a saleable commodity. The dehydration process also helps to prevent corrosion and hydrate formation in pipelines. Operators employ TEG, DEG or EG for dehydration of natural gas streams. Benzene occurs naturally in some gas streams. Within the glycol dehydrator system, benzene and other hydrocarbons are absorbed by the glycol in the absorber. The rate of absorption is proportional to the glycol circulation rate. The still column vent is typically the focus of most emission concerns. During heating of the rich glycol in the still column and reboiler, water and hydrocarbons (including benzene) are emitted as vapours from the still column vent. The still column vent can also be the source of methane, ethane and propane emissions from stripping gas and the processed gas. Operators in the upstream oil & gas industry typically purchase standard sized and equipped glycol dehydrator packages for their operations. In the field, companies operate the dehydrators at conditions that attempt to remove more water to minimize system operating problems. Such practices can increase benzene emissions from glycol dehydrators, due to over-circulation of the glycol and unnecessary use of stripping gas. This is especially common for glycol dehydrators at gas wells where gas production rates decline over time. A report in 2005 (CETAC-West, 2005) identified significant economic potential for optimization of glycol dehydrator systems with low or no capital investment required. The paper estimated 2,400 units available for such investigations. The potential savings for Canada (assuming 50% success rate in optimizing these units) are of approximately $50MM with 372,000 tonnes of CO2E eliminated and 180,000 E3M3 of natural gas savings. Minimizing Benzene Emissions from Glycol Dehydrators Although industry is approaching the limit for benzene emission reductions from dehydrators using the current technology, further reductions may be achieved through:
Performance Evaluation Field operating data can be fed into any of the technical evaluation programs being used to assess the performance of a dehydrator. These programs include GRI-GLYCalcTM and Rich-Lean methods, or other programs such as Prosim, HYSYS, or other “in-house” commercial simulators. Results of these evaluations can be used in a sensitivity analysis whereby specific components of the system can be identified as the most appropriate place for implementing emission control strategies. Care must be taken to ensure that the simulation models are properly reflecting the actual performance of the units. Benzene Emissions Key Factors Of all operating variables affecting benzene emissions, the circulation rate has the greatest impact. This is especially important when considering that operators may have to maintain higher than necessary circulation rates for dehydrating the gas, to overcome some of the inherent physical limitations of the equipment (e.g., the glycol flow distribution across the trays, pump minimum flow requirements etc.). Other important factors are absorber pressure and temperature, use of stripping gas, and existence of a flash tank. In addition, supplementary or “add-on” emission controls are used to remove or destroy pollutants in the still column vent emissions. The most common practices involve the use of condenser and thermal (flare or incineration) systems, either separately or in series. These emission control options could be considered as an optimization of facility design and are best handled on a site-specific basis. Regulatory Requirements To encourage emissions reductions, the Alberta Energy and Utilities Board (AEUB) and Alberta Environment have jointly issued Directive 039, entitled Revised Program to Reduce Benzene Emissions from Glycol Dehydrators dated July 10, 2006. This “Dehydrator” Directive came into effect July 10, 2006. Licensees must comply with the following new requirements by January 1, 2007:
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